The theory of plate tectonics explains the process by which continental landmasses split into two, ultimately giving birth to new oceans. One of the most well-known examples of continental rifts is the East African Rift system, which divides the African plate into the Nubian and Somalian blocks. This continental rift began with the stretching and thinning of the upper part of the continental lithosphere, leading to the development of isolated extensional faults and sub-aerial basins. However, there are instances where continental rifting does not progress to the point of a complete break-up, whereby splitting to sea-floor spreading fails to occur. These "failed rifts" offer valuable insights into the rupture of the lithosphere and the formation of intra-cratonic sedimentary basins.
The Proterozoic basement in northern Somalia underwent significant deformation during the breakup of the Gondwana supercontinent, leading to the development of several intra-cratonic basins during the Mesozoic. The separation of Madagascar from Africa during the Late Jurassic created NW-SE trending rift basins in northern Somalia. A Late Cretaceous rifting event resulted in the formation of WNW-ESE oriented grabens filled with 6-7 km of sediments. Subsequently, the rifting between Africa and Arabia commenced during the Oligocene, leading to the opening of the Gulf of Aden along the northern margin of the Somali peninsula. These multiple deformation events in the past are responsible for the crustal and mantle deformation over northern Somalia.
To unveil the paleo-deformation history of these continental rifts and ongoing dynamic processes, we deployed broadband seismometers in Somaliland and collected gravity data. We analyzed teleseismic earthquakes recorded from September 2020 to August 2022 to estimate crustal thickness and seismic anisotropy within the crust and upper mantle. We conducted 2D gravity modeling to constrain the crustal structure in Somaliland. Our findings revealed that the northern and central Somaliland region has a thinned crust (approximately 22 km) with sediment deposits of 5-6 km in the rift basins, while the crustal thickness is 36.8-38.2 km in southern Somaliland. Analysis of P-wave anisotropy and shear wave splitting indicates a fast axis polarization of the crust oriented at -15° in Central Somaliland and 75-80° for stations near the Gulf of Aden. The thinned crust in central Somaliland has preserved fossil deformation related to the Late Jurassic rifting, which trended NW-SE, while seismic stations near the Gulf of Aden reveal crustal deformation associated with the Oligocene rift event. The observed crustal anisotropy (-85°) in southern Somaliland suggests a far-field response to the WNW-ESE-oriented Late Cretaceous rift stage. The azimuthal anisotropy in the upper mantle (50-56°) aligns with the regional Africa-Arabia plate motion and may result from the lattice-preferred orientation of olivine due to asthenospheric flow.
In summary, this study provides, for the first time, insights into the rift-related extensional strain fabric in the crust and upper mantle deformation inherited from asthenospheric flow in Somaliland. Our findings could have significant implications for understanding the initiation of continental rifting and the formation of intra-cratonic sedimentary basins.