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Chapter from:
AAPG Studies in Geology #44: Geoscience of Rift Systems-Evolution of East Africa
Edited by C.K. Morley
Copyright ©1999 by The American Association of Petroleum Geologists. All rights reserved.
AAPG Studies in Geology #44, Chapter 7: Boundary Fault Angle, with Particular Reference to the Lokichar Fault, Turkana Region, Kenya, by C.K. Morley, Pages 115 - 129

Abstract

Under normal conditions of rifting, boundary faults are predicted to be planar, high-angled with dips between 45° and 70°—this the case for most of the rifts of East Africa. There are, however a number of exceptions. In particular, it seems that in regions of high heat flow and strong volcanic activity half graben boundary faults tend to be lower-angle (30-45°). In the case of the Lokichar Fault (Turkana area, Kenya) lower-angled segments, located around the zone of maximum displacement on the fault, pass into lower displacement areas characterized by very low-angle fault segments (12-20°). The initiation of faults at a low angle cannot be easily explained by rock mechanics theory. Therefore, common explanations for such faults include:

1. rotation of higher angle faults by the “domino” faulting model;
2. rotation of large-displacement faults by isostatic instability created by the faulting; and
3. activation of low-angled pre-existing fabrics.

The Lokichar Fault geometry is inconsistent with any of the above explanations and some other cause of the low-angled nature must be found. In the Turkana area there is a coincidence between the location of the very low-angled segments and regions of intense igneous intrusive activity. If igneous intrusions do play a role in controlling fault dip then there are two possible mechanisms. Both cause reorientation of the stress axes from the simple Andersonian condition, which permits normal faults to form at a lower angle. The mechanisms are:

1. Magma pressure during intrusion and the stresses that remain after cooling of the intrusion may locally create compressional conditions. Approaching dikes the maximum principal stress direction may swing from vertical towards the horizontal.
2. Emplacement of igneous intrusions may heat up and weaken the lower crust permitting it to flow. It may be possible to set up a basal shear stress between the flowing and static crust causing reorientation of the principal stress axes.