Depth-dependent controls on structure , reactivation and 1 geomorphology of the active Thyolo border fault , Malawi rift 2

25 We present new observations of the geometry and pattern of fault growth from the 26 Thyolo fault, an 85 km long border fault in the southern Malawi Rift, from high27 resolution topography and field observations. The rift has a polyphase tectonic 28 history and the Thyolo fault is located towards the edge of the Proterozoic Unango 29 Terrane. Recent activity is demonstrated by an 18.6 ± 7.7 m high fault scarp. 30 Different patterns of segmentation are indicated by fault geometry and fault 31 displacement profiles: two substantial reductions in scarp height do not coincide with 32 surface geometry changes. The surface scarp is divided into two geometrically 33 defined overlapping sections, which are joined by a ~5 km long, fault perpendicular 34 scarp. The scarp height in this linking section is similar to the bounding sections, yet 35 the river drainage network and sediment depocenter distribution is not typical of relay 36 zones. Microstructural and compositional analyses show a 15-180 m thick damage 37 zone with an unusually narrow 0.7 m thick fault core. These features can be 38 explained if the fault exploits weak ductile zones at depth, such as heterogeneity 39 associated with the Unango Terrane boundary, while near surface geometry is 40 controlled by well-oriented, frictionally strong but low-cohesion shallow structures. 41 42