The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25–40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.</jats:p

Fluid flow during unbending: Implications for slab hydration, intermediate-depth earthquakes and deep fluid subduction

We calculate the tectonic stress profile and associated direction of fluid flow during unbending and dehydration of oceanic plates, for a range of critical parameters that affect their combined elasto-plastic and viscous behaviour, such as bending curvature, age, pore fluid pressure and viscous flow laws. In all models, negative pressure gradients are established at Moho depths, down to the base of the slab elastic core. Fluids released at these depths flow downward across the plate, "wetting" or further hydrating the underlying dry levels, and ultimately accumulate at the base of the elastic core, increasing the pore fluid pressure and triggering deep seismicity. The thickness of the "wet" layer increases for low bending curvatures, low pore fluid pressure, old slabs and dry viscous rheologies. "Wetting" of the upper 10-30 km of the slab has important implications for its rheological and anisotropic structure. Unbending favours the redistribution and trapping of significant amounts of fluids in subducting oceanic plates that will subsequently be released at the base of the upper mantle. (C) 2010 Elsevier B.V. All rights reserved

Initiation and growth of strike-slip faults within intact metagranitoid (Neves area, eastern Alps, Italy)

Exhumed strike-slip faults exposed in the Neves area of the Tauern Window (eastern Alps, Italy) formed in the lower brittle crust under hydrous conditions within intact metagranitoids, where any precursor fractures were already healed due to earlier amphibolite-facies metamorphism. Faults initiated as newly formed en-echelon fractures delineating shear bands, with segmentation occurring over scales of 0.001 to 100 m. Due to the initial en-echelon pattern, stepovers between fault segments were almost invariably contractional during subsequent slip accumulation. In initial low-slip (centimeter to decimeter) stages, synthetic slip on the segments was associated with the development of a set of antithetic faults in the contractional stepovers, oriented at an angle of 30 \u201345 to the bounding faults. Slip on this antithetic set did not fully compensate for the decrease in slip on the main faults, implying an additional deformation mechanism within the stepovers (e.g., block rotation and/or out-of-plane movement). In larger faults with slip on the order of a few meters, the antithetic faults within the stepover were crosscut by synthetic sigmoidal faults that connected the overstepped fault segments and accommodated most of the subsequent displacement transfer. This second stage of evolution involving fault linkage is well documented in the Mesule fault, which has a current maximum offset of ~10 m. In the Mesule fault, the total horizontal slip summed along all secondary faults within the stepover accounts almost entirely for the net slip decrease toward the tips of the overstepping faults. However, the boundary faults remain nearly straight in spite of the sigmoidal ramp-like geometry of the connecting faults. Since the adjacent blocks are little deformed and there is no evidence for appreciable volume loss or block rotation in the stepover, signifi cant out-of- plane movement is implied, although it is difficult to quantify. The Neves area provides unusually detailed field constraints on fault initiation, linkage, and displacement accumulation within nonbedded and relatively isotropic granitoid rocks at the base of the brittle crust, where neither a free upper surface nor substantial volume change (e.g., by veining and pressure solution) was a controlling factor in accommodating fault linkage and displacement transfer

Brittle precursors, fluid\u2010rock 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 \u201crigid\u201d 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 \u201cshear localization\u201d. 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

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

The Earth\u2019s crust is generally considered to consist of distinct brittle and viscous (or \u201cductile\u201d) rheological layers, corresponding to Navier-Coulomb failure or viscous flow, with a \u201cbrittle-ductile transition\u201d 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 \u201cductile flow\u201d. 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\u2019s 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 St\ufcwe 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 \u2013 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 \u201ctectonic\u201d 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 (\u201cwet\u201d) or strong (\u201cdry\u201d). 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., St\ufcwe, 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