Marked along-strike variations in dip of normal faults - the Lokichar fault, N-Kenya rift: a possible cause for metamorphic core complexes

The Lokichar fault is a major boundary fault in the northern Kenya rift, mapped from seismic reflection data. The fault is a mixture of high 45-60 degrees, low 20-45 degrees and very low (12-20 degrees) angle segments. The areas of least displacement (up to a maximum 10 km heave) are the very low-angle fault segments (12-20 degrees). The southern higher angle fault segment has a maximum estimated heave of about 20 km. Initiation of normal faults at a low angle cannot be easily explained by rock mechanics theory. 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 (rolling hinge models), and (3) reactivation of low-angle pre existing fabrics. The Lokichar fault geometry is inconsistent with any of the above explanations. The very low-angle segments coincide with regions of intense igneous intrusive activity. Re-orientation of the stress axes from the simple Andersonian condition, could permit normal faults to form at a lower angle; this may happen around intrusive complexes or by setting up a basal shear stress between flowing and static crust. If faults associated with metamorphic core complexes were associated with marked along-strike changes in fault angle the resulting variations in footwall uplift could give rise to the antiformal metamorphic core complex geometry. (C) 1999 Elsevier Science Ltd. All rights reserved.