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

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Authors

Luke Nicholas John Wedmore , Jack Williams , Juliet Biggs, Ake Fagereng , Felix Mphepo, Zuze Dulanya, James Willoughby, Hassan Mdala, Byron Adams

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.

DOI

https://doi.org/10.31223/osf.io/4bs9x

Subjects

Earth Sciences, Geology, Geomorphology, Physical Sciences and Mathematics, Tectonics and Structure

Keywords

Normal faults, border fault, damage zone, High-resolution topography, pre-existing structures, rifts

Dates

Published: 2020-02-26 07:44

Last Updated: 2020-05-27 02:53

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License

CC BY Attribution 4.0 International

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