Post-Cretaceous Sinistral Transpression in Southwest Alxa: Structural and Paleomagnetic Insights into the Long-Term Slip History of the Altyn Tagh Fault

Piercing point studies demonstrate two distinct phases of slip on the Altyn Tagh Fault (ATF) following its initiation ~30 Ma. An initial rapid, large magnitude slip phase transitioned to the present slower, smaller slip phase during the Middle Miocene. Piercing point studies near the Modern NE tip of the ATF, the Neogene shortening and cooling record of the Qilian Shan, and the slip history of faults within the Qilian Shan suggest that the >310 km of pre-Middle Miocene ATF displacement was transferred northeast into the Alxa block. Indeed, recent field mapping revealed numerous NE-SW trending sinistral strike-slip faults showing large Cretaceous and minimal Neogene offsets. These structures may have been kinematically linked to the ATF. We test this proposed linkage with new structural and paleomagnetic data from the Hei Shan, the Alxa range closest to the Modern terminus of the ATF. An ENE-WSW trending sub-vertical fault, which we call the Hei Shan Fault (HSF), can be traced WSW toward the ATF and ESE toward sinistral structures in the Heli and Beida Shan. Paleozoic and Mesozoic strata show left-lateral offset along the HSF, supported by slickenlines and S-C fabrics in the ~30m-thick gouge zone. At least nine fault-bound blocks of Paleozoic basement define a strike-slip duplex along the eastern HSF. Several NW-SE trending thrusts and folds that involve basement and Cretaceous strata occur north and south of the HSF. In the western Hei Shan, minor NE-SW trending strike-slip faults display small (tens of meters) dextral displacements. All of these structures appear to be kinematically linked to the HSF. Upper Neogene alluvial gravels show only minor offset and tilting by the HSF and these secondary structures. Paleomagnetic directions from more than 230 samples from 36 new Hei Shan sampling sites demonstrate that significant vertical axis rotations (relative to a stable Eurasia reference frame) occur only within the fault zones and that block rotations adjacent to and between faults are minor. Counter-clockwise rotations are localized to sites closest to the left-slip HSF, whereas significant clockwise rotations are restricted to right-lateral strike-slip structures and the hanging walls of NE-directed thrusts. Paleomagnetic directions from upper Neogene strata do not show significant rotations. We conclude that the orientation and kinematics of structures in the Hei Shan are most consistent with a left-lateral transpressional duplex that developed after the Cretaceous and was most active prior to the Middle Miocene. These structural and temporal relationships suggest that the HSF was kinematically linked to the ATF during Oligocene to Early Miocene time. Thus, the HSF would have partitioned some of the >310 km of pre-Early Miocene ATF slip onto strike-slip, extensional, and contractional structures within the Hei Shan while transferring the remaining slip north and eastward into the Alxa region.