The kinematics of reactivation of normal faults using high resolution throw mapping

Abstract

Normal reactivation of extensional faults offsetting Cenozoic clastic sediments is investigated using high quality 3D seismic data from offshore Brazil. These faults form complex crestal collapse grabens and result from elliptical doming of the underlying Cretaceous sequence due to Early Cenozoic uplift. The exceptional quality of this dataset allows an extremely detailed analysis of the throw distribution to be conducted on the faults. This, in addition to a reconstruction of the 3D geometry of the fault network, gives insights into the mechanisms and kinematics of reactivation. Two distinct modes of reactivation are recognised from this dataset. The main mode is a classical reactivation by upward propagation of pre-existing structures. A second mode, termed reactivation by dip linkage, is the propagation of an individual fault segment initiated above the pre-existing faults that hard link in the dip direction. For both mechanisms, reactivation processes are selective and only occur on some portions of faults. Factors controlling the preferential reactivation of some segments include: (1) orientation of the pre-existing fault plane relative to the principal stresses responsible for the reactivation, (2) segmentation of the pre-existing network and (3) maximum dimensions and throw values of pre-existing faults and basal tip line geometry associated with a detachment. Reactivation is an important process that may account for part of the scatter in fault-scaling relationships and should be included in fault-growth models.