BORDER FAULT SEGMENTATION AND LINKAGE AT THE PROPAGATING TIPS OF CONTINENTAL RIFTS: INSIGHTS FROM SOUTH MALAWI RIFT

Models of continental break up suggest that rift systems grow by the propagation and linkage of isolated rift segments and by the establishment of a constant length early in the rift history followed by accrual of displacement. Segmentation of actively propagating rift border faults control the expression of rift zones, sediment fill, and sequential evolution over time. The seismic hazard caused by slip along border fault segments makes it vital to understand the processes of segmentation and linkage. However, there is limited insight into segmentation and linkage styles of border faults at the propagating rift tips. In the Malawi Rift, previous studies have focused on the basin-wide regional segmentation and linkage patterns, and sedimentary fill with emphasis on the portion covered by Lake Malawi. Here, we investigate the segmentation and linkage of the border fault systems at the southern tip of the Malawi Rift and the northern tip of the Thyolo Fault in the Shire Graben. We used 30 m-resolution SRTM DEM and aeromagnetic data to characterize the geometry of the faults. We estimate scarp height at every 1 km interval along both western and eastern rift boundaries and plot against fault length along strike to produce minimum displacement-length plot, using moving average trendlines to account for anomalies introduced by inherited fabric, erosion or accuracy of SRTM-DEM.

Our results showed fault length traces > 100 km with orientation NNW-SSE in the north while the fault lengths in the south are < 60 km as the orientation of the faults change to NNE-SSW. The average displacement in the north is slightly greater than the south with increased soft linkage of fault segments towards the rift tips indicated by lack of breaching faults between sets of isolated border fault segments.

Our data suggest that increased number of soft linkages of isolated border fault segments indicate the border faults along the Zomba Graben were formed by splaying border faults due to the rift termination. The atypical increase in the fault displacements at the southernmost tip along both sides of the rift maybe a genetic characteristic of actively propagating continental rift tips due to heterogeneity in terms of lithology and inherited morphology. It also suggests strain transfer due to linkage of the Malawi Rift with the Thyolo Fault of Shire Rift.