TEMPORAL CONSTRAINTS ON ACCOMMODATION ZONE UPLIFT AND NORMAL FAULT PROPAGATION IN LAKE MALAWI

Lake Malawi, at the southern end of the Western Branch of the East African Rift System, is comprised of three half-graben basins formed by normal slip on major border faults. The Central Basin contains a basement high deformed by numerous normal faults. This accommodation zone is in the hanging wall of a border fault and divides the Central Basin into two sub-basins.

This study is focused on the eastern sub-basin, between the NE-SW trending fault array comprising the accommodation zone and a NNW-SSE oriented en echelon fault array on the eastern edge. Structural interpretation integrates three vintages of 2D single-channel reflection seismic survey data. Ages are applied to seismic horizons using an age model derived from scientific drilling data collected in the study area.

Despite continuous movement on these faults, the morphology of the sub-basin has changed little in the last 92 k.y. Here, the 92 ka horizon is a sequence boundary that truncates underlying westward-dipping strata. The overlying sequence displays less variation in thickness than the lower sequence. In the western sub-basin the 92 ka horizon is a correlative conformity and the strata above and below it thicken toward the border fault on the western shore of the lake.

Uplift of the accommodation zone created a longitudinal drag fold in the sub-basin. This syncline is the major depocenter of the sub-basin. Isochron patterns indicate that uplift of the accommodation zone has been continuous since at least 1246 ka.

The sub-basin is defined by fault arrays; however isolated faults propagate through the main depocenter nearly orthogonal to the accommodation zone. Offset of the depocenter on structural maps of seismic horizons indicate these faults have a strike-slip component. Propagation rates are estimated on one of these faults by assuming lateral and vertical propagation are synchronous. This rate increases southeastward from 2.16 m/k.y. to over 10 m/k.y.

Development of the fault array defining the sub-basin has accommodated extension and hanging wall breakup since at least 1246 ka. Extension was also accommodated along individual faults in the sub-basin.