Patterns of displacement along large normal faults: Implications for basinevolution and fault propagation, based on examples from east Africa

Seismic reflection data across six boundary faults from east Africa have re vealed information about the way boundary faults develop with respect to th e synkinematic basin fill. Previous models of basin fill assume that latera l fault propagation and deposition occur simultaneously, which gives rise t o 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, t he basin margins appear to be fixed from a very early stage in the basin hi story (nonpropagating basin model). Depocenters shift not by gradual latera l propagation, but by abrupt changes in location. Such observations indicat e that boundary fault propagation to near-maximum length occurs rapidly dur ing 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 propag ation phase. Mature fault systems may have experienced several phases of ju mps in fault length or the location of fault activity. Major boundary fault s exhibit three basic patterns of evolution; these patterns of evolution ca n have many variants and include (1) simple displacement, where maximum dis placement is approximately at the center of the fault and decreases gradual ly toward the fault tips, (2) variable along-strike displacement that incor porates linkage of in-line or en-echelon, relatively large-displacement fau lts of similar age leaving transverse anticlines and synclines, and (3) asy mmetric propagation where one tip of a fault remains more or less fixed whi le the other tip propagates a relatively long distance. Asymmetric propagat ion 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 al ong-strike from the older fault. Eventually the new fault propagates and li nks with the older fault segment, reactivating the older fault to some exte nt. Additionally, long-lived faults may be composites of fault types 1-3.