Brittle precursors, fluid‐rock interaction and the localization or spreading of shear zones

Heterogeneous ductile shear zones are generally considered to develop by progressive strain localization, implying that shear zones become narrower during their development and that individual zones should affect an ever decreasing volume of rock. We support a diametrically different model: in granitoid plutons and other non-layered rock bodies, shear zones are strongly localized at their very initiation, on pre-existing planar rheological discontinuities, and tend to spread into the adjacent rock with increasing strain. Precursor discontinuities can be either compositional layers (e.g. dykes or veins) or fractures (Fig. 1), with enhanced fluid flow and fluid-rock interaction along these fractures leading to localized compositional and rheological change of the original host rock. Spreading of strain reflects the interplay between two factors: (1) diffusion of fluid away from the central fracture, which broadens the zone of alteration, and (2) development of new fractures both in previously intact rock and in already sheared domains. Cycles of fracturing are driven by local stress concentrations in rocks that remain close to the critical stress state for fracture. Stress concentration can be due to local mechanical instability (e.g. dyke boudinage) or more generally due to inherent problems of strain accommodation during deformation of more “rigid” blocks surrounded by a network of relatively discrete shear zones. As is clear from analogue and numerical models, such blocks between bounding shear zones must also deform internally to maintain strain compatibility. A distributed, more homogeneous background strain may develop in the intervening blocks under higher grade metamorphic conditions, but our field observations demonstrate that more localized shearing of intact granitic protolith in general develops from a brittle precursor. The conference location is particularly appropriate for this topic, as one of the first studies that proposed a brittle precursor to heterogeneous ductile shear zone development was that of Segall and Simpson (1986), who used the Roses shear zones as relevant examples. There are many published studies of shear zone development in previously undeformed granitoid plutons, especially with regard to gradients in microstructure and chemical and/or isotopic changes during “shear localization”. This interest was in part based on the assumption that plutons are relatively homogeneous, allowing a direct comparison between the heterogeneous shear zone and the homogeneous background. However, such intrusive bodies are certainly not homogeneous in detail: they show common compositional boundaries due to enclaves, intrusive contacts and dykes and veins. Cooling plutons also invariably develop a pervasive set of joints due to thermal contraction, typically involving a volume decrease on the order of 15% or more. These joints form conduits for late magmatic or subsequent metamorphic fluids, with the development of veins (especially quartz veins) and localized new mineral growth (commonly biotite in higher temperature cooling joints). These precursor discontinuities act as the controlling loci for localizing shear zones either during pluton cooling (Pennacchioni 2005; Pennacchioni et al. 2010) or later deformation (Mancktelow and Pennacchioni 2005; Pennacchioni and Mancktelow 2007). However, the process is not necessarily limited to precursor magmatic structures (dykes, veins or cooling joints). Deformation under higher grade metamorphic conditions (e.g. upper amphibolite facies, as in Fig. 2) can produce repeated cycles of fracture, fluid rock interaction and ductile shear localized on these brittle precursors. Already developed broader shear zones can themselves be cut by discrete fractures, with fluid-rock interaction and new mineral growth once more changing the local rheology and localizing further shearing to produce new heterogeneous shear zones oblique to the earlier zone (Fig. 2). Brittle precursors localizing ductile shearing have also been proposed for greenschist facies shear zones developed in schists of the Cap de Creus (Fusseis et al. 2006). References Fusseis, F., Handy, M.R., Schrank, C., 2006. Networking of shear zones at the brittle-to-viscous transition (Cap de Creus, NE Spain). Journal of Structural Geology 28, 1228-1243. Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology 27, 645-661. Pennacchioni, G., 2005. Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy). Journal of Structural Geology 27, 627-644. Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780. Pennacchioni, G., Menegon, L., Leiss, B., Nestola, F., Bromiley, G., 2010. Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarsegrained quartz veins. Journal of Geophysical Research-Solid Earth 115. Segall, P., Simpson, C., 1986. Nucleation of ductile shear zones on dilatant fractures. Geology 14, 56-59.