ABSTRACT We generated low-temperature thermochronological data on crystalline rocks from the Chiapas Massif in southern Mexico to constrain the complex relationship among tectonics, exhumation, and sedimentation in the region. Our data show that the first recorded cooling event occurred at ca. 40–25 Ma due to denudation of the sedimentary cover of the Chiapas Massif at slow rates of ~0.1 km/m.y. This was followed by a period of tectonic quiescence from ca. 25 to 14 Ma. Between ca. 14 and 7 Ma, cooling implying exhumation of the massif at rates of up to ~0.7 km/m.y. was renewed, and this was associated with, and possibly responsible for, the Miocene “Chiapanecan” deformational event observed in the Chiapas fold-and-thrust belt to the northeast of the massif. This younger uplift was also accompanied by the onset of arc-related magmatism beneath the massif, between ca. 13 and 9 Ma, along the Tonalá shear zone at the Pacific coast. Since ca. 7 Ma, additional but slower cooling and exhumation are indicated along the length of the Chiapas Massif, and arc magmatism has jumped north by ~125 km from the Tonalá shear zone into the Chiapas fold-and-thrust belt. Concurrently, subsidence and sedimentation have persisted along the offshore Tehuantepec Shelf to the south, suggesting that the Tonalá shear zone has been recently active (despite no magnitude 4 or larger earthquakes), with up-to-the-north vertical displacement. We interpret the exhumation at ca. 40–25 Ma to pertain to displacement of the Chortis block along the paleo–Motagua fault zone, either as a northward propagation of a basement thrust beneath the massif within a regional transpressional setting, or as a deep, ductile crustal thickening and attendant isostatic uplift of the southern flank of the massif during the transpressional passage of the Chortis block. The ensuing quiescence (25–14 Ma) coincided, we believe, with the passage of the “western tail” of Chortis, which is internally deformed and perhaps transferred compressive stress less effectively than had the central, continental core of the Chortis block earlier. Renewed uplift and exhumation of the region began by ca. 14–10 Ma. An onset at ca. 10 Ma is probably the best estimate for the beginning of exhumation of the northwestern and central portions of the Chiapas Massif, whereas the present-day southeastern tip of the massif (potentially an allochthonous sliver belonging to the Chortis block) started to exhume earlier, at ca. 14 Ma. By ca. 13 Ma, arc magmatism had moved into the western Tehuantepec area, marking the onset of subduction of the Cocos plate beneath the Chiapas Massif. Hence, we interpret the main period of uplift of the Chiapas Massif and primary shortening of the Chiapas fold-and-thrust belt (ca. 14–7 Ma) as being driven by the establishment of Cocos subduction beneath the area.

ABSTRACT New K-feldspar 40 Ar/ 39 Ar and apatite fission-track thermochronological data from the crystalline basement of the western Gulf of Mexico (basement core samples from Tamaulipas Arch, Tuxpan, and Jalapa–Santa Ana highs) and K-feldspar 40 Ar/ 39 Ar from field samples of the Chiapas Massif in southern Mexico provide valuable information on the tectonic history of the region, namely, the rifting and postrifting stages of evolution in the Gulf of Mexico. The onset of rifting was probably as early as ca. 216 Ma and was characterized by extensional faulting that led to cooling of the basement footwall blocks by tectonic unroofing. The Tamaulipas Arch and the Jalapa–Santa Ana High were unroofed and cooled until ca. 160 Ma, whereas rocks from the Chiapas Massif were probably affected only until ca. 180 Ma. The thermochronological data suggest that the Tamaulipas Arch and the Chiapas Massif may have been footwalls to low-angle detachments prior to ca. 180 Ma. By ca. 180 Ma, the Chiapas Massif was arguably attached to Yucatán. Rotation of the Yucatán block (and Chiapas Massif) probably started at ca. 167 Ma and unroofed (exhumed) the Tamaulipas Arch very quickly until 155 Ma, when it was unconformably covered by Kimmeridgian sediments along its flanks. The Tamaulipas Arch was progressively buried until the Eocene (ca. 40 Ma), when it was uplifted, and a portion of its sedimentary cover was eroded. A second pulse of uplift occurred in the late Miocene. Our thermochronological data also show that there are along-strike variations in the vertical movements experienced by the Tamaulipas Arch since the Jurassic. This can have important implications for oil maturation of the source rocks in the region, as there might be zones that remained within the oil window for significant amounts of time.

Abstract The Miocene Nanchital conglomerate of the western Chiapas Foldbelt is the coarsest terrigenous clastic depositional Cenozoic unit of the region, probably comprising more proximal sections of hydrocarbon-rich slope-fan reservoirs found in the more distal Sureste Basin of the southern Gulf of Mexico fringe. Traditionally, the felsic igneous and metamorphic components of the conglomerate were assumed to derive from the Permian basement of the nearby Chiapas Massif. However, zircon U–Pb dating of five Nanchital conglomerate clasts from the Chiapas Foldbelt as well as several igneous exposures in SW Tehuantepec indicates that the Nanchital conglomerate's catchment area included the western Isthmus of Tehuantepec for late Middle Miocene and possibly early Late Miocene time, after which the more proximal Chiapas Massif and Chiapas Foldbelt likely became dominant. This study suggests that traditional concerns over the limited extent of quartz-rich clastic source areas feeding terrigenous clastic reservoirs in the Sureste Basin might be overly pessimistic. We propose a temporal framework for viewing Neogene and Quaternary clastic supply to the southern Gulf of Mexico.

ABSTRACT In 1935, Krynine postulated that first-cycle arkose in the humid tropical setting of southern Mexico can be rapidly eroded with minimal chemical weathering and redeposited as second-cycle arkose. Modern quantitative data confirm this hypothesis and highlight exceptions where first-cycle arkosic sediments have been diagenetically altered by intense weathering to yield second-cycle quartz arenites. In this study, extensive sampling of upland source rocks and their derived sediments provided a robust data set with which to quantitatively evaluate the composition and provenance of Holocene sediments. Three upland source terrains were identified: Paleozoic crystalline basement of the Chiapas Massif; Mesozoic to Cenozoic siliciclastic and carbonate rocks of the Chiapas fold belt; and Cenozoic sedimentary rocks in the foothills of the fold belt. Holocene sediments from these source terrains are grouped into seven facies (A–G) based on their provenance and geographic location. Facies A consists of feldspathic sediments from the Mezcalapa-Grijalva River that are sourced from the Chiapas Massif. Facies B consists of lithic-rich sediments from the same area that are derived from the Chiapas fold belt. Facies A and B consist predominantly of first-cycle sand capable of yielding arkosic deposits. Facies C represents a mixture of Facies A and B sands deposited along the course of the Mezcalapa-Grijalva River. Facies D (from Rio Sierra) and Facies E (from Rio Pedregal) represent second-cycle feldspathic sands of the coastal-plain delta and were derived from Cenozoic sedimentary rocks of the foothills. Mild chemical weathering due to rapid mechanical erosion enabled the creation of these arkosic deposits. They are less feldspathic than their parents and have limited occurrence due to mixing with less feldspathic first-cycle sands downstream from their sources. Facies F (from Rio Zanapa) and Facies G (from Lagunas Rosario and Enmedio) represent second-cycle quartzose sands of the low-lying savanna that were also derived from Cenozoic sedimentary rocks in the foothills of the fold belt. Intense, long-term (>10,000 yr) chemical weathering of these sands has precluded the formation of arkoses, instead yielding quartz arenites. They are more weathered than the delta sands (Facies D, E) with a greater loss of feldspar and carbonate detritus. They are enriched in silica and depleted in alumina, CaO, Na 2 O, and K 2 O relative to Facies A arkoses due to loss of feldspars and mafic minerals. Second-cycle sediments eroded from Tertiary sedimentary rocks in the foothills (Facies D–G) contain detrital serpentine and chromite with high abundances of Cr and Ni, suggesting an ultramafic component in their provenance. Cr and Ni are effective tracers for second-cycle components in sands of mixed provenance.

Paleomagnetic studies of Paleozoic sedimentary and plutonic rocks demonstrate that the Yucatan Block did not lie between the North and South American plates in the Pangean assembly during the Permian. In the Middle Permian, the Yucatan Block lay in an inverted orientation on the western margin of Pangea at 6–10°S, probably forming part of the NW coast of South America. Subsequently the block rotated in a series of counterclockwise motions as the North and South American plates separated. By 230 Ma, Yucatan had rotated ∼47° counterclockwise and moved slightly northward, to the equator. Counterclockwise rotation continued through the Jurassic: ∼41° between 230 Ma and ca. Oxfordian time, and another ∼47° between the Oxfordian and Tithonian, at which time, the approximate present orientation with respect to North America was achieved. Passage of the Yucatan Block from NW South America into the gap created by the separation of North and South America is a motion consistent with the left-lateral translations along the Mojave-Sonora or similar megashear. The fact that Yucatan has exhibited a counterclockwise motion throughout its Mesozoic history suggests that the microplate may have acted in a ball-bearing fashion between the larger North and South American plates. The Permian sedimentary remanence appears to be carried dominantly by magnetite and maghemite in red bed strata deposited on the margins of Silurian plutons within the marine Santa Rosa basin of the Maya Mountains. This remanence decays exceedingly linearly to the origin of orthogonal-axes plots. Biostratigraphy indicates a late Pennsylvanian to Middle Permian sedimentary age for the Santa Rosa strata, and the presence of both polarities of remanence in a 110 m magnetostratigraphic sequence of four polarity intervals demonstrates a post-Kiaman, i.e., Middle Permian, magnetization age. Another remanence is exhibited by Silurian plutons, also dual polarity, but corresponding to a paleopole ∼60° counterclockwise of that recorded in the Middle Permian sedimentary rocks. Exceedingly uniform K/Ar ages of 231 ± 7 Ma characterize all Maya Mountains plutons and a volcanic complex, indicating a 230 Ma resetting of the K/Ar radiometric systems in plutons dated as Late Silurian by U/Pb (Steiner and Walker, 1996). Furthermore, metamorphic aureoles are developed in the Permian Santa Rosa strata that border the Silurian plutons, suggesting that the 230 Ma resetting was a postintrusion event that involved the margins of the plutons. The spatial relationship between the pluton and sedimentary poles is reminiscent of that between the North American Late Triassic and Permian reference paleopoles. Therefore the combination of 230 Ma reset K/Ar systems, metamorphic aureoles in strata younger than the plutons, and a magnetization resembling a Late Triassic remanence all suggest that a 230 Ma hydrothermal event remagnetized the igneous rocks and reset their radiometric systems. This probably was an event related to the initial breakup of Pangea. The Maya Mountains plutons yield a paleopole that is statistically identical to that of the Chiapas Massif to the south. Both plutonic complexes exhibit somewhat dispersed dual polarity magnetization populations, suggesting that both were remagnetized at ca. 230 Ma. Importantly, the identical remanences in these widely separated plutonic complexes indicate that the Yucatan Block (including the Chiapas Massif) has been a structural entity since at least 230 Ma.

ABSTRACT Provenance determinations of sediment deposited in circum–Gulf of Mexico basins rely on understanding the geologic elements present in the basement provinces located from northeast Mexico to Honduras. Relevant geologic features of these provinces are herein summarized in text and pictorial form, and they include the Huizachal-Peregrina uplift, western Gulf of Mexico, Huayacocotla, Zapoteco, Mixteca, Xolapa, Juchatengo, Cuicateco, Mixtequita, south-central Chiapas, southeast Chiapas, western Guatemala, central Guatemala, Maya Mountains, and the Chortis block. We recognized basement elements of local character that serve as fingerprints for specific source areas. However, many elements are ubiquitous, such as 1.4–0.9 Ga, high-grade metamorphic rocks that occur both as broad exposures and as inliers in otherwise reworked crust. Xenocrystic and detrital zircon of Mesoproterozoic age is very common and hence not diagnostic of provenance. Neoproterozoic rocks are very scarce in Mexican basement provinces. However, Ediacaran–Cambrian detrital zircon grains are found in Mexican Paleozoic strata; these were possibly derived from distant sources in Gondwana and Pangea. Ordovician–Silurian magmatism is present in approximately half the provinces; magmatic detrital zircon of such age is somewhat informative in terms of provenance. More useful populations are detrital zircon grains with Ordovician–Silurian metamorphic overgrowth, which seem to be mainly sourced from the Mixteca region or the southern Chiapas Massif. Devonian basement has only been discovered in the Maya Mountains of Belize, and detrital zircon of such age seems to be characteristic of that source. A similar case can be made about Carboniferous zircon and the Acatlán Complex, Middle Pennsylvanian zircon and Juchatengo plutons, and Late Triassic zircon and the basement exposed in central Guatemala. In all these cases, the age and geographic extent of the zircon source are restricted and serve as a distinct fingerprint. Plutons of Permian–Early Triassic age are widespread, and detrital zircon grains from them are rather nonspecific indicators of source area. Future dating of detrital white mica using 40 Ar- 39 Ar could help in recognizing Carboniferous–Triassic schist from more restricted schist occurrences such as west Cuicateco (Early Cretaceous) and central Guatemala (Late Cretaceous).

ABSTRACT We redefine the “Chontal arc” of the southern Isthmus of Tehuantepec, Mexico, as the Chontal allochthon. The Chontal assemblage is composed of Upper Cretaceous low-grade metavolcanic and metasedimentary rocks included in the Chivela lithodeme. By means of field observations, laser-ablation detrital zircon geochronology, and trace-element geochemistry, we constrained the provenance and tectonic setting of these rocks. We concluded that they form an allochthon emplaced during a Paleogene transpressive event. Basement structure in the greater Oaxaca-Chiapas area was assessed by qualitative interpretation of Mexican State aeromagnetic maps. Chivela lithodeme sediments include a contribution from Albian–Turonian volcanic arc rocks no longer present in the region, likely sourced from the Chortís block or from the Greater Antilles Arc as it collided with southern Yucatan. Maastrichtian basic intrusive units, basalt flows, and pillow lavas with pelagic sediments in the Chontal are subalkaline, plotting in the normal mid-ocean-ridge basalt (N-MORB) field of discrimination diagrams. The igneous rocks are interpreted as pertaining either to the inception of the paleo–Motagua fault zone (left step in the fault trace), or to local backarc extension behind the Chortís block just before it began to migrate eastward, in a basin we call the Chontal basin. The Chontal allochthon was thrust northward onto parautochthonous strata flanking the Mixtequita and Chiapas Massif basements. Chontal allochthon rocks were later intruded by Miocene granitoids related to the inception of Cocos plate subduction arc magmatism. Rocks of the Chontal allochthon have been previously linked to the Cuicateco belt of eastern Oaxaca, but this is challenged here on the basis of lithologic type, chronology, tectonic associations, structural styles, and discontinuous anomaly trends in aeromagnetic maps.

ABSTRACT The supercontinent of Pangea formed through the diachronous collision of Laurussia and Gondwana during the late Paleozoic. While magmatism associated with its formation is well documented in the Variscan orogeny of Europe and Alleghanian orogeny of the United States, little is known about the Sonora orogeny of northern Mexico. This paper reports geochronology (U-Pb zircon), whole-rock geochemistry, and Lu-Hf zircon isotope data on basement cores from the western Gulf of Mexico, which were used to develop a tectonomagmatic model for pre- to post-Pangea amalgamation. Our results suggest the existence of three distinct phases of magmatism, produced during different stages of continental assembly and disassembly. The first phase consists of Early Permian (294–274 Ma; n = 3) granitoids with geochemical signatures indicative of a continental arc tectonic setting. This phase formed on the margins of Gondwana during the closure of the Rheic Ocean, prior to the final amalgamation of Pangea. It likely represents a lateral analogue of late Carboniferous–Early Permian granitoids that intrude the Acatlán and Oaxacan Complexes. The second phase of magmatism includes Late Permian–Early Triassic (263–243 Ma; n = 13) granitoids with suprasubduction geochemical affinities. However, Lu-Hf isotope data indicate that these granitoids formed from crustal anatexis, with ε Hf values and two-step Hf depleted mantle model ages (T DM[Hf] ) comparable to the Oaxaquia continental crust into which they intrude. This phase of magmatism is likely related to coeval granitoids in the Oaxaca area and Chiapas Massif. We interpret it to reflect late- to postcollisional magmatism along the margin of Gondwana following the assembly of Pangea. Finally, the third phase of magmatism includes Early–Middle Jurassic (189–164 Ma; n = 2) mafic porphyries, which could be related to the synchronous suprasubduction magmatism associated with the Nazas arc. Overall, our results are consistent with Pangea assembly through diachronous collision of Laurussia and Gondwana during subduction of the Rheic Ocean. They suggest that postorogenic magmatism in the western termination of the Rheic suture occurred under the influence of a Panthalassan subduction zone, before opening of the Gulf of Mexico.

ABSTRACT The Gulf of Mexico is best understood as a subsidiary basin to the Atlantic, resulting from breakup of Pangea. The rifting process and stratigraphy preceding opening of the gulf are, however, not fully understood. We present new stratigraphic, sedimentologic, and provenance data for the Todos Santos Formation (now Todos Santos Group) in southern Mexico. The new data support a two-stage model for rifting in the Gulf of Mexico. Field and analytical evidence demonstrate that strata assigned to the Todos Santos Group in Mexico belong to two unrelated successions that were juxtaposed after rotation of the Yucatán block. An Upper Triassic fluvial siliciclastic succession in the western Veracruz basin is intruded by the San Juan del Río pluton (194 Ma, U-Pb) along the Valle Nacional fault. We refer to this succession as the Valle Nacional formation (informal) of the Todos Santos Group, and correlate it with El Alamar Formation of northeast Mexico and the Eagle Mills Formation of the northern Gulf of Mexico. Triassic red beds register an early rifting phase in western equatorial Pangea. Sandstone composition indicates that the Valle Nacional formation is mostly arkoses derived from multiple sources. Paleocurrent indicators in fluvial strata of the Valle Nacional formation are S-SW directed, but restoration of paleomagnetically determined counterclockwise rotation indicates a W-SW–flowing fluvial system. Triassic rifting in the Valle Nacional formation and the Central Cordillera of Colombia Triassic extensional event, the record of which is preserved in mid-crustal levels, may represent conjugate margins. The Early–Middle Jurassic Nazas continental volcanic arc predated the Jurassic rifting phase that led to opening of the gulf. A record of arc magmatism is present in eastern Mexico underlying Middle Jurassic synrift successions, and it is present in La Boca and Cahuasas formations in the Sierra Madre Oriental and La Silla Formation north of the Chiapas Massif. These units have a similar age range between ca. 195 and 170 Ma. Arc magmatism in eastern Mexico is correlated with the Jurassic Cordilleran arc of Sonora, California, and Arizona, as well as the Jurassic arc of the Central Cordillera of Colombia. La Boca and La Silla units record intra-arc extension driven by slab rollback. The Jurassic rifting phase is recorded in the Jiquipilas formation of the Todos Santos Group and is younger than ca. 170 Ma, based on young zircon ages at multiple locations. The informal El Diamante member of the Jiquipilas formation records the maximum displacement rift stage (rift climax). Coarse-grained, pebbly, arkosic sandstones with thin siltstone intercalations and thick conglomerate packages of the Jericó member of the Jiquipilas formation are interpreted as deposits of a high-gradient, axial rift fluvial system fed by transverse alluvial fans. These rivers flowed north to northeast (restored for ~35° rotation of Yucatán). The Concordia member of the Jiquipilas formation records the postrift stage. Thick synrift successions are preserved in the subsurface in the Tampico-Misantla basin, but they cannot be easily assigned to the Triassic or the Jurassic rifting stages because of insufficient study. The Todos Santos Group at its type locality in Guatemala marks the base of the Lower Cretaceous transgression. Overall, three regional extensional events are recognized in the western Gulf of Mexico Mesozoic margin. These include Upper Triassic early rifting, an extensional continental arc, and Middle Jurassic main rifting events that culminated with rotation of Yucatán and formation of oceanic crust in the gulf.

Abstract A database of 134 apatite fission track (AFT), and apatite and zircon (U–Th)/He analyses has been assembled for eastern Mexico. Most of these samples have reset ages and track lengths reflecting rapid cooling. Time–temperature histories were modelled for 99 localities, and were converted to depth using a constant gradient of 30°C km −1 . Maps of these results reveal smooth temperature patterns in space and time, indicating that heating was due to regional burial rather than hydrothermal circulation. Cooling began by 90 Ma in the west and 50 Ma along the eastern edge of the Sierra Madre Oriental. These ages mimic the duration of the Mexican Orogeny, which verifies that most of these AFT ages have event significance. The elongate Mayrán Basin, a part of the Mexican foreland basin system, formed and grew across and above the eastern toe of the active Sierra Madre Oriental. This basin subsided between at least 70 and c. 40 Ma, and reached a minimum depth of 6 km. It was a both a catchment and routing system for sediment from US and Mexican sources. The shape of the basin suggests that early outflow was directed through the Burgos Basin into the Gulf of Mexico (GoM). By 50 Ma, some outflow potentially routed southwards through the Tampico Misantla Basin area. The Mayrán Basin subsided until 40 Ma, and then began to uplift and erode. This inversion mobilized the stored sediment and redeposited it into the GoM, filling the offshore Bravo Trough. Volcanism swept eastwards between 90 and 40 Ma, driven by northeastward-directed flat-slab subduction, which may also have driven the contraction. Local subsidence during contraction suggests there was dynamic pull-down created by the underriding flat slab. Subsidence ceased at c. 40 Ma, as volcanism swept back westward and asthenosphere replaced the flat slab. The crust rebounded, creating an ensuing period of massive erosion which peaked around 20 Ma. Southern Mexico was relatively quiet until rapid uplift began in Oaxaca in late Oligocene–early Miocene time. Uplift progressed eastwards to the Chiapas Massif in the late Miocene, commensurate with the eastward translation of the Chortis Block.

Unravelling end-Cretaceous paleobathymetric dip-profiles along strike in the northern Gulf of Mexico continental margin (nGoM) is important because it provides us with the initial framework in which to assess the evolution of Cenozoic clastic systems and burial history. Light can be shed on the problem by integrating seismic and potential field data with analytical basin modelling techniques and palinspastic-kinematic paleo-geographic analysis. Progressive palinspastic reconstruction of the Gulf is critical to setting up the appropriate lithospheric models to assess subsidence history. Kinematically, the opening of the GoM involved an early rift stage of northwest–southeast stretching (with minor counterclockwise [CCW] rotation) between North America and Yucatán (Triassic–Early Oxfordian), followed by a drift stage of seafloor spreading between the opposing rifted margins that entailed significant CCW rotation of Yucatán Block. This Stage 2 rotation was accommodated by transform motion of Yucatan/Chiapas Massif along the foot of the very narrow eastern Mexican margin, but transform motion stopped once the Central Gulf Spreading Ridge passed any point along this margin. Thus, the crustal boundary along eastern Mexico is ultimately an igneous, constructional contact that is overlain by the entire stratigraphy visible on seismic, and no transform faults are to be expected ( Pindell, 1985 ). The rift stage was asymmetric ( Pindell et al. , 1986 ; Marton and Buffler, 1994) such that Yucatán collapsed off the mainly northwest-vergent Alleghenian Orogen of the southern USA. The nGoM margin (foot-wall) underwent large and rapid tectonic subsidence during the rift stage (because the crust was highly stretched), but little thermal subsidence during the drift stage and thereafter (because the lithosphere was NOT stretched much). In contrast, the Yucatán hanging wall underwent little syn-rift subsidence (because the crust was not stretched much), followed by considerable postrift (Late Jurassic and younger) thermal subsidence (because the lithosphere was stretched). During the rift stage, thick red beds, possibly with lacustrine or even marine intervals, effectively buried basement in most nGoM areas and, toward the end of the rift stage (Callovian–Early Oxfordian), gave way to salt deposition across much of the half-open Gulf basin. Original salt thickness is generally considered to exceed what could have been achieved by thermal subsidence alone (~2km) during Callovian–Early Oxfordian time (presumed agespan of salt deposition); thus, salt accumulation was coeval with Stage 1 tectonic subsidence (syn-rift) and/or involved the marine inundation of pre-existing, isolated, sub-sea level accommodation space, which, by Oxfordian time, was almost definitely filled to sea level with salt. Together, red beds and salt probably are 5–10km thick beneath much of the nGoM rifted margin. Oxfordian onset of CCW rotational seafloor spreading in the Gulf split the pre-existing red-bed/salt basin into the Louann and Campeche halves. Backstripping shows that ocean crust was generated near its typical 2.6km depth below sea level, and not at an Icelandic-setting near sea level. The continent-ocean boundary typically is marked by a large step up from continental to oceanic crust ( i.e. , the rifted continental crust was already buried by red beds and salt far thicker than 2.6km ocean-generation depth, so the basement step to ocean crust is UP). As spreading ensued, a central, widening “chasm” was produced that was floored by oceanic crust and that, once Smackover open marine conditions were established, received no new depositional salt. To our knowledge, truly autochthonous salt cannot be shown to overlie definite oceanic crust; thus, initial spreading was effectively coeval with the onset of Smackover open marine conditions, and there may have been a causal relationship between the onset of spreading and the breaking of the evaporitic sill, wherever that was (Florida Straits or Veracruz Basin are equally viable guesses). As seen south of the Middle Grounds margin, the immediately adjacent shoulders of these salt walls halo-kinetically collapsed into the widening chasm, but, given the enormous width of the nGoM rifted margin, an important question is to assess how far north into the salt basin such early collapse occurred. The Red Sea analogue shows that the salt could have supported shelf platforms at least into the Cretaceous; we typically observe minor (<20km) extrusion of salt across the step up onto oceanic crust, but locally, such as at Sigsbee Escarpment, salt may have collapsed much farther (100km) onto the ocean crust, possibly as early as Late Jurassic–Cretaceous times. In such places, the term “parautochthonous salt” applies, because the salt still underlies most stratigraphy although it acquires a tapered-wedge cross-sectional geometry as it collapses. Because the nGoM margin was the footwall during Jurassic asymmetric rifting, thermal subsidence had far less influence on paleobathymetry than is commonly believed. Thus, determination of paleobathymetry can be roughly gauged by structural analysis of halokinesis. Thus, for large areas of the nGoM margin, we propose (1) that a relatively shallow, “supra-salt platform” persisted until the Late Paleocene onset of the well-known Wilcox growth faulting, and (2) that the Upper Jurassic–Cretaceous supra-platform section remained shallow, and was never deeply buried until halokinetic collapse began. This contrasts sharply with the Campeche Salt margin of Mexico, which was drowned to truly basinal depths in the Late Jurassic-Early Cretaceous due to far higher rates of post-rift thermal subsidence and weak clastic sediment supply. Thus, in the north but not in the south, we envision a very broad, relatively shallow supra-salt platform with a thinner-than-often-assumed Upper Jurassic-Cretaceous section that extended well beyond much of today's coastline. In this case, the true continental slope and rise would have been located much farther out than the Stuart City carbonate trend (which is often inferred as the paleo-shelf edge). This platform may have been stepped due to early halokinesis, particularly at Sigsbee, and along its outer reaches probably sloped or ramped down to the area of oceanic crust. Given this scenario for the paleobathymetry, it should not be surprising to find early Paleogene sands at the foot of that platform slope ( e.g. , Perdido area). The sands could have been transported there from (1) the north or northwest by shelf bypass across the suprasalt platform, or (2) the west, out of a proto-Rio Grande river system, or both. By the end of the Paleocene, salt collapse in updip areas of the supra-salt platform began due to differential burial by prograding clastics, producing syn-depositional counter-regional faults and basin-facing half-grabens at the Wilcox and younger fault trends, which controlled the [new, syntectonic] position of the paleo-shelf edge. Such collapse fed downslope shortening behind (landward of) the Paleogene sands at the foot of the true continental slope. We infer detachment on salt, such that the Mesozoic marine supra-salt section was cut both updip and downdip by at least some of the faults. Apparent rafting of the Mesozoic shelf section at the landward limit of the Wilcox trend ( e.g. , Anderson and Fiduk, 2003 , and also observed in NE Mexico on seismic by the authors) demonstrates that the salt (and inferred Upper Jurassic–Cretaceous shallow shelf) was mobile during end-K/early T time. The concept of the Mesozoic supra-salt platform in the nGoM: (1) requires significant changes to commonly-accepted Late Jurassic through Paleocene paleobathymetric and paleogeographic maps of nGoM , and therefore of reservoir and source rock distribution; (2) indicates the need to develop maturation models for the inner shelf areas that do not assume a pre-existing deep-water setting outboard of the Sligo/Stuart City “reef trends” prior to Tertiary clastic deposition; and (3) provides a new paleogeographic context for assessments and models of Cenozoic deltaic and progradational depositional systems along the northern Gulf of Mexico.

Abstract Gravity data from Central America and the adjacent offshore regions were analyzed in conjunction with seismic reflection/refraction models, seismicity studies, geologic mapping, and well data to determine a gravity-based crustal structure for the region. Analysis of the gravity data included the construction of a Bouguer and three isostatic residual gravity-anomaly maps, and 2-D gravity models across Guatemala/Belize, Nicaragua/Honduras, and Panama, respectively. The isostatic residual-gravity anomaly map that emphasizes anomalies caused by crustal and upper mantle sources was used to correlate gravity anomalies with known geologic/tectonic features and to emphasize possible previously unknown geologic features in the upper crust. These include granitic rocks in central Guatemala that are related to the Chiapas Massif in southeastern Mexico, basement uplifts under the carbonate platform sediments in northern Guatemala, and thin or dense sediments in the El Salvador depression. In Nicaragua and Honduras, the Nicaragua depression thickens toward the Costa Rican border, and the Mosquitia Basin is seen to consist of a series of depositional centers, possibly pull-apart basins. In Panama, Quaternary volcanic rocks are shown to occur close to the Choco Block boundary in the Gulf of Mexico. However, based on isostatic residual-gravity maxima over mafic igneous material in northern Panama, the boundary between the Chorotega and Choco Blocks may extend 150 km west of its present position. The 2-D gravity models indicate that the crustal thickness of the Maya and Chortis Blocks is approximately 36–38 km and approximately 22 km under Panama. However, the density of the upper mantle is higher under Panama than it is under Guatemala and Nicaragua. To model a large-amplitude gravity maximum along the Middle America subduction zone, a combination of an ophiolitic complex and a steeply dipping Cocos Plate was required. Isostatic residual-gravity anomalies indicate that the ophiolitic complex is not continuous along the Middle America trench but occurs in discontinuous steps. To explain a regional gravity maximum over the Nicaraguan volcanic belt, a deep (>5 km) mafic (?) body was required; however, its exact position cannot be determined from gravity modeling alone. Gravity modeling indicates that subduction does not occur between the Nazca and Caribbean Plates; however, this solution is not unique. If the boundary between the two plates is a transform boundary, this boundary must be dipping at a steep angle. To model a large amplitude gravity maximum and minimum along the northern coast of Panama, a subducting plate (Caribbean) was required, with the Caribbean upper mantle being denser than that beneath Panama.