Surface and crustal deformation mechanism of the Dobi graben and surrounding area in the Afar Depression, Ethiopia

Abstract

Our Study used the Advanced Synthetic Aperture Radar (ASAR), C- Band (h = 5.6 cm) of the ENVISAT satellite ASAR data, Landsat Operational Land Imager (OLI), and Shuttle Radar Topography Mission (SRTM) digital elevation models, as well as the integration of geophysical ground base magnetic survey data, aeromagnetic, and Satellite gravity data, to investigate the time series surface deformation and crustal structure of the Dobi graben and surrounding area. Results from our fault population analysis using SRTM DEM aided by satellite imageries suggest that the direction of the faults' lateral propagations are highly influenced by the two regional volcanic rifts, the Red Sea Rift (RSR) and Gulf of Aden Rift (GAR). Additionally, the Dmax/Lmax ratio of the faults is calculated as ~0.03, which indicates the normal faults in the region can be characterized by the constant displacement fault growth model. Our fault population analysis also indicates that the possible presence of melt material in the lower crust likely acts as a barrier for lateral propagations of the faults. Results from our InSAR analysis suggest that an extension process with a creeping mechanism associated mainly with normal faulting presumably causes subsidence within the graben and uplifting in the rift shoulder. The abnormal, continuous uplifting in the horst area might be associated with the temporary reactivation of normal faulting in the region. Finally, our 2D magnetic and gravity forward modeling revealed 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 crusts to be at depth of between 10 and 12 km. Additionally, we found two ~5 km wide zones where melt and mafic dike intrusions 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. We propose that, while the upper crust beneath the Dobi graben is stretching mechanically, the lower crust is stretching ductily, aided by the presence of melt and the intrusion of mafic dikes.