The interplay between fracture and flow in the localization of crustal deformation

The Earth’s crust is generally considered to consist of distinct brittle and viscous (or “ductile”) rheological layers, corresponding to Navier-Coulomb failure or viscous flow, with a “brittle-ductile transition” occurring over a specific and relatively limited depth interval. Depending on the assumed geothermal gradient, a compositionally layered crust could have several such brittle-ductile transitions, but the model still implies that large regions of the crust deform exclusively by either brittle fracture or viscous crystal-plastic flow. However, it is becoming increasingly clear from field observation that, in reality, there is an intimate interplay in space and time between precursor heterogeneities (either structural or compositional), brittle fracture, fluid-rock interaction and more distributed “ductile flow”. In particular, there are now several well-documented examples of brittle precursors localizing subsequent ductile deformation under high grade metamorphic conditions ranging from upper amphibolite (Mancktelow and Pennacchioni 2005, 2007) to even eclogite facies (Austrheim and Boundy 1994). This interplay between fracture and crystal-plastic creep and/or diffusion occurs over a wide range of scales, from 100’s of kilometres down to individual grains. Localization of strain in the crust can lead to the development of zones of very large relative displacement (such as low-angle thrusts and detachments, and steep strike-slip faults). The mechanics of this localization on a narrow zone and its repeated reactivation can only be considered in terms of a cyclical interaction between fracture, flow, and variation in local pore-fluid pressure. These relatively planar and discrete faults and shear zones are commonly observed to cross-cut layering and foliation at a small angle. Small-scale examples from the field, as well as numerical models, show that viscous localization is strongly controlled by existing compositional and rheological heterogeneity (such as bedding, dykes, veins etc), whereas fractures may crosscut such compositional layering at small angles. This suggests that major crosscutting faults, which may now be dominated by mylonitic fabrics characteristic of crystal-plastic flow (e.g., the Periadriatic Fault in the European Alps), could also have had a large-scale, brittle precursor that controlled subsequent ductile localization. On a smaller scale, flanking structures (Passchier 2001; Grasemann and Stüwe 2001) developed around brittle fractures of limited length are particularly clear examples of interacting brittle-ductile deformation, because their geometry can only be explained if discrete slip occurred synchronously with more distributed surrounding ductile flow (Exner et al. 2004). Flanking structures that formed in calcite marbles under amphibolite facies conditions (e.g., on the island of Naxos, Greece) demonstrate that brittle fracturing can play an important role even in weak rocks at high temperatures – conditions generally taken to imply exclusively ductile or viscous behaviour. Such flanking structures are common in mylonitic shear zones (e.g., in mylonites in the footwall of the major Simplon low-angle normal fault in the central Alps) and demonstrate the delicate balance between fracture and flow in such high strain zones, with switches back and forth varying locally in space and through time. This behaviour is not totally unexpected. The reduction of bulk porosity and permeability in rocks with depth raises the local pore fluid pressure from hydrostatic to near lithostatic (as usually assumed in metamorphic petrology), with the result that rocks are generally critically stressed and close to failure. Only minor local changes in the controlling parameters (strain rate, pore fluid pressure, dynamic or “tectonic” pressure) can cause a switch between fracture and flow. In natural examples, the interplay between fracture and flow is observed in middle to lower crustal rocks irrespective of whether they are weak (“wet”) or strong (“dry”). Excellent examples of interacting fracture and flow from glacier-polished outcrops of granodiorite in the Neves area of the eastern Alps developed under wet conditions, with very common quartz vein development and marked fluid-rock interaction along fractures. The deviatoric stress during both flow and fracture was low (<10 MPa), as demonstrated by little deformed calcite porphyroclasts in quartz mylonites, which did not even significantly twin during crystal plastic flow of the matrix quartz under upper amphibolite facies conditions (Mancktelow and Pennacchioni 2010). In contrast, in dry lower crust, such as from the Mont Mary area of the western Alps, stresses were high (as indicated by very small recrystallized quartz grain sizes; Fitz Gerald et al. 2006) and seismic fracture was associated with pseudotachlyte development. Pseudotachylytes subsequently act as rheologically weak layers that strongly localize ductile shearing under dry upper amphibolite facies conditions (Pennacchioni and Cesare 1997). References: Austrheim, H., Boundy, T.M., 1994. Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust. Science 265, 82-83. Exner, U., Mancktelow, N.S., Grasemann, B., 2004. Progressive development of s-type flanking folds in simple shear. Journal of Structural Geology 26, 2191-2201. Grasemann, B., Stüwe, K., 2001. The development of flanking folds during simple shear and their use as kinematic indicators. Journal of Structural Geology 23, 715-724. Fitz Gerald, J.D., Mancktelow, N.S., Pennacchioni, G., Kunze, K., 2006. Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust. Geology 34, 369-372. 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. Mancktelow, N.S., Pennacchioni, G., 2010. Why calcite can be stronger than quartz. Journal of Geophysical Research 115. Passchier, C.W., 2001. Flanking structures. Journal of Structural Geology 23, 951-962. Pennacchioni, G., Cesare, B., 1997. Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary Nappe, Italian Western Alps). Journal of Metamorphic Geology 15, 777-791. 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.