Evolution of a Neoproterozoic to Palaeozoic intracratonic setting, Officer Basin, South Australia

The Neoproterozoic basins of central Australia share many features of architecture and sedimentary fill, suggesting common large-scale extrinsic causal mechanisms. In an attempt to improve understanding of these mechanisms we have gathered and analysed new deep seismic reflection data and re-evaluated existing seismic and well-log data from the eastern Officer Basin, the largest and most poorly known of Australia's intracratonic basins. The Officer Basin is asymmetric and has a steep thrust-controlled northern margin paralleled by sub-basins as much as 10 km in depth. Further south the basin shallows gradually onto a broad platform. The basin rests on a thick crust (≈42 km) that is pervaded by a complex of northward-dipping surfaces most of which terminate erosionally against the sediments of the Officer Basin and are interpreted as prebasinal features, possibly faults. Some appear to have been zones of crustal weakness which were reactivated as thrust complexes and played a major role in basin evolution. The sedimentary succession can be subdivided into six megasequences separated by major tectonically and erosionally enhanced sequence boundaries. The megasequences have distinctive sequence stacking patterns suggesting that they were deposited in response to episodic subsidence induced by a major extrinsic tectonic event. The basin initially formed as part of a giant sag basin which incorporated all the present-day intracratonic basins (Amadeus, Georgina, Ngalia, Officer and Savory Basins) in a single large ‘superbasin’ perhaps as a response to mantle processes. Subsidence then ceased for ≈100 Myr producing a regional erosion surface. Beginning in the Torrensian or Sturtian five more major events of varying regional significance influenced the basin's evolution. Four were compressional events, the first of which activated major thrust complexes along the present basin margins, forming deep foreland sub-basins with elevated intervening basement blocks. Once activated, the thrust complexes and sub-basins persisted throughout the life of the intracratonic basins. From this epoch the intracratonic basins of central Australia were decoupled from the giant sag basin and became interrelated but independent features. Available information suggests that the Officer, Amadeus, Georgina, Ngalia and Savory Basins are related and are perhaps products of major tectonic events associated with the assembly and ultimate dispersal of the Proterozoic supercontinent. The closing phases of these basins were strongly influenced by events occurring along the newly created active eastern margin of the Australian continent in the Palaeozoic.     

Testing long-term patterns of basin sedimentation by detrital zircon geochronology, Centralian Superbasin, Australia

Detrital zircon geochronology of Neoproterozoic to Devonian sedimentary rocks from the Georgina and Amadeus basins has been used to track changes in provenance that reflect the development and inversion of the former Australian Superbasin. Through much of the Neoproterozoic, sediments appear to have been predominantly derived from local sources in the Arunta and Musgrave inliers. Close similarities between the detrital age signatures of late Neoproterozoic sedimentary rocks in the two basins suggests that they were contiguous at this time. A dominant population of 1.2–1.0 Ga zircon in Early Cambrian sediments of the Amadeus Basin reflects the uplift of the Musgrave Inlier during the Petermann Orogeny between 560 and 520 Ma, which shed a large volume of detritus northwards into the Amadeus Basin. Early Cambrian sedimentary rocks in the Georgina Basin have a much smaller proportion of 1.2–1.0 Ga detritus, possibly due to the formation of sub‐basins along the northern margin of the Amadeus Basin which might have acted as a barrier to sediment transfer. An influx of 0.6–0.5 Ga zircon towards the end of the Cambrian coincides with the transgression of the Larapintine Sea across central Australia, possibly as a result of intracratonic rifting. Detrital zircon age spectra of sedimentary rocks deposited within this epicontinental sea are very similar to those of coeval sedimentary rocks from the Pacific Gondwana margin, implying that sediment was transported into central Australia from the eastern continental margin. The remarkably consistent ‘Pacific Gondwana’ signature of Cambro‐Ordovician sediments in central and eastern Australia reflects a distal source, possibly from east Antarctica or the East African Orogen. The peak of the marine incursion into central Australia in the early to mid Ordovician coincides with granulite‐facies metamorphism at mid‐crustal depths between the Amadeus and Georgina basins (the Larapinta Event). The presence of the epicontinental sea, the relative lack of a local basement zircon component in Cambro‐Ordovician sedimentary rocks and their maturity suggest that metamorphism was not accompanied by mountain building, consistent with an extensional or transtensional setting for this tectonism. Sediments deposited at ～435–405 and ～365 Ma during the Alice Springs Orogeny have detrital age signatures similar to those of Cambro‐Ordovician sedimentary rocks, reflecting uplift and reworking of the older succession into narrow foreland basins adjacent to the orogen.     

Interplay of tectonics and sea-level changes in basin evolution: an example from the intracratonic Amadeus Basin, central Australia

Abstract The Amadeus Basin, a broad intracratonic depression (800 times 300 km) in central Australia, contains a complex Late Proterozoic to mid-Palaeozoic depositional succession which locally reaches 14 km in thickness. The application of sequence stratigraphy to this succession has provided an effective framework in which to evaluate its evolution. Analysis of major depositional sequences shows that the Amadeus Basin evolved in three stages. Stage 1 began at about 900 Myr with extensional thinning of the crust and formation of half-grabens. Thermal recovery following extension was well advanced when a second less intense crustal extension (stage 2) occurred towards the end of the Late Proterozoic. Stage 2 thermal recovery was followed by a major compressional event (stage 3) in which major southward-directed thrust sheets caused progressive downward flexing of the northern margin of the basin, and sediment was shed from the thrust sheets into the downwarps forming a foreland basin. This event shortened the basin by 50–100 km and effectively concluded sedimentation. The two stages of crustal extension and thermal recovery produced large-scale apparent sea-level effects upon which eustatic sea-level cycles are superimposed. Since the style of sedimentation and major sequence boundaries were controlled to a large degree by basin dynamics, depositional patterns within the Amadeus and associated basin are, to a large degree, predictable. This suggests that an understanding of major variables associated with basin dynamics and their relationship to depositional sequences may allow the development of generalized depositional models on a basinal scale. The Amadeus Basin is only one of a number of broad, shallow, intracratonic depressions that appeared on the Australian craton during the Late Proterozoic. The development of these basins almost certainly relates to the breakup of a Proterozoic supercontinent and in large part, basin dynamics appears to be tied to this global tectonic event. Onlap and apparent sea-level curves derived from the sequence analysis appear to be composite curves resulting from both basin dynamics and eustatic sea-level effects. It thus appears likely that sequence stratigraphy could be used as a basis for inter-regional correlation; a possibility that has considerable significance in Archaean and Proterozoic basins.     

Gravity anomalies, subsidence history and the tectonic evolution of the Malay and Penyu Basins (offshore Peninsular Malaysia)

The tectonic subsidence and gravity anomalies in the Malay and Penyu Basins, offshore Peninsular Malaysia, were analysed to determine the isostatic compensation mechanism in order to investigate their origin. These continental extensional basins contain up to 14  km of sediment fill which implies that the crust had been thinned significantly during basin development. Our results suggest, however, that the tectonic subsidence in the basins cannot be explained simply by crustal thinning and Airy isostatic compensation.
The Malay and Penyu Basins are characterized by broad negative free-air gravity anomalies of between −20 and −30  mGal. To determine the cause of the anomaly, we modelled four gravity profiles across the basins using a method that combines two-dimensional flexural backstripping and gravity modelling techniques. We assumed a model of uniform lithospheric stretching and Airy isostasy in the analysis of tectonic subsidence. Our study shows that the basins are probably underlain by relatively thinned crust, indicating that some form of crustal stretching was involved. To explain the observed gravity anomalies, however, the Moho depth that we calculated based on the free-air gravity data is about 25% deeper than the Moho predicted by assuming Airy isostasy (Backstrip Moho). This suggests that the Airy model overestimates the compensation and that the basins are probably undercompensated isostatically. In other words, there is an extra amount of tectonic subsidence that is not compensated by crustal thinning, which has resulted in the discrepancy between the gravity-derived Moho and the Backstrip Moho. We attribute this uncompensated or anomalous tectonic subsidence to thin-skinned crustal extension that did not involve the mantle lithosphere. The Malay and Penyu Basins are interpreted therefore as basins that formed by a combination of whole-lithosphere stretching and thin-skinned crustal extension.     

The evolution of the high‐elevated depocenters of the northern Sierras Pampeanas (ca. 28˚ SL), Argentine broken foreland,South‐Central Andes: the Pipanaco Basin

The Pipanaco Basin, in the southern margin of the Andean Puna plateau at ca. 28°SL, is one of the largest and highest intermontane basins within the northernmost Argentine broken foreland. With a surface elevation >1000 m above sea level, this basin represents a strategic location to understand the subsidence and subsequent uplift history of high‐elevation depositional surfaces within the distal Andean foreland. However, the stratigraphic record of the Pipanaco Basin is almost entirely within the subsurface, and no geophysical surveys have been conducted in the region. A high‐resolution gravity study has been designed to understand the subsurface basin geometry. This study, together with stratigraphic correlations and flexural and backstripping analysis, suggests that the region was dominated by a regional subsidence episode of ca. 2 km during the Miocene‐Pliocene, followed by basement thrusting and ca. 1–1.5 km of sediment filling within restricted intermontane basin between the Pliocene‐Pleistocene. Based on the present‐day position of the basement top as well as the Neogene‐Present sediment thicknesses across the Sierras Pampeanas, which show slight variations along strike, sediment aggradation is not the most suitable process to account for the increase in the topographic level of the high‐elevation, close‐drainage basins of Argentina. The close correlation between the depth to basement and the mean surface elevations recorded in different swaths indicates that deep‐seated geodynamic process affected the northern Sierras Pampeanas. Seismic tomography, as well as a preliminary comparison between the isostatic and seismic Moho, suggests a buoyant lithosphere beneath the northern Sierras Pampeanas, which might have driven the long‐wavelength rise of this part of the broken foreland after the major phase of deposition in these Andean basins.     

The formation of a failed continental breakup basin: The Cenozoic development of the Faroe‐Shetland Basin

Ultra‐large rift basins, which may represent palaeo‐propagating rift tips ahead of continental rupture, provide an opportunity to study the processes that cause continental lithosphere thinning and rupture at an intermediate stage. One such rift basin is the Faroe‐Shetland Basin (FSB) on the north‐east Atlantic margin. To determine the mode and timing of thinning of the FSB, we have quantified apparent upper crustal β‐factors (stretching factors) from fault heaves and apparent whole‐lithosphere β‐factors by flexural backstripping and decompaction. These observations are compared with models of rift basin formation to determine the mode and timing of thinning of the FSB. We find that the Late Jurassic to Late Palaeocene (pre‐Atlantic) history of the FSB can be explained by a Jurassic to Cretaceous depth‐uniform lithosphere thinning event with a β‐factor of ~1.3 followed by a Late Palaeocene transient regional uplift of 450–550 m. However, post‐Palaeocene subsidence in the FSB of more than 1.9 km indicates that a Palaeocene rift with a β‐factor of more than 1.4 occurred, but there is only minor Palaeocene or post‐Palaeocene faulting (upper crustal β‐factors of less than 1.1). The subsidence is too localized within the FSB to be caused by a regional mantle anomaly. To resolve the β‐factor discrepancy, we propose that the lithospheric mantle and lower crust experienced a greater degree of thinning than the upper crust. Syn‐breakup volcanism within the FSB suggests that depth‐dependent thinning was synchronous with continental breakup at the adjacent Faroes and Møre margins. We suggest that depth‐dependent continental lithospheric thinning can result from small‐scale convection that thins the lithosphere along multiple offset axes prior to continental rupture, leaving a failed breakup basin once seafloor spreading begins. This study provides insight into the structure and formation of a generic global class of ultra‐large rift basins formed by failed continental breakup.     

Late Triassic tectonic inversion in the upper Yangtze Block: Insights from detrital zircon U–Pb geochronology from south‐western Sichuan Basin

The Sichuan Basin and the Songpan‐Ganze terrane, separated by the Longmen Shan fold‐and‐thrust belt (the eastern margin of the Tibetan Plateau), are two main Triassic depositional centres, south of the Qinling‐Dabie orogen. During the Middle–Late Triassic closure of the Paleo‐Tethys Ocean, the Sichuan Basin region, located at the western margin of the Yangtze Block, transitioned from a passive continental margin into a foreland basin. In the meantime, the Songpan‐Granze terrane evolved from a marine turbidite basin into a fold‐and‐thrust belt. To understand if and how the regional sediment routing system adjusted to these tectonic changes, we monitored sediment provenance primarily by using detrital zircon U‐Pb analyses of representative stratigraphic samples from the south‐western edge of the Sichuan Basin. Integration of the results with paleocurrent, sandstone petrology and published detrital zircon data from other parts of the basin identified a marked change in provenance. Early–Middle Triassic samples were dominated by Neoproterozoic (~700–900 Ma) zircons sourced mainly from the northern Kangdian basement, whereas Late Triassic sandstones that contain a more diverse range of zircon ages sourced from the Qinling, Longmen Shan and Songpan‐Ganze terrane. This change reflects a major drainage adjustment in response to the Late Triassic closure of the Paleo‐Tethys Ocean and significant shortening in the Longmen Shan thrust belt and the eastern Songpan‐Ganze terrane. Furthermore, by Late Triassic time, the uplifted northern Kangdian basement had subsided. Considering the eastward paleocurrent and depocenter geometry of the Upper Triassic deposits, subsidence of the northern Kangdian basement probably resulted from eastward shortening and loading of the Songpan‐Ganze terrane over the western margin of the Yangtze Block in response to the Late Triassic collision among Yangtze Block, Yidun arc and Qiangtang terrane along the Ganze‐Litang and Jinshajiang sutures.     

Pro- vs. retro-foreland basins

Alpine‐type mountain belts formed by continental collision are characterised by a strong cross‐sectional asymmetry driven by the dominant underthrusting of one plate beneath the other. Such mountain belts are flanked on either side by two peripheral foreland basins, one over the underthrust plate and one over the over‐riding plate; these have been termed pro‐ and retro‐foreland basins, respectively. Numerical modelling that incorporates suitable tectonic boundary conditions, and models orogenesis from growth to a steady‐state form (i.e. where accretionary influx equals erosional outflux), predicts contrasting basin development to these two end‐member basin types. Pro‐foreland basins are characterised by: (1) Accelerating tectonic subsidence driven primarily by the translation of the basin fill towards the mountain belt at the convergence rate. (2) Stratigraphic onlap onto the cratonic margin at a rate at least equal to the plate convergence rate. (3) A basin infill that records the most recent development of the mountain belt with a preserved interval determined by the width of the basin divided by the convergence rate. In contrast, retro‐foreland basins are relatively stable, are not translated into the mountain belt once steady‐state is achieved, and are consequently characterised by: (1) A constant tectonic subsidence rate during growth of the thrust wedge, with zero tectonic subsidence during the steady‐state phase (i.e. ongoing accretion‐erosion, but constant load). (2) Relatively little stratigraphic onlap driven only by the growth of the retro‐wedge. (3) A basin fill that records the entire growth phase of the mountain belt, but only a condensed representation of steady‐state conditions. Examples of pro‐foreland basins include the Appalachian foredeep, the west Taiwan foreland basin, the North Alpine Foreland Basin and the Ebro Basin (southern Pyrenees). Examples of retro‐foreland basins include the South Westland Basin (Southern Alps, New Zealand), the Aquitaine Basin (northern Pyrenees), and the Po Basin (southern European Alps). We discuss how this new insight into the variability of collisional foreland basins can be used to better interpret mountain belt evolution and the hydrocarbon potential of these basins types.     

Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction

This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.     

Models of the rapid post‐rift subsidence in the eastern Qiongdongnan Basin,South China Sea: implications for the development of the deep thermal anomaly

The Qiongdongnan Basin is one of the largest Cenozoic rifted basins on the northern passive margin of the South China Sea. It is well known that since the Late Miocene, approximately 10 Ma after the end of the syn‐rift phase, this basin has exhibited rapid thermal subsidence. However, detailed analysis reveals a two‐stage anomalous subsidence feature of the syn‐rift subsidence deficit and the well‐known rapid post‐rift subsidence after 10.5 Ma. Heat‐flow data show that heat flow in the central depression zone is 70–105 mW m^?2, considerably higher than the heat flow (<70 mW m^?2) on the northern shelf. In particular, there is a NE‐trending high heat‐flow zone of >85 mW m^?2 in the eastern basin. We used a numerical model of coupled geothermal processes, lithosphere thinning and depositional processes to analyse the origin of the anomalous subsidence pattern. Numerical analysis of different cases shows that the stretching factor β_s based on syn‐rift sequences is less than the observed crustal stretching factor β_c, and if the lithosphere is thinned with β_c during the syn‐rift phase (before 21 Ma), the present basement depth can be predicted fairly accurately. Further analysis does not support crustal thinning after 21 Ma, which indicates that the syn‐rift subsidence is in deficit compared with the predicted subsidence with the crustal stretching factor β_c. The observed high heat flow in the central depression zone is caused by the heating of magmatic injection equivalently at approximately 3–5 Ma, which affected the eastern basin more than the western basin, and the Neogene magmatism might be fed by the deep thermal anomaly. Our results suggest that the causes of the syn‐rift subsidence deficit and rapid post‐rift subsidence might be related. The syn‐rift subsidence deficit might be caused by the dynamic support of the influx of warmer asthenosphere material and a small‐scale thermal upwelling beneath the study area, which might have been persisting for about 10 Ma during the early post‐rift phase, and the post‐rift rapid subsidence might be the result of losing the dynamic support with the decaying or moving away of the deep thermal source, and the rapid cooling of the asthenosphere. We concluded that the excess post‐rift subsidence occurs to compensate for the syn‐rift subsidence deficit, and the deep thermal anomaly might have affected the eastern Qiongdongnan Basin since the Late Oligocene.     

Numerical models of a subsidence mechanism in intracratonic basins: application to North American basins

McKenzie's model of sedimentary basin evolution and its modification, widely used in geophysics, sometimes fails to explain discrepancies between predicted and observed values of extension, thinning and subsidence of the Earth's crust, as for the North Sea. We develop a numerical model of sedimentary basin evolution based on the mechanism suggested by Lobkovsky. In the course of rifting, accompanied by thinning of lower parts of the lithosphere, the roof of the underlying asthenosphere moves upward. the material of the mantle lifts and partially melts owing to the reduction of pressure. the density difference between the melt and the crystalline skeleton results in the filtration of the lighter melt and its accumulation in the form of a magmatic lens. Due to changed P-T conditions, the material of the lens undergoes the gabbro-eclogite phase transformation. the resultant anomalously heavy eclogite lens sinks in the surrounding material. This induces a viscous flow, changing the surface topography and forming a sedimentary basin. We construct a 2-D numerical model describing a viscous flow induced by subsidence of a heavy body and compute changes of surface topography. to compute the flow we employ the Galerkin-spline approach, with modifications allowing for density discontinuities and time dependence of the phase transformation. We apply the model to the cases of the Illinois, Michigan and Williston basins. the computed and tectonic subsidence curves agree well for these cases. the proposed model is compatible with the seismic structure of the crust and upper mantle below these basins. the model is also consistent with gravity data. the approach is applicable to other intracratonic basins.     

Teleseismic delay-time tomography of the upper mantle beneath southeastern India: imprint of Indo–Antarctica rifting

A 3-D P -velocity map of the crust and upper mantle beneath the southeastern part of India has been reconstructed through the inversion of teleseismic traveltimes. Salient geological features in the study region include the Archean Dharwar Craton and Eastern Ghat metamorphic belt (EGMB), and the Proterozoic Cuddapah and Godavari basins. The Krishna–Godavari basin, on the eastern coastal margin, evolved in response to the Indo–Antarctica breakup. A 24-station temporary network provided 1161 traveltimes, which were used to model 3-D P -velocity variation. The velocity model accounts of 80 per cent of the observed data variance. The velocity picture to a depth of 120 km shows two patterns: a high velocity beneath the interior domain (Dharwar craton and Cuddapah basin), and a lower velocity beneath the eastern margin region (EGMB and coastal basin). Across the array velocity variations of 7–10 per cent in the crust (0–40 km) and 3–5 per cent in the uppermost mantle (40–120 km) are observed. At deeper levels (120–210 km) the upper-mantle velocity differences are insignificant among different geological units. The presence of such a low velocity along the eastern margin suggests significantly thin lithosphere (<100 km) beneath it compared to a thick lithosphere (>200 km) beneath the eastern Dharwar craton. Such lithospheric thinning could be a consequence of Indo–Antarctica break-up.     

Modelling rates and distribution of subsidence due to dynamic topography over subducting slabs: is it possible to identify dynamic topography from ancient strata?

Dynamic topography formed over subducting oceanic lithosphere has been proposed as a mechanism to explain certain otherwise anomalous long-wavelength patterns of subsidence inferred from ancient strata. Forward modelling of mantle flow in response to a subducting slab predicts amplitudes and distributions of dynamic topography that may occur due to various subducting slab geometries and histories. Plotting calculated dynamic topographies at a point against time produces tectonic subsidence curves. These subsidence curves show features such as evolution from convex to concave shape, amplitudes up to ~2000 m, subsidence rates up to ~220 m Myr⁻¹, and a general decrease in subsidence amplitude away from the subduction zone, over a distance of ~2000 km. On many convergent continental margins, dynamic topography is likely to be superimposed on other subsidence mechanisms. In back-arc basins, subsidence due to dynamic topography should be distinguishable from that due to extensional tectonics based simply on the temporal subsidence evolution expressed in the subsidence curve shapes. In a foreland basin setting, comparing dynamic topography models with forward models of flexural loading suggest the two processes can generate similar temporal subsidence patterns, but that dynamic topography causes subsidence over significantly greater wavelengths. Matches between calculated subsidence due to dynamic topography and backstripped subsidence patterns from Upper Cretaceous strata of the Western Interior Basin, USA, support the hypothesis that a long-wavelength 'background subsidence' was caused by dynamic topography.     

Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region

Seismic reflection profiles and well data are used to determine the Cenozoic stratigraphic and tectonic development of the northern margin of the South China Sea. In the Taiwan region, this margin evolved from a Palaeogene rift to a latest Miocene–Recent foreland basin. This evolution is related to the opening of the South China Sea and its subsequent partial closure by the Taiwan orogeny. Seismic data, together with the subsidence analysis of deep wells, show that during rifting (～58–37 Ma), lithospheric extension occurred simultaneously in discrete rift belts. These belts form a >200 km wide rift zone and are associated with a stretching factor, β, in the range ～1.4–1.6. By ～37 Ma, the focus of rifting shifted to the present‐day continent–ocean boundary off southern Taiwan, which led to continental rupture and initial seafloor spreading of the South China Sea at ～30 Ma. Intense rifting during the rift–drift transition (～37–30 Ma) may have induced a transient, small‐scale mantle convection beneath the rift. The coeval crustal uplift (Oligocene uplift) of the previously rifted margin, which led to erosion and development of the breakup unconformity, was most likely caused by the induced convection. Oligocene uplift was followed by rapid, early post‐breakup subsidence (～30–18 Ma) possibly as the inferred induced convection abated following initial seafloor spreading. Rapid subsidence of the inner margin is interpreted as thermally controlled subsidence, whereas rapid subsidence in the outer shelf of the outer margin was accompanied by fault activity during the interval ～30–21 Ma. This extension in the outer margin (β～1.5) is manifested in the Tainan Basin, which formed on top of the deeply eroded Mesozoic basement. During the interval ～21–12.5 Ma, the entire margin experienced broad thermal subsidence. It was not until ～12.5 Ma that rifting resumed, being especially active in the Tainan Basin (β～1.1). Rifting ceased at ～6.5 Ma due to the orogeny caused by the overthrusting of the Luzon volcanic arc. The Taiwan orogeny created a foreland basin by loading and flexing the underlying rifted margin. The foreland flexure inherited the mechanical and thermal properties of the underlying rifted margin, thereby dividing the basin into north and south segments. The north segment developed on a lithosphere where the major rift/thermal event occurred ～58–30 Ma, and this segment shows minor normal faulting related to lithospheric flexure. In contrast, the south segment developed on a lithosphere, which experienced two more recent rift/thermal events during ～30–21 and ～12.5–6.5 Ma. The basal foreland surface of the south segment is highly faulted, especially along the previous northern rifted flank, thereby creating a deeper foreland flexure that trends obliquely to the strike of the orogen.     

Tectonic and sedimentological evolution of the Pliocene–Quaternary basins of Zakynthos island, Greece: case study of the transition from compressional to extensional tectonics

Pliocene–Quaternary basins of the Ionian islands evolved in a complex tectonic setting that evolved from a mid to late Cenozoic compressional zone of the northern external Hellenides to the rapidly extending Pliocene–Quaternary basins of the Peloponnese. The northern limit of the Hellenic Trench marks the junction of these two tectonic regimes. A foreland-propagating fold and thrust system in the northern external Hellenides segmented the former Miocene continental margin basin in Zakynthos and permitted diapiric intrusion of Triassic gypsum along thrust ramps. Further inboard, coeval extensional basins developed, with increasing rates of subsidence from the Pliocene to Quaternary, resulting in four principal types of sedimentation: (1) condensed shelf-sedimentation on the flanks of rising anticlines; (2) coarse-grained sedimentation in restricted basins adjacent to evaporitic diapirs rising along thrust ramps; (3) larger basins between fold zones were filled by extrabasinal, prodeltaic mud and sand from the proto-Acheloos river; (4) margins of subsiding Quaternary basins were supplied at sea-level highstands by distal deltaic muds and at lowstands by locally derived coarse clastic sediment.     

Quantitative analysis of Miocene to Recent forearc basin evolution along the Colombian convergent margin

The Colombian accretionary complex forms the active convergent margin of the North Andes block of South America beneath which the east Panama Basin of the Nazca plate is subducted at a rate of 50–64 km Myr^?1. Multichannel seismic reflection data, collected as part of RRS Charles Darwin cruise CD40, image a series of well-developed forearc basins along the length of the margin, bounded on their oceanward side by an active accretionary complex and on their landward side by oceanward-dipping continental basement. Sedimentary sequences within the forearc basins indicate successive landward migration of the basin depocentre as the structural high bounding its oceanward edge is forced upward and landward by continued growth of the accretionary complex. Uplift beneath the oceanward side of the basins has resulted in progressive landward rotation of the older sedimentary sequences. Prominent seismic reflectors across the basins show a complex onlap–offlap relationship between successive sequences that reflects the interplay between tectonic uplift, sediment supply, differential sediment compaction and basement subsidence due to loading. A numerical model has been devised to investigate how Miocene to Recent forearc basin stratigraphy is controlled by progressive growth of the accretionary complex resulting in elevation of the outer-arc high and landward motion of the rear of the complex up the seaward-dipping backstop formed by the leading edge of the continental lithosphere. The effects of sediment accretion are modelled by treating the accretionary complex as a doubly vergent, noncohesive Coulomb wedge, where forces exerted by the proto- and retro-wedges must be balanced for the system to be in equilibrium. The model generates synthetic basin-fill architecture over a series of steps, each of which represents the deposition of individual sedimentary sequences and their subsequent deformation due to wedge growth. The model accounts for differential sediment compaction and the flexural response of the underlying lithosphere to changes in load distribution over time. Forearc basin evolution is simulated for various rates of sediment supply to the forearc and accretionary complex growth until the synthetic basin-fill geometry matches the observed geometry. The model enables either the rate of accretionary wedge growth or the rate of sediment supply to the forearc basin to be established. The technique is generally applicable to those convergent margins with forearc basins that have developed between an actively accreting wedge and a seaward-dipping backstop. Other examples include Peru, S. Chile, Sumatra and Barbados.     

Crust‐scale 3D model of the Western Bredasdorp Basin (Southern South Africa): data‐based insights from combined isostatic and 3D gravity modelling

The southern South African continental margin documents a complex margin system that has undergone both continental rifting and transform processes in a manner that its present‐day architecture and geodynamic evolution can only be better understood through the application of a multidisciplinary and multi‐scale geo‐modelling procedure. In this study, we focus on the proximal section of the larger Bredasdorp sub‐basin (the westernmost of the five southern South African offshore Mesozoic sub‐basins), which is hereto referred as the Western Bredasdorp Basin. Integration of 1200 km of 2D seismic‐reflection profiles, well‐logs and cores yields a consistent 3D structural model of the Upper Jurassic‐Cenozoic sedimentary megasequence comprising six stratigraphic layers that represent the syn‐rift to post‐rift successions with geometric information and lithology‐depth‐dependent properties (porosities and densities). We subsequently applied a combined approach based on Airy's isostatic concept and 3D gravity modelling to predict the depth to the crust‐mantle boundary (Moho) as well as the density structure of the deep crust. The best‐fit 3D model with the measured gravity field is only achievable by considering a heterogeneous deep crustal domain, consisting of an uppermost less dense prerift meta‐sedimentary layer [ρ = 2600 kg m^?3] with a series of structural domains. To reproduce the observed density variations for the Upper Cenomanian–Cenozoic sequence, our model predicts a cumulative eroded thickness of ca. 800–1200 m of Tertiary sediments, which may be related to the Late Miocene margin uplift. Analyses of the key features of the first crust‐scale 3D model of the basin, ranging from thickness distribution pattern, Moho shallowing trend, sub‐crustal thinning to shallow and deep crustal extensional regimes, suggest that basin initiation is typical of a mantle involvement deep‐seated pull‐apart setting that is associated with the development of the Agulhas‐Falkland dextral shear zone, and that the system is not in isostatic equilibrium at present day due to a mass excess in the eastern domain of the basin that may be linked to a compensating rise of the asthenospheric mantle during crustal extension. Further corroborating the strike‐slip setting is the variations of sedimentation rates through time. The estimated syn‐rift sedimentation rates are three to four times higher than the post‐rift sedimentation, thereby indicating that a rather fast and short‐lived subsidence during the syn‐rift phase is succeeded by a significantly poor passive margin development in the post‐rift phase. Moreover, the derived lithospheric stretching factors [β = 1.5–1.75] for the main basin axis do not conform to the weak post‐rift subsidence. This therefore suggests that a differential thinning of the crust and the mantle‐lithosphere typical for strike‐slip basins, rather than the classical uniform stretching model, may be applicable to the Western Bredasdorp Basin.     

The effects of the spinel—garnet phase transition on the formation of rifted sedimentary basins

We have examined the effects of the spinel-garnet phase transition on subsidence of extensional sedimentary basins. For a constant positive Clapeyron slope ( dP/dT ), the phase boundary moves downwards in the syn-rift and upwards in the post-rift phase. For a non-linear Clapeyron curve ( dP/dT > 0 above 900°C and dP/dT < 0 below 900°C), theory predicts for the reaction of the spinel-garnet phase transition, the direction of phase boundary movement is dependent on the stretching factor, the position of the Clapeyron curve and the lithospheric thickness. A smaller syn-rift and larger post-rift subsidence are predicted for a deeper phase boundary and a thicker lithosphere. The model with a non-linear Clapeyron curve is applied to the subsidence histories of a young extensional basin (Gulf of Lion) and an old continental margin (eastern Canada). The observed syn-rift uplift and the larger post-rift subsidence can be reasonably explained by this model, where the optimum depth of the phase boundary for eastern Canada (˜90 km) is consistent with the estimate from seismic observations and is larger than that for the Gulf of Lion (˜ 50 km). The depth of the spinel-garnet phase boundary is sensitive to the composition of mantle rocks and increases with the extraction of basaltic components from the lithosphere, compatible with our result that the phase boundary is deeper for an older and thicker lithosphere. Thus the surface movement associated with the rifting for these areas may reflect the chemical evolution of the continental lithosphere.     

Supersequences, superbasins, supercontinents – evidence from the Neoproterozoic–Early Palaeozoic basins of central Australia

Neoproterozoic sedimentary basins cover a large area of central Australia. They rest upon rigid continental crust that varies from c. 40–50 km in thickness. Whilst the crust was in part formed during the Archaean and early Palaeoproterozoic, its final assembly occurred at approximately 1.1 Ga as the Neoproterozoic supercontinent, Rodinia, came into being. The assembly process left an indelible imprint on the region producing a strong crustal fabric in the form of a series of north dipping thrusts that pervade much of the thick craton and extend almost to the Moho. Following a period of stability (1.1–0.8 Ga), a large area of central Australia, in excess of 2.5 × 10⁶ km², began to subside in synchroneity. This major event was due to mantle instability resulting from the insulating effect of Rodinia. Initially, beginning c. 900 Ma, a rising superplume uplifted much of central Australia leading to peneplanation of the uplifted region and the generation of large volumes of sand‐sized clastic materials. Ultimately, the decline of the superplume led to thermal recovery and the development of a sag basin (beginning at c. 800 Ma), which in turn resulted in the redistribution of the clastic sediments and the development of a vast sand sheet at the base of the Neoproterozoic succession. The superbasin generated by the thermal recovery was short lived (c. 20 M.y.) but, in conjunction with the crustal fabric developed during supercontinent assembly, it set the stage for further long‐term basin development that extended for half a billion years well into the Late Palaeozoic. Following the sag phase at least five major tectonic episodes influenced the central Australian region. Compressional tectonics reactivated earlier thrust faults that had remained dormant within the crust, disrupting the superbasin, causing uplift of basement blocks and breaking the superbasin into the four basins now identified within the central Australian Neoproterozoic succession (Officer, Amadeus, Ngalia and Georgina Basin). These subsequent tectonic events produced the distinctive foreland architecture associated with the basins and were perhaps the trigger for the Neoproterozoic ice ages. The reactivated basins became asymmetric with major thrust faults along one margin paralleled by deep narrow troughs that formed the main depocentres for the remaining life of the basins. The final major tectonic event to influence the central Australian basins, the Alice Springs Orogeny, effectively terminated sedimentation in the region in the Late Palaeozoic (c. 290 Ma). Of the six tectonic episodes recorded in the basinal succession only one provides evidence of extension, suggesting the breakup of east Gondwana at the end of the Rodinian supercontinent cycle may have occurred at close to the time of the Precambrian–Cambrian boundary. The central Australian basins are thus the products of events surrounding the assembly and dispersal of Rodinia.