Marking the northern margin of the Tibet Plateau, the Altyn Tagh fault system not only truncates various geological units, controlling both the geometric characters and tectonic framework in northern Tibet, but also comprises the main passage where the landmass easternward extrude. It is crucial to study the fault system's forming, active ages and fault trace evolvement to probe the dynamic mechanism of the Tibet Plateau. Here, we take advantage of the soft sediment deformation revealed in Middle Jurassic lacustrine strata in southern Subei to explore such questions. There are generally 26 deformation layers found which can be further subdivided into 4 categories in the field: load and pillow structure, liquefied diapir and crater, liquefied breccia and convoluted structure. The grain size statistic shows the grains mainly range from 0.2 similar to 0.3mm (60%), highly comparable with the modern earthquake ejected and experimental grains size, thus we consider them as earthquake induced liquefaction. Based on the empirical statistical relation between the earthquake magnitude and the maximum liquefaction distance, we presume the minimum magnitude is 6 similar to 6.5, whereas, in view of the specific deformation structure with different magnitude, we suggest the maximum magnitude may exceed 7.5. The 26 deformation layers represent an earthquake episode in Middle Jurassic, with at least 4 earthquakes. The Altyn Tagh fault system was highly active in Middle Jurassic and the fault trace had already reached Subei area at least. Given the Triassic active age deduced from the mylonite in Xorkol and the volcano activities in Cretaceous and Jurassic, soft sediment deformation in Yumen area in Cretaceous, the Altyn Tagh fault may have experienced at least three active ages : the Late Triassic, the Middle Jurassic and the Cenozoic, with the fault length expand from several hundred kilometers to more than 1000 kilometers and to 2000 kilometers.
