Inverted Magma-rich Versus Magma-poor Rifted Margins: Implications for Early Orogenic Systems

Mountain belts are often the result of former inverted rifts or rifted margins. Up to date, relics of magma-poor rifted margins have been found in orogens allowing the investigation of how these margins reactivate and control the formation of mountain belts. In contrast, magma-rich rifted margins have barely been recognized within orogenic systems, and consequently how these margins reactivate and control subsequent orogenesis is poorly understood. We use a thermo-mechanical model to investigate the mechanical behaviour of reactivated magma-rich versus magma-poor rifted margins during early orogenesis (i.e. margin inversion). Input data for our modelling experiments are obtained from two natural laboratories. One is the Demerara Plateau characterized by a mildly shortened magma-rich rifted margin. Its décollement level is observed at the top of the syn-kinematic volcanics, which propagates downwards into a frozen incipient subduction. The other study-case is the Basque-Cantabrian Belt consisting of a reactivated magma-poor hyper-extended rift system with the décollement level at the bottom of the syn-rift sediments. Our modelling results, for inverted magma-rich rifted margin, show that the syn-kinematic volcanics undergo subduction as they are mechanically coupled with the underlying rifted lithosphere. Hence, only post-rift sediments are accreted within the early orogenic accretionary wedge. In contrast, both the syn- and post-rift sediments from a reactivated magma-poor rifted margin are expected to be preserved within the accretionary wedge. Therefore, we conclude that the presence or absence of syn-rift sediments within an accretionary prism may represent a robust indicator to determine the nature of reactivated rifted margins within orogens. We believe that our results may strongly contribute to recognize, the so far poorly identified, magma-rich rifted margins within present-day orogenic systems.