2D thermomechanical numerical models of passive margin reactivation

Subduction is a key driving mechanism in Plate Tectonics, but how it initiates is still poorly understood.

Subduction initiation is thought to be a complex and evolving tectonic process. It consists of stages of lithospheric contractional deformation that may reactivate inherited structures, potentially localizing deformation in a proto-plate boundary and leading to subduction.

During the Cenozoic, the most common site to initiate subduction was at passive margins (Lallemand and Arcay, 2021). Hence, the importance of understanding the main controlling mechanisms that lead to subduction initiation at these locations. These processes are thought to be dependent on various factors, such as the presence of a weak zone (e.g., a serpentinized exhumed mantle layer, serpentine-filled normal and detachment faults), a pre-existing stress/strain field, and the structure/architecture of the rifted margin.

Using high-resolution 2D geodynamic numerical models carried out with the code LaMEM, this work investigates the mechanisms that may control the reactivation on magma-poor rifted margins. In particular, by testing different parameters (e.g., length of the passive margin, presence of a serpentinized layer), different deformation regimes (e.g., strain-rates) and the thermomechanical state of the system that may lead to subduction initiation in these locations.

Furthermore, seismic reflection lines were interpreted in order to better understand and characterize the magma-poor rifted margin archetype (West Iberian rifted margin) and its tectonic structure distribution. The data interpreted helped constrain fault distribution and their geometry in numerical models conducted.

Preliminary results show that serpentine-filled tectonic structures (e.g., inherited normal and detachment faults from the rifting process) facilitate the reactivation of the rifted margin by localizing compressive-induced deformation. Additionally, models show that the presence of serpentinized exhumed mantle in a hyperextended domain, constrain the localization of deformation in this section, leading to an earlier subduction initiation. Therefore, we can infer that the presence of serpentinized exhumed mantle and/or the existence of rift inherited tectonic structures, deeply weakens the passive (or rifted margin). Notwithstanding, results also show that there is a first order dependence on the thermal age of the continental lithosphere (e.g., strength and thickness) for the locus of strain localization. Followed by a second order dependence on passive margin length as a constraining mechanism for the locus of subduction.