Deformation at the frictional-viscous transition: Evidence for cycles of fluid-assisted embrittlement and ductile deformation in the granitoid crust

Abstract

Mid-crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional-to-viscous transition (FVT). Here we investigate an exhumed continental midcrustal basement section in order to envisage the relationship between ductile and brittle deformation at the FVT. Our detailed study from km-to micro-scale shows that, under greenschist metamorphic conditions, deformation is accommodated by a dense network of highly-localized ductile shear zones. In the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine-grained polymineralic matrix consisting of quartz, feldspar, sheet silicates and epidote. During viscous granular flow mass transfer processes under the presence of fluids promote a chemo-mechanical mixing, resulting in grain size reduction and reaction softening. Coeval with this ductile deformation, fluid-assisted embrittlement occurs, as indicated by biotite-coated fractures, cataclasites and injection of non-cohesive polymineralic gouge material into secondary fractures inside the host rock. The embrittlement during predominant ductile deformation occurs in cycles, i.e. prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation. We interpret this fluid-assisted cyclic embrittlement evidenced by injection of the fluidized material into off-fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo-seismic activity. With exhumation and associated cooling, localized deformation persists in the ultrafine-grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure-dependent frictional flow; always showing cycles of slow interseismic flow and fast seismic injection events. Overall, in the granitic crust of the Aar-massif, brittle and ductile deformation coexist up to deformation temperatures of minimum 450°C, indicating that the FVT has to be placed in a rather wide range from 8 km up to > 18-20 km in the granitoid crust.

Permeability and seismic velocity anisotropy across a ductile-brittle fault zone in crystalline rock

“…• C and 150 to 250 MPa peak conditions during Alpine metamorphism (Steck, 1968;Rolland et al, 2009;Wehrens et al, 2016Wehrens et al, , 2017. Our thin section observation shows fracturing of feldspar and undulose extinction along with subgrain boundaires in quartz, which are consistent with the inferred metamorphic temperatures.…”

“…This case study from the GTS is a hybrid between the two end member systems. During the two ductile deformation phases slip was accommodated along localized foliated shear zones that are mylonitic and ultramylonitic (Challandes et al, 2008;Rolland et al, 2009;Belgrano et al, 2016;Wehrens et al, 2016Wehrens et al, , 2017. The transition from highly foliated and extremely recrystallized fault cores towards the host granodiorite represents a strain gradient, which is ∼ 0.5 m thick in this study (Figure 5a).…”

“…Detailed deformation histories of the Grimsel region are available (Challandes et al, 2008;Rolland et al, 2009;Belgrano et al, 2016;Wehrens et al, 2016Wehrens et al, , 2017, which are summarized here. From argon-argon and rubidium-strontium dating, as well as field relations, beginning around 21 Ma and continuing until approximately 10 Ma, ductile deformation resulting from 5 transpression created NNE-SSW, E-W, and NW-SE striking shear zones with steep dips to the south (Figure 1a).…”

“…The shear zone penetrated by the borehole at GTS is characterized by foliation aligned with the mylonitic fault core that developed under viscous flow deformation conditions (Wehrens et al, 2016). The foliation intensity is highest nearest the mylonitic fault core and decreases into the host granodiorite.…”

“…Previous studies from the laboratory (cm) to field outcrop (km) scale have developed into generalized models for the mechanical and Figure 1. a) Geologic map of the Grimsel pass region (after Wehrens et al, 2016Wehrens et al, , 2017, b) borehole orientations (this study -red borehole) with respect to the underground research laboratory (after Krietsch et al, 2017;Amann et al, 2017), c) the borehole in this study depicting the location of the damage zone with coring and stress measurement locations projected along the borehole. The shear zone at the base is divided into three components: 1) transition zone (TZ), 2) mylonitic fault core (MFC), and 3) damage zone (DZ).…”