Evolution of fault patterns within a zone of pre-existing pervasive anisotropy during two successive phases of extensions: an experimental study

Abstract

The aim of the present work is to study the influence of preexisting pervasive strength anisotropy on the development of faults during two phases of extensions. Two different series of experiments are performed by deforming rectangular three-layered models either by orthogonal extension followed by oblique extension (series 1) or by oblique extension followed by orthogonal extension (series 2). The model represents a rectangular zone of rifting. The final fault architecture after two successive phases of extension is primarily controlled by the orientation of the pervasive strength anisotropy. The mode of far-field stress (orthogonal or oblique) plays a role in fault initiation during both the phases of extension. The growth of the faults which are orthogonal or oriented more obliquely (β = 45°/60°) with respect to the rift normal is controlled by the direction of extension. However, the less oblique faults (β = 15°) develop as strike-slip faults irrespective of the direction of extension. The phase 1 faults reactivate during the phase 2 extension only when they are parallel to the preexisting pervasive anisotropy. New faults parallel to the rift axis form only if the phase 2 extension is orthogonal (series 2). It is found to happen irrespective of the orientation of the strength anisotropy and of the 1st phase faults. Those faults act as linking faults for the highly oblique (β = 45°/60°) phase 1 faults. New faults are formed following the anisotropy during both orthogonal and oblique phase 2 extension only if the anisotropy is oriented at low angle (β = 15°) with the rift normal. The different fault patterns developed in the experiments can be matched well with natural examples reported from Karonga basin, Malawi rift, Kenya.