Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi. (October 2020)

• Record Type: Journal Article
• Authors: Wedmore, Luke N.J.; Williams, Jack N.; Biggs, Juliet; Fagereng, Åke; Mphepo, Felix; Dulanya, Zuze; Willoughby, James; Mdala, Hassan; Adams, Byron A.
• Is Part Of: Journal of structural geology. Volume 139(2020)
• Publication Date: 2020-10
• Subjects: High-resolution topography -- Pre-existing structures -- Normal faults -- Rifts -- Border fault -- Damage zone; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2020.104097
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: We present new insights on the geometry, initiation and growth of the Thyolo fault, an 85 km long active border fault in the southern Malawi Rift, from high-resolution topography, field and microstructural observations. The Thyolo fault is located towards the edge of the Proterozoic Unango Terrane, and is the border fault of the Lower Shire Graben, which has experienced four phases of extension since the Jurassic. Recent activity is demonstrated by an 18.6 ± 7.7 m high fault scarp, with two substantial reductions in scarp height along strike. However, the segment boundaries suggested by these displacement measurements do not coincide with changes in fault strike. Elsewhere, a ~5 km long fault perpendicular scarp joins two overlapping sections, yet the scarp height in this linking section is similar to the bounding sections, and there is no evidence of significant pre-linkage strain accumulation. Microstructural analyses along the fault show a 15–45 m thick footwall damage zone with a 0.7 m thick core. We suggest that favourably-oriented, pre-existing shallow structures control changes in surface geometry and the narrow fault core, whereas exploitation of weak ductile zones at depth, possibly associated with the terrane boundary, control the displacement profile of the fault. Highlights: The Thyolo fault, Malawi, is a rift border fault with a polyphase tectonic history. Satellite and field data confirm recent activity on an 18.6 ± 7.7 m high scarp. The fault is segmented, but scarp height minima do not align with geometry changes. Pre-existing shallow structures control the surface geometry and 0.7 m wide fault core. Fault displacement is affected by viscous reactivation along a terrane boundary.