Tectonic-thermal history and hydrocarbon potential of the Pearl River Mouth Basin,northern South China Sea:Insights from borehole apatite fission-track thermochronology

The Pearl River Mouth Basin(PRMB)is one of the most petroliferous basins on the northern margin of the South China Sea.Knowledge of the thermal history of the PRMB is significant for understanding its tectonic evolution and for unraveling its poorly studied source-rock maturation history.Our investigations in this study are based on apatite fission-track(AFT)thermochronology analysis of 12 cutting samples from 4 boreholes.Both AFT ages and length data suggested that the PRMB has experienced quite complicated thermal evolution.Thermal history modeling results unraveled four successive events of heating separated by three stages of cooling since the early Middle Eocene.The cooling events occurred approximately in the Late Eocene,early Oligocene,and the Late Miocene,possibly attributed to the Zhuqiong II Event,Nanhai Event,and Dongsha Event,respectively.The erosion amount during the first cooling stage is roughly estimated to be about 455-712 m,with an erosion rate of 0.08-0.12 mm/a.The second erosion-driven cooling is stronger than the first one,with an erosion amount of about 747-814 m and an erosion rate between about 0.13-0.21 mm/a.The erosion amount calculated related to the third cooling event varies from 800 m to 3419 m,which is speculative due to the possible influence of the magmatic activity.

Parameterized thermal model of a mixed mantle convection

Simple parameterized models, either whole mantle convection or layered mantleconvection, cannot explain the tectonic characteristics of the Earth's evolution history, therefore a mixed mantle convection model has been carried out in this paper. We introduce a time-dependent parameter F, which denotes the ratio betWeen the mantle material involved in whole mantle convection and the material of the entire mantle, and introduce a local Rayleigh number Raloc as well as two critical numbers Ra1 and Ra2. These parameters are used to describe the stability of the phase boundary between the upper and lower mantle. The result shows that the mixed mantle convection model is able to simulate the episodic tectonic evolution of the Earth.

Low Temperature Thermal History Reconstruction Based on Apatite Fission-Track Length Distribution and Apatite U-Th/He Age Using Low-T Thermo

Low temperature thermochronology plays a key role in the study of the tectonic evolution of the upper crust.History modeling of apatite fission-track requires the apparent age and the confined track-length distribution of spontaneous tracks.Obtaining length data does not require either thermal neutron irradiation or LA-ICP-MS measurements of the uranium content of the grains.This paper attempts to decouple the apatite fission-track age from the apatite fission-track length,but to combine the fission-track lengths with the respective apatite U-Th/He age to model the thermal history.The experiments were designed and conducted using a new Mathematica®modeling software“Low-T Thermo”.Results of this modeling show that the thermal history modeling of apatite U-Th/He and fission-track ages can constrain the apatite fission-track length thermal history in the He partial retention zone and fission-track partial annealing zone,respectively.It implies that this combination of apatite fissiontrack length and apatite U-Th/He age has not been implemented before but is presented here as an alternative way of determining thermal histories without the addition of apatite fission-track age.

Latest Triassic to Early Jurassic Thrusting and Exhumation in the Southern Ordos Basin, North China: Evidence from LA-ICP-MS-based Apatite Fission Track Thermochronology

The contractional structures in the southern Ordos Basin recorded critical evidence for the interaction between Ordos Basin and Qinling Orogenic Collage. In this study, we performed apatite fission track （AFT） thermochronology to unravel the timing of thrusting and exhumation for the Laolongshan-Shengrenqiao Fault （LSF） in the southern Ordos Basin. The AFT ages from opposite sides of the LSF reveal a significant latest Triassic to Early Jurassic time-temperature discontinuity across this structure. Thermal modeling reveals at the latest Triassic to Early Jurassic, a ~50~C difference in temperature between opposite sides of the LSF currently exposed at the surface. This discontinuity is best interpreted by an episode of thrusting and exhumation of the LSF with -1.7 km of net vertical displacement during the latest Triassic to Early Jurassic. These results, when combined with earlier thermochronological studies, stratigraphic contact relationship and tectono-sedimentary evolution, suggest that the southern Ordos Basin experienced coeval intense tectonic contraction and developed a north-vergent fold-and-thrust belt. Moreover, the southern Ordos Basin experienced a multi-stage differential exhumation during Mesozoic, including the latest Triassic to Early Jurassic and Late Jurassic to earliest Cretaceous thrust-driven exhumation as well as the Late Cretaceous overall exhumation. Specifically, the two thrust-driven exhumation events were related to tectonic stress propagation derived from the latest Triassic to Early Jurassic continued compression from Qinling Orogenic Collage and the Late Jurassic to earliest Cretaceous intracontinental orogeny of QinUng Orogenic Collage, respectively. By contrast, the Late Cretaceous overall exhumation event was related to the collision of an exotic terrain with the eastern margin of continental China at -100 Ma.

Mesozoic Tectonothermal Evolution of the Southern Central Asian Orogenic Belt: Evidence from Apatite Fission-Track Thermochronology in Shalazha Mountain, Inner Mongolia

Mesozoic intracontinental orogeny and deformation were widespread within the southern Central Asian Orogenic Belt(CAOB). Chronological constraints remain unclear when assessing the Mesozoic evolution of the central segment of this region. The tectonic belt of Shalazha Mountain located in the center of this region is an ideal place to decode the deformation process. Apatite fission-track(AFT) thermochronology in Shalazha Mountain is applied to constrain the Mesozoic tectonothermal evolution of the central segment of southern CAOB. The bedrock AFT ages range from 161.8 ± 6.9 to 137.0 ± 7.3 Ma, and the first reported detrital AFT obtained from Lower Cretaceous strata shows three age peaks: P1(ca. 178 Ma), P2(ca. 149 Ma) and P3(ca. 105.6 Ma). Bedrock thermal history modeling indicates that Shalazha Mountain have experienced three stages of differential cooling: Late Triassic–Early Jurassic(~230–174 Ma), Late Jurassic–Earliest Cretaceous(~174–135 Ma) and later(~135 Ma). The first two cooling stages are well preserved by the detrital AFT thermochronological result(P1, P2) from the adjacent Lower Cretaceous strata, while P3(ca. 105.6 Ma) records coeval volcanic activity. Furthermore, our data uncover that hanging wall samples cooled faster between the Late Triassic and the Early Cretaceous than those from the footwall of Shalazha thrust fault, which synchronizes with the cooling of the Shalazha Mountain and implies significant two-stage thrust fault activation between ca. 230 and 135 Ma. These new low-temperature thermochronological results from the Shalazha Mountain region and nearby reveal three main phases of differential tectonothermal events representing the Mesozoic reactivation of the central segment of the southern CAOB. In our interpretations, the initial rapid uplift in the Late Triassic was possibly associated with intracontinental orogenesis of the CAOB. Subsequent Middle Jurassic–Earliest Cretaceous cooling is highly consistent with the Mesozoic intense intraplate compression that occurred in the southern CAOB, and is interpreted as a record of closure of the Mongol-Okhotsk Ocean. Then widespread Cretaceous denudation and burial in the adjacent fault basin could be linked with the oblique subduction of the Izanagi Plate along the eastern Eurasian Plate, creating a northeast-trending normal fault and synchronous extension. However, our AFT thermochronometry detects no intense Cenozoic reactivation information of Shalazha Mountain region.