Neotectonic Activity Of The Suwan Segment Of The Tan-Lu Fault Zone

NNE-striking major normal faults were developed along the Suwan segment of the Tan-Lu Fault Zone during extensional activities of Cretaceous. They are abbreviated by F1、F2、F3 and F4 faults from east to west along the Jiangsu segment of the Tan-Lu Fault Zone. The normal faults controlled development of the NNE-trending eastern graben、western graben and a horst between the two grabens. The F5 fault occurs in the eastern graben. Detailed field investigation shows that active faults widely appear in the Jiangsu segment. The F2、F3 and F5 active faults mainly show thrusting while the northern segment of the F1 fault presents reverse dextral slip. Computation of fault-slip data suggests that activity of the active faults results from ENE—WSW compression which is consistent with present stress filed in this region. Corss-cutting phenomena and OSL dating results demonstrate that the F2、 F3、F5 faults were active in Late Pleistocene, the northern segment of the F1 fault were active in Holocene, and F4 fault were active in Quaternary. Fault displacement data and Quaternary deposition suggest that the eastern graben in the Jiangsu segment had more intense neotectonic activity, and the F2 and F5 faults show more strong activity. Comprehensive analyses suggest that the active faults in the Jiangsu segment are derived mostly from direct reactivity of previous normal faults, and present characteristics of inhomogeneous and discontinuous activities. The F5 active fault with varied dip directions shows formation mechanism of reactivityof normal faults with different dip directions near the eastern boundary of the eastern graben.They are two major normal faults of the Tan-Lu Fault Zone in Jiashan Basin, and one major normal fault along the eastern margin of the Hefei Basin andthe Dabie orogenic belt. Our detailed filed investigation demonstrates the wide-spread presence of active faults in the Anhui segment of the Tan-Lu Fault Zone. They occur along the eastern and western boundary of the Jiashan Basin and the eastern boundaries of the Hefei Basin and the Dabie orogenic belt. The active faults strike from NNE—SSW to NE—SW from north and south, and show similar dips to previous normal faults at the basin boundaries. Kinematics of the active faults is dominated by reverse dextral displacement, but showing increase in reverse component with reduction in dip angles. Inversion of a series of measured fault-slip data indicates NEE—SWW compression for activity of the active faults, which is consistent with present stress field. Many phenomena suggest middle Pleistocene for the most recent time of the activity in the Anhui segment of the Tan-Lu Fault Zone, which is consistent with the absence of strong earthquakes and weak earthquake concentration in the fault zone. We propose that the active faults originated from direct reactivity of the previous basin boundary normal faults. The limited magnitudes of accumulated vertical displacement along the active faults led to no change in topographic patterns formed during the basin development of Cretaceous to Paleogene.