Upper mantle flow beneath the Northwest of China and its lithospheric dynamics

The regional iosotatic gravity anomalies in the Northwest of China (80°-95°E, 35°-45°N) and corresponding problems of mantle dynamics are discussed in detail in this paper. It is assumed that the anomaly of isostatic gravity in this region is caused by the nonunifromity in density and that the boundary deformation is related to the thermal convection in the upper mantle. using the correlation equation between the mantle now and regional anomalies of isostatic gravity as a constraint, we have calculated the patterns of upper mantle convection. The result shows that the regional lithospheric tectonics is strongly correlated with the upper mantle now. The uplift o f the Tianshan and Kunlun Mountains corresponds to the upward flows in the mantle while the depression of the Tarim, Tuopn and Junggar Basins corresponds to the downward now. Meanwhile, we have also worked out a basic framework for the lithospberic dynamics of the Northwest of China. It is reasonable to infer that the ' mantle flow and the horizontal compression from the india Plate may act as the main driving forces for the lithospheric tectonics of this region.

From foredeep to orogenic wedge-top: The Cretaceous Songliao retroforeland basin, China

Songliao Basin,the largest Mesozoic intracontinental nonmarine basin in eastern China,initiated during the latest Jurassic as a backarc extensional basin;rifting failed and thermal cooling controlled subsidence through the early Late Cretaceous.Integrating 2-D and 3D reflection seismic and borehole data with regional geological studies,we interpret sedimentary sequence and structural patterns of the Coniacian-Maastrichtian fill of Songliao Basin as defining a retroforeland basin system developed after 88 Ma(marked by the T11 unconformity in the basin),including(1)significant increase in the thickness of the Nenjiang Formation eastward towards orogenic highlands of the Zhangguangcai Range and the convergent continental margin;(2)a shift of detrital provenance in the basin from north to southeast;and(3)propagation of E-W shortened structures,increasing eastward in amplitude,frequency,and degree of inversion toward the orogen.During latest Cretaceous,foreland basin fill progressively deformed,as the foredeep evolved to a wedge-top tectonic setting,marked by the basin-wide T04 unconformity within the upper Nenjiang Formation at 81.6 Ma.Much of the basin was brought into the orogenic wedge and eroded by the end of the Cretaceous.Late Jurassic/Early Cretaceous backarc rifting of uncratonized basement comprised of accreted terranes likely facilitated and localized the foreland.Synrift normal faults reactivated and extensively inverted as thrust faults are prominent in the eastern 1/3 of the basin,whereas folds developed above detachments in shaley early post-rift strata dominate the western 2/3 of the basin.Songliao foreland development likely was driven by changing plate dynamics and collision along the Pacific margin after 88 Ma.