Exhumation Process of the Fangdoushan-Shizhu Fold Belt in Meso-Neozoic and Its Tectonic Significance in Western Hubei-Eastern Chongqing

Abstract

The exhumation history of Fangdoushan-Shizhu fold belt in the Western Hubei-Eastern Chongqing since Jurassic was modeled by using combined vitrinite reflectance and apatite and zircon(U-Th)/He and apatite fission track dating methods.The results show that all samples have experienced the highest paleogeotemperature in the study area during the late Jurassic-the early Cretaceous,and from then on,the study area was always to drop in paleogeotemperature.According to the ages of apatite fission track and Ro,the denudation may be reach 3500 m during the surface of unconformity in Mesozoic-Cenozoic period.Besides,combining with the results of(U-Th)/He method and vitrinite reflectance,we can get a further understanding:three tectonic phases have been revealed in the whole:rapid cooling-slow cooling-fast cooling,during the first phase,the cooling speed is about 0.75～1.0℃/Ma from the late Jurassic to the late Cretaceous(aboutl30~70Ma),following a quiet phase with the cooling speed of 0.33~0.4℃/Ma during the late Cretaceous to the end of Eocene(about 70~45 Ma),the final phase reach the maximum cooling speed which is about 0.9～1.1℃/Ma lasting from Eocene to nowCabout 45Ma),the three phases are correspondent with the period of three important structural changes since Middle-Trassic in the Middle-upper Yangtze.The test proves an evidence to the analysis of tectonic evolution of orogenic.