THE BASEMENT STRUCTURE, TECTONIC HISTORY,
SEISMICITY, AND SEISMIC HAZARD POTENTIAL
OF THE FLORIDAN PLATEAU REGION
KENNETH M. LORD
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
This project was generously funded by the Florida Power and
I am indebted to John Luke, Jerry Burford, Russ
Gouldy, Orin Pearson, and David Chaney, all of F.P. & L,
continuing support and enthusiasm.
Additional support was given
by Walt Schmidt and the Florida Geological Survey
who provided the
drill rig necessary for the installation of the seismograph bases.
Over the past years,
Doug Smith's enthusiasm, professionalism,
and friendship have made all aspects of my graduate studies both
rewarding and enjoyable.
In addition, numerous other people
provided advice, assistance, and encouragement to me during the
course of this project.
Tony Randazzo, Paul Mueller, Neil Opdyke,
Tom Crisman, and Ann Heatherington have each provided me with
valuable guidance and helpful suggestions.
Thomas spent many
long hours helping me to debug various digital systems.
and John Roche of the Florida Division of Recreation and Parks and
Robert Arnberger of the National Park Service each went to a great
deal of trouble to make their facilities available for this project, as
did Wes Greenman and Sam Langley of the Department of
helped in the difficult tasks of installing and maintaining the
network of seismographs. In addition, lengthy conversations with
Jon Jee proved to be very fruitful. I am grateful to each of these
people for their time, interest, and assistance.
TABLE OF CONTENTS
LIST OF FIGURES ....
Purpose . . . . . . . . . 1
Methodology .................................. 4
THE STRUCTURE OF THE FLORIDAN PLATEAU
Introduction . . . . . . . . .
Methods of Investigation . . . . . ...
Results and Interpretations. . . . . . ...
Summary ..... . . . . . .
TECTONIC EVOLUTION OF THE FLORIDAN
PLATEAU ........ . .... . .. . . 64
Introduction ................ ......... .. .. ... 64
Regional Tectonic History ......................... 64
SEISMICITY AND STRESS IN THE
SOUTHEASTERN UNITED STATES ...............80
. . . . . . . 80
. . . . . . . 81
5 REGIONAL SEISMOTECTONIC PROVINCES........... 104
Introduction. .. . . .. .. .. ..104
Background and Methods ........................105
Results . . . . . . . . . 111
6 SEISMIC ATTENUATION IN THE
FLORIDA REGION ............................ 123
Introduction . . .. .. .. .. . . . .. .. 123
Background and Methods.........................124
Test Cases for Intensity-Absorption Method ....... .128
Results from the Florida Region ..................133
7 SEISMIC HAZARD IN FLORIDA. . . . . .139
Introduction... ............... .................139
Methods . . . . . . . . .139
Results . . . . . . . . . . .143
8 DISCUSSION AND CONCLUSIONS ...................148
Introduction ................................... 148
Discussion . . . . . . . . . 148
Conclusions.. .. .. ... ..... ..... ...... .. ... 158
APPENDIX SEISMIC HAZARD PROBABILITIES FOR SITES
IN FLORID A .. ... .. .. ... .. .... 161
REFERENCES CITED ................................... 194
BIOGRAPHICAL SKETCH............................... 220
LIST OF FIGURES
Drill hole locations and basement lithologies within the
study area. . . . . . . . .. .a. .
Geophysical survey locations within the study area ......... 11
Depth-to-basement of the Floridan Plateau.
. . . . .. 12
4. Bouguer anomaly map of the Floridan Plateau region.
. . .13
Magnetic anomaly map of the Floridan Plateau region........16
Bouguer anomaly field of the Floridan Plateau downward-
continued to basement level........
. Bouguer anomaly field of the Floridan Plateau upward-
continued to 10 km.
. . . .18
Second vertical derivative of the Bouguer anomaly field
of the Floridan Plateau.
. . ..19
Map of drill hole distribution and basement lithologies
with the designated features discussed in the text.
. . .24
Depth-to-basement map with the designated features
discussed in the text. . . . . . . ...
Magnetic anomaly field map with the designated features
discussed in the text.
. . . .26
- S S- a a.
d a Q d q -
Upward-continued Bouguer anomaly map with the
designated features discussed in the text.
. . . .29
Second vertical derivative of the Bouguer anomaly field
with the designated features discussed in the text.
Various previously published configurations for the
Mesozoic structure of the southern and western
Floridan Plateau basement.
. . .30
. . . . .46
Pre-Mesozoic lithotectonic features of the Floridan
. . . . .60
Mesozoic structural features of the Floridan Plateau
. . . . .6 1
Hypothesized sequence of events during Alleghenian
closure resulting in the translocation of Gondwanan
terrane fragments along transform faults in the
Floridan Plateau basement. . . . .....
Hypothesized sequence of events during the formation
of the Late Triassic and Jurassic extensional features of
the Floridan Plateau basement.
Epicentral distribution in the southeastern United States
and northwestern Caribbean from historical and
. . .72
. . . . 82
Pre-Mesozoic allochthonous terranes of the
southeastern United States.
. . . . .93
Selected Mesozoic seismotectonic features of the
southeastern United States.
. . . .95
The University of Florida Seismograph Network.
Modified Mercalli intensity isoseismal map for the
northern Kentucky earthquake of 27 July, 1980.
. .. .. .130
. Modified Mercalli intensity isoseismal map for the
central Idaho earthquake of 28 October, 1983.
Modified Mercalli intensity isoseismal map for the 1886
Modified Mercalli intensity map for the Florida
earthquake of 27 October, 1973...........
The SEISRISK III input file for the calculation of
seismic hazard in peninsular Florida using the
format described in Bender and Perkins (1987)......... 141
The distribution of annual probabilities of exceeding
0.08 g inFlorida.
The distribution of annual probabilities of exceeding
0.02 g in Florida.
...... .. .. .. 145
. . . . . 146
..... .... 131
. . . .134
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
THE BASEMENT STRUCTURE, TECTONIC HISTORY,
SEISMICITY, AND SEISMIC HAZARD POTENTIAL
OF THE FLORIDAN PLATEAU REGION
Kenneth M. Lord
Dr. Douglas L. Smith
This study seeks to reduce subjectivity in probabilistic seismic
hazard assessments in Florida by providing refined characterizations
of the basement structure, tectonic history, regional seismicity, and
Interpretations of digitally filtered gravity
maps supplemented by previously published data reveal a basement
configuration reflecting the probable translocation of fragments of
Gondwanan crust during Alleghenian continental convergence and
the formation of extensional basins along the southwestern half of
the Floridan Plateau during Mesozoic extension.
The Jay Fault zone
may have originally formed as a dextral transform during
cnn ti n Inrcrtl s -lli inn ht wra c clhchloi onlrtl; ooratr\/a1 tar
The Jurassic Tampa Basin is larger than previously mapped
and the South Florida Basin region probably consists of 2-3 separate
The orientations of the Jurassic
extensional basins suggest two episodes of formation, one during the
rotation of the Yucatan block away from the Floridan Plateau and
another during the separation of North and South America.
no evidence of fault reactivation or other tectonic activity on the
Floridan Plateau since the Middle Cretaceous.
This tectonic quiescence is manifested in the unusually low
level of seismic activity on the Floridan Plateau as demonstrated by a
review of regional instrumental and historical records and the
deployment of a network of digital seismographs accompanied by
four years of monitoring.
The nonuniform distribution of regional
seismic activity is probably related to the distribution and
orientations of preexisting mid-crustal faults and to their
propensities for slippage.
Seismic attenuation in Florida appears to be slightly higher
than in the central United States
western United States.
, but significantly lower than the
Although seismic hazard throughout Florida
is extremely low, the greatest probability for damaging ground
motion (> 0.2 g) exists in northwestern Florida and is calculated to be
8.0 x 10-5
/year, indicating a recurrence interval of 12,500 years.
Throughout most of Florida, however, the probability of exceeding
0.2 g is significantly less than 1.0 x 10-5/year, suggesting recurrence
Large construction projects, such as dams, bridges, and nuclear
are designed to withstand specific maximum levels and
durations of strong ground motion.
The projections of maximum
levels and durations of strong motion are made through the use of
site-specific seismic hazard analyses,
which are based on a number
These criteria include the levels of historical and
the identification and characterization of
seismotectonic sources, expected maximum magnitudes of
earthquakes, and expected resultant ground motion at particular
Yegian, 1979; Johnston, 1981
Floridan Plateau has a historically low level of seismic activity and a
rather enigmatic tectonic setting, little of the information needed to
assess or quantify the criteria used for the analysis of seismic hazard
The most reliable indicators of seismic hazard are the regional
levels of historical and instrumental seismicity
The paucity of
reported seismic events in the historical record suggests that the
I T -_. L---- -- -A --
escarpment, suggests that the historical record may not provide a
reliable measure of seismicity.
In addition, because there has been
no regional network of seismographs until recently, there is little
documentation for the level of instrumental seismicity.
Consequently, neither historical nor instrumental records provide
reliable gauges of seismic activity for use in hazard analyses in
Another potential indicator of seismic hazard is the
identification and characterization of seismotectonic sources and
source regions. Seismicity in the eastern United States is generally
attributed to the reactivation of Mesozoic or older faults in a
moderate stress field (Sykes, 1978; Hamilton, 1981; Wentworth and
Mergner-Keefer, 1983; Zoback, 1992); however, the structure of the
Floridan Plateau basement is deeply buried.
basement of the plateau is overlain by Coastal Plain sedimentary
deposits, which range in thickness from about 900 meters in north-
central Florida to over 5000 meters in southern Florida (Wicker and
Smith, 1978). As a result of this thick sedimentary sequence, there
are relatively few drill holes intersecting the basement (Barnett,
1975; Smith, 1982; Chowns and Williams, 1983) and regional
potential field interpretations are difficult (e.g., Oglesby et al., 1973;
Klitgord et al., 1984).
Consequently, current models for the structure
of the Florida basement are necessarily characterized only by
generalizations of large scale features and lithotectonic units, while
The amplitude of ground motion at a particular epicentral
distance from a seismic event is dependent not only upon the seismic
moment of the event
, but also upon individual site characteristics
and seismic attenuation. Seismic attenuation in the eastern and
central United States has been studied on a regional level (Nuttli,
1973A; Jones et al.,
Bollinger, 1979; Campbell, 1981
Herrmann, 1983), but there have been no independent studies of
attenuation in Florida. Accordingly, Nuttli's (1973A) generalized
attenuation coefficients for the eastern United States are employed in
calculations of seismic hazard in Florida.
However, the Floridan
Plateau has been shown to be an allochthonous terrane with respect
to the North American craton (e.g., Smith, 1982)
. This suggests that
the mechanical response of the plateau may differ from that of the
remainder of the eastern United States, which would invalidate the
use of Nuttli's attenuation coefficients.
As a result, estimations of
expected ground motion in Florida may be invalid.
One consequence of the limitations inherent in the historical
and instrumental records in Florida and the enigmatic nature of the
tectonics and structure of the basement is that the criteria used in
regional seismic hazard analyses (i.e., seismicity, seismotectonic
, and seismic attenuation) are poorly documented.
Previous seismic hazard analyses in Florida have assumed, with little
substantiation, that the risk of local seismic activity is negligible and
that Nuttli's eastern United States attenuation coefficients are
analyses and to reexamine the tectonic history of the region.
Specifically, the intention is to provide improved characterizations of
the basement structure, tectonic evolution, regional seismicity, and
seismic attenuation in the Floridan Plateau region, and to incorporate
these into an empirical assessment of seismic hazard in Florida.
Several geophysical methods have been employed and
incorporated into this study.
methods used are discussed in detail.
In each of the following chapters,
The purpose of this section is
to provide a general overview of the sequence of these individual
methods and how each has been utilized in this study.
The first step of the study was to digitize and filter potential-
field data for the entire Floridan Plateau.
The resultant filtered
gravity maps for the Plateau, when interpreted in conjunction with
previously published magnetic,
drill hole, seismic reflection
seismic refraction data, permitted an improved resolution of the
locations, sizes, natures, and spatial relationships between the
various crustal blocks and structural features of the plateau
A tectonic interpretation based on this improved
structural model prompted a general interpretation of the seismic
history of the plateau, and existing seismic reflection studies of the
Cenozoic sedimentary accumulations overlying the basement allowed
a more specific determination of recent levels of seismic activity.
which only encompass the past twenty years in this region.
instrumental coverage in Florida was insufficient, a network of
digital seismographs was established for the purposes of attempting
to detect and characterize any previously unknown local sources of
seismic activity and to characterize the local mechanical response of
the crust to regional events.
The information compiled from these
historical and instrumental sources was then examined within the
context of previously published regional stress field indicators to
develop a lithospheric model for the distribution of earthquakes in
the southeastern United States.
Following this, each of the seismic source provinces in the
southeastern United States, the Caribbean, and the Gulf of Mexico in
which activity could potentially generate significant ground motion
in Florida were identified.
Each province was characterized based on
the compiled earthquake information, the lithospheric models for the
Floridan Plateau and the southeastern United States, and previously
Included in the characterizations were estimates
of maximum magnitude and recurrence intervals.
Subsequently, a method was developed to estimate a local
value of seismic attenuation in Florida.
This method utilizes
isoseismal maps as a means of approximating the decrease in peak
ground acceleration with distance from an event with a known or
reasonably well-approximated magnitude.
(acceleration, distance, and magnitude) are then used to
where attenuation coefficients are well known.
Once validated, the
method was applied to the isoseismal maps from the 1886 Charleston
earthquake and a 1973 earthquake in northeastern Florida in order
to estimate attenuation coefficients for the southeastern Coastal Plain
Finally, the source zone characterizations and estimated
attenuation coefficients were used as input for a probabilistic seismic
hazard assessment for Florida.
This analysis is based on the method
of Cornell (1968), which addresses the effects of all of the possible
earthquakes likely to affect Florida for each site on a grid of sites
around the state.
Based on the distances to each seismic source zone,
recurrence intervals, and seismic attenuation coefficients, a value is
calculated for each site describing the probability that a particular
acceleration will be exceeded at that site within a given time frame.
The resulting grids were contoured to produce probabilistic seismic
hazard maps for Florida.
THE STRUCTURE OF THE FLORIDAN PLATEAU
All regional models describing the structural features of the
Floridan basement have been founded primarily on deep drill hole
data from wells penetrating pre-Cretaceous crystalline rock.
of an intrinsic bias in the spatial distribution of these drill holes,
most of the regional models have been limited in scope to peninsular
Florida, rather than to the entire Floridan Plateau. Similarly, more
localized, yet more detailed, structural models based on seismic
reflection profiles tend to be spatially biased towards offshore areas.
As a result, with the notable exception of Klitgord et al. (1984)
has been little effort to integrate previously published studies into
an examination of the Floridan Plateau as a singlar feature.
Since the work of Klitgord et al., a number of pertinent
geochemical, geophysical, and drill hole studies have been published
(e.g., Nelson et al., 1985A; Ball et al., 1988; Heatherington et al., 1989;
Dobson, 1990; Heatherington and Mueller, 1991; McBride, 1991);
these have yet to be examined within a regional context.
intention in this chapter is to utilize digitally filtered regional
S,- I C 1 .. .. .. .... .-- ... 1 ..l .J. t, I .ta.a.f I...S t.,. .%C
Methods of Investigation
A series of digital geophysical and structure contour maps for
the Floridan Plateau were produced for this investigation using the
methods described in Lord (1992)
These maps cover the rectangular
area encompassed by 24 to 31 degrees north latitude and 80 to 86
degrees west longitude. All of the digitizing, digital filtering
operations, and contouring were done on a IBM 386 compatible,
equipped with a Houston Instruments digitizing tablet. Additional
drafting and figure preparation were done on an Apple Macintosh
IIci equipped with a scanner and laser printer. Commercial software
packages employed include Rockware's Digitize, Golden Software's
Surfer, Lotus 1-2-3, Silicon Beach Software's Superpaint, and a series
of potential-field geophysical programs available through the USGS
(Hildenbrand, 1983; Grauch, 1984; Godson and Mall, 1989).
The first step was to produce a detailed depth-to-basement
map for the plateau.
In this study area, basement is defined by the
unconformity between pre-Late Jurassic rocks and younger
sedimentary accumulations of the Coastal Plain. All available drill
hole depth-to-basement data, shown in Figure 1, were plotted on a
grid representing points with a 9.2 km spacing over the study area.
Sources for these data include Barnett (1975), Smith (1982)
and Williams (1983), Schlager et al. (1984),
Ball et al. (1988), and
A number of additional depth-to-basement determinations
"N(* .."4 +++
* Granitoid Plutonic Rocks
o Felsic Extrusive Rocks
o Metamorphic Rocks
+ Quartzitic Sandstone or Shale
U Quartzitic Sandstone/Shale with Diabase
* Triassic Redbeds with Diabase
x Mafic Extrusive Rocks
E Intercalated Felsic and Mafic Extrusives
ritill1 h1 r d nl mn-l n Iltwvt Hlll;r lnn nalic wAnth;inr rho
al., 1981; Schlager et al., 1984; Shaub, 1984; Dillon et al., 1985; Nelson
et al., 1985A; Lord, 1987
Ball et al., 1988; McNeely, 1988; Mullins et
Winker and Buffler, 1988; Dobson, 1990; McBride, 1991).
2 is a location map compiled from these geophysical surveys.
In those seismic studies in which the original authors had not made
time-depth conversions, drill hole depth data were used to provide
controls for these conversions.
Where this drill hole control is not
available, such as in deeper areas, time-depth conversions are
These depth determinations were then plotted on
the appropriate points on the grid.
Depth-to-basement values were
interpolated to each remaining point and the grid was subsequently
digitized and contoured.
The final map is shown in Figure 3.
A Bouguer anomaly map of the study area was produced as
This map, which is shown in Figure 4, is essentially a digitized
and recontoured version of that produced by Klitgord et al. (1984),
although an older map by Oglesby et al. (1973) was extensively
utilized as well.
The original Klitgord map was constructed through
the compilation of several different maps from several different
studies and, consequently, there are minor inconsistencies at the
borders between these study areas.
In addition, their map has a
small data-less area extending along the northwestern coast of
peninsular Florida (Klitgord et al., 1984, pg. 7760, fig. 8).
For the purposes of digital filtering, it was desirable to produce
a single, continuous data set for the study area. In order to
Geophysical survey locations within the study area.
(Compiled from Antoine and Harding, 1965; Sheridan et al., 1966;
introduce some error, it is believed that any such error will be
As with the depth-to-basement map, the Bouguer anomaly
map was digitized on a rectangular grid with a 9.2 km spacing, which
was chosen to conform to software limitations while maximizing the
number of data points.
The resulting 6696 data points were then
contoured with a 5 milligal contour interval using the Surfer
A magnetic anomaly map (Figure 5) was also constructed, using
data from King (1959)
Gough (1967), and Klitgord et al. (1984).
data were digitized from these original maps and contoured using a
50 nanotesla contour interval.
Unfortunately, there are relatively
large areas of the plateau for which there are no data.
filters used in this study employ Fourier synthesis to transform the
data from the space domain to the frequency (or wave number)
domain. As a consequence of Gibbs phenomenon, that is, the error
introduced during Fourier synthesis near a discontinuity, it was
determined that the data-less areas in the magnetic anomaly map
would result in unacceptable errors should the magnetic data set be
digitally filtered. As a result, the use of an unfiltered magnetic
anomaly map for the plateau is considered to be more desirable for
Because this map represents a compilation from a number of
magnetic surveys (e.g., Klitgord et al., 1984), the anomaly values
from different sections of Figure 5 may be derived from several
magnetic anomaly patterns presented in Figure 5, there is
undoubtedly some error in the relative magnetic values between any
given area and values in other sections of the map.
The Bouguer anomaly data were digitally filtered using several
methods and a series of maps were created from the filtered data.
These digital filters included a downward continuation (Figure 6), an
upward continuation (Figure 7),
(Figure 8) of the Bouguer anoma
filters are explained below.
and a second vertical derivative
ly field. The purposes of these
In each case, several rows and columns
of extrapolated data were added to each side of the grid to reduce
the near-boundary distortion of Gibbs phenomenon.
rows and columns were then removed prior to contouring.
The downward continuation of potential-field data was done
from a level to an irregular surface using the method of Grauch
. This method is used to approximate the relative values of
gravitational potentials at the basement surface.
to-basement is quite variable,
Because the depth-
ranging from about 850 m to over
6000 m, the removal of resultant depth-related deviations in the
observed field through this method of downward-
continuation might be expected to reveal previously unobserved
trends or features.
In addition, downward continuation has the
effect of sharpening the field,
which better outlines source features
The application of downward-continuation filters, either in the
. This necessitates the use of a low-pass filter prior to
performing these operations.
In order to apply this low-pass filter, the gridded data set is
transformed into the frequency, or wave number, domain through
the use of the two-dimensional Fast Fourier transform (or FFT)
FFT is simply an efficient method of calculating the Discrete Fourier
transform (or DFT),
which, in turn, is a digital method of
approximating the continuous Fourier integral function (Cooley and
. The two-dimensional FFT is given by
where G(k,l) is the potential-field function, with k and 1 representing
grid point locations in the frequency domain, m and n are integers
representing grid point locations in the space domain, M is the
number of rows, and N is the number of columns (Hildenbrand,
. Once in the frequency domain, a tapered low-pass filter was
applied; this progressively filtered out wavelengths from 36.8 km to
18.4 km, while wavelengths below 18.4 km (high frequencies) were
The data set was then transformed back to the
space domain through the application of the inverse FFT (or IFFT).
For the actual downward continuation, the method of Grauch
(1984) utilizes a Taylor series expansion to approximate the
potential-field data, f(z)
at some depth, z, from the observed
F(z) = F(O) +
i2 f(O) +
z value for each grid location was taken from the depth-to-
basement data already described.
This calculation was repeated for
each point on the grid, the added boundary data were removed, and
the resultant data set subsequently contoured (Figure 6).
Another filter applied to the Bouguer anomaly field was an
upward-continuation to an elevation of 10 km.
This has the effect of
removing short-wavelength features of the field, which are often
attributable to shallow crustal sources (Figure 7)
. The upward-
continuation is accomplished by taking the FFT of the data as
described above, giving the Fourier coefficients G(k,1l). As described
by Fuller (1967)
these Fourier coefficients are then multiplied by
the wave number response of the filter.
H(k,l) is given by
where z is the continuation distance.
The application of the IFFT to
the gridded data and the removal of the added boundary data then
permitted the construction of the final upward-continued map.
In a similar manner, a second vertical derivative map was also
The second vertical derivative effectively removes long-
frequency domain and the Fourier coefficients subsequently
multiplied by the wave number response of the filter as given by
Again, the data set was then transformed back into the space domain
using the IFFT, the extra boundary data removed, and the grid
Results and Interoretations
In general, these maps complement and augment the results of
many previous investigations.
The depth-to-basement map
accurately depicts the Peninsular Arch, the expected southward
increase in depth, the series of Mesozoic basins in the southern and
western regions of the plateau, and a small previously undescribed
basin under the northeastern edge of the plateau (Figure 3).
While the differences between the original Bouguer anomaly
map (Figure 4) and the downward-continued map (Figure 6) are
relatively subtle, the downward continuation is preferable as it more
accurately delineates the boundaries of anomaly-producing features.
The upward-continued map (Figure 7) delineates a number of long-
wavelength features that are clearly associated with large scale
crustal features, such as the Mesozoic basins, that have been
previously described, but poorly located and characterized.
accumulations of the Florida Platform are relatively homogeneous
with respect to density.
shallow crustal features
Furthermore, it suggests that smaller
, such as plutons, are not particularly
abundant on the plateau, although this does not preclude the
existence of such features with low density contrasts or in the middle
Pre-Mesozoic Features of the Plateau Basement
The lithologies of the pre-Mesozoic basement terranes of the
northeastern portion of the plateau have been moderately well
Basement lithologies from the remaining portions of
the plateau are unresolved; however, more structural detail is
As a result, this section will necessarily be focused on the
pre-Mesozoic lithotectonic terranes of northern peninsular Florida,
while the next section will be focused primarily on the Mesozoic
structural features of western Florida.
In the following discussion,
the location of each of the lithotectonic features or regions within the
study area is designated by a letter on Figures 9-15.
Osceola Granite--reeion A.
The Osceola Granite, previously
known as the Avalon terrane, is the oldest identified terrane in the
Although described as a granite by convention, it
is better characterized as a relatively undeformed granitoid batholith
composed of a range of felsic lithologies (Milton and Grasty, 1969;
Barnett, 1975; Smith, 1982)
. This granitoid complex underlies the
been obtained by Dallmeyer et al. (198
7) using 40Ar/39Ar
techniques on a composite biotite sample from several specimens.
Sm-Nd isotopic data for the Osceola Granite are suggestive of a mixed
source containing both Proterozoic and Archean components
(Heatherington et al., 1989).
The 8 drill holes intersecting granite in east-central Florida are
generally centered around a pentagonal negative gravity anomaly
(Figures 9, 13
14--A), although several of these holes are outside the
boundaries of the anomaly.
The magnetic signature of the terrane is
more complex, as the easternmost of the holes are located along a
north-south oriented, elongated positive magnetic anomaly, while
most of the others are located within the confines of a proximal
negative magnetic anomaly (Figure 11--A).
oriented boundary between these magnetic features bisects the
negative gravity anomaly.
Barnett (1975) modeled the Osceola Granite as constituting the
block-faulted end cap of the Peninsular Arch, a hypothesized
structural high under north-central Florida. Although the Peninsular
Arch has since been described as an erosional feature (Smith, 1982)
and the Osceola Granite shows little petrologic evidence of
metamorphism (Milton and Grasty, 1969), the possibility of block
The randomness and variability in depth-to-
basement for the 8 drill holes is suggestive of block-faulting, as is the
variability in both the gravity and magnetic fields in the area
suggested by the number of drill holes that have bottomed in
weathered diabase throughout the area (Figure 9).
The contacts between the Osceola Granite and the juxtaposed
terranes are not well constrained. A primary reason for this arises
from the inherently poor resolution of isotopically determined ages
for the North Florida Rhyolite to the north (Mueller and Porch, 1983)
and the St. Lucie Metamorphic Complex to the southeast (Bass, 1969).
As a result, the fundamental age relationships between the three
terranes remain undetermined and age-based inferences concerning
the nature of the contacts between them are ambivalent. A number
of contradictory suggestions have been made.
For example, Chowns and Williams (1983), citing the apparent
concordance in ages between the Osceola Granite and the
amphibolite, suggested that they could comprise a conterminous
terrane. Accordingly, the rhyolitic terrane would then overlie both
the granite and amphibolite, and be separated from them by a thrust
fault or unconformity. Alternatively, Bass (1969) believed that the
St. Lucie amphibolite, by reason of metamorphic grade, must
necessarily be separated from all other Florida basement rocks by a
"fundamental discontinuity." This was supported by the presence of
thin layers of unaltered basaltic rock, granite, and diorite
immediately over the amphibolite, which implies a horizontally
faulted contact (Bass, 1969). Another possibility is that the Osceola
Granite represents an intrusion into an older terrane consisting of the
Granite that it is bounded by steeply dipping contacts with the
Samples from several drill holes show that the Osceola Granite
extends northeastward and northwestward beyond the boundaries
inferred from the potential-field gradients; however, to the south,
other holes penetrate rocks indicating that the steep gradients mark
the boundaries of the terrane (Figures 9, 11, 13--A)
the Osceola Granite appears to be truncated
by the Jay Fault, as has been suggested by Smith (1982).
southeast, the presence of both mafic and felsic extrusive rocks in
drill holes near to those intersecting the Osceola Granite clearly
delineates the boundary, which coincides with steep,
trending gravity gradients separating the pentagonal negative
anomaly from a smaller ovoid positive anomaly.
It is unclear
whether the extrusive rocks overlie the granitoid or rather have
been juxtaposed against it.
The eastern boundary of the Osceola
Granite is unresolved, although the steep north-northeast trending
gravity gradient along the eastern side of the pentagonal negative
anomaly suggests that the granitoid may not extend beyond the coast
of peninsular Florida.
St. Lucie Metamorphic Comnlex--reeion B.
A single drill hole
has revealed the presence of an amphibolite grade metaigneous
assemblage at a depth of about 3760 m immediately southeast of the
Osceola Granite, underlying St. Lucie County (Bass, 1969; Milton and
approximate age obtained for the Osceola Granite.
The assemblage is
comprised primarily of amphibolite, with some quartz diorite gneiss
and chlorite schist (Bass, 1969; Thomas et al., 1989).
several names have been applied to this terrane, in this work it will
be referred to as the St. Lucie Metamorphic Complex.
The drill hole is located near the northern edge of an ovoid
east-west oriented negative gravity and magnetic anomaly
anomaly is relatively small
100 km long by 5
, and may delineate the boundaries of the terrane (Figures 11
It is probable that the St. Lucie Complex is bounded to
the southwest by the Jay Fault.
To the north
the terrane extends
only a short distance, as demonstrated by drill holes and inferred
from the gravity and magnetic anomaly.
To the east, the anomaly
extends only to the coast of peninsular Florida and, in an
interpretation of an offshore seismic reflection profile, Sheridan et al.
(1981) found no evidence for the presence of this terrane off the
As a result, it appears that the St. Lucie Metamorphic
Complex encompasses only a relatively small area and may therefore
be a fault-bounded block.
An alternative possibility is that the
metamorphic rocks represent a roof pendant in the proximal felsic
igneous rocks (P
The metamorphic grade of the assemblage is indicative of
regional activity, which suggests that this small terrane represents
an isolated fragment of a once larger unit (Bass, 1969).
Metamorphic Complex and are approximately the same age
Based on these similarities and because there are
no other similar metamorphic terranes known in the Gulf of
Schlager et al. (1984) have inferred the
two to be cogenetic.
North Florida Rhvolite--reeion C.
Overlying the Osceola Granite
to the north and west is a terrane of slightly metamorphosed calc-
alkaline rhyolitic to andesitic volcanic rocks.
The upper surface of
this terrane in northeastern Florida dips southeastward and ranges
in depth from about 1180 m to about 1650 m (Smith, 1982).
calc-alkaline nature (Mueller and Porch, 1983) and trace element
signatures (Heatherington et al.,
1989) of samples from this terrane
each suggest eruption in an ocean-continent convergent margin
In attempting to date two separate samples,
and one andesite, Mueller and Porch (1983) identified disturbed
40Ar/39Ar release patterns from each--despite this,
concordant ages of 412
15 Ma and 418 9 Ma were obtained.
Although the disturbed release patterns suggest these age
determinations to be minima, stratigraphic relationships suggest a
Late Precambrian to Early Paleozoic age for the terrane (Arden,
1974; Chowns and Williams, 1983).
Few structural inferences have been made about the North
From a seismic reflection survey in the Florida
panhandle, Arden (1974) distinguished faulting and gentle folds in
the South Georgia Rift, as suggested by Arden (1974) and Klitgord et
The sublateral extent of this terrane is uncertain. Although the
distribution of drill holes intersecting rhyolite is generally coincident
with an elongated negative gravity and magnetic anomaly (Figures 9
previous geophysical and drill hole studies suggest that
the terrane continues northward under the Suwannee Basin terrane,
described below, and extends into southern Georgia (Barnett, 1975;
Wicker and Smith, 1978; Chowns and Williams, 1983).
To the south,
two drill holes intersecting rhyolite have been interpreted to indicate
a subcrop of this terrane immediately south of the Osceola Granite
(e.g., Chowns and Williams, 1983
rhyolitic samples have also been obtained from the south-central
portion of the Floridan Plateau in the South Florida Basin; it has been
suggested that these samples are either from horsts of this same
felsic volcanic terrane (Thomas et al., 1989) or from part of a
separate southerly Mesozoic bimodal suite (Barnett, 1975; Mueller
and Porch, 1983)
. To the west, felsic basement rocks have been
encountered in the Florida panhandle area; these rocks are undated,
and have been variously correlated to the North Florida Rhyolite
(Arden, 1974; Thomas et al.,1989),
the Osceola Granite (Barnett,
and to the Permian (?) granite of the Wiggins Arch in
Mississippi (Smith, 1983).
Suwannee Basin--region D.
Overlying the north Florida igneous
1983; Thomas et al, 1989).
This terrane extends northward into
southern Georgia and westward at least to the Florida panhandle, as
well as southwestward to the Middle Ground Arch in the north-
central region of the plateau (Ball et al., 1983; Dobson and Buffler,
. The Suwannee Basin has been intersected by numerous drill
holes at depths ranging from 720 m to 1730 m (Smith, 1982) and is
composed of a massive Early Ordovician quartzitic sandstone overlain
by intercalated Middle Ordovician to Middle Devonian sandstones
and shales (Arden, 1974)
. The topographic high of the terrane, in the
north-central portion of peninsular Florida, is an erosional remnant
known as the Peninsular Arch.
The structure of the Suwannee Basin is somewhat better
defined than that of the underlying North Florida Rhyolite, although
the nature of the contact between the two is rather enigmatic. Arden
(1974), from an interpretation of a seismic reflection profile,
interpreted the contact to be a northeast-dipping, Middle to Late
Paleozoic age thrust fault that emplaced the Suwannee Basin over the
volcanics, while others (e.g., Thomas et al., 1989) have suggested the
contact to be an unconformity. Although drilling into the Suwannee
Basin terrane has never penetrated more than about 700 m, gravity
modeling indicates that the eastern Suwannee Basin has a maximum
thickness of about 2500 m (Wicker and Smith, 1978), while seismic
reflection profiles suggest the western Suwannee Basin may be even
thicker (Arden, 1974; Thomas et al., 1989).
profile and numerous drill holes have shown the western section to
have been extensively faulted, mildly folded, and intruded by
diabase during the Triassic evolution of the South Georgia Rift.
difference has prompted some workers to separate the Suwannee
Basin into two distinct terranes, the East and West Suwannee Basins
(e.g., Klitgord et al., 1983; Ball et al., 1988).
The relatively smooth potential-fields in the region occupied by
the Suwannee Basin terrane are bisected by two linear NE-trending
negative gravity anomalies separated by a distinct, northeast-
trending negative magnetic anomaly and low-amplitude positive
These extend from the northeastemmost Gulf of
Mexico to the eastern Florida-Georgia border (Figures 11 and 1
Based on an onshore, local, high resolution gravity
study, Coleman and Stewart (1982) believed the Suwannee Basin
sedimentary deposits to be underlain by two northeast-trending
Paleozoic (?) troughs coinciding with these linear negative gravity
Offshore, however, Dobson and Buffler (1991) have
interpreted basement faulting in a seismic reflection profile and the
presence of Triassic red beds in proximal drill holes as indicative of a
moderately-sized, northeast-trending Triassic rift basin extending at
least to the northwest Florida coast and coinciding with the central
positive gravity anomaly and the associated negative magnetic
anomaly of feature D1.
The absence of corresponding red beds in
onshore drill holes suggests the possibility that feature D1, although
Although the boundary between the Suwannee Basin terrane
and the superimposed Triassic South Georgia Rift is reasonably well
constrained from drill hole data, it is likely that the Suwannee Basin
extends northward and westward under the rift basin and for a
significant distance beyond it.
COCORP seismic reflection profiling
(Nelson et al., 1985A) and gravity modeling (Lord et al., 1992) both
indicate that the Suwannee Basin extends into southern Georgia along
most of its border with Florida, although neither the nature nor the
location of the northern or western boundaries of the terrane have
lay Fault--feature E-E1
. A variety of evidence suggests the
presence of a major crustal boundary, known as the Jay Fault, which
may divide the Floridan Plateau crust.
Northeastern Florida is
generally characterized by northeast-southwest trending gravity and
magnetic anomalies, while those in southwestern Florida are
generally perpendicular to this trend (e.g., King, 1959; Oglesby et al.,
1973; Smith, 1982). Although the boundary between these two
trends is not well defined, it extends through the center of the
plateau in a northwest-southeast direction, close to a landward
extension of the trend of the Bahamas Fracture Zone.
Drill hole evidence and seismic reflection profiles suggest that
the implied extension of the fracture zone also constitutes the
boundaries of several features, including the South Georgia Rift, the
north Florida igneous rocks, and the Suwannee Basin terrane
and in the northeastern Gulf of Mexico (Dobson and Buffler, 1991)
suggest normal faulting along this same trace.
Consequently, the Jay
Fault is generally considered to be a morphological extension of the
Jurassic Bahamas Fracture Zone.
The origin, character, and age of this
extension, however, are unresolved (Barnett, 1975; Smith, 1982;
Klitgord et al., 1984; Smith, 1993).
It has been suggested that the Jay Fault is a Jurassic left-lateral
transform that served to connect spreading centers in the Atlantic
and Gulf of Mexico, while bringing several disparate lithotectonic
units into juxtaposition (Klitgord and Schouten, 1980; Smith, 1982;
Klitgord et al., 1984). Using this model as a starting point, several
authors (Smith, 1982; Chowns and Williams, 1983; Whitelaw and
Smith, 1989) have hypothesized an extension of this proposed
transform boundary across the Floridan Plateau and into Alabama,
where it may merge with the Middle Mesozoic Gulf basin marginal
fault zone (Smith, 1983; Klitgord et al., 1984).
Another possibility for the origin of the feature is that it is a
right-lateral feature formed during the Late Paleozoic closure of the
Iapetus Ocean (Smith et al., 1992; Smith, 1993). The premise of this
model is that the Jay Fault originated in two stages. During the
Paleozoic, the boundary was a right-lateral, strike-slip fault that
served to accommodate differential motion within Gondwana during
closure with Laurentia.
Later, during Triassic and Jurassic extension,
the boundary was reactivated as a normal fault.
This would be
panhandle region, also suggested a right-lateral fault model,
in this model, the fault formed in response to north-south
compressive stresses during the Jurassic.
Inherent in a third model for the nature of the lithotectonic
boundary across Florida is the implication that the Jay Fault has
never been subject to significant lateral motion (Heatherington et al.,
1989; Heatherington and Mueller
Results of trace element
analyses of basement rocks from north and south Florida exhibit
similar patterns of high field strength element depletion and large
ion lithophile element enrichment (Heatherington et al., 1989), as
well as similar Nd model ages for the lithosphere from which these
rocks were derived (Heatherington and Mueller, 1991)
suggests that the volcanic suites of north and south Florida may have
been derived from a single contiguous underlying unit.
It is possible,
then, that this geophysically and lithologically inferred fault may not
be a transform that juxtaposed two entirely disparate lithotectonic
units, but rather that it is simply a normal fault zone that
experienced little or no lateral movement (Heatherington and
The linear nature of the Jay Fault is evident on the depth-to-
basement and upward-continued Bouguer anomaly maps (Figures 10
. The upward-continued Bouguer anomaly map
strongly suggests that the fault acts as boundaries for the Osceola
Granite (A), the St. Lucie Metamorphic Complex (B), the South Georgia
is the Jay Fault linear, but apparently continuous across the Floridan
Plateau as well.
Along the southeastern edge of the plateau, Sheridan's (1981)
seismic reflection profile clearly shows a series of faults with varying
amounts of horizontal offset along the proposed trace of the Jay
Hence, the Jay Fault is more accurately described as a zone of
faulting, rather than as a single fault. An abrupt southward increase
in depth-to-basement marks the zone along most of its length. An
exception to this is in the northwestern part of the plateau where the
South Georgia Rift extension intersects the zone. At this point, a
seismic reflection profile (Dobson and Buffler, 1991) indicates the
presence of a single northeastward-dipping master fault.
although the Jay Fault zone appears to be continuous and linear, the
nature of the Mesozoic faulting changes along strike.
Mesozoic Features of the Plateau Basement
The Floridan Plateau basement is characterized by a variety of
Mesozoic structural features.
Lithic accumulations associated with
these features (i.e., red beds, hypabyssal rocks, etc.) often obscure
the pre-Mesozoic units of the underlying crust and, as a result, the
pre-Mesozoic nature of the basement under these features is largely
While the general locations of Mesozoic features on the
plateau basement are reasonably well established, specific structural
details and subaerial boundaries are poorly constrained.
northeast-striking Jurassic basins and ridges. This series extends to
southern peninsular Florida, where a large Triassic-Jurassic basin
occupies the southern third of the peninsular basement (Klitgord et
South Georgia Rift--reeion F.
Underlying the north-central
portion of the plateau is a 95 km wide, northeast-trending section of
the Early Mesozoic South Georgia Rift.
The South Georgia Rift is a
large, complex graben system extending from the Florida panhandle
area to eastern Georgia (Daniels et al.,
section in Florida overlies the western block-faulted edge of the
Suwannee Basin terrane (Smith, 1982) and has been previously
referred to as the Tallahassee Graben (Smith, 1983) and the
Southwest Georgia Embayment (Barnett, 1975; Miller, 198
. The rift
is filled with arkosic sandstones and mafic igneous rocks with a
maximum thickness of about 1800 m in Florida (Arden, 1974) and it
has been pervasively intruded by diabase sills (Barnett, 1975;
Chowns and Williams, 1983; Smith, 1983).
Milton and Grasty (1969)
acquired three K-Ar ages for samples of diabase sills from northern
Florida. Although these ages are now generally considered to be
suspect, they were found to range from about 180 to 200 Ma.
The onshore boundaries of the South Georgia Rift are
reasonably well constrained from the presence of red beds and
associated hypabyssal rocks encountered in deep drill holes. A
circular negative gravity anomaly with a diameter of about 50 km
accumulations of low density salt in the Jurassic Apalachicola
Embayment, which overlies the South Georgia Rift in this area.
The north-central Floridan extension of the Triassic-Early
Jurassic South Georgia Rift system is a moderately complex feature.
The presence of short-wavelength gravity and magnetic anomalies in
the area of the rift (Figures 11 and 15--F) may indicate it to be
comprised of an assemblage of secondary horst and graben
structures (Barnett, 1975
Limited seismic reflection
profiling shows extensive normal faulting in the rift floor and the
surrounding area; however, 1
definitively detected (Arden,
no large secondary horsts have been
1974; Dobson and Buffler, 1991).
Because there are few seismic reflection profiles in the area
and, as shown in Figure 14, the long-wavelength potential-field
signatures associated with the rift have relatively low gradients, the
lateral boundaries of the rift extension are not definitively mapped
Ball et al. (1988), in a seismic reflection profile just
off the coast of the Floridan panhandle, interpreted a northeast-
dipping master fault with about 1400 m of throw to mark its
southwestern terminal boundary
Subsequent reflection work has
shown the offshore extension of the rift system to consist of two
at least one of which probably extends to the
southwest as far as the Jay Fault zone (Dobson and Buffler, 1991).
Analachicola Embavment/Analachicola Basin-reeions F. G.
Extending from northwest to southeast across the Floridan Plateau
the Apalachicola (or Southwest Georgia) Embayment (F) and the
Middle Ground Arch (H), a basement high that may extend
undisturbed across the Jay Fault zone.
These are located to the
northeast of the Jay Fault zone and are part of a series that extends
northwestward and includes the Chattahochee Arch, the Covington
Embayment, the Conecuh Arch and the Wilcox Embayment, all of
which lie out of this study area (e.g., Miller, 1982; Mitchell-Tapping,
1982; Smith, 1983).
On the southwestern side of the Jay Fault zone are the
Apalachicola Basin (G)
also known as the Northeast Gulf Basin, and
the Tampa Basin (I), also known as the St. Petersburg Basin.
are separated by the Middle Ground Arch (H).
Continuing to the
southeast, the Sarasota Arch (J), also known as the Pinellas Arch,
separates the Tampa Basin (I) from the South Florida Basin (L) (e.g.,
Klitgord et al.,
1984; Ball et al.,
1988; Winker and Buffler, 1988
Dobson, 1990; Dobson and Buffler, 1991). As shown in Figure 16,
there have been significant variations between previous publications
with regard to the locations, sizes, and spatial relationships of these
various Jurassic features.
The Apalachicola Embayment (F), which is bounded along its
northwestern edge by the Chattahoochee Arch, is a Jurassic basement
trough overlying the Triassic South Georgia Rift.
On the basis of drill
holes and electric logs, Mitchell-Tapping (1982) modeled the
Apalachicola Embayment as a graben flanked by two peripheral
discussed circular negative gravity anomaly, there is no definitive
potential-field signature attributable to the Apalachicola Embayment.
The Apalachicola Embayment is contiguous with, and
essentially indistinguishable from, the Apalachicola Basin,except that
the two are separated by a prominent basement hingeline at the
apparent trace of the Jay Fault zone and the Apalachicola basin is
characterized by a much greater depth-to-basement.
the hingeline, dipping intrabasement reflections suggest the presence
of Paleozoic strata with a synclinal structure underlying the
Apalachicola Basin (Dobson, 1990; Dobson and Buffler, 1991).
eastern section of the basin in the study area is marked by a
moderate amplitude positive gravity and magnetic anomaly (G)
basin extends southwestward to the plateau escarpment and may
reach a basement depth of 12 km (Buffler and Sawyer, 1985; Buffler,
. The eastern margin of the basin is marked by a major
bounding fault with a throw of at least 1000 m (Ball et al., 1988).
This fault separates the Apalachicola Basin from the Middle Ground
Middle Ground Arch-region H.
Extending across the Jay Fault
zone, adjacent to the Apalachicola Basin, is the Middle Ground Arch.
This is a broad, topographically positive feature that extends from
the southwestern flank of the Peninsular Arch westward to the edge
of the plateau (Figure 10--H)
. The crest of this ridge dips generally
westward and ranges in depth from about 1500 m under peninsular
Apalachicola and Tampa Basins immediately west of the Jay Fault
zone, the arch has been classified by some as two separate features--
the Middle Ground Arch to the east and the Southern (or DeSoto)
Platform to the west (Winker and Buffler, 1988; Dobson, 1990;
Dobson and Buffler, 1991).
The Middle Ground Arch is marked by one of the northeast-
trending, long-wavelength, low amplitude negative gravity anomalies
(Figure 14--H) previously mentioned as flanking feature Dl.
Although there are few short-wavelength features in the region,
seismic reflection profiles have shown the crest of the Middle Ground
Arch to have been moderately faulted. As suggested by stratigraphic
perturbations, these faults have horizontal throws on the order of
100 m and became inactive after the Late Jurassic (Ball et al., 1988;
Dobson, 1990). In addition, graben structures and fill associated with
the South Georgia Rift extend along the crest of the northeastern end
of the Middle Ground Arch from peninsular Florida to the Jay Fault
zone (Dobson and Buffler, 1991). An intrabasement igneous
intrusive body, possibly Mesozoic in age, has been identified on
seismic reflection profiles and is marked by an oblong low amplitude
negative gravity anomaly (Figure 15-feature HI) (Ball et al., 1988).
Near the southeastern flank of the Middle Ground Arch, a single drill
hole intersecting Paleozoic siltstones (Ball et al., 1988) signifies a
possible westward extension of the Suwannee Basin terrane.
Martin (1978) suggested that the Middle Ground Arch is simply
Basin implies a significant degree of structural control.
seismic reflection line extending across the southeastern boundary of
the arch, on the other hand, does not show any analogous faulting
(Ball et al., 1988).
In addition, the Bouguer anomaly gradient across
the strike of this southeastern boundary is uniform and relatively
low (Figure 1
. These observations suggest the Tampa Basin-Middle
Ground Arch margin to be a broad transition zone with little
structural control, which reinforces Martin's proposal of formation by
differential subsidence for this particular boundary.
Tampa Basin--region I. Southeast of the Middle Ground Arch is
the Tampa Basin, which is also known as the Florida Elbow Basin
(Pindell, 1985). It is essentially an analogue to, and coeval with, the
Apalachicola Basin. The Tampa Basin extends updip northeastward
from the western edge of the plateau although, unlike most the other
Jurassic features in this series, it has not been previously mapped as
extending eastward as far as the Jay Fault zone (e.g., Klitgord et al.,
1984; Buffler, 1989)
. It reaches a depth-to-basement of at least
5500 m (Figure 10--I) and Dobson and Buffler (1991) have modeled
it to be significantly deeper (greater than 9000 m).
Seismic reflection profiles are characterized by dipping
intrabasement reflections in the basin that, as in the basement of the
Apalachicola Basin, imply an underlying extensively faulted Paleozoic
synclinal structure (Ball et al., 1988; Dobson, 1990).
None of the
three offshore wells intersecting basement in the Tampa Basin,
intercalated with Jurassic diabase (Dobson and Buffler, 1991). In
addition, some of this faulting perturbed the Cretaceous sediments
immediately overlying these igneous rocks.
Thus, it is likely that at
least some of the basement faulting and structure originated during
the Jurassic evolution of the basin, rather than during the Paleozoic.
The Bouguer anomaly signature of the Tampa Basin is quite
distinct (Figure 14--1). It is marked by a 50 mgal long-wavelength
positive gravity anomaly. This positive anomaly averages about 160
km wide and 275 km long, suggesting that the Tampa Basin is
significantly larger than shown on previous basement maps;
however, a corresponding positive magnetic anomaly appears to be
somewhat smaller (Figure 11--I).
As discussed above, the
northwestern boundary of the basin is probably a sloping margin.
That margin appears to crest approximately 40 km south of the
northernmost expression of the gravity anomaly, although shallow
dip interpretations are difficult from the available seismic reflection
Pindell (1985) has speculated that a large strike-slip fault, the
Florida Elbow Fault, extends northwestward from the Jay Fault zone,
across the Tampa Basin, and into the northeastern Gulf of Mexico.
Although there is no gravity or seismic evidence for such a fault, the
steep eastward-striking magnetic gradient which bisects the
northern half of the basin may be the result of a structure along that
The relative absence of short-wavelength gravity anomalies
diameter moderate amplitude negative gravity anomalies with
corresponding positive magnetic anomalies may indicate the
presence of mid-crustal intrusive bodies similar to that under the
Middle Ground Arch (Figures 11 and 15--feature II).
The northeastern boundary of the Tampa Basin, as suggested
by the positive gravity anomaly, appears to correspond extremely
well with the presumed trace of the Jay Fault zone (Figure 14). This
places the easternmost section of the Tampa Basin under western
peninsular Florida and shifts the boundary significantly more
northeastward than had been previously modeled, although Ball et
al. (1988) identified a basement monocline only 30 km off the west
coast of Florida that was suggested to be the boundary. In addition
to the positive gravity anomaly, the presence of Jurassic diabase in
several drill holes in the same area of western peninsular Florida
(Figure 9) and a corresponding positive magnetic anomaly (Figure
11) are also suggestive of a northeastward extension of the basin as
far as the Jay Fault zone.
The southeastern boundary of the basin, which separates it
from the Sarasota Arch, is better constrained with respect to location.
Along this boundary, both of the potential-field gradients are
relatively high, which is suggestive of a morphologically distinct
feature. Although not confirmed by seismic data, stratigraphic
correlations from well logs also suggest the presence of a large down-
to-the-north fault or series of faults along this boundary; this fault
Tampa Basin and the South Florida Basin.
It extends from the Jay
Fault zone under peninsular Florida westward possibly to the edge of
the Florida Escarpment (Figure 10--J)
although Klitgord et al. (1984)
have suggested that the arch may be truncated by an extension of
the Cuba Fracture Zone just east of the escarpment.
basement of the Sarasota Arch averages 4 km and there is little
evidence of any significant dip along the crest.
There have been four
drill holes intersecting basement on the Sarasota Arch--three
bottomed in granitic rocks and one intersected diabase (Figure 9).
Although seismic profiles in the southern quarter of the
Floridan Plateau tend to be poor quality as a consequence of the
depth and the attenuation of seismic energy by the overlying karstic
limestone, a number of vertical faults have been observed on seismic
profiles crossing the Sarasota Arch (Shaub, 1984; Ball et al., 1988).
These faults are small, with throws on the order of 10s of meters,
and, as suggested by perturbations in the overburden, became
inactive by the Early Cretaceous.
Klitgord et al. (1984) interpreted a northeast-trending zone of
circular gravity and magnetic anomalies along the northwest edge of
the Sarasota Arch to be the result of a series of shallow
intrabasement intrusive bodies. Although several short-wavelength
circular negative gravity anomalies exist in this area (Figure 13--
region J), not all of these correlate well with the circular magnetic
anomalies (Figure 11-region J).
Therefore, while two or three of the
intrusions, a linear chain spatially associated with the Sarasota Arch
The Sarasota Arch is marked by a distinct long-wavelength
negative gravity anomaly (Figure 14--J), although there seems to be
no particular associated magnetic signature (Figure 11). As
described above, the northwestern boundary of this negative
anomaly correlates well with the large bounding fault inferred by
Ball et al. (1988). Although the anomaly appears to be partially
truncated near its northeastern boundary, it generally extends
southwestward from the region of the Jay Fault. The southwestern
boundary of the negative anomaly could be interpreted to occur
slightly east of the escarpment; however, a seismic reflection profile
along the crest of the Sarasota Arch (Ball et al., 1988) shows no
evidence for truncation at an extension of the Cuba Fracture Zone. As
a result, it is likely that the southwestern boundary of the arch is
coincident with the Florida Escarpment.
The southeastern boundary of the Sarasota Arch, as interpreted
from a seismic reflection profile near the escarpment, is a
southeastward-dipping, relatively steep, block faulted margin
(Shaub, 1984) that corresponds to the boundary of the negative
gravity anomaly. However, this gravity boundary is not continuous.
As discussed below, a 60 km wide negative gravity anomaly extends
southeastward from the boundary (Figure 14 --feature L4) and is
consistent with the probable presence of a large basement horst
recognized a possible basement high along the Florida Escarpment
extending southeastward from the Sarasota Arch. This high, the
Sheffield Arch, was identified from a limited number of profiles with
no deep well control and is, therefore, not well characterized.
There is a positive gravity anomaly oriented generally along
the axis of this proposed arch (Figures 14 and 15 -K). This positive
anomaly is approximately 180 km long and 120 km wide. It has no
concomitant magnetic signature. Although the positive gravity
anomaly could be the result of a morphologically positive feature
such as the Sheffield Arch, all of the other positive basement
features of the Floridan Plateau have been characterized by negative
Consequently, the characterization of a large,
continuous basement high in this area is unresolved.
South Florida Basin--region L.
Underlying southern peninsular
Florida is a Triassic-Jurassic structural feature known as the South
Based on a series of two-dimensional gravity profiles,
Wicker and Smith (1978) determined that the basin ranges in depth
from about 3000 m in south-central peninsular Florida to more than
5500 m at the southern edge of the peninsula.
The basin is generally
considered to underlie much of the southern portion of the Floridan
Plateau and to extend to its southern and eastern edges (e.g., Barnett,
1975; Smith, 1983; Klitgord et al., 1984).
The western extent of the
basin and the presence of any significant intrabasinal structures are
Triassic alkali basalts and rhyolites (Barnett, 1975; Mueller and
Porch, 1983; Heatherington and Mueller, 1991)
it has been suggested
that the occurrences of rhyolite represent horsts of an underlying
southerly extension of the Late Precambrian-Cambrian North Florida
Rhyolite (Thomas et al., 1989)
Isotopic dating of the rhyolites
(Milton and Grasty, 1969; Barnett, 1975) and the basalts (Milton,
Mueller and Porch, 1983) suggest that the two are coeval, with
ages of about 180 Ma.
This supports the model that the basalts and
rhyolites comprise a Mesozoic bimodal suite and almost certainly
discounts the premise that the rhyolites in the region of the South
Florida Basin are part of the North Florida Rhyolite.
There are only a few drill holes intersecting basement within
the South Florida Basin and all of these are in southwestern and
south-central peninsular Florida (Figure 9).
In addition, there is only
one published seismic reflection profile of the basement in the basin
(Figure 2). As a result, the basement model for this area is
necessarily based almost exclusively on potential-field data and is,
therefore, rather speculative.
The northeastern boundary of the South Florida Basin is the Jay
In their seismic reflection profile along the eastern edge
of the Floridan Plateau, Sheridan et al. (1981) found this boundary to
be a large, down-to-the-south, normal fault with an associated series
of smaller faults at the landward extension of the Bahamas Fracture
The boundary is marked by a northwest-trending zone of
southern margins of the South Florida Basin are not morphologically
well defined; however, the basin is considered to extend at least as
far as the Florida Escarpment.
Although the entire area bounded by the Jay Fault zone, the
Sarasota Arch, and the Florida Escarpment is commonly considered to
be a singular basin, there are four or five (depending on whether
feature K is included) individual long-wavelength potential-field
features within these bounds (Figures 11 and 14-L1-L4). For
example, the northeastern corner of this region is marked by an
oblong positive gravity and magnetic anomaly oriented along the Jay
Fault zone (LI).
Immediately southeast of this feature is a similarly
oriented negative gravity and magnetic anomaly (L2) which, in turn,
is separated from a second negative gravity anomaly (L3) by a
second positive gravity anomaly (L4)
. This long-wavelength
variability and the shapes of these features suggest the probability
of several northwest-southeast oriented basins within the area
traditionally considered to be the South Florida Basin.
All of the South Florida Basin area drill holes which have
successfully intersected basement have been located proximal to a
negative gravity and magnetic anomaly, specifically, feature L2.
suggests that the anomaly may mark a region characterized by a
relatively shallow basement.
In addition, the southern end of
Sheridan's reflection profile, near the southeastern corner of the
plateau, was located on a positive gravity and magnetic anomaly and
gravity and magnetic anomalies within the South Florida Basin
denote high depth-to-basement areas, while the negative gravity and
magnetic anomalies are characteristic of shallow basement arches.
Blake Plateau Basin--feature M.
Along the northeastern edge
of the Floridan Plateau is an area that, as suggested by a series of
seismic refraction profiles (Sheridan et al., 1966), is characterized by
an increase in depth-to-basement (Figure 10--M).
This feature may
represent a shallow salient of the Blake Plateau Basin, which is the
southernmost of the four major Jurassic-Early Cretaceous basins of
the eastern United States continental margin (e.g., Grow and
Sheridan, 1982; Dillon et al., 1985).
signature attributable to this salient.
There is no particular gravity
The western boundary of the
main body of the Blake Plateau Basin is located just east of the
eastern continental margin of the Floridan Plateau, slightly out of this
study area. Although the data are limited, there is no evidence that
the salient is fault-bounded. Rather, it appears to have formed
simply by differential subsidence.
Cenozoic Structural Features of the Floridan Plateau
There are only a few reports of Tertiary structural features of
tectonic origin on the Floridan Plateau.
These include small faults
resulting from halokinesis off the panhandle of peninsular Florida
(e.g., Addy and Buffler, 1984; Ball et al., 1988), as well as slump
faults and related features near the western Florida escarpment (e.g.,
tectonic origins; however, in each of these cases, subsequent workers
have failed to confirm these origins.
The most predominant of these reported features include
several surficial anticlinal and synclinal structures in northwestern
peninsular Florida and southwestern Georgia. Patterson and Herrick
(1971) summarized the reports of these features, which conflicted
with respect to locations and descriptions in many cases, and
concluded that there is no evidence to support the presence of any
surficial structural feature in this area, with one possible exception.
This exception is a small dome in southwestern Georgia, slightly
outside of this study area, known as the Gordon Anticline. The other
reported features were attributed to the differential erosion and
dissolution of the near-surface Tertiary carbonates.
A Tertiary feature that is regularly mentioned in the literature
is the Ocala Uplift, a reported anticlinal structure in central
peninsular Florida. As in the previous instances, a tectonic origin for
this feature was dismissed by Winston (1976), who attributed the
apparent anticlinal structure of the surficial carbonates as having
been caused by differential erosion and dissolution.
The only other purported Tertiary feature is the Bronson
Graben, described as a small northwest-trending structure in north-
central peninsular Florida with an associated series of surficial
normal faults paralleling it. This was proposed by Vernon (1951) on
the basis of lineations in aerial photographs and discrepancies in drill
(1989) to extend Appalachian fault provinces along the length of the
, there has been no field recognition of these
features, and it is unlikely that Vernon's observations are indicative
of lithospheric movement during the Tertiary.
It appears, then, that
with the possible exception of the Gordon Anticline, there is no
evidence for any Tertiary structure on the Floridan Plateau
attributable to regional tectonic stress.
A compiled map of the pre-Mesozoic lithotectonic terranes of
the Florida Plateau basement is shown in Figure 17
peninsular Florida, southern Georgia, and the Florida Panhandle, the
subcrops of Precambrian and Paleozoic rocks apparently represent
isolated pieces of more extensive terranes that were fragmented
during the collision and subsequent separation of Laurentia and
Although these fragments were not significantly
metamorphosed during this process,
there is evidence allowing the
suggestion of thrust faulting, block faulting, and faulted contacts.
Potential-field gradients, lithologic contrasts, and the limited existing
seismic reflection data suggest that all of the contacts between these
pre-Mesozoic terranes may be faulted, with the possible exception of
the boundary between the Osceola Granite and the North Florida
Figure 18 represents a compilation of the Mesozoic structural
U. -- 0 *********n* -ee e- ** -* **
nne-ae a *'\ ** ---- *. at* n C..
... .... ........ ....T rram .. .... .
----- --- N
..... rn e ... ..
lay Fault -- "'"rt" Fla""
Zo"e a..e--* R- h lite
'- St. Lucie
's's' ? S
.,,', ,, ~
1^\ 1 1
- \-\9 \
Rhyolite and Andesite
Felsic and Mafic Extrusive
Sandstone and Shale
Felsic Plutonic Rocks
Indicates Unknown Boundary
Possi ble Faulted Contact
plateau. Although the linear nature of the fault zone suggests that it
originated as a strike-slip fault, the present structure of the zone is
independently determined by each of the bordering Mesozoic
extensional features and consequently changes along strike.
southeastern region of the plateau, the fault zone primarily consists
of a series of southwestward-dipping normal faults.
northwest along much of its length, the zone alternates between
accommodating significant vertical offsets at southwestward-dipping
Jurassic basin boundaries and having little or no vertical offset
where intersected by basement arches.
In the northwest, the zone
changes to accommodate the northeastward-dipping normal
bounding faults of the South Georgia Rift.
The southwestward extension of the South Georgia Rift into
Florida resulted in block faulting in the western Suwannee Basin
terrane. Although there is some suggestion of intrarift horst and
graben formation, neither these structures nor the lateral master
bounding faults of the rift extension have been definitively located
There is a small ancillary rift basin (or basins)
southeast of the main rift.
As mentioned above, the southwestern
boundary of the South Georgia Rift is a large northeastward-dipping
normal fault at the Jay Fault zone.
Overlying the South Georgia Rift in this area is the Jurassic
Apalachicola Embayment, which, as suggested in previous studies,
may also be bordered by lateral master faults.
extensional basins bordering the southwestern side of the Jay Fault
, the Apalachicola Basin, the Tampa Basin, and the
South Florida Basin, are separated by two wide basement ridges, the
Middle Ground Arch and the Sarasota Arch.
Each of the basins
the probable exception of the northwestern edge of the Tampa Basin,
is fault-bounded along its lateral boundaries.
In addition, it is likely
that each extends from the Jay Fault zone southwestward at least as
far as the Florida Escarpment.
Although there is little corroborating
seismic reflection data for the South Florida Basin, potential-field
signatures suggest the probability that this area is underlain by
several isolated basins
, rather than being a singular feature.
basement low under the northeast edge of the Floridan Plateau is
interpreted to be a subsided landward salient of the Jurassic Blake
With the exception of gravity-induced slump features near the
Floridan Escarpment and small faults in the northeastern Gulf
produced by halokinesis, there is no evidence of any
structural feature on the Floridan Plateau.
There may be a small
anticlinal structure, the Gordon Anticline, in southwestern Georgia
just north of this study area.
TECTONIC EVOLUTION OF THE FLORIDAN PLATEAU
As a consequence of the depth of burial of the Floridan Plateau
basement and the lithotectonic complexity of the region, past
interpretations of tectonic history have necessarily been based on an
inadequate knowledge of basement structure.
The intention in this
chapter is to provide a reinterpretation of the tectonic history of the
Floridan Plateau within the context of the new interpretations of
lithologic and structural relationships presented in Chapter 2 and to
incorporate the findings of recently published geochemical,
geophysical, and tectonic studies into this interpretation.
Regional Tectonic History
Tectonic reconstructions for the continental blocks) underlying
the Floridan Plateau and the adjacent Bahamas Platform are based
primarily on sparse information from deep drill hole samples and, as
a consequence, the origin of these blocks has been the subject of
The Suwannee Basin terrane and, by association, the
assemblages to have Gondwanan affinities (e.g., Cramer, 1971
In addition, paleomagnetic data and ages of detrital
zircons (Opdyke et al.,
Mueller et el.
,1993) are also
inconsistent with a Laurentian origin for this terrane.
As a result, a
correlation between these terranes and Gondwana prior to the
Alleghenian closure of the lapetus Ocean has been reasonably well
established; however, the specific position of the Floridan Plateau
with respect to Gondwana during the Paleozoic has not been
A number of investigations have suggested that the pre-
Mesozoic basement terranes of the northeastern Floridan Plateau
originated along Africa's western margin near Senegal.
investigations include continental reconstructions (Bullard et al.,
1965; Wilson, 1966
Dietz et al., 1970; LePichon and Fox, 1971
Pindell and Dewey, 1982; Klitgord et al.,
1983; Van Siclen, 1984; Pindell, 1985; V
1983; Roussell and Liger,
enkatakrishnan and Culver,
1988), paleontological analyses from Florida and Africa (Cramer
Pojeta et al.,
comparisons of isotopic abundances
(Dallmeyer, 1987, 1988; Dallmeyer et al.,
Heatherington et al., 1993), paleomagnetic data (Van der Voo et al.,
1976; Opdyke et al., 1987),
and stratigraphic correlations between
Florida and west Africa (Smith, 1982; Chowns and Williams, 1983).
Although a proposed tectonic history for Florida based on any one of
these lines of evidence would be inconclusive, the cumulative effect
There are specific units in western Africa that can be
correlated to their apparent counterparts underlying the Floridan
These units provide the sole record of the pre-Mesozoic
tectonic history of the Floridan Plateau.
The extensive North Florida
Rhyolite of north Florida and south Georgia may be correlated to
similar felsic volcanic rocks which are present throughout the
western margin of Africa (Dillon and Sougy, 1974; Dallmeyer, 1987
Dallmeyer and Villeneuve, 1987
Heatherington et al.,
Geochemical analyses of the Florida suite suggest emplacement in a
convergent plate margin setting,
with an ocean-continent subduction
environment being the most likely setting (Mueller and Porch, 1983).
The St. Lucie Metamorphic Complex has been correlated to
units of the central Rockelide Orogen in Guinea, proximal to the Bove
Basin (Chowns and Williams
,1983; Dallmeyer and Villeneuve, 198
In addition to similarities in petrology and degree of metamorphism,
both of these metamorphic units appear to have been affected by an
extensive thermotectonic event, Pan-African II, that occurred along
the northwestern margin of Gondwana about 550 Ma ago (Dallmeyer
and Villeneuve, 1987)
In addition, there is no evidence that the St.
Lucie Metamorphic Complex was thermally affected by the
subsequent Caledonian or Hercynian orogenies.
This is consistent
with a central Rockelide origin for the complex, but not with a more
northerly origin, where the adjacent Mauritanides were strongly
deformed during the later orogenies (Roussel et al., 1984), nor with a
There are a number of similarities that support the correlation
of the Osceola Granitoid Complex of east central Florida with the Coya
Granite of Senegal, which is exposed in the northern Rockelides.
are post-tectonic, Early Paleozoic plutons with similar petrographic
and petrologic characteristics (Dallmeyer et al.,
, as well as numerous continental reconstructions, suggest
that during the Early Paleozoic, the Osceola and Coya Granites were
likely to have formed as part of the series of granitoids emplaced
along Gondwana's northwestern margin during Pan-African II
orogenies (Dillon and Sougy, 1974; Dallmeyer et al., 1987).
The Suwannee Basin Complex of north-central Florida was
deposited in an Early Paleozoic sedimentary basin.
The lower portion
of the sampled sequence of this complex consists of quartz
sandstones overlain by graptolite-bearing black shales, which are
indicative of a restricted marine environment.
stratigraphic and paleontological similarities, t
Based largely on
he Suwannee Basin
was correlated by a number of workers to the Bovy Basin of western
Guinea (Cramer, 1971, 1973
Pojeta et al., 1976; Smith, 1982; Chowns
,1983; Venkatakrishnan and Culver, 1988).
Subsequently, a paleomagnetically determined Lower Ordovician
paleolatitude of 49 degrees south (Opdyke et al., 1987) and a
comparison of plateau ages for muscovite (ca. 505 Ma) from both
basins (Dallmeyer, 1987) have provided further support for this
These deposits probably represent the disjointed
Although the present day basal contact of the Suwannee Basin
terrane is likely to be a simple unconformity, there is some evidence
to suggest that it may instead be a northward-dipping thrust fault.
Although the Suwannee Basin has been only mildly metamorphosed,
seismic reflection profiles suggest the presence of low angle faulting
within the terrane (Arden, 1992,
personal communication) and the
basal contact has the appearance of a ramped thrust fault (Arden,
1974). In addition, thrust faults are common in the central
Rockelides, which are in close proximity to the Bove Basin (e.g.,
Williams and Culver, 1982).
Smith (1993) has postulated that the pre-Mesozoic terranes of
the plateau basement, including the Suwannee Basin, the North
Florida Rhyolite, the Osceola Granite, and the St. Lucie Metamorphic
Complex, were fragmented and repositioned to their present day
configurations during the late stages of AUeghenian closure.
scenario, fragments of these Gondwanan terranes were moved
laterally into their relative positions,
with separate fragments being
translocated into eastern Florida, southeastern Georgia, and
northwestern Florida, along a series of left and right lateral strike-
These strike-slip faults,
including the precursor to the Jay
Fault, formed in response to the differential stresses in Gondwana
resulting from a deformational collision with Laurentia.
is demonstrated in Figure 19, which illustrates the formation of
strike-slip faulting in the Florida basement and the relative
Ala. Ga. S.C.
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. Plateau ,
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There are other lines of evidence that support this scenario. It
is unlikely that the numerous Triassic and Jurassic features of the
plateau basement would share a common linear boundary such as
the Jay Fault zone (Chapter 2), unless this boundary existed as a pre-
existing zone of weakness. The continuity and linearity of this zone
of weakness is strongly suggestive of a long strike-slip fault zone,
which is consistent with Smith's model.
In addition, the
metamorphic Catoche terrane encountered in DSDP holes 150 kms.
west of the Florida Escarpment in the Gulf of Mexico is petrologically
similar to the St. Lucie Metamorphic Complex and displays similar
40Ar/39Ar age spectra (Dallmeyer, 1984)
, it appears
to be another detached fragment of the same Gondwanan source
terrane (Schlager et al.,
If one accepts a Gondwanan origin for
the Catoche terrane, this necessitates northward to westward net
transport relative to the present orientation of North America for
emplacement of the terrane fragment to the west of the Florida
Because Mesozoic extensional trends suggest only
southeastward transport, then northward to westward movement of
the Catoche terrane must have occurred during or after closure, but
prior to Mesozoic extension in some manner similar to that described
Although significant northward and westward movement along
a series of strike-slip faults is suggested, the amount of lateral offset
along the Jay Fault zone may have been rather limited.
Granite and is probably another fragment of the same terrane
In addition, Heatherington and Mueller (1991) have
found little evidence for differences in Nd model ages for the
lithospheric components from which the volcanic suite of the South
Florida Basin and the north Florida volcanic rocks were derived.
Each of these suggests that there was not enough net strike-slip
movement along this section of the Jay Fault during the Paleozoic or
Mesozoic to bring separate lithospheric units into juxtaposition.
Most Late Paleozoic-Early Mesozoic reconstructions of Pangaea
place the continental basements of the Floridan Plateau, the Yucatan,
and the Bahamas Platform in the reentrant at the junctions between
the juxtaposed North American
, South American, and African plates.
With the formation of a Late Triassic interior rift system, North
America began to rift away from Gondwana along the traces of the
Paleozoic continental margins (Van der Voo et al., 1976; Mullins and
Lynts, 1977; Pindell and Dewey
et al., 1984; Van Siclen, 1984).
,1982; Klitgord et al., 1983; Klitgord
The hypothesized sequence of events
on the Floridan Plateau during this period are shown in Figure 20.
A large graben system, the South Georgia Rift, formed across
southern Georgia nearly along the trace of the Alleghenian suture
between Florida and North America (Nelson et al., 1985B).
configuration of the rift was largely controlled by the presence of
Continental Rift Basin
Fault Zone (accommodates normal
and lateral motion)
aT Yyi a &lah a _a -v aL a a_ C a. a a 4 a a tie a Ca
across the Triassic northern margin of the Yucatan block, as
suggested by the presence of red beds in the Todos Santos Formation
of southern Mexico (Buffler and Sawyer, 1985; Molina-Garza et al.,
. With a Late Triassic-Early Jurassic shift in the drifting
the South Georgia Rift became an aulacogen and the
, Yucatan, and western Bahamas basements were left
appended to the southeastern margin of North America (Mullins and
Pindell and Dewey, 1982
Burke et al.,
1984; Klitgord et al.,
1984; Van Siclen, 1984).
Smith (1982) has suggested that this shift in the drifting
geometry was caused by the formation of a Triassic hot spot and
triple junction associated with the opening of the Atlantic Ocean near
the southern tip of Florida.
This model is supported by the ages,
lithologies, and isotopic signatures of the bimodal rhyolitic and
basaltic suite in the South Florida Basin region.
The basalts of this
terrane have yielded 40Ar/39Ar ages of about 190 Ma and trace
elemental analyses have been found to be consistent with a mantle
source, mixing with Precambrian continental lithosphere, and
deposition in an extensional environment (Mueller and Porch, 1983;
Heatherington and Mueller, 1991).
Both the configuration of the Floridan Plateau basement and
recent continental reconstructions suggest that the South Florida
Basin suite is more likely to be associated with the opening of the
Gulf of Mexico, rather than the Atlantic Ocean.
The alternating basins
of the South Florida Basin terrane to the northwest.
In addition, as
suggested by the position of the salient between Africa, North
America, and South America, it is likely that during the Triassic and
the continental crust of the western Bahamas block was
located immediately to the east of the South Florida Basin terrane
(e.g., Mullins and Lynts,
1977), where it would have effectively
isolated the terrane from the Atlantic oceanic rift zone (Figure 20).
Although these reconstructions suggest that the South Florida Basin
was geographically removed from the Atlantic Ocean and that the
bimodal volcanics were erupted as a consequence of the opening of
the Gulf of Mexico basin, this does not negate a Late Triassic-Early
Jurassic shift in oceanic rifting geometry as a causal mechanism for
the failure of the South Georgia Rift.
A number of Late Triassic reconstructions of the Gulf of Mexico
region indicate that the north-central Gulf region was occupied by
the Yucatan (or Maya) Peninsula block which, as supported by recent
paleomagnetic data (Molina-Garza et al., 1992), later rotated
counterclockwise away from North America during its separation
with South America (e.g., Pindell, 1985; Ross and Scotese, 1988;
Rowley and Pindell, 1989). These reconstructions suffer from a gap
in the northeastern Gulf of Mexico. There have been two general
hypotheses that account for this gap.
One hypothesis is that the gap was occupied by the crustal
blocks now occupying the Florida Straits and the South Florida Basin
Florida during the Late Jurassic.
This displacement would have
occurred along a left-lateral strike-slip fault (the Florida Elbow Fault)
which would necessarily extend across the southwestern quarter of
the Floridan Plateau (Figure 16B)
. The second hypothesis is that any
apparent gaps are accountable entirely to attenuation and subsidence
of continental crust in the eastern gulf during the Jurassic (e.g.,
Buffler and Sawyer, 1985; Dunbar and Sawyer, 1987
Winker and Buffler, 1988).
In contrast to the obvious potential-field signature attributable
to the Jay Fault zone, there is no suggestion of a Florida Elbow Fault
extending across southwestern Florida in either Bouguer anomaly
maps or seismic reflection surveys.
In addition, the Middle Jurassic
basins and arches of the southwestern Floridan Plateau appear to
extend undisturbed across the proposed trace of the Florida Elbow
Hence, there is little supporting evidence for this fault or any
similar structure across the southwestern plateau, and the
translocation of the Florida Straits block from the eastern gulf to its
present location by some other path seems unlikely.
Another consideration with respect to the apparent gap is that
the southwestern half of the Floridan Plateau is comprised of
attenuated (i.e., stretched and thinned) continental crust, as
suggested by the prominent extensional basins and the drill holes
intersecting granitic basement (Figure 1).
The depth-to-basement in
the Tampa, Apalachicola, and South Florida Basins demonstrates that
and 1.8 (Pindell, 1985). Consequently, the model that attributes an
apparent gap in the northeastern Gulf of Mexico exclusively to the
attenuation and subsidence of continental crust is preferred.
As previously mentioned, restoration of the Jurassic crustal
attenuation of the southwest Floridan Plateau results in the
relocation of the South Florida Basin region into the proximity of the
eastern gulf. Potential-field maps indicate that the South Florida
Basin region is comprised of several separate basins which trend
northwest-southeast, perpendicular to the trend of the other
Mesozoic features of the southern and western Floridan Plateau
(Figure 18). The northwest-southeast orientations of these basins
and their proximity to the Yucatan during the Late Triassic-Early
Jurassic suggests that they may have formed in response to
southwestern extensional stress as the Yucatan block rotated
counterclockwise away from the Floridan Plateau (Figure 20).
Subsequently, as North America and South America continued
to separate, a series of basement horsts and grabens formed around
the periphery of the Gulf of Mexico basin (Pindell and Dewey, 1982;
Burke et al., 1984; Klitgord et al., 1984; Buffler and Sawyer, 1985;
Pindell, 1985; Winker and Buffler, 1988). A few of the horsts in the
southeastern United States, such the Wiggins Arch underlying
southern Alabama and Mississippi, have been proposed to be
stranded blocks left behind by the departing Yucatan block or South
American continent (Smith et al., 1981; Pindell and Dewey, 1982;
North American crust (e.g., Pindell, 1985; Ball et al., 1988; Winker
and Buffler, 1988; Dobson and Buffler, 1990; Reitz, 1991).
The Gulf of Mexico existed as a restricted basin from about 165
Ma until about 150 Ma, resulting in the deposition of extensive salt
deposits (Pindell and Dewey, 1982; Salvador, 1987). During this
period, stress in the western Floridan Plateau was reoriented to a
north-south or northwest-southeast extensional regime, as
delineated by the northeast-southwest orientations of the horsts and
salt-containing basins of the southwestern Floridan Plateau. This
shift in the stress regime is likely to have been associated with the
continuing separation of the North American and South American
The mechanisms of relative motion between North and South
America during the Jurassic are not well constrained, partly because
much of the intervening crustal structure was subsequently
destroyed during the eastward migration of the Caribbean plate. It
is probable that the relative motion between the two continents was
accommodated by strike-slip movement along a series of northwest-
southeast or north-south oriented transform faults (Pindell and
Dewey, 1982; Klitgord et al., 1984; Van Siclen, 1984; Pindell, 1985;
It has been suggested that the Bahamas Fracture Zone/Jay
Fault system acted as an important transform fault in the eastern
Gulf of Mexico during the Jurassic, not only by accommodating
However, as mentioned, the plateau basement southwest of
the Jay Fault zone appears to be composed entirely of attenuated
continental crust, rather than of stranded individual blocks, as had
, similarities between Nd isotope
signatures of basement samples from north and south Florida imply
little net strike-slip motion along the Jay Fault zone (Heatherington
and Mueller, 1991).
Consequently, while some sinistral motion along
the Jay Fault zone during the Jurassic resulting from crustal
attenuation along its southwestern margin is likely, its proposed
roles as the southern boundary of North American continental crust
and as a major transform boundary seem implausible.
Tensional deformation in the Gulf of Mexico region ceased by
the Late Jurassic (Salvador, 1987)
indicating that the lithotectonic
blocks of the Floridan Plateau had reached their present relative
positions by that time.
A review of existing seismic reflection
profiles reveals no evidence of continued faulting or other
tectonically-induced deformation in the sedimentary column
overlying the Floridan Plateau after the Early Cretaceous, despite the
presence of numerous pre-existing potential zones of weakness.
Thus, as soon as the region became established as a passive margin,
the level of crustal stress on the plateau appears to have diminished
of Mexico associated with the collision of Cuba and the Bahamas
during the Early Paleogene has been documented; however, there is
no evidence for any associated tectonic activity or significant
horizontal lithospheric stress on the Floridan Plateau (Pindell, 1985).
Rather, the undisturbed Upper Cretaceous and Tertiary strata on the
plateau document an extended, nearly continuous period of shallow
marine carbonate and plastic deposition which occurred in response
to regional subsidence.
SEISMICITY AND STRESS IN THE
SOUTHEASTERN UNITED STATES
Several hypotheses have been developed to explain seismic
activity in the intraplate setting of the eastern United States;
however, specific causal mechanisms and conditions remain
enigmatic. The relative paucity of earthquakes and the tectonic
complexity of the region result in seismic hazard assessments that
are necessarily rather subjective and often the source of debate. An
improved understanding of seismicity and seismic hazard in the
eastern United States is necessary and can best be acquired from the
integration of a variety of observations.
The intention in this chapter is to characterize epicentral
density patterns in the southeastern United States and to provide a
summary of the disparate hypotheses and observations relating to
the seismicity of the region. A further objective is to evaluate
possible causes for the nonuniform distribution of seismicity, with
particular emphasis on the seismotectonic differences between the
active South Carolina-north Georgia seismic zone and the juxtaposed,
yet seismically quiescent south Georgia-Floridan Plateau region.
The characterization of epicentral distribution in the
southeastern United States was accomplished through the integration
of a literature review of historical and instrumental records and the
establishment of a network of digital seismographs in Florida.
seismograph network has been in operation since 1989.
a review of previously published seismic reflection profiles and an
overview of the tectonic setting,
as discussed in Chapters
2 and 3,
respectively, are used as general indicators of prehistoric seismic
activity for the Floridan Plateau.
Epicentral Distribution in the Southeastern United States
Compilations of historic reports of seismic activity in the
southeastern United States, except for Florida, have been provided
by Bollinger (1973A, 1975)
. In these compilations,
reported events during the period between 1754 and 1974 are
identified. Of these,
76 reports are from eastern Tennessee, 73 are
from North Carolina, and 451 are from South Carolina, while only 34
are from Georgia and 18 are from Alabama (Figure 21).
Nearly all of
the Georgia and Alabama events occurred in the northern portions of
The remainder of the reports are from Virginia,
Virginia, Maryland, and Kentucky.
Based on this compilation,
Bollinger identified several different seismically active zones in the
n Florida Historical Events
, Instrumentally Located
+ Historical or Poorly
fl* ... n ) I t.ia a a_ 1 41 .a Ia. ai a -
* t 4 4f!L -L *
350 kilometers. It covers an area of more than 145,000 square
kilometers and trends perpendicular to the structural grain of the
Appalachians (Bollinger, 1973A,B)
Although epicentral distribution
within this zone is diffuse, it includes Charleston, S.C., where an 1886
event had an estimated body-wave magnitude of 6.8 (Bollinger,
, 1983) and a lengthy aftershock sequence, including more than
300 events during the subsequent 35 year period (Tarr, 1977).
Another zone, the Southern Appalachian seismic zone, extends
from central Alabama northeastward to western Virgina along the
Blue Ridge and Valley and Ridge provinces. It encompasses 162,000
square kilometers and, with respect to epicentral density, is more
active than the South Carolina-Georgia seismic zone.
with Modified Mercalli (MM) intensities greater than VI have
occurred in this zone since 1874, including the intensity VIII event
of Giles County, Virginia in 1897.
In addition to the historic reports of seismic activity, epicentral
distribution patterns have also been determined from instrumentally
During the period from 1977-199 1
United States Seismological Network (SEUSSN) located 1162 events,
which are also shown in Figure 21 (SEUSSN Contributors, 1992).
These instrumental records generally corroborate the previously
determined epicentral distribution patterns of Bollinger, implying
that the effects of population distribution, local ground response, and
reporting error have not significantly biased the historical record.
(1983) identified only 33 reports of earthquakes in Florida; these
reports were later used to construct a seismicity map of the state
(Reagor et al., 1987).
Most of these reports do not represent tectonic
activity in Florida (Smith and Randazzo,
1989; Lord and Smith, 1991).
Rather, many are attributable to events that occurred elsewhere and
others, based on their descriptions, are more likely to have been
blasting, military activities,
or atmospheric phenomena.
conclusion of these reviews was that Florida has experienced only 6
possible low-intensity historic events.
A subsequent review of
newspaper reports has revealed that 1 of these 6 possible events (on
21 June 1893) was felt in the Charleston area before being felt in
Florida and, therefore, is not likely to have originated on the Floridan
The remaining 5 possible events are shown on Figure 21.
Instrumental records also support the premise that the
Floridan Plateau, which includes south Georgia, is characterized by an
unusually low level of seismic activity. In fifteen years of operation,
the SEUSSN has never detected an event on the Floridan Plateau. In
addition, during the three years that the state-wide network of
digital seismographs has been in operation, there were no events
located on the plateau and only a single local event was detected.
This was a mb 3.8 event located in the Gulf of Mexico, 340 kilometers
west-southwest of Sarasota, Florida on March 31
In addition to historical and instrumental records, a review of
existing seismic reflection surveys from around the plateau (Chapter
plateau basement appears to have been in a low-seismicity state for
a relatively long period of time.
Consequently, historical records, instrumental records, and
geophysical data each demonstrate a pronounced difference in levels
of seismic activity between proximal regions.
To the north, in the
southern Appalachians and southern Coastal Plain, there is a
moderate to high level of seismic activity, including several historic
large magnitude events.
Immediately to the south, however, there is
an unusually low level of seismic activity, with only occasional low
intensity events. As inferred from Figure 21, the boundary between
these two regions extends approximately east-west across south-
central Georgia and Alabama and is a well-defined example of the
nonuniform nature of seismicity in the southeastern United States.
Several plausible explanations for this nonuniform distribution
of seismicity can be presented, although they are not mutually
The first is that this difference is caused by variability in
the lithospheric stress field, as suggested by Hatcher et al. (1987).
Another possible reason is that there are mechanical differences (i.e.,
lithology or structure) between adjacent zones (Wheeler and
This would imply that local seismogenic features
exert primary control over the distribution of seismic activity in the
southeastern United States.
A final possibility is that there is a
systematic variability in the propensities of pre-existing fault zones
This would be attributable to variations in either the
Lithospheric Stress in the Southeastern United States
While the stress regime in central and eastern North America
has been established to be primarily northeast-southwest
compressive (Zoback and Zoback, 1980; Zoback, 1992), there have
been a number of contradictory observations.
This suggests the
possibilities of local perturbations in the regional field and/or the
influence of a local stress field. As a result, the sources) of
seismogenic stress in the southeast is still rather speculative.
Stress orientations in the Coastal Plain and southern
Appalachian provinces have been determined from a variety of
sources, such as the orientation of recent faulting (Schafer, 1979;
Zoback and Zoback, 1980; Wentworth and Mergner-Keefer, 1981,
1983), hydrofracturing in deep wells (Zoback and Zoback, 1980), in
situ measurements (Hooker and Johnson, 1969; Zoback et al., 1978),
fault plane solutions (Tarr, 1977; Tarr and Rhea, 1983; Johnston et
al., 1985; Zoback, 1992), borehole elongation (Plumb and Cox, 1987)
and crustal modeling (Kuang et al., 1989; Richardson and Reding,
Within these provinces, there is general agreement that the
maximum stress is compressional and horizontal.
indicators suggest that the maximum compressional stress direction
is oriented northeast-southwest, many are also indicative of a
northwest-southeast oriented field.
A number of indicators have been used to infer northeast-
One of these is shallow focal plane solutions
addition, Plumb and Cox (1987) measured borehole elongations from
eastern Tennessee to Canada, including a number from west of the
Appalachians, and found nearly their entire study area to be under
While Plumb and Cox had no
measurements from the southeastern Coastal Plain, Hooker and
Johnson (1969) measured in situ stresses in 5 boreholes in Georgia,
Virgina, and North Carolina.
Their measurements were inconclusive,
as their orientations ranged from nearly north-south to nearly east-
Focal mechanisms from the southern Appalachians
demonstrate that at depths averaging between 9 and 15 kms, the
basement under the Appalachian detachment is under northeast-
southwest compression (Johnston et al.,
Using a variety of
indicators, Zoback and Zoback (1989) found the eastern and central
United States to be under a relatively uniform northeast-southwest
More recently, M.L Zoback (1992) found that of
the 32 central and eastern United States earthquakes used in her
investigation, 25 indicated slip compatible with northeast-southwest
Alternatively, some of the stress field indicators suggest a
component of horizontal compressive stress oriented approximately
Schafer (1979) came to this conclusion while
investigating recent surficial fault motion in the western Valley and
Ridge of eastern Tennessee, as did Wentworth and Mergner-Keefer
1983) and Prowell (1989) in studies of Cenozoic faulting from
characterized the Appalachian and Coastal Plain provinces as being
under northwest-southeast compression.
Finally, Bollinger (1983)
and Wentworth and Mergner-Keefer (1983) both concluded that the
1886 Charleston earthquake was probably a thrust event along a
northeast-trending structure--another indication of northwest-
Within Florida, stress orientations have been deduced only
from a single report of recent tectonic faulting (Vernon, 1951). In
Vernon proposes northwest-striking normal faulting in
the west-central portion of the peninsula as a source of aerial photo
lineations and inconsistencies in drill logs.
The report has
subsequently been cited as evidence for northeast-southwest
extensional stress on the Floridan Plateau (York and Oliver, 1976;
Zoback and Zoback, 1980; Prowell, 1983)
No other reports of these
features exist, and no physical recognition of them has been
achieved, although northwest-trending jointing is common in that
Accordingly, the orientation of the stress field
under the Floridan Plateau is unknown, although Zoback and Zoback
(1989) indicate that southeastern Georgia and the Atlantic
continental margin off Florida's northeast coast are under north-
northeast to south-southwest compression.
In an attempt to explain the contradictory indicators of stress
direction in the southern Appalachians and Coastal Plain,
Oliver (1976) suggested that seismogenic stresses in the eastern
perturbed by the strain release during the 1886 event and has not
Bollinger (1983) hypothesized that there were two
sets of perpendicular seismogenic features around Charleston, one
northwest-southeast and one northeast-southwest, along which
earthquakes occur without regard to the orientation of the stress
McCartan and Gettings (1991) proposed that young mid-crustal
plutons locally disrupt the stress field by concentrating stress.
Hatcher et al. (1987) suggested that the eastern and central United
States could be divided into a number of individual blocks based on
geophysical signatures and that a southern group of blocks, including
the Floridan Plateau, are decoupled from the regional stress regime.
Although each of these may explain certain observations or
particular local phenomena, none resolves all of the contradictory
observations throughout the eastern United States.
The total stress field at a given site is the sum of regional and
Regional stress, as the name suggests, is that stress
regime which prevails over a large area and is produced by tectonic
effects (e.g., Kuang et al.,
Local stress is that induced by
surficial topographic relief or shallow crustal features or phenomena.
Remnant stress is that stress remaining in a rock after an applied
stress has been removed (Sbar and Sykes, 1973).
The primary source of regional stress in eastern North America
appears to be ridge-push from the mid-Atlantic ridge (Yang and
Aggarwal, 1979: Zoback and Zoback, 1980; Kuang et al.,
resistance to plate motion (Zoback and Zoback, 1980) and lithospheric
stress induced by asthenospheric convective flow (Voight, 1969; Sbar
Either of these would also produce a northeast-
southwest maximum compressional stress direction.
The contradictory indicators of stress direction suggest the
possibility of a local stress stress field in the eastern United States.
Several possible sources of local stress have been proposed.
Bollinger (1973B) and Barosh (1981) have suggested isostatic uplift
and flexure as a causal mechanism for compression; however, Brown
(1978) was unable to find a correlation between vertical crustal
movement and seismicity along the eastern seaboard.
the rebounding Appalachians are the source of the uplift in this
model, then the predicted extensional stress along the crest of the
mountains is not observed (Zoback and Zoback, 1980).
proposed source of local northwest-southeast compressional stress is
gravity-induced backsliding southeastward along the Appalachian
decollement surface (Seeber and Armbruster, 1981).
This model also
predicts extension along the apex of the Appalachians, which is not
observed (Zoback and Zoback, 1980).
Finally, Kuang et al. (1989)
considered the primary sources of local stress to be topographic
relief and density contrasts within the lithosphere.
their model, these would also be likely to induce northwest-
Seismotectonic Features of the Southeastern United States