Crustal architecture and nature of continental breakup along a transform margin: a case study from East Africa transform margin (Tanzania-North Mozambique)

The Tanzania-North Mozambique continental margin is one of the most conspicuous transform margins in the world. The most distinguishing feature associated with this margin is offshore Davie Fracture Zone (DFZ). So far, the DFZ has been generally described as a very long linear oceanic fracture zone, commonly linked with rifting and breakup between Eastern and Western Gondwana. We carried out a combined study of gravity, magnetic and seismic data to determine the crustal architecture, geometry and the kinematic nature of continental breakup along a transform margin. The nature of DFZ and its role and kinematic linkage with overall continental breakup processes were also investigated. Our datasets include ION GXT’s deep reflection seismic data together with magnetic and gravimetric investigations and public domain dataset (Lamont-Doherty laboratory, USA). This study is primarily focused on the transform margin segment along Tanzania-North Mozambique corridor. The entire margin, from Somalia to the South Africa, is highly segmented into both rifted and transform segments. The Somalia-Majunga (Madagascar) and North Mozambique-Antarctica conjugate margins were developed as a result of normal fault controlled extension. These two extensional margins were connected through dextral Davie Strike-Slip System (DSS) forming a transform margin along the Tanzania-North Mozambique segment. The transform margin along the Tanzania is mostly transtensional. Classical synrift signatures, as observed in the extensional margins, are not as prominent in this transform margin segment. However, smaller scale rift signatures are observed in deep water, which roughly resembles hyper-extended distal margin architecture. We interpret a possible fossilized incipient spreading center within a very large pull-apart system with some remnant fossil asthenospheric diapir. These features also suggest that the extension was reached at quite advanced stages of rifting, which was mostly controlled by magmatic and thermal weakening. It appears that the incipient spreading were initiated with large pull-apart systems, but eventually failed as the hard-linkage were established between northern and southern extensional margins through DSS as Madagascar started to move from North to South. Along north Mozambique, the transform margin becomes almost a pure strike-slip system from its transtensional Tanzanian counterpart. Here, the total extension is highly localized within a narrow zone. Therefore, post breakup kinematic linkage between northern and southern extensional margin and dextral movement along the Davie Strike-Slip fault controls the complex nature of continental breakup along the transform margin. The oceanic-continental boundary suggests a narrow zone of continental crust in the transform margin of Tanzania-North Mozambique corridor in comparison to the wide zone of extended continental crust seen in the Somalia region and Southern Mozambique region in the north and south respectively. Our forward gravity modelling carried out along select ION lines and magnetic anomaly patterns have led to underpinning the newly interpreted ocean-continent boundary.