The Cathaysia Block, located in southeastern China, is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in the Cathaysia Block. However, the timing and kinematics of the active-passive margin transition need to be better constrained. The SW Cathaysia Block, near the coastal area of the South China Sea, is selected for studying the transition. There are two major geological units in this region: the Nanling Range and the Yunkai Terrane. The Nanling Range is a magmatic belt composed of granitic plutons with formation ages ranging from Caledonian to Cretaceous. The Yunkai Terrane is a metamorphic terrane of Caledonian age. Thirty zircon fission-track (ZFT) data and thirty apatite fission-track (AFT) data were obtained from the granitic plutons in the SW Cathaysia Block. The distribution of ZFT ages shows two episodes of exhumation of the granitic plutons: the first episode is found to occur during 170 Ma – 120 Ma and affect the SW part of the Nanling Range; the second episode, a more regional exhumation event, occurred during 115 Ma- 70 Ma. The AFT dating results show a general cooling sequence from south to north during Late Cretaceous – middle Eocene, contrary to the conventional passive margin model. Numerical geodynamic modeling of subduction zone indicates that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The numerical model results of lithospheric deformation associated with subduction process reveal that granitic crust is easily deformed compared to gneissic crust. The results could be used to explain the observation that the granite-dominant Nanling Range was exhumed earlier than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic- Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. On the other hand, the numerical models of crustal extension with pre-existing faults dipping ocean-ward demonstrate a trend of fault reactivation sequence from south to north. Assuming that granite exhumation in the Cathaysia Block was mainly a product of rifting and fault reactivation, the numerical models support the AFT results. The integrated FT dating and numerical model results suggest that roll-back of the subducting paleo-Pacific Plate slab during Late Cretaceous is likely to be the driving force of the transition from Mesozoic subduction to Cenozoic extension in the Cathaysia Block. The timing of the transition is suggested to have taken place at ~ 92 Ma, according to a rapid cooling as revealed by the thermal history modeling of AFT length data. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Both the SW South China Sea and southern Beaufort Sea represent areas of extended continental crust, located on continental margins associated with oceanic spreading centres and zones of continent ocean transition (COT). Multichannel seismic reflection data are interpreted to characterize the COT in the SW South China Sea and the Southern Beaufort Sea. Based on the modelling and subsidence analysis, these two areas, SW South China Sea and southern Beaufort Sea, are compared with each other and the process of formation of hyper-extended crust in marginal oceanic basins can be conceptually modelled. It is noted that the initial weak thinning of the continental crust happened and the ductile middle/lower crust is coupled with the brittle upper crust. As extension continued, the continental crust is thinned down to ~10km, which is in accord with depth-dependent lithosphere thinning. Major crustal thinning is unlikely to result from brittle, high-angle normal fault in the upper crust. The degrees of lower crustal extension are so high and very high amounts of lower crustal extension, presumably achieved by ductile flow, would be required to have affected the crust within the COT. The seafloor spreading centre existed in the area adjacent to the research regions, so the extension within the COT occurred prior to the onset of seafloor spreading and the lower ductile flow is away from the continent and towards the oceanic crust. The interpretations require that the continental lithosphere prior to seafloor spreading must have been very weak given the evidence for significant lower crustal flow, inferred shallow depth of the brittle ductile transition and the fact that the COT continued to extend after the cessation of seafloor spreading.

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

The 2008 Wenchuan Ms 8.0 earthquake resulted in huge casualty and heavy strike to the local economy. Consequently, Chengdu Plain, with dense population and short distance to the highly active Longmen Shan fault zone needs an evaluation of the risk of earthquake as soon as possible. As part of a big project, we conducted mapping of gravity anomaly of Chengdu Plain in order to locate the possible buried fault zones beneath the Chengdu Plain. In this project a total of 640 km2 area was surveyed, and 230 relative gravity data have been collected along six profiles with each extending to about 70 km across the plain area. 140 more spots were measured around the city area in order to have a more precise delineation of geological structures beneath the city region. The Bouguer gravity anomaly has been calculated to delineate the large-scale anomaly trend on the surface. Data analyses presented that a coincidence between the sedimentation thickness and the residual gravity anomaly. The minimum residual anomalies suggested locations where depo-centers used to located (Pengzhou-Chongzhou, across Xinsheng County). The long-wave trend of Bouguer anomaly which reflects the deeply buried Moho discontinuity presented a northwest dipping trend of this density boundary underneath the plain region. / published_or_final_version / Earth Sciences / Master / Master of Philosophy

published_or_final_version / abstract / toc / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Earth Sciences / Master / Master of Philosophy

published_or_final_version / Earth Sciences / Master / Master of Philosophy