Thermal Backstripping from Vitrinite Reflectance Profiles in Comparison with an Extensional Modelfor Passive Continental Margin Basins on NorthernContinental Shelf of South China Sea

Three typical vitrinite reflectance (R0) proriles from Pearl River Mouth, Qiongdongnan and Beibuwan basins on the nortbern condsental shelf or the South China Sea display two or more different sub-linear slope relationships between IgR0 and deptb. According to the reconstruction of sedimentary and burial histories for these wells, a relatively large R0 gradient corresponds to a high depositional rate, while a relatively small R0 gradieut represeuts a low depositional rate. In this study, a modiried thermal backstripping model for paleokeat fIow using vitrinite reflectance data has been used to reconstruct paleotbermal history by the linear-segmeut regression. Tke study results indicate that the gentle parts of a IgR0 profile rerlect abnormai periods or high paleoheat flow, in contrast, the steep parts of a IgR0 profile reflect periods of peleoheat flow decay.In order to have an alternative approach and correlative study, an improved two-layer extensional model for the formation and evolution or rifting basin iu pesive contiueutal margin bas been applied. The model simulates basin subsidence process due to stretcbing and thinuing of continental lithospbere and thermal effects by asthenospbere upwelling, and determines the relationship between subsidence and paleoheat flux through the geological time. The simulation results suggest that these basins have undergone two or three rirting and thermal events, and it is clear that the large R0 gradient segment reflects a response to high paleoheat flow during the early, rapid subsidence stage, while the low R. gradient segmeut is a result or the tbermal decay during the thermal subsidence because of thermal contraction of a cooling Iitbospbere. The results also suggest that the maturity profiles of these basins can be employed to visual understanding tke paleogeothermal characteristics of rifting basins.

Basement tectonics and flexural subsidence along western continental margin of India

The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse postrift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that:(1) either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or(2) a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin(MOB) during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India(WCMI). Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan-Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono-climatic interactions in Asia.

Numerical modeling of the anomalous post-rift subsidence in the Baiyun Sag,Pearl River Mouth Basin

The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea,with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity.According to the theoretical models for the rifted basins,the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor.A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor.Using the 2D forward modeling based on the flexural cantilever model,we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles.By doing this the lithospheric stretching factor was obtained,and then the theoretical post-rift subsidence was calculated.The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping.Along the 1530 line in the Baiyun Sag,the anomalous post-rift subsidence is over 2 km in the sag center,and varies slightly to the north and south edges of the sag.This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south.The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (≤13°) can we simulate the shape of the backstripped profile.

Cenozoic Subsidence Features of Beitang Sag and Relationship with Tectonic Evolution

Based on the application of the EBM basin modeling software and 2-D seismic profiles, the Paleogene and Neogene subsidence histories of the Beitang （北塘） sag are simulated with the back-stripping technique, and the relationship between subsidence character and tectonic revolution is discussed. Moreover, the result of the basin modeling reveals that the subsidence history of the Beitang sag has the characteristics of several geological periods, and these succeeding periods have shown certain inheritance and difference characteristics. At the early （Es3） and middle （Es2-Es1） rifting periods, the subsidence reaction of the Beitang sag was mainly in the charge of tectonic activity, while at the late （Ng-Nm＋Q） rifting period-post rifting period and post rifting subsidence-acceleration period-the subsidence type is mainly that of thermal subsidence or regional depression effect; from the beginning of the subsidence history to the end, the reason for the basin subsidence has changed from tectonic activity to non-tectonic activity.