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The origin of intermediate-depth seismicity in subducting oceanic lithosphere is still debated. A key for interpretation is provided by deep-seated pseudotachylytes (quenched frictional melts produced during seismic slip along a fault), exhumed counterparts of the actual deep seismicity, that can record relevant information on the seismic processes at hypocenter depths. Pseudotachylytes crosscutting the dry ophiolitic peridotite/gabbros of Moncuni (Lanzo ultramafic Massif, W. Alps)[1] have been interpreted to have formed at intermediate-depth (ca. 70 km) conditions under high differential stress and proposed as an analogue for the lower plane of the double seismic layer of subducting plates.Moved by these observations, we investigated by numerical simulations the potential of a subducting dry slab to achieve the high differential stress required for brittle failure in absence of fluid-mediated embrittlement during the plate bending and unbending. We performed pseudo-2D thermo-mechanical simulations of free subduction of a dry slab considering a visco-elasto-plastic rheology. We tested a homogeneous dry plate and a dry plate with weak circular inclusions representing partially hydrated volumes in the first 40 Km of the slab. The effect of low temperature plasticity (LTP) in olivine was also tested. In the unbending portion of the subducting slab the stress field describes two arcs - the outer one in compression and the inner one in extension - matching the two planes of seismicity. However, the homogeneous slab can only reach a differential stress of around 1 GPa, that is not high enough for triggering earthquakes. The presence of weak inclusions, with degraded elastic properties, but still high viscosity, induces a local amplification of the stress field. Differential stresses in excess of 4 GPa are obtained considering inclusions with a shear modulus decreased by 60-70% relative to the surrounding material but similar viscosity. Increase of the spatial density of inclusions determines a general increase of stress due to local interactions of the stress fields. The LTP of olivine, when considered in the simulations, introduces a stress cut off hampering the differential stress around weak inclusions to rise above 1.5 GPa. However, if the effect of pressure and strain hardening are considered, differential stresses above 3 GPa are achieved, that are high enough for brittle failure at intermediate-depth conditions. The modeled slab with scattered weak inclusions is compatible with a dry and strong peridotitic mantle with partially serpentinized domains, most likely related to faulting during slab bending.Our results show that brittle failure can occur at intermediate depths during subduction in relatively dry rocks, confirming the hypothesis developed from field interpretations. Further advanced microstructural investigations (e.g. TEM, HR-EBSD) on selected mantle pseudotachylytes, as well as on experimental analogues, can help to better understand the behavior of intermediate-depth earthquakes, hopefully providing new insights into the processes of stress accumulation and release during the seismic cycle. [1]: Pennacchioni et al., 2020, Record of intermediate-depth subduction seismicity in a dry slab from an exhumed ophiolite, Earth Planet. Sc. Lett. 548, 11649

Recognition of seismically induced microstructures is important to unravel the different deformation processes during seismic cycles, especially at the base of the upper crust where many earthquakes nucleate. Deformed quartz veins related to a strike-slip shear zone within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intense kinking in elongated quartz grains. The kink band boundaries are inclined into the general dextral sense of shear. Cathodoluminescence (CL) images reveal that the entire thin section contains a very high density of intragranular, sub-planar microstructures developed as thin dark CL lamellae accompanied with nanometre-scale fluid inclusions. Based on the oscillating orientation variation across low angle boundaries (misorientation angle 1-9°) these lamellar microstructures are referred as short-wavelength undulatory extinction microstructures - SWUE (Trepmann and Stöckhert, 2013, Solid Earth, 4). Only grains with SWUE, orientated parallel to the foliation, are kinked. In general, kinked microstructures mainly develop in strongly anisotropic material or within lamellar minerals, i.e. micas. Deformation at high stresses (e.g. at greenschist conditions or during coseismic loading) can produce a strong anisotropic microstructure in quartz by the development of deformation lamellae. Trepmann and Stöckhert (2013) showed in deformation experiments of quartz that SWUE preserve evidence of an earlier coseismic stress peak, even when overprinted during subsequent crystal plastic creep deformation at lower stress. The SWUE in the deformed Schober quartz veins are interpreted in a similar way. These microstructures were primary deformation lamellae developed during coseismic loading. Pseudotachylyte veins within the analysed shear zone next to the sample give evident for a seismic event. Subsequent overprint by ongoing creep at lower stresses is recorded by the vein quartz mylonites. The densely spaced sub-planar microstructures cause a high anisotropy of the quartz grains, which finally were kinked. Electron Backscatter diffraction data give evidence of different slip system active in the formation of the deformation lamellae/SWUE and the subsequent kinking. The opposite direction of the Burges vectors (based on Weighted Burges Vector analysis, Wheeler et al., 2009, Journal of Microscopy, 233) in the kink band domain is consistent with sinistral shearing along the anisotropic deformation lamellae/SWUE in the dextral sheared kink band. Kinked micas (muscovite and biotite) in the mica-rich host rock, next to the kinked quartz vein sample, point to seismic induced kinking. However, the question arise if the kinking of micas and quartz was caused instantaneously during the same seismic event or if the quartz kinking is related to post-seismic creep at transient high stresses during ongoing deformation at lower greenschist facies conditions under dextral sense of shear (Bestmann et al., JGPR – Solid Earth, 126).

The seismogenic environments build up the highest differential stresses on Earth. Differential stress of as much as hundreds of MPa to few GPa is accumulated during the interseismic loading stage and it is abruptly released in a sequence of fast, high-stress events (earthquake rupture tip propagation, frictional fault slip, thermomechanical interactions) determining large-magnitude, local stress changes on and near the fault plane. A major challenge to obtain a quantification of these stresses is represented by their heterogeneity in space and time. However, they can be exceptionally recorded in exhumed fault rocks bearing pseudotachylytes (quenched coseismic frictional melts).Here, for the first time, we provide a measure of the residual elastic stress preserved in the lattice of seismically shocked garnets crosscut by a pseudotachylyte fault vein by means of HR-EBSD (high-angular resolution electron backscattered diffraction). The thin (3 mm-thick) pristine pseudotachylyte was produced during a single seismic event at mid-crustal conditions (ca. 500 MPa, 500 °C) within felsic gneisses in the hanging wall of the Woodroffe Thrust (Musgrave Ranges, central Australia). Centimetric garnets of the host rock are intensely fractured and extremely comminuted close to the pseudotachylyte. A local enrichment in Mn is associated with healing of the cracks close to the pseudotachylyte and with the growth of epitaxial garnet in the cataclastic portions. HR-EBSD maps (ca. 30 x 50 µm2) were acquired in the garnet at increasing distance from the pseudotachylyte and in a cataclastic domain in contact with it. Residual stresses reach up to 5 – 6 GPa in contact with the pseudotachylyte and decrease to a few hundred MPa in less than a millimeter from it. Similar high stresses are recorded also in the clasts of a cataclasite flanking the pseudotachylyte, while newly grown high-Mn garnet surrounding the clasts records lower stresses. High stress domains in the mapped areas, a few micrometers in size, are bounded by straight bands of stress sign inversion that become less regular and more closely spaced towards the pseudotachylyte. Geometrically necessary dislocations (GND) are more abundant close to the pseudotachylyte and are linked with the high-stress domains.Microstructures, stress gradients and magnitudes all suggest that the extreme residual stresses recorded in proximity of the pseudotachylyte belong to the stage of propagation of the earthquake rupture, in agreement with theoretical predictions of the stress fields at the tip of a propagating fracture. The partial preservation of the stress in the strained lattice of the garnet is made possible by the quasi-instantaneous healing of the cracks that produced a load-bearing framework to maintain the elastic strains and by the presence of high GND densities produced during the high-stress – high-strain rate rupture propagation event.

Faults can act as conduits for the migration of hydrothermal fluids in the crust, affecting its mechanical behaviour and possibly leading to earthquake swarm activity. To date, there are still few constraints from the geological record on how fault-vein networks develop through time in high fluid-flux tectonic settings. Here, we describe small displacement (

Many crystalline rocks of the continental crust contain coarse-grained quartz as a main mineral (e.g., granitoids). Incipient deformation of coarse quartz, which likely controls the accumulation of bulk strain in heterogeneously deformed crustal rock volumes, commonly develops microshear zones (MSZs) of localized recrystallization. At mid-crustal conditions, where quartz deformation is mostly accomplished by subgrain rotation recrystallization, grains of MSZs can show an abrupt change in crystallographic orientation (large misorientation angle) with respect to the host quartz that is still not fully understood. We analyzed MSZs (20–200 µm thick) from deformed coarse-grained (millimeter grain size) quartz veins in the Austroalpine Schobergruppe (Eastern Alps). Electron backscatter diffraction analysis reveals that the MSZs are characterized by a nearly 90° misorientation angle between the c-axes of the host and new grains, which also share one {m} and one {1122} pole, compatible with Japan twinning. This abrupt switch of the c-axis orientation can promote geometrical softening and shear localization. So far, Japan twinning has been interpreted as a growth feature. We show that deformation-induced twinning in quartz, including Japan and Dauphiné twinning, can play an important role in initiation of crystal-plastic deformation within the crust.

The strike-slip Davenport Shear Zone in Central Australia developed during the Petermann Orogeny (~ 550 Ma) in an intracontinental lower crustal setting under dry sub-eclogite facies conditions (~ 650 °C, 1.2 GPa). This ca. 5 km wide mylonite zone encloses several large low-strain domains, allowing a detailed study of the initiation of shear zones and their progressive development. Quartzo-feldspathic gneisses and granitoids contain compositional layers, such as quartz-rich pegmatite dykes, mafic bands and dykes, which should preferentially localize viscous deformation if favorably orientated. This is not observed, except for long, continuous and fine grained dolerite dykes. Instead, many shear zones, typically a few millimeters to centimeters in width but extending for tens of meters, commonly exploited pseudotachylytes and are sometimes parallel to a network of little overprinted fractures. The recrystallized mineral assemblage in the sheared pseudotachylyte is similar to that in the host gneiss, without associated hydration due to fluid-rock interaction. Lack of localization in quartz-rich, coarser grained (typically >50 µm) rocks compared to mafic dykes, precursor fractures and pseudotachylytes implies that localization in the dry lower crust preferentially occurs along elongate, planar fine-grained layers. Transient high stress repeatedly initiated fractures, providing finer grained, weaker, planar precursors that localized subsequent ductile shear zones. This intimate interplay between brittle and ductile deformation suggests a local source for lower crustal earthquakes, rather than downward migration of earthquakes from the shallower, usually more seismogenic part of the crust.

The Woodroffe Thrust (WT) in the Musgrave Ranges (central Australia) is a shallowly south-dipping crustal-scale mylonitic zone extending E-W for over 600 km. The WT, developed during the intracontinental Petermann Orogeny (630-520 Ma), placed hanging wall lower-crustal granulite to upper-amphibolite facies rocks of the Fregon Subdomain (FS) over footwall amphibolite-facies mid-crustal gneisses and granitoids of the Mulga Park Subdomain (MPS). The WT mylonites largely affect the MPS and to a minor extent the FS. Towards the WT, the hanging wall hosts the largest volumes of supposedly deep-seated, tectonic pseudotachylytes (pst) worldwide, also partially involved in mylonitization adjacent to the WT. The WT has been inferred to have only a very small difference in pressure (depth) over the ca. 60 km of N-S exposure along the transport direction, from 1.0 – 1.3 GPa to 0.8 – 1.1 GPa, thus representing effectively a very shallowly dipping structure[1]. However, it was noted that these pressure estimates had to be considered with some caution due to not always ideal mineral compositions. Here we present new pressure constraints in northern outcrops from the eastern segment of the thrust suggesting a more complex geometry than previously inferred, with significant variation in depth along the structure.Pseudotachylyte-bearing peraluminous gneisses, from two localities ca. 80 km apart (Sentinel Bore, SB, to the east and Kelly Hills, KH, to the west) in the immediate hanging wall of the WT, were investigated to establish the ambient conditions during seismic faulting. The gneisses display mm-thick alternation of quartz-feldspar and cordierite-sillimanite-rich layers, including sparse garnet, magnetite, ilmenite, and biotite. Along microfractures of the pst damage zone (i) sillimanite was fractured and remained unaltered; (ii) cordierite broke down to either an andalusite + quartz + biotite symplectite overgrown by kyanite (SB), or just kyanite (KH); and (iii) K-feldspar developed flame perthites. The pst at SB and KH also show a different mineralogy. At SB, pst assemblages include (i) andalusite (pseudomorphosed by biotite) + quartz intergrowths rimmed by plagioclase and K-feldspar; (ii) sillimanite microlites overgrowing sillimanite clasts; (iii) microlitic kyanite, and (iv) poikilitic garnet as the latest grown phase. At KH, pst assemblages include (i) cordierite + quartz intergrowths; (ii) sillimanite microlites overgrowing sillimanite; (iii) microlites of kyanite, and (iv) poikilitic garnet. Andalusite is absent at KH.The newly identified andalusite, stable in pst, sheared pst and along microfractures in the host rock at SB indicates pressures ≤ 0.5 GPa during seismic faulting, i.e. significantly lower than in the more southern portion close to Mount Woodroffe (ca. 60 km to the SW of SB)[2]. The absence of andalusite at KH implies a complex undulating geometry for the WT. 1: Wex et al., 2017, Geometry of a large‐scale, low‐angle, midcrustal thrust (Woodroffe Thrust, central Australia). Tectonics, 36(11), 2447-2476.2: Hawemann et al., 2018, Pseudotachylytes as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia). Solid Earth, 9, 629-648

Fault zone architecture controls, for instance, the nucleation, propagation and arrest of individual seismic ruptures, the moment magnitude of the mainshocks and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the architecture of crustal-scale seismogenic sources is still poorly known. Here, we examine the architecture of the >40-km-long, Mesozoic seismogenic Bolfin Fault Zone (BFZ) of the Atacama Fault System (Northern Chile). The exceptionally well-exposed BFZ cuts through plutonic rocks of the Coastal Cordillera and was seismically active at 5-7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple fault core strands consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone as wide as 75 m. These cataclastic units are associated with a damage zone, up to 150-m-thick, which comprises strongly altered and brecciated rock volumes, and with clusters of epidote-rich fault-vein networks located at the linkage of the BFZ with other faults. The architecture of the BFZ is the result of fault core widening by cyclic co-seismic frictional melting and post-to-inter-seismic fault healing due to hydrothermal (chlorite + epidote ± K-feldspar) mineral precipitation plus pervasive, possibly associated with mainshocks and aftershocks, damaging of the surrounding rocks. Additionally, we interpret the epidote-rich fault-vein networks as an exhumed seismic source of fluid-driven earthquake swarm-type sequences in agreement with seismological observations of presently active magmatic and hydrothermal regions.

Earth's mantle rocks are poly-aggregates where different mineral phases coexist. These rocks may often be approximated as two-phase composites with a dominant phase and less abundant one (e.g. bridgmanite-ferropericlase composites in the lower mantle). Severe shearing of these rocks leads to a non-homogeneous partitioning of the strain between the different phases, with the composite developing a laminar fabric of weak and thin material where strain localizes. The resulting bulk rock is a mechanically anisotropic media that is hardened against normal stress, while significantly weakened against fabric-parallel shear stress.Due to the large scale difference between the laminar gran-scale fabrics and regional-to-global geological processes, Earth’s rocks are idealised as homogeneous materials instead of multi-phase bodies in numerical models. Thus, a characterization of the rheology evolution of the bulk composite is necessary to better understand large-scale geological processes in which anisotropy may play a fundamental role. Recent three-dimensional numerical (de Montserrat et al. 2021) studies have shown that the degree of lateral interconnectivity of the weak and thin layers is rather limited, thus estimating the rheology of a composite with laminar fabrics by the idealized Voigt and Reuss averages for fibres yield a strong underestimation of the strength of the composite. Instead, we use a combination of numerical results and micro-mechanics to develop an empirical framework to estimate the evolution of the (anisotropic) rheology of such composites.We apply this rheology framework to study the effects of fabric-induced directional-weakening/hardening on global mantle convective patterns. First order effects of extrinsic anisotropy of lower mantle material observed in our two-dimensional models are a decrease of the wavelength of convective cells, and up to a ~50% increase in the average mantle flow velocity caused by the weakening of the flow-parallel component of the viscosity tensor. The latter is particularly evident in mantle plumes, where the ascent and transfer of hot lower mantle material to lower depths is enhanced by the near-alignment of the weak fabrics with the plume channel.

Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the urn:x-wiley:02634929:media:jmg12553:jmg12553-math-0004 of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. urn:x-wiley:02634929:media:jmg12553:jmg12553-math-0005 and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid-saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid-saturated to fluid-undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load-bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid-undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid-mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid-undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn-kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids.

Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

In the last 10 years, many attempts have been mad to use the titanium-in-quartz geothermobarometer (TitaniQ) to constrain the ambient conditions during mylonitization of quartz in metamorphic rocks. However, most of the studies have shown that the TitaniQ is not readily applicable. First, the application of the TitaniQ calibrations1-2 is possible if two of the relevant variables (temperature, pressure and Ti activity) can be fixed. But the results of both calibrations can deviate by >100°C. Secondly, several studies have shown that deformation/recrystallization processes, the availability of aqueous fluids, the amount of strain and the duration of deformation result in microstructures with a heterogeneous distribution of Ti concentrations [Ti]. Therefore, in most cases, homogenous and complete equilibration of the [Ti] at the ambient conditions of deformation does not occur. In quartz mylonites, the microstructure is commonly complex as result of strain partitioning and total accumulated strain. For such a complex rock the challenge for applying TitaniQ is to identify those domains where Ti re-equilibration to the syn-kinematic ambient conditions, did possibly occur. Identifying such domains requires the strict integration of correlated high-resolution analysis by optical microscopy, SEM-CL, EBSD and Ti-in-qtz analysis using secondary ion mass spectrometry (SIMS). This integrated information especially provides a robust interpretative tool for the interplay between grain-scale deformation, fluid-rock interaction, geochemical exchange and the evolution of the crystallographic preferred orientation during progressive strain.We present the study of the deformation microstructures of quartz veins (Schober Group, Eastern Alps) as key example of such an integrated data collection to unravel characteristic deformation processes responsible for the partial or complete resetting of the Ti-in-quartz system under retrograde conditions. The Schober quartz veins developed at amphibolite facies conditions (510-590 °C, 0.5-0.6 GPa) and were overprinted by deformation at lower greenschist facies. Subgrain rotation (SGR) recrystallization was the dominant recrystallization mechanism during mylonitization. During deformation complete resetting of the initial [Ti] of 3-4 ppm down to 0.2-0.6 ppm occurred in domains (e.g. pressure shadows) where sufficient fluids were available and could percolate through the microstructures. High strain and pervasive quartz dynamic recrystallization did not necessarily result in homogeneous and complete re-equilibration of the [Ti]. Our study reveals that subgrain boundaries were locally pathways for partial [Ti] reset.Using the example of mylonitized quartz veins from the Schober Group in the Austroalpine domain of the Eastern Alps, we aim at showing that the initial Ti-in-qtz and corresponding CL signature of the quartz vein is reset to different degrees even at high strains and pervasive dynamic recrystallization, depending on the availability of fluids and its repartitioning. (1) Huang, R., Audétat, A., 2012. The titanium-in-quartz (TitaniQ) thermobarometer: a critical examination and re-calibration. Geochim. Cosmochim. Acta 84, 75–89.(2) Thomas, J.B., Watson, E.B., Spear, F.S., Shemella, P.T., Nayak, S.K., Lanzirozzi, A., 2010. TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz. Contrib. Mineral. Petrol. 160, 743–759.

Rocks of the Earth's crust and mantle commonly consist of different minerals with contrasting mechanical properties. During progressive, high-temperature (ductile) deformation, these rocks develop extrinsic mechanical anisotropy linked to strain partitioning between different minerals, amount of accumulated strain, and bulk strain geometry. Extrinsic anisotropy plays an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain- and rock-scale fabrics causing this anisotropy cannot be directly simulated in large-scale numerical simulations. For two-phase aggregates-a good rheological approximation of most Earth's rocks-we propose a method to indirectly approximate the extrinsic viscous anisotropy by combining (a) 3D mechanical models of rock fabrics, and (b) analytical effective medium theories. Our results confirm that weak inclusions induce substantial weakening by forming a network of weak thin layers with limited lateral connectivity. Consequently, even when the inclusion phase is extremely weak, structural weakening is not larger than 30-60%, less than in previous estimates. On the other hand, the presence of strong inclusions does not have a profound impact on the effective strength of the aggregate, and lineated fabrics only develop at relatively low viscosity contrasts. When rigid inclusions become clogged, however, the aggregate viscosity can increase over the theoretical upper bound. We show that the modeled grain-scale fabrics can be parameterized as a function of the bulk deformation and material phase properties and combined with analytical solutions to approximate the anisotropic viscous tensor.

The over 600 km long Woodroffe Thrust developed at lower to mid-crustal levels during the intracontinental Petermann Orogeny at ca. 560–520 Ma. Ductile deformation with a top-to-north shear sense was accommodated along a shallowly (≤30°) south-dipping surface. Metamorphic conditions during deformation are established along a 60 km N-S transect, providing an ideal framework for studying variation in microstructure and crystallographic preferred orientations with changing temperature (ca. 520–620 °C) and pressure/depth in dominantly dry felsic crust. In the Woodroffe Thrust mylonites, dynamic recrystallization of quartz was dominated by subgrain rotation, whereas feldspar underwent grain size reduction by neocrystallization. Differential stress, estimated from quartz grain size piezometry, decreases with increasing metamorphic grade (i.e., deeper structural levels), and indicates a long-term average strain rate of around 10−11–10−12 s−1. We propose a qualitative rheological model to explain the observed cyclic interplay between ductile shearing (mylonitization) and brittle fracturing (pseudotachylyte formation) in the relatively dry middle crust. The model involves the downward migration of earthquake ruptures from the overlying seismogenic zone, which transiently triggers seismic slip at mid-crustal levels.

How major crustal‐scale seismogenic faults nucleate and evolve in crystalline basements represents a long‐standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio‐temporal evolution of the Bolfin Fault Zone (BFZ), a >40‐km‐long exhumed seismogenic splay fault of the 1000‐km‐long strike‐slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5–7 km depth and ≤ 300°C in a fluid‐rich environment as recorded by extensive propylitic alteration and epidote‐chlorite veining. Ancient (125–118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long‐term far‐stress field associated with the oblique ancient subduction. The large‐scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy‐pinned segments during fault growth., Key Points We describe the structural evolution of the Bolfin Fault Zone, a >40‐km‐long exhumed seismogenic fault of the Atacama Fault SystemSeismic faulting exploited magmatic foliations and dykes well‐oriented with respect to the subduction‐related stress fieldThe sinuous geometry of the Bolfin Fault Zone results from hard linkage of anisotropy‐pinned fault segments during fault growth

Rocks of the Earth's crust and mantle commonly consist of aggregates of different minerals with contrasting mechanical properties. During progressive, high temperature (ductile) deformation, these rocks tend to develop an extrinsic mechanical anisotropy related to the strain competition of the different minerals, the amount of accumulated bulk strain and the bulk strain geometry. Extrinsic anisotropy is thought to play an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain-scale and rock-scale associated with this anisotropy cannot be directly implemented in large-scale numerical simulations. For two-phase aggregates -a good rheological approximation of most Earth's rocks- we propose a methodology to indirectly approximate the extrinsic viscous anisotropy by a combination of (i) 3-D mechanical models of rock fabrics, and (ii) analytical effective medium theories. The resulting 3-D mechanical models, confirm that the weak least abundant phase induces substantial rock weakening by forming an inter-connected network of thin layers in the flow direction. 3-D models further suggest, however, that the lateral inter-connection of these weak layers is quite limited, and the maximum structural weakening is considerably less than previously estimated. Ont the other hand, presence of hard inclusions does not have a profound impact in the effective strength of the aggregate, with lineations developing only at relatively low compositional strength contrast. When rigid inclusions become clogged, however, the aggregate viscous resistance can increase over the theoretical upper bound. We show that the modelled grain-scale fabrics can be parameterised as a function of the bulk deformation and material phase properties and can be combined with analytical solutions to approximate the anisotropic viscous tensor. At last, the resulting parameterisation of the extrinsic viscous tensor is implemented in a bi-dimensional global mantle convection code. Preliminary results show that extrinsic is responsible for an increase of the upwelling speed of hot material from the lowermost mantle, different convective cell shapes, and deflection of mantle plumes at the uppermost mantle.

Intermediate-depth subduction seismicity is still hiding most of its secrets. While plate unbending is recognised as the main stress loading mechanism, the processes responsible for earthquake nucleation are still unclear and depend upon the question whether failure occur in a wet dehydrating slab or in a completely dry lithosphere. The recent observation of subduction-related pseudotachylytes (quenched frictional melts produced during seismic slip along a fault) in the dry ophiolites of Moncuni (Lanzo Massif, W. Alps)1, an exhumed example of the actual intermediate-depth seismicity, and the interpretation of seismic data from various double-plane seismic zones in subducting slabs2 suggest that the seismogenic portions of subducting oceanic slabs can be dominantly dry. In absence of a fluid-mediated embrittlement (i.e. dehydration embrittlement), a dry and strong slab requires extremely high differential stress for brittle failure to occur.Here we investigate with numerical simulations the potential of a subducting dry oceanic slab of building up the high differential stress required for failure. We performed pseudo-2D thermo-mechanical simulations of free subduction of a dry slab in the asthenosphere considering a visco-elasto-plastic rheology. We tested both a homogeneous dry plate and a dry plate with scattered weak circular inclusions representing domains of partial hydration in the first 40 km of the slab.The stress field in the unbending portion of the slab describes two arcs, the outer one in compression and the inner one in extension, in agreement with the two planes of seismicity. For the homogenous plate the maximum values of differential stress are around 1 GPa, i.e. not high enough for triggering earthquakes. The presence of weak inclusions induces a stress amplification, which can be of several folds if elastic properties of the inclusions are sufficiently degraded, but still maintaining a high viscosity. For inclusions with a shear modulus decreased by 60-70% relative to the surrounding material, but similar viscosity, stress values in excess of 4 GPa are obtained, high enough for brittle failure at 100 km of depth. This inclusion rheology is compatible with that of a slightly hydrated and serpentinized meta-peridotite. These meta-peridotite domains are likely to be found in the oceanic lithosphere around faults related to slab bending which represent the main pathways for fluid infiltration in the slab.We conclude that extremely high deviatoric stresses can be achieved in dry and strong subducting plates in presence of scattered domains of meta-peridotite acting as local stress amplifiers. These previously unreported stress values may explain brittle seismic failure at intermediate depth conditions. References:1: Pennacchioni et al., 2020, Record of intermediate-depth subduction seismicity in a dry slab from an exhumed ophiolite, Earth Planet. Sc. Lett. 548, 1164902: Florez and Prieto, 2019, Controlling factors of seismicity and geometry in double seismic zones, Geophys. Res. Lett. 46, 4171-4181

On-fault processes during earthquakes contribute to seismic rupture propagation and slip. Here we investigate clast fragmentation in an experimental pseudotachylyte (solidified seismic melt) produced with a rotary shear machine. We slid for 0.44 m (corresponding to Mw ≥ 6 earthquakes), at slip rates > 1 m/s, pre-cut samples of quartz + phyllosilicates + plagioclase + sillimanite + garnet -bearing ultramylonite, that hosts pseudotachylytes in nature. The ultramylonite minerals extensively preserved as clasts in the experimental pseudotachylyte are quartz, plagioclase, and sillimanite. Garnet is scarcely preserved, despite having a melting temperature similar to plagioclase, probably due to having low thermal shock resistance. This selective clast survival is identical to the one found in the natural pseudotachylytes. Based on these experimental observations and assuming non-equilibrium melting, the preservation of a mineral, as a clast, in the melt appears to be controlled by its thermal shock properties as well as by its melting temperature. Since the mechanical effects of rupture propagation in these experiments were negligible, we conclude that, for Mw ≥ 6 earthquakes, (i) frictional slip and heating of the slipping zone plus (ii) thermomechanical properties of minerals, rather than fault rupture processes, control mineral comminution and clast survival in frictional melts.

In the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred long-term shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ1 and σ3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes., Geology, 48 (11), ISSN:0091-7613, ISSN:1943-2682

The origin of intermediate-depth subduction seismicity is a topic of research since long time. While plate unbending is considered as one of the main stress loading mechanisms, the processes responsible for earthquake nucleation are still unclear and depend on the question of whether failure occurs in the wet dehydrating portion of the slab or in the predominantly dry portion. Recently, the seismogenic portions of subducting oceanic slabs have been proposed to consist of dominantly dry metaperidotite that deforms by seismic brittle failure in absence of fluid-mediated embrittlement. In this work, we quantify by numerical modelling the differential stress achievable during unbending of a subducting slab. We show that the presence of discrete hydrated domains in a dry, strong slab amplifies the differential stress to high seismogenic values (ca. 3-4 GPa in the 100-200 km depth range) at intermediate depths. We also consider the effects of low temperature plasticity in olivine that can hinder the build-up of high differential stress to the first 100 km of depth.

Study locations, stereoplots, detailed outcrop maps, and high resolution photographs.

Study locations, stereoplots, detailed outcrop maps, and high resolution photographs.

The origin of earthquakes in the lower crust at depth of 20-40 km, where dominantly ductile deformation is expected, is highly debated. Exhumed networks of lower crustal coeval pseudotachylytes (quenched frictional melt produced during seismic slip) and mylonites (produced during the post- and interseismic viscous creep) provide a snapshot of the earthquake cycle at anomalously deep conditions in the crust. Such natural laboratories offer the opportunity to investigate the origin and the tectonic setting of lower crustal earthquakes.The Nusfjord East shear zone network (Lofoten, northern Norway) represents an exhumed lower crustal earthquake source, where mutually overprinting mylonites and pseudotachylytes record the interplay between coseismic slip and viscous creep (Menegon et al., 2017; Campbell and Menegon, 2019). The network is well exposed over an area of 4 km2 and consists of three main intersecting sets of ductile shear zones ranging in width from 1 cm to 1 m, which commonly nucleate on former pseudotachylyte veins. Mutual crosscutting relationships indicate that the three sets were active at the same time. Amphibole-plagioclase geothermobarometry yields consistent P-T estimates in all three sets (700-750 °C, 0.7-0.8 GPa). The shear zones separate relatively undeformed blocks of anorthosite that contain pristine pseudotachylyte fault veins. These pseudotachylytes link adjacent or intersecting shear zones, and are interpreted as fossil seismogenic faults representing earthquake nucleation as a transient consequence of ongoing, localised aseismic creep along the shear zones (Campbell et al., under review).The coeval activity of the three shear zone sets is consistent with a local extensional setting, with a bulk vertical shortening and a horizontal NNW-SSE extension. This extension direction is subparallel to the convergence direction between Baltica and Laurentia during the Caledonian Orogeny, and with the dominant direction of nappe thrusting in the Scandinavian Caledonides. 40Ar‐39Ar dating of localized upper amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord East network yielded an age range of 433–413 Ma (Fournier et al., 2014; Steltenpohl et al., 2003), which indicates an origin during the collisional (Scandian) stage of the Caledonian Orogeny.We propose that the Nusfjord East brittle-viscous extensional shear zone network represents the rheological response of the lower crust to the bending of the lower plate during continental collision. (Micro)seismicity in the lower crust in collisional orogens is commonly localized in the lower plate and has extensional focal mechanisms. This has been tentatively correlated with slab rollback and bending of the lower plate (Singer et al., 2014). We interpret the Nusfjord East shear zone network as the geological record of this type of lower crustal seismicity.

During exhumation, metamorphic rocks change their rheological behavior from dominantly ductile to brittle. Especially at the “brittle-ductile transition” at the bottom of the brittle crust, which coincides roughly with the domain where most “shallow” earthquakes nucleate, rocks exhibit a close interplay between ductile flow and fracturing.In the Neves area (Tauern window, Eastern Alps) the exhumation across the brittle-ductile transition of amphibolite-facies meta-granitoids during the Alpine cycle is recorded by the association of pseudotachylyte veins and localized low-grade mylonites (stage-2 deformation). The stage-2 structures exploited the precursor amphibolite-facies foliation within meter-thick mylonites (stage-1 deformation) and were in turn overprinted by epidote-chlorite-bearing shear fractures and veins (stage-3 deformation). The kinematics and orientation of stage-1 and stage-3 structures indicate a slight rotation of the regional shortening direction from 345° to about 360°. This implies that stage-2 mylonites and pseudotachylytes developed at a high angle to the shortening direction.The syn-kinematic metamorphic assemblage of stage-2 mylonites includes quartz, oligoclase (Ab75), biotite, epidote, and minor muscovite and K-feldspar; garnet was not stable. This assemblage constrains the deformation at upper greenschist facies condition and temperatures of around 400 °C. During mylonitization the coarse-grained (mm-sized) amphibolite-facies quartz recrystallized by subgrain rotation to ultra-fine (~ 3 µm average grain size determined from EBSD maps) aggregates. Such a small grain size yields differential stress > 200 MPa during stage-2 mylonitization, considering the piezometer of Cross et al., 2017 1.Pseudotachylytes are in a close spatial association with stage-2 mylonites and share the same sense of shear. There is no evidence of a ductile overprint of pseudotachylytes. The stage-2 structures developed at a very high angle to the inferred shortening direction, which implies that the coseismic slip occurred on planes with a very low friction coefficient (estimated stage-2 mylonite and pseudotachylyte. Mylonites progressively formed the mica-rich foliation planes, continuous over large distances, that provided the weak mechanical anisotropy eventually leading to coseismic slip. Reference:1: Cross, et al., 2017, The recrystallized grain size piezometer for quartz: An EBSD‐based calibration. Geophys. Res. Lett., 44(13), 6667-6674.

Quartz veins in poly-metamorphic settings often accommodate the latest deformation state and therefore can provide important information. Identification of microfabric (microstructure and crystallographic preferred orientation, CPO) evolution of quartz during mylonitization, and especially of the grain-scale interplay between brittle and crystal-plastic processes, has different relevant implications: e.g., on understanding the efficiency of fluid mobility through deforming quartz that can dramatically influence the rheology and the degree of chemical exchange. However, in order to interpret the microstructure and the related deformation processes it is necessary to relate these especially to the deformation temperature. Particularly the CPO and the Ti-in-qtz geothermometry is used to constrain the deformation temperature. However, both methods have to be applied with great caution because even when many times used some fundamental processes are not fully understood yet. Here we present results from deformed quartz veins from the Prijakt Nappe (Autroalpine Unit, Schober Mountains, Central Eastern Alps). These veins localized ductile shear and eventually seismic faulting (recorded by the occurrence of pseudotachylytes) within Eo-Alpine eclogite-facies shists. The veins formed shortly after the eclogitic peak, but the temperature of their deformation remains unconstrained. CL imaging reveals critical details for understanding the role of microfracturing and fluid-rock interaction during initial stages of shear localization, the onset of dynamic recrystallization and the resetting of the Ti-in-quartz geochemistry. Even when optical-light-microscopy and EBSD analysis indicate crystal plastic deformation by subgrain rotation CL and orientation contrast (OC) imaging gives evidence of brittle stage of deformation at least for some of the deformation microstructure. Microshear zones show a bulk dark-CL, but still bright tones in cores of new recrystallized grains similar to the CL signature of the host coarse quartz crystals. CL dark tones also match with the pattern of subgrain boundaries. This reflects fluid permeability pathways along subgrain and grain boundaries (identified by widespread fluid inclusions) and the associated partial resetting of Ti concentrations. The CPO of the new grains within the micro-shear zones rotate with the sense of shear around the kinematic Y-axis and cannot be related to the activity of specific slip systems. In contrast the partial single girdle of c-axis within the ultramylonite with its elongated substructured grains and its characteristic layered microstructure can be related to the activity of several slip systems. Misorientation axis analysis indicates that prism

Coeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550 °C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition.

The length, thickness and strain gradients of small-scale (10−3–10−1 m thick) ductile shear zones are pre-determined by the presence of surface precursors (e.g. fractures or compositional layers) and associated fluid-rock interaction. Fractures, with their surrounding host-rock damage zone and concurrent tectonic under-pressure during dilation, provide efficient fluid pathways and networks with diffusive fluid-rock interaction at their margins. The fluid-induced, gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types, including single homogeneous-to-heterogeneous shear zones and paired shear zones. As a consequence, geochemical differences between shear zone and host rock may reflect fluid-rock interaction during the precursor brittle history rather than during slip, especially considering that shear zones with even a small component of stretch may be over-pressured and therefore unable to drain fluids from the surrounding rocks. Support from field observation for this interpretation is given by (1) the lack of correlation between shear zone thickness and accumulated displacement; and (2) the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors. Most small-scale shear zones in massive rocks (granitoids) are initially neither thickening nor narrowing with increasing strain. This concept may also apply to more foliated rocks, but does not necessarily hold for larger-scale shear zones.

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

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

At Arvogno, Vigezzo valley in the Central Alps, Italy, pegmatite dikes are unique in the scenario of a tertiary alpine pegmatite field because they show marked geochemical and mineralogical niobium&ndash, yttrium&ndash, fluorine features. These pegmatites contain AB2O6 aeschynite group minerals and ABX2O8 euxenite group minerals as typical accessory minerals including aeschynite-(Y), polycrase-(Y), and samarskite-(Y). They are associated with additional typical minerals such as fluorite, Y-dominant silicates, and xenotime-(Y). The Y&ndash, Nb&ndash, Ti&ndash, Ta AB2O6 and ABX2O8 oxides at the Arvogno pegmatites did not exhibit any textural and compositional features of oxidation or weathering. They are characterized by low self-radiation-induced structural damage, leading to the acquisition of unit-cell data for aeschynite-(Y), polycrase-(Y), and samarskite-(Y) by single-crystal X-ray diffraction. Aeschynite-(Y) and polycrase-(Y) crystals allowed for both to provide space groups whereas samarskite-(Y) was the first crystal from pegmatites for which cell-data were obtained at room temperature but did not allow for the accurate determination of the space group. According to the chemical compositions defined by Ti-dominant content at the B-site, the cell parameters, respectively, corresponded to polycrase-(Y), aeschynite-(Y), and the monoclinic cell of samarskite-(Y). Emplacement of Alpine pegmatites can be related to the progressive regional metamorphic rejuvenation from east to west in the Central Alps, considering the progressive cooling of the thermal Lepontine Barrovian metamorphic dome. Previous studies considered magmatic pulses that led to emplace the pegmatite field in the Central Alps. As an example, the pegmatites that intruded the Bergell massif were aged at 28&ndash, 25 millions of years or younger, around 20&ndash, 22 m.y.

We investigate the influence of unmelted clasts on the( 40)Ar-Ar-39 age record of a Miocene pseudotachylyte within the early Oligocene Riesenferner pluton (eastern Alps). This case study is ideal to investigate the effect of inherited Ar retained in survivor clasts because, although seismic faulting was only slightly younger (

Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (, Key Points Rock‐scale layering in the Earth's mantle produces negligible extrinsic anisotropyRock‐scale layering cannot be detected with seismic anisotropy but mainly with seismic wave scatteringGrain‐scale layering can generate substantial extrinsic anisotropy around the transition zone

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

Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U + HREE content. The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution–reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

Exhumed faults within the tonalitic Adamello pluton (Southern Alps) were seismic at depth as indicated by the presence of pseudotachylytes (solidified friction-induced melts). During cooling of tonalite, early-formed joints were first exploited by localized ductile shear zones associated with deposition of quartz veins (at ~ 500 °C), and later by pseudotachylyte-bearing cataclastic faults (at ~ 250–300 °C ambient temperature). Adjacent to pseudotachylytes, quartz of the host tonalite shows pervasive thin (1–10 μm wide) healed micro-fractures and ultra-fine (1–2 μm grain size) recrystallized aggregates along micro-shear zones. Under cathodoluminescence (CL) the healed micro-fractures have a darker gray shade than the host “magmatic” quartz that reflects a change in Ti concentrations ([Ti]) as indicated by NanoSIMS measurements. [Ti] vary from 35–55 ppm in the CL-lighter host quartz to 10–13 ppm along the CL-darker healed micro-fractures. These [Ti] were inherited by the ultra-fine recrystallized aggregates that overprinted both the magmatic quartz and the healed micro-fractures during the high temperature transient related to frictional seismic slip. Based on Ti-in-quartz thermometry, we infer that micro-fracture healing occurred at higher temperatures than the ambient temperatures of faulting (250–300 °C at 0.2 GPa), for which [Ti]

At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution–precipitation, and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by the formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we use electron backscattered diffraction to (i) investigate the microstructure of a granodiorite mylonite, developed at ∼ 450°C during cooling of the Rieserferner pluton (Eastern Alps); and (ii) assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain-size reduction and anti-clustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain-size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450°C, show that grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain-size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust., Solid Earth, 9 (6), ISSN:1869-9510, ISSN:1869-9529

Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not fit this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560–520Ma) and is interpreted to be at least 600km long in its E–W strike direction, with an approximate top-to-north minimum displacement of 60–100km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P–T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. We also establish a qualitative increase in the abundance of fluids in the footwall along an approx. 60km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100°C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100m) of the Woodroffe Thrust mylonitic zone., Solid Earth, 9 (4), ISSN:1869-9510, ISSN:1869-9529

The structural evolution of the Oligocene Rieserferner pluton (Eastern Alps), developed during pluton cooling and exhumation, includes localized deformation structures consisting of: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins, and related ductile shear zones (formed at ∼600 °C); (ii) shallowly dipping joints with associated epidote and quartz veins, and related ductile shear zones (∼450 °C); (iii) steeply dipping mafic dykes and calcite + white mica-bearing brittle-ductile faults (∼300 °C); (iv) steeply dipping pseudotachylyte-bearing faults (