Episodic weakening and strengthening during synmetamorphic deformation in a deep-crustal shear zone in the Alps

Abstract

The Greiner shear zone (western Tauern Window) deformed a variety of metasedimentary, metavolcanic, and plutonic lithologies at conditions of c. 525-575 ~ and 30-40km depth. Microstructural relationships point to a succession of weakening and strengthening episodes. Stage I involved softening via a change in deformation mechanism. Grain-size reduction in plagioclase-rich horizons locally produced rocks with an average grain size of <30 #m and microstructural features consistent with deformation via grain boundary diffusion creep (GBDC; a fluid-assisted deformation mechanism similar to pressure solution, which may result in superplastic behaviour). At constant stress, GBDC will result in a significant increase in strain rate relative to neighbouring layers. Stage II involved reaction-induced strengthening. Rapid bulk diffusion rates associated with GBDC allowed rapid growth of large (up to 20 cm) hornblende cystals. This growth of large cross-cutting crystals shut down grain-size-sensitive flow mechanisms in the plagioclase matrix and locally 'locked' the shear zone, shifting ductile deformation to weaker horizons. Stage III involved reaction-induced softening. Local variations in bulk composition and/or fluid availability caused large hornblende grains in some horizons to be partially replaced by biotite. These biotite-rich layers localized subsequent deformation, whereas adjacent layers with intact hornblende record minimal strain. Deformation and metamorphism together exert control on fluid availability, diffusion rates, and reaction kinetics, and these factors collectively control fabric development and rheology. The effects of interaction between small-scale deformational and metamorphic processes are difficult to predict, but can have an important influence on shear zone behaviour at depth. The resulting complexities need to be accounted for in models of crustal strength-depth relationships and shear zone rheologies.

Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

“…, 2004). Strain hardening and permeability reductions through the growth of more rigid and coarser grained mineral assemblages can also be driven by fluid‐mediated metamorphic reactions (Steffen et al. , 2001).…”

“…, 1996), to the growth of coarse‐grained actinolite. The growth of coarse‐grained actinolite in effect reduces the surface area available for fluid‐rock interaction thereby decreasing fluid permeability, diffusion rates and reaction kinetics (Steffen et al. , 2001).…”

Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

“…, 2003). Rocks in shear zones do not behave uniformly during deformation; they are subjected to regimes of alternating strengthening and weakening during metamorphism (Steffen et al. , 2000).…”

Deformation of Granitic Rocks: Experimental Studies and Natural Examples

“…Evidence of transient fracturing has been observed even in granulite grade anorthosites, and it may serve to localize subsequent recrystallization (e.g., Lafrance et al 1998). Another complication is the possibility that layers and zones of fine-grained plagioclase in amphibolite grade rocks, which at first appear to be dynamically recrystallized, may contain zoning patterns inconsistent with recrystallization as well as a random LPO, strongly suggestive of dissolution and reprecipitation (e.g., Steffen et al 2001;Lapworth et al 2002). Finally, it is not uncommon for high temperature microstructures indicative of regime 2 dislocation creep, with subgrain rotation recrystallization, to be overprinted by later, lower temperature deformation involving grain boundary bulging.…”