THE ROLE OF CRUSTAL STRENGTH ANISOTROPIES IN STRAIN LOCALIZATION DURING INCIPIENT CONTINENTAL RIFTING: AN EXAMPLE FROM THE OKAVANGO RIFT ZONE, BOTSWANA

Fault nucleation and growth is investigated through length-height profiles of 7 normal faults of the Okavango Rift Zone, Botswana, using 1029 topographic profiles obtained from the Shuttle Radar Topography Mission (30 m spatial resolution). Faults preserve 3 different across-scarp profile geometries: asymmetric, linear, and composite which exhibit irregular “saw-tooth” shapes, indicating degradation by incision. Fault trace lengths (L) vary from 55 to 237 km and fault trace heights (H’) vary from 17 to 141 m. Normal faults show low D/L ratios compared to “typical” normal faults (D_max/L=10^-3). The intrinsically long L and lower D_max reflect nucleation of fewer faults along favorably oriented preexisting mechanical strength anisotropies in the crust. Fluid circulation within fault zones reduces the shear stress necessary to reactivate these faults. Subsequent slip events trigger a pore fluid wave that promotes along strike propagation of the fault with low displacement. This creates a positive feedback in which fewer, longer, faults suppress nucleation of new faults. Eventually these faults develop strong cohesive fault rocks and are abandoned in favor of forming new faults and the process is repeated. In this way the preexisting basement structures greatly influence the earliest stages continental rifting as defined by the location and growth of master border faults.