Spatial and temporal controls on the development and evolution of the Tanzanian continental margin
Sii How Theng, Pollux
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The East African seaboard has historically been considered to be a passive continental margin formed following rifting and continental break-up in the Middle Jurassic. Whilst much of the margin conforms to the standard passive margin model of pre-, syn- and post-rift sequences and a rift-drift subsidence history, the occurrence of anticlines forming the core to the islands of Zanzibar and Pemba attest to a more complicated tectonic evolution. Regional interpretation of a grid of high fidelity 2D and 3D seismic data, including a subset of long-offset, deep lines provides new insights into the margin’s crustal structure and shows geometries not normally attributed to passive margin development. These comprise a gently folded seabed, bedrock subcrop, a series of angular unconformities in the shallow section and an underlying zone of intense deformation associated with contractional reactivation of a precursor normal fault. This is consistent with the margin having undergone a hitherto unrecognized phase of structural inversion in the Neogene. Likewise, inversion and transpression structures are recognised offshore along NNW-SSE striking lineaments such as the Davie-Walu Trough, documenting additional contractional phases during the Cretaceous. Inboard of the zone of structural inversion, the Pemba Channel represents a protected remnant of extension and is still influenced by an E-W extensional regime, something that is substantiated by surface GPS data and earthquake focal mechanisms. The short-lived compressional events are envisaged to be related to external horizontal forces and far-field stresses associated with regional tectonism, particularly within the East African Rift System. However the crustal structure and basement fabric also play a role in the localisation of these stresses. Crustal identification along the margin supports lineaments set up during the initial NNW-SSE extension and N-S dextral southwards motion of Madagascar which may have reactivated under appropriately directed stress.