Importance of rifted margin inheritance during continental collision revealed by numerical modelling

A significant part of accommodated localized deformation in continent-continent collision zones occurs along mechanically weak fault zones inherited from earlier tectonic events, in particular through polyphase rifting of continental margins. Besides the pre-existence of weak zones, the inherited thermal, rheological and geometric characteristics of continental plate margins may affect collision dynamics and promote or impede the subduction or accretion of continental lithospheric slivers. Therefore, the implication of previous rifting dynamics is required when investigating the structural and mechanical evolution of continental collision systems.

In this work, we test the impact of rift-inherited rifted margin architecture on continental collision by using geodynamic numerical modelling. We apply the two-dimensional finite difference numerical code Norma with a locally refined fully staggered Eulerian grid measuring 1000 x 150 km and a Lagrangian marker field tracking deformation. The numerical experiments undergo initial extension of continental lithosphere, followed by a phase of tectonic quiescence and subsequent convergence, ultimately culminating in continental collision. Depending on the amount of extension and whether oceanic lithosphere developed or not, the initial phase of convergence is characterized by oceanic subduction. Our parametric study includes the variation of the thermal conditions of the continental lithosphere, the amount of extension, and the duration of tectonic quiescence, all affecting the rheological and morphological characteristics of the tectonically accreted rifted continental margins.

Modelling results demonstrate that a warmer initial geotherm produce highly-extended wide (>100 km) continental margins with several individual continental crustal slivers in contrast to narrow rifted margins in case of a cold and strong lithosphere. Upon tectonic inversion, a short previous phase of thermal relaxation of the rifting-related mantle upwelling leads to subduction initiation at the former spreading ridge, while >20 Myr of tectonic quiescence results in subduction along one of the continental margins. Ultimately, the inherited crustal and rheological architecture of the extended lithosphere and its thermal state influence the dynamics during orogeny, resulting in either single- or double-verging orogenic wedges. Our study provides further insight into the specific conditions of pre-collisional rifted margins of natural orogens.