HR: 1340h
AN: T43B-1406    [Abstracts]
TI: Controls on the Geometry and Evolution of a Highly Asymmetric Rift System; Southern South Africa
AU: * Paton, D
EM: dpaton@mines.edu
AF: Colorado School of Mines, Department of Geology, Golden, CO 80401
AU: * Paton, D
EM: dpaton@mines.edu
AF: University of Edinburgh, School of Geosciences, Edinburgh, EH9 3JW United Kingdom
AB: Three Mesozoic basins (Algoa, Gamtoos and Plettenburg) in southern South Africa form a highly asymmetric rift system. This system initially formed through the negative structural inversion of the Paleozoic Cape Fold Belt and the influence of pre-existing structures in controlling basin geometry and evolution in space and time is presented. The asymmetric configuration is apparent from the consistent S to SW dip of the bounding faults. The St Croix Fault (Algoa Basin) is a shallow dipping (<24°) listric fault with a displacement of ~12.5 km. In contrast, both the Gamtoos and Plettenburg faults (Gamtoos and Pletmos basins respectively) are higher angle (40°-65°) planar faults to >12,000 m depth and have displacements of 16.5 and 12 km. Estimated cumulative displacement rates for the Gamtoos and Plettenburg faults are 0.55 mm/yr-1 and 0.43 mm/yr-1. Two models for underlying basin geometry are presented. The first invokes a south dipping decollement from which all of the normal faults branch. The second proposes a combination of a south dipping detachment surface in the NE (Algoa) and planar normal faults that extend to the brittle-ductile transition zone in the SW (Gamtoos and Pletmos). These models raise important questions regarding: 1) layer and partitioning of stress within the upper and lower crust; and 2) the accommodation of planar faults with such large displacements within the southern South African lithosphere. In addition, the evolution of the Gamtoos and Plettenburg faults are established using seismic stratigraphy coupled with onshore observations. This study reveals that these faults grew through the coalescing of co-linear, isolated depocentres to form very long and under-displaced faults during the early syn-rift interval. The faults were at least 100 km long with displacements of <800 m within 5 Myrs of rift initiation. During the subsequent 25 Myrs of rifting there was no appreciable increase in fault length, although cumulative displacement increased to ~16.5 km. Although the final Displacement-Length (D-L) values of the faults conform to established models, the near vertical D-L growth profiles do not conform to previous models. The atypical evolution of the faults is considered in a context of reactivating the underlying, pre-extensional structures.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8020 Mechanics, theory, and modeling
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005