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Mechanisms driving the long-term dynamics of plate interfaces remain poorly-constrained. To date, the rheology of the crust is considered to be controlled by solid-state diffusion processes such as crystal plastic deformation (dislocation creep). Yet, most minerals formed at high-pressure conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite) and can only be deformed plastically at unrealistically high stresses or temperatures. A growing number of studies point to the crucial role of fluid-rock interactions and mineral transformations in the development of crustal shear zones of low viscosity. The rock weakening is interpreted as being induced by dissolution and precipitation processes at grains boundaries in chemical disequilibrium. Here, we tackle the eclogite rheology conundrum by performing the first deformation experiments at high-pressure conditions (> 2 GPa) on a two-phase aggregate representative of the lower crust.Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, 2.1 GPa and a shear strain rate of 10⁻6 s⁻¹. The starting material consists of mixed powders of plagioclase and clinopyroxene separated from an undeformed gabbro (Kågen, Norway) and hot-pressed with a grain size lower than 100 µm. Experiments have been conducted with 0.2% added water.Mechanical data indicate that the samples are first very strong with a peak differential stress between 1.0 and 1.4 GPa. Then, a significant weakening is observed with a stress decrease of 0.5 GPa. The high-strain samples are characterized by a strain gradient and a reaction gradient, both increasing toward the center of the shear zone. The nucleation of new phases leads to a drastic grain size reduction and phase mixing. The intensities of both are positively correlated with the strain intensity. The nature, distribution and fabric of the reaction products vary also progressively with strain intensity. At the peak stress, the reaction products are restricted to grain boundaries where they form corona structures, while in the high-strain samples, they occur throughout the sample replacing most of the starting material. The primary plagioclase and clinopyroxene grains show incipient dynamic recrystallization, whereas reaction products never do. The nano-porosity reported in the samples attests to the presence of free-fluid phase along the reactive grain boundaries, despite the high-pressure conditions. This nano-porosity requires grain boundary sliding (GBS) processes to form, as indicated by the spatially associated quadrupole junctions.Our results show that strain at eclogite-facies conditions is preferentially localized by GBS-accommodated dissolution and precipitation creep in reactive zones. We suggest that this dominant deformation process take place in rock at chemical disequilibrium in the presence of a free-fluid phase. Therefore, deformation along deep plate interfaces should be initiated and governed by transient and local transformation weakening, allowing long-term deformation at far lower stresses than dislocation creep.

The Lyngen Magmatic Complex (LMC) is the lowest unit of the Lyngsfjellet Nappe (Upper Allochthon, North Norwegian Caledonides). The fabrics of the LMC rocks range from undeformed to mylonitic. The undeformed rock is a gabbro-norite formed primarily by anorthite-rich (93%) plagioclase, enstatite, and augite. Two deformation events are distinguished in the LMC: (D1) an earlier shearing that has produced a N—S trending vertical foliation with sub-horizontal stretching lineation and dextral sense of shear, and (D2) a top-to-SE-directed thrust contact with the lower nappe series at the base of the meta-gabbro-norite. In the thrust contact region, the early vertical foliation is rotated into a flat-lying orientation and shows an ESE-trending stretching lineation. Deformed fabrics of D1 have developed successively from lower amphibolite, to epidote-amphibolite, and to greenschist metamorphic grades, i.e., on a retrograde temperature-path. The fabrics of the thrust contact have also developed from amphibolite to greenschist conditions.Rock fabrics associated to D1 are dominantly located in the northern portion of the LMC (from Lyngstuva to the north side of the Kjosen fjord). The amphibole compositions of these rocks vary from core to rim, showing a trend from pargasitic to actinolitic composition, consistent with the transition from high- to low-temperature (amphibolite to greenschist facies). U-Pb dating of titanite associated with the greenschist grade in meta-gabbro-norite assemblages indicates a date of 485±9 Ma. This date is interpreted as a deformation/metamorphic age, because the analysed titanite forms from pargasite breakdown and is aligned parallel to the deformed fabric. As this deformation event is synchronous with the crystallization age of the LMC (481±6 Ma, Augland et al., 2014), the deformation associated to the N—S oriented stretching lineation and vertical foliation is linked to sea floor strike slip movements during back-arc spreading of the LMC. D2-rock-fabrics are dominantly located in the southern portion of the LMC and represent typical structures of nappe stacking during the Scandian collisional stage of the Caledonian orogeny. Close to the lower boundary of the LMC, garnet-bearing amphibolites, allow refining the P and T conditions for this unit. Thermobarometric estimates result in conditions of 650°C and 10kbar. This temperature is in contrast with the Raman spectroscopy values averaging around 530°C for the graphite bearing sediments below the lower contact of the LMC, i.e. sediments between the meta-gabbro-norite and the underlying Reisa nappe. The temperature difference between the two deformation events indicates re-heating of the meta-gabbro-norite during the Scandian thrusting.The D1 structural relationships described in the LMC appears common for supra-subduction zone settings, and could potentially be observed at deeper mantle sections as reported in younger analogue tectonic settings as the Wadi al Wasit area of the Oman ophiolite. D2 appears linked to out-of-sequence thrusting at the base of the LMC with respect to the surrounding nappes, contributing to the north Norwegian Caledonides nappe transport sequence.

Grain size is a critical parameter for viscous deformation processes and controls the rheological behaviour (weakening and strain localizations in mylonites) of polycrystalline aggregates. Recently, Ghosh et al., (2022) used Tana quartzite (~200 μm) to develop a new grain-size-insensitive flow law (stress exponent, n = 2; activation energy, Q = 110 kj/mol). The n = 2 is interpreted to indicate the serial operation of grain interior (dislocation glide) and grain boundary (dissolution-precipitation, DPC + Grain boundary sliding, GBS) accommodation processes. It is shown that all the previous results obtained from coarse-grained (∼100 - 200 μm) quartz aggregates (high total strain, significant recrystallization) plot within a factor of ∼5 times the strain rate predicted by this flow law. Moreover, a number of earlier studies with fine-grained (3.6 to 12 μm) quartz reported an n-value range of 1.7 to 2.5 (Kronenberg and Tullis 1984; Fukuda et al., 2018), similar to the values obtained from coarse-grained samples. A deviation from the grain-size-insensitive creep-dominated process is expected at those grain-size ranges and poses the question: how much weakening is possible by grain-size-reduction within the continental crust?Using a Griggs-type apparatus, we deformed fine-grained (~ 3-4 μm) as-is and 0.1 wt.% H2O-added novaculites, representing a fully recrystallized shear zone material, at the same condition as coarse-grained Tana quartzite (~200 μm). The as-is novaculite is ~3.5 times stronger than the H2O-added novaculite. A similar strength difference (~4 times) was also observed between the as-is and H2O-added Tana quartzite. The H2O-added novaculite is ~4 times weaker than the H2O-added Tana. The as-is novaculite is ~4.5 times weaker than the as-is Tana quartzite. Thus, the addition of 0.1 wt.% H2O causes a similar order of weakening as the smaller grain size; the as-is novaculite has a similar strength as the data predicted from the H2O-added Tana flow law. The maximum strength difference (more than an order of magnitude) arises between the end-member samples i.e., the as-is Tana and H2O-added novaculite, which indicate the combined effect of grain size and H2O. The n (1.91 to 2.19) and Q (118 kj/mol) values measured from novaculites are similar to the Tana and the Tana flow law can express all the novaculite data within the error associated with the Griggs-type apparatus, only by manipulating the A-value of the flow law. Therefore, the n and Q values are largely grain-size-insensitive, while the strength differences due to grain size and added-H2O can be expressed by varying the A-value. Further, the similarity in the n and Q values indicates that the same deformation mechanisms (i.e., dislocation creep accommodated by grain boundary processes) operate in both materials. Microstructural analysis of the novaculite reveals the development of a core-shell structure of individual grains due to grain boundary migration (GBM). A combination of deformation mechanisms (dislocation glide, GBM, GBS) are mutually dependent and interact to achieve the bulk sample strain. The variations in A-value may reflect the efficiency of the GBS mechanism depending on grain size and H2O content. Finally, the implications of these results on crustal strength will be discussed.

The trigger mechanism of intermediate depth earthquakes (30 - 300 km) is a long-standing debate. Many studies showed that these seismic events nucleate along a double-seismic zone within the subducting slab. The seismic events of the lower plane coincide with the depth of major dehydration reactions in the lithospheric mantle. Consequently, it is thought that these events are related to the release of fluids. Several scenarios are currently discussed that might lead to brittle deformation. Among these are dehydration embrittlement and dehydration-driven stress transfer.Antigorite is one of the most important candidates for fluid release due to its high H2O content and stability limits within the lower Wadati-Benioff zone. The release of water through antigorite dehydration can be calculated by equilibrium thermodynamics and is mainly a function of temperature. This does, however, not account for deformation (e.g., stored internal strain energy) leading to local variations in the free energy of minerals. In this study we aim to explore the effect of shear stress on the stability of antigorite.We performed high-pressure and high-temperature experiments in a Griggs rig. We used intact drill cores of two different starting materials for our experiments: a foliated and an isotropic antigorite-serpentinite. Both starting materials did not contain relict olivine and/or orthopyroxene. We run our experiments at 620 to 650 °C with a confining pressure of 1.5 GPa and a strain rate of 10-6 s-2. Subsequent analyses of the experimental runs revealed no dehydration products within the bulk sample. However, we observed the formation of ultra-fine grained (< 100 nm) olivine and orthopyroxene along narrow zones, which are orientated 30 to 40 ° with respect to the compression axis. These zones are similar in all runs and independent of the starting material microstructure. We thus propose that shear stress localization within our cylindrical sample triggered the dehydration. Within subduction zones local variations in stress field due to mineralogical or textural heterogeneities could promote dehydration, eventually leading to seismic events through stress transfer.

According to well-established hypotheses based on field observations of natural faults, viscous deformation may localize following pre-existing brittle fractures. The weak behaviour can be explained by brittle grain size reduction and phase mixing, which may activate grain size sensitive processes in the viscous field. To prove this hypothesis, it is necessary to perform experiments to observe the strain and stress evolution in faulted and non-faulted rocks. Pec et al. (2012) performed experiments on granitic rocks by shearing manually crushed granitic powder between coarse solid granitic forcing blocks. However, in their study, there are unavoidable boundary conditions between the forcing blocks and the gouge, and a comparison to an intact rock without fracture is difficult.In our study, we reduce the boundary conditions to a minimum and can directly compare the stresses and microstructural evolution during deformation of intact and fractured granitic ultramylonites at 650°C, confining pressure of 1.2GPa, and a constant displacement rate of 10-8m/s. We perform these experiments on initially solid cylindrical samples in two experimental sets: In set A, we slowly apply the load and confining pressure, to ensure an intact rock sample is deformed. In set B, we create fractures before the experiment starts but already in the closed system of the experimental setup. Once experimental P/T conditions are reached, both experimental sets are deformed to different finite strains to investigate the associated microstructural evolution. The deformation is disseminated in the set A experiments, but localizes strongly along the fracture in experimental set B. The strain is accommodated by viscous granular flow incorporating an impressive grain size reduction of up to 1000x and dissolution/precipitation processes. In addition, the stress records show that in experiments A, initially a 30% higher yield stress has to be overcome before steady state flow, while in set B steady state flow is reached directly without a strain softening increment. In both sets, steady state stresses range around 300MPa, i.e. far below the confining pressure.Applying microstructural observations and mechanical data of our experiments to deformation of granitoid crust in nature reveals that fractures serve to reach mechanical steady state earlier compared to non-fractured crust. As a matter of strain, however, both settings may yield at the same mechanical strengths of resulting shear zones. It is important to note that polymineralic fine-grained ultramylonites are up to four times weaker than monomineralic quartz, presenting an important behaviour of efficient strain localization and rheological properties substantially below those of the end member minerals. Pec, M., Stünitz, H. and Heilbronner, R., 2012. Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures. Journal of Structural Geology, vol. 38, pp. 200-221. https://doi.org/10.1016/j.jsg.2011.09.

Quartz rheology in the presence of H2O is crucial for modelling (numerical and geophysical) the deformation behavior of the continental crust and gives important insights into crustal strength. Experimental studies in the past have determined stress exponent (n) values for flow law between ≤ 2 to 4, while the values for activation energy (Q) vary from ~120 to 242 kJ/mol. Here, we investigated the quartz rheology under high-pressure and high-temperature conditions, using a new generation hydraulically-driven Griggs-type apparatus. In order to develop a robust flow law for quartzite, we performed constant-load coaxial deformation experiments of natural coarse-grained (~ 200 μm) high purity (> 99 % SiO2) quartzite from the Tana quarry (Norway). Our creep tests were carried out at 750 to 900 °C on the as-is (no added H2O) and 0.1 wt.% of H2O added samples under 1 GPa of confining pressure. In contrast to earlier strain rate stepping experiments, the constant-load procedure needs lower strain at each step (≤1−2%) to achieve steady-state conditions. As a consequence, there is a very low amount of recrystallization. Importantly, we can determine the Q-value independently of the stress exponent (n). Microstructures from the deformed samples were characterized using polarized light microscopy (LM), SEM-cathodoluminescence (CL), and Electron backscatter diffraction (EBSD).Our creep results for both the as-is and 0.1 wt.% H2O-added samples yield Q = 110 kJ/mol and n = 2. Our microstructural analysis suggests that the bulk sample strain is accommodated by grain-scale crystal-plasticity, i.e., dislocation glide (dominantly in prism Our new flow law predicts strain rates that are in much better agreement with the inferred natural values than the earlier flow laws. It further suggests that the strength of the continental crust considering quartz rheology is significantly lower than previously predicted.

The rheology of mafic rocks buried at high to ultra-high-pressure conditions remains enigmatic. Minerals stable at these conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite). In the laboratory, they show plastic deformation only at very high temperature (e.g. > 1000°C for pyroxene and garnet). Yet, viscous shear zones in mafic rocks metamorphosed at amphibolite and eclogite-facies conditions are commonly reported in fossil collisional and subduction zones. These shear zones localize and accommodate large amounts of strain by weakening of the host rocks. This weakening is interpreted as being induced by a transition from grain size insensitive to grain size sensitive creep, in particular with the activation of the dissolution–precipitation creep. However, the exact interplay between deformation, mineral reaction and fluid/mass transfer remains poorly-known.We have conducted a first series of deformation experiments at eclogite-facies conditions on a 2-phase aggregate representative of mafic rocks. Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, and 2.1 GPa with a shear strain rate of 10⁻6 s⁻¹. The starting material consists of mixed powders with < 100 µm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with ‘as is’ (dried at 110°C) starting material and with 0.2% added water.The mechanical data indicate that the samples are first very strong with a peak differential stress at 1.4 GPa. Then, a significant weakening is observed with a stress decrease by 0.5 GPa. The high-strain sample is characterized by a strain gradient increasing toward the center of the shear zone. Metamorphic reactions occur throughout the sample, but the high-strain areas contain considerably more reaction products than the low-strain areas. The nucleation of new phases leads to a drastic grain size reduction and phase mixing, whose intensities are positively correlated with the strain intensity. The nature, distribution and fabric of the mineral products vary also progressively with the strain intensity.In the low-strain areas, dissolution-precipitation processes mainly occur along grain boundaries: plagioclase is rimmed by zoisite and a secondary plagioclase more albitic in composition while clinopyroxene is rimmed by amphibole. In the mid-strain areas, dissolution-precipitation processes are more pervasive: amphibole and a secondary more sodic clinopyroxene occurs in pressure shadows of primary clinopyroxene, while primary plagioclase is completely replaced by a fine-grained mixture of zoisite and quartz. Reaction products show a strong shape-preferred orientation parallel to the shear direction. In the high-strain areas, dissolution-precipitation leads to the nucleation of a fine-grained mixture of garnet and secondary clinopyroxene, quartz and kyanite. Most reaction products have subhedral shape with no clear preferred orientation. Hydrous minerals (amphibole and zoisite) are not observed. Our preliminary results indicate that strain at eclogite-facies conditions is preferentially accommodated and localized by dissolution-precipitation processes. Further micro-structural and geochemical analyses are required to quantify the exact interplay between the physical and chemical processes controlling the dissolution-precipitation creep.

Mafic rocks are a key constituent of the oceanic and lower continental crust. Strain localisation and fabric development in these rocks is controlled by the active deformation mechanisms. From studies of natural rocks it has been established that strain localisation and weakening in mafic rocks is directly related to fluid availability and resultant mineral reactions. Understanding the interplay between reactions, fluid availability, and deformation aids in quantifying the stresses and rates of deformation processes. We have conducted an experimental investigation to constrain the weakening mechanisms in gabbro. Shear experiments were performed in a Griggs-type apparatus at 800-900°C, and 1.2-1.5 GPa with a shear strain rate of 10⁻⁵s⁻¹. The starting material consists of mixed powders with

Rheological models of Earth’s granitoid mid- to upper crust are commonly based on the physico-chemical properties of the most abundant rock forming minerals quartz and feldspar. However, there is increasing field evidence that deformation in these rocks localizes in ultrafine-grained polymineralic shear zones, which are weaker than any of the end member minerals. Especially at the brittle to viscous transition, the localization and deformation mechanisms, i.e. the role of incipient brittle deformation vs. continuous viscous strain localization, is not yet fully understood.To fill this gap in knowledge, ultramylonite samples with granitic composition from the Central Aar Granite (Aar Massif, Central Switzerland) were deformed using a Griggs type apparatus. The foliation of the ultramylonitic starting material was oriented 45° to the compression direction, to investigate the influence of grain size and composition on strain localization in the different mylonite bands. Two types of coaxial experiments were conducted at 650°C, and 1.2 GPa confining pressure: A) Discrete fractures were created before the shear deformation starts; B) No fractures were induced during an early stage of the experiment.All experiments have in common that strain is accommodated in 20-100 µm wide viscous shear zones with elongated grains and minor grain size reduction. In these shear zones, most strain is further localized in 10-20 µm wide zones, showing dramatic grain size reduction down to few tens of nanometres. In the experimentally generated shear zones, both, brittle and viscous processes are active. In terms of overall rock strength, all newly formed ultrafine-grained shear zones are up to three times weaker than comparable experiments on pure quartz or coarser grained granites – which agrees well with field observations. Furthermore, pre-fractured type A) is up to two times weaker than the non-fractured type B), and the orientation and number of shear zones is also fundamentally different between the two experiment types.This study confirms two weakening factors promoting different types of strain localization at the brittle to viscous transition: 1) The existence of fractures and their interconnectivity – facilitating highly-localized grain size reduction; 2) Initial sample heterogeneity by polymineralic composition and ultrafine grain size – generating grain size reduction along strain gradients by activating viscous processes. Further quantitative microstructural analyses will reveal the role of chemistry and the deformation mechanisms on the localization behaviour.

The effect of composition on microstructural development and mechanical strength was tested using mica-quartz-aggregates during deformation experiments.This study used two chemically different biotite minerals mixed with quartz: (1) high F-phlogopite and (2) intermediate biotite in order to investigate the role of biotite-bearing systems for the development of shear zones and strain accommodation. Shear experiments (Griggs-type apparatus) were performed using mica (30 vol. %) and quartz (70 vol. %) assemblages at 750 and 800°C, 1000 MPa and a shear strain rate of ~10-5 s-1.Mechanical results for the F-phlogopite-bearing assemblage indicate strong samples, approximately equivalent to pure quartz samples (Richter et al., 2018), deforming at differential stresses of 764-1097 MPa). F-phlogopite flakes are preferentially oriented parallel to the main shear direction, but poorly interconnected. Most of the strain is accommodated by quartz behaving as an interconnected network. Cathodoluminescence imaging reveals that quartz recrystallizes mainly by local pressure-solution and its strength controls the overall strain accommodation.In contrast, intermediate biotite assemblages are significantly weaker and deform for lower differential stresses of 290-327 MPa, as expected for natural rocks. Biotite flakes form an interconnected network accommodating most of strain.The interconnectivity of biotite grains thus plays a major role in weakening quartz-biotite assemblages. However, at similar P-T-strain and grain size conditions, the capacity of biotite grains to interconnect may also depend on its chemical composition, particularly considering the effect of trace elements incorporation (as fluorine) on the strength of the biotite interlayer bounds (Dahl et al., 1996, Figowy et al., 2021). This led us to conclude that different types of mica, behaving differently, strongly affect strength, deformation mechanism, and microstructure of the rock due to their structure, composition and stability fields.

Natural quartzite samples, as-is and with 0.1 wt% of added H2O, have been deformed up to ∼30% bulk strain in axial shortening experiments with constant strain rate of ∼10−6 s−1 at 900 °C and 1 GPa, and in strain rate stepping ∼10−5 to ∼10−7 s−1 at 900 °C and 1–1.5 GPa, in order to investigate the role of H2O in deformation and recrystallization of quartz. H2O-added samples showed ∼30 MPa lower mean strengths than as-is samples. Samples weaken slightly after 15% strain with mean flow stresses in the range of 154–227 MPa, and stress exponent (n) values between 1.45 and 2.13. The original quartz grains have been deformed plastically (dislocation glide). Discrete mode I cracks without detectable offset have developed in addition to plastic strain. Deformation was associated with recrystallization of up to 20% of the material in the most deformed parts of the samples. New grains were nucleated by both cracking and subgrain rotation, and were largely reconstituted by H2O-promoted grain boundary migration, related to dissolution-precipitation processes. This reconstitution of material is documented by a change in luminescence to blue, caused by trace elements exchange in quartz structure. The blue luminescence is prominent along healed cracks and high angle grain boundaries while it was not observed along the low angle boundaries formed by subgrain rotation. Compared to the as-is samples, the crack-related recrystallization is more frequent in the H2O-added samples. The low stress exponent values may indicate dissolution-precipitation and grain boundary sliding processes to accommodate incompatibilities at grain boundaries arising from an insufficient number of active slip systems. We suggest that the ubiquitous presence of H2O in nature may promote recrystallization of quartz by combinations of cracking, dislocation glide and creep and dissolution-precipitation processes.

The mechanical properties of quartz are fundamental to control the plastic behaviour of the continental crust. Our understanding of quartz rheology is still limited in the following respects: i) the large variability of flow law parameters in the earlier literature (stress exponent n = 4 to ≤ 2 and activation energy Q = 120 to 242 kJ/mol), and ii) the difficulty to identify the rate-limiting deformation mechanism, if several mechanisms are operating simultaneously. These two issues are connected and cannot be resolved separately. The present study has carried out constant-load experiments to constrain the flow law parameters of quartz. A new generation hydraulically-driven Griggs-type apparatus has been employed, resulting in reproducible mechanical data, even at very low strain rates (10−8 to10−9 s−1; so far, closest to the natural ones). Furthermore, the Q-value in constant load experiments can be estimated without prior knowledge of the n value. Our new n (= 2) and Q values (= 110 kJ/mol) are fairly low. We calculated an A-value of 1.56 × 10−9 /MPa/sec. Microstructural analysis suggests that the bulk sample strain in our experiments is achieved by crystal plasticity, i.e., dislocation glide with minor recovery by sub-grain rotation, accompanied by grain boundary migration. Micro-cracking helps to nucleate new grains. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution-precipitation creep and grain boundary sliding. These grain boundary processes are responsible for the n-value that is lower than expected for dislocation creep ( 3). The new flow law can consistently estimate strain rates (especially at low stresses) in excellent agreement with documented natural case studies and predicts a rapid drop in strength of quartz-bearing rocks in the continental crust below a depth of ∼10 km or at a temperature of ∼300 °C and higher.

International audience; Quartzite samples of high purity with a grain size of ∼200 μm have been experimentally deformed by coaxial shortening in a solid medium apparatus at 900°C and at confining pressures ranging from 0.6 to 2 GPa. Most samples have been shortened by ∼30% with 0.1 wt.% added H2O. The samples deformed dominantly by crystal plasticity (dislocation creep), and there is a systematic decrease of flow stress with increasing confining pressure. Strain rate stepping tests yield stress exponents of n∼1.4. The strain determined from individual grain shapes matches that determined from bulk shortening. In addition to plastic strain, mode I cracks developed in all samples, principally in the grain boundary regions. Recrystallized material, visible through cathodoluminescence colours, forms by two mechanisms: (1) progressive subgrain rotation and (2) cracking, nucleating small new grains. After high-angle boundaries have been established, grain boundary migration takes place, and a distinction of new grains nucleation origin (subgrain rotation or cracking) is impossible. At higher pressure, there is more recrystallized material forming in the deformed samples, and it is inferred that the inverse pressure dependence of flow stress is caused by enhanced grain boundary migration at higher pressure, consistent with previous studies.

Mafic rocks consist of strong minerals (e.g. clinopyroxene, plagioclase) that can only be deformed by crystal plastic mechanisms at high temperatures (>800°C). Yet, mafic rocks do show extensive deformation by non-brittle mechanisms when they have only reached lower temperatures (~650°C). In many of such cases, the deformation is accommodated by an interaction of deformation with simultaneous mineral reactions. Here we show that dissolution-precipitation creep plays a major role in deformation of gabbro lenses at mid and upper amphibolite facies conditions.The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C, and depths of ∼26-34 km. The Kågen gabbro on south Arnøya is comprised of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and metamorphism from the low strain gabbro lenses to high strain mylonites at margins of the lenses. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene, amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the extension direction. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern NorwayHöpfl Stephan1, Konopásek Jiří1, Stünitz Holger1,2 Bergh G., Steffen1UiT Norges arktiske universitet, Institutt for geovitenskap, stephan.m.hopfl@uit.no 1Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway2Institut des Sciences de la Terre (ISTO), Université d’Orleans, Orleans 45100, France The Balsfjord Series is located in the central part of Troms–Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician–Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Tromsø Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Tromsø Nappe Complex features metamorphic peak ages of ca. 455–450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE–SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting. Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Tromsø Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Tromsø Nappe Complex, the Balsfjord series and the underlying RNC.

Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO’s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.

Amorphous materials are frequently observed in natural and experimentally produced fault rocks. Their common occurrence suggests that amorphous materials are of importance to fault zone dynamics. However, little is known about the physico-chemical impact of amorphous materials on fault rheology. Here we present deformation experiments on mafic fault rock, where amorphous material forms due to intense mechanical wear during the experiments. The experiments are run at temperatures from 300 to 600 °C, confining pressures of 0.5 or 1.0 GPa, and at constant displacement rates of (d˙ax) 2 ·10−7, 2 ·10−8 or 2 ·10−9 ms−1, resulting in bulk strain rates (γ˙) of ≈3 ·10−4, 3 ·10−5 and 3 ·10−6 s−1. At these conditions, the mafic rock material undergoes intense brittle deformation and cataclastic flow, but sample strength significantly decreases with increasing temperatures – a feature commonly attributed to viscous deformation processes. Microstructural analyses show that after an initial stage of homogeneous cataclastic flow, strain localizes into narrow (2–10 μm wide) ultra-cataclastic bands that evolve into amorphous shear bands. With the data presented in this research paper, we argue that the temperature sensitivity recorded in the mechanical data is caused by viscous deformation of the amorphous material. We suggest that with the formation of amorphous materials during brittle deformation, fault rheology becomes significantly temperature-sensitive. This has important implications for our understanding of fault strength and weakening due to the presence of amorphous materials. In addition, weak material along faults will lead to stress concentrations that may trigger seismic rupture.

Rock deformation experiments are used to compile mechanical data sets for minerals and rocks and to study microstructure and texture development.The Griggs apparatus, a solid medium piston cylinder machine was designed about 60 years ago to investigate rock deformation mechanisms and rheology at elevated confining pressures. In a typical experiment today, the confining medium is NaCl, with confining pressures up to 3 GPa, temperatures up to 1100°C, and displacement rates between 10-8 and 10-2 ms-1 (equivalent to strain rates of 10-7 to 10-3 s-1). In axial tests, the cylindrical samples are 12 to 15 mm long with a diameter of 0.625 mm. In shearing test, split cylinder assemblies are used with 0.5 to 1 mm thick samples introduced along the 45° pre-cut. Reasonable total strains are limited to 30% axial shortening or shear strains of gamma 4. (Higher strains can be attained but are difficult to analyse mechanically. Unlike for gas rigs, torsion is not available for solid medium machines).As of now, the operational fleet of solid medium deformation apparatus comprises worldwide over 20 machines in different labs (mainly in Europe, U.S.A. and Japan), providing the scientific community with an ever-growing rheological and microstructural data base.In view of numerous developments in experimental design, as well as improvements of hardware and software for data acquisition and processing, the experimental community was recently invited to a two-day workshop, hosted by the experimental group of Orléans University. The main goal was to discuss the following points:- how to further improve the apparatus, increase its scope and improve calibrations; - how to further improve data processing, and the precision and reliability of the results;- how to maintain consistency among the labs and through time (backwards compatibility);- how ensure compatibility of results from axial and shearing experiments;- how to make the data available to the community.The poster represents a condensed report of the meeting highlighting a few issues of special interest to the structural community. Visitors to the poster are invited to share their thoughts and to give us feed-back.

Strain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: amphibole, garnet and zoisite. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

The ability of water to enhance plastic deformation of a quartz aggregate has been experimentally demonstrated during the sixties (e.g. Griggs and Blacic 1965), however the processes involved are still questioned. Notably the processes combining the effect of water and pressure during the deformation are still not completely understood. Pressure strongly influences the strength of fine-grained (3.6 - 4.9 µm) wet quartz aggregates (Kronenberg and Tullis 1984), where diffusion creep operates (Fukuda et al. 2018) but its effect on coarser-grained material expected to deform only by dislocation creep is not well constrained. To re-assess the effect of pressure on quartz crystal plastic deformation, natural wet quartzite samples from the Tana quarry in northern Norway (grain size ≈ 150 µm) have been deformed using a Griggs-type apparatus at varying confining pressures (from 0.6 to 2.0 GPa). All the samples with 0.1 wt. % H2O added were shortened coaxially up to 30% strain at constant strain rate (≈10-6 s-1) and temperature (900°C).All mechanical records show that quartzite flow stresses decrease systematically with increasing pressure. These results allow to determine the strength of quartzite as a function of water fugacity, such as introduced in the flow law by Kohlstedt et al. (1995) to account for both pressure and water effects. In our case, the fugacity coefficient is m≈1 when using a stress exponent of n=2.Microstructure and image analyses of samples reveal that the bulk strain results mainly from crystal plastic deformation of original grains whereas the recrystallization processes are limited volumetrically (less than 5%) and restricted to the boundaries of original grains. Deformation is not strongly partitioned into recrystallized domains compared to flattened original grains. Optical and SEM-cathodoluminescence images revealed the presence of cracks in conjunction with recrystallization (even for high-pressure samples) and associated chemical/fluid interaction, but the cracks do not contribute significantly to the bulk strain of the samples.In order to determine the amount of water used for the deformation and the redistribution of H2O during deformation, the H2O content of the quartzite has been calculated from FTIR (Fourier Transform InfraRed spectroscopy) measurements for both, grain interiors and grain boundaries. The H2O concentrations decrease inside grains with the onset of deformation with respect to the starting material. H2O is primarily stored in the grain boundary region. There is no systematic correlation with pressure. Thus, pressure dependence of H2O weakening is not restricted to fine-grained materials at high pressure and temperature. Deformation redistributes water from the grain interiors to their grain boundaries.References:Fukuda, J., Holyoke III, C.W., and Kronenberg, A.K. (2018). J. Geophysical Res.: Solid Earth, 123(6), 4676-4696.Griggs, D. T., and Blacic J. D. (1965). Science, 147(3655), 292‑295.Kohlstedt, D. L., Evans B., and Mackwell S. J. (1995). J. Geophysical Res.: Solid Earth, 100(B9), 17587-17602.Kronenberg, A. K., and Tullis J. (1984). J. Geophysical Res.: Solid Earth, 89(B6), 4281‑4297.

Since quartz is among the most abundant minerals in continental crust and one of the first to show plasticity with increasing pressure and temperature, understanding its mechanical behavior is crucial for estimates on crustal strength and modeling of geodynamic processes. Since discovery of significantly lower mechanical strength of quartz as a consequence of H2O presence in the crystal (Griggs & Blacic, 1965), remarkable amount of work has been done in order to improve knowledge about processes and mechanisms responsible for so called H2O weakening effect. As the weakening effect depends on molecular H2O, it is a disequilibrium weakening process that is difficult to incorporate into existing flaw laws.In order to estimate mechanical behavior of quartz in presence of H2O, we performed deformation experiments in solid-medium Griggs-type apparatus in coaxial setting under controlled laboratory conditions using very pure natural quartzite from Tana quarry in northern Norway. Behavior of as-is and 0.1 wt% H2O added samples was studied in 1) eight shortening experiments at 900 °C, 1 GPa and constant strain rate of 10-6 s-1 reaching 5% and 30% strain, 2) six strain rate stepping experiments covering 10-5, 10-6 and 10-7 s-1, 3) two temperature stepping experiments covering 750, 850 and 950 °C and 4) one hot-pressing experiment maintaining the starting experimental conditions for 14 hours.There is a negligible strength difference between the as-is and H2O added samples. Both H2O added and as-is strain rate steeping experiments had shown surprisingly low stress exponent, with the highest value of 2.26. Temperature stepping experiments gave activation energy values of 177 kJ/mol and 198 kJ/mol. In all studied samples, strain increases towards the sample centers exhibiting grain size decrease from initial 250 – 300 µm. Three principal deformation mechanisms contributing to the bulk strain were identified: 1) crystal plasticity of original grains manifested by flattening, undulatory extinction, and development of subgrains, 2) cracking of original grains demonstrated by fluid inclusion trails and minor grain offset and 3) dynamic recrystallization via subgrain rotation recrystallization indicated by misorientation analysis from EBSD data. FTIR spectroscopy was applied to evaluate H2O speciation, quantity and distribution. Regardless of added H2O, most of deformed original grains showed relative H2O concentration between 0 and 400 H/106Si, implying significant decrease of H2O content from original 600 to 2000 H/106Si measured in undeformed grains. Average H2O concentration in grain boundaries showed 750 H/106Si for as-is samples and 1300 H/106Si for H2O added. Plasticity is most visible in CL-images, as well as higher degree of grain fragmentation and crack density in samples with added H2O. Ubiquitous presence of fluid along the grain boundaries, demonstrated by FTIR results, may have facilitated sliding along grain boundaries which, in turn, could explain the low stress exponent derived from strain rate stepping experiments.REFERENCES:Griggs, D. T. & Blacic, J. D. (1965): Quartz: Anomalous Weakness of Synthetic Crystals. Science 147(3655):292-95.

To investigate the role of strong and weak secondary phases on the recrystallized grain size of quartz, we performed grain size analyses on quenched samples from general shear experiments on quartz-garnet and quartz-muscovite mixtures. Six general shear experiments were conducted in the Griggs apparatus; three with mixtures of quartz-garnet (vol.% garnet 5, 15, 30) and three with mixtures of quartz-muscovite (vol.% muscovite 5, 10, 25). The starting powders for both set of experiments were synthetic mixtures of quartz-muscovite or quartz-garnet with 0.1 wt.% water added. The quartz-garnet experiments were conducted at 900°C, a pressure of 1.2 GPa, and a shear strain rate of ~10-5 s-1, while the quartz-muscovite experiments were conducted at 800°C, a pressure of 1.5 GPa, and a shear strain rate of ~10-5 s-1. At these deformation conditions quartz is stronger than muscovite and weaker than garnet. We observed that the bulk strength of the aggregate decreases with a greater volume percent of muscovite and increases with a greater volume percent of garnet. Garnet at these conditions does not deform plastically. The presence of secondary phases within the deforming aggregate causes stress concentrations and partitioning of strain rate between the different phases relative to the measured bulk stress and strain rate. The degree of partitioning is primarily related to the rheology and volume percent of the phases. Due to the piezometric relationship between recrystallized grain size and stress, we can use the quartz recrystallized grain size to determine the local stress of quartz in the experiments and compare it to the measured bulk stress. The results from these analyses will provide new insight into the effect of strain partitioning in general and of strong and weak secondary phases on quartz rheology.

Strength profiles through the crust and upper mantle typically show the brittle to viscous transition as a change in deformation mechanism from frictional sliding to crystal plastic (dislocation creep) mechanisms. Even though such a change may conceivably take place, experimental evidence and natural observations indicate that a transition from semi-brittle to diffusion creep mechanisms rather than dislocation creep is more common.In experiments we have carried out on granitoid and mafic rock material we can distinguish 3 main processes for the brittle to viscous transition: (1) Grain size comminution by cracking produces a sufficiently small grain size (sub-micron) to cause a switch to diffusion creep. (2) Amorphous material forms (aseismically) from mechanical wear at high stresses (high dislocation densities or high work rate) without melting. The amorphous material is observed to be weak and deforms viscously. (3) Nucleation of new minerals as a consequence of metastability of existing minerals at given P,T, fluid-conditions produces fine-grained and well-mixed aggregates causing a switch to diffusion creep as in (1).The viscously deforming part of the crust or upper mantle is not the region where most earthquakes occur, because low stresses commonly are associated with viscous deformation. However, the transitions observed in experiments described above are transformational processes the material progressively evolves over a period of time in terms of microstructure, grain size, and/or composition, i.e., they are deformation-history-dependent transitions. In other words, during the transformation, only parts of the material deform by viscous processes while others have not evolved and are still brittle (and stronger). The bulk material strength of partially transformed rock depends on the connectivity of the weaker transformed material. The weaker material causes stress concentrations at the tips of transformed zones. The coalescence of transformed zones and/or a sufficiently large amount of transformed material is expected to cause catastrophic failure and thus seismic rupture. In such a way, transformation to viscously deforming weaker material may cause seismic behavior rather than according to the conventional view, where material properties change as a result of seismic deformation first, leading to creep.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. In order to study the mutual effect of deformation and mineral reactions, we have conducted shear experiments on fine-grained plagioclase-pyroxene assemblages in a Griggs-type solid-medium deformation apparatus. Experiments were performed at a constant shear strain rate of 10−5 s−1, a confining pressure of 1 GPa and temperatures of 800, 850 and 900 °C. While the peak stress of plagioclase + orthopyroxene assemblages reaches values between those of the end-member phases, the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase (plagioclase). This weakening correlates with the coeval development of high-strain shear zones where new phases are preferentially produced, including new pyroxene, plagioclase and amphibole. The reaction products mostly occur as intimately mixed phases within fine-grained and interconnected shear bands, together with different compositions with respect to the starting material. This indicates that deformation significantly enhances the kinetics of mineral reactions, which in turn strongly weaken the deforming sample, here attributed to a switch to grain-size-sensitive diffusion creep through phase nucleation and grain size reduction. Such an interplay between deformation and mineral reactions may have strong implications for the initiation, development, and durability of shear zones in the lower crust.

Unaltered mafic rocks consist of mechanically strong minerals (e.g. pyroxene, plagioclase and garnet) that can be deformed by crystal plastic mechanisms only at high temperatures (>800°C). Yet, many mafic rocks do show extensive deformation by non-brittle mechanisms when they have been subjected to lower temperature conditions. In such cases, the deformation typically is assisted by mineral reactions. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro exposed on south Arnøya is comprised of almost undeformed gabbro lenses with sheared margins wrapping around them. The shearing has taken place at temperatures of 690 ± 25 °C and pressures of 1.0 to 1.1 GPa. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene (with different composition), amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the stretching direction. Synchronous deformation and mineral reactions of clinopyroxene suggest that mafic rocks can become mechanically weak during a general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation; the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses and low stress exponents.

Natural quartz single crystals were experimentally deformed in two orientations: (1) ⊥ to one prism plane and (2) in O + orientation at 900 and 1000°C, 1.0 and 1.5 GPa, and strain rates of ~1 × 10 A6 s A1. In addition, hydrostatic and annealing experiments were performed. The starting material was milky quartz, which consisted of dry quartz with a large number of fluid inclusions of variable size up to several 100 μm. During pressurization fluid inclusions decrepitated producing much smaller fluid inclusions. Deformation on the sample scale is anisotropic due to dislocation glide on selected slip systems and inhomogeneous due to an inhomogeneous distribution of fluid inclusions. Dislocation glide is accompanied by minor dynamic recovery. Strongly deformed regions show a pointed broad absorption band in the ~3400 cm A1 region consisting of a superposition of bands of molecular H 2 O and three discrete absorption bands (at 3367, 3400, and 3434 cm A1). In addition, there is a discrete absorption band at 3585 cm A1 , which only occurs in deformed regions and reduces or disappears after annealing, so that this band appears to be associated with dislocations. H 2 O weakening in inclusion-bearing natural quartz crystals is assigned to the H 2 O-assisted dislocation generation and multiplication. Processes in these crystals represent recycling of H 2 O between fluid inclusions, cracking and crack healing, incorporation of structurally bound H in dislocations, release of H 2 O from dislocations during recovery, and dislocation generation at very small fluid inclusions. The H 2 O weakening by this process is of disequilibrium nature because it depends on the amount of H 2 O available.

International audience; The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal mis-orientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

Grain growth of quartz was investigated using two quartz samples (powder and novaculite) with water under pressure and temperature conditions of 1.0–2.5 GPa and 800–1100 ∘C. The compacted powder preserved a substantial porosity, which caused a slower grain growth than in the novaculite. We assumed a grain growth law of dn-d0n=k0fH2Orexp⁡(-Q/RT)t with grain size d (µm) at time t (seconds), initial grain size d0 (µm), growth exponent n, a constant k0 (µmn MPa−r s−1), water fugacity fH2O (MPa) with the exponent r, activation energy Q (kJ mol−1), gas constant R, and temperature T in Kelvin. The parameters we obtained were n=2.5±0.4, k0=10-8.8±1.4, r=2.3±0.3, and Q=48±34 for the powder and n=2.9±0.4, k0=10-5.8±2.0, r=1.9±0.3, and Q=60±49 for the novaculite. The grain growth parameters obtained for the powder may be of limited use because of the high porosity of the powder with respect to crystalline rocks (novaculite), even if the differences between powder and novaculite vanish when grain sizes reach ∼70 µm. Extrapolation of the grain growth laws to natural conditions indicates that the contribution of grain growth to plastic deformation in the middle crust may be small. However, grain growth might become important for deformation in the lower crust when the strain rate is −12 s−1.

International audience; Strain localization in viscously deformed rocks commonly results in fine-grained shear zones where massive fluid circulation is regularly observed. Recently attributed to strain-induced pumping, this phenomenon may have major implications for the distribution of ores deposits and rock rheology. However, although grain size reduction and/or creep cavitation have been proposed as important processes, the source mechanism of fluid concentration remains unresolved, particularly at high pressure. Here we use secondary ion mass spectrometry to document the H 2 O content of fine-grained olivine across an experimental shear zone, which developed with grain size reduction during a H 2 o-saturated shear experiment at 1.2 GPa and 900 °C. Through data interpolation, the olivine matrix reveals high fluid concentrations where shear strain is localized. These concentrations far exceed the predicted amount of H 2 o that grain boundaries can contain, excluding grain size reduction as a unique source of water storage. Instead, we show that H 2 o increases per unit of grain boundary across the shear zone, suggesting that cavitation and "healing" processes compete with each other to produce a larger pore volume with increasing strain rate. This provides an alternative process for fluids to be collected where strain rate is the highest in deep shear zones. On Earth, both the crust and mantle incorporate aqueous fluids that interact with solid rock materials in many ways. Commonly referred to as fluid-rock interactions, they strongly affect rock deformation and petrogenetic processes, giving rise, for instance, to hydrolytic weakening, pressure-solution creep or metamorphic reactions , including partial melting. During strain localization, this type of interactions may involve one or several chemo-physical processes that help to channelize fluid flow in ductile shear zones where grain size is substantially reduced. When rocks deform by sub-solidus viscous creep, strain indeed partitions into fine-grained shear bands that recurrently develop in the presence of massive fluid circulation, as revealed by the enrichment of hydrous phases 1-4 in the shear zones. Although the source process of such fluid flow localization is unknown at present, it may have critical implications for rock mechanics and distribution of ore deposits in deep Earth environments 5. Primarily attributed to seismic pumping of a pre-existing fault 1 , fluid infiltration is commonly inferred using evidence for dissolution-precipitation, which suggests a long-term process rather than a co-seismic one 3,5-7. Rutter and Brodie 8 and Wark and Watson 9 proposed that high fluid contents could occur in ductile shear zones as a result of fluid permeation in response to grain size reduction; because of high pressure and high temperature, supercritical fluids may distribute along grain boundaries as a uniform boundary film 9. However, recent observations of syn-tectonic water accumulation along mantle shear bands 4 do not support such a "passive" process for attracting fluids towards ductile shear zones. Instead, they suggest that fluids are dynamically driven by the deformation itself. There is abundant documentation of micro-pores produced during deformation of fine-grained material, including within natural shear zones in the middle/lower crust 5,7,10-15 and during deformation experiments on ceramics, metals and natural rocks 16-20. Also referred to as creep cavitation, the production of these micro-pores results either from grain boundary sliding (GBS) or, in a minor extent, from Zener-Stroh cracking if dislocations interact with grain boundaries 17. In both cases, the micro-cavities arise from limitations of the material to flow, particularly when diffusive mass transfer is slow at low temperature. This led several authors to propose that

International audience; Coesite was found in quartz aggregates, experimentally deformed at confining pressures of 1.0-1.5 GPa and temperatures between 600 °C and 900 °C. The confining pressure (Pc) and, in most cases, the mean stress (σm) of the experiments were below those of the quartz-to-coesite phase transformation. Yet, coesite formed when the maximum principal stress (σ1) was within the P-T range of the coesite stability field. In one sample, the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz. It is inferred that this sample started to deform with σ1 above the quartz-to-coesite phase transformation and, with on-going deformation, σ1 decreased to values in the quartz stability field due to strain weakening. In all cases, σ1 triggered the quartz-to-coesite reaction as well as the reverse reaction, suggesting that σ1 is the critical parameter for the quartz-to-coesite transformation - not Pc or σm. With progressive deformation, the coesite laths rotated towards the shear plane as more rigid particles with the sense of shear. In case of back reaction, new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite. The experiments cover different degrees of pressure ‘overstepping’, different temperatures, and different experimental durations at P and T and deformation always enhances the reaction kinetics. The observation that σ1 is critical for a pressure-dependent phase transformation (also for reversals) poses questions for the thermodynamic treatment of such phase transformations.

Small-scale shear zones within the Permian Truzzo meta-granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 μm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/10 6 Si and very little broad band absorption

Field studies and seismic data show that semi-brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so-called frictional - viscous transition. To understand the physical and chemical processes accommodating semi-brittle flow, we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space (temperature, T = 300, 400, 500 and 600°C, confining pressure, Pc ≈ 300, 500, 1000 and 1500 MPa, shear strain rate, γ ≈ 10-3, 10-4, 10-5 and 10-6 s-1, to finite shear strains, = 0 - 5). The experiments have been carried out using a granular material with grain size smaller than 200 μm with a little H2O added (0.2wt%). Only two experiments (performed at the fastest strain rates and lowest temperatures) have failed abruptly right after reaching peak strength ( ~ 1400 MPa). All other samples reach high shear stresses ( ~ 570 – 1600 MPa), then weaken slightly (by Δ ~ 10 – 190 MPa) and continue to deform at a more or less steady state stress level. Clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed. In order to express this relationship, the strain rate – stress sensitivity has been fit with a stress exponent, assuming γ  n and yields high stress exponents (n ≈ 10 - 140), which decrease with increasing temperature. The microstructures show widespread comminution, strain partitioning and localization into slip zones containing, at first, nanocrystalline and partly amorphous material, later, during continued deformation, fully amorphous material in slip zones. Despite the mechanical steady state conditions, the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below γ = 5. Within the slip zones, the fault rock material progressively transforms from a crystalline solid to an amorphous material. We present and interpret the experimental results both in terms of sliding friction and viscous flow, and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation.

This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~ 760–790 °C and ~ 9.4–11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730 °C and 9.5–10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610 °C and 5.5–6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605 °C and 9.2–10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439 ± 1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640 ºC and 11.7–13 kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~ 440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

Fluid inclusions in quartz are known to mod- ify their shapes and microstructures (textures) during weak plastic deformation. However, such changes have not been experimentally demonstrated and criteria are not available to relate them to paleostress conditions. To address these issues, quartz crystals containing natural CO2-H2O-NaCl fluid inclusions have been experimen- tally subjected to compressive deviatoric stresses of 90-250 MPa at 700C and *600 MPa confining pres- sure. Strains of up to 1% cause the inclusions to develop irregular shapes and to generate microcracks in crystal- lographic planes oriented subperpendicular to the major compression axis, r1. The uniform alignment of the microcracks imparts a planar fabric to the samples. The microcracks heal and form swarms of tiny satellite inclusions. These new inclusions lose H2O by diffusion, thereby triggering plastic deformation of the surrounding quartz via H2O-weakening. Consequently, the quartz samples deform plastically only in domains originally rich in inclusions. This study shows that fluid inclusions deformed by deviatoric stresses may indeed record information on paleostress orientations and that they play a key role in facilitating crystal-plastic deformation of quartz.

While it is widely observed that mafic rocks are able to exeprience high strains by viscous flow, details on their rheology and deformation mechanisms are poorly constrained. Here, rock deformation experiments on four different, water-added plagioclase-pyroxene mixtures are presented: (i) plagioclase(An60-70) – clinopyroxene – orthopyroxene, (ii) plagioclase(An60) – diopside, (iii) plagioclase(An60) – enstatite and (iv) plagioclase(An01) – enstatite. Samples were deformed in general shear at strain rates of 3 × 10−5 to 3 × 10−6 s−1, 800 °C and confining pressure of 1.0 or 1.5 GPa. Results indicate that dissolution-precipitation creep (DPC) and grain boundary sliding (GBS) are the dominant deformation mechanisms. Coinciding with sample deformation, syn-kinematic mineral reactions yield abundant nucleation of new grains; the resulting intense grain size reduction is considered crucial for the activity of DPC and GBS. In high strain zones dominated by plagioclase, a weak, non-random and geometrically consistent crystallographic preferred orientation (CPO) is observed. Usually, a CPO is considered a consequence of dislocation creep, but the experiments presented here demonstrate that a CPO can develop during DPC and GBS. This study provides new evidence for the importance of DPC and GBS in mid-crustal shear zones within mafic rocks, which has important implications on understanding and modelling of mid-crustal rheology and flow.

The mechanism of shear zone formation in lower crustal, relatively “dry” rocks is still poorly understood. We have studied the high-temperature deformation of the Hasvik gabbro (northern Norway) which commences by fracturing. The 10–20 µm wide fractures show little displacement. The fine-grained plagioclase and orthopyroxene in the fractures lack a crystallographic preferred orientation (CPO) or a systematic crystallographic orientation with respect to the host grains. Fractures grade into narrow shear zones, which are composed of fine (10–20 µm), equant grains of recrystallized plagioclase, amphibole, and pyroxene. Recrystallized plagioclase and pyroxene have compositions different from the magmatic grains, suggesting that they have formed by nucleation and growth. Based on conventional plagioclase-amphibole thermobarometry, the shear zones have formed at temperatures and pressures of 700–750°C and 0.5–0.6 GPa. The observed primary minerals cut by fractures suggest high-temperature fracturing in the absence of high pore pressures, which implies a high strength of the lower crustal gabbros and high stresses at fracturing. The shear zones are characterized by the lack of CPO and a small grain size, suggesting that the mechanism of deformation of the fine-grained plagioclase and orthopyroxene has been grain boundary sliding accommodated by diffusive mass transfer. The amphibole grains have strong CPOs, which most likely result from oriented growth and/or rigid body rotations during deformation. The process that initiated the fracturing and subsequent viscous creep in the Hasvik gabbro may have resulted from a process of coseismic loading followed by creep during decaying stress in the lower crust.

This study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high-U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high-grade metamorphism, or represent partially reset ages due to high-T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1-forming event lost its Zr-in-rutile and U–Pb signatures during the S2-forming event. It records Zr-in-rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr-in-rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.

Shear zones channelize fluid flow in 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 (temperature = 700–730 °C, pressure = 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

Accepted manuscript version, licensed CC BY-NC-ND 4.0. Published version available at https://doi.org/10.1016/j.jsg.2018.05.021. Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10−5s−1 using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H2O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) “wet” porphyroclasts (grain size ∼100–200 μm), containing initially a high density of μm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ∼3 to 4.5), in order for the microstructures and H2O distribution to approach some state of “equilibrium”. The H2O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H2O content, while H2O content of initially dry clasts increases during deformation. H2O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H2O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H2O storage capacity of the grain boundary network is determined as a H2O-boundary-film of ∼0.7 nm thickness.

International audience; In order to address geological processes at great depths, rock deformation should ideally be tested at high pressure (> 0.5 GPa) and high temperature (> 300 °C). However, because of the low stress resolution of current solid-pressure-medium apparatuses, high-resolution measurements are today restricted to low-pressure deformation experiments in the gas-pressure-medium apparatus. A new generation of solid-medium piston-cylinder ("Griggs-type") apparatus is here described. Able to perform high-pressure deformation experiments up to 5 GPa and designed to adapt an internal load cell, such a new apparatus offers the potential to establish a technological basis for high-pressure rheology. This paper provides video-based detailed documentation of the procedure (using the "conventional" solid-salt assembly) to perform high-pressure, high-temperature experiments with the newly designed Griggs-type apparatus. A representative result of a Carrara marble sample deformed at 700 °C, 1.5 GPa and 10-5 s-1 with the new press is also given. The related stress-time curve illustrates all steps of a Griggs-type experiment, from increasing pressure and temperature to sample quenching when deformation is stopped. Together with future developments, the critical steps and limitations of the Griggs apparatus are then discussed.

International audience; Shear experiments on quartz gouge were performed at elevated confining pressures (predominantly 1.5 GPa) and temperatures (500 °C - 1000 °C) at shear strain rates of 3.5·10−6 s−1 to 2·10−3 s−1 to study the brittle-to-viscous transition. An unsystematic temperature dependence of strength at low temperatures changes towards a clear temperature weakening dependence above 650 °C. The transition from a pressure strengthening to a pressure weakening relationship takes place continuously between 650 °C and 800 °C. Strain rate stepping experiments reveal power-law breakdown at low temperatures (∼650 °C). Between 800 °C and 1000 °C, a stress exponent of n = 1.9 ± 0.6 and an activation energy of Q = 170 ± 72 kJ/mol indicate a combination of diffusion and dislocation creep. The Goetze criterion is confirmed as the upper stress limit for viscous deformation mechanisms. Localised deformation in the form of semibrittle shear bands with Riedel geometry at low temperatures changes to homogeneous deformation with a pervasive foliation accompanied by a continuous texture evolution between 700 °C and 1000 °C. Fracturing dominates at low temperatures accompanied by increasing amounts of dissolution and precipitation in fine-grained zones with increasing temperature. Above 650 °C, dislocation and diffusion creep are the dominating deformation processes, with dislocation creep being favoured in larger grains while dissolution-precipitation is active in the fine-grained fraction.

An edited version of this paper was published by AGU. Copyright (2017) American Geophysical Union. Source at https://doi.org/10.1002/2017JB014333. To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron‐rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas‐medium deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine‐grained olivine‐pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.

Palaeozoic–Mesozoic brittle normal faults onshore along the SW Barents Sea passive margin off northern Norway give valuable insight into fault and fluid flow processes from the lower brittle crust. Microstructural evidence suggests that Late Permian–Early Triassic faulting took place during multiple phases, with initial fault movement at minimum P–T conditions of c . 300 °C and c . 240 MPa ( c . 10 km depth), followed by later fault movement at minimum P–T conditions of c . 275 °C and c . 220 MPa ( c . 8.5 km depth). The study shows that pore pressures locally reached lithostatic levels (240 MPa) during faulting and that faulting came to a halt during early (deep) stages of rifting along the margin. Fault permeability has been controlled by healing and precipitation processes through time, which have sealed off the core zone and eventually the damage zones after faulting. A minimum average exhumation rate of c . 40 m Ma −1 since the Late Permian is estimated. It implies that the debated Late Cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation, which caused subsequent isostatic recalibration and uplift of the marginal crust. The studied faults may be used as analogues of basement-involved fault complexes offshore, revealing details about the offshore nature of faulting, including past and present basement and fault zone permeability.

To investigate the relationships between deformation, cracking, and partial melting in the lower continental crust, axial compression and hydrostatic experiments were performed on K-feldspar single crystals at temperatures of 700° and 900°C and confining pressures between 0.75 and 1.5 GPa. Sample deformation was carried out at a constant strain rate of ~ 10−6 s−1. The samples deformed at 700°C show typical brittle behavior with formation of conjugate fractures and peak stresses that increase with confining pressure. Samples deformed at 900°C show formation of shear fractures, peak stresses below the Goetze criterion, and inverse confining pressure dependence of peak stress, indicating that along the fractures deformation was not dominantly friction controlled. Microstructural and chemical analyses reveal the presence of melt (

[1] We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnoya, Norway. Over a distance of ∼1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (∼1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

International audience; For the past decades, grain size reduction leading to diffusion creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle. However, the mechanisms of grain size reduction in this regime are still poorly understood (e.g., Platt, 2015). Here we show new experimental results that document grain size reduction and material weakening during wet olivine diffusion creep. While occurring for both, mono-phase and two-phase aggregates, grain size reduction is coeval with strain localisation and local phase mixing in olivine–pyroxene aggregates. Based on evidence of fluid inclusions and cracks filled with a fine-grained phase mixture, we conclude that grain size reduces as a result of fluid-assisted nucleation that takes place in the presence of an aqueous fluid during diffusion creep. Cavitation induced by grain boundary sliding (creep cavitation) can be inferred, and may play a critical role for olivine grain size reduction. Amongst their implications for rock rheology in general, our findings highlight a key process for strain localisation in the ductile uppermost mantle.

Recrystallization of perthites in granulite facies ( T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with diffusion creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during diffusion creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive creep, resulting in strain localization.