Influence of pre-existing pervasive fabrics on fault patterns during orthogonal and oblique rifting: An experimental approach

Abstract

Pervasive mechanical anisotropy (e.g. metamorphic foliation) in the pre-rift basement rocks often appears to control the orientation of new faults in natural rift systems, and thus tend to influence formation of fault-bounded sedimentary basins. However, the extent to which this influence works in nature has remained debatable, and contrasting ideas exist regarding the issue of structural inheritance in natural rifts like the East African Rift system. Earlier experimental works have investigated different kinematic aspects of faulting in simulated orthogonal and oblique rift systems, including the role of a crustal-scale pre-existing zone of weakness on continent-scale rifting. These experiments have tested the influence of discrete pre-existing weak zones (e.g. pre-existing faults or an ancient orogenic belt) on localization of subsequent rifting, but have not considered the control of pervasive fabric anisotropy (e.g. metamorphic foliation, etc.) in pre-rift basement rocks on the fault patterns during rifting. Here we present a series of simple, roughly scaled, analog model experiments where pervasive strength anisotropy in the basement was created by brushing plaster of Paris on a viscous (pitch) substratum. This way a transverse anisotropy of tensile strength was generated in the plaster of Paris layer due to thickness variation across the brush marks. The basement was overlain by a dry sand pack, simulating weak sedimentary rocks overlying a foliated crystalline basement. Orthogonal and oblique extension of the models at various angles, and with differently oriented strength anisotropy, exhibited the control of the anisotropy on the orientations of new faults in the sand layer. In general, anisotropy could significantly control the fault pattern only when it was at a high angle (>45°) with respect to the maximum instantaneous horizontal stretching direction (εH1), otherwise they largely disregarded the anisotropy and formed orthogonal to εH1. Faults never formed orthogonal to the displacement vector (D) in oblique rifting, unless the fault-controlling anisotropy was perpendicular to D. Faults strongly following the anisotropy were much longer than those not following that trend. Faults oblique to the anisotropy were short, segmented and tortuous in nature, and mainly grew by segment linkage. The experimental results support many field observations made in the East African Rift system, and are likely to be applicable to other rift systems like Gondwana basins of India, where field data on such structural inheritance are meager.