How Do Variably Striking Faults Reactivate During Rifting? Insights From Southern Malawi

Abstract

Abstract Crustal extension is commonly thought to be accommodated by faults that strike orthogonal and obliquely to the regional trend of the minimum compressive stress ( σ 3 ). Activation of oblique faults can, however, be conceptually problematic as under Andersonian faulting, it requires preexisting crustal weaknesses, high fluid pressures, and/or stress rotations. Furthermore, measurements of incremental fault displacements, which are typically used to identify oblique faulting, do not necessarily reflect regional stresses. Here, we assess oblique faulting by calculating the stress ratio ( σ 3 / σ 1 , where σ 1 is the maximum compressive stress), slip tendency, and effective coefficient of friction ( μ s ′) required to reactivate variably striking normal faults under different trends of σ 3 . We apply this analysis to NW and NNE striking active faults at the southern end of the Malawi Rift, where NE‐SW, ENE‐WSW, E‐W, and SE‐NW σ 3 trends have previously been proposed. A uniform σ 3 trend is inferred for this region as recent joints sets do not rotate along the rift. With a NE‐SW trending σ 3 , NW‐striking faults are well oriented, however, NNE‐striking faults require μ s ′ < 0.6 to reactivate. This is inconsistent with a lack of frictionally weak phyllosilicates detected in the fault zone rocks. With an ENE‐WSW to E‐W trending σ 3 , all faults can reactivate at μ s ′ > 0.55. These σ 3 trends are also comparable to a focal mechanism stress inversion, regional joint orientations, and previously reported geodetically derived extension directions. We therefore conclude that unlike typical models of oblique rifting, the southern Malawi Rift consists of faults that all strike slightly oblique to σ 3 .