Upper lithospheric structure of the Dobi graben, Afar Depression from magnetics and gravity data

Abstract

We employed high-resolution ground magnetic, aeromagnetic and satellite gravity data to image the upper lithospheric structure beneath the Dobi graben and surroundings. This graben is located in the East Central Block (ECB) of the Afar Depression (AD) which is enclosed between the Red Sea and the Gulf of Aden rifts. Our aim is to understand the styles of crustal extension beneath the Dobi graben. We used two-dimensional (2D) radially-averaged power spectral analysis of both the aeromagnetic and the satellite gravity data to image deeper magnetic and density sources. We used the spectral analysis to estimate the depth to the boundary between the upper and lower crust, and the depth to Moho. Subsequently, we performed 2D forward modeling using the high-resolution ground magnetic and the satellite gravity data to produce a NE-SW trending model for the upper lithosphere beneath the Dobi graben and surroundings. We found the crust to be thinner beneath the Dobi graben reaching a thickness of only ∼23 km. We also found the boundary between the upper and lower crust to be at depth between 10 and 12 km. Additionally, we found two ∼5 km wide zones where melt and mafic dikes are possibly present within the lower crust. These zones are centered beneath a relay zone on the southwestern side of the Dobi graben and beneath a narrow (∼2 km wide) graben just to the northeast of the Dobi graben. Our model agrees with previous findings, especially Vp/Vs ratios of 1.85 or higher calculated from broadband passive seismic receiver function studies, which indicate the presence of melt in the lower crust beneath the graben. Because of this and the occurrence of earthquakes above ∼15 km deep seismogenic zone, we propose that, while the upper crust beneath the Dobi graben is mechanically stretching, the lower crust is ductily stretching aided by the presence of melt and the intrusion of mafic dikes.