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

Abstract

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 basins in the east African rift system. The Lake Malawi rift basin is a complex of intrabasins and intrabasin highs formed by three major fault segments, each about 100 km in length. The basin has been actively subsiding since the late Miocene, and has a maximum depth of about 3 km. Unlike the majority 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 reflection 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 the 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 interval, the central segment accrued the least total displacement of the three segments. This contradicts the common notion that the location of maximum displacement remains at a fixed location from fault inception or that the largest fault in any population of faults always maintained the highest displacement rate. Consistent with observations on smaller faults, there is a marked increase in the displacement gradient on individual faults in the regions of overlapping segments. Although there is an observable asymmetry in individual segment displacement-profiles, there is a clear evolution toward a flattened ‘bell shaped’ total displacement profile for the fault system that is consistent with the shape and scaling relationships of displacement vs. length that are observed on a wide range of individual normal faults. This suggests that the irregularities in the shape and scaling relationships observed 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 sufficient time to evolve. This in turn describes a process for maintaining a self-similar scaling observed in large populations of faults, which span several orders of magnitude in fault length.