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

Abstract

A section of anatectic felsic rocks from a high-pressure (>13 kbar) continental crust (Variscan Bohemian Massif) preserves unique evidence for coupled melt flow and heterogeneous deformation during continental subduction. The section reveals layers of migmatitic granofels interlayered with anatectic banded orthogneiss and other rock types within a single deformation fabric related to the prograde metamorphism. Granofels layers represent high strain zones and have traces of localized porous melt flow that infiltrated the host banded orthogneiss and crystallized granitic melt in the grain interstices. This process is inferred from: (1) gradational contacts between orthogneiss and granofels layers; (2) grain size decrease and crystallographic preferred orientation of major phases, compatible with oriented growth of crystals from interstitial melt during granular flow, accommodated by melt-assisted grain boundary diffusion creep mechanisms; and (3) pressuretemperature equilibria modeling showing that the melts were not generated in situ. We further argue that this porous melt flow, focused along the deformation layering, significantly decreases the strength of the crustal section of the subducting continental lithosphere. As a result, detachment folds develop that decouple the shallower parts of the layered anatectic sequence from the underlying and continuously subducting continental plate, which triggers exhumation of this anatectic sequence.

Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia

“…In contrast, the exhumation of migmatites in the core of antiforms was certainly associated with crystallization of melt and reequilibration, as demonstrated by the presence of leucosomes in the axial planar of F2 folds (Figures e, b, and b). Thus, these axial planar leucosomes can mark the timing of the D2 folding (e.g., Weinberg et al, ; Závada et al, ). Although we have no specific age constraints for these leucosomes in the current area, we propose that the upright F2 folding follows rapidly the 400‐ to 380‐Ma extension in the deep crust, probably in the early Late Devonian.…”

The recognition of former melt flux through high‐strain zones

“…Compared to the more pronounced CPOs formed by solid‐state, dislocation creep dominated deformation (e.g. Berger & Stünitz, ; Závada et al., ), the activity of GBS is suggested to result in weak to little CPO (e.g. Bestmann & Prior, ; Jiang, Prior, & Wheeler, ; Svahnberg & Piazolo, ; Viegas, Menegon, & Archanjo, ).…”

“…High‐strain zones in which high volumes of leucosome (>10%) crystallized are easily recognised due to the presence of distinct pockets, seams and dykes of leucosome at the outcrop scale (e.g. Brown & Solar, ; Carvalho, Sawyer, & Janasi, ; Collins & Sawyer, ; Sawyer, Dombrowski, & Collins, ; Weinberg, Hasalová, Ward, & Fanning, ; Závada et al., ). Thin‐section‐scale features indicative of the former presence of melt in analogue modelling of high‐strain zones include segregations of felsic minerals in shear bands, pockets, and/or fractures (e.g.…”

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

“…Migmatites and granites are the main constituents of the continental crust and their petrogenesis and emplacement are intimately linked to orogenic evolution (Thompson and Connolly, 1995;Sawyer, 1998;Brown, 2001;Foster et al, 2001;Vanderhaeghe, 2009;Sawyeret al, 2011;Weinberg, 2016;Závada et al, 2018). Various heat sources have been proposed to cause high-temperature metamorphism and partial melting of orogenic roots comprising an increase in radioactive heat production of the thickened crust, an increase in the basal heat flux associated with delamination of the lithospheric mantle and heat advection through the emplacement of mantle-derived magmas (Houseman et al, 1981;England and Thompson, 1984;Henk et al, 2000;Annen and Sparks, 2002;Vanderhaeghe et al, 2003;Vanderhaeghe and Duchêne, 2010;Ueda et al, 2012).…”