H 2 O-fluid-saturated melting of subducted continental crust facilitates exhumation of ultrahigh-pressure rocks in continental subduction zones

Abstract

We present two-dimensional numerical models of plate subduction and collision inspired by the Scandinavian Caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra-high pressure metamorphic rocks. Three possible reactions were tested: low temperature solidus representing H 2 O-fluid-saturated partial melting, and two end-member reaction curves for dehydration melting. Thermo-mechanical effects of partial melting were implemented as (1) a viscosity decrease as a determined rheologically critical melt percentage was reached (here 0.1), (2) a change in effective heat capacity and adiabatic heating/cooling accounting for a latent heat term in the heat equation. Among the 3 tested reactions, only H 2 O-fluid-saturated partial melting drastically modifies the collision dynamics from the non-melting reference model holding all other parameters constant. A substantially low general viscosity truncation (here 10 17 Pa s) is needed to properly resolve the effect of partial melting on deep collision processes. Low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment, where migmatites exhume from UHP conditions at rates and with pressure-temperature paths similar to the natural values acknowledged for the Norwegian Caledonides. High temperature melting has no drastic influence on early collision dynamics. While positive buoyancy remains the first order driver for the exhumation of buried continental rocks, exhumation initiates in these cases with eduction subsequent to slab detachment. Melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 • C. Some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone. Even if minor in terms of amount of magma produced, H 2 O-fluid-saturated partial melting at UHP conditions could therefore have a dramatic rheological effect and actually limits continental rocks subduction and facilitates their exhumation.

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…The style of folding and deformation is also similar to that explaining exhumation of eclogites in Scandinavian Caledonides that were exhumed due to progressive weakening of continental crust along eclogite facies (hydrated) shear zones (Jolivet et al, 2005). This style, schematically presented by Labrousse et al (2011Labrousse et al ( , 2015, is similar to the detachment folds developed above a weak layer (e.g., Blay et al, 1977;Solar and Brown, 2001;Barraud et al, 2004). This folding can be observed on the outcrop scale in domain A (Fig.…”

“…The asymmetrical folding of the layered sequence of anatectic rocks can explain melt percolation triggered exhumation of the lowdensity felsic crust during continental subduction (Labrousse et al, 2011(Labrousse et al, , 2015. The style of folding and deformation is also similar to that explaining exhumation of eclogites in Scandinavian Caledonides that were exhumed due to progressive weakening of continental crust along eclogite facies (hydrated) shear zones (Jolivet et al, 2005).…”

“…www.gsapubs.org | Volume 10 | Number 2 | LITHOSPHERE ECC is probably also driven by buoyancy, as modeled by Labrousse et al (2015), but likely takes place already at much shallower depths of ~50 km. While the model of Labrousse et al (2015) shows geometries of exhumed partially molten crustal batches at the scale of several kilometers, the detachment folding can explain backflow of partially molten rocks along the subduction interface at scales ranging from meters to hundreds of meters.…”

“…In contrast, based on a different numerical modeling study, Butler et al (2015) argued that melting in subducted crust has only minor, if any effect on exhumation, at least for large UHP terranes, for example the Western Gneiss Region in Norway. However, numerical model results might be biased by the approach chosen to introduce the melt-enhanced weakening (Jamieson and Beaumont, 2011;Labrousse et al, 2015;Butler et al, 2015;Maierová et al, 2016). Furthermore, low resolution of these models (~1 km grid spacing) does not allow simulation of mesoscale to microscale deformation mechanisms coupled with porous melt flow and melt segregation (see Wallner and Schmeling, 2016).…”

“…Labrousse et al (2015) suggested that melting of subducted upper continental crust at ultrahigh-pressure (UHP) conditions triggers its subsequent exhumation. Labrousse et al (2015) showed that the partial melting of subducting crust in the subduction channel starts at depths exceeding 100 km, depending on the water content. Increasing amount of buoyant partially molten crust subsequently detaches from the subducting plate and rises upward in the form of a diapir-like structure.…”