Soft-sediment Deformation in the Late Triassic and the Indosinian Tectonic Movement in Longmenshan

Abstract

The Upper Triassic Series of the Longmenshan region includes the Carnian Ma'antang Formation,the Norian Xiaotangzi Formation and the Rhaetian Xujiahe Formation.Field study firstly revealed existence of abundant soft-sediment deformations triggeried by earthquakes in several horizons of the Xiaotangzi and Xujiahe formations.Deformation includes liquefaction-induced deformation(liquefied breccia,liquefied droplet,liquefied diapir,and liquefied homogeneous layer,etc.),plastic deformation(convolute deformation,soft pudding),and gravity related load,ball-and-pillow and pillow beds.The above soft-sediment deformations were responses to the Late Triassic tectonic movement in the Longmenshan region and are the records of seismic events associated with the breakup,collision and obduction strike-slip between the Songpan-Ganzi terrane and the Yangtze plate in the Indosinian period.Integrated analysis of paleoseismic events and sedimentary events displays a complete tectonic movement process in the Longmenshan region during the Indosinian period:at the middle stage of the Late Triassic(the middle stage of the Indosinian tectonic period),breakup of the Songpan-Ganzi terrane and the Yangtze plate resulted in a near NS-trending paleo-fault,and earthquake generated by the fault movement triggered a series of soft-sediment deformations in the neritic sediment of the Xiaotangzi Formation;at the late stage of the Late Triassic(the late stage of the Indosinian tectonic period),the intracontinental subduction of the Songpan-Ganzi terrane and the Upper Yangtze plate occurred,and the earthquake induced lake sediment deformations in the Xujiahe Formation;and at the late stage of the Late Triassic(the last stage of the Indosinian tectonic period),the Songpan-Ganzi terrane thrusted onto the Upper Yangtze plate through sinistral strike-slip obduction to form the Songpan-Ganzi Mountain and the western Sichuan Foreland Basin,their boundary is the Wenchuan-Maoxian fault now,and the epeirogeny and orogeny associated with paleoseism occurring in the Indosinian period.Based on the locations of the identified liquefaction-induced soft-sediment deformations and their distances to the Wenchuan-Maoxian fault,it was estimated that the magnitude of the paleoseism occurred in the Norian Xiaotangzi Formation was about Ms7.2.The current records of liquefaction-induced deformations in the Xiaotangzi Formation are far from the farthest liquefaction point away from the Wenchuan-Maoxian fault,so the actual magnitude of the paleoseism was far larger than Ms7.2 and there should have been stronger seismic events.Actually large catastrophic earthquakes(e.g.,Wenchuan large earthquake of Ms8 on May 12,2008) in Longmenshan had already occurred frequently during the Late Triassic,and the currently active seismic belt in Longmenshan is the continuation of the Mesozoic paleoseismic belt.