Stress re-orientation along zones of weak fabrics in rifts: An explanation for pure extension in ‘oblique’ rift segments?

Details Stress re-orientation along zones of weak fabrics in rifts: An explanation for pure extension in ‘oblique’ rift segments? Stress re-orientation along zones of weak fabrics in rifts: An explanation for pure extension in ‘oblique’ rift segments?

Sep 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010e%26psl.297..667m&link_type=abstract

Earth and Planetary Science Letters, Volume 297, Issue 3-4, p. 667-673.

Physics

5

Scientific paper

The Cenozoic East African Rift System (EAR) displays strong interaction between rift faults and zones of weakness in Precambrian basement and Late Carboniferous-Permian Karroo rifts. Assuming E-W regional extension, NW-SE and NE-SW trending rift segments following older zones of weakness are expected to show oblique extensional or transtensional displacement. Yet, the earthquake focal mechanism-derived modern maximum horizontal stress (Shmax) pattern in the Western Branch of the EAR displays similar orientations to dominant foliation direction in Proterozoic orogenic belts. The oblique segments are undergoing almost pure extension. Localized stress deflections near zones of strength anisotropy are commonly described in studies of geomechanics, in-situ stresses and fracture patterns. The correspondence between foliation and Shmax direction in the Western Branch suggests re-orientation of stress at a larger scale than has previously been recognized. The stress guide effect appears best developed in the Ubendian Belt-Rukwa Rift where the Shmax direction from the regional N-S direction to a NW-SE trend and NW-SE trending foliations are consistently steeply dipping. However, this effect is considered only one of a number of possible causes of stress deflection in the East African Rift. Precambrian basement foliation is not oriented consistently enough to re-orient stress on a large-scale everywhere (particularly the Kenya Rift). The effects of stress rotation along oblique fabrics are: 1) faults normally predicted to exhibit oblique slip are actually near pure dip-slip, 2) faults along oblique trends can be optimally oriented with respect to the (local) stress field, and develop in a zone of reduced cohesive shear strength (with respect to isotropic rocks), 3) where foliations are inclined from vertical the maximum principal stress will be similarly inclined leading to initiation of extensional faults at angles considerably lower than 60°, and 4) faults will strike parallel to the foliation but cross-cut foliations in cross-section.

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