Characteristics of Spatial Distribution and Activities of the Changxing-Fenghua Fault in Hangzhou Bay

The Changxing-Fenghua fault (CFF) is a regional fault trending in NW-SE,which diagonally goes across the Hangzhou Bay region.The CFF changed the NE-SW trending tectonic frameworks which were formed in the Indosinian movement.The CFF plays an important role for the development of mineral resources and oil resources.It consists of a series of NW-SE-trending faults which are discontinuous,parallel to each other or dextral en-echelon array.Secondary NW-SE-trending faults of NE-SW-or E-W-trending deep preexisting faults are easily misdeemed from the CFF.Therefore,the distribution,even the determination of the CFF are challenged in recent decades.We analyzed the distribution and activity of the CFF in the Mesozoic and Cenozoic according to the TM remote sensing image,aeromagnetic anomaly map,Bouguer gravitational anomaly map and artificial seismic profiles of the Tongxiang-Pinghu depression,Changhe depression and Ningbo basin.The TM remote sensing image indicates the new NW-SE-trending fractures assemble in Shangyu city,the northern part of Simin Shan and the Ningbo city.And the difference from the previous understanding is that the NW-SE-trending fractures around Fenghua city are only secondary fractures of preexisting deep faults.Although inconspicuous in the aeromagnetic anomaly map and discontinuous on the ground surface,the CFF is a remarkable boundary of positive and negative abnormality on the Bouguer anomaly map.The fracture distribution and the Bouguer anomaly map suggest the CFF starts in Chang-xing County and expends southeastwardly to the East China Sea.The CFF spreads as en-echelon fractures and passes by the Tongxiang,Haining,Yuyao and Ningbo cities,rather than the Fenghua city as people suspected from the aeromagnetic anomaly map before.Artificial seismic exploration showed that the CFF controlled the deposition process and rebuilt the structures of continental basins from the Cretaceous to Eogene.The CFF cut across old structures with dextral horizontal displacement on the ground surface and strata as a normal fault on the artificial seismic profile.Therefore,the CFF is a tenso-shear fault and belongs to the NW-trending active fault zones of the eastern Chinese continent in the Mesozoic and Cenozoic.