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Abstract

Seaward dipping reflector or SDR complexes comprise piles of individual basaltic flows and interbedded sediments that are thought to have formed subaerially at the flanks of tholeiitic shield volcanoes like those in the Afar or at larger magmatic complexes like Iceland. During the rift process, these flows subsequently acquire very steep true dips (up to 25°) almost always in the seaward direction. Past explanations for the acquisition of these dips involve progressive burial, loading and flexure by subsequent flows (e.g., Pálmason, 1980), or listric, landward-dipping faulting and magmatic dilation (Geoffroy, 2005). These factors no doubt play a role, but we feel that such models fall short of a full explanation by exceeding reasonable amounts of flexure and the amount by which huge blocks of continental crust can rotate by faulting alone. Such models also do not provide an explanation for how the topmost SDR layer subsides rapidly to the depth of normal oceanic crust as the latter begins to form, a problem that has been apparent since Mutter et al. (1982) and Hinz (1981).

Here, we present and discuss some of the main observations visible in long-offset, depth imaged seismic reflection records of SDR complexes at magma rich passive margins and propose a new but simple magmatic evacuation model for their production. Having conducted a global review of seismic data imaging SDR complexes, we present a line from the southern Brazilian margin to serve as a template example that shows most of the important criteria worldwide. After identifying the main observations, we propose a simple model of magmatic evacuation, similar to salt or mud evacuation in sedimentary sequences, to explain the observations. We hope that this simple proposal will spawn new avenues of research to refine and support the general model.

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