Magmatism and deformation during continental breakup

Abstract

Since the 1960s, plate tectonic theory has provided a fundamental framework that explains how the configuration of the continents and ocean basins changes through time. Within this scheme, heat is lost from the Earth as continents split apart and new oceans form, a process clearly visible from the conjugate margins of continents ruptured in the past that now flank many of the globe’s mid-ocean ridges. Here I discuss observations and models of rifting processes in the Afar depression of Ethiopia, where the tectonic and volcanic processes responsible for splitting continents is still ongoing. Understanding the interaction between magma intrusion and mechanical extension is fundamental in trying to understand how continents break apart. The break-up of continents and subsequent formation of ocean basins is a fundamental component of plate tectonics that has shaped the geological record and distribution of natural resources (e.g. oil and gas) along continental margins such as the edges of the Atlantic Ocean. Continental rifts are initially characterized by relatively broad zones of mechanical extension in which faulting, ductile stretching and heating of the tectonic plate accommodates strain and defines the primary architecture of the rift zone during and after rifting (McKenzie 1978). Ultimately, however, the locus of strain must shift towards a narrowing zone that becomes the new seafloor spreading centre. It is here that magma formed from decompression melting of the mantle intrudes and creates new ocean floor (Delaney et al. 1998). Despite the importance of continental break-up in plate tectonic theory, it remains unclear how and when the transition from mechanical to magmatic extension of the plate occurs (Ebinger 2005). It also remains ambiguous how important the thermal, chemical and physical structure of the mantle are in aiding the melting of the passively upwelling mantle, and in so doing supplying magma that is intruded into the tectonic plate and/or erupted to the surface (Shillington et al. 2009).