Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi

Luke N J Wedmore*, Jack N Williams, Juliet J Biggs, Åke Fagereng, Felix Mphepo, Zuze Dulanya, James Willoughby, Hassan Mdala, Byron Adams

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

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.
Original languageEnglish
Article number104097
JournalJournal of Structural Geology
Early online date21 Jun 2020
DOIs
Publication statusE-pub ahead of print - 21 Jun 2020

Keywords

  • high-resolution topography
  • pre-existing structures
  • normal faults
  • rifts
  • border fault
  • damage zone

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