Strain Accommodation and its Relationship to Pre-existing Structures along the Karonga Fault, Malawi

Abstract:

The Livingstone border fault, with its 7 km of total displacement, accommodates most of the strain in the northern portion of the Malawi Rift. Its hanging wall is also breaking up, as suggested by the 2009 earthquake sequence in Karonga. This hanging wall block is underlain in part by the NW- and N-striking Mughese Shear Zone. The superposition of new faults on the pre-existing structures makes this area an ideal location to study the effect of the orientation of pre-existing structures on the accommodation of strain in the hanging wall in the western flank of the northern Malawi Rift. We used gravity and aeromagnetic data and remote sensing to map the Precambrian macro-scale structural fabric of the greater Karonga region. Moreover, we mapped mesostructures within the Precambrian and younger rocks. In the northern portion of the Karonga fault, a single east-dipping fault zone with a mean strike of 32° and a 59° dip cuts the Precambrian foliation that has a mean strike of 301° and 79° dip, accommodating the majority of strain in this region. South of the city of Karonga, the Precambrian foliation assumes a NNW average strike that is steeply dipping. Here the Karonga fault disperses from a single fault with a 2 km damage zone to several distinct east- and west-dipping faults over 6 km that strike in the same overall direction as the foliation planes from the Mughese Shear Zone. Karoo rift structures (horsts and grabens) and their associated rock formations could also be reactivated in this area. These relationships suggest that within the Malawi Rift, strain can be accommodated differently based on the nature and orientation of pre-existing structures. The structural fabric surrounding the southern portion of the Karonga fault seems to favor reactivation and strain distribution, whereas strain is localized in the northern portion of the fault zone.