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Crustal structure and tectonic evolution of the anza rift, northern Kenya

by L.C. Greene, D.R. Richards, R.A. Johnson
Tectonophysics ()

Abstract

The Anza trough is a Mesozoic rift located in northern Kenya that\nappears to be the failed third arm of a paleo-triple junction which\nallowed the separation of Madagascar from Africa during the Jurassic.\nThe rift is oriented NW-SE and its tectonic evolution is related\nto that of the Mesozoic southern Sudan rift system. We analyzed seismic\nand gravity data from the southwestern side of the Anza rift including\nthe Chalbi Desert to gain a better understanding of rift structure.\nGravity data delineate the main rift basins as well as a small sub-basin\non the southwest side of the main rift. Normal faulting evident on\nthe NW end of a 42-km-long, NW-SE oriented Vibroseis� profile, marks\nthe western boundary of the sub-basin. This sub-basin is offset from\nthe trend of the main Anza trough; the western boundary may be a\ncomplex fault zone accommodating a change in direction of the main\nrift trend. Gravity values increase to the NW in the faulted area,\nsuggesting shallowing of basement. A strong NW-dipping reflection\nfrom 0.5 s to almost 3 s is interpreted as a pre- to mid-Cretaceous\nunconformity. The configuration of the unconformity and the normal\nfaulting strongly resembles the half-graben geometry imaged in the\nEast African Rift. Numerous discontinuous reflections can be seen\ndeeper in the section between 6 and 9 s, but a distinct reflection\nMoho cannot be interpreted with certainty. In addition to seismic\nand gravity data, regional geologic and well data lead us to conclude\nthat there are probably Jurassic marine sediments in the bottom of\nthe Anza rift.

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