A geotectonic history of the northern Mozambique Basin including the Beira High – A contribution for the understanding of its development

The interpretation of a comprehensive two dimensional (2D) seismic reflection data set discloses several rifting events for a typical passive rifted margin. Two major phases have been identified: Rift I phase is in agreement with the initial rifting of Gondwana postulated for the early Jurassic. This rift resulted in break-up stage 1 governed by a north-eastern motion of Antarctica along the reactivated Pebane shear zone.Indications for lava flows were identified in the seismic profiles along the Offshore Zambezi Depression. They form a link between early rifting, the initial break-up and early sea-floor spreading. The nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post-rift magmatism that occurred when the Antarctica Plate (with the Beira High) drifted dextrally from the west to east until the Mid-Jurassic times. It is more likely that the break-up in stage 1 resulted from high tension due to strain relief of Rift I phase. Above all, the extensional deformation occurred in a narrow-rift mode. Therefore, the V-shape of the Offshore Zambezi Depression suggests a possible rift-failure structure. This is best explained by the Reeves and de Wit model (2000). This model postulates that the motion of Antarctica changed to southward direction at around 170 Ma. Consequently a “rift jump” from the Offshore Zambezi Depression in the northwest to the south-eastern edge of the Beira High occurred, and Rift II phase may have started leading to the break-up in stage 2. During this stage (Rift II phase) the extension migrated towards the east, thus thinning the crust, and exhuming the sub-continental mantle in the continental–ocean transition zone. The Rift II phase shows a sequence of half-graben morphologies confining the syn-rift infill that is subdivided in three units: syn-rift I, syn rift-II and rift sag. All three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping, low-angle detachment faults.With respect to the presence of the two rift phases, it is deduced that break-up and sea-floor spreading are diachronous within Rift I and Rift II segments.

► Gondwana broke-up in two stages governed by two rift phases.
► Rift 1 with lava flows occur only in the deepest Offshore Zambezi Depression.
►  Rift 2 mainly the eastern Beira High is highly deformed by normal faults showing asymmetric half-graben morphology.
► The Break-up Unconformity (BU) is formed at the transition Mid to Late Jurassic.