| Summary: | Accurate mapping of first-order tectonic features such as oceanic fracture zones and continental margins is vital for the production of reliable plate reconstructions. These reconstructions allow for a better understanding of the palaeo-configuration of continental fragments within Gondwana and ultimately provide insight into how and why supercontinents break apart. Detection of spreading lineaments within the heavily sedimented Western Somali Basin (WSB) has been achieved using a novel technique based on directional derivatives of free-air gravity. This new lineament dataset allows for the construction of a high-resolution plate tectonic reconstruction of the WSB, which is in good agreement with ocean magnetic data and the position of the abandoned WSB spreading centre. The model also reveals a change in spreading direction, from NNW-SSE to N-S, during the Late Jurassic. This controversial spreading direction change places the origin of Madagascar within the Tanzania Coastal Basin (TCB), inboard of the Davie Fracture Zone (DFZ), which was previously believed to be the continent-ocean transform margin of the WSB. This tight-fit of Gondwana fragments prior to continental breakup necessitates a reassessment of both the crustal nature of the TCB, which is shown to be partly oceanic in nature, and of the nature of the margins surrounding the WSB. The northern margins of the WSB are likely orthogonally rifted margins. However, the western margins are likely highly segmented and/or obliquely rifted margins. The model also predicts a large transform offset along the Rovuma Basin. Systematic gravity modelling and combined seismic investigations along the Rovuma basin reveals the ‘Rovuma Transform Margin’, which offsets the obliquely rifted margins of northeast Mozambique and Tanzania. The discovery of this transform margin confirms the initial SSE plate motion predicted from gravity lineament analysis and plate reconstructions, and shows that the breakup of the Gondwana supercontinent occurred not just along pre-existing lithospheric weaknesses associated with the Karoo rift system, but also along newly developed highly oblique deformation zones as well. The final breakup of the Gondwana supercontinent, which followed extensive and episodic Karoo aged rifting, was coincident with extensive magmatism in Mozambique and may therefore have been triggered by the interaction of several facilitators of continental breakup (i.e. oblique rifting, pre-existing weaknesses, and magmatism). The oblique breakup of Gondwana along the TCB led to the development of a segmented mid-ocean ridge system within this basin, offset by SSE trending fracture zones. These fracture zones were incompatible with the N-S spreading that followed the Late Jurassic change in plate motion, resulting in the abandonment of mid-ocean ridge segments and compression within this basin. This compression led to the formation of the 250 km long Tanzania Coastal Basin thrust belt, the largest intraplate oceanic thrust belt yet discovered. The cessation of compression within the TCB followed the development of the DFZ, which propagated from south to north. This structure was subsequently dominated by transpression throughout its history, suggesting it was not perfectly compatible with plate driving forces. Formation of the DFZ along aligned weak rifted margins and young oceanic crust may have resulted in the mismatch of plate motions and driving forces, and also suggests a first order ‘top-down’ control on plate motions during the breakup of Gondwana.
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