Conjugate Margins Research Articles

Crustal Structure of the Camamu‐Almada Margin Along the Northeastern Rift Segment of Brazil From an Integration of Deep‐Penetration Seismic Reflection Profiles, Refraction, and Gravity Modeling

AbstractThe Camamu‐Almada segment of the northeastern Brazil rifted‐passive margin covers an area of 22,000 km2 and includes: (a) thin to ultra‐thin metamorphic continental crust; (b) a transitional zone of unknown composition; and (c) Albian and younger oceanic crust. We investigate the structural‐magmatic evolution of the region by interpreting 19,000 line km of two‐dimensional reflection seismic data, 105 offshore well logs, and 15 seismic refraction stations. Interpretation of these data suggests that the 100 km‐wide rifted zone is structurally complex and composed of at least three discrete crustal blocks: (a) 10–40 km‐wide and 4–6 km‐thick zones of hyperextended continental and possibly unroofed lower continental crust with an average density of 2.88 g/cc; (b) two 20 km‐wide sub‐circular zones of exhumed mantle with an average density of 3.1 g/cc; and (c) 40–60 km‐wide and 6–7 km‐thick proto‐oceanic crust that formed in situ. These blocks are similar in thickness and composition to rifted zones described by previous workers along its conjugate margin in Gabon, West Africa, and on the rifted‐passive margin of the western Mediterranean.

Continental rifting in the South China Sea through extension and high heat flow: An extended history

We present a new extensional tectonic model for the Cenozoic history of SE Asia and the opening of the South China Sea (SCS), proposing a feedback mechanism by which intracontinental rifts initiate and propagate without invoking mantle plumes.Four principal tectonic models have been proposed for SCS opening: 1) Slab pull from subduction of a Proto South China Sea (PSCS); 2) Extrusion tectonics from the India-Asia collision; 3) Basal drag from a mantle plume; and 4) Backarc rifting. Each model was developed around different particular data, and all tend to perpetuate independently through selective data prioritisation. We present a new GPlates model, showing that the geological and geophysical correlations between the opposing SCS conjugate margins best agrees with a common initial development on the South China Margin, and that regional development via protracted extension since the Mesozoic is in agreement with available paleomagnetic data for Borneo.The geodynamic mechanism for protracted lithospheric extension in SE Asia is via the development of progressive feedback processes, initiated by Mesozoic slab rollback and migration of the subduction zone beneath South China, leading to intracontinental thinning and extension. This in turn drove passive asthenospheric upwelling, increasing heat flow and crustal ductility, and enhancing further extension as a wide rift rather than narrow crustal neck. Subsequently, following sufficient continental extension, SCS oceanic spreading occurred. This feedback mechanism (involving shallow, not deep mantle processes) may enhance and enable intracontinental rifting elsewhere.

The paleo-lithospheric structure and rifting-magmatic processes of the northern South China Sea passive margin

The northern South China Sea has complex rifting-magmatic styles, which are significantly distinct from the Atlantic end-member passive margins that initiated rifting in a craton, and was argued to originate from a subducted setting. Here we propose a new kinematic method to calculate the paleo-lithospheric structure and reconstruct the rifting processes along a ∼ 400 km seismic line in the northern South China Sea margin. The detailed paleo-tectonic setting and its linkage with the rifting-magmatic processes are explored. Seismic-gravity analysis reveals a “boudin-type” passive margin with a wide distal domain (>200 km) comprising discrete hyperextended zones of the Baiyun Sub-basin, Liwan Sub-basin and a narrow continent-ocean transition (<30 km). The calculated pre-rift lithospheric thickness declines abruptly from ∼ 100–80 km in the proximal margin to ∼ 40 km in the distal margin, implying that the pre-rift margin was affected by a flat to steep slab of the subducted paleo-Pacific. The steep slab lying beneath the proximal margin formed an arc where the high-density lower crust probably has a pre-rift age. The flat slab beneath the Baiyun Sub-basin to the southern conjugate margin formed a forearc. As the slab steepened and retreated to beneath the Baiyun Sub-basin in rift phase 1 (∼56–38 Ma) and Liwan Sub-basin in rift phase 2 (∼38–33 Ma), the mantle convection and slab dehydration weakened the above lithosphere successively, promoting seaward migration and younging of strong faulting, magmatic underplating, intrusion, and eruption. When slab broke suddenly at ∼ 33 Ma in the continent-ocean transition, the deep hot asthenosphere upwelled to rupture the lithosphere abruptly and induced intense magmatism. Modelling results further show that the paleo-crustal thickness and basement erosion are both fundamentally important in assessing and understanding the crustal thinning.

Crustal structure and tectonic evolution of the Newfoundland Ridge, Fogo Basin, and southern Newfoundland transform margin

The southern Newfoundland transform margin is a crustal-scale transfer zone that juxtaposes continental crust of the Grand Banks with the oceanic domain of the Fogo Basin, offshore SE Newfoundland. This crustal feature is generally accepted as a left-lateral transfer zone between the much-studied Scotian and Newfoundland margins, but relatively little is known about either its crustal-scale architecture or tectonic evolution. Interpretation of newly available regional seismic data from the Newfoundland Ridge and Fogo Basin, offshore Canada provides evidence for approximately 500 km of left-lateral displacement and associated segmented extension between the Late Triassic (210 Ma) and latest Jurassic (145 Ma) as a result of differential extension rates between the conjugate margins of Nova Scotia–Morocco and Newfoundland (Grand Banks)–Iberia. We show that the southern Newfoundland transform margin represents a 100–150 km wide zone of distributed left-lateral displacement across extended continental crust at the SW edge of the Grand Banks, and that crust beneath the Newfoundland Ridge and Fogo Basin is unlikely to be oceanic in origin. Kinematic restorations of regional cross sections and crustal area balancing provide the basis for map-based reconstructions of the area. Our results, which integrate existing plate reconstructions, crustal thickness data, and established age relationships, help to constrain the early structural evolution of the North Atlantic as the African and North American plates separated during Triassic to Jurassic rifting.

Oceanward rift migration during formation of Santos–Benguela ultra-wide rifted margins

Abstract This paper presents the time and space evolution of crustal deformation and their respective sedimentary infill of the 600 km wide, asymmetric conjugate rifted margin of the Santos–Benguela basins. Based on a geoseismic transect obtained with interpretation of long-offset seismic reflection and tied by wells, we interpret six main synrift unconformities, corresponding to different deformation phases processed from the Valanganian to Early Albian. Confined by these unconformities, sedimentary growths with progressively young relative ages towards the boundary with the oceanic crust are interpreted as evidence of oceanward rift migration. The combination of this information with crustal structure derived from long-offset seismic reflection illuminating the deep crust of the Santos–Benguela conjugate margins, resulted in a complete view of sedimentary infill, internal compartments, and crustal structure. These data were used to guide a dynamic model of rifting resulting in a simulated lithospheric section. We show that the margin architecture can be explained by the combination of an early, protracted phase of distributed deformation, followed by basinward rift migration. Distributed deformation lasted from the Valanginian to Early Aptian (135–117 Ma), initiating with isolated lakes that later coalesced into a wide basin-scale lake (&gt;450 km). From the Mid Aptian to Early Albian (117–110 Ma), rift migration formed the main structural compartments and unconformities, as well as the distal hinge zones we observe today in the seismic lines. During this time, the inner proximal margins were left behind to thermally subside, whereas outer proximal and distal margins were tectonically active. Coexistence of these two processes explains the enigmatic simultaneous formation of proximal sag-like geometries, with late synrift accumulation of a salt layer up to 3 km thick, with tectonically active faults in the distal margin, promoting crestal block uplift that could explain the deposition of Late Aptian, shallow water, pre-salt carbonate rocks.

Jurassic-recent stratigraphy, structure, and hydrocarbon potential of the Mesozoic-Cenozoic rifted-passive margin of the Tarfaya-Dakhla basin of southern Morocco

We describe the structure, stratigraphy, and hydrocarbon potential of the 1000-km-long Tarfaya-Dakhla Basin (TDB) along the Atlantic rifted-passive margin of Morocco. Our dataset consists of a 4885-line km grid of depth-converted industry seismic data tied to three DSDP wells in the deepwater area and exploratory wells on the slope and shelf. Based on mapping of this seismic grid, the tectonostratigraphic events that have affected this margin include: 1) a Late Triassic-Early Jurassic rift phase that separated the Moroccan margin from its conjugate margin in eastern Canada began during the Late Triassic-Early Jurassic; 2) early passive margin sedimentation consists of Middle Jurassic-Early Cretaceous passive margin phase consisting of a massive, 14-km-thick, reefal-carbonate platform formed above the rifted margin during the Early Jurassic and was subsequently buried by the 6-km-thick, Early Cretaceous (Berriasian) Boujdour deltaic complex. During this period continued high rates of clastic sedimentation resulted in margin steepening and gravitational collapse during the Valanginian and formed a 480-km-long and 68-km-wide passive margin fold belt characterized by a zone of updip normal faults of 480 km in length and 30 km in width and a zone of downdip folds and thrusts of 117 km in length and 15 km in width. Geochemical data from DSDP 397 and 369 wells and two pseudo-wells were used to evaluate source rock potential and hydrocarbon prospectivity of the Jurassic to Recent section. Basin modeling reveals that the Jurassic source rocks known from DSDP drilling are presently overmature in the study area with hydrocarbon generation beginning in the Upper Jurassic-Lower Cretaceous. The Lower Cretaceous source rocks are immature except for localized areas, and the Upper Cretaceous is immature across the study area.

Northwest Pacific-Izanagi plate tectonics since Cretaceous times from western Pacific mantle structure

Northwest Pacific-Izanagi subduction histories along Eurasia are poorly constrained due to extensive subduction, which partially consumed the western Pacific plate and the entire Izanagi plate, its hypothesized conjugate margin. Here we reconstruct NW Pacific-Izanagi plate tectonics since Cretaceous times by mapping and structurally restoring (i.e. unfolding) the subducted western Pacific slabs from regional and global tomography, and re-creating the vanished Izanagi plate as its conjugate rift flank. Unfolding of the western Pacific slabs based on their cross-sectional areas, corrected for ‘tomographic smearing’, reveals that 2230 to 5000 km of western Pacific plate was subducted between Kamchatka and the southern Marianas. We add our restored western Pacific and Izanagi plates to a global plate model to reveal that Izanagi subduction under Eurasia after the mid-Cretaceous was limited between the present Bohai Bay-Yellow Sea, China, and northern Russia. The southern limit of Izanagi subduction was a NW-SE sinistral transform that intersected Eurasia near present Qingdao, China, and segmented eastern Eurasia continental magmatism during the late Cretaceous; we call this transform the ‘Qingdao line’. We reconstruct a low-angle Izanagi-Pacific ridge-trench intersection with Eurasia at ∼50 ± 10 Ma between Bohai Bay-Yellow Sea and northern Russia. The ∼50 Ma Pacific plate motion change initiated subduction along the Qingdao line transform, forming the Izu-Bonin-Marianas arcs and reorganizing the Bohai Bay-Yellow Sea faults. We show tomographic and geodynamic modeling evidence that a 4000 km-long, laterally-continuous, NE-SW trending, seismically-slow ‘slab gap’ at 1000 ± 250 km depth between present northern Sakhalin and central China is the tomographic signature of the subducted Izanagi-Pacific ridge.

The continent-to-ocean transition in the Iberia Abyssal Plain

Abstract Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by &amp;gt;100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.

Overview on GPlates: focus on plate reconstruction

Plate tectonic reconstructions have been employed in geosciences since 1970s, in the context of hydrocarbon exploration, regional geology and paleobiology. Such studies have given valuable inputs for climate and geodynamic computations, present-day mantle structure, models of plate motion, and the interpretation of the drift of hotspots, true polar wander (TPW), sea level and stratigraphic signals. However, geodynamic models generated in the past by incorporating global plate tectonic reconstructions have limitations. To overcome this, GPlates software brings forward a new era of interactive plate tectonic reconstruction software integrated with GIS databases that incorporates a wide variety of geological and geophysical data. Besides modelling tectonic and crustal evolution, GPlates has also been used in visualizing paleogeography and paleobathymetry, in understanding deep carbon cycle, subduction zone initiation, mantle evolution, investigating earthquakes and predicting future supercontinents. the software has been widely used in hydrocarbon exploration along the passive conjugate margins such as from the margins of South America and Africa and has provided promising results in acquiring new and more reliable prospecting criteria for petroleum systems. Additionally, it has become an integral tool for paleolatitude calculations, modelling of paleoclimate and paleoenvironment. This article reviews key plate reconstructions that have been carried out using GPlates, the typical constraints and the set of input parameters.

Definition of tectono-sedimentary elements for rifted continental margins of the Norwegian and Greenland seas

Abstract The geology of the conjugate continental margins of the Norwegian and Greenland Seas reflects 400 Ma of post-Caledonian continental rifting, continental breakup between early Eocene and Miocene times, and subsequent passive margin conditions accompanying seafloor spreading. During Devonian-Carboniferous time, rifting and continental deposition prevailed, but from the mid-Carboniferous, rifting decreased and marine deposition commenced in the north culminating in a Late Permian open seaway as rifting resumed. The seaway became partly filled by Triassic and Lower Jurassic sediments causing mixed marine/non-marine deposition. A permanent, open seaway established by the end of the Early Jurassic and was followed by the development of an axial line of deep marine Cretaceous basins. The final, strong rift pulse of continental breakup occurred along a line oblique to the axis of these basins. The Jan Mayen Micro-Continent formed by resumed rifting in a part of the East Greenland margin in Eocene to Miocene times. This complex tectonic development is reflected in the sedimentary record in the two conjugate margins, which clearly shows their common pre-breakup geological development. The strong correlation between the two present margins is the basis for defining seven tectono-sedimentary elements (TSE) and establishing eight composite tectono-sedimentary elements (CTSE) in the region.

Crustal structure and lateral variations in the Gulf of Mexico conjugate margins: From rifting to break-up

The Gulf of Mexico opened as a Late Triassic-Mid Jurassic continental rift that was first largely covered by the Mid-Jurassic Louann Salt and later split apart by a triangular-shaped oceanic crust. Salt in the Gulf of Mexico largely hampers the imaging and interpretation of underlying pre-salt and crustal geometries, which are fundamental for assessing the early kinematic evolution of the margin. To better define these deep geometries and their lateral variations, we built three seismic-based crustal-scale cross-sections across the Florida-Yucatan conjugate margins, in the areas where the Louann Salt is thinner. They were used, together with magnetic and gravity anomalies data, to build a crustal domains map for the study area. Cross-sections show a meaningful along-strike variation: the South Florida-East Yucatan area is characterized by a narrower rifted continental crust that evolves sharply to oceanic crust whereas in the North Florida and central-western Yucatan areas, the rifted continental crust is wider and the transition to the oceanic crust corresponds to a narrow magmatic or exhumed mantle domain. Bulk continental crust extension was determined using the area balancing method. Estimated horizontal extension values vary from a minimum of ∼120 km in the South Florida-East Yucatan conjugate to a minimum of ∼240 km in the North Florida-Central Yucatan conjugate, being systematically larger in the northern margin. Based on our observations and considering previous models, we propose that the study area evolved from an early rift involving magmatism, to a magma-poor margin, with continental break-up (OCT formation) being characterized by mantle exhumation and associated magmatism along the North Florida and central-western Yucatan areas. As in other analogues such as the Gulf of Guinea, the thick evaporite deposited during the late rift, while mantle was being exhumed.

Retreat of the Great Escarpment of Madagascar From Geomorphic Analysis and Cosmogenic 10Be Concentrations

AbstractThe eastern margin of Madagascar has a prominent relief change from the flat coastal plain to the low‐relief high plateau, characterizing a typical great escarpment topography at a passive margin. A quantification of the spatial distribution of erosion rates is necessary to understand the rate of landscape evolution. We present catchment‐averaged erosion rates from detrital cosmogenic 10Be concentrations, systematically covering distinct morphological zones of the escarpment. Erosion rates are differentiated across the escarpment, where the high plateau and the coastal plain are slowly eroding with an average rate of 9.7 m/Ma, and the escarpment basins are eroding faster with an average rate of 16.6 m/Ma. The Alaotra‐Ankay Graben related basins have the highest erosion rate with an average rate of 27 m/Ma. The spatial pattern of erosion rates indicates a retreating escarpment landscape. Retreat rates calculated from the 10Be concentrations are from 182 to 1,886 m/Ma. The rates of escarpment retreat on Madagascar are consistent with a model of a steady retreat from the coastline since the time of rifting, similar to the Western Ghats escarpment on its conjugate margin of the India Peninsula.

Crustal architecture and evolution of the southwestern South China Sea: Implications to continental breakup

The South China Sea (SCS) is a marginal basin characterized by propagating breakup and subsequent seafloor spreading during the Cenozoic. Our study investigates the crustal architecture and tectonic evolution of the Xisha Islands - Northwest Borneo conjugate margins using a 1000 km long seismic profile and two shorter seismic lines. This system is characterized by wide margins showing a succession of rift basins with thinned continental crust, associated with large extensional detachment faults soling on top of the lower crust. In relation with crustal extension, several syn-rift sequences have been identified. The lower syn-rift unit (syn-rift I) shows well expressed wedge-shape growth strata. The upper one (syn-rift II) presents a more uniform thickness, and is slightly offset by normal faults dipping toward the depocenters. To illustrate the evolution of the southwestern SCS margins during continental breakup, we restore two conjugate sections near the oceanic magnetic anomaly C5En (18.8 Ma). The conjugate COT presents an asymmetrical hyper-extension architecture in cross sections, with the southern margin being the lower plate margin. In map view, the spatial variation of crustal structures illustrates a bayonet shape characterized by an initial N–S extension coeval with the first phase of seafloor spreading in the east sub-basin in the SCS. The extension direction changed shortly later (circa 23Ma) to NW-SE opening in the southwest sub-basin in relation to a ridge jump. We suggest that variable extension direction is linked to changes in the compressional tectonic regime along Borneo and Palawan.

Lateral coexistence of ductile and brittle deformation shapes magma-poor distal margins: An example from the West Iberia-Newfoundland margins

A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain.

Reply to comment on ‘The challenge in restoring magma-rich rifted margins: The example of the Mozambique-Antarctica conjugate margins’ (Gondwana Research, 2021, 95 29–44)

Reply to comment on ‘The challenge in restoring magma-rich rifted margins: The example of the Mozambique-Antarctica conjugate margins’ (Gondwana Research, 2021, 95 29–44)

Provenance of the Incipient Passive Margin of NW Laurentia (Neoproterozoic): Detrital Zircon from Continental Slope and Basin Floor Deposits of the Windermere Supergroup, Southern Canadian Cordillera

Abstract The origin of the passive margin forming the paleo-Pacific western edge of the ancestral North American continent (Laurentia) constrains the breakup of Rodinia and sets the stage for the Phanerozoic evolution of Laurentia. The Windermere Supergroup in the southern Canadian Cordillera records rift-to-drift sedimentation in the form of a prograding continental margin deposited between ~730 and 570 Ma. New U-Pb detrital zircon analysis from samples of the post-rift deposits shows that the ultimate source area was the shield of NW Laurentia and the near uniformity of age spectra are consistent with a stable continental drainage system. No western sediment source area was detected. Detrital zircon from postrift continental slope deposits are a proxy for ca. 676-656 Ma igneous activity in the Windermere basin, likely related to continental breakup, and set a maximum depositional age for slope deposits on the eastern side of the basin at 652±9 Ma. These results are consistent with previous interpretations. The St. Mary-Moyie fault zone near the Canada-U.S. border was most likely a major transform boundary separating a rifted continental margin to the north from intracratonic rift basins to the south, resolving north-south variations along western Laurentia in the late Neoproterozoic at approximately 650-600 Ma. For Rodinia reconstructions, the conjugate margin to the southern Canadian Cordillera would have a record of rifting between ~730 and 650 Ma followed by passive margin sedimentation.

Neoproterozoic to Jurassic tectono-metamorphic events in the Sierra Nevada de Santa Marta Massif, Colombia: insights from zircon U-Pb geochronology and trace element geochemistry

ABSTRACT The Sierra Nevada de Santa Marta Massif (SNSM) in the Northern Andes affords an insight into lower crustal section structural relations. This polymetamorphic basement comprises high-grade gneisses, which alternate with imbricate slivers of medium-to high-grade schists. New field observations coupled with LA-ICP-MS U-Pb zircon geochronology and trace element geochemistry reveal some hallmarks on the evolution of this strongly remobilized crustal section. The Proterozoic Rodinia-type basement (1100 − 880 Ma) recorded at least two separated tectono-metamorphic events (M0 and M1) and was subsequently affected by a magmatic episode during the Ordovician (~450 Ma; M2). During the Permian, subduction of Panthalassian oceanic crust beneath Pangaea led to the installation of a magmatic-arc and probably coeval metamorphism (~288-266 Ma; M3). Middle-Late Triassic roll-back extension of the upper plate led to anatexis and metamorphism (M4) reaching peak conditions at ca. 239 Ma with concurrent crystallization of garnet and sillimanite, followed by a decreasing geothermal gradient at ca. 202 Ma. Early Jurassic arc build-up under a rejuvenated convergence was contemporaneous to metamorphism on an accretionary wedge (M5) at ca. 185 Ma. The SNSM Massif basement units yield metamorphic ages that compare to similar tectono-metamorphic provinces in NW-Gondwana, including the Central Cordillera of Colombia, the Guajira Peninsula, the Mérida Andes, the Perijá Range, the Santander Massif, and the Central American and Mexican terranes to the south of the Ouachita-Marathon Suture (Oaxaquia, Mixteca, and Maya blocks). Age equivalence of these events is best explained by the existence of a conjugate margin with Laurentia until the Late Triassic ~216 Ma, which preceded the inception of a Colombian marginal seaway during the break-up of western Pangaea. Furthermore, the presence of Ordovician metamorphic rocks in the SNSM Massif confirms the extent of a Famatinian arc along NW-Gondwana during the Early Palaeozoic.

The transition from salt diapir to weld and thrust: Examples from the Northern Flinders Ranges in South Australia

AbstractThe interactions between salt diapirs, thrust welds and thrusts in contractional belts are poorly understood due to, first, the inability of seismic data to distinguish between thrusts and welds or resolve associated sub‐resolution deformation, and second, the paucity of good field examples. The Warraweena area in the Northern Flinders Ranges of South Australia contains examples of Neoproterozoic to Early Cambrian squeezed diapirs linked by steep reverse faults formed during the Delamerian Orogeny. Benefiting from good field exposures, we use geological mapping, cross‐section construction and conceptual structural models to assess the three‐dimensional geometry and evolution of the structures, the lateral transition from diapirs to linking faults and the variability of associated meso‐ and small‐scale deformation. Three discrete diapirs consist of narrow outcrops of Callanna Group megabreccia (Willouran in age) up to 5‐km long. Their diapiric origin is confirmed by local development of caprock, steepening of flanking strata in composite halokinetic sequences and reworked diapir and roof debris in adjacent strata. The surrounding rocks display only background levels of small‐scale deformation. In contrast, the linking faults show no evidence of precursor diapirism, have fault‐related anticlines up to 100s of m in wavelength in their hanging walls, and an associated increase in small‐scale deformation (i.e. millimetre to metre scale folds, fractures and shear fabrics). The transitions from diapirs to faults occur within less than 200 m as short thrust welds at the diapir terminations. The exposed structures are analogous to those found on the subsurface of other salt basins such as the Gulf of Mexico and the South Atlantic conjugate margins. The results of this work can aid geoscientists evaluating three‐way traps against squeezed diapirs, welds or faults, and can help them to predict the style and abundance of both halokinetic and small‐scale structures that are below seismic resolution.

Evolution of shallow and deep-water deposits included in the regressive drift succession of the Potiguar Basin (Brazilian Equatorial Margin) during the Late Cretaceous to Holocene

The regressive drift succession (late Campanian–Holocene) corresponds to the last major sedimentary cycle of the Potiguar Basin and includes shallow and deep-water deposits that have potential plays similar to other hydrocarbon discoveries in the Brazilian Equatorial Margin and the African conjugate margin. This study aimed to perform a stratigraphic analysis of regressive drift succession, seeking a better understanding of its evolution and characterization of potential hydrocarbon reservoirs. Based on the interpretations of five exploratory wells and seven seismic lines, this study identified five depositional sequences. In the most proximal regions, sequences 1 and 2 exhibit a dominance of slope facies and sigmoidal/tangential-oblique seismic patterns. In contrast, sequences 3, 4, and 5 show complex sigmoid-oblique seismic patterns. In addition, these last depositional sequences display significant sedimentation of the shelf facies. Sequence 3 marks a remarkable carbonate development in the shelf, whereas a notable siliciclastic input characterizes Sequence 4. Sequence 5, in turn, has its configuration defined by extensive migration of distal deposits toward the proximal portions. Finally, in the distal regions, the five depositional sequences exhibit a preponderance of hemipelagic deposits, with the presence of gravitational flow deposits that occur mainly in the two oldest sequences (1 and 2).

Middle-Late Triassic metamorphism of the Guajira Arch-basement: Insights from zircon U–Pb and Lu–Hf systematics

The Guajira Arch is located in the northernmost portion of South America and holds the record of several stages of magmatism and metamorphism. We performed LA-ICP-MS U–Pb zircon geochronology coupled with Lu–Hf isotope geochemistry on 11 samples from a high-grade gneissic basement hosting interleaved complexes of younger meta-granitoids and meta-sediments to unravel its tectono-metamorphic evolution. The oldest rocks correspond to a Rodinia-type basement at ca. 1100-880 Ma, affected by mafic magmatism during the Permian at ca. 272 Ma, anatexis during the Carnian at ca. 230 Ma, updoming and the formation of an extensional detachment fault during the Norian at ca. 224 Ma concluding with Jurassic magmatism since ca. 190 Ma. Trace element geochemistry shows that Triassic zircons are depleted in HREE and yield lower Th/U ratios than Jurassic zircons enriched both in HREE, Yb, and Y. Hf systematics indicates a Triassic to Jurassic evolution towards more radiogenic compositions. The lithological associations, structural setting, U–Pb ages, and Hf isotope geochemistry of the Triassic-Jurassic rocks at the Guajira Arch characterize an active margin setting of a hot orogenic belt in NW-Gondwana, comprising the Central Cordillera of Colombia, the Santa Marta Massif, the Merida Andes, the Perijá Range, the Santander Massif and the para-autochthonous terranes south of the Ouachita-Marathon Suture (Oaxaquia, Acatlán, and Maya), and thus vindicate the existence of a conjugate margin with Laurentia until the Late Triassic, preceding the breakup of western Pangaea. The Triassic anatectites and Jurassic granitoids from the Guajira Arch yield Proterozoic Hf TDMi ages which are indistinguishable from the autochthonous Proterozoic basement and thus render an allochthonous origin by terrane accretion highly improbable. Instead, these data point to melting of the continental crust by the incorporation of primitive material reflecting a two-stage process at the boundary of a large underlying mantle convection cell (i) Triassic slab steepening, subduction arrest, updoming anatexis at ca. 230-223 Ma and (ii) Early Jurassic arc-magmatism at ca. 190-165 Ma due renewed convergence. Furthermore, we propose that the structural setting, lithology, and isotope geochemistry of the Guajira Arch units, fit into the style of a cordilleran core-complex emplaced in a roll-back extensional setting preceding Pangaea break-up.