Discovery of the late-Quaternary activity along the eastern segment of Maidan Fault in Southwest Tianshan and its tectonic implication

The late-Quaternary deformation characteristics of the boundary fault zones are critical to understanding the crustal deformation of the Tianshan Mountains. Based on remote sensing image interpretation, field surveys, trenching and optically stimulated luminescence dating methods, we obtain the reliable activity evidences of the Maidan Fault in late-Quaternary. The Maidan Fault is the boundary fault of the Tianshan Mountains and Tarim Basin. The fault, with a total length of 400 km and the maximum width about 15~17 km, comprises a series of secondary faults. During the late Quaternary, the fault was still very active. The fault dislocated the lateQuaternary landform surfaces, forming obvious scarps on the surfaces. The height of the scarps range several to hundred meters. Trench excavation shows that paleoearthquakes occurred on the faults during late Holocene. The vertical displacement caused by the last paleoearthquake event is above 2 m. The different late Quaternary landforms with different vertical displacement heights indicate that several strong earthquake events have occurred on the Maidan Fault since the late Quaternary. The discovery of activity on the Maidan Fault shows that the deformation does not focus solely on the newly born reverse fault and fold belt. Faults at the root of Kalpin nappe system have also participated in absorbing and partitioning some of the tectonic deformation. This phenomenon may explain why the shortening rate got by geology method of the Kalpin nappe structure is much less than that obtained by GPS. This deformation mode of the Tianshan orogenic belt is obviously different from the piggyback propagation as considered previously. The activities of the Tianshan root faults migrated to the frontal faults of the piedmont nappe, and the root fault activity weakened gradually. But the activity in Kalpin nappe structure does not accord with this mode. The root faults and the frontal faults of the Kalpin nappe structure are all obviously active, which indicates the nappe structure in the southwestern Tianshan is an out-of-sequence, or a non-sequence thrust system. This kind of structure mode brings new challenges to us in constructing seismogenic tectonic models and assessing seismic risk. ©, 2014, State Seismology Administration. All right reserved.