Tectonic Thermal Evolution History Of The Central Uplift Of The South Yellow Sea Basin From CSDP-2 Drilling Well

The central uplift is the largest tectonic unit of the South Yellow Sea Basin(SYSB),which is not only the key study area of regional tectonic evolution,but also the potential area of current resource exploration.Based on the comprehensive description of drilling cores,experimental analysis,interpretation of logging and seismic data of the first deep whole-core drilling CSDP-2 well in central uplift of SYSB,the burial history and tectonic thermal evolution history have been studied by a serious of research methods and simulation techniques.The first-stage drilling results of the CSDP-2 well have revealed the Neogene,Triassic,Permian,Carboniferous and Devonian.The burial history of central uplift has been studied based on the establishment of stratigraphic framework,seisimic interpretation and estimation of eroded thickness.Dominated by regional transgression during Carboniferous,the central uplift gradually turned into the stage of marine deposition after the late Devonian continental deposition,but the deposition rate was relatively lower.The deposition rate is significantly accelerated during the early Permian,then the alternative marine and continental deposition has formed because of regional uplift and regression at the end of early Permian.The study area suffered intense tectonic compression influenced by the Indosinia tectonic movement during Triassic.The detachment surface interlaid in the Permian argillaceous strata interval drilled by CSDP-2 may be the result of tectonic thrust during this period.The central uplift experienced strong denudation from the middle-late Cretaceous,and the eroded thickness is 1220 m calculated by sonic-logging data and 1400 m calculated by vitrinite reflectance.The general subsidence happened in Neogene and the unconsolidated sedimentary strata formed.CSDP-2 well revealed the constant temperature zone at a depth of about 80 m,of which the temperature is 15.8?.The neutral point depth range is roughly in the vicinity of 1100 m,and the average geothermal gradient is about 24.11~24.28?/km.The thermal evolution history of the central uplift was studied by means of paleo-geothermal gradient method,fluid inclusion temperature measurement and apatite fission track(AFT)stochastic inversion.Results show that the paleotemperature of Paleozoic is about 40? at the end of Carboniferous and there is a significant temperature-rise process during the early Peimian and then tends to be steady.There is another short-term temperature-rise process at the end of Permian.The paleotemperature generally keep stable during Triassic and then rapidly rise from the late Triassic to early-middle Jurassic when the central uplift has not yet suffered large-scale uplift and denudation,and modeling results from vitrinite reflectance show that the highest paletemperature of Carboniferous and Permian can respectively reach 190? and 170? and the heat flow can reach 126 m W/m~2.The maximum paleogeothermal gradient is about 59?/km during this time,which is much higher than the present geothermal gradient.The paleotemperature and heat flow both began to decline from the late Jurassic followed by a slight rise,which is resulted from the rift development and rapid continental deposition during the early Cretaceous.During the mid-late Cretaceous,the denudation rate accelerated and resulted in the paleotemperature continuously decreased,and the paleo-heat flow began to decline until the end of Cretaceous when the rift development of the SYSB roughly finished.The AFT thermal history inversion results from the Permian samples show that it has not yet entered the annealing zone during the early Cretaceous,and the paleotemperature is about 100~140?,corresponding to the results of fluid inclusion temperature measurement.The AFT inversion results reveal two distinct cooling events,the first happened from the late Cretaceous to the early Paleocene when the paleotemperature declined to about 80?,and then followed by a slow cooling process until the late Oligocene when the other rapid cooling event happened and continued to the early Miocene.Since the Neogene,the paleotemperature slowly rised to the current level.Based on the theory of advanced Mc Kenzie extension model,the geodynamic modeling and traditional geothermometer simulation are integrated and applied to reconstruct the tectono-thermal evolution history of the northern depression of the South Yellow Sea Basin during Mesozoic-Cenozoic.Modeling results show that the central uplift and northern depression,which are both the secondary tectonic units of SYSB,experienced similar thermal evolution history since Cretaceous and characterized by the heat flow trend of rift basin,showing a distinct increase process during the rifting stage and a continus decrease process during the post-rifting stage.By contrast,the paleo-heat flow of the central uplift is generally lower than the north depression,the maximum value of the former can reach 70 m W/m~2 at the end of the Late Cretaceous while the paleo-heat flow of north depression is about 71.7~75.5 m W/m~2 during the early Paleocene and the highest is about 80 m W/m~2.