Structural controls on the interaction between basin fluids and a rift flank fault: constraints from the Bwamba Fault, East African Rift

Hollinsworth, A. D., Koehn, D. , Dempster, T. J. and Aanyu, K. (2019) Structural controls on the interaction between basin fluids and a rift flank fault: constraints from the Bwamba Fault, East African Rift. Journal of Structural Geology, 118, pp. 236-249. (doi: 10.1016/j.jsg.2018.10.012)

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We present petrographic and structural analyses of a basement-hosted border fault in the East African Rift. Understanding the mechanical evolution and fluid-rock interaction of rift-flank faults is integral to developing models of fluid flow in the crust, where hydraulic connections may occur between basement faults and basin sediments. The Bwamba Fault forms the flank of the Rwenzori Mountains Horst in western Uganda, and has locally reactivated older mylonitic fabrics in the basement gneisses. The fault core features discrete mineralised and veined units. Shear fabrics and fault scarp striations indicate predominately normal kinematics, with minor strike-slip faulting and fabrics. Transient brittle failure was accompanied by two phases of fluid ingress, associated with veining and mineralisation. The first was localised and strongly influenced by host lithology. The second involved widespread Fe-oxide and jarosite mineralisation. The latter signals the onset of a hydraulic connection between Fe- and S-rich sedimentary rocks in the adjacent Semliki Rift Basin and the Bwamba Fault, involving co-seismic and or post-seismic fluid injection into the fault at ca. 150–200 °C, and 2.5–3 km depth. Such evolving permeability connections between basin sediments and basement faults are important for local hydrocarbon and geothermal systems, and may be typical of active rifts.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Koehn, Dr Daniel and Dempster, Dr Tim and Hollinsworth, Mr Allan
Authors: Hollinsworth, A. D., Koehn, D., Dempster, T. J., and Aanyu, K.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Journal of Structural Geology
ISSN (Online):1873-1201
Published Online:01 November 2018
Copyright Holders:Copyright © 2018 The Authors
First Published:First published in Journal of Structural Geology 118:236-249
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
683431NERC Oil and Gas Collaborative Doctoral Training Centre 2014Daniel KoehnNatural Environment Research Council (NERC)NE/M00578X/1SCHOOL OF GEOGRAPHICAL & EARTH SCIENCES