MESOZOIC N-S EXTENSION IN THE EASTERN ALTYN TAGH RANGE ON THE NORTHERN MARGIN OF THE QINGHAI-TIBET PLATEAU

Lying in the eastern Altyn Tagh Range along the northern margin of the Qinghai-Tibet Plateau, the >300-km long, E-W-trending Lapeiquan fault is a south-dipping normal fault, with its dip angle as low as <30°.The central segment of the fault is a 30~50 m thick ductile shear zone with a well-developed mylonitic fabric and stretching mineral lineation, whereas the eastern and western segments are characterized by cataclastic deformation.The kinematic indicators such as asymmetric boudinage structure, asymmetric folds and minor brittle and ductile faults within the fault zone all indicate a hanging wall-south normal-slip sense of shear.The age of the Lapeiquan fault is constrained by two lines of evidence.First, a sequence of Early-Middle Jurassic sediments is locally present in the hanging wall of the Lapeiquan fault.The clasts of the Jurassic strata, particularly the stromatolite-bearing, cherty limestone and purple quartzite, can be correlated with those in the footwall of the fault.The Early-Middle Jurassic strata were probably deposited in an extensional basin above the Lapeiquan fault.Second, ⁴⁰Ar ³⁹Ar thermochronological analyses indicate two prominent cooling events in the Lapeiquan footwall.The older event occurred in the latest Triassic-earliest Jurassic between~220 and 187 Ma, while the younger event occurred in the late Early Cretaceous at~100 Ma.Because the 220~187 Ma cooling ages are prevalent in the Lapeiquan footwall, they represent the main stage of faulting.The authors interpret the younger phase of fault motion at~100 Ma to have been related to fault reactivation.The deformation was aided by motion along the south-dipping Qiashikan normal fault that merges with the eastern Lapeiqaun fault. The Mesozoic extension in the Qinghai-Tibet Plateau and other parts of East Asia may be summarized as back-arc extension resulting from northward subduction of the Tethyan oceanic plate and the westward subduction of the Pacific plate.