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

Abstract

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.

Hybrid local and teleseismic P-wave tomography in North Tanzania: role of inherited structures and magmatism on continental rifting

“…Recent geophysical (Calais et al, 2008;O'Donnell et al, 2013;Albaric et al, 2014;Selway et al, 2014;Roecker et al, 2017;Weinstein et al, 2017;Tiberi et al, 2019;Plasman et al, 2017Plasman et al, & 2019 and geochemical studies (Hui et al, 2015;Lee et al, 2016;Muirhead et al, 2016;Baudouin et al, 2016) have demonstrated the role of fluids, thermal and compositional modifications at different scales in this region. However, due to the different target structures imaged by each method and their inherent resolution, the relative role of hydration, inherited rheology, suture zones, thermal state and their interplay in the resulting surface deformation are difficult to assess.…”

“…Variations of Moho geometry and lower crust structures in this area have been imaged in previous geophysical studies (e.g. Tesha et al, 1997;Plasman et al, 2017Plasman et al, & 2019Tiberi et al, 2019) and will be discussed in the following sections. (Fig.…”

“…The NTD system is thus a complex geodynamic zone, involving magmatism and tectonic processes which interact at different depths. In the crust, the evolution of the rift and its morphology is strongly linked to inherited structures, particularly the Proterozoic belts and the craton boundaries (Albaric et al, 2014;Weinstein et al, 2017;Roecker et al, 2017;Plasman et al, 2017Plasman et al, & 2019Tiberi et al, 2019). In the mantle, Archean and Proterozoic units interact with the mantle upwelling implying thermal and compositional modifications in the mantle lithosphere (Ritsema et al, 1998;Huerta et al, 2009;Koptev et al, 2018;Tiberi et al, 2019, Muirhead et al, 2020.…”

“…2017) who related this velocity reduction within the rift valley to a thick sequence of sediments in the upper 5 km (Birt et al, 1997;Ebinger et al, 1997) and active volcanism in the mid-crust laterally delimited by deeply rooted rift border faults in the western side (Le Gall et al, 2008;Weinstein et al, 2017). Low Vs anomalies, probably related to deep sedimentary or volcanic sequences (Mana et al, 2015;Plasman et al, 2019), are also detected in the mid-crust below the EW transverse volcanic belt, except below the Ngorongoro volcanic complex where a high-velocity pattern is detected at 10 km depth (Figs. 3…”

“…In particular, they showed that the seismicity localized at 30 km depth delimits the top of both a high conductive and high Vp/Vs ratio zone. Several processes have been proposed to explain this lower crustal swarm such as magma upwelling (Plasman et al, 2019), a shear zone at the boundary between two major tectonic units and deep fluid pressure arising from melts (Albaric et al, 2014). Finally, little to no seismicity is recorded North-East and South-East of the studied region.…”