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The dynamics of growing collisional orogens are mainly controlled by buoyancy and shear forces. However, the relative importance of these forces, their temporal evolution and their impact on the tectonic style of orogenic wedges remain elusive. Here, we quantify buoyancy and shear forces during collisional orogeny and investigate their impact on orogenic wedge formation and exhumation of crustal rocks. We leverage two-dimensional petrological–thermomechanical numerical simulations of a long-term (ca. 170 Myr) lithosphere deformation cycle involving subsequent hyperextension, cooling, convergence, subduction and collision. Hyperextension generates a basin with exhumed continental mantle bounded by asymmetric passive margins. Before convergence, we replace the top few kilometres of the exhumed mantle with serpentinite to investigate its role during subduction and collision. We study the impact of three parameters: (1) shear resistance, or strength, of serpentinites, controlling the strength of the evolving subduction interface; (2) strength of the continental upper crust; and (3) density structure of the subducted material. Densities are determined by linearized equations of state or by petrological-phase equilibria calculations. The three parameters control the evolution of the ratio of upward-directed buoyancy force to horizontal driving force, FB/FD=ArF , which controls the mode of orogenic wedge formation: ArF≈0.5 causes thrust-sheet-dominated wedges, ArF≈0.75 causes minor wedge formation due to relamination of subducted crust below the upper plate, and ArF≈1 causes buoyancy-flow- or diapir-dominated wedges involving exhumation of crustal material from great depth (>80 km). Furthermore, employing phase equilibria density models reduces the average topography of wedges by several kilometres. We suggest that during the formation of the Pyrenees ArF⪅0.5 due to the absence of high-grade metamorphic rocks, whereas for the Alps ArF≈1 during exhumation of high-grade rocks and ArF⪅0.5 during the post-collisional stage. In the models, FD increases during wedge growth and subduction and eventually reaches magnitudes (≈18 TNm-1) which are required to initiate subduction. Such an increase in the horizontal force, required to continue driving subduction, might have "choked" the subduction of the European plate below the Adriatic one between 35 and 25 Ma and could have caused the reorganization of plate motion and subduction initiation of the Adriatic plate. [ABSTRACT FROM AUTHOR]

A popular idea is that accretion of sediment at a subduction zone commonly leads to the formation of a subduction channel , which is envisioned as a narrow zone located above a subducting plate and filled with vigorously circulating accreted sediment and exotic blocks. The circulation can be viewed as a forced convection, with downward flow in the lower part of the channel due to entrainment by the subducting plate, and a 'backflow' in the upper part of the channel. The backflow is often cited as an explanation for the exhumation of high-pressure/low-temperature metamorphic rocks from depths of 30 to 50 km. Previous analyses of this problem have mainly focused on the restricted case where the walls bounding the flow are artificially held fixed and rigid. A key question is if this configuration can be sustained on a geologically relevant timescale. We address this question using a coupled pair of corner flows. The pro-corner accounts for accretion and deformation directly above the subducting plate, and the retro-corner corresponds to a deformable region in the overlying plate. The two corners share a medial boundary , which is fully coupled but is otherwise free to rotate and deform. Our results indicate that the maintenance of a stable circulating flow in a narrow pro-corner (<15°) requires an unusually large viscosity ratio, μretro/μpro > 103. For lower viscosity ratios, the medial boundary would rotate rearwards, converting the initially narrow pro-corner into an obtuse geometry. For a stable narrow corner, we show that the backflow within the corner is caused by downward convergence of the incoming flow and an associated downward increase in dynamic pressure, which reaches a maximum at the corner point. The total pressure is thus expected to be much greater than predicted using a lithostatic gradient, which means that estimates of depth from metamorphic pressure would have to be adjusted accordingly. In addition, we show that the velocity fields associated with a forced corner flow and a buoyancy-assisted channel flow are nearly identical. As such, structural geology studies are not sufficient to distinguish between these two processes. [ABSTRACT FROM AUTHOR]

Two prominent phases of deformation characterize the structure and current geometry of the western-central Alps: (1) nappe emplacement and stacking associated with single-vergent, NW-directed thrusting, and (2) backfolding associated with doubly-vergent, SE-directed movement. For the Monte Rosa region, early subduction and delamination of crustal units has been proposed to explain the nappe stack sequences that emplace ophiolite units onto continental crustal units, associated with areas of high-pressure metamorphism. However, the reasons for switching to late-stage backfolding associated with major uplift to lower grade metamorphic conditions is still ambiguous. This change in deformation style of the Alpine orogeny could potentially be explained by the relative weakening of the orogenic, wedge-shaped, lid with respect to the lower wedge in which nappe stacking took place. In this study, we present simple two-dimensional numerical simulations of two-wedge viscous corner-flow in order to quantitatively understand the mechanisms that control the large-scale geometries of the Alpine orogeny. Our initial model configuration consists of a lower wedge (representative of the orogenic wedge above the subducting European plate) and an upper wedge (representative of the overriding Adriatic orogenic lid). Our model utilizes the M2Di routines (Räss et al., 2017) which employ a direct-iterative approach to solve the system of partial differential equation for 2D viscous Stokes flow. The solver relies on a MATLAB based finite difference method to solve incompressible viscous flow adapted for both linear and power-law viscous rheology. We employ a semi-Lagrangian backward characteristic advection scheme in order to advect material phases; and define material boundaries using the level-surface approach that enables tracking of interfaces over large strains and limits numerical diffusion. We aim to: (1) benchmark numerical solutions against analytical corner-flow solutions valid for simple geometries and linear viscous flow, (2) quantify the impact of variable viscosity contrasts between the two wedges, (3) quantify the impact of an evolving deformable interface between the two wedges, and (4) quantify the impact of linear and power-law viscous flow on the results. Moreover, we compare our model results with the current geometry and tectono-metamorphic history of the Monte Rosa region in the Western Alps. [ABSTRACT FROM AUTHOR]