Thermotectonic History of the Southern Malawi Rift and Northern Shire Graben Border Faults: Insights From Apatite Fission Tracks and Remote Sensing Analyses

Abstract:

One of the fundamental problems in continental rift propagation is how strain is accommodated and transferred between rift systems. The Malawi Rift is an early stage magma-poor rift that trends N-S and seems to terminate to the south at the NW-SE trending Shire Graben. The Shire Graben is considered a Paleozoic-Mesozoic rift because the sedimentary rocks in the basin are of those eras. However, the steep scarps of its border faults and the recent earthquakes along strike (e.g. 2007 M4.8 earthquake) suggest that the rift has been reactivated and that strain from the Malawi Rift is being transferred along the Shire Graben. Although there are known ages for the onset of rifting along the Livingstone Fault in the north, and of cooling of the Chilwa Alkaline Province in the south, there remains a knowledge gap in geochronological data to constrain ages of strain accommodation and transfer in southern Malawi and Shire Graben. This study aims to answer the following question: What are the cooling ages associated with the Cenozoic rifting in the southern Malawi Rift and Shire Graben?

We used 30-m SRTM-DEM and previous data to map faults in the southern Malawi and northern Shire Graben. Apatite fission track dating is being conducted to record upper crustal (60-130 °C) cooling and exhumation ages related to tectonic events associated with footwall uplift using 12 rock samples (gneisses, syenite, foliated granite, and felsic dikes) collected from the footwall blocks of several major extensional border faults.

The structural characterization of faults and other lineaments in the southern Malawi Rift show changes in fault parameters (length, strike, relief, segmentation and linkage) that may be associated to the spatial stress influence of the faults in the Shire Graben. Thermochronological results will provide new data and basis to compare ages of uplift between the southern Malawi Rift and Shire Graben. Moreover, it will allow us to compare the ages of uplift between the southern and northern Malawi Rift to test existing models based on structural architecture and sediment fill that suggest break-up started earlier in the northern part of the Malawi Rift than in the south.