Late Paleozoic Earthquake Events in the Nixiong Area and Its Geological Significance, Western Lhasa Block

The tectonic background of Lhasa terrane located at the northern margin of Gondwana in the Late Paleozoic is still in dispute. Seismites have been newly discovered in the Late Paleozoic strata (Laga Formation and Xiala Formation) in Nixiong area in the western section of the Lhasa terrane. The seismite unit of the first member of Laga Formation is found in pebbled fine sandstone, which is ice-rafting deposit formed by the melting of sediment-laden ice blocks. In this unit, we have found massive seismites related sedimentary and structure features. Furthermore, typical seismite rock types, including seismic corrugated rock, shattering rock, and plastic breccia are recognized. The authors have also found soft sediment deformation structures related to earthquake in the tidal-flat sedimentary in the second member of Laga Formation, such as convolute bedding, sand ball-pillow structure. Chert limestone with micrite veins, located at the bottom of Xiala Formation, is another seismite horizon. Based on the paragenetic relationship between chert limestones and seismite horizons of the Angjie Formation and the Xiala Formation, it can be inferred that partial chert limestones in the Late Paleozoic in Lhasa terrane may be closely related to the earthquake. Combined with the regional data, it is concluded that a series of seismic events in the Late Paleozoic recorded the rifting process at the northern margin of Gondwana, and proved a new seismic activity for Late Paleozoic basin evolution of the Lhasa terrane.