Patterns of Displacement Along Large Normal Faults: Implications for Basin Evolution and Fault Propagation, Based on Examples from East Africa

ABSTRACT Seismic reflection data across six boundary faults from east Africa have revealed information about the way boundary faults develop with respect to the synkinematic basin fill. Previous models of basin fill assume that lateral fault propagation and deposition occur simultaneously, which gives rise to along-axis onlap of the basin fill onto the prerift basement (propagating basin model). Seismic reflection data from boundary faults in east Africa show only rare evidence for along-axis broadening of the basins. Instead, the basin margins appear to be fixed from a very early stage in the basin history (nonpropagating basin model). Depocenters shift not by gradual lateral propagation, but by abrupt changes in location. Such observations indicate that boundary fault propagation to near-maximum length occurs rapidly during the very early stages of rifting, probably by linkage of numerous small faults. Displacement then builds (following the b-value earthquake model) along the fault during basin development, out of phase with the main propagation phase. Mature fault systems may have experienced several phases of jumps in fault length or the location of fault activity. Major boundary faults exhibit three basic patterns of evolution; these patterns of evolution can have many variants and include (1) simple displacement, where maximum displacement is approximately at the center of the fault and decreases gradually toward the fault tips, (2) variable along-strike displacement that incorporates linkage of in-line or en-echelon, relatively large-displacement faults of similar age leaving transverse anticlines and synclines, and (3) asymmetric propagation where one tip of a fault remains more or less fixed while the other tip propagates a relatively long distance. Asymmetric propagation can occur smoothly and gradually or, alternatively, there can be a jump in the location of the fault, and a new fault of different age can form along-strike from the older fault. Eventually the new fault propagates and links with the older fault segment, reactivating the older fault to some extent. Additionally, long-lived faults may be composites of fault types 1–3.