Softening trigerred by eclogitization, the first step toward exhumation during continental subduction

Abstract

Direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn-exhumation deformation. Most ultrahigh-pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation. Whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces, low viscosity in the subduction channel, overburden removal by erosion and normal faulting, a basic question is seldom considered: why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface? This event, seminal to exhumation processes, must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it. Our field observations in the Bergen arc show that Caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the Caledonian subduction. Thus, the sequence of deformation preserved in the Bergen Arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation. This observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions, i.e. underthrusting of the subducting slab. In this context of simple shear, metamorphic reactions assisted by fracturating, fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab. These tectonic units can then be incorporated into the channel circulation and start their upward travel

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).…”

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

“…Fluid migration in subducted crust is suggested by the formation of veins bearing eclogite-facies minerals (Philippot and Kienast, 1989;Rubatto and Hermann, 2003), and Caledonian-aged eclogite-facies shear zones bounding Grenvillian-aged granulites in the Bergen Arcs region of Norway (Jolivet et al, 2005). These shear zones suggest that hydration weakening occurred under eclogite-facies conditions.…”

“…Examination of exposed continental and oceanic (U)HP terranes shows that they vary along a continuum from being completely pervasively deformed (Terry and Robinson 2004) to consisting of low-strain regions or blocks on a variety of scales separated by high-strain shear zones (Labrousse et al, 2002;Jolivet et al, 2005). Despite their rapid burial and exhumation along a lithosphere-scale shear zone, there are many examples of units within UHP terranes that have accumulated remarkably little strain: well-preserved magmatic fabrics have been reported in (U)HP granites , gabbros Krabbendam et al, 2000) and pillow basalts (Bearth, 1959;Puga et al, 1995;Oberhänsli et al, 2002).…”

Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction–collision system, Bergen Arcs, Norway

“…The Bergen Arcs high pressure (HP) rocks form, together with the Western Gneiss Region (WGR), one of the largest HP-to UHP provinces in the world. The Bergen Arcs HP rocks are famous for their preservation of km-scale coherent eclogite facies deformation patterns, and for recording progressive eclogitization at depth (Austrheim 1987, Austrheim and Boundy 1994, Jolivet et al 2005. They offer an unique opportunity to study metamorphic and tectonic processes in the eclogite facies root zones of collisional orogenic belts.…”