Anomalous passive subsidence of deep‐water sedimentary basins: a prearc basin example,southern New Caledonia Trough and Taranaki Basin,New Zealand

Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.     

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.     

The synrift phase of the early Domeyko Basin (Triassic,northern Chile): Sedimentary,volcanic, and tectonic interplay in the evolution of an ancient subduction‐related rift basin

The geodynamic setting along the SW Gondwana margin during its early breakup (Triassic) remains poorly understood. Recent models calling for an uninterrupted subduction since Late Palaeozoic only slightly consider the geotectonic significance of coeval basins. The Domeyko Basin initiated as a rift basin during the Triassic being filled by sedimentary and volcanic deposits. Stratigraphic, sedimentological, and geochronological analyses are presented in order to determine the tectonostratigraphic evolution of this basin and to propose a tectonic model suitable for other SW Gondwana‐margin rift basins. The Domeyko Basin recorded two synrift stages. The Synrift I (~240–225 Ma) initiated the Sierra Exploradora sub‐basin, whereas the Synrift II (~217–200 Ma) reactivated this sub‐basin and originated small depocentres grouped in the Sierra de Varas sub‐basin. During the rift evolution, the sedimentary systems developed were largely controlled by the interplay between tectonics and volcanism through the accommodation/sediment supply ratio (A/S). High‐volcaniclastic depocentres record a net dominance of the syn‐eruptive period lacking rift‐climax sequences, whereas low‐volcaniclastic depocentres of the Sierra de Varas sub‐basin developed a complete rift cycle during the Synrift II stage. The architecture of the Domeyko Basin suggests a transtensional kinematic where N‐S master faults interacted with ~NW‐SE basement structures producing highly asymmetric releasing bends. We suggest that the early Domeyko Basin was a continental subduction‐related rift basin likely developed under an oblique convergence in a back‐arc setting. Subduction would have acted as a primary driving mechanism for the extension along the Gondwanan margin, unlike inland rift basins. Slab‐induced dynamic can strongly influence the tectonostratigraphic evolution of subduction‐related rift basins through controls in the localization and style of magmatism and faulting, settling the interplay between tectonics, volcanism, and sedimentation during the rifting.     

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.     

Constraining provenance,thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon,New Zealand

The Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance >200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region.     

Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi-disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data

Four Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA^®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km^?1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.     

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.     

Post‐orogenic thermal history and exhumation of the northern Appalachian Basin: Low‐temperature thermochronologic constraints

Apatite fission‐track (AFT) thermochronology and (U‐Th)/He (AHe) dating, combined with paleothermometers and independent geologic constraints, are used to model the thermal history of Devonian Catskill delta wedge strata. The timing and rates of cooling determines the likely post‐orogenic exhumation history of the northern Appalachian Foreland Basin (NAB) in New York and Pennsylvania. AFT ages generally young from west to east, decreasing from ~185 to 120 Ma. AHe single‐grain ages range from ~188 to 116 Ma. Models show that this part of the Appalachian foreland basin experienced a non‐uniform, multi‐stage cooling history. Cooling rates vary over time, ~1–2 °C/Myr in the Early Jurassic to Early Cretaceous, ~0.15–0.25 °C/Myr from the Early Cretaceous to Late Cenozoic, and ~1–2 °C/Myr beginning in the Miocene. Our results from the Mesozoic are broadly consistent with earlier studies, but with the integration of multiple thermochronometers and multi‐kinetic annealing algorithms in newer inverse thermal modeling programs, we constrain a Late Cenozoic increase in cooling which had been previously enigmatic in eastern U.S. low‐temperature thermochronology datasets. Multi‐stage cooling and exhumation of the NAB is driven by post‐orogenic basin inversion and catchment drainage reorganization, in response to changes in base level due to rifting, plus isostatic and dynamic topographic processes modified by flexure over the long (~200 Myr) post‐orogenic period. This study compliments other regional exhumation data‐sets, while constraining the timing of post‐orogenic cooling and exhumation in the NAB and contributing important insights on the post‐orogenic development and inversion of foreland basins along passive margins.     

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.     

Microbial‐dominated carbonate platforms during the Ladinian rifting: sequence stratigraphy and evolution of accommodation in a fault‐controlled setting (Catalan Coastal Ranges,NE Spain)

The Upper Muschelkalk sedimentary record constitutes a major transgressive pulse of north‐eastern Iberia during the Ladinian. This record is arranged in two transgressive–regressive (T–R) sequences formed by two stepped microbial‐dominated carbonate ramp systems where accommodation was mainly controlled by extensional faults. This study seeks to gain new insights into how the evolution of syn‐rift subsidence controls the creation of accommodation space, the depositional styles and, especially, the palaeogeographical domains where specific microbialites developed (thrombolites and stromatolites). Thrombolite bodies (at least 40 m thick) display two types of architecture, biostromal and mud‐mounded and stromatolite bodies (at least 7 m thick) consist of tabular and domed, head‐shaped morphologies. Domed and mounded forms are usually developed during stages of increasing accommodation rates, low‐to flat‐nelief forms tend to grow in association with periods of low accommodation rates. A sea‐level fall of at least 50 m occurred at the end of the Early Ladinian leaving the platform subaerially exposed. As a result, a prominent karst with significant erosional incisions and profuse collapse breccia fillings was formed in the inner and middle ramp settings. The resultant subaerial unconformity bounds T–R sequences 1 and 2. Subsidence curves display two stages of rapid/decelerated total subsidence, constituting two discrete rift/post‐rift pulses in the large Triassic rifting period: (i) Buntsandstein – Middle Muschelkalk, and (ii) Late Muschelkalk – Imon Formation (Rhaetian). The second pulse is characterized by a rapid syn‐rift subsidence during the Late Muschelkalk, and a decelerated post‐rift subsidence throughout the deposition of Keuper facies and Imon Formation. The Late Muschelkalk rapid syn‐rift pulse of total subsidence produces gains in accommodation, which controls the development of the stromatolites and thrombolites (biostromes and mud‐mounds).     

Development of sedimentary basins: differential stretching,phase transitions,shear heating and tectonic pressure

Classical models of lithosphere thinning predict deep synrift basins covered by wider and thinner post‐rift deposits. However, synextensional uplift and/or erosion of the crust are widely documented in nature (e.g. the Base Cretaceous unconformity of the NE Atlantic), and generally the post‐rift deposits dominate basins fills. Accordingly, several basin models focus on this discrepancy between observations and the classical approach. These models either involve differential thinning, where the mantle thins more than the crust thereby increasing average temperature of the lithosphere, or focus on the effect of metamorphic reactions, showing that such reactions decrease the density of lithospheric rocks. Both approaches result in less synrift subsidence and increased post‐rift subsidence. The synextensional uplift in these two approaches happens only for special cases, that is for a case of initially thin crust, specific mineral assemblage of the lithospheric mantle or extensive differential thinning of the lithosphere. Here, we analyse the effects of shear heating and tectonic underpressure on the evolution of sedimentary basins. In simple 1D models, we test the implications of various mechanisms in regard to uplift, subsidence, density variations and thermal history. Our numerical experiments show that tectonic underpressure during lithospheric thinning combined with pressure‐dependent density is a widely applicable mechanism for synextensional uplift. Mineral phase transitions in the subcrustal lithosphere amplify the effect of underpressure and may result in more than 1 km of synextensional erosion. Additional heat from shear heating, especially combined with mineral phase transitions and differential thinning of the lithosphere, greatly decreases the amount of synrift deposits.     

The role of pre‐existing structures during rifting,continental breakup and transform system development,offshore West Greenland

Continental breakup between Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, which are connected via the Davis Strait, a region mostly comprised of continental crust. This study contributes to the debate regarding the role of pre‐existing structures on rift development in this region using seismic reflection data from the Davis Strait data to produce a series of seismic surfaces, isochrons and a new offshore fault map from which three normal fault sets were identified as (i) NE‐SW, (ii) NNW‐SSE and (iii) NW‐SE. These results were then integrated with plate reconstructions and onshore structural data allowing us to build a two‐stage conceptual model for the offshore fault evolution in which basin formation was primarily controlled by rejuvenation of various types of pre‐existing structures. During the first phase of rifting between at least Chron 27 (ca. 62 Ma; Palaeocene), but potentially earlier, and Chron 24 (ca. 54 Ma; Eocene) faulting was primarily controlled by pre‐existing structures with oblique normal reactivation of both the NE‐SW and NW‐SE structural sets in addition to possible normal reactivation of the NNW‐SSE structural set. In the second rifting stage between Chron 24 (ca. 54 Ma; Eocene) and Chron 13 (ca. 35 Ma; Oligocene), the sinistral Ungava transform fault system developed due to the lateral offset between the Labrador Sea and Baffin Bay. This lateral offset was established in the first rift stage possibly due to the presence of the Nagssugtoqidian and Torngat terranes being less susceptible to rift propagation. Without the influence of pre‐existing structures the manifestation of deformation cannot be easily explained during either of the rifting phases. Although basement control diminished into the post‐rift, the syn‐rift basins from both rift stages continued to influence the location of sedimentation possibly due to differential compaction effects. Variable lithospheric strength through the rifting cycle may provide an explanation for the observed diminishing role of basement structures through time.     

A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

The Paradox Basin is a large (190 km × 265 km) asymmetric basin that developed along the southwestern flank of the basement‐involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull‐apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement‐cored uplifts of the Late Cretaceous–Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre–Paradox system are exemplary of typical ‘immobile’ foreland basin systems. Along the southwest‐vergent Uncompahgre thrust, ～5 km of coarse‐grained syntectonic Desmoinesian–Wolfcampian (mid‐Pennsylvanian to early Permian; ～310–260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian‐modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift‐parallel barrier ～200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite‐shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (～300–260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end‐Permian two‐dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian–Permian subsidence, highlighting the similarities between the Paradox basin‐fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust‐bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins.     

Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins1

Multiple episodes of extensional tectonism dominated the formation of Mesozoic fault-bounded basins on the Grand Banks of Newfoundland, the Irish Continental Shelf and the central North Sea. A range of structural and stratigraphic responses in the Jeanne d'Arc, Porcupine and Moray Firth basins support widespread synchronous tectonic controls on sedimentation during one of these episodes, the Late Cimmerian. Rifting was preceded by a phase of related tectonism during which subsidence rates began to vary across broad areas but without significant fault block rotation. This Late Cimmerian ‘onset warp’ pattern of subsidence is considered to have been essential in the establishment of restricted anoxic basins from latest Oxfordian through Kimmeridgian (sensu gallico) time and the development of one prolific layer of organic-rich source rocks. The most prominent and widely recognized structural/lithostratigraphic response to Late Cimmerian rifting was the deposition of sediment wedges. Tithonian to early Valanginian strata generally thicken- into northerly trending faults in the Jeanne d'Arc and Porcupine basins, indicating that extensional stress was orientated WNW-ESE across a very broad area. The misalignment of this regional Late Cimmerian extensional stress with local inherited structural fabric may be responsible for transpressional uplift of individual fault blocks in the Outer Moray Firth basin. Sedimentological responses to Late Cimmerian rifting were varied, though a common lithofacies stacking pattern is recognized. Variably thick conglomerates and/or sandstones were widely deposited at the start of rift deformation, while palaeoenvironments ranged from alluvial and braid plain to submarine fan even within individual basins. The relatively coarse basal sediments fine upwards into a second layer of commonly organic-rich shales and mark The widest variations in palaeoenvironments and sediment thicknesses occurred during the last phase of Late Cimmerian rift tectonism, though all three basins show evidence of decreasing water depths, increasing oxygen levels and increasing grain size. This lithofacies stacking pattern of relatively coarse to fine to coarse (reservoir/source/reservoir) and the development of bounding unconformities are largely attributable to progressive changes in rift-controlled subsidence. Rift basin subsidence rates are interpreted to increase from a low at initiation of faulting to a mid-rift peak, followed by slowing subsidence to the end of extension. A number of counteracting crustal mechanisms that may account for progressive variations in rift-induced subsidence are considered.     

Rifting and pre‐rift lithosphere variability in the Orphan Basin,Newfoundland margin,Eastern Canada

The Orphan Basin, lying along the Newfoundland rifted continental margin, formed in Mesozoic time during the opening of the North Atlantic Ocean and the breakup of Iberia/Eurasia from North America. To investigate the evolution of the Orphan Basin and the factors that governed its formation, we (i) analysed the stratigraphic and crustal architecture documented by seismic data (courtesy of TGS), (ii) quantified the tectonic and thermal subsidence along a constructed geological transect, and (iii) used forward numerical modelling to understand the state of the pre‐rift lithosphere and the distribution of deformation during rifting. Our study shows that the pre‐rift lithosphere was 200‐km thick and rheologically strong (150‐km‐thick elastic plate) prior to rifting. It also indicates that extension in the Orphan Basin occurred in three distinct phases during the Jurassic, the Early Cretaceous and the Late Cretaceous. Each rifting phase is characterized by a specific crustal and subcrustal thinning configuration. Crustal deformation initiated in the eastern part of the basin during the Jurassic and migrated to the west during the Cretaceous. It was coupled with a subcrustal thinning which was reduced underneath the eastern domain and very intense in the western domains of the basin. The spatial and temporal distribution of thinning and the evolution of the lithosphere rheology through time controlled the tectonic, stratigraphic and crustal architecture that we observe today in the Orphan Basin.     

Sequence architecture and depositional evolution of the northern continental slope of the South China Sea: responses to tectonic processes and changes in sea level

The continental slopes of the South China Sea (SCS), the largest marginal sea on the continental shelf of Southeast Asia, are among the most significant shelf‐margin basins in the world because of their abundant petroleum resources and a developmental history related to sea floor spreading since Late Oligocene time. Based on integrated analyses of seismic, well‐logging and core data, we systematically document the sequence architecture and depositional evolution of the northern continental slope of the SCS and reveal its responses to tectonism, sea‐level change and sediment supply. The infill of this shelf‐margin basin can be divided into seven composite sequences (CS1–CS7) that are bounded by regional unconformities. Composite sequences CS3 to CS7 have formed since Late Oligocene time, and each of them generally reflects a regional transgressive–regressive cycle. These large cycles can be further divided into 20 sequences that are defined by local unconformities or transgressive–regressive boundaries. Depositional–geomorphological systems represented on the continental slope mainly include shelf‐edge deltas, prodelta‐slope fans, clinoforms of the shelf‐margin slope, unidirectionally migrating slope channels, incised slope valleys, muddy slope fans, slope slump‐debris‐flow complexes and large‐scale soft‐sediment deformation of bedding. Changing sea levels, reflected by evidence from sequence architecture in the study area, are generally comparable with those of the Haq (1987) global sea level curve, whereas the regional transgressions and regressions were apparently controlled by tectonic uplift and subsidence. Composite sequences CS3 and CS4 formed from Late Oligocene to Middle Miocene time and represent continental‐slope deposition during a time of northwest‐northeast seafloor spreading and subsequent development of sub‐basins in the southwest‐central SCS. The development of composite sequences CS5 to CS7 after Middle Miocene time was obviously influenced by the Dongsha Movement during convergence between the SCS and Philippine Sea plates. Climatic variations and monsoon intensification may have enhanced sediment supply during Late Oligocene‒Early Miocene (25–21 Ma) and Late Pliocene‒Pleistocene (3–0.8 Ma) times. This study indicates that shelf‐edge delta and associated slope fan systems are the most important oil/gas‐bearing reservoirs in the SCS continental‐slope area.     

LA‐ICP‐MS dating of detrital zircon grains from the Cretaceous allochthonous bauxites of Languedoc (south of France): Provenance and geodynamic consequences

The Cretaceous of southern France is characterised by a long erosional hiatus, outlined with bauxite deposits, which represent the only remaining sedimentary record of a key period for geodynamic reconstructions. Detrital zircons from allochthonous karst bauxites of Languedoc (Southern France) have been dated using LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), in order to specify the age of deposition and to constrain the provenance of the weathered material. We analysed 671 single detrital zircons grains from three karst bauxitic basins, stretching from close to the Variscan Montagne Noire to the present‐day Mediterranean Sea. Analytical results provide Variscan (300–350 Ma) and Late Proterozoic (550–700 Ma) ages as primary groups. In addition, Middle‐, Late Proterozoic and Early Archean (oldest grain at 3.55 Ga) represent significant groups. Mid‐Cretaceous zircons (118–113 Ma) provide a pooled age of 115.5 ± 3.8 Ma, which constitutes the maximum age for bauxite deposition. Results also suggest a dual source for the Languedoc bauxite: one generalised sedimentary source of regional extent and a localised source in the Variscan basement structural high, that has been progressively unroofed during Albian. Integration of these new findings with previously published thermochronological data support the presence of an Early Cretaceous marly cover on the Variscan basement, which has been weathered and then, removed during the Albian. The Languedoc bauxite provide a spatial and temporal link between the uplift of southern French Massif Central to the north, and the Pyrenean rift and its eastward extension to the south. These new results allow to constrain the timing and distribution of uplift/subsidence during the mid‐Cretaceous events in relation with the motion of the Iberian plate relative to Eurasia.     

Location,extent, and magnitude of dynamic topography in the Late Cretaceous Cordilleran Foreland Basin,USA: New insights from 3D flexural backstripping

Mantle-induced dynamic topography (i.e., subsidence and uplift) has been increasingly recognized as an important process in foreland basin development. However, characterizing and distinguishing the effects (i.e., location, extent and magnitude) of dynamic topography in ancient foreland basins remains challenging because the spatio-temporal footprint of dynamic topography and flexural topography (i.e., generated by topographic loading) can overlap. This study employs 3D flexural backstripping of Upper Cretaceous strata in the central part of the North American Cordilleran foreland basin (CFB) to better quantify the effects of dynamic topography. The extensive stratigraphic database and good age control of the CFB permit the regional application of 3D flexural backstripping in this basin for the first time. Dynamic topography started to influence the development of the CFB during the late Turonian to middle Campanian (90.2–80.2 Ma) and became the dominant subsidence mechanism during the middle to late Campanian (80.2–74.6 Ma). The area influenced by >100 m dynamic subsidence is approximately 400 by 500 km, within which significant (>200 m) dynamic subsidence occurs in an irregular-shaped (i.e., lunate) subregion. The maximum magnitude of dynamic subsidence is 300 ± 100 m based on the 80.2–74.6 Ma tectonic subsidence maps. With the maximum magnitude of dynamic uplift being constrained to be 200–300 m, the gross amount of dynamic topography in the Late Cretaceous CFB is 500–600 m. Although the location of dynamic subsidence revealed by tectonic subsidence maps is generally consistent with isopach map trends, tectonic subsidence maps developed through 3D flexural backstripping provide more accurate constraints of the areal extent, magnitude and rate of dynamic topography (as well as flexural topography) in the CFB through the Late Cretaceous. This improved understanding of dynamic topography in the CFB is critical for refining current geodynamic models of foreland basins and understanding the surface expression of mantle processes.     

Rapid outer marginal collapse at the rift to drift transition of passive margin evolution,with a Gulf of Mexico case study

Interpretation of long‐offset 2D depth‐imaged seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading and thermal subsidence of the margin. This collapse post‐dates rifting and stretching of the crust, but occurs roughly ten times faster than thermal subsidence of young oceanic crust, and thus is tectonic and pre‐dates the ‘drift stage’. We term this middle stage of margin development ‘outer margin collapse’, and it accords with the exhumation stage of other authors. Outer continental margins, already thinned by rifting processes, become hanging walls of crustal‐scale half grabens associated with landward‐dipping shear zones and zones of low‐shear strength magma at the base of the thinned crust. The footwalls of the shear zones comprise serpentinized sub‐continental mantle that commonly becomes exhumed from beneath the embrittled continental margin. At magma‐poor margins, outer continental margins collapse and tilt basinward to depths of about 3 km subsea at the continent–ocean transition, often deeper than the adjacent oceanic crust (accreted later between 2 and 3 km). We use the term ‘collapse’ because of the apparent rapidity of deepening (<3 Myr). Rapid salt deposition, clastic sedimentation (deltaic), or magmatism (magmatic margins) may accompany collapse, with salt thicknesses reaching 5 km and volcanic piles 1525 km. This mechanism of rapid salt deposition allows mega‐salt basins to be deposited on end‐rift unconformities at global sea level, as opposed to deep, air‐filled sub‐sea depressions. Outer marginal collapse is ‘post‐rift’ from the perspective of faulting in the continental crust, but of tectonic, not of thermal, origin. Although this appears to be a global process, the Gulf of Mexico is an excellent example because regional stratigraphic and structural relations indicate that the pre‐salt rift basin was filled to sea level by syn‐rift strata, which helps to calibrate the rate and magnitude of collapse. We examine the role of outer marginal detachments in the formation of East India, southern Brazil and the Gulf of Mexico, and how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading. We suggest that backstripping estimates of lithospheric thinning (beta factor) at outer continental margins may be excessive because they probably attribute marginal collapse to thermal subsidence.     

Normal fault growth and early syn-rift sedimentology and sequence stratigraphy: Thal Fault, Suez Rift, Egypt

This paper investigates the tectono‐stratigraphic development of a major, segmented rift border fault (Thal Fault) during ca. 6 Myr of initial rifting in the Suez Rift, Egypt. The Thal Fault is interpreted to have evolved by the progressive linkage of at least four fault segments. We focus on two contrasting structural settings in its hangingwall: Gushea, towards the northern tip of the fault, and Musaba Salaama, ca. 20 km along‐strike to the south, towards the centre of the fault. The early syn‐rift stratigraphic succession passes upwards from continental facies, through a condensed marginal marine shell‐rich facies, into fully marine shoreface sandstone and offshore mudstone. Regionally correlatable stratal surfaces within this succession define time‐equivalent stratal units that exhibit considerable along‐strike variability in thickness and facies architecture. During the initial ca. 6 Myr of rifting, the thickest stratigraphy developed towards the centre of the array of fault segments that subsequently hard linked to form the Thal Fault. Thus, a displacement gradient existed between fault segments at the centre and tip of the fault array, suggesting that the fault segments interacted, and a fixed length was established for the fault array, at an early stage in rifting. Towards the centre of the Thal Fault the early syn‐rift succession shows pronounced thickening away from the fault and towards a series of intra‐block antithetic faults that were active for up to ca. 6 Myr. This indicates that a large proportion of fault‐controlled subsidence during the initial ca. 6 Myr of rifting occurred in the hangingwalls of antithetic intra‐block faults, and not the present‐day Thal Fault. The antithetic faults progressively switched off during rifting such that after ca. 6 Myr of rifting, fault‐activity had localised on the Thal Fault enabling it to accrue to the present‐day high level of displacement. Aspects of the development of the Thal Fault appear to be in contrast to many models of fault evolution that predict large‐displacement rift‐climax faults to have always had the greatest displacement during fault population evolution. This study has implications for tectono‐stratigraphic development during early rift basin evolution. In particular, we stress that caution must be taken when relating final rift‐climax fault structure to the early tectono‐stratigraphy, as these may differ considerably.