MAGNETIC AND GRAVITY IMAGING OF THE LITHOSPHERIC STRUCTURE BENEATH THE DOBI GRABEN, AFAR DEPRESSION

We employed high-resolution ground magnetic, aeromagnetic and satellite gravity data to image the lithospheric structure beneath the NW-trending Dobi graben and surroundings. This graben is located in the center of the East Central Block (ECB) in the Afar Depression (AD) enclosed between the Red Sea propagator and the Gulf of Aden propagator (GAP). The upper crust beneath the Dobi graben is currently actively rifting through mechanical stretching as evidenced from the 1989 swarm of intermediate magnitude (5.7 < Ms < 6.3) earthquakes. Our aim is to understand the styles of crustal extension accommodation at deeper depth beneath the Dobi graben. We first enhanced the aeromagnetic data to image the base of the surface formations (graben sediment fill and Afar Stratoid Series basalts). Then, we used two-dimensional (2D) radially-averaged power spectral analysis of the aeromagnetic and gravity data to image deeper magnetic and density sources, especially the boundary between the upper and lower crust and the Moho. Subsequently, we performed 2D forward modeling using the high-resolution ground magnetic and the satellite gravity data to produce 2D NE-SW trending lithospheric cross-section of the Dobi graben and surroundings. We found the crust to be thinner beneath the graben reaching a thickness of ~31 km. We also found the boundary between the upper and lower crust to be at 12 to 16 km depth. Additionally, we found two zones of partial melting within the lower crust with a width ~3.2 km beneath the southwestern side of the Dobi graben and ~4.5 km beneath a narrow (~2 km wide) graben just to the northeast of the Dobi graben. Our model is in accord with previous findings, especially the Vp/Vs ratios of 1.85 or higher calculated from broadband passive seismic receiver function studies indicative of the presence of partial melting in the lower crust. We propose that, while the upper crust beneath the Dobi graben is mechanically stretching, the lower crust is ductily stretching aided by partial melting.