Oil Prospects in the Mozambique Channel: Incipient Subduction meets Passive Margin

Spectrum’s Anongporn Intawong, Phillip Hargreaves, Karyna Rodriguez, Neil Hodgson, and Paolo Esestime originally produced this abstract for PETEX 2018. 

Davie Ridge Overview

The Davie Ridge has been observed as a prominent morphological feature in the Mozambique Channel. It shows a relative gravity low bounded to the east by a gravity high striking across the continental margin of Madagascar and Mozambique. The ridge was discovered by Heirtzler and Burroughs (1971) noticing a bathymetric elevation in the Mozambique Channel; they proposed that the Davie Ridge represents the expression of a transform fault resulting from the relative southward motion of Madagascar with respect to Africa. The entire feature was subsequently termed the ‘Davie Fracture Zone’ by Scrutton (1978), even though there is no evidence of sea floor spreading in the East African Ocean.

It has long been widely understood that Madagascar was first disconnected from the Kenya-Somalia part of the African continent during the Middle to Late Jurassic, by dextral transcurrent movement along the Davie Ridge after the Gondwana breakup in the Late Liassic. However, most of plate tectonic reconstructions have assumed an inactive transform fault of the Davie Ridge.

A continuation of the Davie Ridge as a transform fault has been interpreted to extend northwards to the Karimbas Graben in northern Mozambique and Walu Ridge in Kenya by Rabinowitz (1971) based on an observed gravity high running along the graben and ridge until its intersection with the continental margin of Kenya. Recent contrary studies by Klimke and Franke (2016), and Klimke et al. (2018) show no geological evidence of the existence of a major transform fault in the Karimbas Graben and Walu Ridge based on field and reflection seismic data. The Karimbas Graben and Walu Ridge were subsequently interpreted to be tectonically unrelated to the Davie Ridge and to the southward motion of Madagascar. Consequently, the Davie Ridge has been proposed to be a conjugate transform margin to the Gunnerus Ridge offshore Antarctica, with a much more southerly pre-break-up position of Madagascar within Gondwana than most published plate reconstruction.

Incipient Subduction Davie Ridge

Recently acquired and public domain 2D reflection seismic data covering offshore central Mozambique Channel, south of the Rovuma Basin and northeast of the Zambezi Delta (Angoche Basin), are used to understand the crustal nature, to identify basin architectures, and to assess potential hydrocarbon prospects within this area.

The northeast Zambezi Delta region demonstrates a volcanic rifted margin style with deformation of Seaward Dipping Reflectors (SDRs) identified between the inboard uplifted continental crust and outboard oceanic crust representing a passive margin. The margin shows a narrow area of SDRs comparing to other volcanic rifted margins, for instance the South Atlantic margin of Namibia and South Africa. The crustal architecture consequently appears to be faulted semi-stratified volcanic layers which are interpreted to be oceanic crust in the outboard.

The Davie Ridge is clearly related to neotectonics. The ridge is composed of more than one ridge making ‘the Davie Zone’ a better term, based on its display on gravity and reflection seismic data. This zone comprises sedimentary ridges and trenches, and it is made up of thick sedimentary layers, younger well stratified and older compressed sediments (Fig 1). The compressed sediments have been observed by Klimke et al. (2018) and this study all the way along the gravity minimum of the Davie Zone (Fig 1 and 2), and we have interpreted this to be a crustal thickening accretionary wedge front of a forearc basin which stratigraphically continues in to the Morondava Basin of Madagascar. We believe that these features were developed during an early stage of subduction where the passive margin of the African plate moves underneath the compressed continental crust of the Madagascar-Seychelles-Mascarene microplate.

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Oil Prospects in the Mozambique Channel: Where Incipient Subduction meets Passive Margin