Apatite fission-track dating and thermal-history modeling were carried out on samples from the Dabashan （大巴山）, a fold-thrust belt, northeast of the Sichuan （四川） Basin and east of the Tibetan Plateau. A first cooling event in the Late Cretaceous is followed by a prolonged period of ther- mal stability with exhumation rates of 〈0.025 mm/a, as determined from age vs. elevation relationships. The preservation of age vs. elevations relationships and the lack of distinct age changes across tectonic structures indicate that the Dabashan fold-thrust belt formed prior to the Late Cretaceous, consistent with the current view of Triassic-Early Cretaceous shortening. Relatively short mean track lengths （-12 μm） indicate that the samples remained in the partial annealing zone for a prolonged time. The knick points in the best-fitting temperature-time models suggest that the onset of late-stage accelerated cooling commenced at 〈11 Ma. Related exhumation rates are 0.3-0.2 mm/a assuming geothermal gra- dients of 20 and 30 ℃/km. We speculate that this late-stage event results from eastward growth of the Tibetan Plateau and overstepping of the Sichuan Basin, it is likely responsible for the youthful mor- phology of the Dabashan.
There are lots of Neoproterozoic-lower Paleozoic outcropping bitumen distributed at the northern Longmen Shan trust belt in Southwest China,indicative of existing paleo-oil reservoir.The bitumen develops from the Sinian to Permian strata,mainly along the thrust fault plane and fracture system in
沈传波GE XiangDavid SELBYRUAN XiaoyanHU ShouzhiPENG Lei
The exact determination of isotopic ages of hydrocarbon accumulation,reconstruction and destruction periods using traditional isotopic dating methods is complex because of the small numbers of minerals that correlate with hydrocarbons.The quantitative and direct study of hydrocarbon geochronology is therefore an important scientific problem for isotope geochronology and petroleum geology.This study obtains two isotopic ages from a quartz vein sample associated with bitumen in a reverse fault located in the Bankeng paleo-reservoir on the southern margin of the middle Yangtze block (the northern margin of the JiangnanXuefeng uplift) by inclusion 40Ar/39Ar stepwise crushing in a vacuum.Two different and good linear inverse isochrons that correspond to two age plateaus were determined.The two ages correspond to primary inclusions of about 228 Ma and secondary inclusions of about 149 Ma.These inclusion groups represent two distinct kinds of fluids.Quartz veins associated with bitumen in faulted paleo-reservoirs,which have a strict response relationship with tectonization and hydrocarbon accumulation,are the unified products of tectonic processes,hydrocarbon accumulation and reconstruction.Therefore,they can be used to constrain the hydrocarbon accumulation,reconstruction and destruction periods that are controlled by multiphase and complicated tectonic actions.The evolutionary processes of hydrocarbon accumulation can be divided into two periods consisting of a primary oil and gas reservoir formation period in the late Indosinian epoch (about 228 Ma) and a period of oil and gas reservoir reconstruction in the early Yanshan epoch (about 149 Ma).This study quantitatively reconstructs the hydrocarbon accumulation and destruction chronological framework of a giant hydrocarbon accumulation belt along the southern margin of the middle Yangtze block (the northern margin of the Jiangnan-Xuefeng uplift) controlled by multiphase and complicated tectonism.The two ages associated with hydrocarbons here correspond to the special c
LIU ZhaoQianMEI LianFuQIU HuaNingSHEN ChuanBoTANG JiGuangYUN JianBing
