Constraints on the 3D Sediment and Crustal Architecture of the Weakly Extended Malawi Rift from the Onshore/Offshore Wide-Angle Refraction Experiment

Current models of continental rifting increasingly point to the important role of weakening mechanisms like the addition of magmatic products in overcoming the disparity between the magnitude of tectonic forces available for rifting and the forces required to break strong, cold lithosphere. However, many rifts have limited volcanism. To understand the controls on rifting in magma-poor systems, we conduct 3D first arrival time tomography from active-source wide-angle refraction data collected in the Malawi Rift to constrain crustal structure along and across the rift. The Malawi Rift represents a weakly extended rift system located within the southernmost portion of the EARS. The only surface magmatism present occurs within the Rungwe Volcanic Province (RVP) located at the northern termination of the Malawi Rift. We utilize active-source data collected in Lake Malawi as a part of SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania). Over 86,000 unique air gun shots were recorded on an array of 33 offshore "lake" bottom seismometers and 55 onshore seismometers. The resulting ray-coverage encompasses the entire northern section of the Malawi Rift spanning the North and Central basins of Lake Malawi, portions of the surround plateaus, as well as the RVP. First arrivals are picked for all shot-receiver pairs with sufficient signal-to-noise ratio and included in a 3D first-arrival tomography model. Direct arrivals (Ps and Pg) and reflections (PmP) are observed on the majority of instruments, with clear arrivals observed to offsets >220 km. Data and preliminary models indicate variations in fault structure and overall sediment thickness between and within rift basins. The North Basin is characterized by a series of synthetic intrabasin faults and sediments thickening to the east along the Livingstone border fault. The Central Basin is characterized by sediments thickening to the west along the Nkhata border fault in the south near Usisya, Malawi and then transitioning to eastward thickening at the northern termination of the Central Basin near the Manda, Tanzania.