1 Introduction
How the mantle lithosphere deforms during rifting remains one of the most debated issues in plate tectonics. However, two mechanisms that complement each other are preferred by the community. These are the magma assisted diking or weakening of the mantle (Buck, 2006; Piccardo et al., 2007) and depth-dependent thinning (DDT) (Huismans & Beaumont, 2011; Huismans & Beaumont, 2008, 2014; Kusznir et al., 2005; Lavier et al., 2019; Royden & Keen, 1980). DDT is the process by which the mantle lithosphere initially thins passively and weakens through the localization of deformation along lithosphere-scale shear zones and the upwelling of a buoyant mantle (Huismans & Beaumont, 2011). This latter mechanism is preferred for magma poor margins where little volcanism is expressed at the rift surface before the transition to oceanic crust. Observations of exhumed or denuded mantle lithosphere in the southern European Alps (Lanzo peridotite) show that melt migration through porous mantle lithosphere at depths between 50 and 15 km can also weaken the mantle by reducing viscosity (Kaczmarek & Müntener, 2008; Piccardo et al., 2007) and likely lead to DDT (Huismans & Beaumont, 2011; Lavier et al., 2019; Ros et al., 2017; Svartman Dias et al., 2015). The mantle lithosphere in this model thins and delaminates, reducing the lithosphere’s yield stress and allowing the continental rift to split the lithosphere into two plates (Davis & Kusznir, 2016; Svartman Dias et al., 2015, 2016). This mode of rifting is supported by both global surveys of crustal and lithospheric thinning factors (Kusznir & Karner, 2007) and numerical models of magma-poor rifted margins (e.g., Huismans and Beaumont, 2011, Ros et al., 2017; Lavier et al., 2019). In this article, we address interactions between mantle deformation and magmatic processes that control magma-poor rifted margin evolution, focusing on the rift-to-drift transition using field geological evidence, seismic imaging, and numerical modeling.
Despite the rich literature on fossil rifted margins, progress on this issue requires further observations to spatially link (both in depth and along the OCT) structures and magmatic emplacements prior to the establishment of a stable seafloor spreading center. One locality that can provide these observations is the Deep Ivory Coast Basin (DICB; also referred to as the Ivorian margin or Deep Ivorian Basin) rifted margin off West Africa. The DICB is a small magma-poor margin situated between two transform margins associated with the St. Paul and Romanche Transform Zones (Basile et al., 1993; Mascle & Blarez, 1987). The DICB has recently been the subject of a high-resolution, 3D seismic experiment, making it a convenient natural laboratory for studying the whole rift-to-drift transition. In this paper we interpret seismic sections from the Ivory Coast margin that clarifies the mechanisms controlling the transition from rift to drift. In addition, we test our hypotheses using numerical experiments simulating lithospheric extension with boundary and initial conditions consistent with magma-poor margins. Because modern rifts and preserved rifted margins only preserve snapshots or end-states of rifted margin evolution, numerical models are crucial for understanding how these systems evolve through time and in situ.
1.1 Mantle deformation processes