Growth of a normal fault system: observations from the Lake Malawi basin of the east African rift

We have studied the growth history of the Usisya normal fault system, which bounds the west side of the Lake Malawi basin, one of the largest rift bas ins in the east African rift system. The Lake Malawi rift basin is a comple x of intrabasins and intrabasin highs formed by three major fault segments, each about 100 kin in length. The basin has been actively subsiding since the late Miocene, and has a maximum depth of about 3 km. Unlike the majorit y of previous studies of fault systems, we are able to define the temporal evolution of this fault system using the patterns of sediment infill. This is due almost exclusively to the: availability of a high-density seismic re flection network in combination with several clearly identifiable temporal marker beds. An analysis of the seismic reflection data reveals that growth of the basin-bounding faults occurred in the following sequence: 1) In the early stages of rifting (starting about 8.6 Ma), the northern fault was th e most active: 2) then, extension shifted to the southernmost fault segment and lasted until about 2.5 Ma, 3) during the interval between about 2.3 Ma and 1.6 Ma, the central segment was most active. Prior to the last interva l, the central segment accrued the least total displacement of the three se gments. This contradicts the common notion that the location of maximum dis placement remains at a fixed location from fault inception or that the larg est fault in any population of faults always maintained the highest displac ement rate. Consistent with observations on smaller faults, there is a mark ed increase in the displacement gradient on individual faults in the region s of overlapping segments. Although there is an observable asymmetry in ind ividual segment displacement-profiles, there is a clear evolution toward a flattened 'bell shaped' total displacement profile for the fault system tha t is consistent with the shape and scaling relationships of displacement vs . length that are observed on a wide range of individual normal faults. Thi s suggests that the irregularities in the shape and scaling relationships o bserved in complex fault systems will eventually smooth-out. Moreover, the observed growth pattern suggests that the profile of the fault system will progress toward that of isolated faults, provided the system is allowed suf ficient time to evolve. This in turn describes a process for maintaining a selfsimilar scaling observed in large populations of faults, which span sev eral orders of magnitude in fault length. (C) 2000 Elsevier Science Ltd. Al l rights reserved.