Divergent Continental Margins Research Articles

Interactions between pre‐existing structures and rift faults: Implications for basin geometry in the northern South China Sea

AbstractThe northern South China Sea (SCS) margin evolved from the Mesozoic convergent to Cenozoic divergent continental margin, and thus, it developed on a heterogeneous crystalline basement with inherited Mesozoic structures. Pre‐existing structures and their interactions with rift faults have historically not been described or interpreted in the intensely stretched Baiyun sub‐basin. Large‐scale 3D seismic reflection data allow us to identify four types of Mesozoic tectonic fabrics within the basement and explain their genesis: (1) Thin, isolated and north‐dipping seismic reflections 1, interpreted as thrust faults representing orogenic processes. Tilted thick seismic reflections 2 are formed by reactivation of seismic reflections 1 during post‐orogenic extension, which are all related to the NW‐ward subduction of the palaeo‐Pacific plate. (2) Thin, isolated and shallowly dipping seismic reflections 3 and low‐amplitude, semi‐transparent and chaotic seismic reflections 4 represent the low‐angle thrust system and the associated nappe units, which are related to the shift from NW‐ to NNW‐ward subduction of the paleo‐Pacific plate. Subsequently, we investigate the structural interaction between Mesozoic intra‐basement and Cenozoic rift structures. Syn‐rift, post‐rift and long‐term faults are developed in Cenozoic strata, and quantitative statistical and qualitative analyses revealed two main types of structural interactions between them and underlying intra‐basement structures: (1) Rift faults develop with inheritance of intra‐basement structures, including fully and partially inherited faults. (2) Rift faults modify intra‐basement structures, although they are controlled by intra‐basement structures at an earlier stage. Finally, our results reveal the control of pre‐existing structures on the geometry of the Baiyun sub‐basin, especially the selective reactivation of NE‐trending shear zones (SR2), which are influenced by the regional stress field and the width and dip of the shear zones.

Influence of Surface Processes on Postrift Faulting During Divergent Margins Evolution

AbstractWe used thermomechanical numerical models to simulate the formation and evolution of divergent continental margins since continental rifting, taking into account surface processes of erosion of the continental escarpment. We found that the degree of crust–mantle coupling, the magnitude and extent of erosion of the coastal landscape, and the preexistence of weakness zones in the continental crust are important elements that control the reactivation of faults along divergent margins during the postrift phase. The numerical experiments indicated that the presence of a lower crust with a relatively low viscosity, facilitating the decoupling of the upper crust and the development of hyperextended margins, could also contribute to the development and/or reactivation of normal faults in the interior of the continent when the margin is continually subjected to differential denudation and consequent flexural response of the lithosphere. This effect is suppressed in scenarios where the lower crust presents a relatively high viscosity, consequently inducing the coupling of the upper crust with the lithospheric mantle. In this case, the long wavelength of the flexural response of a coupled lithosphere to erosional unloading has a minor impact on the reactivation of faults in the upper crust. As an application of our model, we propose that the combination of a decoupled continental lithosphere, escarpment retreat due to erosion, and the preexistence of shear zones parallel to the coast contributed to the development of postrift Cenozoic tectonism in southeastern Brazil.

Resolving lithostratigraphic complexities in the Crayfish Group, Otway Basin using chemostratigraphy

The Otway Basin comprises a significant part of the eastern Australian Southern Rift System, a divergent passive continental margin formed during the Cretaceous separation of the Australian and Antarctic continents. Early rifting activity resulted in the development of many half grabens within the Otway Basin, which are largely infilled by sediments of the Casterton Formation and Crayfish Group. Despite over 20 years of exploration and hydrocarbon production from these units however, their lithostratigraphic characterisation and nomenclature remain ambiguous, with structural complexity and prevalent lateral facies changes leading to confusion in their basin-wide correlation. Deposited in a largely non-marine, fluvial/lacustrine environment, repeating cycles of sandstones and shales of the Crayfish Group can be difficult to resolve using petrology, palynology and wireline log data. The use of chemostratigraphy is favoured as an investigative tool in this situation since changes in provenance, lithic composition, facies, weathering and diagenesis are reflected in the mineralogy of the sediments, resulting in variations in their inorganic geochemistry. Uniform sedimentary successions can thus potentially be differentiated into unique sequences and packages based on their characteristic geochemistry, aiding in the resolution of complex structural relationships and facies changes. In this study, we present new inorganic geochemistry data for four key wells in the South Australian (SA) Penola Trough and interpret the geochemistry data consistent with, and building on, the chemostratigraphic schema of Forbes et al. to demonstrate its utility and robustness. We then undertake inter-well wireline log correlations across the SA Penola Trough using the wells with chemostratigraphic data as controls.

Seismic constraints on a remnant Mesozoic forearc basin in the northeastern South China Sea

The northeastern of the South China Sea (SCS) has undergone tectonic inversion from a Mesozoic convergent continental margin to a Cenozoic divergent continental margin. The remnant subduction-related structures, such as volcanic arc and forearc basin, likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion. We applied ocean bottom seismometers (OBS) data, combining with multi-channel seismic data, gravity data, drilling data, and petrologic observations, to reveal geophysical and structural features of the Dongsha Basin, a Mesozoic basin in the northeastern SCS. The Mesozoic strata are characterized by large thickness (~1–8 km), relatively high P-wave velocities (3.5–5.5 km/s), high densities (2.55–2.65 g/cm3), alternating folds, and thrust faults, which are significantly different from the features of Cenozoic strata. The Dongsha Basin is flanked by a Mesozoic granitic arc to the northwest and an inferred Palaeo-Pacific subduction zone to the southeast, indicating that it is geographically located in the Mesozoic forearc region. Considering the similar Mesozoic strata detected in the southern margin of the SCS and the contemporaneous accretionary complex outcrops observed in the Palawan Continental Terrane, we propose that the Mesozoic strata in these two regions originated from a common Mesozoic forearc basin, which was broken by the forearc rifting and subsequently separated by the spreading of Cenozoic SCS. Accordingly, a continuous Palaeo-Pacific subduction system is reconstructed, including the “arc-forearc-accretionary wedge” tectonic sequences. Reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore South China, we speculate that rollback of the Palaeo-Pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the SCS.

Post-rift influence of small-scale convection on the landscape evolution at divergent continental margins

After decades of geological and geophysical data acquisition along with quantitative modeling of the long-term evolution of the landscape at divergent continental margins, the search for an explanation for the formation and evolution of steep escarpments bordering the coast is still a challenging task. One difficult aspect to explain about the evolution of these escarpments is the expressive variability of denudation rate through the post-rift phase observed in many margins. Here I propose that the interaction of small-scale convection in the asthenosphere with the base of the continental lithosphere can create intermittent vertical displacements of the surface with magnitude of a few hundreds of meters at the continental margin. These topographic perturbations are sufficient to produce an expressive variability in the rate of erosion of the landscape through the post-rift phase similar to the exhumation history observed along old divergent margins. I show that the vertical motion of the surface is amplified when a mobile belt is present at the continental margin, with lithospheric mantle less viscous than the cratonic lithosphere and, consequently, more prone to be partially eroded by the convective asthenosphere. I conclude that the influence of small-scale convection is not the primary explanation for the formation of high topographic features at divergent continental margins, but can be an important component contributing to sustain a preexistent escarpment. The present results are based on numerical simulations that combine thermochemical convection in the mantle, flexure of the lithosphere and surface processes of erosion and sedimentation.

Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K–Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean–continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

Tectonic variation and structural evolution of the West Greenland continental margin

Because of its geographic extent of over 2500 km (1553 mi), the West Greenland margin provides a much understudied example of a divergent continental margin, both with respect to hydrocarbon exploration and academic studies. A seismic interpretation study of representative two-dimensional reflection profiles from the Labrador Sea, Davis Strait, and Baffin Bay was undertaken to identify sedimentary and structural components to elucidate the tectonic development of the margin. Nine horizons were interpreted from six representative seismic lines in the area. Margin-scale tectono-stratigraphy was derived from isochron maps, the geometry of mappable faults and their associated stratal architecture. Rifting began in Early to Late Cretaceous at ca. 145–130 Ma, which was followed by two pulses of volcanism in Eocene and Paleocene ages. The transition to the drift stage includes a typical subsidence phase but also erosion, uplift, and deposition of Neogene postrift packages. The shift in the position of depocenters in the Davis Strait and the Labrador Sea during Paleocene and Miocene times is evidence for structural modification of the basin bounding faults. Drift stage deformation suggests a possible anticlockwise rotation in the orientation of the spreading axis in Baffin Bay culminating in an ultraslow sea-floor spreading. Sea-floor spreading on the West Greenland margin started in the south at 70 Ma in the Labrador Sea and propagated northward into the Baffin Bay by 60 Ma. Prospective petroleum systems include thick Cretaceous age strata, with structural traps provided by grabens and inversion structures. Our structural model provides insight into a margin that is highly variable in its structural configuration, further modified by other processes such as magma-assisted rifting that may result in elevated regional heat flow, which has considerable impact on hydrocarbon maturation. Further constraining the implications of heat flow associated with volcanic activities in comparison to that associated with lithospheric stretching will be critical in future exploration.

Fluid flow during early compartmentalisation of rafts: A North Sea analogue for divergent continental margins

High-quality 3D seismic data tied to eighteen (18) boreholes are used to investigate the styles of faulting and associated fluid flow features in Triassic–early Jurassic rafts of the Broad Fourteens Basin, Southern North Sea. The study area is presented as an analogue for continental margins experiencing early stage gravitational gliding, i.e. prior to complete separation and downslope translation of individual rafts. In such a setting, and for present-day stress conditions, fault slip data indicate that chasms and faults separating rafts in the Broad Fourteens Basin comprise structures subject to dip slip and strike-slip reactivation. Chasms and faults sub-parallel to these latter chasms comprise the most significant bypass areas for fluid sourced from pre-salt strata. Faults sub-parallel to the main chasms show limited propagation into Early Cretaceous and Cenozoic strata draping the rafts, a character further stressed by the depth of occurrence of fluid pipes and dim spots. This is an important observation, and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where gravitational gliding is limited such as West and Equatorial Africa, Southeast Brazil and parts of the Gulf of Mexico.

The breakup sequence and associated lithospheric breakup surface: Their significance in the context of rifted continental margins (West Iberia and Newfoundland margins, North Atlantic)

Regional (2D) seismic-reflection profiles and borehole data are used to characterise the syn- to post-rift transition in the shallow offshore Porto Basin, and in deep-offshore regions of West Iberia and Newfoundland (East Canada). The interpreted data highlight the development of a regional stratigraphic surface at the time of complete lithospheric breakup between West Iberia and Newfoundland. This surface, usually called “breakup unconformity”, is renamed in this work as Lithospheric Breakup Surface (LBS), on the basis that: (1) it is not always developed as an unconformity and (2) all lithosphere is involved on the breakup process, not only the continental crust. Depositional changes occur across the LBS in association with Late Aptian lithospheric breakup, which is marked by the deposition of a breakup sequence (BS) rather than a single stratigraphic surface. Stratigraphic correlations between strata in shallow and deeper parts of the two margins lead us to propose the breakup sequence (BS) as representing the transitional period between lithospheric breakup and the establishment of thermal relaxation as the main process controlling subsidence on divergent continental margins. The results in this work are important for other continental margins as they demonstrate that during lithospheric breakup significant quantities of sediment bypassed the inner proximal margins of West Iberia and Newfoundland on their way to the outer proximal margin. In addition, the interpreted data show that complete lithospheric breakup between conjugate margins is recorded by similar tectono-stratigraphic events. In Iberia and Newfoundland, these events are associated with reservoir successions in sediment overfilled basins and with carbon-rich strata (‘black shales’) in sediment-starved basins.

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

Abstract The tectonosedimentary development of the South Atlantic is compared with the Central Atlantic margins, which are associated with major episodes of magmatism during the Mesozoic continental break-up. Subsequently, the Cenozoic break-up in the North Atlantic led to the formation of the volcanic Norwegian–Greenland conjugate margins. The DSDP boreholes in the magma-poor Iberian–Newfoundland margins have confirmed the occurrence of exhumed mantle at the ocean–continent transition. This possibility has been suggested for the South Atlantic margins, but still lacks confirmation from drilling. The Red Sea and the Gulf of Aden may be considered as natural laboratories to study the break-up processes and formation of divergent continental margins. Using key geological and geophysical data, we compare some of the structures observed in incipient stages of basin formation between the African and the Arabian plates with the structures observed in older sedimentary basins associated with the Gondwana break-up. We also analyse deep seismic reflection profiles and potential field data at the continent–ocean boundary of these conjugate margins, using palinspastic reconstructions to define the corresponding seismic pairs. We conclude that the Red Sea and the Gulf of Aden display remarkable differences to the Iberian–Newfoundland margins, and notable similarities with the South Atlantic margins.

Geological structures of ridges with relation to the definition of three types of seafloor highs stipulated in Article 76

The ridge like seafloor highs with various geological origins can be classed into mid-ocean ridges, transverse ridges related to transform faults, hot spot/mantle plume originated ridges, microcontinent rifted from major continent, intra-plate arc formed by interaction of two oceanic plates, and tectonic ridges uplifted by later tectonic activity. Those ridges moved towards the convergent continental margins along with the underlain plate drifting and formed so-called accreted ridges commonly trending at a high angle to the continental margins. At divergent continental margins, the continental crusts were extended and thinned accompanying with magmatism, which formed high terrains protruding or parallel to the coastal line. The ridges worldwide have various origins and the crustal thicknesses and structures of them are diversity. The crusts beneath the microcontinents, and the transverse ridges along the transform margin, and the seafloor highs beside the passive continental margins are continental, while the crusts of other ridges are oceanic. Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS) has classed the seafloor highs worldwide into three legal categories, namely oceanic ridges, submarine ridges and submarine elevations, for the purpose to delineate the outer limit of the coastal States’ continental shelf beyond 200 nautical miles. To define the categories of the legal seafloor highs to which the ridges with various geological origins belong, the continuities in morphology and geology including the rock types, crustal structures, origins and tectonic setting of the ridges and the coastal States’ land mass with its submerged prolongation should be taken into account. If a ridge is continuous both in morphology and geology with the coastal States’ land mass and its submerged prolongation, it is a submarine elevation stipulated in Article 76. If it is discontinuous in morphology, the ridge should be regarded as oceanic ridges. If a ridge is continuous in morphology but discontinuous in geology with the coastal States’ land mass and its submerged prolongation, then it is a submarine ridge as stipulated in Article 76.

Geochemical Indications of Possible Gas Hydrates in the Northeastern South China Sea

Abstract Gas hydrate, mainly composed of hydrocarbon gas and water, is considered to be a clean energy in the 21st century. Many indicators such as BSRs (Bottom-Simulating Reflections), which are thought to be related to gas hydrate, are found in the South China Sea (SCS) in recent years. The northeastern part of the SCS is taken as one of the most potentials in the area by many scientists. It is situated in the conjunction of the northern divergent continental margin and the eastern convergent island margin, whose geological settings are much preferable for gas hydrate to occur. Through this study, brightness temperature anomalies recorded by satellite-based thermal infrared remotely sensed images before or within the imminent earthquake, the high content of hydrocarbon gas acid-degassed from subsurface sediment and the high radioactive thermoluminescence value of subsurface sediment were found in the region. Sometimes brightness temperature anomalies alone exist in the surrounding of the Dongsha Islands. The highest content of hydrocarbon gas amounts to 393 μL methane per kilogram sediment and the highest radioactive thermoluminescence value is 31752 unit; their geometric averages are 60,5 μL/kg and 2688.9 unit respectively. What is more inspiring is that there are three sites where the methane contents are up to 243, 268 and 359 μL/kg and their radioactive thermoluminescence values are 8430, 9537 and 20826 unit respectively. These three locations are just in the vicinity of one of the highest confident BSRs identified by predecessors. Meanwhile, the anomalies are generally coincident with other results such as headspace gas anomaly in the sediment and chloride anomaly in the interstitial water in the site 1146 of Leg 184. The above-mentioned anomalies are most possibly to indicate the occurrence of gas hydrate in the northeastern SCS.

Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins

Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins

Investigating Large Igneous Province Formation and Associated Paleoenvironmental Events: A White Paper for Scientific Drilling

No abstract available. doi:10.2204/iodp.sd.6.01.2008

Large igneous provinces

Catastrophic massive volcanism and contemporaneous environmental change have punctuated Earth history for at least the past 3.5 billion years. Geodynamic and magmatic processes associated with large igneous province (LIP) formation interact with crustal structure and tectonic setting to produce various expressions of LIPs, the most common of which are oceanic plateaus, magma‐dominated divergent continental margins, and continental flood basalts. Environmental and biotic perturbations simultaneous with LIP formation include climate changes, mass extinctions, accelerated evolutionary rates, oceanic anoxic events (OAEs), and variations in ocean chemistry. Exploring these relationships promises exciting science.The LIP workshop, hosted by the Integrated Ocean Drilling Program (IODP) and Joint Oceanographic Institutions, was held at the University of Ulster in Coleraine, U.K. Eighty scientists from 16 nations discussed strategies for advancing understanding of LIPs and associated environmental changes using the three new IODP platforms and related technologies. Over 4 days of plenary and breakout sessions, including examination of the UNESCO World Heritage Giant's Causeway exposure of the North Atlantic LIP, scientists who approach LIPs through field, laboratory, and modeling studies shared their understanding of the world's LIPs; discussed outstanding problems related to LIP origin, emplacement, and environmental consequences; and outlined a global mission to address these problems via drilling in conjunction with complementary geoscientific studies.

Tectonic history and the biogeography of the freshwater fishes from the coastal drainages of eastern Brazil: an example of faunal evolution associated with a divergent continental margin

The eastern Brazilian coastal drainages are of great biogeographical significance, because of their highly endemic fish faunas. Phylogenetic patterns suggest a close biotic relationship between the rivers that flow into the Atlantic and those on the adjacent upland crystalline shield. However, little has been said on the dynamics of the geological processes causally related to the cladogenetic events between these areas. Distributional and phylogenetic patterns suggest a close association with the geological history of the passive continental margin of South America, from the Cretaceous to the present day. In this area megadome uplifts, rifting, vertical movements between rifted blocks and the erosive retreat of the South American eastern continental margin are hypothesized as the main geological forces controlling the distribution of freshwater fishes. The tectonic activity associated with the break-up of Gondwana and separation of South America and Africa formed six megadomes that control most of the current courses of the main crystalline shield river basins. Except for basins located at the edges of such megadomes, these river systems developed long, circuitous routes over the ancient Brazilian crystalline shield before emptying into the recently opened Atlantic Ocean. Initial cladogenetic events between upland crystalline drainages and Atlantic tributaries were probably associated with vicariant processes, and some ancient basal sister-groups of widespread inclusive taxa are found in these coastal hydrographic systems. Later, generalized erosive denudation resulted in an isostatic adjustment of the eastern margin of the platform. These, along with reactivations of ancient rifts led to vertical movements between rifted blocks and gave rise, in southeastern Brazil, to taphrogenic (rift related) basins. These basins, such as the Taubaté, São Paulo, Curitiba and Volta Redonda basins, among others, captured adjacent upland drainages and fauna. The fossil fishes from the Tremembé Formation (Eocene-Oligocene of Taubaté Basin) exemplify this process. Other taphrogenic systems of Tertiary age were also identified in other segments of the Atlantic continental margin, such as in Borborema province, in NE Brazil, with marked influence over drainage patterns. At the same time, erosive retreat of the eastern margin of the platform successively captured upland rivers, which became Atlantic tributaries evolving associated to main rift systems. The continued nature of these processes explains the mixed phylogenetic and distributional patterns between Atlantic tributaries and the upland crystalline shield areas, especially in the southeastern continental margin, represented by successively, less inclusive sister-groups associated with cladogenetic events from the Late Cretaceous to the present.

Sediment-hosted micro-disseminated gold mineralization constrained by basin paleo-topographic highs in the Youjiang basin, South China

The Youjiang basin is a Devonian–Triassic rift basin on the southern margin of the Yangtze Craton in South China. Strong syndepositional faulting defined the basin-and-range style paleo-topography that further developed into isolated carbonate platforms surrounded by siliciclastic filled depressions. Finally, thick Triassic siliciclastic deposits covered the platforms completely. In the Youjiang basin, numerous sediment-hosted, micro-disseminated gold (SMG) deposits occur mainly in Permian–Triassic chert and siliciclastic rocks. SMG ores are often auriferous sedimentary rocks with relatively low sulfide contents and moderate to weak alteration. Similar to Carlin-type gold ores in North America, SMG ores in the Youjiang basin are characterized by low-temperature mineral assemblages of pyrite, arsenopyrite, realgar, stibnite, cinnabar, marcasite, chalcedony and carbonate. Most of the SMG deposits are remarkably distributed around the carbonate platforms. Accordingly, there are platform-proximal and platform-distal SMG deposits. Platform-proximal SMG deposits often occur in the facies transition zone between the underlying platform carbonate rocks and the overlying siliciclastic rocks with an unconformity (often a paleo-karst surface) in between. In the ores and hostrocks there are abundant synsedimentary–syndiagenetic fabrics such as lamination, convolute bedding, slump texture, soft-sediment deformation etc. indicating submarine hydrothermal deposition and syndepositional faulting. Numerous fluid-escape and liquefaction fabrics imply strong fluid migration during sediment basin evolution. Such large-scale geological and fabric evidence implies that SMG ores were formed during basin evolution, probably in connection with basinal fluids. It is well known that basinal fluids (especially sediment-sourced fluids) will migrate generally (1) upwards, (2) towards basin margins or basin topographic highs, (3) and from thicker towards thinner deposits during basin evolution. The isolated carbonate platform (as a basin paleo-high) and related syndepositional fault system, together with the unconformity-related facies succession, may have controlled the migration pathway of ore-forming basinal fluids and subsequently determined the location of SMG deposits in the Youjiang basin. Unlike Carlin-type gold deposits, SMG mineralization in the Youjiang basin may represent an integral aspect of the dynamic evolution of extensional basins along divergent continental margins.

Flexural partitioning of the Late Archaean Witwatersrand foreland system, South Africa

The Witwatersrand Basin is part of a retroarc foreland system formed during the Late Archaean in response to crustal thickening along the northern margin of the Kaapvaal Craton. This foreland system may be partitioned into foredeep, forebulge, and back-bulge flexural provinces. The preserved sedimentary fill occupies the region of the flexural foredeep, and displays a typical transition from underfilled (deep and shallow marine) to overfilled (fluvial and alluvial fan) phases. Loading in the Witwatersrand thrust belt took place in relation to the oblique convergence of two compressional stress fields. As a result, the foredeep is restricted along the strike to a distance of <400 km, and the forebulge is constrained at the intersection between the areas of influence of the two stress fields. The apex of the forebulge is outlined by the gravity line of −130 mgal, which matches the shape of the theoretical depositional limit of the foredeep. The back-bulge region of the foreland system is superimposed on the divergent continental margin of the Kaapvaal Craton, and hosts the Mozaan Group of the Pongola sequence. The Mozaan and Witwatersrand successions are correlative and display in-phase stratigraphies, as being part of the same foreland system; the gap between their areas of occurrence may be related to the uplift and erosion in the forebulge region. The forebulge migrated a relatively short distance down dip (i.e., <100 km), causing the erosion and wedging out of the underlying Dominion Group. This erosional surface displays all the diagnostic features of a basal (forebulge) unconformity, with an increasing stratigraphic gap towards the foreland, and a correlative conformity in the more proximal parts of the basin. The Witwatersrand foreland system formed on a newly cratonized continental lithosphere, which gave the basin a small flexural wavelength (i.e., ∼490 km from the center of orogenic loading to the axis of the back-bulge) relative to most Phanerozoic counterparts. The forebulge unconformity may be associated with the lack of dynamic loading, which is probably a common feature for all Precambrian retro-foreland systems due to the lower rates of subduction associated with the early stage of plate tectonic processes.

Stratigraphic record of an Atlantic‐type global tectonic cycle in the Palaeoproterozoic Ashburton Province of Western Australia

New descriptive and interpretive stratigraphic frameworks for the Ashburton (and Gascoyne) Province of the Capricorn Orogen are based on sequence stratigraphy. Two megasequences are recognised. A lower megasequence (Megasequence VIIIL) is interpreted to record the opening, to the northwest, of an Atlantic‐type ocean. Four supersequences record rift and thermal‐subsidence stages of a divergent continental margin, with north‐ or northeast‐trending ocean‐margin and northwest‐trending failed‐rift geometries. There is no evidence for a southern oceanic regime. Rather, the Gawler Craton of South Australia is interpreted to have lain to the southwest of the Pilbara Craton, such that the cratons were a contiguous divergent continental margin. An upper megasequence (Megasequence VIIIU) is interpreted to record the conversion of the divergent margin to a convergent (hinterland) margin during southeast‐dipping subduction. Eight supersequences are recognised. These record backarc, remnant‐ocean, retroarc‐satellite and indent‐linked hinterland basins. All the basins preserve the failed‐rift geometry. To the southwest, collision‐indentation of the hinterland (Pilbara‐Gawler) continent by an indenter (Yilgarn Craton) is interpreted to have begun during the remnant‐ocean stage. An east‐ to east‐southeast‐trending megashear between the Pilbara and Yilgarn Cratons is interpreted to have been the boundary transform fault on the northern margin of that indenter.

Regional extension as a geologic trigger for diapirism

Initiation of diapirs is one of the least understood aspects of salt tectonics. Differential sedimentary loading or erosion are both effective but not universal. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional extension. Extended salt basins typically develop salt structures, whereas nonextended basins typically do not. Even in salt basins overprinted by inversion or orogenic contraction, the diapirs were initiated during extension on divergent continental margins or in intracontinental rifts. Regional extension thins brittle overburden by forming grabens and half grabens above flowing salt. These fault structures (1) differentially load the salt by their surface relief and (2) weaken the overburden by fracturing and thinning it. Diapiric walls of pressurized salt rise in reaction to the shitting positions of fault blocks in extending overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventually the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively as an independent intrusion. Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had trivial displacements. Physical modeling shows that extension of the basement has only indirect influence on diapirism by creating space for extension of the overburden, which is the direct cause of diapirism, whether extension is thick-skinned or thin-skinned and whether the salt was deposited before, duing, or after rifting.