FAULT GROWTH DURING NASCENT STAGES OF CONTINENTAL RIFTING

Fault growth during incipient continental rifting is investigated using topographic scarps of normal faults of the Okavango Rift Zone (ORZ), Botswana, the youngest branch of the East African Rift System. The ORZ is located within the NE-trending Proterozoic Damara and Ghanzi orogenic belts, situated between the Congo and Kalahari cratons. The main faults of the system define a half graben filled by fluvio-deltaic sediments of the Okavango and Kwando rivers. Major normal fault traces have azimuths (~050) parallel to the azimuths of the dominant NE trending (040-070) structural fabric of the basement.

Topographic profiles (30 m. spatial resolution) extracted from Shuttle Radar Topography Mission were used to define fault scarps. 7 main fault traces were analyzed using displacement (D) - length (L) ratios. Fault lengths vary from 55 to 236 km and fault scarps heights vary from 3 to 141 m. Kunyere, Thamalakane, and Gumare faults exhibit D/L profiles with “parabolic” shapes. Mababe and Chobe faults show “linear” profiles. The Phuti and Linyanti faults exhibit irregular “saw-tooth” profiles. ORZ faults have high D/L ratios plotting on or to the right of D_max/L=10^-3 defined as the typical D/L ratios for normal faults (Kim & Sanderson, 2005). The intrinsically long L and lower D_max of ORZ normal faults, when compared to “typical” normal faults, may reflect several factors: 1) Fault scarp height may record a minimum throw along older faults affected by aggradation (e.g., Linyanti), degradation (e.g., Chobe), or multiple displacement (e.g., Kunyere). 2) Fault lengths are less susceptible to such modifications as shown by good agreement between SRTM and geophysical methods whereas D values for these faults based on basement throw (Kinabo, et al., 2008) show similar or greater displacement than recorded by topographic fault scarps. 3) D/L fault traces with significantly greater displacement at one tip may reflect fault block rotation or strain transfer along fault linkages. 4) A strong possibility exists that ORZ faults exploited pre-existing weakness of the planar basement fabrics enabling propagation of initially long faults with subsequent incremental increases in displacement as the rift evolves (e.g., Walsh et al., 2002 ).