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Dynamo-thermal subsidence and sag–salt section deposition as magma-rich rifted margins move off plume centres along incipient lines of break-up
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 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 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 New low-temperature thermochronological data analyses (apatite fission track and apatite and zircon [U-Th]/He) on rocks from the southern (Pacific) margin of Mexico between Acapulco and the western Gulf of Tehuantepec, where pre–middle Eocene arc and forearc complexes are expected but missing, show that this continental margin was subjected to an important Tertiary exhumational event. Exhumation is constrained to ca. 32–20 Ma in the west (Acapulco) and to ca. 19–11 Ma in the east (Puerto Angel) and was thus eastwardly diachronous. The diachroneity is interpreted as relating to the migration of the Chortis block, representing the western end of the Caribbean plate. The amount of exhumation along the trend is constrained to roughly 4–5 km (~0.3–0.6 km/m.y.). These magnitudes and rates are much less than previous estimates of 2.5–4 km/m.y. during the Oligocene, which are likely overestimated. These faster rates have been employed in a competing model for arc removal by orthogonal subduction erosion (i.e., Chortis block not involved), which is accordingly questioned. The exhumation was not due to shearing or fault-related uplift as the Chortis block migrated, but rather to the inception of subduction along Mexico in the wake of Chortis block migration. A four-part history applies to southern Mexico that is eastwardly diachronous: (1) inception of arc magmatism as the Chortis block first moved over the ~150 km depth contour of the Farallon/Cocos Benioff zone; (2) uplift and exhumation of basement as southern Mexico encountered and overrode the site of the Farallon/Cocos Benioff zone; (3) northward migration of arc magmatism as the Chortis block left the cross section and North America continued to advance further onto the Cocos plate, producing flat slab subduction geometry; and (4) resumption of forearc subsidence once the Mexican margin had acquired a subduction zone hanging-wall geometry. The missing arc terrane along southern Mexico is the Chortis block.
Strontium isotope dating of evaporites and the breakup of the Gulf of Mexico and Proto–Caribbean Seaway
ABSTRACT New and existing strontium isotope data are given for several widespread evaporites from western equatorial Pangea. The data indicate evaporite deposition occurred on proximal margins of the Gulf of Mexico at ca. 169 Ma (Bajocian, not Callovian as commonly thought) and 166 Ma in Trinidad (Bathonian-Callovian boundary). The 166 Ma age may also apply to undated evaporite on the Bahamian margin, conjugate rift of Trinidad, and now in Cuba. We show that: (1) the Trinidadian (and Bahamian?) evaporite pertains to rifting rather than to Late Jurassic–Cretaceous carbonate platform deposition; (2) the Mata Espino-101B evaporite (a borehole in Veracruz Basin, Mexico) is not Paleocene but Bajocian (halite) or Bathonian (gypsum) and hence is not related to possible Paleogene Gulf of Mexico desiccation; (3) evaporite deposition may have offlapped basinward in the Gulf of Mexico (Bathonian–early Oxfordian in more distal areas), because most Atlantic opening models preclude the Gulf of Mexico from being large enough by 169 Ma to accommodate the mapped expanse of autochthonous salt deposition; and (4) a 3–9 m.y. hiatus (the Norphlet window) is apparent in proximal areas around the Gulf of Mexico between evaporite and upper Oxfordian marine successions, caused perhaps by proximal margin uplift (flexural or thermal) or by Gulf of Mexico water level remaining below paleo–sea level (evaporation?) during Bathonian–early Oxfordian time. Although a 20–30 m.y. hiatus may exist below evaporite in the U.S. coast, cordilleran Mexico was tectonically active into the Middle Jurassic, and pre-salt continental deposits are closer in age to salt deposition there. Pre-salt strata along Campeche–northern Yucatán remain undated. Our data do not resolve if the evaporite was sourced from the Atlantic, the Pacific, or both, but the fact that the Trinidadian evaporite is younger than Gulf of Mexico evaporite, and the presence of Bajocian marine and evaporite sections across Mexico perhaps favor the Pacific as the source.
ABSTRACT Jurassic northward migration of Mexico, which lay on the southern part of the North America plate, resulted in temporal evolution of climate-sensitive depositional environments. Lower–Middle Jurassic rocks in central Mexico contain a record of warm-humid conditions, indicated by coal, plant fossils, and compositionally mature sandstone deposited in continental environments. Paleomagnetic data for central Oaxaca and other regions of central and eastern Mexico indicate that Lower and Middle Jurassic rocks were deposited at near-equatorial paleolatitudes. In the Late Jurassic, the Gulf of Mexico formed as a subsidiary basin of the Atlantic Ocean when the Pangea supercontinent ruptured. Upper Jurassic strata across Mexico, including eolianite and widespread evaporite deposits, indicate dry-arid conditions. Available paleomagnetic data (compaction-corrected) from southern and northeast Mexico for Upper Jurassic strata indicate deposition at ~15°N–20°N. As North America moved northward during Jurassic opening of the Atlantic Ocean, different latitudinal regions experienced coeval Middle–Late Jurassic climatic shifts. Climate transitions have been widely recognized in the Colorado Plateau region. The plateau left the horse latitudes in the late Middle Jurassic to reach temperate humid climates at ~40°N in the latest Jurassic. Affected by the same northward drift, the southern end of the North America plate represented by central Mexico gradually reached the arid horse latitudes in the late Middle Jurassic as the Colorado Plateau was leaving them. As a result, Late Jurassic epeiric platforms developed in the circum–Gulf of Mexico region after a long period of margin extension and were surrounded by arid land masses. We propose that hydrocarbon source-rock deposition was facilitated by arid conditions and wind-induced coastal upwelling.
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 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.
The basins, orogens and evolution of the southern Gulf of Mexico and Northern Caribbean
Abstract We present an updated, internally consistent synthesis of the Permo-Triassic assembly and Mesozoic evolution of the Gulf of Mexico, Mexico, Florida–Bahamas and northern South America (Guiana margin and northern Andes), incorporating advances at regional, field and geochronological levels. The recently determined Bajocian age for salt deposition (using 87 Sr/ 86 Sr isotopes) is integrated by modifying the plate kinematic framework with a new Equatorial Atlantic reconstruction that expands the gap between the Americas by 180 km over many kinematic frameworks. NW–SE synrift lithospheric extension along western Florida–Bahamas is estimated at 40%, implying thinned continental crust beneath Great Bank, Bahamas, the conjugate for the Guianas Basin margin. In cordilleran Mexico (excluding the Yucatán Block), we propose two new means by which continental crust migrated into the ‘Colombian overlap position’ of Pangaean reconstructions. The first involved Jurassic–earliest Cretaceous sinistral displacement of the Oaxaca Block along a NW–SE ‘North Oaxaca Transfer’ through or adjacent to the Cuicateco Belt. The second applies to the continental crust in eastern Mexico to the north of Cuicateco, a region we refer to as ‘peninsular Mexico’. There, most Mesozoic basement faults trend NW–SE, and the common occurrence of Permian mid-crustal anatectic basement directly beneath Mesozoic red beds, salt and marine strata suggests extreme extension prior to the onset of sedimentation. Because these Mesozoic sedimentary sections typically sum to 3–8 km in thickness, the post-rift crust of peninsular Mexico probably averaged about 25 km in thickness before later orogenesis. Our reconstructions suggest that this Triassic–Middle Jurassic extension approached 100%, beginning with overthickened Alleghanian (Permian) crust about 50 km in thickness in palaeo-northern Mexico, and was accompanied by a significant sinistral component broadly distributed across the rift array. The updated model provides an exploration and kinematic framework for the entire region.
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 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.
Integrated Cretaceous–Cenozoic plate tectonics and structural geology in southern Mexico
Abstract The structural evolution of southern Mexico is described in the context of its plate tectonic evolution and illustrated by two restored crustal scale cross-sections through Cuicateco and the Veracruz Basin and a third across Chiapas. We interpret the Late Jurassic–Early Cretaceous opening of an oblique hyper-stretched intra-arc basin between the Cuicateco Belt and Oaxaca Block of southern Mexico where Lower Cretaceous deep-water sediments accumulated. These rocks, together with the hyper-stretched basement beneath them and the Oaxaca Block originally west of them, were thrust onto the Cretaceous platform of the Cuicateco region during a Late Cretaceous–Eocene orogenic event. The mylonitic complex of the Sierra de Juárez represents this hyper-stretched basement, perhaps itself an extensional allochthon. The Chiapas fold-and-thrust belt is mainly Neogene in age. Shallowing of the subduction angle of the Cocos Plate in the wake of the Chortis Block, suggested by seismicity and migrating arc volcanism, is thought to play an important role in the development of the Chiapas fold-and-thrust belt itself, helping to explain the structural dilemma of a vertical transcurrent plate boundary fault (the Tonalá Fault) at the back of an essentially dip-slip fold-and-thrust belt.
Role of outer marginal collapse on salt deposition in the eastern Gulf of Mexico, Campos and Santos basins
Abstract Outer marginal collapse (OMC), a recently proposed process by which top-rift and base-salt unconformities formed near sea level may subside rapidly to 2.5–3 km at continental margins as mantle exhumation or seafloor spreading begins, needs further examination. We examine salt deposition at three margins and find that the differing positions and volumes of salt can be related to different durations of salt deposition as OMC and subsequent mantle exhumation proceed. Along NW Florida, salt is thin but deep and is interpreted as having formed at the start of OMC, before drowning further to abyssal depths. In the Campos Basin, salt is thick and extends across tens of kilometres of interpreted exhumed mantle, interpreted as having formed during the entire period of OMC before spreading onto mantle during exhumation. In the Santos Basin, salt is thick and extends across c. 100 km of interpreted exhumed mantle and/or oceanic crust, arguably requiring ‘lateral tectonic accommodation’, whereby salt deposition persists near global sea level across the conjugated salt basin during mantle exhumation beneath mobile salt. The supposition that OMC can account for salt deposition in three different basins without invoking problematic 1.5–2 km-deep subaerial depressions provides further support for the process.
Compelling evidence from eastern Mexico for a Late Paleocene/Early Eocene isolation, drawdown, and refill of the Gulf of Mexico
Continental Margin Formation and Creation of “Lateral Tectonic Accommodation Space” for Salt Deposition, Campos and Santos Basins, São Paulo Plateau, Brazil
Abstract Within the scope of our ongoing seismic reflection interpretations of basement at magmatic continental margins, and in particular those of the South Atlantic, we report on our current views concerning the São Paulo Plateau offshore Brazil, which involves the Campos, Santos, and Pelotas basins. In addition, much can be gleaned by integrating the African conjugate margin, which we also consider to a lesser extent. Our broad-brush view for this segment of South Atlantic rifting is that: A broad zone of mixed magmatic/thinned continental crust formed between opposing “hinge zones” of the two larger plates during an initial period of intracontinental extension, thereby generating the synrift section in numerous grabens and half-grabens; This basinal area of thin, mixed crust began to subside without much further faulting for an interval, thereby generating “sag sections” along both continental margins; Plate divergence was renewed or accelerated and the thin crustal region and sag section broke up in what effectively could be viewed as a second rifting episode, and which created an irregular zone of deeper basement comprising faulted subsag crust, exhumed mantle, and magmatic build ups outboard of the respective sag sections; and Plate divergence was finally taken up by more normal styles of seafloor spreading. Definition of crustal type is hindered in seismic reflection interpretation by whether or not there should be much observable difference in the crustal structure resulting from rifting of a pre-existing oceanic plateau versus rifting of already-thinned continental crust. After all, Iceland is covered by normal faults, and if extension continued as magmatic supply waned, we might expect the resulting surface to have many geometries of rifted continental crust. However, the 080° trending Atlantic fracture zone fabric east of the São Paulo Plateau appears to us to dominate the crustal fabric under the Plateau as well, and thus we strongly suspect spreadingrelated processes in the development of the Plateau east of the sag section, at azimuths similar to the well-mapped parts of the South Atlantic Ocean. Within this 3-staged context, outlined as a general overview of “basin zones” and basin opening kinematics, our main objective here is to propose the idea of the generation of “lateral tectonic accommodation space” for salt accumulation in the central South Atlantic. We envision that salt flowed seaward at both conjugate margins as tectonic extension and/or seafloor spreading continued, and that the “lateral accommodation space” created by the extension was continuously filled with migrating salt while new salt was deposited. Further, the hypothetical instantaneous salt deflation at the depositional surface as a result of the tectonic extension and salt migration was constantly replaced by the deposition of new salt across the entire basin. This mechanism allows all the salt to be deposited effectively at global sea level, and it entirely avoids poorly supported models, in our view, involving progressive seawater seepage through semi-permeable marine barriers into enormous, deep (>1 km below global sea level), air-filled depressions. While we acknowledge that the basin’s depositional surface needn’t have remained immediately at global sea level at all times during salt deposition, we present the hypothesis as though it were, simply to provide an end member view that opposes the air-filled hole hypothesis.