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

Abstract

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.

The Effect of Magma Poor and Magma Rich Rifted Margins on Continental Collision Dynamics

“…Accretion of margin material observed in our models is consistent with field observations (Manatschal, 2004; Manatschal & Müntener, 2009; Mohn et al., 2014) and modeling studies (Gómez‐Romeu et al., 2023) that show that magma‐poor margin material can be found in mountain ranges such as the Alps. In the Scandinavian Caledonides, the presence of both magma‐rich and magma‐poor margin material in different units along the orogeny has been suggested (e.g., Andersen et al., 2012; Jakob et al., 2019; Kjøll, Andersen, et al., 2019).…”