Buoyancy versus shear forces in building orogenic wedges

Abstract

Abstract. 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 TN m−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.

Horizontal Force Required for Subduction Initiation at Passive Margins With Constraints From Slab Detachment

“…In this study we build upon our previous work presented in Candioti et al (2021). Candioti et al (2021) presented twodimensional (2D) petrological-thermomechanical numerical models that simulate several, subsequent geodynamic processes in a single and continuous simulation, including thermal convection in the upper mantle.…”

“…In this study we build upon our previous work presented in Candioti et al (2021). Candioti et al (2021) presented twodimensional (2D) petrological-thermomechanical numerical models that simulate several, subsequent geodynamic processes in a single and continuous simulation, including thermal convection in the upper mantle. They modelled 1) the formation of hyperextended passive continental margins bounding an embryonic ocean, 2) SZI via thermal softening during convergence of the ocean-margin system, 3) subduction and closure of an embryonic ocean, 4) formation of a collisional orogen and 5) SD of subducted lithosphere.…”

“…These studies do not investigate the formation of a new convergent plate boundary associated with SZI. 2) Models that investigate both SZI and the associated formation of a new convergent plate boundary, without a priori prescribing an already existing weak plate boundary in the initial model configuration (e.g., Thielmann and Kaus, 2012;Auzemery et al, 2020;Kiss et al, 2020;Candioti et al, 2021). The plate boundary formation must be numerically simulated in these models and, hence, these models can be used to determine the horizontal force required for SZI at, for example, passive margins.…”

“…An important step to reduce the driving force necessary for SZI by thermal softening is to consider more realistic passive margin architectures. Including mechanical heterogeneities and numerically modelling the architecture of passive margins prior to convergence, instead of designing these margins ad hoc, Candioti et al (2020) and Candioti et al (2021) report significantly smaller force magnitudes between 15 and 18 TN m −1 for thermal softening induced SZI. Although the plate driving force magnitudes in these models are significantly reduced, a minimum value for horizontal forces required for SZI via thermal softening has not been quantified yet in a systematic way.…”

“…Although the plate driving force magnitudes in these models are significantly reduced, a minimum value for horizontal forces required for SZI via thermal softening has not been quantified yet in a systematic way. Here, we generate the passive margin architecture with a numerical lithosphere rifting simulation and apply this architecture as initial configuration for convergence simulations to investigate SZI at passive margins (see also Candioti et al, 2020Candioti et al, , 2021. Advantages of using a numerically calculated passive margin architecture as initial configuration are that 1) such architecture is thermomechanically feasible and consistent with the applied convergence model, 2) it is isostatically stable and 3) the initial temperature field, including thermal convection in the upper mantle, is consistent with the architecture and applied thermal parameters.…”