Geochemical and geophysical effects of tectonic activity in faulted areas of the North China Craton

Abstract

Fluid geochemistry in active fault zones has been proven to be sensitive to tectonic activity. The North China Craton (NCC) has attracted much attention because of its complex and intense tectonic activity. In this study, fluid geochemistry in the primary active fault zones in the NCC was investigated, including inference of its tectonic activity. Stronger degassing from soil and springs has been observed in the northeastern Tibetan Plateau (NETP) and the Zhang-Bo seismic zone (ZBSZ) than in the other seismic zones. Both geological soil gas and deepderived gas (crust- or mantle-derived gas) from springs were concentrated there. Also, a comprehensive analysis has indicated that the development of new fractures might have occurred widely beneath the NETP and ZBSZ because of the strong regional tectonic activity there. The 3He/4He and 4He/20Ne of gas from the springs in the ZBSZ suggest that the low-velocity zone 20– 40 km deep might be a magmatic intrusion derived from the mantle. However, crust-derived gas accompanied with a negligible mantle-derived component has been detected in the Diebu-Bailongjiang fault (DBF), the West Qinling fault (WQLF), and the Liupanshan fault (LPSF) in the NETP. There, the occurrence of more new fractures was probable, in accordance with the obvious δ18O shift of the water from the springs in the fault zones. This suggests that a channel flow, also depicted by the low-velocity zone 20– 40 km deep, could have formed within the crust and that the probable leading front reached the LPSF.