Muriel Rocher

Widening of normal fault zones due to the inhibition of vertical propagation

Abstract In this paper, we document the early stage of fault-zone development based on detailed observations of mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults contained in a weak layer. This restriction involves a flat-topped throw profile along the fault plane and modifications of the fault structures near the restricted tips, with geometries ranging from planar structures to fault zones characterized by abundant parallel fault segments. The ‘far-field’ displacement (i.e. the sum of the displacement accumulated by all the fault segments and the folding) measured along the restricted faults exhibiting this segmentation may have flat-topped shapes or triangular shapes when fault-related folding is observed above the layer-parallel faults. We develop a model from the observations. In this model, during the course of restriction, a fault forms as a simple isolated planar structure, then parallel fault segments successively initiate to accommodate the increasing displacement. We assume that, eventually, the fault propagates beyond the layer-parallel fault. This model implies first that fault widening is controlled by the fault capacity to propagate vertically in the layered section. Likewise, owing to restriction, fault growth occurs with non-linear increases in maximum displacement, length and thickness.

Widening of Normal Fault Zones During Vertical Propagation

In this paper, we document the early stage of faulting, based on detailed observations on mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults. This restriction involves a modification of the fault structures: far from the restricted tip, fault structures correspond to a simple planar slip surface, near the restricted tips, their structures range from a planar structure to a complex fault zone characterized by abundant parallel fault segments. Based on the observations, we developed a model of fault zone evolution in which fault zone complexity, specifically the number of sub parallel segments, increases to accommodate increasing strain, during restriction. Eventually, the fault should finally propagate beyond the layer-parallel faults with a complex geometry inherited from the period of restriction. This model implies that fault widening is controlled by the host rock and formerly developed fractures. Wide fault zones are expected in layered rocks with strong mechanical heterogeneities and with preexisting joints and layer-parallel faults. Likewise, fault growth occurs with non-linear increasing in maximum displacement, length, and thickness, due to restriction. Such a model of fault impacts on the vertical permeability and the seismic behavior of the rock.