TECTONICS, VOL. 4, NO. 1, PAGES 1-39, JANUARY 1985

ALLEGHENIAN RECONSTRUCTION AND SUBSEQUENT EVOLUTION OF THE GULF OF MEXICO, BAHAMAS, AND P ROTO-CARI B BEAN

1 James L. Pindell

Exploration Research Division, Pennzoil Company, Houston, Texas

Abstract. A detailed model for the of Mexico from Late Triassic to earliest evolution of the Gulf of Mexico, the Cretaceous time, but seafloor spreading Bahamas and the Proto-Caribbean is built was delayed until the Late Callovian. This within the framework provided by a detailed divided a single Gulf-wide salt basin into initial Alleghenian (western Pangean} the Louann and Campeche salt provinces. reconstruction and an accurate subsequent The Yucatan block progressively rotated relative-motion history between North about 43 degrees counterclockwise away America and Gondwana (northern Africa and from the Texas-Louisiana margin around a South America). The Alleghenian pole in northern Florida. The reconstruction closes all pre-Jurassic Tamaulipas-Golden Lane-Chiapas fault zone oceans; accounts for Jurassic attenuation of of eastern Mexico is interpreted as the continental crust by restoring that remains of an initially intracontinental attenuation to original pre-rift continental transform system along which Yucatan thicknesses; incorporates an improved migrated. Attenuated continental crust Equatorial Atlantic fit between northern beneath southern Florida and the western Brazil and the Guinea margin of Africa; Bahamas, termed here the Florida Straits quantitatively removes changes in shape of block, migrated approximately 300 km out northern South America due to Late of the eastern Gulf, approximately along Cretaceous and Cenozoic accretion and Central Atlantic flow lines. These internal deformation; includes pre-Mesozoic rotations are consistent with recently continental crust presently underlying the suggested magnetic anomaly trends in the western Bahamas and southern Florida; and Gulf of Mexico (Shepherd et al., 1982; S. correlates Late Paleozoic geology of Hall, personal communication, 1984). The Yucatan with its neighboring continental Proto-Caribbean formed synchronously by a masses. Extension occurred within the Gulf fan-like rotation of Yucatan away from Venezuela. 1 Now at Department of Geological INTRODUCTION Sciences, Durham University, United Kingdom. Evolutionary models of the Gulf of Mexico must integrate the geology of the Copyright 1985 circum-Gulf region (Figure 1) into a by the American Geophysical Union. quantitative kinematic framework defined by a Late Paleozoic continental reconstruction Paper number 4T1357. of western Pangea (North and South 0278-7407/85/004T- 1357510.00 America, Africa and Yucatan} and an Pindell: AlleghenianReconstruction, Gulf of Mexico Evolution

88 s s LIMIT OF SALT

c c LOWER CRETACEOUS

MAFIC

EDGE OF BLAKE MARATHON PRE-MESOZOIC

c KEY WELLS TO BASEMENT

LOU•NN OVERTHRUST 72

s

s

s

s

s

GOLDEN LANE

c c

ß CHORTIS KM 500 1000 Fig. ]. General inctonic and locality map of the Gulf region, features described in text. MSM, Mojave-Sonora Megashear; TMVBL, Trans-Mexican Volcanic Belt Lineament; SU, Sabine Uplift; MU, Monroe Uplifq JD, Jackson Dome; WA, WigginsArch; MOA, Florida Middle Grounds Arch; SO, Sierra Guaniguanico,presently western Cuba but was northeastern corner of Yucatan block prior to Paleogene;BSMA, Blake Spur Magnetic Anomaly; M-25, M-2], M-J6, M-l], Central North Atlantic magnetic anomalies. accurate subsequent relative motion history common prior to collision, and the majority between North and South America. The of Triassic deposition upon each consists of absence of any indication of Upper red beds that are lithologically similar and Permian-Triassic marine conditions in the difficult to correlate biostratigraphically. circum-Gulf region suggests that the initial Because of repeated Paleozoic orogeny reconstruction must recognize complete in the Appalachians, and because the terms Paleozoic ocean closure. In turn, the way Appalachian, Ouachita, Marathon and in which closure is achieved in the Huastecan describe geographic portions of a reconstruction must provide an internally single continuous, but diachronous, belt of consistent rearrangement of interpreted Late Paleozoic deformation, the term tectonic provinces and metamorphic zones Alleghenian orogenesis is used here to from the various continental blocks. The describe collectively the diachronous event rearrangement of provinces is made that produced the Alleghenides, those Late difficult by the fact that Jurassic Mississippian-Middle Permian tectonic continental breakup has followed features and metamorphism that were approximately the lines of Late Paleozoic created by final ocean closure between continental suturing; only rarely can Gondwana and American portions of pre-Permian geology be tied across Laurussia [Graham et al., 1975; Kluth and Alleghenian sutures in the reconstruction Coney, 1981; Pindell and Dewey, 1982; because the opposing continents had little in Dewey, 1982; Bradley, 1982]. Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 3

Before a plausible Alleghenian Venezuela in order to match the Louann and reconstruction may be attempted, one must Campeche rift-related salt provinces in the (1) identify all pre-Mesozoic continental Gulf (Figure 1). This positioning of Yucatan crust involved in Alleghenian orogenesis, (2) provides the necessary geometry of res tore to original crustal thickness the opposing margins for a tight fit between attenuation undergone by each margin during Gondwana and Laurussia. Discussed below Late Triassic and Jurassic rifting, and (3} are considerations that constrain and retract, where possible, post-Permian substantiate reconstructions of western offsets on intracontinental fault systems Pangea in the complex area between North such as those in Mexico [Anderson and and South America. These collectively Schmidt, 1983] and in northwest South produce the Alleghenian paleoreconstruction America [Dewey and Pindell, 1984]. In of Figure 2. Table 1 defines abbreviations, addition, proper realignment of the and interprets geological features, circum-Atlantic continents by closure of the portrayed in Figure 2. Central-North and the South Atlantic The Florida Straits Block Oceans is critical to Alleghenian paleogeography, by providing the primary Several lines of evidence suggest that framework on which to build more detailed southern Florida and much of the Bahamas aspects of the western Pangean Platform are underlain by pre-Mesozoic reconstruction. continental crust, a block (or blocks} In this study, a detailed Alleghenides referred to here as the Florida Straits reconstruction is derived, following the block (Figure 1). above methodology, which serves as a The nature and age of basement beneath starting point from which is modeled the Bahamas has been debated for decades, subsequent evolution of the Gulf of Mexico, the Bahamas and the Proto-Caribbean Sea. as geophysical and well data have not yet provided a conclusive answer. However, The evolutionary model is based upon a framework of relative motions between gravimetric, magnetic, and seismic data North America and South America-Africa [Uchupi et al., 1971] suggest that presumably pre-Mesozoic, attenuated [Klitgord and Schouten, 1982], the latter of continental crust underlies the western half which remained a single continent of the platform, whereas the eastern half throughout the opening of the Gulf. is oceanic. Basement of the oceanic Basement geology, intracontinental extension, and general Jurassic portion must have been raised to the photic sedimentation patterns of the Gulf region zone early on to initiate carbonate bank are reviewed and interpreted, and are development, presumably by plate motions integrated into the evolving plate kinematic along transform faults, or by volcanism, framework. The evolutionary model is during the separation of North America and Gondwana. Recently acquired seismic data discussed, and implications are derived. along the northern Bahamian margin (J. W. THE ALLEGHENIAN RECONSTRUCTION Ladd, personal communication, 1984} indicate that the western Bahamas are underlain by A consensus is emerging that the Late attenuated continental crust as far east as Paleozoic reconstruction of circum-Atlantic Tongue of the Ocean. A similar eastward continents and blocks in the Gulf and extent is suggested by the Punta Alegre Caribbean realm must achieve total closure evaporites of northern Cuba [Pardo, 1975] in the Gulf of Mexico area [Pindell and and by the possible salt occurrence [Lidz, Dewey, 1982; Buffler et al., 1980, 1981; 1973] beneath Exuma Sound (Figure 1), if it Dewey, 1982; Dietz and Holden, 1970; is assumed that evaporite sequences Freeland and Dietz, 1972; Salvador and pertaining to continental breakup are Green, 1980; Pilger, 1978; Walper, 1980; formed primarily in shallow water, on White, 1980]. The reconstruction must attenuated continental crust due to initial explain deformation patterns in the subsidence during rifting. The southern Alleghenian orogenic belt and the apparent margin of the Bahamas was imbricated in absence of Upper Permian to Late Triassic thrusts as Cuba collided with the Bahamas marine sedimentary rocks in the in the Paleogene [Gealey, 1980; Dickinson s tratigraphies of the margins involved. and Coney, 1980], providing surface Further, it seems most probable that the exposures in northern Cuba of early Yucatan block, but not the Chortis block, Bahamian stratigraphy. The eastward fit between the U.S. Gulf Coast and extent of the Punta Alegre closely matches Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution

THE ALLEGHENIDES (Late Permian) presentT

NORTH AMERICA o h basins er • highs

ß wells to basement / MFB O isolated basement

exposures

/c

YAQUI AFRICA

25N

//,' / Guy a n a ,/' Shield ,direction X ,""•/ ,/ ofclosure • .-':':I 'SOUTH' '-" AMERICA,,'"' "o / ', -'.... "

1000 km at 25 North

Fig. 2. Late Permian Alleghenides reconstruction derived in text, abbreviations and numeric labels defined in Table 1. Total rotation parameters for closure defined in Table 2. Chortis block and Mexico south of Trans-Mexican Volcanic Belt not included because their Permian posit,•ons are unknown. the extent of continental crust suggested by c9ntinental crust underlies southern Florida, Uchupi et al. [1971]. the southern Florida Shelf, and the western In the subsurface of the southern half of the Bahamas (Figure 1), and must Florida Shelf, the recent drilling of granitic be included in Alleghenian reconstructions. basement rocks of the Tampa Arch (Figure The fact that much of this crust overlaps 1) at depths of 3 to 3.7 km (R. N. Erlich, Africa when the Atlantic Ocean is closed personal communication, 1984) also indicates that crust of the Florida Straits demonstrates the existence of attenuated block has migrated some distance into its continental crust. Further, seismic present position, either by internal sections presented in Schlager et al. extension during rifting or by translation [1984] show a block-faulted base ment from the site of the present eastern Gulf beneath the Triassic to Recent sedimentary of Mexico along plate boundaries, as in section of the northern wall of the Florida Pilger [1978]. It will be seen that both Straits Channel. The Jurassic rhyolites and mechanisms probably occurred. basalts of the South Florida Volcanic Province of Klitgord et al. [1984] probably Stretching Analysis of Rifted Margins intrude and cover this stretched continental crust, and reflect intracontinental volcanism Areas presently underlain by during Mesozoic rifting between Africa and pre-Mesozoic continental crust that must be southeastern North America. integrated into the Alleghenian assembly are It appears that the Florida Straits block shown in Figure 3, excluding western (or blocks) of attenuated pre-Mesozoic Africa, and include the U.S. Gulf Coast Pindell: Alleõhenian Reconstruction, Gulf of Mexico Evolution 5

TABLE 1. Key and Interpretive Glossary to Figure 2

Abb re via tio n Expl anat ion

AB Anadarko Basin. Carboniferous extensional and right-lateral strike-slip basin, compressed in Late Pennsylvanian and/or Early Permian. ApB Appalachian Basin. Devonian to Carboniferous foreland sedimentary basin resulting from load of Appalachian thrust sheets. ArB Arkoma Basin. Carboniferous foreland basin resulting from load of Ouachita thrust sheets to the south. ArdB Ardmore Basin. Subbasin of Anadarko Basin, sits amidst dextral strike-slip zone. ArM Arbuckle Mountains. Dextral shear zone in North American foreland, probably related to Wichita Uplift. Active during Pennsylvaniano BB Bove Basin. Platform sequence (terrigenous} of Paleozoic age, rocks correlatable to Florida's Suwannee Basin. BRP Blue Ridge Province. Metamorphic thrusted nappes in central Appalachians, thrusting caused by collision of Africa with North America. BWB Black Warrior Basin. Carboniferous foreland basin resulting from load of Appalachian thrust sheets. BZ Brevard Zone. High-angle reverse and/or strike-slip fault in southern Appalachians, possible zone of lateral escape during Alleghenian compression. CA Cincinnati Arch. Foreland bulge caused by lithospheric flexure from load of Appalachians. CB Chihuahua Block. Zone north of Mojave-Sonora Megashear but south of Rio Grande which underwent basement reactivation during Jurassic (sinistral shear) and Cretaceous (compression). In Figure 2, a homogeneous simple shear totalling 100 km across the block has been restored to the northwest. CP Coahuila Platform. Block accreted to North America during Alleghenian assembly, possibly originally part of Gondwana but stranded during rifting, may represent Late Paleozoic arc created by closure between North America and Gondwana. CC Cordillera Central, Colombia. Presumably formed northwest margin of South America throughout Paleozoic to Late Cretaceous. Age and chemistry of Paleozoic and Mesozoic plutonic rocks questionable, but may have been Late Paleozoic arc prior to ocean closure. Mesozoic-Cenozoic arc volcanism has occurred here as well. Ch-SRG Chuacus-Santa Rosa Group. Chuacus Group are metamorphosed (in early Carboniferous) igneous and sedimentary rocks of Guatemala belonging to Yucatan block. Metamorphism relates to continental collision. The Upper Pennsylvanian to Lower Permian Santa Rosa Group unconformably overlies the Chuacus, and was deposited in a shallow sea behind the main Alleghenian thrust belts. DB Delaware Basin. Extensional, right-lateral strike-slip basin of Early Permian age. Relates to the escape of the Mexican peninsula during continental collision. 6 Pindell: AlleghenianReconstruction, Gulf of Mexico Evolution

TABLE 1. (continued)

Abbreviation Explanation

D-EPS Desmoinesian-Lower Permian Sediments. Postorogenic shallow-water molasse deposited behind Ouachita thrust zone. Correlated with Santa Rosa Group (Yucatan Block} and Permian of Colombia. DR U Devil's River Uplift. Right-lateral shear zone created by northwestward migration of Marathon region along southern side of Llano Uplift during ocean closure. EC Eastern Cordillera. Uplifted since Late Miocene due to compression and dextral strike-slip motion caused by Panama-Colombia collision. Was shallow-water sea and received Permian molasse from adjacent uplifted Cordillera Central during collision. emf Eagle Mills Formation. Predominantly Upper Triassic continental red beds filling grabens and obscuring basement. FWB Fort Worth Basin. Pennsylvanian foreland basin related to loading of Ouachita structural belt. G Guajira Block. A northern extension of Cordillera Central (see Cordillera Central). GP Guinea Plateau. Extended and submerged pre-Mesozoic continental crust which must be included in Alleghenian reconstruction. [B Illinois Basin. Intracontinental basin initiated during Early Paleozoic, reactivated in Alleghenian orogeny with the intersection of Mississippi Embayment and Rough Creek fault trends. KB Kerr Basin. Early Permian foreland basin, related to loading by Marathon thrust sheets (Ouachita structural belt}. LU Llano Uplift. Stable buttress of North America during collision, foreland bulge to the Ouachita structural belt. MA Merida Andes. Uplifted since Late Miocene during Andean Orogeny. Was shallow-water sea and received Permian molasse from nearby uplifted Cordillera Central (Santa Marta and Guajira). MB Mar fa Basin. Early Permian foreland basin related to loading of Marathon thrust sheets. ME Mississippi Embayment. Structural low, floored by Precambrian rift, only slightly reactivated during Alleghenian collision and Mesozoic rifting, but underwent extension and volcanism in C r e taceou s. M FB Mauritanides Foreland Basins. Devonian to Permian foreland basins related to loading by Mauritanides thrust sheets during collision. MiB Midland Basin. Shallow, cratonic basin adjacent to Delaware Basin, cause uncertain. MGA Middle Grounds Arch of Northwest Florida Platform. Structural basement high, or horst, defines edge of pre-Mesozoic continental crust in northeast Gulf. MM Marathon Mountains. Metamorphosed Paleozoic continental rise and slope sediments and orogenic flysch thrust up onto North American shelf sequence during Late Pennsylvanian to Early Permian time. Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 7

TABLE 1. (continued)

Abb re via tio n Expl an at ion

MPG Mexican Paleozoic Geosyncline. Foreland basin related to emplacement of Huastecan thrust sheets during Early to Middle Permian time. MSB Mississippi Slate Belt. Dextral shear zone created by migration of Ouachita region and Yucatan along southern end of Appalachians during Carboniferous ocean closure. MU Monroe Uplift. Basement high, possibly stranded continental remnant of Gondwana. MyM Maya Mountains. Deformed Upper Paleozoic igneous and sedimentary rocks. Shales and sands (Maya Series)correlated to Santa Rosa of Guatemala, Desmoinesian-Early Permian sediments of U.S. Gulf Coast, and Lower Permian of Colombia. ND Nashville Dome. Southern portion of Cincinnati Arch (see Cincinnati Arch). OD Ozark Dome. Structural basement high, foreland bulge related to loading of Ouachita thrust belt. Paraguana Block. Northward extension of Cordillera Central (see Cordillera Central}. PMSM Proto-Mojave-Sonora Megashear. Possible? zone of dextral continental escape during collision. PT Pedregosa Trough. Foreland basin related to emplacement of Huastecan thrusts in Permian time. May also be site of dextral transform motion allowing continental escape of Mexican peninsula. PTFS Pedregosa Trough Fault System. Postulated zone of dextral slip enhancing development of Pedregosa Trough and allowing continental escape of Mexican peninsula. RCFZ Rough Creek Fault Zone. Zone of intracontinental dextral shear, accommodating shortening in southern Appalachians during Carboniferous. SG Sierra Guaniguanico, Cuba. Prior to Paleogene Cuba-Bahamas collision, this continental crust belonged to northeast margin of Yucatan. SM Santa Marta Block. 'Northern extension of Cordillera Central (see Cordillera Central}. TA Tampa Arch. Pre-Mesozoic basement high off southwest Florida Platform, forms western portion of Florida Straits Block. VRP Valley and Ridge Province. Folded platform cover in front of primary Appalachian thrust sheets. VVB Val Verde Basin. Early Permian foreland basin related to loading by Marathon thrust sheets. WA Wiggins Arch. Metamorphic pre-Mesozoic basement high, originally part of Gondwana. WU Wichita Uplift. Dextral shear zone related to minor Pennsylvanian northwestward displacement of Llano block relative to midcontinent. Franklin Mountains. Upper Paleozoic carbonate shelf of North America [Bridges, 1970]. 8 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution

TABLE 1. (continued)

Abbreviation Explanation

Bisbee, Arizona. Upper Paleozoic carbonate shelf of North America [Bridges, 1970]. Cananea. Upper Paleozoic carbonate shelf of North America [Bridges, 1970]. Sierra Los Ajos. Upper Paleozoic carbonate shelf of North America [Bridges, 1970]. E1 Tigre. Upper Paleozoic carbonate shelf of North America [Bridges, 1970]. Solitario. Surface exposure of southern continuation of Marathon belt [Bridges, 1970]. Mina Plomosas-Placer de Guadalupe. Marathon equivalent, Ordovician to Devonian limestone, dolostone, chert, Carboniferous shale and carbonate, Permian reefs and shale [King, 1975]. Sierra del Cuervo. Wolfcampian highly deformed but little metamorphosed shale and sand [King, 1975]. Sierra de la Mojina. Lower Cretaceous conglomerate with metasedimentary clasts with ages of 330-350 Ma [King, 1975]. 10 Villa Ahumada Borehole. 1600 m of shale and silt, upper part Wolfcampian [Bridges, 1970]. 11 Ciudad Victoria (west of). Thrust slices of Precambrian gneiss, schists, Silurian to Devonian black shale, cleaved Carboniferous-Permian flysch, ultramafics [King, 1975; De Cserna, 1976; Salas, 1970]. 12 Aramberri. Pre-Mesozoic phyllites, schists, metavolcanics [King, 1975]. 13 Catorce. Ultramafic (ophiolite?) obducted with the Huastecans [de Cserna, 1976]. 14 Potrero de la Mula. Granodiorite plutons dated at 206 Ma, dating is questionable [King, 1975]. 15 Las Delicias. Permian volcaniclastic shale and greywacke, reefy carbonates, highly folded, and intruded by Triassic (cooling age) granite [King, 1975]. 16 Sierra del Carmen. Phyllite, marble, quartzite, greenschists, 263-275 Ma on metamorphics [King, 1975]. 17 Borehole. Bottomed in Paleozoic schists [King, 1975]. 18 Borehole. Bottomed in Paleozoi½ slate and quartzite [King, 1975]. 19 Borehole. Bottomed in granite-gneiss, 358 Ma [King, 1975; Flawn et al., 1961]. 2O Borehole. Bottomed in Paleozoic granite [King, 1975]. 21 Cajamarca Group. Exposures of orthogneiss, tuffs, basic lavas, metatonalites. Me tamorphic cooling ages: biotite, 239 Ma; muscovite, 214 Ma [Irving, 1971, 1975; Shagam, 1975]. 22 Granitic schists, metamorphism 215 Ma [Irving, 1975]. 23 Granitic schists, metamorphism 220 Ma [Irving, 1975]. 24 Borehole. Yucatan no. 1. Meta-andesite, 330 Ma and 290 Ma [Marshall, 1974]. 25 Borehole. Yucatan no. 4. Metaquartzite [Marshall, 1974]. 26 Borehole. DSDP 537. Phyllite, 449 Ma and 456 Ma [Schlager et al. 1984 ]. Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 9

TABLE 1. (continued)

Abb revia tion Explanat ion

27 Borehole. DSDP 538A. Amphibolite gneiss, 496 Ma and 348 Ma. Also diabase, 190 Ma and 165 Ma on different sills [Schlager et al. 1984]. 28 Borehole. Pre-Jurassic(?) granodiorite R. N. Erlich, personal communication, 1984). 29 Borehole. Pre-Jurassic(?) granite (R. N. Erlich, personal communication, 1984). 30 Borehole. Mississippian rhyolitic tuffs [Nicholas and Waddell, 1982]. 31 Borehole. Desmoinesian-Early Permian shallow-water, undeformed clastics and carbonates [Nicholas and Waddell, 1982]. 32 Borehole. Granite and phyllite, metamorphic ages of 270 Ma to 325 Ma [Cagle and Khan, 1983; J. Cagle, personal communication, 1984]. 33 Borehole. Precambrian at 8,877 feet [Case and Holcombe, 1980]. 34 E1 Baul. Paleozoic sediments metamorphosed in Late Paleozoic, granite intrusion 287 Ma [Feo-Codecido et al., 1984].

margin and eastern Mexico, northern South should be accounted for, which reduces the America, the Yucatan block, Florida, the apparent stretching factor in the flexural Blake Plateau, and the Florida Straits wings of margins [Watts, 1981]. Within the block. The Antilles and the Panama-Costa stretched zone itself, the effects of flexure Rican isthmus are post-Alleghenian, are not well understood because the crust Cretaceous and Cenozoic island arc undergoes complex changes in flexural complexes built on Late Mesozoic ocean rigidity during rifting. Hence, an Airy floor. The continental crust of the Chortis model is used here in highly attenuated block (Figure 1) entered the Caribbean portions of margins. From lithospheric region from the Pacific realm during the cross sections, total crustal extension may Late Cretaceous-Paleogene [Dickinson and be estimated, assuming plane strain and Coney, 1980; Wadge and Burke, 1983] and, conservation of continental crust, and hence, does not need to be considered in discounting subsidence in the wings due to the reconstruction. The margins of the flexure, by defining a line that divides pre-Mesozoic blocks were considerably equally the cross-sectional area of attenuated during Mesozoic rifting, so that continental crust on its oceanward side and their present geographic dimensions greatly the cross-sectional area of sediment and exceed their former, prerift dimensions, raised mantle on its landward side (Figure and this extension must be restored in 4). By estimating the position of this line proper reconstructions of western Pangea. intermittently along each margin, the The cross-sectional geometry of restored, prerift geometries may be continental crust along each margin may be approximated. estimated by isostatically balancing various U.S. Gulf Coast and eastern Mexico. thicknesses of sediment, crust, upper Figure 4 illustrates the restoration of mantle, and water, at several locations attenuation in the case of the U.S. Gulf across the margin, with an equivalent Coast, where crustal attenuation across a column of "normal" continental crust. The 760-km-wide zone was extreme. Sediment depth to the sediment-crust interface can thicknesses of 13.4 to 14.6 km beneath 0 to be determined seismically (reflection and 1 km of water in the Gulf Coast basin refraction), but identification of the [Antoine et al., 1974] suggest beta values crust-upper mantle boundary is more approaching 4. In northern Louisiana, difficult. Where possible, a component of southern Mississippi and east Texas, as far subsidence due to flexure at the margin north as the Talco-South 10 Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution

U.S.GULF COAST /-• SUMU •"-•

BLAKE PLATEAU

FLORIDA STRAITS BLOCK

1.2

20

o o b

10

SOUTH AMERICA

PRESENT LIMIT OF PRE-MESOZOIC CRUST PRE-RIF-r LIMIT OF PRE-MESOZOIC CRUST DIRECTION OF EXTENSION KM

300 600 900 30

• 10

0 ' 100 90 ,80 70 60

Fig. 3. Areas of pre-Mesozoic continental crust in the Gulf-Caribbean region and amount of synrift extension and/or subsequent change in shape. Prerift shapes are used in the Alleghenian reconstruction of Figure 2. Abbreviations as in Figure 1.

Arkansas-Pickens-Gilbertown fault system, system (Figure 1). Similar analyses at other shallow listric faults and rift structures locations along the margin lead to the indicate that basement attenuation occurred, prerift limit of pre-Mesozoic crust shown but was relatively minor compared to in Figure 3. southern Louisiana and offshore. Along eastern Mexico, basement drops Subsidence has been due largely to flexure off quickly along the eastern side of the caused by sediment loading in the Gulf, Tamaulipas Arch, the Golden Lane high, and although significant rifting occurred in the offshore along Veracruz (Figure 1}. As will East Texas, North Louisiana and Central be discussed later, this curvilinear line Mississippi salt basins. Nunn [1982] de fines a mar gin viewed he re as the estimated beta values in these basins remains of a transform fault between between 1.5 and 2, but these are perhaps Mexico and Yucatan which formed as the overestimates because flexure was ignored. latter migrated away from the U.S. Gulf Integration across the 760-km-wide Gulf Coast. Hence, crustal attenuation due to Coast Plain and margin yields a net rifting is believed to be negligible. north-south extension due to rifting of The Yucatan block. Basement beneath about 400 km, or an average beta factor of the Yucatan Peninsula lies beneath a 2.1 (Figure 4}. Thus, the prerift limit of 4-km-thick predominantly Late Jurassic to pre-Mesozoic continental crust through Cretaceous carbonate-evaporate section central Louisiana lies at about 30.5 N, [Lopez Ramos, 1975], except in the approximately along the Tepetate fault southern, orogenically active portion and Pindell: Alleõhenian Reconstruction, Gulf of Mexico Evolution 11

z•

(m•l) "I:IA:I"I Y:IS MO"I:I•] Hld:10 12 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution

along the eastern basement high. The 1981] and water depths average 1 km. thickness of this section suggests a 20% These values indicate beta values of about extension of crust, but the absence of a 3. The extension appears to be in a well developed rift-related clastic section southeast direction, parallel to the may indicate that much of this deposition is divergence of Africa from North America. due to flexure-induced subsidence relating Thus, in the Alleghenian reconstruction the to the cooling and loading of the Blake Plateau must be restored to about surrounding oceanic crust. Greater but 33% of its present size in a northwest uncertain sedimentary thicknesses occur direction. beneath the Campeche Shelf, where rifting Beneath the northern half of the Florida probably was more severe. Northwest of Straits block, the postrifting sedimentary the Campeche Escarpment, an 80-kin-wide section is 7 to 8 km thick (J. W. Ladd, rifted mar gin is over lain by 7 to 8 km of personal communication, 1984}, but in the sediment and about 3 km of water [Bufflet South Florida Basin (south of Tampa Arch, et al., 1980], suggesting a beta factor of Figure 1} and north of Cuba thicknesses about 5 and 64 km of extension within this may reach 12 km [Meyerhoff and Hatten, narrow belt. Sediment thicknesses beneath 1974]. In the area of Cay Sal Bank, the 12 Tabasco and westernmost Campeche (6 km) km sediment thickness is partly due to and offshore beneath Campeche Bay (8.5 thrust imbrication pertaining to the km, under 2 km of water) [Buffler et al., Paleogene Cuba-Bahamas collision 1984] indicate beta values of 1.5 to 3.7 [Dickinsonand Coney, 1980; Gealey, 1980], over a 400-km-long, northwest trending, whereas the 9 km thick section in the cross-section. In addition to stretching, Andros area ahead of the thrusts Cenozoic faulting also has altered the [Meyerhoff and Hatten, 1974] is largely due original shape of Yucatan. First, to inc.reased:•-su•bsidenceand sedimentation continental, terrigenous shelf sediments and rates ca•'u.s.ed:_•byforeland loading. In the crust originally belonging to the An•r•ø's?'-•':•tii••:iPaleocene-Middle Eocene northeastern Yucatan block but presently in de

(5 = Guaj•ra /,e, At=Aruba / \ / x C= Curacao / \ / B = Bona I re / / \\ N Haracalbo/ . ?, \ ^v- A,,•

• ,

Lesser ' Anf•lles and SoAm enezuelan Bsn

c/o:• -NoAm • S•an/Or,enfe 360 (Cayman) Colomb,[n e• Bsn Fig, 5. Vector-triangle diagram showingtrend and magnitudeof Neogene offsets between blocks and plates of the circum-Caribbean region, after Dewey and Pindell [1984]. Restoration of offsets derives pre-Late Miocene shape of northern South America, shown in Figure 6.

the shape of northern South America due to Quantifying the late Neogene deformation accretion and to internal strike-slip motions of northern South America to determine its may be quantified, as outlined in the next paleogeography at 9 Ma is achieved by section. restoring known and inferred post-nine Ma offsets upon faults and plate boundaries Change in the Shape of Northern South separating terranes and plates of the America southern Caribbean realm [Dewey and Pindell, 1984]. Late Neogene motions Three episodes of tectonism have between the terranes and plates can be significantly altered the Jurassic shape of described by a vector-triangle diagram northern South America since the breakup (Figure 5), the restoration of which of Pangeao The first was the accretion of produces the 9 Ma paleogeography of Figure Cretaceous aged basement of the Western 6. The cause of this uplift and deformation, Cordillera of Colombia and Ecuador in Late and drastic paleogeographic change, is, most Cretaceous time [Barrero, 1979; Mooney, likely, the progressive collision of the 1980]. The second was the accretion of the Panama arc with western Colombia [Pindell Netherlands-Venezuelan Antilles volcanic arc and Dewey, 1982; Wadge and Burke, 1983], (Aruba, Curacao, Bonaire, Aves, Roques, and the subduction of buoyant, young crust and Orchila) against the northern margin of produced at the Galapagos spreading center, Venezuela in Late Cretaceous to P aleogene which includes the Carnegie and Cocos time [Maresch, 1974; Gealey, 1980; Beets aseismic ridges, beneath the Cordilleran et al., 1984]. The third has been internal terrane of Colombia and Ecuador. deformation, mainly by motion along several The islands offshore northern Venezuela strike-slip faults, of northern South (Aruba to Orchila, Figure 6, inset) are America over the last 9 Ma amounting composed largely of Upper Cretaceous essentially to 290 km of northeastward plutonic and volcanic rocks intruding and migration of the Andean Cordilleran overlying a Lower Cretaceous mafic series terranes of Ecuador, Colombia and which probably represents oceanic crust northwestern Venezuela, relative to cratonic [Maresch, 1974; Beets et al., 1984]. Hence, South America [Dewey and Pindell, 1984]. the islands comprise an intraoreanic arc The effects of each episode on northern which developed and collided with Venezuela South American geography are approximated subsequent to the breakup of Pangeao La below. Blanquilla (southern portion of Aves Ridge), Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution

Fig. 6. Pre-Late Miocene paleogeography of blocks and plates of circum-Caribbean realm, derived by restoring offsets defined in Figure 5. Inset: Blocks of northern South America which have undergone relative motion, on the faults shown, over the last 9 Ma. VC, thrust front of Villa de Cura and other nappes. Heavy line is pre-Campanian shape of northern South America used in Alleghenian reconstruction, derived by removing Aruba-Orchila and Western Cordillera accreted terranes from pre-Late Miocene paleogeographic reconstruction. Modified after Dewey and Pindell [1984].

Margarita (origin uncertain} and Tobago Basic Igneous Complex, Diabase Group} (accreted to leading edge of Caribbean overlain by deep-water abyssal sediments Plate at an unknown time) also are may represent oceanic crust accreted to composed entirely of post-Paleozoic rocks. the Cordillera Central in the Late In Venezuela, the ultramafic bearing Villa de Cretaceous [Mooney, 1980; Irving, 1975; Cura Klippe and associated nappes have Shagam, 1975; Feininger and Bristow, 1980; been interpreted as the oceanic forearc Barrero, 1979; Goossens and Rose, 1973; [but see Beets et al., 1984] of this Duque-Caro, 1979]. Rocks typical of Netherlands-Venezuelan Antilles arc, which island-arc volcanism are present was obducted onto northern Venezuela [Henderson, 1979], but these probably during Late Cretaceous to Paleogene postdate the accretion of the basement arc-continent collision [Maresch, 1974; complex to South America and, hence, Gealey, 1980]. Because pre-Mesozoic represent arc volcanism along the newly basement occurs near the coast to the accreted South American margin rather north of the Villa de Cura, the precollision than volcanism at a preexisting arc of edge of northern South America apparently Pacific provenance that collided with lies along the coast or offshore within the Cordillera Central. The cause of accretion Bonaire Basin, but the offshore islands may have been due to the young age and must be excluded from Pangean consequent buoyancy of the oceanic crust; reconstructions. accretion occurred only a short time after In the Western Cordillera of Colombia formation. The basement complex may, in and Ecuador (Figure 6, inset), fact, be an occurrence of oceanic crust pre-Cretaceous rocks are absent. that was affected by the mid-Cretaceous B" Cretaceous basic igneous rocks (Pinon, extrusion event of the Caribbean Plate Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 15

[Burke et al., 1978] that, rather than Africa and South America prior to Aptian entering the Caribbean, was accreted to plate divergence in the Equatorial Atlantic western South America. In addition to the and throughout the opening of the Gulf of Western Cordillera, the Neogene-accreted Mexcico. Panama arc also must be removed from South America. The Yucatan Block The removal of post-Paleozoic accreted terranes and restoration of Neogene The Yucatan block played an important deformation produces the pre-Campanian role in the evolution of the Gulf of Mexico. shape of northern South America shown in Its rotated prerift position against Texas Figure 6, inset. This shape is believed to and Louisiana as portrayed by Pindell and represent the Jurassic shape of northern Dewey [1982] accounts for such problems South America, although Sierra de Santa as the Ouachita salient and the apparent Marta and the Guajira Peninsula may have absence of Late Permian to Triassic marine migrated northeastward during the Late rocks in the Gulf region by allowing Cretaceous-Paleogene(?} from the area of complete continental closure by the the Lower Magdelena Valley [Duque-Caro, Permian. Further, magnetic anomaly 1979]. Also, deformation prior to the lineations in the central Gulf of Mexico are Campanian is possible, but the stable shelf oriented nearly east-west and diverge to conditions that prevailed during the Early the west [Shepherd et al., 1982; S. Hall, Cretaceous over much of northern South personal communication, 1984]. This trend America [Maresch, 1974; Irving, 1975] is clearly distinct from the Central North suggests relative tectonic quiescence at Atlantic trend and is consistent with an that time. origin of the Gulf of Mexico by counterclockwise rotation of Yucatan, as The Equatorial Atlantic Assembly suggested by Pindell and Dewey [1982]. Rock types also constrain this initial The opening of the Equatorial Atlantic position of Yucatan. The Chuacus Group in between northeast Brazil and the Guinea Central Guatemala is a metamorphosed margin of Africa postdated the opening of (greenschist to amphibolite} collection of the Gulf of Mexico. Because Africa•s marbles, quartzites, greenstones, schists, relative motion with respect to North serpentinites and volcanic rocks, with America during the opening of the Gulf can granodioritic intrusions, whose age is known be traced by magnetic anomalies in the only to be pre-Permian on the basis that it Central Atlantic, the pre-rift reconstruction is unconformably overlain by the Permian of South America with northern Africa Santa Rosa Group [Roper, 1978; Van den defines, through the three-plate circuit, the Boom, 1972; Anderson et al., 1973]. relative relationship between North and Preserved within a section apparently of South America during Gulf evolution. the Chuacus Group that was only Pindell and Dewey [1982] and Klitgord et moderately metamorphosed, however, are al. [1984] suggested that the Carboniferous (probably Lower mid-Cretaceous South America-northern Carboniferous) fossils [Van den Boom, Africa fit of Rabinowitz and LaBrecque 1972]. Thus, regardless of the age-range of [1979], which is tighter than the South the Chuacus stratigraphy, the age of Atlantic fit of Bullard et al. [1965], metamorphism apparently is Carboniferous. existed also during the Jurassic, and should Isotopic evidence [Gomberg et al., 1968] on be used when modelling the Gulf of Mexico. the Rabinal Granite, a probable However, this tighter fit overlaps synmetamorphic melt from the granitization Precambrian crust between Liberia and of a Chuacus arkose [Van den Boom, 1972], northernmost Brazil by about 30 km, produces possible ages of 1075 Ma plus or demonstrating the need for minor minus 25 and 345 Ma plus or minus 20. The improvement. A revised Equatorial Atlantic latter is compatible with a Lower reconstruction is shown in Figure 7, which Carboniferous age. It appears, therefore, aligns the prerift limits of continental crust that the deformation and metamorphism along the northern Brazil and Guinea exhibited by the Chuacus Group can be margins. These limits were determined by attributed to Alleghenian orogenesis. removing marginal sediments and restoring Alleghenian orogenesis as modeled by Dewey the inferred syn-rift extension, as in Figure [1982] would suggest a prolongedperiod of 4 for the Gulf margin. This fit describes deformation and metamorphism for the the interplate relationship between northern rocks of Yucatan, as they occupied a shear 16 Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 17

zone between North and South America older ages are similar to many of those from latest Devonian to earliest Permian recovered from the basement of Florida time. Further support for proximity of [Smith, 1982]. That Florida belonged to northwestern South America and North Gondwana prior to Alleghenian collision is America by Late Devonian time is based on clear from its pre-Carboniferous the presence of Appalachian basin marine Afro-South American fauna [Cramer, 1971; invertebrate fauna of that age in Colombia Pojeta et al., 1976]. Similarities between and Venezuela [Boucot, 1975; Barrett, Yucatan and Florida would imply the same 1983]. Sediments of the unconformably for Yucatan. Lithologies and Upper overlying Santa Rosa Group (Permian) Paleozoic isotopic ages of 270 to 325 Ma probably were shed from exposed portions on rocks (phyllite, granite) from a well of the Chuacus, and other structural highs penetrating the Wiggins Arch (Figure 1) in the orogenic zone, into shallow marine [Cagle and Khan, 1983; J. Cagle, personal molasse basins during the final stages of communication, 1984] are similar to those orogenesis, as they are far less deformed recovered from the Yucatan block and may and relatively unmetamorphosed. suggest that the Wiggins Arch was part of In the Maya Mountains of Belize (Figure Gondwana as well. All lie to the south of 1), a variably deformed, argillaceous to the proposed Alleghenian suture. conglomeratic sequence of Pennsylvanian-Middle Permian age overlies Synthesis of the Alleghenian Reconstruction older (?}gneissic basement with granitic intrusions [Dawe, 1984; Nelson, 1984; The above considerations provide Bateson and Hall, 1977; Dixon, 1957]. At geological and quantitative geometrical least the upper part of the sedimentary constraints on reconstuctions of western section consists of shallow-water, Pangea. Although Middle to Late Paleozoic fossiliferous molassic sands, shales and convergence between Laurussia and conglomerates, and is lithologically and Gondwana is poorly understood, patterns of biostratigraphically correlative with the Alleghenian orogenesis and Late Paleozoic Santa Rosa Group of Central Guatemala sedimentation appear to be best explained [Anderson et al., 1973]. These deposits are by complete ocean closure during very similar in lithology and age to the eontinent-continent collision, within which Des moines ian to Lower Permian late to the Yucatan block filled the gap between post-orogenic molasse deposits in the Venezuela and the U.S. Gulf Coast margin subsurface of the U.S. Gulf Coast [Woods (Figure 2}. The probable suture zone, from and Addington, 1973] and to the Lower east to west, lies between the Appalachians Permian strata of northwest South America and the Mauritanides of western Africa; [Shagam, 1975]. These localities align in a crosses Georgia between the Suwannee continuous belt behind the main Alleghenian Basin and the Southern Appalachians; thrusted zone to the north, providing continues north of the Wiggins Arch and support for the reconstruction from a Sabine Uplift, following approximately the sedi mentological perspective. trend of the Gilber town-South In the subsurface of northern Yucatan, Arkansas-Mexia graben system; is offset by meta-andesite/dacite was drilled (Yucatan the Texas Lineament along the Devil's River No. 1, see Figure 1} and isotopically dated Uplift {essentially a tear fault); swings to as 330 Ma and 290 Ma [Marshall, 1974]. the south of the Marathons but again is The andesite may represent an occurrence offset, possibly by a proto-Pedregosa of the Alleghenian arc that theoretically Trough fault system, and passes to the should have existed; the isotopic age is northwest of the Coahuila Platform and eonsistent with a thermal resetting during then south between the Huastecan belt of the period of closure between Gondwana and Mexico and northwest South America. Laurussia. At Cato•:he and a neighboring Whether the Chortis block was part of knoll of the Yucatan block, DSDP holes Mexico during Late Paleozoic time or 538A and 537 (Figure 1} reached Paleozoic whether it arrived later after migration basement as well [Schlager et al., 1984]. along the Cordillera is unknown and, hence, Hole 538A sampled amphibolite gneiss with Chortis is excluded from the isotopic ages of 496 and 348 Ma, and hole recons truction. 537 obtained phyllite with ages of 449 and Accretionary complexes that were 456 Ma. The 348 Ma age can be attributed developed during ocean closure include the to thermal resetting during the Alleghenian Ouachitas, the Marathons and the orogeny [Schlager et al., 1984], whereas the Huastecans. All of these accretionary 18 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution complexes overthrust Paleozoic shelf rocks northern Gulf includes a collection of highs of North America, suggesting that final (Sabine, Monroe, Wiggins) separated by convergence was achieved by southeast intervening basins (Gulf Coast Salt Basins). dipping subduction with North America as The Sabine and Wiggins basement highs the downgoing plate. The case for consist of Paleozoic metamorphics, Alleghenian accretionary prisms in the volcanics and granite [Nicholas and Waddell, southern Appalachians is questionable, but 1982; Cagle and Khan, 1983], which may be these may lie to the east beneath the interpreted as remnants of the leading edge coastal plain. of Gondwana which were sutured to and Alleghenian thrust-loaded foreland basins overthrusted onto the margin of North which developed on North America include America during the Alleghenian Orogeny and the Appalachian, Black Warrior, Arkoma, subsequently left behind during Late Fort Worth, Ker, Val Verde, Marfa, Triassic to Early Jurassic rifting [Pindell Pedregosa basins and the Mexican Paleozoic and Dewey, 1982; Smith et al., 1981]. The "Geosyncline." Distal, foreland bulges same applies for Florida, on the basis of include the Cincinnati Arch, Nashville Dome, Early Paleozoic fauna in Suwannee Basin Ozark Dome, and Llano Uplift. [Cramer, 1971], and for the Coahuila Possible remnants of the expected arc Platform of northeastern Mexico, which between Gondwana and North America possesses Late Paleozoic granodioritic include the Upper Paleozoic granodiorites, intrusions [King, 1975] possibly andesites or volcanics presently found in representative of an arc. Basement in the the Central Cordillera of Colombia [Irving, attenuated margin of northwestern Yucatan 1975], the Chuacus Group of Guatemala, block is unknown. Yucatan No. 1 well of Yucatan, the Coahuila Extensive salt deposits overlie the zones Platform, the Sabine Uplift, and the of attenuated continental crust, and seaward basement of Florida. Zones of Late limits of the Louann and Campeche salt Paleozoic strike-slip motion leading into the provinces match reasonably the North American foreland which may have ocean-continent boundaries [Buffler et alo, allowed escape of continental fragments 1981], although oceanward halokinesis has include the Rough Creek Fault Zone, the occurred, particularly in the Sigsbee Anadarko Basin/Wichita Uplift, the Delaware Escarpment [Lehner, 1969]. It seems likely Basin [Dewey, 1982] and, possibly, the that the onset of eraplacement of oceanic Pedregosa Trough. Closure was achieved by crust by seafloor spreading in the Gulf at east to west oblique collision from the oceanic isostatic depths (2.6 km depth) latest Devonian through the middle Permian, allowed sufficiently open marine circulation with probable migrations of various blocks to terminate salt deposition, and split the within the suture zone. once continuous salt province into the distinct provinces known today. INTERPRETIVE REVIEW OF PRIMARY Unfortunately, the halokinesis on both sides GULF-REGION GEOLOGICAL FEATURES of the Gulf prevents direct matching of opposing margins for paleogeographic The generalized tectonic map of the Gulf reconstructions. of Mexico region (Figure 1) shows primary The oceanic portion of the Gulf of tectonic features that any model of Mexico was created by primarily Late Gulf-region evolution must explain. These Jurassic seafloor spreading at a ridge features are reviewed and interpreted system that must be included in below. plate-boundary reconstructions. Linear magnetic anomaly trends in the central Gulf Basement fan slightly to the west and generally par allel the U.S. Gulf Coast mar gin and the Three types of crust exist in the Gulf Campeche Escarpment of Yucatan [Shepherd region: normal-thickness continental crust, et al., 1982; S. Hall, personal variably-attenuated continental crust, and communication, 1984]. This leads oceanic crust. Prior to seafloor spreading independently to the conclusion that Yucatan in the Gulf, relative plate motions were originated from the Texas-Louisiana margin accommodatedby continental attenuation (as and rotated counterclockwise to its present much as 500 km between the southern U.S. position, and further substantiates the and Yucatan, see above). This attenuation geological arguments presented earlier for must be incorporated when modeling Gulf this initial position in Alleghenian evolution. Attenuated basement around the paleogeography. Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 19

When the Equatorial Atlantic is closed structure responsible for these (see above} and South America-Africa is discontinuities has been termed the then refitted to North America following Mojave-Sonora Megashear (MSM) [Silver Central North Atlantic flow lines [Klitgord and Anderson, 1974]. The timing of motion and Schouten, 1982], overlap occurs between along the postulated MSM is constrained the Florida Straits block and the Guinea between the first major motions of the and Demerara Plateaus of Africa and South breakup of Pangea (Late Triassic, first America, respectively. Also, the closure of stretching, but early Mid-Jurassic is first Yucatan against the Texas-Louisiana margin significant motion) and deposition of the leaves an oceanic hole in the northeastern Oxfordian Zuloaga Group (Smackover Gulf with a southeasterly magnetic anomaly equivalent}, as the latter apparently masks trend that is discordant with the trend in the fault zone in northeast Mexico and is the central Gulf [Shepherd et al., 198:2]. It not offset. A 700 to 800 km offset during appears that crust of the Florida Straits Bajocian-Callovian time (about :20-25 million block migrated to the east-southeast, in years) indicates displacement rates between addition to undergoing internal extension, :2.8 to 4 cm/yr. It appears as though, in during Gulf evolution. This motion relative effect, blocks or slices of Cordilleran to the remainder of Florida was left-lateral Mexico migrated more or less with South strike-slip, as discussed below, while motion America during initial breakup, maintaining relative to Yucatan was extensional, as a land bridge from North to South America defined by the eastern Gulfrs magnetic at least until Callovian time, when saline anomalies. waters finally entered the Gulf of Mexico region and evaporated to form the once Fault Systems of the Gulf Region co•.tinuous Louann and Campeche salt deposits. From which ocean the saline U.S. Gulf Coast. In the U.S. Gulf Coast waters entered, the Pacific or the Atlantic, margin, the Mexia-Talco-South is unknown. Another potential, but Arkansas-Pickens-Gilbertown fault system unproved, zone of major Mesozoic offset defines the northern limit of significant across Mexico is along the Trans-Mexican normal faulting in basement. This system Volcanic belt. This area, during Late coincides closely with the probable Cenozoic time, has been the site of Alleghenian suture. Assuming Yucatan significant calc-alkaline volcanism, possibly originated from the Texas-Louisiana margin because it is a zone of weakened crust and rotated to its present position, the [Mooser, 1969; Anderson and Schmidt, direction of extension in the U.S. Gulf 1983]. Coast was north-south. Therefore, the The number of major faults through Mexia-Tal½o fault zone owes its origin to Mexico and their offsets are poorly differential subsidence and minor dextral understood; proposed offsets are based shear between the basement of the stable largely on geometry alone. It is very Llano area and the basement of the Gulf possible that all of Mexico, since it resided Coast during attenuation. Similarly, motion on the western, convergent margin of in the Mobile Bay system may have been Pangea during breakup, experienced severe slightly sinistral. internal shearing of the sort seen along the Mexico. In most reconstructions of western North American margin today. If western Pangea, much of Mexico overlaps studies of North America/Farallon relative South America, if Mexico is kept in its motion for Late Jurassic-Early Cretaceous present position with respect to North time are accepted, Farallon subduction America. Understanding the emplacement of beneath southwestern North America Mexico into this "overlap position • has been possessed a strong left-lateral oblique problematic because known evidence for component [Engebretson, 198:2, but see large-scale motions upon one or more Duncan and Hargraves, 1984]. This is in intracontinental transform faults across agreement with the postulation of a simple Mexico and the southwesternmost U.S. is shear regime for Mexico during the Late meager. Evidence in favor of such large Jurassic [Beck, 1983], and possibly Middle offset faults includes the interruption of Jurassic as well. northeasterly striking Precambrian tectonic Along the eastern Mexican margin, belts, and a 700 to 800 km sinistral continental basement falls off abruptly along displacement of stratigraphic columns a linear trend defined by the eastern side having "provocative similarities" [Anderson of the Tamaulipas Arch, the Golden Lane and Schmidt, 1983]. The theoretical high, and offshore at Veracruz. If extended 20 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution

to the south, this trend crosses into Florida region. Basement structure in Chiapas, approximately along the northern Florida and the southeasternmost U.S. can flank of the Chiapas Massif. Sediment only be inferred from gravimetric, magnetic thickness adjacent to this flank exceeds and borehole information, but basement 4500 metres [Viniegra, 1971], suggesting the appears to be irregular and to consist of a existence of a major structural break. The collection of horstlike Paleozoic highs linear trend as a whole is perpendicular to separated by grabens filled with red beds magnetic anomaly trends in the Gulf of [Smith, 1983; Barnett, 1975; Klitgord et al., Mexico [Shepherd et al., 1982; S. Hall, 1984]. Primary structural features include personal communication, 1984], and defines the Jay Fault, the Georgia Embayment a small circle about a pole located in the •graben• system, and the Florida Elbow vicinity of northern Florida. It is suggested basin which separates the Middle Grounds that this trend, termed here the and Tampa Arches (Figure 1). Tamaulipas-Golden Lane-Chiapas fault zone, The northwest trending Jay fault [Smith, was a right-lateral transform zone between 1983] aligns with the Pickens-Gilbertown Yucatan and eastern Mexico, that allowed fault system and the Bahamas fracture migration of Yucatan away from the zone of Klitgord et al. [1984], and defines Texas-Louisiana margin. This is further a steep, down to the south drop-off in supported by the conspicuous absence of basement. Sinistral motion along this fault salt diapirs marginal to the central portion is speculative, but it clearly defines the of this trend, in the area of the Mexican northern limit of significant basement Ridges [Buffler et al., 1979], in contrast to attenuation, as do the Pickens-Gilbertown areas of the Gulf where attenuated fault system in the Gulf Coast and the continental basement exists with thick salt Bahamas fracture zone across the Florida deposits and diapirs [Buffler et al., 1980, Peninsula. This attenuation probably 1981]. A transform origin for the eastern occurred in the Middle Jurassic, as it did Mexican margin which postdated salt throughout the entire Gulf of Mexico. North deposition explains the absence of salt. In of the Jay, relatively minor basement addition, the Tamaulipas Arch, during Late structures such as the Covington Jurassic time, was a linear basement high Embayment and Conecuh and Pensacola of Paleozoic rocks that supplied significant Arches formed due to general extension amounts of debris to proximal deposits of between North America and Gondwana. the Huizachal and Zuloaga Groups during Development of the northeast trending, initial rifting and platform subsidence Triassic Georgia Embayment system is [Sandstrom, 1982; Stabler, 1982; Meyer and older than the other fault zones, and is Ward, 1982; Bracken, 1982]. The linearity often referred to as an extensional rift on and positive structural relief of the Arch the basis of its Triassic red beds and during the Jurassic suggest strike-slip basalts filling narrow troughs [Barnett, faulting as a possible cause of its uplift. 1975]. If the system•s formation were due In Chiapas, coarse, Upper Jurassic-Lower to crustal extension, then a well-developed Cretaceous Todos Santos red beds fill Jurassic sedimentary section related to northwest trending structural valleys and basin subsidence could be expected. ridges whose relief reached 1000 to 2000 m However, such a section is absent across at the time of deposition [Burkart and southern Georgia and north-central Florida Clemons, 1971]. This scenario accords with [Barnett, 1975; Smith, 1983] and, therefore, models of strike-slip faulting equally as it is suggested that the Georgia Embayment well as it does with rifting, and the system probably was formed by strike-slip northwest trend of the basins is shear, which caused local uplift and erosion, perpendicular to the expected extension and deposition of Upper Triassic red beds direction between North and South America. into associated strike-slip basins. On the Finally, definition of a Mexico-Yucatan basis of geometry alone, the suggested shear zone along the northeast side of the sense of offset is right-lateral (discussed Chiapas Massif avoids the often invoked later). hypothesis that rr_•ajor displacement has Basement structure beneath southern occurred across the Isthmus of Florida and the south Florida shelf is Tehuantepec; crystalline rocks cross the poorly known due to the thick post-Triassic isthmus from southern Mexico into the sedimentary section. As mentioned earlier, Chiapas Massif with no obvious structural the eastern part of Florida Straits block break [King, 1969; Case and Holcombe, overlaps Africa when the continents are 1980 ]. reassembled. This is true even after 80% Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution 21

internal extension is restored to the forearc tectonically loaded the southern northwest (see above). In addition, the Florida/Bahamian margin, depressing the Yucatan block alone cannot fill the oceanic carbonates of the latter to oil-maturation portion of the Gulf of Mexico; a small depths. Oil has since migrated upward into oceanic swath remains in the northeastern the fractured serpentinites that presently deep Gulf. Therefore, it is apparent that form small reservoirs in and offshore crust of the Florida Straits block migrated northern central Cuba [Wassal, 1956]. The out of the eastern Gulf, in addition to causes of other possible "events" during the undergoing severe internal extension. Cretaceous, including the Gulf-wide Hypothesized here is the existence of a mid-Cretaceous unconformity [Buffler et fault (Florida Elbow Fault, Figure 1) along al., 1980], volcanism and local plutonism in which the Florida Straits block, subsequent the Gulf Coast [Zartman, 1977; Smith et to crustal attenuation south of the Jay al., 1981], uplift of the Wiggins, Monroe and Fault, migrated 300 km east-southeast out Sabine highs, and reactivation of the of the eastern Gulf to its present position Mississippi Embayment [Ervin and beneath the south Florida shelf and western McGinnis, 1975] are unclear. In addition to half of the Bahamas. The Florida Elbow these plate-tectonic deformations, much of Fault runs from the southern escarpment of the northern and western Gulf has the Paleozoic Florida Middle Grounds Arch, experienced severe sedimentary gravity continues through the Florida Elbow basin sliding and halokinesis. and crosses Florida near Lake Okeechobee, underlies the northwest Providence Channel, EVOLUTION OF THE GULF OF MEXICO and defines the northern margin of Great REGION Bahama Bank. Such a trend is readily seen on a magnetic anomaly contour map Figures 8 through 14 outline a model for [Klitgord et al., 1984, figure 5]. It is the Mesozoic plate-tectonic evolution and further suggested that sinistral motion early sedimentary history of the Gulf of along the fault system produced the Florida Mexico region derived from the Alleghenian Middle Grounds escarpment by translating reconstruction and the interpretations of the Tampa Arch away from the Middle fault systems and internal extension Grounds Arch. summarized above. The model is developed within the plate-kinematic framework Tectonism Following Gulf Formation defined by rotation parameters in Table 2. An Upper Oxfordian deposition map shows Since the Gulf of Mexico's formation, generalized lithofacies in the which was completed by or during the paleogeographic framework defined by the earliest Cretaceous, the Gulf region has evoiutionary model at that time and further experienced at least two periods of constrains the proposed evolutionary plate-tectonic deformation. The first was model. the eastward advance of thrust sheets of the Sierra Madre Oriental x•f Mexico Late Triassic (Figure 1} in Late Cretaceous to Eocene time [Dengo, 1975]. The cause of thrusting In the Late Triassic, western Pangea is still debated, but the load of the thrusts, began to rift along widespread, poorly which travelled across Jurassic salt, defined zones of intracontinental depressed the western portion of the block-faulting and dike emplacement (Figure eastern Mexican margin, reactivating the 8}. Minor associated motions, recorded by Tampico-Tuxpan and Veracruz sedimentary the Late Triassic geology outlined below, basins by flexural loading. The Laramide led to the pre-Atlantic, early Middle orogenesis also elevated and reactivated the Jurassic relationship between Africa and Tamaulipas-Golden Lane highs, probably due North America defined by Klitgord and to lithospheric flexure in response to Schouten [1982]. loading by the Sierra Madre to the west, In the southern U.S., north-south or by sedimentation offshore to the east, extension produced an extensive graben or both. The second period of tectonism system filled with Eagle Mills red beds. was the early Paleogene arrival and Along the eastern United States, red beds, collision of Cuba (Greater Antilles} with the dikes and sills of the Newark Group and its Bahamas/southern Florida shelf [Dickinson equivalents were deposited and emplaced in and Coney, 1980; Gealey, 1980]. The a belt of grabens paralleling the coast from eraplacement of thrust sheets of the Cuban the Piedmont out to the continental shelf. 22 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution 23

f 24 Pindell: AlleghenianReconstruction, Gulf of Mexico Evolution

z Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 25

1,1,1 26 Pindell'. Alleghenian Reconstruction, Gulf of Mexico Evolution

o

/ /

/ / Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 27

NORTH AMERICA

AFRICA

SOUTH AMERICA

Shallowcarbonate r•J Riftbasin red beds • Deepcarbonate/shale • $alt/evaporite !• Terrigenousshelf • Shallowmicrite OXFORDIAN DEPOSITION •J Volcanicarc/trough • Ooliticcarbonate

Fig. 13. Depositional facies of the Gulf region at the close of Smackover deposition, plotted on Late Jurassic plate reconstruction. CP, Coahuila Platform. Other abbreviations and conventions as in Figure 8.

This system has generally been attributed Plateau/eastern Florida. A possible origin to extension between Africa and North for the Georgia Embayment is that of America, but the distribution and dextral shear, which would have allowed orientation of dikes with 200-220 Ma Florida to escape laterally from the isotopic ages fit a right-lateral shear model Africa-Blake Plateau convergence. A within the arcuate zone (pre-Atlantic shear shearing origin for the Georgia Embayment zone, Figure 8} along the eastern U.S. and is suggested by the absence of a Jurassic western African margins [Swanson, 1982]. sedimentary section there, as would be It is not implied that each graben of the expected had the Embayment formed purely Newark series is a pull-apart basin sensu by extension. stricto, but extension was probably To the west, southeasterly tectonic accompanied by dextral slip. Dextral shear transport of blocks within Mexico began at along the eastern U.S. is compatible with this time, along one or several shear zones, north-south extension along the U.S. Gulf or by internal strain. This motion Coast; thus, the Eagle Mills and Newark effectively maintained a land bridge from systems may be attributed to the same North to South America until the Callovian relative plate motion. or later. A speculative cause of this early Lying amidst the eastern and southern motion is left-lateral oblique subduction of U.S. shear and extensional zones, the Kula/Farallon Plate. respectively, is the complex region of continental blocks comprising Florida, the Middle Jurassic Blake Plateau and the western Bahamas. Dextral shear between North America and Divergence between Africa and North Africa would have led to convergence America began in early Middle Jurassic between western Africa and the Blake time (Figures 9 and 10} by seafloor 28 Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution

Z Pindell: Alleõhenian Reconstruction, Gulf of Mexico Evolution 29

spreadinõ north of the Blake Plateau, and Collectively, the motions are responsible by continental attenuation in the area of for the emplacement of central Cordilleran Blake Plateau and the Florida Straits Mexico into its present position with block. The Blake Spur Maõnetic Anomaly respect to North America, which was (BSMA) can be symmetrically correlated formerly occupied by northwest South about the mid-Atlantic Ridõe with the America. western marõin of Africa, and aliõns with The postulation that Yucatan rotated out the eastern boundary of the present Blake of the Gulf realm in turn implies that a Plateau (Fiõure 11). Hence, it is evident corresponding, synchronous rotational that internal stretchinõ within the Blake opening occurred between Yucatan and Plateau accounts for diverõence between Venezuela (Figures 9, 10, 11, and 12). This North America and Africa until the time basin may be termed the "Proto-Caribbean" when the BSMA formed (approximately 170 as it occupied the future site of the Ma). The crust to the north of the Blake Caribbean but was subducted beneath the Plateau that was created by seafloor true Caribbean Plate during Late spreadinõ prior to formation of the BSMA Cretaceous to Eocene time [Dickinson and produces the Central North Arianticks Coney, 1980]. Whether this Proto-Caribbean asymmetry with respect to the present basin was marine during Middle Jurassic ridõe axis. An early ridõe apparently was time is debatable. However, the fact that abandoned as the spreadinõ center jumped it was subducted suggests that it was to the site of the BSMA floored by oceanic or extremely attenuated 19711, associated with which was ridõe continental crust, and that it was probably formation between the Blake Plateau/Florida marine. Furthermore, Middle Jurassic Straits block and Africa. marine sedimentary rocks are found in the In the developinõ Gulf of Mexico, northern ophiolitic belt of Cuba [Pardo, Yucatan% proõressive diverõence from 1975] that were accreted to the Greater Texas-Louisiana was achieved by severe Antilles arc at the leading edge of the continental attenuation until probably Caribbean Plate during subduction of the Callovian or early Oxfordian time (postsalt), Proto-Caribbean. Finally, Middle Jurassic, at which time seafloor spreadinõ beõan. terriõenous, partially marine sediments (San Durinõ this early rift staõe in Gulf Cayetano Group) [Meyerhoff, 1964] are development, intracontinental extension was preserved upon presumably continental crust accompanied by subaerial deposition of the in the Sierra Guaniguanico of western Cuba; Werner red beds and anhydrites. the latter is interpreted here as a portion In the west, southeastward transport of of the rifted margin of the northeastern blocks accelerated along the Mexican corner of the Yucatan block which was Cordillera, preventing the incursion of obliquely overridden and caught up by Cuba significant amounts of sea water into the in the early Paleoõene Bahamas-Cuba Gulf realm (Figures 10 and 11). One collision. That this margin of Yucatan probable zone of concentrated sinistral bordered the Proto-Caribbean also suggests shear strain was the Mojave-Sonora a marine character for the latter during megashear [Anderson and Schmidt, 1983; the Middle Jurassic. Silver and Anderson, 1974], although strike-slip fault motions may have occurred Late Callovian (Latter Middle Jurassic) as far inland as the "Rio Grande lineament" (Figures 8 and 9), a term used here to By the Callovian (Figure 11), basement define that line which bounds the attenuation had produced a broad, subsiding northeastward extent of significant platform of continental crust. As this basement reactivation during both Jurassic platform subsided through sea level, or as and Laramide tectonism. Based on a tectonic barrier to either the Pacific or geometry, it is suggested that left-lateral juvenile Atlantic was repeatedly breached, offsets within the "Chihuahua block" north flooding of the Gulf realm and subsequent of the Mojave-Sonora meõashear total evaporation produced the once continuous approximately 100 km. Motions upon the Louann and Campeche salt deposits. The Mojave-Sonora and faults within the "Yaqui" Punta Alegre and (?) Exuma Sound salt may block (of Anderson and Schmidt [1983]) to also correlate with the Louann and the south collectively approach 800 kin. Campeche, but may be slightly older These motions were completed by Oxfordian (Bathonian) and relate to the opening of the time, as the Zuloaga Group (Oxfordian) Central North Atlantic rather than to the masks the Mojave-Sonora Meõashear. Gulf of Mexico. Such is the case with the 30 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution

TABLE 2. Finite Rotation Parameters

Anomaly/Age Latitude Longitude Anglea Source

Equatorial Atlantic Closure

Jurassic to Early 52.08øN 34.03øW 51.39 e Cretaceous GondwanaWith Respectto North Americab

Permo-Triassic c 65.55øN 13.69øW -77.15 e ( Pangea )

Mid-Jurassic 66.95øN 12.02øW -75.55 f (pre-Atlantic)

Blake Spur 67.02øN 13.17øW -72.10 f Mag. Anomaly

Late Callovian 67.13øN 15.49øW -65.93 g (Louann Salt)

Anomaly •.-21 66.50øN 18.10øW -61.92 f (Tithonian)

Anomaly M-16 66.10ON 18.40øW -59.79 f (Berriasian) YucatanWith Respectto North Americad

Permo-Tria ssic 29.74øN 80.36øW -43.41 e ( Pangea )

Mid-Jurassic 29.50øN 81.40øW -41.00 e

Blake Spur time 29.50øN 81.40øW -34.00 e

Late Callovian 29.50øN 81.40øW -20.00 e

Anomaly-2] time 29.50øN 81.40øW -9.00 e

Anomaly-16 time ...... b

a Positive angle is anticlockwise rotation looking down at pole. b South America and northern Africa are treated as a single plate. c Stage-pole from Permo-Triassic to Mid-Jurassic is 26.7øN, 63 4øW 2 5 ø with respect to North America d Yucatan parameters measured in Mercator and contain inherent error. e This study. f Klitgord and Schouten [1982]. Klitgord (personal communication,1982). g Interpolated between M-25 and BSMA of Klitgord and Schouten [1982]. h Early Cretaceous and Present position.

Senegal Basin and Guinea Plateau salt but the majority of evidence [Scott, 1984] deposits [Jansa and Weidmann, 1982] which indicates a late Middle Jurassic age, opposed the eastern Florida Straits block at probably late Callovian [Salvador and Green, that time (Figure 10). A Callovian age has 1980; Imlay, 1980]. Seismic stratigraphic not actually been proved for the Louann, studies [Buffler et al., 1980; 1981] suggest Pindell' Alleghenian Reconstruction, Gulf of Mexico Evolution 3l

correlation of the Campeche and Louann locally, by salt migration where salt deposits. The Louann (and probably accumulations were sufficiently thick. The Campeche) possesses shallow-water Norphlet effectively blanketed basement characteristics, and its clean, pure nature features (except Sabine and Wiggins) implies rapid deposition, perhaps on the [Nicholas and Waddell, 1982; Cagle and order of only hundreds of thousands of Khan, 1983] so that by Smackover time years (C. Schreiber, personal (Figure 13), a true platform or carbonate communication, 1983). The environmental ramp had developed iBudd and Loucks, picture associated with the Gulf of Mexico's 981. salt deposits consists of a subsiding In the western Gulf, migration of basin/platform, during lithospheric Yucatan continued along the attenuation, located possibly behind a Tamaulipas-Golden Lane-Chiapas trans form foundering tectonic barrier to marine fault (TGLC). To the north of the migrating waters. Flooding by marine waters junction of TGLC and the central Gulf ridge produced numerous or continued system, TGLC evolved as a fracture zone shallow-water influxes perhaps only tens of separating zones of differential subsidence. meters deep, the evaporation of which led The abrupt topographic low, or freeboard, to salt deposition at rates which easily east of TGLC received enormous volumes matched rates of basin subsidence of sediment (for example, Burgos Basin [Schreiber and Hsu, 1980]. east of Montetrey, Mexico) [King, 1969]. Except for local occurrences such as the As strike-slip motion ceased along the Buckner facies of the northern Gulf Coast, Tamaulipas Arch, it subsided and eventually Gulf-wide salt deposition ceased by early was onlapped by Upper Jurassic OxfordJan time at the onset of relatively carbonates. open marine circulation. Open circulation Along the Pacific margin, left-lateral probably was structurally controlled by the migration of Mexican blocks may have establishment of the Gulf of Mexico's occurred at this time primarily along the mid-ocean ridge system (Figures 1! and !2). Trnn.•-Maxic•nn Vo!can. ic Belt lineament. The emplacement of oceanic crust at typical The portion of proto-Mexico referred to isostatic elevations (2.6 km below present here as the Guerrero block could not yet sea level) produced a deep central Gulf have reached its present position because trough sufficient to allow open marine of overlap with South America. Models of circulation throughout most of the Gulf. Caribbean evolution [Wadge and Burke, Furthermore, it was not until about this 1983; Pindell and Dewey, 1982] assume a time that the separation between South pre-Eocene connection between the Guerrero America and North America, incorporating and pre-Mesozoic Chortis blocks; Chortis all basement attenuation in the Gulf, was may have been connected to the Guerrero sufficient to accommodate the present size block at this time. Arc-related volcanism of the Yucatan block. The most likely within Colombia is indicated for the Late entrance for marine waters into the Gulf Jurassic by radiometrically determined ages during the Late Jurassic is between Florida on presumably subduction-related plutons and Yucatan, as DSDP leg 77 documented [Tschznz et al., 1974]. Convergence between Jurassic extension and marine sedimentation Chortis-Guerrero and Colombia is, [Schlager et al., 1984], and the therefore, seen as likely. This can only Proto-Caribbean certainly had become have been possible if the Kula/Farallon marine by Oxfordian time. Spreading at Plate convergence rate with South America the Gulf's ridge system isolated the main exceeded the spreading rate between North salt provinces, accounted for continued and South America, and if the separation between the U.S. Gulf Coast and Guerrero-Chortis terrane essentially Yucatan without further basement belonged to the Kula/Faralton Plate, as the attenuation within their margins, and Salinian block of western California translated the Florida Straits block to its essentially belongs to the Pacific Plate present position beneath the western today. Bahamas and southern Florida (Figures 11, 12 and 14). Norphlet and Smackover Early Fithonian (Late Jurassic) deposition on the attenuated Gulf Coast margin was not controlled by primary Ox for dian-Kim meridgian seafloor rifting but by differential subsidence spreading at the Gulf ridge system between adjacent basement blocks within a separated the Yucatan and Florida Straits thermally subsiding platform and, perhaps blocks from the U.S. Gulf Coast margin and 32 Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution isolated the three main salt provinces North America. Likewise, the Guerrero (Figure 13). Figure 13 summarizes block of Mexico may have reached its final lithofacies deposited in the Gulf area during position as well. Termination of its Oxfordian time (primarily Smackover and southeastward migration may relate to equivalents). A postulated triple junction cessation of a shearing component of off of DeSoto Canyon in the northeast Gulf subduction, as is indicated by the Early connected the central and eastern Gulf Cretaceous initiation of head-on convergence ridge systems to the postulated Florida (northeast relative motion) between North Elbow transform. Southeastward migration America and the Kula/Farallon plates of the Florida Straits block may have [Engebretson, 1982]. The termination of produced the steep, east-southeast trending seafloor spreading in the Gulf of Mexico portion of the northwestern Florida affixed the Yucata•_ and Florida Straits Escarpment. In the southwestern Gulf, blocks to the North American Plate. Thus, transform motion between Yucatan and continued spreading between North and Mexico continued along the Golden South America occurred at a ridge Lane-Chiapas portion of TGLC. No salt was system(s} in the Proto-Caribbean basin. deposited along the Mexican margin where This ridge must have been connected to the seafloor spreading occurred. In Chiapas, mid-Atlantic Ridge via a long left-lateral deposits of the Todos Santos "rift facies • transform, and must have extended out into are younger (Late Jurassic-Early the Pacific realm between Yucatan and Cretaceous) [Anderson et al., 1973] than Colombia. those to the north (Werner and Beginning at magnetic anomaly M-21 and equivalents). This is because shear along ending at M-4, fracture zone traces in the the active transform between Yucatan and western Central North Atlantic possess a eastern Mexico continued longer in the kink that indicates a westward shift in the south; that portion of TGLC north of the Central North Atlantic pole position during southward migrating central-Gulf spreading that time interval [Klitgord and Schouten, center progressively became a fracture 1982; Klitgord et al., 1984]. Such a shift zone, typified by simple thermal subsidence should produce compression at any long and carbonate deposition. left-lateral transforms in the system. In Mexico, shearing, perhaps localized Assuming that the eastern Bahamas is along the Trans-Mexican Volcanic Belt underlain by oceanic crust (see above), the lineament, brought the Guerrero block (and origin of the eastern Bahamas platform Chortis-Nicaragua Rise) near its final (oceanic basement uplift to, or formation position. To the east, the Proto-Caribbean at, the photic zone} is probably due either continued to open, fanlike, between Yucatan to compression at the long left-lateral and Venezuela. The mid-ocean ridge in this Bahamian transform zone, similar to the basin must have been joined in some way Recent 3 km uplift of basement at the with the plate boundary separating the Mussau Ridge of the eastern Caroline Plate Florida Straits and Yucatan blocks, which in [Hegarty et al., 1983] and the Pliocene turn must have been connected to the emergence of the Romanche transform in Central North Atlantic ridge system by a the Equatorial Atlantic [Bonatti et al., long transform along the south side of the 1977], or to extension and volcanism along Bahamas. A single triple junction is the transform zone (hot spot}. However, portrayed connecting these plate boundaries due to the kinematic prediction of in the northern Proto-Caribbean, for compression arising from the temporary simplicity, but Paleogene subduction of this change in the Central Atlantic pole position, crust has eliminated direct evidence for a compressional origin is favored here this proposition. (Figure 14).

Berriasian (Earliest Cretaceous) Pos t-Berriasian Development

Horizontal plate motions associated with Throughout most of the Cretaceous, the opening of the Gulf of Mexico were carbonate banks developed across nearly all completed by the earliest Cretaceous of the shelf margins of the Gulf, Bahamas, (Figure 14). It was not until this time that and Proto-Caribbean region. Terrigenous South America had migrated sufficiently far clastics were largely restricted to the arc from North America to accommodate systems bounding the Pacific realm. Yucatan in its present position relative to During the Late Cretaceous, however, Pindell: Alleghenian Reconstruction, Gulf of Mexico Evolution 33 thrusting, uplift and erosion in the Sierra Jurassic marine sediments in Gulf-region Madre Oriental and the Rocky Mountain stratigraphic columns, and (3) the geometry Overthrust provided enormous quantities of of Alleghenian orogenesis, which indicates terrigenous clastics to the wes tern and the former existence of a buoyant central Gulf realm. The Florida Banks (continental) mass directly south of the were protected from clastic deposition by Ouachita-Marathon thrust belts during the DeSoto Canyon bathymetric barrier. collision (the so-called Llanoria Plate). In The probable cause of this orogenesis is addition, Late Paleozoic geology correlates the arrival of buoyant oceanic masses at well across this reconstruction of North western North American trench systems. America, Yucatan and South America. Also associated with this orogenesis was Finally, the rotation required to move the migration of the present Caribbean Yucatan from its suggested prerift position Plate, led by the Greater Antilles arc to its present position both explains and is system, into the Proto-Caribbean realm, supported by the majority of post-Triassic thereby subducting crust of the Gulf-region geology, providing further Proto-Caribbean basin. The Caribbean iterative evidence favoring the proposed Plate as a whole probably is a buoyant Alleghenian recons tructiono mass of Pacific origin [Burke et al., 1978]. This study illustrates the importance of Orogeny and clastic deposition has occurred basing regional models of evolution within a progressively eastward along the margins of framework provided by paleopositions and the Proto-Caribbean, associated with the relative motions of major plates. In the northeastward migration of arc systems at case of the Gulf of Mexico, the motions of the leading edge of the Caribbean Plate Yucatan, the Florida Straits block, and [Dickinson and Coney, 1980; Pindell and blocks within Mexico are constrained by the Dewey, 1982]. The Caribbean Plate presence and motions of the larger North interacted with the southern Yucatan shelf and South American plates. The and northwest South America in the Late significance of internal continental Cretaceous, the western and central attenuation prior to seafloor spreading has Bahamas and north-central Venezuela in the been stressed, and evolutionary models of Paleogene, and with northeastern Venezuela other regions such as the Mediterranean and the easternmost Bahamas in the also should incorporate internal extension Neogene [Pindell and Barrett, in press, during initial rifting. In the Gulf of 1985]. Mexico, only about half of the separation between Yucatan and the U.S. Gulf Coast DISCUSSION was accomplished by seafloor spreading, whereas the other half was achieved by This evolutionary model of the Gulf of intracontinental extension. Mexico, Bahamas and Proto-Caribbean was Thick occurrences of salt in extensional constructed by integrating Gulf-region regimes around the world are deposited geology within a framework of empirical primarily upon rifted, subsiding continental geometric and plate-kinematic constraints, crust. Rift-related salt deposits in the including the North America-Africa Gulf of Mexico occur primarily in realms relative-motion history, the. initial of attenuated continental crust as defined Africa-South America reconstruction, by seismic reflection and refraction restoration of continental attenuation in methods [Buffler et al., 1980, 1981; Ibrahim rifted margins, and a revised shape of et al., 1981]. Thus, reconstructing northern South America. Any model of Gulf boundaries defining the original depositional evolution is highly dependent upon the limits of salt, prior to halokinesis, is useful prerift position of Yucatan with respect to for estimating paleogeography at the onset North America; this position largely defines of seafloor spreading. The Gulf of Mexico the rotation required to move Yucatan to provides an example which, despite salt its present position. That Yucatan migration beneath Sigsbee Escarpment, originated between Venezuela and the U.S. suggests that in areas of poorly known Gulf Coast margin is strongly suggested by basement structure, the occurrence of salt (1) the close match between the Triassic deposits may be used to roughly shape of Yucatan and the continental void approximate the extent of attenuated remaining between Venezuela and the U.S. continental crust. Gulf Coast margin after consideration of the above geometric constraints, (2) the Acknowledgments. This work was done absence of Upper Permian to Middle largely during the author's employment by 34 Pindell' Alleõhenian Reconstruction, Gulf of iMexico Evolution

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