Influence of passive margin orientation during continental subduction on slab breakoff dynamics
Résumé
Slab pull is the main driving force for plate motion, and is caused by the negative buoyancy of the subducting oceanic lithosphere. The progressive consumption of oceanic lithosphere at trench can lead to the closure of oceans and to the arrival at trench of continental lithosphere (e.g., Tethys). The positive buoyancy force that results from continental lithosphere subduction acts as an opposite force to oceanic lithosphere slab pull, leading to strain localization within the slab, necking and ultimately to slab breakoff. Detachment of subducted lithosphere in convergence settings is expected to have a clear signature at the surface, which includes changes in topography, exhumation of HP rocks, etc... In this study, we present the first series of 3D analogue models of subduction with variable geometries for the continental-oceanic transition zone (COT). For this, we systematically change the angle between the direction of the continental to oceanic lithosphere transition of the subducting plate and the direction of the trench. We analyze the impact of the COT geometry on the timing and depth of the subsequent slab breakoff as well as its role on the rate of slab break-off lateral propagation. We show that slab breakoff occurs in a limited range of depths (160-200 km) for the model with a COT parallel to the trench while in models with non-parallel COT slab breakoff depth largely varies along the subduction zone (180-345 km depth in model H-250 and 165-320 km depth in model H-110). The timing of slab-breakoff is also dependent on the initial geometry. The full detachment along the COT takes around 14 Myr after initiation of continental subduction in models with a COT parallel to the trench, while it largely increases for the non-parallel models, reaching values up to around 30 Myr. Lateral propagation of slab detachment is also influenced by the initial configuration of the subducting plate. In the model with the COT parallel to the trench, stretching and necking develop almost simultaneously along the entire width of the subduction zone. In non-parallel models, the slab does not deform at the same time.