Stephen J. Vincent

Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China

Abstract The Bohai Basin is one of a family of early Cenozoic extensional basins that lie along the eastern margin of Asia from Russia to Vietnam. Initial extension was probably triggered by subduction roll-back of the oceanic Pacific Plate from the Asian continent. There were two phases in the Bohai Basins rift history. The earlier, Paleocene-early Eocene phase resulted in the deposition of the Kongdian Formation and the fourth (lowest) member of the Shahejie Formation in a series of elongate half grabens. These half grabens have master faults with a NNE-SSW orientation. Secondary normal faults are typically clockwise oblique to the master faults, indicating a component of dextral transtension. Deposition was focused in the west and south of the present basin. These rocks are mainly alluvial fan and fluvial red beds. The architecture of the basin underwent an important change at ca. 43–45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half grabens as the Kongdian Formation, but the Bozhong Depression in the central part of the basin originated at this time, and became the major depocentre. The Bozhong Depression superficially resembles a pull-apart basin. It formed when continued transtension of the earlier Tertiary fault systems to the east and west created an extensional overlap between them. During this phase, the basin as a whole had a geometry with elements typical of both pull-apart and transtensional basins. Regional extension in many east Asian basins ended at the end of the Oligocene, probably because of the onset of transpression within eastern Asia, caused by the collision of Australia with the Philippine Sea Plate. Dextral transpression caused minor inversion of some of the earlier normal faults in Bohai, but as a whole the basin began to subside in a post-rift phase of thermal subsidence that has lasted until the present day.

Insights from the Talysh of Azerbaijan into the Paleogene evolution of the South Caspian region

The age and mode of formation of the South Caspian Basin are disputed. An ~10-km-thick, predominantly middle Eocene clastic and volcanic succession is exposed in the Talysh mountains of Azerbaijan at its western margin. Here, high-K alkali basalts pass laterally to the east and southeast into volcanogenic sandstone-dominated turbidity current and debris-flow deposits. These southeasterly directed depositional systems accumulated in water depths generally greater than 200 m and fed directly into the western South Caspian Basin. New Ar-Ar ages cluster around 39 Ma, with an upper, 1400-m-thick volcanic interval being deposited in 2.2 ± 0.2 m.y. We interpret that this rapid deposition and magmatism records a major back-arc extensional/transtensional event in the Talysh, north of the north-dipping Neotethyan subduction zone. This event is recognized across much of southwest Asia and may indicate a period of significant basin formation within the adjacent South Caspian Basin. A transition into Upper Eocene–Lower Oligocene strata, dominated by fine-grained turbidity current and hemipelagic sediments with slope instability features, is interpreted to mark the end of rifting and volcanism in the Talysh and the start of the Arabia-Eurasia collision. Overlying Oligocene coarse clastic rocks are interpreted as the erosional products of localized topography created by the further propagation of compressional deformation into the Talysh region.

LATE CENOZOIC TECTONICS OF THE KEPINGTAGE THRUST ZONE : INTERACTIONS OF THE TIEN SHAN AND TARIM BASIN, NORTHWEST CHINA

The Kepingtage (Kalpin) thrust zone, northwest China, is an actively deforming part of the India-Asia collision system. It lies south of the Tien Shan, has an area of ∼16,000 km2, and consists of arcuate, emergent imbricates. Overall vergence is toward the interior of the Tarim Basin to the south. Thrust sheets typically expose Upper Cambrian to Permian platformal strata. Thrusting is largely thin-skinned; thin Upper Cambrian evaporites are likely to be the main decollement horizon. The Kepingtage thrust zone is the only margin of the Tarim Basin to deform in this style in the Cenozoic. It is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the Tarim Basin. Thick (up to 10 km) Mesozoic and Cenozoic clastic successions are present and deformed in these regions. Such successions are not present in the Kepingtage thrust zone or the adjacent Bachu Uplift within the Tarim Basin, because of episodic activity on steep, northwest-southeast trending thrusts which define the margins of the Bachu Uplift. These Mesozoic-Cenozoic strata may have suppressed the ability of regions along strike from the Kepingtage thrust zone to deform by thin-skinned thrusting utilizing the Upper Cambrian decollement. The along strike variation in active thrusting at the southern margin of the Tien Shan is an example of syntectonic sedimentation controlling thrust belt deformation style. A balanced section across the thrust zone indicates ∼28% shortening, equivalent to ∼35 km. This is equivalent to an average slip rate of ∼1.8 mm yr−1 and a strain rate of ∼4.4 × 10−16 s−1, assuming deformation began at circa 20 Ma. Active deformation is focused along the frontal thrust, the Kepingtage Fault. The northern boundary of the Kepingtage thrust zone is formed by the South Tien Shan Fault. This major, north dipping thrust is seismically active, with published earthquake focal depths at midcrustal levels (14 and 18 km). It juxtaposes upper Carboniferous sedimentary rocks of different facies and different Paleozoic deformation histories. Speculatively, it represents a reactivation of a late Paleozoic thrust, which originated at the boundary between the platformal interior of the Tarim Block and a deeper-water foreland basin to its north.

Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Abstract Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.

EVOLUTION OF THE MINLE AND CHAOSHUI BASINS, CHINA : IMPLICATIONS FOR MESOZOIC STRIKE-SLIP BASIN FORMATION IN CENTRAL ASIA

The Minle and Chaoshui Basins of northern China are pull-apart basins that originated during Jurassic to Early Cretaceous strike-slip and extensional deformation in the western North China block (Alashan), and the region to its southwest, known as the Hexi Corridor. The basins are separated by the Longshou Shan, which has been a positive topographic feature since Jurassic time. Lower Cretaceous successions in the Minle and Chaoshui Basins display similar rift-related, fining-upward motifs. These comprise alluvial conglomerates and sandstones that pass upward into lacustrine mudrocks, followed by a final fine-grained alluvial phase with extensive paleosol development. The upper part of each succession includes a second lacustrine interval, which is related either to a short-lived humid climatic period, or downstream damming of the exit for the fluvial systems of both basins. North-south–trending normal faults and rare synsedimentary rotation and slump structures affect the basin fills. The Minle and Chaoshui Basins are only two of a series of Late Jurassic–Early Cretaceous strike-slip–related basins in this region that nucleated on three basement blocks, Tarim, Qilian Shan, and Alashan. All of the basins are characterized by a nonmarine, alluvial-lacustrine fill, controlled by a combination of strike-slip and normal faults. From an analysis of the distribution, orientation, and motion sense of subsurface and exposed faults we conclude that three types of strike-slip–related basins are present: pull-apart basins (Minle, Huahai-Jinta, Chaoshui), transtensional basins (Southwest Badanjilin, northern Wuwei), and a third type that developed between divergent strike-slip faults of opposing motion senses. We term this type “extrusion fault-wedge basins” (Jiuquan, Bayanhaote, western Wuwei). The cause of Late Jurassic to Early Cretaceous strike-slip and extension in this region is interpreted to be the Lhasa block–Asia collision. Compression arising from this event is speculated to have caused eastward extrusion of the crust of the Hexi Corridor and Alashan. Deformation was accommodated by a 25° counterclockwise rotation of the Alashan with respect to the remainder of the North China block; thrusting occurred in the intervening Helan Shan. The extension in the Hexi Corridor occurred at the same time as vast foreland basins formed to the west (Tarim) and east (Ordos), and while extension affected much of eastern Mongolia and northeastern China.

Fault reactivation in the Junggar region, northwest China : the role of basement structures during Mesozoic-Cenozoic compression

Basement structures exposed at the margins of the Junggar Basin were created during the Altaid orogeny in the Late Palaeozoic. The most prominent structures are backstops to subduction-accretion complexes (North Tien Shan Fault), or major thrusts within these complexes (Dalabute and Kelameili faults). Both types of basement structure are far more common in accretionary, Turkic-type, orogens such as the Altaids than true sutures. Probably the only suture sensu stricto in the Junggar area is cryptic, and lies under the Junggar Basin’s thick Mesozoic-Cenozoic cover. The exposed Palaeozoic fault zones have been reactivated by Mesozoic-Cenozoic compressional events, which are the long-distance expression of orogenies at the southern margin of Asia. Latest Palaeozoic and Mesozoic events reactivated a larger number of fault zones than have been affected by the Cenozoic India-Asia collision, possibly because of an increase in the strength of the Junggar basement over time, following Late Permian rifting. Cenozoic strain is partitioned between strike-slip motion on basement structures within the Palaeozoic orogenic belts around the Junggar Basin, and numerous thrusts and transpressive faults in regions marginal to and within the basin itself. Most major strike-slip faults are reactivated structures, and occupy narrower zones than their Palaeozoic precursors. Thrust zones follow the Palaeozoic basement grain, but active faults have propagated into the Mesozoic-Cenozoic cover.

The exhumation of the western Greater Caucasus: a thermochronometric study

This study provides 39 new thermochronometric analyses from the western part of the Greater Caucasus, a region in which existing data are extremely limited and of questionable quality. The new results are consistent with field studies that identify Triassic to Middle Jurassic (Cimmerian) and Oligo-Miocene (Alpine) orogenic erosional events. An inverse relationship between the fission track and depositional ages of Oligo-Miocene sedimentary samples also implies some degree of Eocene erosion of the Greater Caucasus and intermediate sediment storage. Cooling ages and field relationships within the core of the range, west of Mt Elbrus, require ~5 km of Permo-Triassic exhumation and restrict the overall amount of Cenozoic exhumation to ~2.5 km. Current exhumation rates are typically low, and do not support a Plio-Pleistocene increase in climate-driven denudation. High (~1 km Ma−1) rates of exhumation are restricted to the southern flank of the range in northwest Georgia. Despite a general lack of significant seismicity within the study region, this exhumation peak is close to the largest instrumentally recorded earthquake in the Caucasus (Ms = 7.0). This may suggest that exhumation is associated with the decoupling of the sedimentary succession from its crystalline basement in the southern part of the range and the inversion of the largely Jurassic fill of the Greater Caucasus basin. Rates of exhumation are compatible with this being driven by active shortening. Further sampling and analysis are required to provide a higher-resolution, low-temperature thermochronology of Alpine exhumation, to isolate the drivers for Palaeogene Dziruli Massif cooling and uplift, and to constrain better the extent of the current, localized phase of rapid exhumation.

Structural Features of Northern Tarim Basin: Implications for Regional Tectonics and Petroleum Traps

The rhombus-shaped Tarim basin in northwestern China is controlled mainly by two left-lateral strike-slip systems: the northeast-trending Altun fault zone along its southeastern side and the northeast-trending Aheqi fault zone along its northwestern side. In this paper, we discuss the northern Tarim basins structural features, which include three main tectonic units: the Kalpin uplift, the Kuqa depression, and the North Tarim uplift along the northern margin of the Tarim basin. Structural mapping in the Kalpin uplift shows that a series of imbricated thrust sheets have been overprinted by strike-slip faulting. The amount of strike-slip displacement is estimated to be 148 km by restoration of strike-slip structures in the uplift. The Kuqa depression is a Mesozoic-Cenozoic foredeep depression with well-developed flat-ramp structures and fault-related folds. The Baicheng basin, a Quaternary pull-apart basin, developed at the center of the Kuqa depression. Subsurface structures in the North Tarim uplift can be divided into the Mesozoic-Cenozoic and the Paleozoic lithotectonic sequences in seismic profiles. The Paleozoic litho-tectonic sequence exhibits the interference of earlier left-lateral and later right-lateral strike-slip structures. Many normal faults in the Mesozoic-Cenozoic litho-tectonic sequence form the negative flower structures in the North Tarim uplift; these structures commonly directly overlie the positive flower structures in the Paleozoic litho-tectonic sequence. The interference regions of the northwest-trending and northeast-trending folds in the Paleozoic tectonic sequence have been identified to have the best trap structures. Our structural analysis indicates that the Tarim basin is a transpressional foreland basin rejuvenated during the Cenozoic.

Along-strike variations in the composition of sandstones derived from the uplifting western Greater Caucasus: causes and implications for reservoir quality prediction in the Eastern Black Sea

Abstract Oligo-Miocene outcrops along the southern margin of the western Greater Caucasus preserve a record of sediments shed from the range into the northern and central parts of the Eastern Black Sea. Sandstones in the Russian western Caucasus are significantly more quartz-rich than those located farther SE in western Georgia. The latter contain appreciably more mudstone and volcanic rock fragments. Oligo-Miocene turbidite systems derived from the Russian western Caucasus in the Tuapse Trough and central Eastern Black Sea may therefore form better-quality reservoirs at shallow to moderate depths than sediments derived from west Georgian volcaniclastic sources in the easternmost part of the basin. Palynomorph analysis indicates sediment derivation predominantly from Jurassic and Cretaceous strata in the Russian western Caucasus and from Eocene strata, and an increasing proportion of Cretaceous strata upsection, in western Georgia. An Eocene volcaniclastic source is proposed for the increased rock fragment component in west Georgian sandstones. Eocene volcaniclastic rocks are no longer exposed in the Greater Caucasus, but similar rocks form the inverted fill of the Adjara–Trialet Basin farther south in the Lesser Caucasus. The former presence of a northern strand of this basin in the west Georgian Caucasus is supported by earlier thermochronological work. Supplementary material: A sample data table, petrographic data table, petrographic key, QFL sandstone compositional plot and palynomorph reworking Stratabugs™ charts are available at www.geolsoc.org.uk/SUP18662.

The formation and inversion of the western Greater Caucasus Basin and the uplift of the western Greater Caucasus: implications for the wider Black Sea region

The western Greater Caucasus formed by the tectonic inversion of the western strand of the Greater Caucasus Basin, a Mesozoic rift that opened at the southern margin of Laurasia. Subsidence analysis indicates that the main phase of rifting occurred during the Aalenian to Bajocian synchronous with that in the eastern Alborz and, possibly, the South Caspian Basin. Secondary episodes of subsidence during the late Tithonian to Berriasian and Hauterivian to early Aptian are tentatively linked to initial rifting within the western, and possibly eastern, Black Sea and during the late Campanian to Danian to the opening of the eastern Black Sea. Initial uplift, subaerial exposure, and sediment derivation from the western Greater Caucasus occurred at the Eocene-Oligocene transition. Oligocene and younger sediments on the southern margin of the former basin were derived from the inverting basin and uplifted parts of its northern margin, indicating that the western Greater Caucasus Basin had closed by this time. A predominance of pollen representing a montane forest environment (dominated by Pinacean pollen) within these sediments suggests that the uplifting Caucasian hinterland had a paleoaltitude of around 2 km from early Oligocene time. The closure of the western Greater Caucasus Basin and significant uplift of the range at approximately 34 Ma is earlier than stated in many studies and needs to be incorporated into geodynamic models for the Arabia-Eurasia region.