Long fault, narrow damage: the Bilila-Mtakataka fault, Malawi
Abstract
The development of faults and their damage zones in mechanically anisotropic crust is still poorly understood. Here, we describe the fault zone geology and fracture network around the Bilila-Mtakataka fault (BMF), an active normal fault in the amagmatic southern Malawi rift, where the crust experienced successive Precambrian orogenies at amphibolite to granulite facies metamorphic conditions. Previous studies have proposed that these events created pre-existing weaknesses that allowed the BMF to achieve its 110 km length for a relatively small total displacement (~500 m). The fault is expressed on the surface by a ≤25 m high soil-mantled scarp, which previous geomorphic analysis indicates formed in ≥2 Mw 7.5-8.0 earthquakes. At two locations, Mua and Kasinje, bedrock-eroding rivers form exposures up to 350 m from the scarp. At Kasinje, the fault separates foot- and hanging-wall granitic gneisses, with fault-parallel foliations. The hanging wall is covered by sediments in Mua, and the fault cross-cuts gently dipping footwall gneissic foliation. At Kasinje, macroscale fractures around the fault are dominantly foliation-parallel and only clearly visible within 5 m of the scarp. At Mua, macroscale fractures in a large range of orientations extend less than 20 m into the footwall.
Quantitative microstructural analysis of samples collected along fault-perpendicular transects indicate a 3 m wide zone of scattered cataclasites adjacent to the BMF scarp, and no systematic variation in microscale fracture intensity with distance into the fault's footwall. Whilst this may reflect a >350 m wide damage zone, the fracture intensity varies considerably within each thin section, indicating that fracturing was not primarily controlled by distance from the fault. We also observe <20 m thick zones of increased fracturing for other higher-displacement faults in Malawi, suggesting narrow damage zones are common there. Thus we conclude that despite the BMF's 110 km length and potential for Mw >7 earthquakes, it lacks a clear damage zone of more than a few metres. We attribute this to rapid fault growth by exploitation of a pre-existing crustal weakness. If true, this suggests that deep-seated crustal anisotropies can significantly modify fault zone damage, and thus also fluid flow and the seismic energy budget.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR32A..04W
- Keywords:
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- 1209 Tectonic deformation;
- GEODESY AND GRAVITY;
- 5104 Fracture and flow;
- PHYSICAL PROPERTIES OF ROCKS;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS