Rheologic Transitions During Exhumation of High-Pressure Metamorphic Rocks

Abstract:

The exhumation of deeply buried rocks typically involves dynamic feedbacks between deformation and metamorphic reactions (+ fluid and/or melt) that influence rheology and facilitate or drive large-magnitude exhumation. The evolution of grain-scale to terrane-scale processes during decompression can be seen in rocks exhumed from oceanic and continental subduction and from orogenic crust. In the Sivrihisar (Turkey) high-P/low-T (oceanic subduction) complex, microstructures record deformation and syn-kinematic reactions during decompression from eclogite to blueschist facies conditions; this transformation resulted in dramatic strength reduction that promoted strain localization along the subduction interface. In quartz-rich rocks, qz was deformed in the dislocation creep regime and records transitions in microstructure and slip systems during near-isothermal decompression from 2.5 to 1.5 GPa; these transitions may be related to decreasing water fugacity over tens of km of decompression. High-to ultrahigh-P eclogite in exhumed continental subduction zones such as the Western Gneiss Region (Norway) record decompression from >2.5 GPa to <1 GPa. Eclogite shows dramatic textural evidence for decompression, including partial melting and decomposition into hbl-gneiss, resulting in weakening and strain localization. In collisional orogens that are underlain by partially molten crust, upper crustal extension/transtension drives rapid ascent of the deep crust to form migmatite-cored domes. The exhuming deep crust entrains HP relics such as eclogite (e.g. Montagne Noire dome, France) as it traverses much of the orogenic crust, from >1.2 GPa to (in some cases) <0.1 GPa in a single, geologically rapid event during which the partially molten crust reaches the near-surface. In summary, decompression of subducted or deeply buried crust systematically leads to rheologic transitions and feedbacks between deformation and metamorphism in the presence of aqueous fluid and/or melt.