Kinematics Of Active Faults And Block Interactions Along The Northern Tibetan Margin And Southern Beishan,China

The active block theory suggests that the China continent comprises a series of first-or second-order active blocks.The block interactions constitute the basic characteristics of tectonic deformation in China continent since the late Cenozoic,and play an important role in controlling the occurrence and style of great or strong earthquakes.Recognition of boundaries of different blocks and their interactions is the key to understand outward expansion of the Tibetan Plateau,tectonic response of the surroundings blocks,and seismic hazard assessment.With the northward growth of the Tibetan Plateau,the Qilian Shan-Hexi Corridor and the Altyn Tagh fault?ATF?system represent the northern edge of the Tibetan Plateau.The Hexi-Dunhuang Corridor area is located between the Qilian Shan,Tarim and Beishan-Alxa blocks,and thus is an ideal place for studying the interactions of three blocks.In this thesis,we selects several faults or fault systems in Hexi-Dunhuang Corridor area and in the southern Beishan to document their structural geometry,kinematics,timing,and downward architecture for further discussing the block interactions and tectonic response to the outgrowth of the Tibetan Plateau.The main results of the thesis are included as follows:?1?The NE-striking Sanweishan fault?SWSF?is characterized by sinistral strike-slip with a top-to-the-NW thrusting component.Offset geomorphic markers and dating results yield Pleistocene strike-slip and vertical uplift rates of 0.06-1.25 mm/a and 0.05-0.08 mm/a,respectively.The E-W trending Nanjieshan fault?NJSF?is dominated by N-and S-directed thrusting and km-scale folding with variable components of sinistral strike-slip.The calculated total N-S crustal shortening rate across the NJSF is?0.3 mm/a.Slowly rates of deformation for the SWSF and NJSF account for about 10%of the total intraplate strain accommodated along the northeasternmost ATF system.Over a 1000-km length,the northward expansion of the Tibetan Plateau occurs by progressive NE-ward growth of a transpressional duplex rooted SE into the ATF.?2?The Beihewan Fault?BHWF?along the southeastern margin of the Beishan block is dominantly strike-slip,with local thrust or normal displacements.Average rates of left-lateral motion and thrusting since the Holocene are estimated to be?2.69mm/a and?0.35 mm/a,respectively.Close examination of high-resolution satellite imagery west and east of the BHWF reveals other Quaternary surface fault scarps,pressure ridges,offset drainages and truncated lithological strike belts.Collectively,these features constitute a previously unrecognized 150 km-long sinistral deforming belt along the southeastern Beishan and northern Hexi Corridor boundary that we call the Southeast Beishan Wrench Belt.Magnetotelluric data indicate that the BHWF is a sub-vertical low-resistivity zone that penetrates into the lower crust.Existing geological and geophysical evidence do not support subsurface linkage between the BHWF and Altyn Tagh-Jinta-Nanshan Fault system further south in the Hexi Corridor.?3?The Jiujing Fault?JJF?in the southern Beishan block is a late Pleistocene to Holocene oblique-slip normal fault.Ttrench results reveal that the most recent surface-rupturing event occurred on?14 ka.The JJF is a connecting structure between the regional E-W-trending Quaternary Jinmiaogou and Hongqishan left-lateral fault systems.The overall regional structure is an evolving sinistral transtensional duplex,which is kinematically compatible with NE-directed maximum horizontal stress?SHmax?.Cosmogenic ²⁶Al/¹⁰Be burial dating of the deepest sediments in the Jiujing Basin indicates that the basin began to form at?5.5 Ma.Our results provide new insights into the regional network of structures responsible for Latest Miocene-Recent crustal reactivation in the southern Beishan with implications for crustal stability,earthquake hazards and the timing of Beishan reactivation relative to other deforming regions north of Tibet.?4?Remote sensing image interpretation reveals that the Liuyuan fault?LYF?system in the southwestern Beishan is an oblique thrust strike-slip fault system.The displaced Quaternary sediments at the range front suggest the Quaternary activity of the LYF.The geometry and kinematics of the LYF system and its structural relationship with bounding sinistral strike-slip faults to the N and S suggest a dominated regional strike-slip transpressional duplex,which is compatible with a NE-directed regional compressional maximum stress.Collectively,we suggest that the northern Qilian foreland thrust and the Altyn Tagh Fault are still the main tectonic structures of high slip rates at the margin of the Tibetan Plateau,where the strong earthquakes occurred.The block interactions reactivated the pre-existing structure grains in the blocks outside of the plateau,and may also generate some young new fault systems,such as the Beihewan,Jiujing and Liuyuan fault systems in the Beishan.Sinistral strike-slip transpressional and transtentional deformations in the Tarim and Beishan-Alxa blocks co-absorbed reactivated crustal deformation derived from the expansion of the plateau.The stable blocks north of the Tibetan Plateau are undergoing block rejenuvation,but the amplitude and distribution of reactivation still need to be further investigated.