Syn-rift Subsidence Research Articles

Dynamo-thermal subsidence and sag–salt section deposition as magma-rich rifted margins move off plume centres along incipient lines of break-up

Magma-rich rifting generally occurs over mantle plumes or rising convection cells; magma-poor rifting does not. Plumes produce up to 2 km dynamic elevation, outcompeting synrift subsidence in active and ancient magma-rich rifts, as evidenced by top-rift unconformities. Magma-rich margins show anomalously fast and large post-tectonic subsidence, evidenced by thick, short-lived little-faulted sag–salt sections. Syndepositional thermal subsidence cannot be responsible alone, and lack of faulting argues against tectonic subsidence. We argue that dissipation of dynamic topography (dynamic subsidence) is the additional component with thermal subsidence. We depict the formation of the magma-rich central South Atlantic and Gulf of Mexico salt-bearing margins relative to former plumes, showing that sag–salt deposition occurs as the margins migrate off plume flanks while supra-plume rifting continues. However, the pre-sag–salt basement must first be extended or eroded during dynamic elevation to fall below its pre-uplift elevation. ‘Dynamo-thermal subsidence’ creates ‘dynamo-thermal accommodation’, the sum and combined result of dynamic plus thermal subsidence, respectively. It is fast enough to negate the need for deep, sub-sea-level, subaerial depressions for accumulation of thick sag–salt sections. Also, differences in dynamic elevation during salt deposition explain differences in autochthonous salt thickness. Supplementary material : Supplementary figures, seismic data, structural restorations, a compilation of igneous activity and an extended figure caption are available at https://doi.org/10.6084/m9.figshare.c.5908896

Detachment-controlled subsidence pattern at hyper-extended passive margin: Insights from backstripping modelling of the Baiyun Rift, northern South China Sea

How detachment fault controls the basin subsidence at hyper-extended passive margin remains mysterious. Based on a detailed geological interpretation and backstripping on 29 high-resolution multi-channel seismic profiles, we calculate the basin-scale tectonic subsidence across the Baiyun Rift to study the relationship between tectonic subsidence and detachment faulting. Our results confirm that detachment faulting is the primary factor controlling the syn-rift subsidence. A decaying subsidence rate, or deficit subsidence is observed in the eastern Baiyun Rift from the early to late Eocene, which is in marked contrast to the intensive brittle stretching. We attribute this to the doming of the detachment fault footwall involving the syn-rift magmatism during its hyper-extension process. While in the western Baiyun Rift the average subsidence rate is over 45 m Myr−1 higher than that in the eastern as the response of flexural loading in the late Eocene. During the post-rift stage, the subsidence rate shows a similar oceanward increasing trend, and an anomalous rapid subsidence event during the middle Miocene in the eastern Baiyun Rift, which can be ascribed to the magma-assisted weakened rheological condition of the lithosphere deriving from the syn-rift detachment faulting. We propose that this relationship between subsidence pattern and detachment faulting has global applicability for other hyper-extended rift systems developing detachments and magmatic-related dome structures.

The deep mantle upwelling beneath the northwestern South China Sea: Insights from the time-varying residual subsidence in the Qiongdongnan Basin

Deep hot mantle upwelling is widely revealed around the Qiongdongnan Basin on the northwestern South China Sea margin. However, when and how it influenced the hyper-extended basin is unclear. To resolve these issues, a detailed analysis of the Cenozoic time-varying residual subsidence derived by subtracting the predicted subsidence from the backstripped subsidence was performed along a new seismic reflection line in the western Qiongdongnan Basin. For the first time, a method is proposed to calculate the time-varying strain rates constrained by the faults growth rates, on basis of which, the predicted basement subsidence is obtained with a basin- and lithosphere-scale coupled finite extension model, and the backstripped subsidence is accurately recovered with a modified technique of backstripping to eliminate the effects of later episodes of rifting on earlier sediment thickness. Results show no residual subsidence in 45–28.4 Ma. But after 28.4 Ma, negative residual subsidence occurred, reached and remained ca. −1000 m during 23–11.6 Ma, and reduced dramatically after 11.6 Ma. In the syn-rift period (45–23 Ma), the residual subsidence is ca. −1000 m, however in the post-rift period (23–0 Ma), it is positive of ca. 300 to 1300 m increasing southeastwards. These results suggest that the syn-rift subsidence deficit commenced at 28.4 Ma, while the post-rift excess subsidence occurred after 11.6 Ma. Combined with previous studies, it is inferred that the opposite residual subsidence in the syn- and post-rift periods with similar large wavelengths (>102 km) and km-scale amplitudes are the results of transient dynamic topography induced by deep mantle upwelling beneath the central QDNB, which started to influence the basin at ca. 28.4 Ma, continued into the Middle Miocene, and decayed at ca. 11.6 Ma. The initial mantle upwelling with significant dynamic uplift had precipitated considerable continental extension and faulting in the Late Oligocene (28.4–23 Ma). After ca. 11.6 Ma, strong mantle upwelling probably occurred beneath the Leizhou–Hainan area to form vast basaltic lava flow.

Reconstruction of the Cenozoic tectono-thermal history of the Dongpu Depression, Bohai Bay Basin, China: Constraints from apatite fission track and vitrinite reflectance data

The Dongpu Depression is an important oil-producing area in the Bohai Bay Basin, China. The exploration focus in this area has recently shifted to intervals with greater depths (>3500 m) because of decreasing hydrocarbon production at depths shallower than 3500 m. Deep petroleum resource evaluation is becoming important and the tectono-thermal history is a major part of this evaluation. In this study, Cenozoic tectono-thermal histories of the Dongpu Depression are reconstructed by coupling an inversion method of vitrinite reflectance and apatite fission track data from 13 wells. Moreover, the maturation history of the deep source rocks was modeled based on these histories. The results clearly show that the Dongpu Depression experienced two heat flow maxima during the deposition of the middle parts of the Eocene Shahejie 3 member and the Oligocene Dongying Formation, which exhibit maximum heat flow values of 70.5–77.5 mW/m2 and 64.8–70.0 mW/m2, respectively. These two periods of high heat flow are linked to the rifting events of the Dongpu Depression, corresponding to the intense rifting substage (45-38 Ma) and the later rifting substage (38-27 Ma). Tectonic subsidence analysis revealed that the Dongpu Depression experienced initial syn-rift subsidence during the Paleocene, followed by subsequent thermal subsidence since the Neogene. The maturation history indicates that these source rocks experienced only one hydrocarbon generation phase in the mid-northern sags, and experienced a hydrocarbon generation peak during the Oligocene Dongying Formation deposition.

Early Cretaceous subsidence of the Naturaliste Plateau defined by a new record of volcaniclastic-rich sequence at IODP Site U1513

The Naturaliste Plateau is a submarine continental ribbon rifted from the southwest Australian margin during the Early Cretaceous breakup of East Gondwana. It occupied a key position near the juncture of Greater India and the boundary between Australia and Antarctica. However, details of the Early Cretaceous evolution of the plateau are not well known because of limited data. Drilling at Site U1513 during IODP Expedition 369 recovered the first complete Lower Cretaceous succession on the eastern Naturaliste Plateau. The succession includes syn-rift volcanic rocks, Hauterivian to early Aptian volcaniclastic-rich sedimentary rocks, and Albian claystone strata. The 235-m thick volcaniclastic-rich sequence represents the missing post-breakup record in the southwest Australian rifted margin. It spans the transition from syn- to post-rift phase during the final stages of breakup between Greater India and Australia-Antarctica. We report the lithological, petrophysical, geochemical, paleontological, and paleomagnetic characteristics of the sequence, and then synthesize the results to define the Early Cretaceous depositional environment and subsidence history of the Naturaliste Plateau. From the early Hauterivian, weathered volcanic products were eroded and re-deposited locally as a volcaniclastic-rich sequence, with a major contribution from the southern Naturaliste Plateau. The depositional environment evolved from a shelf to upper bathyal condition during the Hauterivian through early Barremian with a decreasing sedimentation rate. This period is defined as a late syn-rift subsidence phase by NW-SE trending extension. After the final breakup with Greater India, the plateau remained at upper bathyal depths with little deposition until the early Aptian. Mid–lower bathyal depths inferred from the Albian claystone strata suggest that the post-rift thermal subsidence commenced during the late Aptian. This two-phase post-rift subsidence reflects the proximity or high temperature of mantle plume, possibly the Kerguelen plume, and its westward migration relative to the southwest Australian rifted margin.

Early Cretaceous tectonostratigraphic evolution of the Erlian Basin, NE China: A record of Late Mesozoic intraplate deformation in East Asia

Abstract The Erlian Basin is one of the largest intracontinental Meso-Cenozoic basins in Northeast China. The basin is filled with extra-thick Cretaceous sediments and offers unique opportunities to untangle the Late Mesozoic intraplate deformation in East Asia. However, despite decades of petroleum exploration in the basin, the Early Cretaceous chronostratigraphy and structural evolution of the Erlian Basin have been poorly studied. Based on newly obtained zircon U-Pb geochronological data, well lithological logs, and seismic reflection profiles, the present study carries out a detailed analysis of the tectonostratigraphic evolution of the Erlian Basin during the Early Cretaceous. The depositional ages of the Lower Cretaceous strata in the basin are refined, and the Early Cretaceous evolution of the basin is reestablished. The Lower Aershan, Upper Aershan, Lower Tenggeer, Upper Tenggeer, and Saihan formations are constrained to the middle Valanginian (ca. 138–135 Ma), late Valanginian (ca. 135–133 Ma), Hauterivian–early Aptian (ca. 133–121 Ma), middle Aptian (ca. 119–115 Ma), and latest Aptian–Albian (post ca. 115 Ma), respectively. The Early Cretaceous syn-rift subsidence of the basin was intermittent rather than continuous, and was punctuated by multiphase contractional deformation during the earliest Cretaceous (prior to ca. 138 Ma), middle Valanginian (ca. 135 Ma), latest Valanginian (ca. 133 Ma), middle Aptian (ca. 120 Ma), late Aptian (ca. 115 Ma), and Early/Late Cretaceous boundary. We tentatively attribute the extensional and contractional deformation in the Erlian Basin to a series of continental collisions along Asian margins: Karakoram-Lhasa/Qiangtang collision, Kolyma-Omolon/Siberia collision, Siberia/Amuria collision, Chukotka/Kolyma-Omolon collision, West Philippines/South China collision, and Okhotomorsk/East Asia collision.

Multiproxy reconstruction of the middle Miocene Požega palaeolake in the Southern Pannonian Basin (NE Croatia) prior to the Badenian transgression of the Central Paratethys Sea

The Pannonian Basin of Croatia is the largest back-arc extensional basin on the European continent, located between the Alps, Carpathians and Dinarides. Syn-rift subsidence started at ~18 Ma and predated the onset of the Miocene Climate Optimum (MCO; 17–14.7 Ma). In this paper, we investigate the evolution of the fluvial-lacustrine palaeoenvironment just prior to the transgression of the Central Paratethys Sea. Studies are based on a 17-m-thick section of the Požega palaeolake in the Southern Pannonian Basin using palaeobiological (mollusks, ostracods, palynomorphs, fish otoliths), mineralogical and sedimentological data. Our results reveal fluctuating freshwater and brackish conditions and a pulsating water budget, which we relate to phases in which the basin was, alternatively, hydrologically open and closed. We infer that the alternating environmental conditions can be best-explained by dry and wet periods, respectively. Droughts produced stagnant conditions, algae overproduction and increased salinity levels, supporting the survival of sporadically invading brackish water species. The wet periods, on the other hand, led to lake highstand episodes, expansion of lake area, and to the widespread development of profundal facies. The lake basin eventually became overfilled and connected through outflowing rivers with the Central Paratethys sea. However, the absence of brackish-marine assemblages in the Požega palaeolake nevertheless indicates that there was no marine water inflow from the Central Paratethys at that time.

Deeply concealed half-graben at the SW margin of the East European Craton (SE Poland) — Evidence for Neoproterozoic rifting prior to the break-up of Rodinia

Baltica was one of continents formed as a result of Rodinia break-up 850–550 Ma. It was separated from Amazonia(?) by the Tornquist Ocean, the opening of which was preceded by Neoproterozoic extension in a network of continental rifts. Some of these rifts were subsequently aborted whereas the Tornquist Rift gave rise to splitting of Rodinia and formation of the Tornquist Ocean. The results of 1-D subsidence analysis at the fossil passive margin of Baltica provided insight in the timing and kinematics of continental rifting that led to break-up of Rodinia. Rifting was associated with Neoproterozoic syn-rift subsidence accompanied by deposition of continental coarse-grained sediments and emplacement of continental basalts. Transition from a syn-rift to post-rift phase in the latest Ediacaran to earliest early Cambrian was concomitant with deposition of continental conglomerates and arkoses, laterally passing into mudstones. An extensional scenario of the break-up of Rodinia along the Tornquist Rift is based on the character of tectonic subsidence curves, evolution of syn-rift and post-rift depocenters in time, as well as geochemistry and geochronology of the syn-rift volcanics. It is additionally reinforced by the high-quality deep seismic reflection data from SE Poland, located above the SW edge of the East European Craton. The seismic data allowed for identification of a deeply buried (11–18 km), well-preserved extensional half-graben, developed in the Palaeoproterozoic crystalline basement and filled with a Neoproterozoic syn-rift volcano-sedimentary succession. The results of depth-to-basement study based on integration of seismic and gravity data show the distribution of local NE–SW elongated Neoproterozoic depocenters within the SW slope of the East European Craton. Furthermore, they document the rapid south-eastwards thickness increase of the Neoproterozoic succession towards the NW–SE oriented craton margin. This provides evidence for extensive crustal thinning occurring prior to the break-up of Rodinia and formation of the Tornquist Ocean.

Symmetry during the syn- and post-rift evolution of extensional back-arc basins: The role of inherited orogenic structures

Rheological heterogeneities in the lithosphere have first order control on the topographical expression of tectonic processes. Pre-existing orogenic suture zones localise extensional deformation resulting in asymmetric basins. Such crustal geometries are often in contrast with the more symmetric regional lithospheric structure observed beneath extensional basins. We study such (a)symmetries and their controlling parameters by conducting a series of 2D thermo-mechanical numerical experiments of the extension of an overthickened, hot lithosphere that contains a weakness zone. The modelling shows that syn-rift subsidence is low to moderate creating asymmetric half grabens where extension migrates in space and time, grouped in an overall symmetrical appearance on a larger scale. The initial lithospheric mantle asymmetry is attenuated by the lateral heat conduction and further dynamic evolution of the thermal anomaly during the “post-rift” phase, resulting in differential vertical movements of the crust including additional 2–3 km subsidence in the basin centre. The modelling shows that the initial crustal and lithospheric thicknesses, rate of extension and surface processes strongly control the thermo-mechanical evolution of the extensional system. The numerical modelling yields new insights into the mechanics of coupling between near-surface kinematics and the evolution of deep lithospheric structure in the Pannonian basin and the Aegean, two of Europe's largest back-arc systems.

Geodinámica de los márgenes conjugados del Atlántico Sur central: implicaciones para el potencial de hidrocarburos

Using a new margin restoration model for the central South Atlantic we highlight margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust. We interpret these parameters as diagnostic of strain evolution during rifting. Pre-salt subsidence patterns and sequence isopachs of late syn-rift and early post-rift sequences are symptomatic of these complex extension behaviours. Inherited basement fault trends partition extensional strain during stretching with resultant rift-related structural styles recording the heterogeneity of the pre-rift crust. Correspondingly the seismic and gravity expression of the transition from continental crust to oceanic crust differs systematically along the length of the ‘conjugate’ basins. The presence of a conspicuous coast parallel positive linear gravity anomaly, referred to as the ‘terminal horst’, defines the basinward extent of attenuated continental crust in the more symmetrical Campos Basin and its interpreted conjugate. In the ‘asymmetrical’ Santos and Benguela Basins the ocean–continent transition at the present-day OCB (ocean–continent boundary) becomes equivocal with little gravity expression. Consequently the relationship of primary salt basin edges to the basinward extent of crustal extension appears more complex. Pre-existing crustal and lithospheric mantle scale heterogeneities are considered to impart a first-order control on whole crust deformation and ultimately ‘sag’ basin development. These structurally defined heterogeneities are believed to partition crustal strain and juxtapose endmember mechanisms of pure-shear and simple shear deformation styles as recorded by the complex distribution of syn-rift subsidence patterns within the pre-salt basins of the central South Atlantic. These conclusions challenge the necessity for invoking dominantly depth-dependent processes as a mechanism to explain apparent pre-salt, syn-rift subsidence anomalies. The complex interplay of structure and stratigraphy with an overlay of palaeoclimatic models provides important insights into hydrocarbon play fairways along the margin. In particular the impact on reservoir and source rock distribution and heat flow are discussed.

Present-Day Heat Flow, Thermal History, and Tectonic Subsidenceof the Jianghan Basin

Thermal gradient distribution of the Jianghan Basin is obtained based on systematic steady-state and oil-test temperature data. The present geothermal gradient and terrestrial heat flow of 18 wells are calculated in the Jianghan Basin. A total of 9 measured terrestrial heat flow values are reported based on detailed thermal conductivity data and systematic steady-state temperature data. These values vary from 41.9 to 60.9 mW/m2, with a mean value of 52.3 ± 6.3 mW/m2. However, thermal history analyses, derived from vitrinite reflectance (RO) and apatite fission track (AFT) data, indicate that the Jianghan Basin was at a stable stage before the Indosinian movement. The heat flow increased and reached its maximum value of ∼68 mW/m2 during the Late Indosinian to Early Yanshan period. Since the Late Himalayan period, rapidly decreasing basement heat flow made the Jianghan Basin cool down. Furthermore, geohistory analysis reveals that the tectonic subsidence of the Jianghan Basin from the Cretaceous to Early Miocene was characterized by initial synrift subsidence followed by thermal subsidence. The thermal and tectonic subsidence history have directly effects on petroleum source rock maturation, so their analyses are of great significance forpetroleum exploration and hydrocarbon source assessment in the Jianghan Basin.

Role of rift transection and punctuated subsidence in the development of the North Falkland Basin

The results of well-constrained seismic interpretation and new mapping of three-dimensional (3D) seismic data volumes demonstrates that the North Falkland Basin consists of two superimposed failed rift basins: a Late Jurassic NW–SE-striking Southern Rift Basin (SRB); and an Early Cretaceous north–south-striking Northern Rift Basin (NRB). The SRB is best developed in coastal waters of the Falkland Islands, where it comprises a series of extensional sub-basins that are transected by faults belonging to the more substantive NRB. Regional interpretation demonstrates that the NRB consists of a southward-tapering, asymmetric extensional basin containing a thick (in excess of 10 km) sequence of sediments. Its syn-rift subsidence history was controlled by a major west-dipping normal fault array comprising several fault segment precursors, which, together with corresponding antithetic faults, effectively subdivides the hanging wall into a series of sub-basins throughout its length. The NRB initially developed in a fluvial and later lacustrine environment before becoming predominantly marine in the Tertiary. A prograding delta system filled the basin from the north during the early post-rift phase. Contemporaneously, sediment was shed off the segmented basin-bounding fault via long-established feeder drainage systems through breached relay ramps into the depocentre. The resultant sediment dispersal led to deposition of numerous lacustrine turbidites that created the Sea Lion fans and its affiliates, the location of which mimics, and is thus interpreted to have been controlled by, the underlying syn-rift sub-basins. Post-rift subsidence was punctuated by an important, but short-lived, phase of basin inversion during the Aptian that created a large, broad and gentle north–south-striking anticline that runs along the central basin axis. Whilst the episode of basin inversion arrested subsidence, it did not inhibit petroleum prospectivity. The syn-rift lacustrine source intervals did subsequently pass through the critical moment in the Cretaceous leading to hydrocarbon maturation and the migration of waxy oil, a process that continues to the present day.

Cretaceous anomalous subsidence and its response to dynamic topography in the Songliao Basin, Northeast China

To understand the formation mechanisms and evolution of the Songliao Basin, we applied backstripping and strain rate inversion methods to 20 wells and reinterpret two seismic sections in the Songliao Basin. The obtained data were used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of this area. The predicted post-rift subsidence based on the uniform stretching model that followed earlier lithospheric thinning events was much lower than the subsidence provided by backstripping. The anomalous subsidence during the post-rift stage was between 200m and 800m in the Songliao Basin, and the depocenter of anomalous subsidence moved eastward between ca. 110 and 80Ma before returning to the west between ca. 80 and 65Ma. Regional variations in the subsidence rates suggest a possible deficit in the negative buoyancy (mantle loading) that is induced by the sinking slab beneath the Songliao Basin. In addition, the anomalous post-rift subsidence was primarily driven by downward drag pressure from the subducting western Izanagi slab and asthenospheric mantle flow beneath the Songliao Basin. The basement rifting in the Songliao Basin during the early Cretaceous and the normal faulting that occurred during the post-rift stage resulted in a secondary contribution to the anomalous post-rift subsidence. Unlike the Songliao Basin, several early Cretaceous rift basins south of the Yanshan Belt were only filled during the syn-rift subsidence, and the anomalous post-rift uplift of the Yanshan belt was driven by deep mantle flow.

Geothermal regime and source rock thermal evolution in the Chagan sag, Inner Mongolia, northern China

The Chagan sag has the greatest oil and gas exploration potential among other sags in the Yingen Ejinaqi Basin, Inner Mongolia. The average geothermal gradient in the Chagan sag is 33.6 °C/km, whereas the heat flow ranges from 65.9 mW/m2 to approximately 85.5 mW/m2, with an average value of 74.5 mW/m2. Thermal history reveals that the Chagan sag experienced the following 4 stages of thermal evolutions: (1) a rapidly increasing geothermal gradient stage from the Early Cretaceous Bayingebi Formation depositional period to the Early Cretaceous Suhongtu Formation depositional period; (2) a geothermal gradient peak stage during the Early Cretaceous Yingen Formation depositional period; (3) a high geothermal gradient continuation stage during the Late Cretaceous Wulansuhai Formation depositional period; and (4) a thermal subsidence stage during the Cenozoic. Tectonic subsidence analysis reveals that the area experienced an initial synrift subsidence during the Early Cretaceous followed by a subsequent long-term thermal subsidence since Late Cretaceous. Thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of source rock maturation. The maturation histories of the 3 sets of source rocks in the sag were modeled on the basis of the present geothermal field, thermal history and tectonic subsidence history. The results reveal that the hydrocarbon generation of the Chagan sag was controlled by the Early Cretaceous geothermal fields, and the source rock maturity reached the maximum at the end of the Yingen Formation depositional period. Moreover, the maturation evolution degree shows a difference for the 3 sets of source rocks. The source rocks of the Bayingebi 1 and 2 Formations reached the middle mature and dry-gas stage. In contrast, the source rocks of the Suhongtu 1 Formation only reached the early mature and middle mature stage. This work may provide new insights for the understanding of the oil and gas exploration potential of the Chagan sag.

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).

Facies heterogeneity and depositional models of a Ladinian (Middle Triassic) microbial-dominated carbonate ramp system (Catalan Coastal Ranges, NE Spain)

The Ladinian sedimentary successions record a major transgressive pulse of northeastern Iberia during the Middle Triassic. This record is arranged in two transgressive–regressive (T–R) sequences formed by two fault-block microbial and shoal-dominated carbonate ramps. The main factor controlling the creation of accommodation space is the differential rapid syn-rift subsidence, which induces the generation of palaeogeographical barriers that played a key role in the sedimentary architecture and microbial development.The facies architecture of these ramps was reconstructed by means of facies analysis and sequence stratigraphy. T–R sequence 1 corresponds to a fault-block carbonate ramp system dominated by microbialites and oolitic shoals. T–R sequence 2 represents a fault-block carbonate ramp system mainly characterised by lagoons and shoals.The diverse types of microbialites (stromatolites and thrombolites) are widely represented in the inner and middle ramp settings, reaching a width of 45 km and an average thickness of 40 m. T–R sequence 1 contains the most noticeable microbial deposits, stromatolites in the inner ramp (up to 7 m thick) and thrombolites in the middle ramp (up to 40 m thick). T–R sequence 2 is mainly characterised by the occurrence of oolitic stromatolites in the inner ramp and internal shoals and sheltered lagoons in the middle ramp. Microbialites exhibit a fabric-selective ancient porosity which is currently occluded by coarse calcite and quartz cements. The most common types of ancient porosity are interlaminar, vuggy and mouldic (grain dissolution and evaporite replacement).The studied carbonate ramps can be used as an analogue for ancient microbialite reservoirs in rapidly subsiding depositional areas, with a high diversity of microbial deposits formed in restricted to open marine conditions, with a wide array of ancient porosity and a well-known sequence stratigraphic context.

Structural records of the Late Cretaceous–Cenozoic extension in Eastern China and the kinematics of the Southern Tan-Lu and Qinling Fault Zone (Anhui and Shaanxi provinces, PR China)

Our study of the Late Cretaceous–Cenozoic extension in Eastern China is a kinematic analysis of faults (≈1500 striated fault data, 175 stress solutions at 90 sites) that has permitted us to separate stress fields belonging to six tectonic events. A WNW–ESE extension (1) has affected Eastern China from the Campanian to the Late Palaeocene–Early Ypresian. Under this extension, the Qinling (QLFZ) and Southern Tan-Lu (STLFZ) Fault Zones have been dextral transtensional fault zones. A first transpressional event (2) (NNE–SSW shortening) has occurred during the Late Ypresian–Lower Lutetian; the QLFZ and the STLFZ have been dextral transpressional fault zones. A NE–SW extension (3) has affected the Weihe graben during the Palaeogene; after a second transpressional event (4), the NE–SW extension (5) has been rejuvenated during the Late Miocene and the QLFZ has been a normal fault zone again and the STLFZ a sinistral transtensional fault zone; synrift subsidence has occurred in the Weihe graben. The second transpressional event (4) (WNW–ESE shortening) has occurred during the Late Oligocene–Early Miocene, during a period of uplifting and exhumation. At that time, the STLFZ and the QLFZ have become sinistral transpressional fault zones. A period of extension (6) has taken place during the Late Pliocene–Quaternary; the extensional direction has trended ≈NNW–SSE; during this event, the QLFZ has been a sinistral transtensional fault zone and the STLFZ a dextral transtensional fault zone. These results are compared with those previously published in order to evaluate the regional significance of these tectonic events.

Lower Aptian ammonite biostratigraphy in the Maestrat Basin (Eastern Iberian Chain, Eastern Spain). A Tethyan transgressive record enhanced by synrift subsidence

The present paper analyses the stratigraphic distribution of ammonites collected in the Lower Aptian sediments of the Maestrat Basin (E Spain). The faunal successions obtained from the systematic sampling of ten selected sections located in several sub basins led us to identify four biostratigraphic units (from base to top: Deshayesites oglanlensis, Deshayesites forbesi, Deshayesites deshayesi and Dufrenoyia furcata Zones) that are directly correlatable with the current Mediterranean standard zonation. This study further provides essential biochronologic information to accurately date the different lithostratigraphic units included in the interval studied. Thus, the upper part of the Xert Formation can be tentatively attributed to the lowermost Aptian (D. oglanlensis Zone). The Forcall Formation, which has yielded most of the ammonites, practically extends within most of the Lower Aptian. Its lower and middle parts (Cap de Vinyet and Barra de Morella Members) correspond to the upper part of the D. oglanlensis Zone and the lower part of the D. forbesi Zone, respectively. The base of the Morella la Vella Member (upper part of the Forcall Formation) includes the local record of Oceanic Anoxic Event 1a (OAE 1a) and correlates with the Roloboceras hambrovi horizon (middle/upper part of the D. forbesi Zone). The boundary between the Forcall Formation and the Villarroya de los Pinares Formation is diachronous and varies depending on the sub-basins. The base of the Villarroya de los Pinares Formation can be dated as the middle-upper part of the D. furcata Zone in the Galve, Perello and Salzedella sub-basins.

Geothermal regime and hydrocarbon kitchen evolution of the offshore Bohai Bay Basin, North China

The offshore Bohai Bay Basin, located in the central Bohai Bay Basin, north China, one of the most petroliferous basins in China. In this article, based on formation-testing temperature, drill-stem tests, and bottom-hole temperature data, 80 thermal gradient values at the depth interval of 0 to approximately 3000 m (~9843 ft) in the offshore Bohai Bay Basin were obtained. The basinwide average thermal gradient is 31.8 4.6C/km. Based on the above thermal gradient data and the corresponding average weighted thermal conductivity data, 80 measured terrestrial heat flow values were obtained. These values range from 33.5 to 84.0 mW/m2, with an average value of 60.8 8.7 mW/m2. The heat flow and thermal gradient distribution in this region generally show higher values in the uplifts and lower ones in the sags. A thermal history, derived from vitrinite reflectance and apatite fission-track) data, indicates that Paleogene cooling occurred after a period of much higher paleogeothermal gradient (3854C/km). Tectonic subsidence analysis reveals that the area experienced initial synrift subsidence during the Paleogene followed by subsequent thermal subsidence since the Neogene. Thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of petroleum source rock maturation. The maturation and hydrocarbon expulsion histories of the Paleogene Shahejie 3 Formation (E2s3), which is the most important source rock in the offshore Bohai Bay Basin, are modeled. Results show that the Shahejie 3 Formation is in a high mature stage at the present day, and the Bozhong and Qikou sags are the most important kitchens. The Huanghekou sag became the third most important hydrocarbon kitchen in the early Neogene. Based on this hydrocarbon kitchen evolution, oil and gas mainly accumulated after 12 Ma. The evolution of kitchen areas may provide new insights for the understanding of the oil and gas exploration potential of the offshore Bohai Bay Basin.

Margin segmentation prior to continental break-up: A seismic–stratigraphic record of multiphased rifting in the North Atlantic (Southwest Iberia)

A dense grid of multichannel (2D) seismic profiles, tied to borehole, dredge and outcrop data are used to analyze the multiphased rifting, structural architecture and rift-locus migration across the southwest Iberian margin. In the study area, three distinct sectors show different structural evolutions from the Late Triassic to the Late Jurassic–earliest Cretaceous. The three structural sectors are distinguished by: 1) the presence of incipient tilt-blocks on the inner proximal margin, which denotes limited syn-rift subsidence; 2) developed tilt-blocks on the outer proximal margin; 3) evidence of significant fault-related subsidence on the outer proximal margin during the Middle Jurassic, followed by an Oxfordian–Tithonian/Berriasian(?) rift phase leading to seafloor spreading; and 4) marked crustal stretching on the distal margin, where highly-rotated tilt-blocks overlain by thick Late Triassic to Late Jurassic units are observed. This work demonstrates that significant subsidence occurred in Southwest Iberia several millions of years prior to the latest Jurassic–earliest Cretaceous extensional episode leading to continental breakup. The magnitude of early-subsidence episodes approaches that of the last subsidence pulse preceding continental breakup. Across the southwest Iberian margin the observed structural sectors differ from each other in terms of the age of rift climax of syn-rift strata. We interpret the multiple extensional pulses recorded in Southwest Iberia as resulting not only from continental rifting between Iberia and Newfoundland, but also between Nova Scotia and Morocco. Thus, it is considered that pre-breakup units in the deep-offshore basins of Iberia comprise multiple rift-related sequences whose distribution and relative thickness depends on local subsidence rates, on the diachronous northward-migration of rifting, and on the relative crustal stretching experienced by individual sub-basins.