Cenozoic Rift Basins Research Articles

Distribution Patterns and Genesis of Geological Fractures/Microfaults in the Qiongdongnan Basin, North of the South China Sea

The Qiongdongnan Basin (QDNB), located in the north of the South China Sea, is a Cenozoic rift basin with abundant oil and gas resources. Large flake hydrates have been found in the core fractures of Quaternary formations in the deep-water depression of the QDNB. In order to understand the spatial distribution patterns of these fractures, their geneses in sedimentary basins, and their influences on gas migration and accumulation, such fractures have been observed using high-resolution 3D seismic images and visualization techniques. Four types of fractures and their combinations have been identified, namely bed-bounded fractures/microfaults, unbounded fractures, fracture bunches, and fracture clusters. Bed-bounded fractures/microfaults are mainly short and possess high density; they have developed in mass transport depositions (MTDs) or Meishan and Sanya Formations. The unbounded fractures/microfaults that occur in Miocene–Pliocene formations are mainly long and discrete, and are dominantly caused by strong tectonic movements, the concentration of stress, and sustained intense overpressure. The fracture bunches and fracture clusters that occur in Oligocene–Early Miocene formations have commonly developed with the accumulation of large numbers of fractures and may be related to the release of pressure, diapirs, and basement fault blocks (228.9 ± 1 Ma). In this study, six fluid charging or leakage models are proposed based on distinct fracture types, assuming the uniform conductivity of each fracture. In a 3D space view, a vertical decrease in the fracture scale (number or density) will more likely result in gas supply than dispersion, thus promoting the accumulation of gas in the reservoirs. Nevertheless, the fractures above the Bottom Simulating Reflect (BSR)/seismic anomaly are excessively developed, and bed-bounded fractures within a particular layer, such as MTDs, can easily cause seabed leakage. These results are useful for explaining the vertical migration of gas/fluids in areas and formations with less developed gas chimneys, faults, diapirs, and other structures, particularly in post-rifting basins.

The hydrocarbon source potential of Permo-Triassic rocks of the Indochina Block adjacent to the Paleo-Tethys suture in Thailand

The main Thailand petroleum systems are found in the syn-rift and post rift sections of Cenozoic rift basins. The Khorat Plateau area is a separate gas province with Permian limestone reservoirs possibly sourced from Triassic lacustrine, and Permo-Carboniferous marine shales. However, these source rocks and those of equivalent age west of the Khorat Plateau are not well described in the literature. This study investigated the source potential of outcropping Permo-Triassic shales from the western margin of the Khorat Plateau and the Khao Khwang Fold and Thrust Belt (KKFTB). The key formations sampled are: Permian: Alum Shale, Nong Pong, Sap Bon, Khao Khad, and Nam Duk formations; Triassic: Pang Asok and Hua Hin Lat Formation. Most potential Permo-Triassic source rocks sampled exhibited little potential due to the dominance of Type IV, non fluorescent amorphous, vitrinite and intertinite mascerals and low <1 % TOC content. Probably the Type IV kerogens are not original, but the product of subsurface alteration of other kerogen types (most likely Type III). The complex burial histories of the formations indicates such alteration could have occurred post-maturation. Probably maximum burial in the KKFTB was attained during the Triassic Indosinian Orogeny when upper diagenetic to anchizone conditions were attained. Significant burial also occurred when the Jurassic-Cretaceous Khorat Group (>2 km thick) was deposited. This later burial could be responsible for later maturation in areas north of the KKFTB. Simply in terms of TOC content >1, the Hua Hin Lat and Khao Khad formations have the most promising source potential.

Controls on Late Miocene to Early Quaternary continental sedimentation during the development of the Sohag basin, Nile Valley, Egypt

Sohag Basin is a Late Cenozoic rift basin that follows the course of the River Nile and contains a sequence of Early Miocene to Quaternary times. The present study outlines the impact of the allocyclic factors controlling a sequence of a fluvio-lacustrine succession dated from the Late Miocene to the Early Pleistocene and is subdivided into Units II and III. Unit II is made mostly of fluvial siliciclastic sediments (FA1), interpreted as channels and floodplains. Unit III consists of alluvial conglomerates (FA2) followed upwards by lacustrine carbonates (FA3) covering the whole basin. These two units are arranged in two depositional sequences (DSQ-1 and DSQ-2), bounded by sequence boundaries. Both sequences are composed of the Low- and High-Accommodation System Tracts (LAST, HAST), and they are distinguished primarily by the facies associations and the nature of the contacts that separate them. Allogenic controls strongly influenced the general evolution of the fluvio-lacustrine system. Tectonics determined the variations in thickness and lithofacies of Units II and III, where the basin was affected by the reactivation of basin-bounding faults, and suggested that the basin physiography changed from the full-graben to half-graben stage. The variation in the lacustrine facies associations and the pedogenic features show the response to tectonics and local climate conditions. Provenance played a role in the drainage pattern evolution. Using the Facies architecture and sequence stratigraphic framework, a depositional model was constructed and strongly influenced by tectonic subsidence and uplift of the basin concurrent with the reactivation of the Gulf of Suez-Red Sea rifting.

Mapping the Recent Vertical Crustal Deformation of the Weihe Basin (China) Using Sentinel-1 and ALOS-2 ScanSAR Imagery

The Weihe Basin, located in central China, is a Cenozoic rift basin that is characterized by a complicated geological background, with intense tectonic movement and relatively significant seismic activity. The faulting behaviors, slip rates, geometrical structures, and possible surface traces of the active faults in the Weihe Basin are still not well known. The goal of this work is to provide a more complete picture of recent vertical ground deformation of the basin and to identify active faults. We derived two basin-wide average InSAR deformation maps during 2015–2019 using C-band Sentinel-1A/B and L-band ALOS PALSAR2 ScanSAR imagery. The basin-wide vertical and east–west deformation components decomposed from ascending and descending InSAR measurements show that vertical movement dominates the deformation of the Weihe Basin. Subsidence and uplift maxima both occurred over the Xi’an region at rates of about −146 and 20 mm/y, respectively. A subsidence of ~38 mm/y was found at Sanyuan, Fuping, Weinan, Lantian, Qianxian, and Xingping while an uplift of ~11 mm/y was found at Fufeng, Huxian, Jingyang, Fuping, Pucheng, and Huaxian. The significant vertical deformation in these regions is spatially linked to previously identified or unmapped faults. A slip rate of ~13.7 mm/y on faults F20, F6, F10, and F7 explained the observed uplift of up to 5 mm/y in the Fufeng and Wugong areas. Furthermore, surface fault traces were clearly identified for faults F7-1, F8-1, F20, F25, and F26 based on discontinuities in the InSAR-derived vertical deformation measurements. Our results provide an accurate and economical way to delineate the surface deformation and fault movement and the associated geohazards over the Weihe Basin, and offer independent unprecedented data for further geological and geophysical interpretation.

Cenozoic basement-involved rifting of the northern South China Sea margin

The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.

Anomalous Post‐Rift Subsidence in the Bohai Bay Basin, Eastern China: Contributions From Mantle Process and Fault Activity

AbstractThe Bohai Bay Basin is the largest Cenozoic rift basin in eastern China, which exhibits a high rate of post‐rift subsidence deviating from the theoretical exponentially decay trend of thermal subsidence. The driving force for this phenomenon remains an outstanding question. Here we quantify the spatial and temporal distribution of the anomalous post‐rift subsidence by removing the thermal subsidence related to earlier stretching events from the observed tectonic subsidence. A multi‐episodic finite extension model is employed to estimate the stretching factors during rifting for nine profiles and 48 wells covering the basin. Our results show that the anomalous subsidence commenced at 12 Ma and the average anomalous subsidence rate accelerated from ∼19 m/Myr during the late Miocene to ∼75 m/Myr during the Quaternary, reaching ∼400 m at present. The anomalous subsidence is compared with published changes in dynamic topography arising from mantle flow. The temporal evolution of the dynamic topography generally fits the evolution of the anomalous subsidence, whereas variation in dynamic topography exhibits smaller amplitude and larger wavelength than our results. Small‐scale convection in the shallow mantle might play a role in generating short‐wavelength topography disturbances in the past tens of million years. Besides, the spatial distribution of the Quaternary anomalous subsidence generally coincides with fault movements in the past ∼2 Myr. We suggest mantle processes as well as fault activities might be possible mechanisms accounting for additional accommodation in the basin.

Origin of synchronous extension and inversion in a rift basin: The Phitsanulok Basin, central Thailand

Abstract The Phitsanulok Basin is a Cenozoic rift basin in central Thailand. The basin opened in the Oligocene to Early Miocene with the development of N-S and NNW-SSE trending normal faults. In the Middle Miocene the northern part of the basin continued to experience extension, while the southern part was affected by compressive inversion. Extension resumed in the Late Miocene to Pliocene, followed by post-rift subsidence in the Quaternary. The Middle Miocene anomalous deformation is attributed to activities of the NE-SW Uttaradit Fault and NW-SE Mae Ping Fault, respectively at the northern and southern basin margins. Both faults had limited activities during the Oligocene to Early Miocene rifting, where E-W extension was probably accompanied by vertical maximum principal stress. In the Middle Miocene, the maximum principal stress rotated to a N-S direction and produced sinistral motion on the Uttaradit Fault and dextral motion on the Mae Ping Fault. The strike-slip motions contributed to a counter-clockwise rotation of the continental block west of the Phitsanulok Basin, creating the synchronous extension and contraction. The kinematic model introduces the possibility of local inversion in a rift basin being originated from short-term switch between the maximum and intermediate principal stress axes and reactivation of long-lived strike-slip faults, as opposed to major change in regional stress related to plate movements. The model implies that the segment of the Mae Ping Fault south of the Phitsanulok Basin was inactive during the Oligocene to Early Miocene and was reactivated as a dextral fault in the Middle Miocene.

Crustal structure beneath the Ethiopian Plateau and adjacent areas from receiver functions: Implications for partial melting and magmatic underplating

Receiver function measurements of crustal thickness (H) and Vp/Vs provide important information on the formation and evolution of continental plateaus and rift zones such as the Ethiopian Plateau, the Afar and Main Ethiopian Rifts in northeastern Africa. Unfortunately, the presence of a low-velocity sedimentary layer atop most of the Cenozoic rift basins may lead to strong reverberations that can mask the P-to-S conversions from the Moho, resulting in unreliable H and Vp/Vs measurements. Here we estimate H and Vp/Vs beneath the Afar Depression (AD), Ethiopian Plateau (EP), and the Main Ethiopian Rift (MER) by stacking receiver functions. Reliable H and Vp/Vs measurements are obtained at a total of 69 stations, among which 18 stations are pre-processed by applying the reverberation removal technique. In comparison to the MER and EP, the AD is characterized by a thinner crust and higher Vp/Vs, and the thinnest crust and highest Vp/Vs are found in the Red Sea Rift (RSR) in central AD. Gradual variations of H and Vp/Vs between the previous and present axes of the RSR support a gradual rather than jumpy migration model of the axis. The overall high Vp/Vs values in the study area can be interpreted as a combined result of continental flood basalts at the surface, magmatic intrusion, and crustal partial melting. The relatively low amplitude of the P-to-S converted phases from the Moho observed in the southern part of the western Ethiopian Plateau and the AD with the exception of the RSR may be attributed to magmatic underplating.

Angular Unconformity of the Late Quaternary Strata in the Hetao Basin, North of the Ordos Block (West China): Timing and Its Tectonic Implications

Following the uplift of the Tibet Plateau and the continuous subduction of the Pacific Plate, graben faulting began to appear around the Ordos Block in the Cenozoic. The Hetao Basin is a Cenozoic rift basin between the Ordos Block and the Yinshan Mountains, and Late Quaternary sedimentary strata, which have lacustrine facies, are widely distributed inside this basin. However, the evolution of the Hetao Basin and its related fault systems has been debated for a long time due to the lack of tectonic evidence. In this study, four sections named Haolaigou, Bianqianghao, Huhesala, and Hazigai are selected along the north margin of the Hetao Basin. With the lithology and structural analysis of the Upper Pleistocene series in these sections, two new angular unconformities are found within the 10 m thick sedimentary sequence of the lacustrine sediments. Based on the dating results, we speculate that these two upper and lower angular unconformities are formed between 33 ka BP and 40 ka BP, and 60 ka BP and 80 ka BP, respectively. The angular unconformities also provide tectonic constraints for the latest and ongoing tectonic activity in the Quaternary. This tectonic movement begins at around 80 ka BP and causes two different unconformities of the lower strata with varying degrees of deformation (tilt) but also leads to the final death of the ancient lake.

The Influence of Pre‐existing Basement Faults on the Cenozoic Structure and Evolution of the Proximal Domain, Northern South China Sea Rifted Margin

AbstractIn this paper, we investigate the structure of the Cenozoic rift basin in the proximal domain of the northern South China Sea (SCS) margin, with an emphasis on the influence exerted by pre‐existing basement faults on rift development. Results of our work indicate that the overall rift architecture of the proximal domain of the northern SCS margin is associated with extensional faults arranged in two main structural trends, WNW‐ to EW‐trending and ENE‐trending. Structural evidence presented in this paper suggests that extensional faults of the two main structural trends mainly reactivated two pre‐existing basement fault systems mapped in previous works, a WNW‐ to EW‐striking thrust fault system (PFS1) and an ENE‐striking thrust fault system (PFS2). During the first rifting phase, both pre‐existing fault systems were reactivated; however, strains and depocenters were mainly localized on the ENE‐striking normal fault system that reactivated the PFS2. On the contrary, in the second rifting phase, extension was mainly accommodated by WNW‐ to EW‐striking normal faults that reactivated the PFS1, while most of the earlier ENE‐striking normal faults became inactive or less active. These different degrees of reactivation of the two pre‐existing fault systems during the two rifting phases suggest a clockwise rotation of the regional extension direction from NNW‐SSE in the Syn‐rift stage 1 to N‐S in the Syn‐rift stage 2. This study implies that apart from Cenozoic tectonic processes, pre‐existing structures have also played a key role during rifting evolution across the SCS margin.

Synrift magmatism in a Cenozoic rift basin from 3D seismic data, Wichianburi Sub-basin, Petchabun Basin, Thailand: part 2. How rift structure and stratigraphy modify intrusion morphology

The Latest Oligocene–Pleistocene basic sheet intrusions in the Late Oligocene–Pliocene Wichianburi Sub-basin (onshore Thailand) and Eocene intrusions of the Ceduna Sub-basin (offshore southern Australia) provide examples of igneous intrusion architecture in rift and post-rift basin settings respectively that have been imaged by 3D seismic reflection data. These examples indicate that rift-related intrusions are overall smaller, have a higher aspect ratio, and tend to be more planar or transgressive in a single direction than the saucer-shaped sills that dominate in the post-rift setting of the Ceduna Sub-basin. The long-axis trend and dip direction of most sills in the rift setting investigated tend to conform with rift structure (i.e. fault strike directions and bedding dip and strike directions), and give a strong orientation bias, whereas orientations in the post-rift basin are more varied. The few saucer-shaped sills that formed in the Wichianburi Sub-basin are asymmetric and incompletely developed. A third area, the Kora region of the Taranaki Basin, is a region where intrusions are related to a volcanic centre. This gives the intrusions a mixture of circumferential and radial intrusions related to magmatic processes (e.g. loading by the volcanic edifice, magma chamber pressure) and rift-related influences. Rift-related sills commonly form stacked arrays. Fundamental factors that affect the morphology of sills include magma composition, volume, reservoir pressure and the mechanical stratigraphy of the host rocks. In rifts extra complexities affecting some sills, or parts of sills, include rift structure (faults, rotated, folded bedding), basin size and morphology, stress distribution and sedimentary architecture. Supplementary material: Thumbnail time–structure maps of 107 sills from the Wichianburi Sub-basin mapped from 3D seismic reflection data are available at https://doi.org/10.6084/m9.figshare.c.4620101

Synrift magmatism in a Cenozoic rift basin, from 3D seismic data, Wichianburi Sub-basin, Phetchabun Basin, Thailand: part 1. Distribution and timing of intrusions and forced folds

The Late Oligocene–Pliocene Wichianburi Sub-basin, Phetchabun Rift, Thailand has undergone three phases of development: Late Oligocene–Early Miocene ( c. 16 Ma) east–west extension on predominantly NNW–SSE- to north–south-striking faults; 16 Ma– c. 3 Ma NE extension on NNW–SSE-, north–south- and NE–SW-trending faults; and 3 Ma to Present post-rift subsidence. The basin was also affected by intrusive and extrusive igneous activity from 24 to 2 Ma. The relationships among the structure, rift basin stratigraphy and sheet intrusions are unusually well documented as a result of extensive hydrocarbon exploration drilling and 3D seismic data, together with the production of hydrocarbons from igneous sills. The oldest igneous activity ( c. 24–13 Ma) lies SW of the Wichianburi Sub-basin, where extrusive rocks exposed at the surface have been dated by K–Ar and 40 Ar/ 39 Ar methods. The overall igneous activity youngs from the south of the basin to the north (11–9 Ma). Numerous domal folds occur across the basin; the sill and laccolith dimensions, the rate of fold development, the variable timing of fold formation and the fold geometry all indicate that the folds are related to magma emplacement, not tectonic inversion. Fold formation occurred at c. 16–15 and 12–11 Ma in single or multiple stages. The timing of folding, coupled with the radiometric dating of some sills (K–Ar, 39 Ar/ 40 Ar), indicates that the greatest amount of intrusive activity occurred between 16 and 11 Ma. The 16 Ma date coincides with the switch from east–west to NW–SE extension, which promoted dilatancy within the upper crust along a range of synrift fault orientations and some pre-rift trends. The burial history of the main depocentre shows that Miocene sill emplacement occurred at depths &lt;1 km, but was still able to cause the local maturation of hydrocarbon source rocks. Sill emplacement affected the petroleum system by forming migration pathways, important reservoirs and also creating forced fold traps.

Palaeogene–Neogene sedimentary and tectonic evolution of the Yinchuan Basin, western North China Craton

ABSTRACTThe Yinchuan Basin is one of the Cenozoic rift basins around the Ordos Block in the North China Craton and is also the frontier zone of the north-eastern expansion of the Tibetan Plateau. Due to its special geotectonic position, the views on formation mechanisms and evolution process of the Yinchuan Basin are still controversial. Herein, we present a detailed study of the Palaeogene–Neogene sediment sequence in and around the basin coupled with activities of faults. Our results demonstrate that the Yinchuan Basin mainly underwent two evolutional stages during the Palaeogene–Neogene. At the first stage, from the Eocene to the late middle Miocene, the Yinchuan Basin was dominated by a NW–SE extension with deposits of the Sikouzi, Qingshuiying, and Zhangenbu Formations. The incipient Yinchuan Basin was developed on pre-existing basement faults and deposits of the Sikouzi Formation (E2s). The Qingshuiying Formation (E3q) consists mainly of lacustrine mudstones with unified eastward palaeocurrents in the basin and its periphery, indicating that a unitived lake existed in the studied area during the Oligocene. The alluvial fan conglomerates in the lower part of the Zhangenbu Formation (N1z) developed on both sides of the Helan Mountains as a response to the initial uplift of the mountains. At the second stage, from the late Miocene to the Pliocene, the sedimentation of the Yinchuan Basin was dominantly controlled by a NE–SW compression that led to a brief uplift of the study area. After a short sedimentary discontinuity, the Ganhegou Formation (N1-2g) was deposited in the basin. The evolutional features of the two stages of the basin are consistent with that of the regional tectonics. It is suggested that the tectonic evolution of the Yinchuan Basin in the early and late stages was mainly controlled by the Pacific plate’s subduction and the northeastward expansion of the Tibetan Plateau, respectively.

인도네시아 칼리만탄 남동부 바리토 분지의 석유 탐사 잠재성 고찰: 인접한 쿠타이 분지와의 석유시스템 비교

The Kutai and Barito basins are the Cenozoic rift basins in southeastern Kalimantan, Indonesia and are very mature for hydrocarbon exploration history. Both basins have experienced similar structural evolution and depositional history. Despite the similarity between two basins, large oil and gas fields were found in the Kutai Basin, whereas not common in the Barito Basin and localized in its northeastern part. Depositional history of both basins evolved from lacustrine syn-rift to marine post-rift stages through fluvio-deltaic transitional environment. However, different type of petroleum system has been identified in the Barito and Kutai basins. Syn-rift lacustrine to fluvio-deltaic sequence (Tanjung Formation) is an essential element of petroleum system in the Barito Basin and post-rift deltaic sequence (Balikpapan and Kampung Baru formations) is a key element in the Kutai Basin. Such difference in the petroleum system type is associated with difference in burial history, affected by spatio-temporal variation of uplifting and basin subsidence. The differential uplifting of the Meratus Mountains allowed the syn-rift Tanjung Formation to reach thermal maturation only in the northeastern Barito Basin. The uplifting of the Central Kalimantan Mountains, however, caused erosion of the syn-rift sequence in the Kutai Basin and supplied enough sediments to form the post-rift deltaic sequence with thermal maturity. Future exploration potential of the Barito Basin still remains in the well-known, syn-rift Tanjung Formation and needs to be further evaluated from the pre-Cenozoic fractured basement, the post-rift Warukin, and the Berai limestone reservoirs particularly in the western region of the Barito Basin.

Depositional sequence response to Paleogene Lake level and Paleoclimate changes in the northern Liaoxi depression, eastern China

Bohai Bay Basin is a Cenozoic rift basin containing abundant oil and gas resources in northeastern China. By combining 3D seismic data and well logs, three composited sequences are identified from regional unconformity relationships, and seven sequences can be further identified from local unconformities. Major depositional systems include fan delta, braided delta, fluvial delta, shallow lake, deep lake, and sublacustrine fan. Through identifying onlap points in the ramp of the Liaoxi depression, a lake level change curve is constructed. Pollen and algae data are used to analyze paleoclimate which shows two large cool-warm cycles in Paleogene Liaoxi depression. Lake level change is controlled by both tectonic subsidence and climate change. Paleoclimate change is the main factor controlling lake level during stable tectonic periods, while tectonic subsidence is the main controlling factor controlling lake level during active tectonic periods. Depositional evolution corresponds with lake level change, which indicates that lake level stays relatively high with deep or shallow lacustrine and delta front deposits developed, and lake level stays relatively low with alluvial fan, braid delta plain, fan delta, and shallow lacustrine deposits developed.

Benthic foraminiferal fauna and sediment provenance of Eocene syn-rift sequences in Taiwan: implication for onset of Asian epi-continental marginal seas off China coast

Early Cenozoic rift basins developed commonly on the Mesozoic basement along the SE Asia Continent. However, Eocene–Oligocene sequences were only exposed widely onshore Taiwan. We study benthic foraminifera and determine U–Pb geochronology of detrital zircon grains in the Early Cenozoic sequence onshore Taiwan for understanding if there was any significant change in faunal content and sediment provenance between the Eocene syn-rift sequences and the overlying Oligocene post-rift strata across the breakup unconformity. The Eocene syn-rift sequences are mostly of thick, cross-bedded sandstones of fluvial-deltaic depositions without marine fauna. Intensive study on the Eocene stratigraphy in more than ten sections all over the Taiwan mountain belt in the last three decades, shallow marine benthic foraminifera of Gaudryina hayasakai-Hanzawaia asterizana-Planularia okinoshimaensis-Heterolepa praecinta assemblage were found only in very limited exposures in the Western Foothills, central Taiwan. This study shows that the Eocene benthic foraminiferal assemblage is similar to the Shihtsaoan fauna in the overlying Oligocene post-rift marine strata. This indicates no significant faunal break across the breakup unconformity. As a consequence, Gaudryina hayasakai Zone of the Shihtsaoan age can extend downward to the Eocene and the Shihtsaoan Stage would be equivalent to the Hsuehshanian Stage established previously from northern Taiwan. The Taiwan Eocene marine sequences could be deposited in a normal-fault bounded narrow seaway or gulf connecting southeastward to the open Huatung Basin. U–Pb geochronology of detrital zircons in the Taiwan Eocene syn-rift sandstones shows a prominent age peak centered in Yenshanian (100–155 Ma) similar to the Paleocene–Oligocene fluvial-deltaic deposits in the East China Sea and is also comparable to the U–Pb zircon age spectrum of modern river sands of the Fujian Province, SE China. Petrographic study also suggests a prevailing sediment provenance of the Cathaysia granites and volcanics. These Cathaysia detritals were transported to the Taiwan Eocene grabens through some SE-flowing rivers. Structural reconstructions in previous study indicated that the mountain belt with the Eocene sequences in NE Taiwan had been shortened by imbricated thrusting westward for 160–200 km wide during the arc-continent collision in the last 6.5 Ma. Integrating paleontology and sedimentology evidence reveal that the Eocene coast was located along the modern shelf break of the East China Sea ~ 350–400 km east off the modern Chinese coast line. This implies that in the Eocene time (~ 40 Ma) there was no wide Asian epi-continental marginal seas off Chinese coast until the onset sedimentation of the Oligocene post-rift strata during opening of the South China Sea in the Early Oligocene time (~ 33 Ma).

Geometry and Evolution of the Cangdong Sag in the Bohai Bay Basin, China: Implications for Subduction of the Pacific Plate

The Cangdong Sag is a complex Cenozoic rift basin at the center of the Bohai Bay Basin. Cenozoic structures in the Cangdong Sag can be subdivided into the Cangdong Fault System in the west and the Xuxi Fault System in the east. The geometry of the boundary faults varies along the axes of half-grabens. According to the cross-sectional strata geometry, unconformity and planar structural pattern, the Cenozoic structural evolution of the Cangdong Sag can be divided into four distinct stages: (1) major Paleocene initial rift, (2) latest Paleocene–early Eocene intensive rift, (3) late Eocene–Oligocene strike–slip superimposed rift, and (4) Neogene to present-day post-rift depression. The extensional deformation was mainly derived from horizontal stress induced by the upwelling of asthenosphere. The strike–slip structure of the Cangdong Sag provides important information related to the subduction of the Western Pacific Plate. It was found that the strike–slip movement of the southern Xuxi Fault Zone was activated during the deposition of the third member of Shahejie Formation to the Dongying Formation; therefore, ~43 Ma probably marks the time when the Western Pacific Plate initially changed its subduction direction from northwest to nearly west.

Modelling basalt weathering at elevated CO 2 concentrations: implications for terminal to post-magmatic rifting in the Deccan Traps, Kachchh, India

Abstract Deccan volcanism was synchronous with rifting along the west coast of India. Pre- and synmagmatic rifting has been widely reported in the Deccan Volcanic Province, but extension post-dating magmatism, and predating India–Eurasia collision, is less well known. A recent study in the Kachchh area of western India documented weathering of basalts to kaolinite at the base of Cenozoic rift basins, with rift flanks relatively less altered to smectites, and this was attributed to post-magmatic rifting. This study models basalt weathering under open- and closed-system conditions to simulate rainwater interacting with basalts either on topographical slopes (within rifts) or on flat-topped hills (flow tops). Both systems were modelled under p C O 2 conditions ranging from low, present-day values to higher values more appropriate for the end-Cretaceous–early Paleocene time, after basalt emplacement. The results show that if p C O 2 exceeded values of 10 −2.5 , basalts would be altered to kaolinite in both open and closed systems. Existing p C O 2 estimates in the aftermath of Deccan volcanism fall below this value, implying that the differential basalt weathering was more likely to have been caused by terminal to post-magmatic rifting. This indicates that extensional tectonics along the Indian west coast in the Kachchh region continued even after cessation of Deccan volcanism.

Syn- and post-depositional sand bodies in lignite – the role of coal analysis in their recognition. A study from the Frimmersdorf Seam, Garzweiler open-cast mine, western Germany

The Lower Rhine Basin is a Cenozoic-age rift basin at the NW end of the European Cenozoic Rift Basin. In Middle Miocene times, fluctuating sea-levels in the North Sea and the repeated spreading of paralic mires resulted in the deposition of marine sand units (Frimmersdorf and Neurath sands), alternating with thick lignite beds (Morken, Frimmersdorf and Garzweiler seams). Within the Frimmersdorf Seam, the presence of sand bodies has been interpreted both in terms of syn-depositional emplacement in an estuarine setting, as well as post-depositional sand injection and soft-sediment deformation partly linked to tectonic activity within the Lower Rhine Basin. While understanding of the causal mechanisms has improved, early recognition of sand concentrations in future mine area remains difficult. Therefore, organic petrological and inorganic geochemical analyses of sand and lignite samples were carried out in order to assess their efficacy as proxies for such studies. The results reflect the complexity of the depositional environment, which was influenced by both fresh and marine waters. Ash yields of the lignite samples are variable and sulphur contents as well as Ca/Mg ratios depend on the distance from the overlying marine Neurath Sand. Humic substance values of the lignites are dependent on the sampling position within the seam, and may be related to sand body deposition. Moderate contents of gelified macerals and the small size of huminite particles reflect the depositional environment during peat formation, but are not influenced by sand body deposition, not even when in close proximity to the sands. While the geochemical and petrological methods provide much information on the depositional environment of the Miocene-age Lower Rhine Basin, and also on marine versus freshwater deposition of the sands, they do not facilitate the differentiation of syn- and post-depositional sand emplacement in the Frimmersdorf Seam.

Neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern China

Mantle convection could have a significant effect on basin evolution; however, research quantifying this relationship is controversial. To understand the formation mechanism and evolution of the Cenozoic rift basins in eastern China, we applied the back-stripping technique and strain rate inversion modeling to 119 wells from Sangjiang Basin, Bohai Bay Basin, North Yellow Sea Basin, South Yellow Sea Basin, East China Sea Shelf Basin, and four basins within the northern South China Sea margin. The modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern China. Residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences. Our results show that the residual subsidence since 20Ma in eastern China generally increases from ∼100 to 300m in terrestrial areas to ∼1.2–1.8km onthe continental shelf. Our observed residual subsidence is generally consistent with the present-day dynamic topography that is predicted from mantle flow models, and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography. The migration pathway of the residual subsidence since 20Ma is consistent with the movement direction of the Pacific and Philippine plates. Therefore, we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern China.