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Fault segmentation, deep rift earthquakes and crustal rheology: Insights from the 2009 Karonga sequence and seismicity in the Rukwa-Malawi rift zone

Fagereng, Ake 2013. Fault segmentation, deep rift earthquakes and crustal rheology: Insights from the 2009 Karonga sequence and seismicity in the Rukwa-Malawi rift zone. Tectonophysics 601 , pp. 216-225. 10.1016/j.tecto.2013.05.012

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Abstract

The Rukwa–Malawi rift zone has a record of seismic events down to depths in excess of 30 km, deep for a zone of active continental extension. This deep seismicity, as well as the presence of long (~ 100 km) border faults, has previously been explained by the long-term bulk rheology of intact, old, cold, anhydrous strong crust in east Africa, or the presence of mafic material in the lower crust. The Karonga sequence of 2009 showed a style of faulting different from continuous slip along long border faults, and is interpreted as segmented failure of hanging wall faults. Coulomb stress transfer in this sequence is calculated, and found to be consistent with segmented slip on a fault system synthetic to a nearby border fault and restricted to depths < 12 km. The inferred thermal structure of the Malawi rift indicates that slip at depths in excess of 30 km occurs at temperatures greater than the 350–450 °C commonly inferred at the base of the seismogenic zone. Crustal strength calculations indicate that long border faults and deep seismicity require the presence of a weak zone of localized deformation with increased strain rate (or fluid pressure), within a strong lower crust. A hypothesis is proposed where shallow, segmented frictional failure occurs in regions of relatively strong, intact crust (e.g. the Karonga sequence), whereas long border faults and deep earthquakes are representative of zones of weakness within strong crust. This hypothesis, if correct, implies that seismogenic thickness can vary within thick elastic lithosphere, such that localized weak zones of the crust enable nucleation of larger seismic events, whereas strong, intact crust favors smaller, segmented events and a shallower seismogenic zone.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Faults; Rifts; East African Rift System; Seismicity; Coulomb stress change; Crustal rheology
Publisher: Elsevier
ISSN: 0040-1951
Last Modified: 04 Jun 2017 06:05
URI: http://orca-mwe.cf.ac.uk/id/eprint/56354

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