Spatial and temporal controls on the development and evolution of the Tanzanian continental margin
Abstract
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.