Buoyancy versus shear forces in building orogenic wedges

Abstract. Orogenic belts formed by collision are impressive manifestations of plate tectonics. Observations from orogenic belts, like the Western Alps, indicate an important involvement of the mantle lithosphere, significant burial and exhumation of continental and oceanic crustal rocks and the importance of the plate interface strength that can be modified, for example, by the presence of serpentinites. A popular model for the formation of such belts is the so-called orogenic wedge model. However, most wedge models consider crustal deformation only and do, hence, not consider subduction, the impact of related buoyancy forces arising from density differences between subducted crust and surrounding mantle and the effects of different plate interface strength. Here, we quantify the relative importance of buoyancy and shear forces in building collisional orogenic wedges. We leverage two-dimensional (2D) petrological-thermo-mechanical numerical simulations of a long-term (ca. 170 Myr) lithosphere deformation cycle involving subsequent hyperextension, cooling, convergence, subduction and collision. We compare simulations employing density fields calculated with linearized equations of state with simulations employing density fields calculated by phase equilibria models including metamorphic reactions. Further, we consider serpentinisation of the mantle material, exhumed in the hyperextended basin. Our models show that differences in density structure and in shear strength of serpentinites or upper crust have a strong impact on the evolution of orogenic wedges. Higher serpentinite strength causes a dominance of shear over buoyancy forces, resulting in either thrust-sheet dominated orogenic wedges, involving some diapiric exhumation at their base, or relamination of crustal material below the overriding plate. Lower serpentinite strength (equal importance of shear and buoyancy forces) generates orogenic wedges that are dominated by diapiric or channel-flow exhumation. Deep subduction (> 80 km) and subsequent surface exhumation of continental crust along the subduction interface occurs in these models. Employing phase equilibria density models decreases the average buoyancy contrasts, allows for deeper subduction of continental crust and reduces the average topography of the wedge by several kilometers. A decrease of upper crustal shear strength causes smaller maximal crustal burial depths. Progressive subduction of continental crust increases upward-directed buoyancy forces of the growing wedge and in turn increases horizontal driving forces. These driving forces eventually reach magnitudes (≈ 18 TN m−1) which were required to initiate subduction during convergence. We suggest that the evolving relation between shear and buoyancy forces and the increase of horizontal driving force related to the growing Alpine orogenic wedge has significantly slowed down (or choked ) subduction of the European plate below the Adriatic one between 35 and 25 Ma. This buoyancy-related choking could have caused the reorganization of plate motion and the initiation of subduction of the Adriatic plate. We discuss potential applications and implications of our model results to the Pyrenean and Alpine orogenies.