Formation of SDRs-Ocean transition at magma-rich rifted margins: Significance of a mantle seismic reflector at the western Demerara margin

Abstract

At distal magma-rich rifted margins, seismic reflections several seconds below the ∼10 s TWT expected base of petrological crust for thermally equilibrated lithosphere, are commonly observed. These deep seismic reflectors are often interpreted as the petrological Moho as they distally rise and merge into the imaged oceanic Moho, however, the origin of these deep seismic reflections beneath the expected base of the petrological crust is not understood.We investigate the example of the Jurassic Demerara Plateau located offshore north-east South America, which shows a strong seismic reflection at ∼15 s TWT, beneath 20 km thick SDRs (Seaward Dipping Reflectors). We use combined inversion of gravity and seismic reflection data, together with seismic interpretation, to investigate the depth of the deep reflector, its origin, formation as well as the composition and structure of the overlying material. Results from quantitative techniques show that the deep reflector is located below the SDRs at least 10 km within the mantle at a depth of ∼42 km. These results, together with seismic interpretation, suggest that the deep reflector corresponds to an intra-mantle detachment limiting semi-ductile and sheared magmatic-infiltrated mantle above from depleted ductile asthenospheric mantle below. Distally, the semi-ductile mantle is removed when the asthenospheric mantle focuses to form a stable mid-oceanic ridge. This lateral evolution results from a progressive coupling between the magmatic SDRs above and the asthenospheric mantle below. Therefore, we propose that the intra-mantle detachment, corresponding to a rheological contrast, plays a key role in the transition from distributed lithosphere extension to focused mid-ocean ridge accretion at distal magma-rich rifted margins. In addition, it may represent an indicator to determine the thermal structure of a magma-rich rifted margin.