Quaternary faulting of the Jiangsu part of the Tan-Lu Fault Zone, East China: Evidence from field investigations and OSL dating

Abstract

The main brittle faults in the Jiangsu part of the Tan-Lu Fault Zone, East China, are, from east to west, faults F1, F5, F2, F3, and F4. The details of their Quaternary activity remain controversial. We present structural, depositional, topographical, and geochronological evidence that indicates widespread fault activity during the Quaternary. Our fieldwork revealed that faults F1, F5, F2, and F3 cut Quaternary sediments. Each fault produces a narrow gouge belt in the Quaternary sediments and incohesive fault rocks consisting of gouge, unconsolidated breccia and cataclastic belts in the bedrock. With the exception of dextral faulting along the northern part of the fault F1, the main Quaternary faults show evidence of thrusting with a minor dextral component. Analysis of a large amount of fault-slip data demonstrates that ENE–WSW compression was responsible for the Quaternary faulting. Our OSL dating results, combined with observed changes in Quaternary sediment thicknesses, indicate that except for Holocene activity on the northern part of fault F1, and probable Quaternary activity on the northern part of fault F4, the most recent period of activity on the main faults in the Jiangsu part of the Tan-Lu Fault Zone was the late Pleistocene. These inferences are consistent with the absence of M⩾4.0 earthquakes along the fault zone. Fault displacement data and Quaternary sedimentation suggest that Quaternary activity was more intense in the late Pleistocene, and that faults F2 and F5 were the most intensely active. Our field investigation shows that faults F1–F4 originated from the reactivation of Cretaceous graben-boundary faults whereas fault F5, with its varied dip directions, was derived from the reactivation of antithetic normal faults associated with the boundary faults in the eastern graben.