Download Geology - Central Washington University Geological Sciences

Downloaded from geology.gsapubs.org on July 12, 2010
Geology
Early Paleozoic transform-margin structure beneath the Mississippi coastal plain,
southeast United States
D.L. Harry, J. Londono and A. Huerta
Geology 2003;31;969-972
doi: 10.1130/G19787.1
Email alerting services
click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles
cite this article
Subscribe
click www.gsapubs.org/subscriptions/ to subscribe to Geology
Permission request
click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA
Copyright not claimed on content prepared wholly by U.S. government employees within scope of their
employment. Individual scientists are hereby granted permission, without fees or further requests to GSA,
to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make
unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and
science. This file may not be posted to any Web site, but authors may post the abstracts only of their
articles on their own or their organization's Web site providing the posting includes a reference to the
article's full citation. GSA provides this and other forums for the presentation of diverse opinions and
positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political
viewpoint. Opinions presented in this publication do not reflect official positions of the Society.
Notes
Geological Society of America
Downloaded from geology.gsapubs.org on July 12, 2010
Early Paleozoic transform-margin structure beneath the Mississippi
coastal plain, southeast United States
D.L. Harry* 
J. Londono* 
A. Huerta* 
Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, Alabama 35487-0338,
USA
ABSTRACT
A geophysical transect across the central Gulf of Mexico coastal plain shows that the
early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine
state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the
crystalline crust decreases abruptly across the margin over a distance of ;50 km, from
35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita foldand-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform
margins, but very different from most rifted margins, which display much more gradual
transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally
thick oceanic crust adjacent to the margin. The lack of these features is also characteristic
of modern transform margins. Combined with transects across the margin farther west,
the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies
an ;800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast
Alabama.
Keywords: Gulf of Mexico, North America, rifting, Ouachita orogeny.
Figure 1. Simplified tectonic map of southeastern North America (after Thomas, 1991).
Dashed line shows location of profile in Figure 2. Small letters indicate state names.
INTRODUCTION
The North American Gulf of Mexico coastal plain is underlain by an early to middle Paleozoic passive continental margin that developed following Late Proterozoic rifting along
the southern Laurentian continent. The margin structure is obscured by younger events
that include the late Paleozoic OuachitaAppalachian orogeny, Mesozoic opening of
the Gulf of Mexico, and formation of the
modern passive continental margin. However,
the broad-scale geometry of the margin can be
inferred from the strike of the OuachitaAppalachian orogen and the distribution of synrift sedimentary and volcanic rocks (Cebull et
al., 1976; Thomas, 1976, 1991; Viele and
Thomas, 1989). These observations suggest
that Laurentia moved west-northwest during
Proterozoic rifting, creating a passive continental margin consisting of distinct northnortheast–striking rift segments and eastsoutheast–striking transform segments (Fig.
1). The central Gulf of Mexico coastal plain
overlies the proposed Alabama-Oklahoma
transform segment, which extends ;800 km
from southwestern Arkansas into southeastern
Alabama.
Geophysical data and studies of lithospheric
flexure in central Texas and western Arkansas
confirm the proposed margin segmentation in
these areas (Kruger and Keller, 1986; Keller
et al., 1989; Culotta et al., 1992; Mickus and
Keller, 1992; Harry and Mickus, 1998; Mickus, 1999), but few data constrain the structure
of the Alabama-Oklahoma transform margin.
In this paper geophysical data are used to constrain the structure and history of the central
part of the Alabama-Oklahoma transform margin in eastern Mississippi. Comparison of
these results with modern rift and transform
margins indicates that the subsurface geology
closely matches that of modern transform
margins.
*E-mail and present addresses: Harry—
Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523-1482, USA,
[email protected]; Londono—Department of
Geology and Geophysics, Louisiana State University,
E235 Howe-Russell Complex, Baton Rouge, Louisiana 70803, USA, [email protected]; Huerta—
Department of Geosciences, Pennsylvania State University, 407 Dieke Building, University Park,
Pennsylvania 16802, USA, [email protected].
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; November 2003; v. 31; no. 11; p. 969–972; 3 figures.
969
Downloaded from geology.gsapubs.org on July 12, 2010
Figure 2. Cross section across Paleozoic (Pz) margin and Ouachita suture in central Mississippi (after Harry and Londono,
2004). Top: Bouguer gravity profile. Center: 5:1 vertical exaggeration with seismic refraction (dashed lines), seismic reflection (horizontal arrow at top), and well-data constraints. Vertical arrow indicates location of crust-mantle hinge used to align
profiles in Figure 3. Ouachita facies includes deep-water low-grade metamorphic rocks emplaced on Laurentian margin
during Ouachita orogeny and orogenic clastic deposits involved in thrusting and folding (Viele, 1989). Numbers indicate
density (in kg/m3). Bottom: 1:1 vertical exaggeration.
GEOPHYSICAL CONSTRAINTS ON
MARGIN STRUCTURE
A transect across the Mississippi coastal
plain is shown in Figure 2. Petroleum industry
well and seismic data constrain the upper 10
km in the transect and the thickness of the
upper Paleozoic clastic sequence in the northern and central Black Warrior basin (Harry
and Londono, 2004). The thickness of the
crust, depth to mid-crustal interfaces south of
the Ouachita fold-and-thrust belt, and thickness of Mesozoic and Cenozoic sedimentary
rocks in the Mississippi Interior Salt basin are
constrained by seismic refraction data (Warren
et al., 1966). Other features are constrained
primarily by forward modeling of gravity
data, including the depth to the base of the
Ouachita fold-and-thrust belt, its southern extent, and the geometry of the late Paleozoic
Ouachita suture. Densities in the gravity model are based on empirical relations to seismic
velocity for the appropriate lithologies (Christensen, 1989) and comparison to previous
gravity models in the region (Kruger and Keller, 1986; Mickus and Keller, 1992). These initial density estimates were revised as needed
during the forward-modeling process.
MARGIN STRUCTURE
The Mississippi transect (Fig. 2), like similar transects in Louisiana and Texas (Kruger
970
and Keller, 1986; Mickus and Keller, 1992),
reveals a buried Paleozoic passive continental
margin that was not affected by subsequent
tectonic events. The Ouachita fold-and-thrust
belt is a thin-skinned orogen that was thrust
northward over the passive margin, but it did
not result in significant shortening or crustal
thickening on the margin. Mesozoic extension
was focused south of the orogen, and also did
not significantly modify the Paleozoic
passive-margin structure. Consequently, the
deep crust in this area preserves the structure
of the Paleozoic passive margin at the time of
its formation.
The thickness of the Proterozoic crust decreases abruptly across the margin, from 35
km in the northern Black Warrior basin to
;10 km north of the Ouachita fold-and-thrust
belt 50 km farther south. The thickness of the
Proterozoic crust in the Black Warrior basin is
constrained by seismic refraction and reflection data, which constrain the depths to the
base of the crust and top of the Paleozoic
carbonate-shelf sequence, respectively (Harry
and Londono, 2004). Refraction data also constrain the depth to the base of the crust beneath the Ouachita fold-and-thrust belt (Warren et al., 1966). The depth to the top of the
Paleozoic crust (and, hence, the thickness of
the Paleozoic crust) beneath the Ouachita or-
ogen and central Black Warrior basin is constrained primarily by the gravity model.
The nature of the crystalline crust beneath
the southern Black Warrior basin and Ouachita
orogen is problematic. Figure 2 depicts this as
oceanic crust emplaced in a southwardverging subduction complex beneath the allochthonous terrane that collided with Laurentia during the Ouachita orogeny. However, the
data allow for a variety of interpretations, including (1) complete absence of subducted
oceanic crust beneath the Ouachita fold-andthrust belt, (2) presence of ;7-km-thick oceanic crust (typical of modern oceanic crustal
thicknesses) as shown in Figure 2, or (3) oceanic crust to ;12 km thick, indicating excessive volcanism during margin formation. The
latter alternative is similar to interpretations of
margin structure in western Arkansas and
Louisiana (Keller et al., 1989; Mickus and
Keller, 1992), but it is deemed unlikely in
Mississippi because it requires an unusually
shallow detachment (,10 km) and low densities in the Ouachita orogen compared to the
model of Mickus and Keller (1992) to compensate for the high-density oceanic crust
(Harry and Londono, 2004). The first alternative is also unlikely because the absence of
oceanic crust would require a shallower highdensity basement in the Mississippi Interior
GEOLOGY, November 2003
Downloaded from geology.gsapubs.org on July 12, 2010
Salt basin that is not consistent with the seismic refraction data (Warren et al., 1966).
Thus, the favored interpretation involves
;7-km-thick oceanic crust preserved in the
relict subduction system. The northern extent
of the oceanic crust is poorly constrained. Figure 2 places the ocean-continent transition beneath the central Ouachita orogen, but it may
be as far north as the southern Black Warrior
basin. To be consistent with the gravity data,
the presence of high-density oceanic crust beneath the northern Ouachita orogen and southern Black Warrior basin would require a
slightly thicker upper Paleozoic sequence and
thinner crystalline crust in these areas than
shown in Figure 2, but this is permissible given the available data.
COMPARISON TO MODERN
CONTINENTAL MARGINS
Three aspects of the geophysical transect
indicate that the Paleozoic continental margin
beneath the Mississippi coastal plain is a
transform rather than a rift margin. First, the
steep crustal thickness gradient seaward of the
basement hinge zone and the abrupt transition
from thick to thin crust are strikingly similar
to modern transform margins (Fig. 3). Rift
margins typically have much wider regions of
transitional crust thickness and lack the steep
monotonic gradient. In addition, horsts and
grabens produce large-amplitude shortwavelength variations in crustal thickness near
the basement hinge zone on most rift margins.
The Mississippi Paleozoic margin, like most
transform margins, shows relatively subdued
topography near the hinge zone.
Second, the seismic velocity and density
structures of the margin show no indication of
synrift volcanism, plutonism, or magmatic underplating of the crust. This lack of synrift
magmatism is another characteristic of transform margins. Although amagmatic rift margins exist (e.g., the Iberia margin), they are
rare. The majority of modern rift margins are
associated with widespread synrift magmatism
and mafic underplating (White et al., 1987;
Whitmarsh et al., 2001). Gravity modeling of
the Mississippi Valley graben to the northeast,
the Gulf of Mexico farther south, and the Cretaceous Monroe volcanic province near the
Louisiana-Mississippi border demonstrates
that magmatic underplating and volcanic features should be resolved if they are present
(Harry and Londono, 2004), so their absence
cannot be attributed to an inability of the data
to resolve these features.
Third, the Paleozoic oceanic crust adjacent
to the margin does not appear to be anomalously thick (see preceding discussion). Oceanic crust adjacent to modern rift margins is
commonly thicker than the global average as
GEOLOGY, November 2003
Figure 3. Crustal thickness variations across continental margins. Profiles are
aligned along hinge zone at crust-mantle interface and normalized according to (H
2 Hmin)/(Hmax 2 Hmin), where H is thickness of crystalline crust and prerift sedimentary rocks and Hmin and Hmax are minimum and maximum thicknesses across
each profile. Shading indicates regions encompassing modern rift (dark gray) and
transform (light gray) margins. Rift margins (solid lines)—Carolina Trough (Holbrook et al., 1994; Trehu et al., 1989), northern Bay of Biscay (Le Pichon and Barbier, 1987), southern Bay of Biscay (Fernandez et al., 1998), South China Sea (Nissen et al., 1995), Edoras (Barton and White, 1997), Ethiopia (Makris et al., 1991),
northern Grand Banks (Keen and Dehler, 1997), southern Grand Banks (Keen and
Dehler, 1997; Reid, 1994), southeast Greenland (Korenaga et al., 2000), central Gulf
of Mexico (Harry and Londono, 2004), Hatton Bank (Vogt et al., 1998), Iberia (Bowling and Harry, 2001), Namibia (Bauer et al., 2000; Stewart et al., 2000), Orphan
Basin (Keen and Dehler, 1997), Otway (Finlayson et al., 1998), central Saudi Arabia
(Makris et al., 1991), Nova Scotia (Keen and Potter, 1995), and Wilkes (Eittreim,
1994). Transform margins (dashed lines)—Exmouth Plateau (Lorenzo et al., 1991),
Flemish Cap (Todd and Reid, 1989), southwest Grand Banks (Keen et al., 1990),
Ghana (Sage et al., 2000), Hornsrund (Jackson et al., 1990), and Oman (Barton et
al., 1990).
a result of excess transitory magmatism immediately after continental breakup (White et
al., 1987). In contrast, oceanic crust adjacent
to transform margins is formed at mature
spreading centers in steady-state thermal conditions. This process typically results in either
normal-thickness crust or, if significant heat is
conducted away from the ridge system and
into the adjacent continental lithosphere, crust
that is slightly thinner than the global average
(Bird, 2001).
SUMMARY AND CONCLUSIONS
The late Proterozoic through middle Paleozoic Laurentian passive continental margin beneath the central North American Gulf of
Mexico coastal plain is preserved in a nearly
pristine state beneath upper Paleozoic through
Cenozoic rocks emplaced during the Ouachita
orogeny, opening of the Gulf of Mexico, and
formation of the modern Gulf of Mexico
coastal plain. The thickness of the crust on the
margin changes abruptly over a distance of
;50 km, from ;35 km beneath the northern
Black Warrior basin to ;10 km beneath the
Ouachita orogen. The abrupt change in crustal
thickness and the steep crustal thickness gradient seaward of the basement hinge zone are
similar to modern transform margins and contrast with the more gradual crustal attenuation
that is typical of rifted continental margins.
The lack of synrift magmatism and normal
faulting on the margin is also characteristic of
modern transform margins, and is unusual on
rift margins. The data are inconclusive in regard to the position of the ocean-continent
transition and thickness of the Paleozoic oceanic crust, but favor an interpretation that
places ;7-km-thick oceanic crust adjacent to
the margin beneath the central Ouachita foldand-thrust belt. The volume of postrift magmatism is similar to modern transform
margins.
Thus, the structure and magmatic history of
971
Downloaded from geology.gsapubs.org on July 12, 2010
the margin are strikingly similar to modern
transform margins and quite different from
most rift margins. The margin beneath the
Mississippi coastal plain appears to be an
along-strike continuation of a similar transformmargin structure in western Arkansas and
Louisiana, supporting the contention that the
Paleozoic passive margin beneath the central
Gulf of Mexico coastal plain developed along
a continuous transform segment of a late Proterozoic rift system that extended from western Arkansas to western Alabama (e.g., Thomas, 1976).
ACKNOWLEDGMENTS
We are grateful to Richard Groshong Jr., Jack
Pashin, and Bill Thomas for insightful discussions
and to Peter Clift and an anonymous reviewer for
their helpful comments. This research was supported by National Science Foundation grant OCE9906889.
REFERENCES CITED
Barton, A.J., and White, R.S., 1997, Crustal structure of Edoras Bank continental margin and
mantle thermal anomalies beneath the North
Atlantic: Journal of Geophysical Research,
v. 102, p. 3109–3129.
Barton, P.J., Owen, T.R.E., and White, R.S., 1990,
The deep structure of the East Oman continental margin: Preliminary results and interpretation: Tectonophysics, v. 173, p. 319–331.
Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K.,
Schulze, A., Trumbull, R.B., and Weber, K.,
2000, Deep structure of the Namibia continental margin as derived from integrated geophysical studies: Journal of Geophysical Research, v. 105, p. 25,829–25,853.
Bird, D., 2001, Shear margins: Continent-ocean
transform and fracture zone boundaries: The
Leading Edge, v. 20, p. 150–159.
Bowling, J.C., and Harry, D.L., 2001, Geodynamic
models of continental extension and the formation of nonvolcanic rifted continental margins, in Wilson, R.C.L., et al., eds., Nonvolcanic rifting of continental margins: A
comparison of evidence from land and sea:
Geological Society [London] Special Publication 187, p. 511–536.
Cebull, S.E., Shurbet, D.H., Keller, G.R., and Russell, L.R., 1976, Possible role of transform
faults in the development of apparent offsets
in the Ouachita–southern Appalachian tectonic
belt: Journal of Geology, v. 84, p. 107–114.
Christensen, N.I., 1989, Pore pressure, seismic velocities, and crustal structure, in Pakiser, L.C.,
and Mooney, W.D., eds., Geophysical framework of the continental United States: Geological Society of America Memoir 172,
p. 783–798.
Culotta, R.T., Latham, T., Sydow, M., Oliver, J.,
Brown, L., and Kaufman, S., 1992, Deep
structure of the Texas Gulf passive margin and
its Ouachita-Precambrian basement: Results of
the COCORP San Marcos Arch Survey:
American Association of Petroleum Geologists Bulletin, v. 76, p. 270–283.
Eittreim, S.L., 1994, Transition from continental to
oceanic crust on the Wilkes-Adelie margin of
Antarctica: Journal of Geophysical Research,
v. 99, p. 24,189–24,205.
Fernandez, V.G., Gallart, J., Pulgar, J.A., Gallaste-
972
gui, J., Danobeitia, J.J., and Cordoba, D.,
1998, Crustal transition between continental
and oceanic domains along the North Iberian
margin from wide angle seismic and gravity
data: Geophysical Research Letters, v. 25,
p. 4249–4252.
Finlayson, D.M., Collins, C.D.N., Lukaszyk, I., and
Chudyk, E.D., 1998, A transect across Australia’s southern margin in the Otway basin region: Crustal architecture and the nature of
rifting from wide-angle seismic profiling: Tectonophysics, v. 288, p. 177–189.
Harry, D.L., and Londono, J., 2004, Structure and
evolution of the Mississippi coastal plain and
Gulf of Mexico continental margin, southeast
U.S.A.: Geological Society of America Bulletin (in press).
Harry, D.L., and Mickus, K.L., 1998, Gravity constraints on lithospheric flexure and the structure of the late Paleozoic Ouachita orogen in
Arkansas and Oklahoma, south central North
America: Tectonics, v. 17, p. 187–202.
Holbrook, W.S., Reiter, E.C., Purdy, G.M., Sawyer,
D., Stoffa, P.L., Austin, J.A. Jr., Oh, J., and
Makris, J., 1994, Deep structure of the U.S.
Atlantic Continental Margin, offshore South
Carolina, from coincident ocean-bottom and
multichannel seismic data: Journal of Geophysical Research, v. 99, p. 9155–9178.
Jackson, H.R., Faleide, J.I., and Eldholm, O., 1990,
Crustal structure of the sheared southwestern
Barents Sea continental margin: Marine Geology, v. 93, p. 119–146.
Keen, C.E., and Dehler, S.A., 1997, Extensional
styles and gravity anomalies at rifted continental margins: Some North Atlantic examples: Tectonics, v. 16, p. 744–754.
Keen, C.E., and Potter, D.P., 1995, The transition
from a volcanic to a nonvolcanic rifted margin
off eastern Canada: Tectonics, v. 14,
p. 359–371.
Keen, C.E., Kay, W.A., and Roest, W.R., 1990,
Crustal anatomy of a transform continental
margin: Tectonophysics, v. 173, p. 527–544.
Keller, G.R., Braile, L.W., McMechan, G.A., Thomas, W.A., Harder, S.H., Chang, W.-F., and Jardine, W.G., 1989, Paleozoic continent-ocean
transition in the Ouachita mountains imaged
from PASSCAL wide-angle seismic reflectionrefraction data: Geology, v. 17, p. 119–122.
Korenaga, J., Holbrook, W.S., Kent, G.M., Kelemen, P.B., Detrick, R.S., Larsen, H.-C., Hopper, J.R., and Dahl-Jensen, T., 2000, Crustal
structure of the southeast Greenland margin
from joint refraction and reflection seismic tomography: Journal of Geophysical Research,
v. 105, p. 21,591–21,614.
Kruger, J., and Keller, G., 1986, Interpretation of
regional gravity anomalies in the Ouachita
mountains and adjacent gulf coastal plain:
American Association of Petroleum Geologists Bulletin, v. 70, p. 667–689.
Le Pichon, X., and Barbier, F., 1987, Passive margin
formation by low-angle faulting within the upper crust: The northern Bay of Biscay margin:
Tectonics, v. 6, p. 133–150.
Lorenzo, J.M., Mutter, J.C., Larson, R.L., Buhl, P.,
Diebold, J.B., Alsop, J., Hopper, J., Falvey, D.,
Williamson, P., and Brassil, F., 1991, Development of the continent-ocean transform
boundary of the southern Exmouth Plateau:
Geology, v. 19, p. 843–846.
Makris, J., Henke, C.H., Engloff, F., and Akamaluk, T., 1991, The gravity field of the Red
Sea and East Africa: Tectonophysics,
v. 198, p. 369–381.
Mickus, K.L., 1999, Magnetotelluric observations in
the western Ouachita Mountains, southeastern
Oklahoma: Geophysics, v. 64, p. 1680–1688.
Mickus, K.L., and Keller, G.R., 1992, Lithospheric
structure of the south-central United States:
Geology, v. 20, p. 335–338.
Nissen, S.S., Hayes, D.E., Buhl, P., Diebold, J., Bochu, Y., Weijun, Z., and Yongqin, C., 1995,
Deep penetration seismic soundings across the
northern margin of the South China Sea: Journal of Geophysical Research, v. 100,
p. 22,407–22,433.
Reid, I.D., 1994, Crustal structure of a nonvolcanic
rifted margin east of Newfoundland: Journal of
Geophysical Research, v. 99, p. 15,161–15,180.
Sage, F., Basile, C., Mascle, J., Pontoise, B., and
Whitmarsh, R.B., 2000, Crustal structure of
the continent-ocean transition off the Cote
d’Ivoire–Ghana transform margin: Implications for thermal exchanges across the palaeotransform boundary: Geophysical Journal International, v. 143, p. 662–678.
Stewart, J., Watts, A.B., and Bagguley, J.G., 2000,
Three-dimensional subsidence analysis and
gravity modelling of the continental margin
offshore Namibia: Geophysical Journal International, v. 141, p. 724–746.
Thomas, W., 1976, Evolution of the OuachitaAppalachian continental margin: Journal of
Geology, v. 84, p. 323–342.
Thomas, W.A., 1991, The Appalachian-Ouachita
rifted margin of southeastern North America:
Geological Society of America Bulletin,
v. 103, p. 415–431.
Todd, B.J., and Reid, I., 1989, The continent-ocean
boundary south of Flemish Cap: Constraints
from seismic refraction and gravity: Canadian
Journal of Earth Sciences, v. 26, p. 1392–1407.
Trehu, A.M., Ballard, A., Dorman, L.M., Gettrust,
J.K., Klitgord, K.D., and Schreiner, A., 1989,
Structure of the lower crust beneath the Carolina Trough, U.S. Atlantic continental margin: Journal of Geophysical Research, v. 94,
p. 10,585–10,600.
Viele, G.W., 1989, The Ouachita orogenic belt, in
Hatcher, R., et al., eds., The AppalachianOuachita orogen in the United States: Boulder,
Colorado, Geological Society of America, Geology of North America, v. F-2, p. 555–561.
Viele, G.W., and Thomas, W.A., 1989, Tectonic synthesis of the Ouachita orogenic belt, in Hatcher, R., et al., eds., The Appalachian-Ouachita
orogen in the United States: Boulder, Colorado, Geological Society of America, Geology
of North America, v. F-2, p. 695–728.
Vogt, U., Makris, J., O’Reilly, B.M., Hauser, F.,
Readman, P.W., Jacob, A.W.B., and Shannon,
P.M., 1998, The Hatton basin and continental
margin: Crustal structure from wide-angle
seismic and gravity data: Journal of Geophysical Research, v. 103, p. 12,545–12,566.
Warren, D., Healy, J., and Jackson, W., 1966, Crustal seismic measurements in southern Mississippi: Journal of Geophysical Research, v. 71,
p. 3437–3458.
White, R.S., Spence, G.D., Fowler, S.R., McKenzie,
D.P., Westbrook, G.K., and Bowen, A.N.,
1987, Magmatism at rifted continental margins: Nature, v. 330, p. 439–444.
Whitmarsh, R.B., Manatschal, G., and Minshull,
T.A., 2001, Evolution of magma-poor continental margins from rifting to seafloor spreading: Nature, v. 413, p. 150–154.
Manuscript received 5 May 2003
Revised manuscript received 16 July 2003
Manuscript accepted 22 July 2003
Printed in USA
GEOLOGY, November 2003