Sedimentary Responses of Late Triassic Soft-Sedimentary Deformation to Paleoearthquake Events in the Southwestern North China Plate

Abstract

Tectonic events caused by paleoearthquakes are reflected in sediments. Outcrops and cores from the Chang-7 Member of the Late Triassic Yanchang Formation, Ordos Basin in Northern China, yield a wide variety of soft-sediment deformation structures (SSDSs), many of which are laterally extensive for more than 150 km. They include various types of folds, soft-sediment liquefaction flow deformation (liquefied sand dyke, liquefied breccia), gravity-driven deformation (load structures, ball-and-pillow structures), hydroplastic deformation (loop bedding, convolute deformation), and brittle deformation (intrastratal and stair-step faults, cracks). In most cases, deformation resulted in hybrid brittle-ductile structures exhibiting lateral variation in deformation style. These occur in delta front to semideep-to-deep lake sands and mudstones (shales). The seismites recognized in outcrops and cores indicate earthquakes with magnitudes (Ms) between 6 and 8, which are interpreted as a response to orogenic events related to the collision of the South China Block (SCB) and North China Block (NCB) during the Late Triassic period. Systematic study of the spatial and temporal distribution of these seismites improves the understanding of the tectonic context and evolutionary history of sedimentary basements. This study can provide a new perspective on the evolution of tectonic activities in the basin.