NORMAL AND STRIKE-SLIP FAULTS INTERPLAY IN THE TENDAHO-GOBAAT DISCONTINUITY, AFAR DEPRESSION, ETHIOPIA

We propose through the integration of remote sensing and field studies a new model to account for strike-slip faulting and their relationship with normal faulting in the evolution of overlapping rifts found in the East-Central part of the Afar Depression of Ethiopia. The Afar triple junction is characterized by three structural trends that intersect to form a 200,000 km2 region of diffuse but complex deformation: (a) A NNW-trend parallel to the Red Sea in the north; (b) an E-W trend parallel to the Gulf of Aden in the east; and (c) a NE-trend parallel to the Ethiopian Main Rift in the south. The Gulf of Aden and Red Sea rifts are currently active and propagating into the Afar Depression through a progression of disconnected rifts. The Manda Inakir and Manda Hararo are the most recent of these disconnected rifts, which are propagating in a NW and SE directions parallel to each other creating an overlap zone of more than 120 km in length and 100 km in width, known as the East-Central Block (ECB). Fault plane solutions of seismic activity over the last 40 years within the ECB have shown evidence of strike-slip movement along dominantly NW-trending faults. Models to explain strike-slip faulting in the ECB have advocated for dextral shearing perpendicular to the rifts producing clockwise rotation, sinistral displacement along NW-trending faults and localized NW-trending zones of ongoing extension. However, we found evidence of sinistral displacement along NW-trending faults as well as dextral displacement along NE- to NNE-trending faults and NNE-trending fractures. These are at high angle to the NW-trending normal and sinistral strike-slip faults. Therefore, we propose a tectonic model for the ECB to account for this additional evidence taking into consideration rheological difference due to lithological variation (basalts and rhyolites) as well as block thickness originated from the spatial distribution of localized extensional zones. In our model we explained the presence of dextral strike-slip displacement along NE- to NNE-trending faults as adjustments of faulted blocks due to clockwise rotation of the ECB. This is further supported by the presence of map-scale kink structures as well as rift-in-rift rhomb-shaped grabens enclosed between NE- to NNE-trending faults which occur within NW-trending grabens.