Structural inheritance and strain in the amagmatic southern Malawi rift
Abstract
The effects of pre-existing structure on the geometry and distribution of strain in an early stage, evolving rift is often obscured by later stage sedimentation or magmatism. To explore these effects, we present new field observations from two grabens in the southern Malawi Rift System. This poorly studied amagmatic rift has a short history of extension, minor sedimentary cover, and a range of pre-existing structures relating to the polyphase tectonic history preserved in the Pre-Cambrian basement rocks.
Our observations of high-resolution topography reveal five sub-parallel Late Quaternary fault scarps across the 50 km wide Zomba Graben, with activity across these faults confirmed by additional analysis of river profiles. Scarps are up to 30 m high with an across strike spacing of 10-15 km. Scarp height measurements indicate that 54 ± 24 % of strain is currently accommodated across intra-rift faults. Unlike the Zomba Graben, the ~50 km wide Lower Shire Graben has been active during previous episodes of extension in the Jurassic-Cretaceous and the Karoo. In this graben, 70 ± 14 % of strain associated with the current phase of rifting is localised on the rift border fault, which has an ~30 m high fault scarp. The border fault is segmented, but segmentation patterns do not reflect the surface fault geometry which both follows and cross-cuts the metamorphic foliation. Measurements of CO2 gas flux confirm that there is no detectable magmatic component of rifting in the region. Therefore, we interpret that the observations of localised strain the Lower Shire Graben and distributed strain the Zomba Graben are related to the effect of structural inheritance. We suggest mid-crustal heterogeneities such as pre-existing anastomosing shear zones and crustal terrane boundaries play a more important role in localising strain in an early rift than the structures observed at the surface.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T31B..04W
- Keywords:
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- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8109 Continental tectonics: extensional;
- TECTONOPHYSICS;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS