Crustal structure at a young continental rift: a receiver function study from the Tanganyika Rift

The southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use eleven broadband seismometers to implement receiver function analysis using H-κ stacking, a method sensitive to crustal thickness and VP/VS ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance and phase-weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by VP/VS ratios of 1.75-1.77, implying a felsic bulk composition. Crust beneath the fault bounded basins has high VP/VS (>1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high VP/VS. However, elevated VP/VS values within fault-bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively un-extended Tanzanian craton and Bangweulu Block are 41.6-42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault-bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early-stage continental rifting in the Western rift, Africa

Lithospheric Structure of a Transitional Magmatic to Amagmatic Continental Rift System-Insights from Magnetotelluric and Local Tomography Studies in the North Tanzanian Divergence, East African Rift

Continental break-up is controlled by several parameters and processes (rheology, inherited structures, magmatism, etc). Their impact, chronology and interactions are still poorly known and debated, particularly when rifting interacts with cratons. In order to better understand the rifting initiation in a cratonic lithosphere, we analysed 22 magnetotelluric (MT) soundings collected along two East-West profiles in two different rift segments of the North Tanzanian Divergence. The North Tanzanian Divergence, where the East African Rift is at its earliest stage, is a remarkable example of the transition between magmatic to amagmatic rifting with two clearly identified segments. Only separated by a hundred kilometers, these segments, Natron (North) and Manyara (South), display contrasted morphological (wide versus narrow), volcanic (many versus a few edifices) and seismic (shallow versus deep activity) signatures. Magnetotelluric profiles across the two segments were inverted with a three-dimensional approach and supplied the resistive structure of the upper lithosphere (down to about 70 km). The Natron segment has a rather conductive lithosphere containing several resistive features (Proterozoic Belt), whereas the Manyara segment displays highly resistive blocks probably of cratonic nature encompassing a conductive structure under the axial valley. The joint interpretation of these models with recent local and regional seismological studies highlights totally different structures and processes involved in the two segments of the North Tanzanian Divergence. We identified contrasted CO2 content, magma upwelling or trapping, in depth regarding the Manyara or the Natron branch and the influence of inherited cratonic structures in the rifting dynamics