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

Abstract The tectonic events caused by paleoearthquakes have a good response in sedimentary filling. 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 sedimentary deformation structures (SSDSs), many of which are laterally extensive for more than 150 km. They include various types of folds, soft sediment liquefaction deformation (liquefied sandstone dyke, liquefied breccia), gravity-driven deformation (load structures, ball-and-pillow structures), hydraplastic deformation (loop bedding, convolute deformation) and brittle deformation (intrastratal faults and stair-step microfaults, microcracks). In most cases, deformation is represented by 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 between 6 and 8, which are interpreted as a response to splicing collision orogenic events 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.