Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation

Abstract

Abstract. Many plate tectonic processes, such as subduction initiation, are embedded in long-term (>100 Myr) geodynamic cycles often involving subsequent phases of extension, cooling without plate deformation and convergence. However, the impact of upper mantle convection on lithosphere dynamics during such long-term cycles is still poorly understood. We have designed two-dimensional upper-mantle-scale (down to a depth of 660 km) thermo-mechanical numerical models of coupled lithosphere–mantle deformation. We consider visco–elasto–plastic deformation including a combination of diffusion, dislocation and Peierls creep law mechanisms. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. Our models exhibit realistic Rayleigh numbers between 106 and 107, and the model temperature, density and viscosity structures agree with geological and geophysical data and observations. We tested the impact of the viscosity structure in the asthenosphere on upper mantle convection and lithosphere dynamics. We also compare models in which mantle convection is explicitly modelled with models in which convection is parameterized by Nusselt number scaling of the mantle thermal conductivity. Further, we quantified the plate driving forces necessary for subduction initiation in 2D thermo-mechanical models of coupled lithosphere–mantle deformation. Our model generates a 120 Myr long geodynamic cycle of subsequent extension (30 Myr), cooling (70 Myr) and convergence (20 Myr) coupled to upper mantle convection in a single and continuous simulation. Fundamental features such as the formation of hyperextended margins, upper mantle convective flow and subduction initiation are captured by the simulations presented here. Compared to a strong asthenosphere, a weak asthenosphere leads to the following differences: smaller value of plate driving forces necessary for subduction initiation (15 TN m−1 instead of 22 TN m−1) and locally larger suction forces. The latter assists in establishing single-slab subduction rather than double-slab subduction. Subduction initiation is horizontally forced, occurs at the transition from the exhumed mantle to the hyperextended passive margin and is caused by thermal softening. Spontaneous subduction initiation due to negative buoyancy of the 400 km wide, cooled, exhumed mantle is not observed after 100 Myr in model history. Our models indicate that long-term lithosphere dynamics can be strongly impacted by sub-lithosphere dynamics. The first-order processes in the simulated geodynamic cycle are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyperextended rifted margins, which might have been the case for the Alpine orogeny.

Buoyancy versus shear forces in building orogenic wedges

“…(A9). Previous work has demonstrated that this choice, in combination with suitable flow law parameters, results in a viscosity structure that is consistent with geophysical constraints with respect to the convection dynamics of the mantle and the thermal thickness of the overlying, horizontal lithosphere (Candioti et al, 2020). Similar average grain sizes of the order of 1 mm in the upper mantle have been predicted by whole mantle convection models that include state-of-the-art grain size evolution models (Dannberg et al, 2017;Schierjott et al, 2020).…”

“…However, thermal softening, caused by shear heating, is active, resulting from the conservation of energy and temperature-dependent viscous flow stress. The algorithm has already been used to model deformation processes at various scales (Yamato et al, 2015;Duretz et al, 2016b;Yamato et al, 2019;Petri et al, 2019;Bessat et al, 2020) including upper mantle convection coupled to lithospheric-scale defor-L. G. Candioti et al: Buoyancy versus shear forces in building orogenic wedges mation (Candioti et al, 2020). Appendix A provides a detailed description of the algorithm.…”

“…We here follow the modelling approach of Candioti et al (2020) and we employ a 1600 km wide and 680 km deep (see Fig. 2) model domain.…”

“…A popular geodynamic model explaining the formation of collisional orogens, such as the Western Alps, the Pyrenees or the Himalayas, is the wedge model (e.g. Chapple, 1978;Dahlen et al, 1984;Platt, 1986;Willett et al, 1993;Vanderhaeghe et al, 2003;Malavieille, 2010;Dal Zilio et al, 2020b). The first mechanical models of so-called critical wedges considered a frictional deformation (stresses are controlled by a specific yield criterion) and were L. G. Candioti et al: Buoyancy versus shear forces in building orogenic wedges originally applied to accretionary wedges.…”

Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

“…Experiments with dry dislocation creep result in extensive brittle-plastic deformation of the lithosphere 43,44 . Experiments with composite diffusion-dislocation creep and small constant grain size (1 mm) results in a lithosphere thinned by convective erosion (<90 km) driven by low asthenosphere viscosities of <10 18 Pa•s 16,45 . For constant grain sizes larger than 1 cm, dislocation creep becomes the main deformation mechanism throughout the entire upper manlte 16 .…”