The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite, ± garnet ± K‐feldspar. Amphibole‐plagioclase geothermobarometry and thermodynamic modeling indicate that pristine and mylonitized pseudotachylytes formed at 650–750°C and 0.7–0.8 GPa. Thermodynamic modeling indicates that a limited amount of H2O infiltration (0.20–0.40 wt. %) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites.

Shear zones channelize fluid flow in the Earth's crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (T=700-730° C, P=0.65-0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in both domains is <25 m. Microstructural observations and EBSD analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C'-type shear bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C'-type shear bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistent with synkinematic formation of creep cavities producing dilatancy. Thus, this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain-size sensitive creep, thereby stabilising strain localization in the polymineralic ultramylonites.

[1] Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic composition. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C-820°C, P = 0.75-0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, and titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed at high differential stress (50-100 MPa) by dislocation creep. High stresses are demonstrated by the small grain size (11-17 mm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C-815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pockets. The high stress deformation of quartz overprints an earlier, lower stress deformation, which is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deformation by dislocation creep in quartz. The water partitioned into the segregated melt crystallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystalline water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust.Citation: Menegon, L., P. Nasipuri, H. Stünitz, H. Behrens, and E. Ravna (2011), Dry and strong quartz during deformation of the lower crust in the presence of melt,

[1] The microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain g > 15, in a small temperature window (about 25°C) above 500°C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: g < 1) consist of millimeter-to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 < g < 3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: g = 4 to 15) are pervasively to completely recrystallized to fine (34-40 mm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism hai with subordinate rhomb and basal hai slip. Recrystallization occurred rather abruptly for 3 < g < 4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (g ≈ 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism hai slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism hai slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism hci and prism hai slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth-and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal hai slip would have been dominant.

Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep.

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We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300-400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal \a[ (regions without twinning) to {p} and {p 0 } \a[(pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.