Druse Calcite Crystals Formed by Mesoproterozoic Paleo-Earthquake Activity in the Northern Margin of the North China Craton

The Meso-neoproterozoic Bayan Obo rift is located along the northern margin of the North China Craton,and was associated with the break-up of the Columbia supercontinent.During rift evolution,syn-sedimentary deformation occurred due to tectonic activity and earthquakes.Seismic events are recorded in the Jianshan Formation of the Bayan Obo Group,Inner Mongolia,as soft sediment deformation structures in the central Bayan Obo rift.Druse calcite crystals and collapse breccias in the Jianshan Formation may provide information on the rift evolution.The druse calcite crystals are idiomorphic-columnar in shape and associated with graphite,pyrite,and quartz.δ^(13)C values of the graphite are−20‰,indicative of biogenic deoxygenation and formation in water.The druse calcite crystals are inorganic in origin and formed in water at a temperature of 55℃,based on calciteδ^(13)C andδ^(18)O data.The calcite grew in paleo-caves containing fault breccias,with heat derived from faulting.As such,the druse calcite crystals are important evidence for seismic events.The collapse breccias(i.e.,fault breccias)and other indicators of slip show that displacement occurred from NE to SW,which is different from the paleocurrent direction in the Jianshan Formation.The thickness of the collapse breccia is∼200 m,which represents the height of the fault scarp.The strike of the fault scarp was NE-SW,based on the distribution of the collapse breccia.The Bayan Obo and Yanliao rifts experienced rapid NW-SE extension,and developed similar deformation structures at ca.1.6 Ga related to break-up of the Columbia supercontinent.