--> Abstract: The Formation of Lake Tanganyika, Western Rift, East Africa: Application of a Simple-Shear/Pure-Shear Flexural Cantilever Model of Continental Lithosphere Extension, by C. K. Morley and N. J. Kusznir; #91012 (1992).

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ABSTRACT: The Formation of Lake Tanganyika, Western Rift, East Africa: Application of a Simple-Shear/Pure-Shear Flexural Cantilever Model of Continental Lithosphere Extension

MORLEY, C. K., Amoco Production Company, Houston, TX, and N. J. KUSZNIR,* University of Liverpool, Liverpool, UK

A flexural cantilever simple-shear/pure-shear model of lithosphere extension has been applied to the formation of the Lake Tanganyika rift. The Tanganyika rift formed in the late Tertiary and lies on the western arm of the East African rift. Maximum basin depths are of the order of 7 km. Substantial topographic relief with elevations up to 2.5 km exists on the basin flanks.

Three cross sections across the rift, based on seismic reflection data, have been modeled. Profiles both across and between transfer zones have been examined. The models assume a planar fault geometry and incorporate the rheological, thermal, and flexural isostatic consequences of continental lithosphere extension. During faulting, the footwall and hanging wall blocks are considered as two interacting flexural cantilevers; the response of these cantilevers to the isostatic forces produced by extension generating footwall uplift and hanging wall collapse.

Fault positions and fault displacements obtained from seismic reflection data have been used as input to the models. The models predict substantial footwall uplift for the major rift bounding faults with maximum elevations of up to 2.5 km above regional. Erosion of this footwall uplift has greatly reduced the extension that is observable today on these major rift bounding faults. In order to model the observed rift basin geometry and depths it is necessary to increase the extension on these boundary faults above that observed, and erode the footwall uplift allowing for flexural isostatic rebound. Good agreement between observed and modeled basin cross sections has been achieved. The models predict the posterosional extension now observed on these boundary faults.

The best fit of modeled profiles to observation is achieved using a low value for lithosphere flexural rigidity corresponding to an effective elastic thickness of the order of 3 km. The horizontal stresses within the brittle upper crust generated by the flexural bending associated with footwall uplift and hanging wall collapse generate extensive failure within the upper crust. After failure the maximum horizontal bending stresses are of the order of 1 kb. The low residual flexural rigidity of the lithosphere after extension, as determined for the Tanganyika rift and many other examples, is controlled by the upper crustal brittle failure stress envelope. The modeled flexural rigidities appear to be lower where the Tertiary rift follows the earlier Karoo rift.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)