Important porosity is commonly observed in quartz-rich rocks that viscously deformed at depths of the metamorphic continental crust. Although the presence of such pores – often occurring with angular, pyramidal shapes – has major implications for fluid circulation, mass transfer and rock strength, whether or not they are directly produced by deformation remains unclear. Here we provide detailed observations of plastically deformed, pure quartz aggregates decorated by (sub)micrometric pores in granitic shear bands (western granite, Naxos, Greece). Using electron microscopy, we demonstrate that pores (1) decorate both grain boundaries and intra-grain substructures, (2) do not necessarily align with substructures when decorating grain boundaries, and (3) interconnect through layers of amorphous SiO2. Transmission electron microscopy further reveals dislocation densities one (or two) order(s) of magnitude below the predictions based on the lattice curvature gradients, suggesting the occurrence of residual stress along pore-decorated, partly amorphized substructures. Challenging long-lasting hypotheses, these features are here proposed to result from stress concentration at grain and subgrain boundaries, followed by fluid exsolution into mechanically amorphized quartz. Although involving syn-kinematic processes that remain to be fully understood, our findings corroborate recent studies that emphasise stress-induced amorphization as a general process of lithospheric rock deformation.

Crustal evolution of the North Tianshan during the late Paleozoic remains controversial due to the lack of structural and metamorphic constraints. We present new structural, petrological and geochronological data of the Xiaopu metamorphic complex (XMC) to address this issue. The XMC shows a well-preserved Buchan-type metamorphic sequence made of garnet, andalusite-staurolite and sillimanite zones. Structural observations indicate that garnet, sillimanite, andalusite and staurolite grew syn-kinematically during a transtensional event (D2). Inclusion trails in garnet cores oriented at high angle with the external foliation probably record an earlier deformation stage (D1). Synchronous ductile normal faults and thrusts (D3) overprinted previous structures and control the exhumation of the metamorphic complex. LA-ICP-MS zircon U-Pb ages of pre-D2 gneissic granodiorite, syn-D2 granitic dikes and post-D2 diorites constrain the occurrence of D2 between 332 and 305 Ma. Apatite U-Pb ages of mylonitic rocks from the ductile fault indicate that D3 occurred at ~285 Ma. Phase modelling and geothermobarometers were used to estimate the pressure-temperature (P-T) conditions of the XMC. Peak conditions show progressive temperature increase from andalusite-staurolite schists (~580 °C) to sillimanite migmatites (~680 °C) at nearly constant pressure (~4 kbar), suggesting a significant thermal effect of nearby large intrusive bodies. The schists display a pre-peak heating with slight decompression and the migmatites show retrograde cooling and decompression to ~600 °C and ~3 kbar. Their P-T paths and high/T and low/P thermal regimes are in line with an extensional setting. Monazite U/Th-Pb ages of 313-311 Ma for sillimanite-bearing schists and migmatites suggest monazite growth during the retrogression. 40Ar/39Ar ages of mica from sillimanite schists are in the range of 298-275 Ma, consistent with a later cooling and exhumation associated with D3. These data reveal the crustal transtension during the middle to late Carboniferous (~330-310 Ma) and cooling and exhumation during the early Permian (~285 Ma) of the North Tianshan arc. Such transtensional tectonics probably facilitated decompression melting of the lower crust and lithospheric mantle of the arc. Subsequent ascent and emplacement of the resulting melt could yield high thermal gradient, produce partial melting of the mid-crustal rocks and promote crustal exhumation.

Aseismic slip, particularly in the form of Slow Slip Events (SSEs), plays an undisputed role in the release of stress along faults, occurring slowly and without generating classical seismic waves. SSEs are recognized as critical phenomena influencing various stages of the seismic cycle, including postseismic phases, earthquake triggering or arresting, and interseismic transients. However, the mechanisms governing their underlying physics remain debated. Three primary hypotheses have been proposed: (1) heterogeneities in fault constitutive properties that may drive episodic SSEs; (2) stress interactions arising from geometric complexities (e.g., damage zones) that could explain the full observed slip spectrum; and (3) the influence of fluids circulating along fault zones, which increase pore pressure and reduce normal stress, thereby promoting slip. To investigate these mechanisms, we integrate SSE databases, slab thermal models, and thermodynamic metamorphic modeling.Our study examines nine subduction zones around the Pacific region, using thermal slab models that account for uncertainties in temperature estimations. By using an extensive SSE database (1800 events, Slow Earthquake Database, from the Japanese project “Science of Slow-to-Fast Earthquakes), we compare modeled temperature and pressure conditions with observed SSE distributions. Statistical analysis reveals two distinct temperature ranges where SSEs cluster: approximately 100°C and 350–550°C. Thermodynamic modeling of mafic rocks under subduction conditions indicates that the 100°C cluster aligns with the smectite-to-illite transition, a reaction known to release significant amounts of water. The 350–550°C cluster corresponds to metamorphic transitions from greenschists to amphibolites, which also release considerable water. SSEs are notably absent at pressure-temperature conditions where mafic rocks are fully dehydrated.The water released during such metamorphic reactions increases pore pressure, reduces normal stress, and facilitates slip. While the mechanisms sustaining slow slip—such as nucleation length or dilatant stress—remain debated, our results suggest that water release due to metamorphic reactions is a key trigger for SSEs along subduction interfaces. In addition to the release of fluids, we hypothesize that the change in resistance induced by the change in mineralogical configuration might also play a role in the nucleation of SSEs. These findings highlight the importance of integrating geophysical observations with petrological processes to better understand the dynamics of SSE in subduction zones

Strain localization within crustal shear zones involves intricate feedback between deformation mechanisms, metamorphic reactions and fluid circulation. Despite evidence that these high-deformation zones proceed at least partly through dissolution-precipitation creep, available creep laws so far only account for dislocation creep and/or solid-state diffusion processes. Deciphering the role and the contribution of dissolution-precipitation creep to strain accommodation is now required to further understand the rheological behavior of polymineralic crustal rocks. This study combines high-resolution microstructural and compositional analytical techniques to track the progressive deformation of the K & aring;gen metagabbros, at and below grain scale. The K & aring;gen metagabbros preserved a strain and re-equilibration gradient over outcrop-scale, where metamorphic re-equilibrations and fluid infiltration took place at constant pressure and temperature representative of lower crustal conditions (ca. 1 GPa - 660 +/- 25 degrees C). The comparison and quantification of chemical and microstructural information in this shear zone, through pixel-per-pixel and grain-per-grain correlated EPMA and EBSD maps, enables tracking grain-scale deformation mechanisms as well as the interplay between grain size reduction, mineral reactions, phase mixing and material transfer. Dissolution-precipitation creep appears dominant for strain accommodation in the K & aring;gen metagabbros shear zone. As deformation progresses, nucleation of new metamorphic minerals (clinopyroxene, plagioclase, amphibole) allow for grain size reduction and compositional homogenization through dissolution, transport and precipitation processes associated with fluid ingression along grain boundaries. Intracrystalline plastic deformation is here insignificant for strain accommodation. Thermodynamic modeling and textural analysis reveal that re-equilibration processes are spatially controlled by microdomains where equilibrium is reached locally, on a scale of similar to 100 mu m.

Despite extensive research over the years, the weakening mechanisms that govern strain localization along deep subduction interfaces are still debated. These mechanisms span from the downdip boundary of the seismogenic zone (∼350°C) to the mechanical coupling transition with the upper plate mantle near sub-arc depths (>600°C). Current thermo–mechanical models posit that rock rheology is primarily stress- and rate-temperature-sensitive in the absence of mineral reactions. Strain is accommodated by stable creep, within several km-thick shear zones and at very low strain rates (< 10-11 s-1). However, geophysical observations of active subduction zones have outlined, over the last two decades, that deep plate interfaces are likely to be dominated by unstable creep characterized by episodic events of aseismic slips (“slow slip events”) occurring at relatively high strain rates (> 10-7 s-1). Meanwhile, geological (i.e. petro-structural) observations of deep subduction interfaces have shown that strain is generally localized within < 10–100’s m-thick shear zones. These shear zones are also known to concentrate metamorphic reactions and episodic fluid flow that have both significant influence on the rock strength. Yet, quantifying the effects of these chemo–mechanical transformations on the transient aseismic slips of deep plate interfaces remains hindered by the complexity of integrating geophysical and geological observations and the general lack of high-pressure deformation experiments. Drawing on novel deformation experiments conducted at 2 GPa (eclogite-facies conditions) using a new generation Griggs-type apparatus, we reveal that unstable creep can be steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC) during mineral reactions. These changes of rheology can cause rock weakening by several orders of magnitude if intergranular fluid transfer is efficient. Such a weakening is a transient process since reaction rates tend to be intermittent / episodic at great depths. Moreover, we show that fluid concentration during viscous strain localization promotes extensive fracturing that may correspond to tremors (i.e., low frequency earthquakes) observed during slow slip events. Indeed, thermodynamic modeling of mafic and sedimentary rocks along pressure/temperature (P/T) gradients of active subduction zones worldwide reveals that slow slip events and tremors preferentially occur in horizons undergoing major dehydration reactions, and thus potential transient changes in rock rheology.

Deep crustal shear zones, fundamental to the dynamics of terrestrial plate tectonics, exhibit complex processes of initiation and evolution that are yet to be comprehensively quantified across both long and short temporal scales. Conventionally, thermo–mechanical models posit that crustal rock behaviour is dominated by monomineralic aggregates undergoing processes like intracrystalline plastic deformation by dislocation creep. However, high-pressure and temperature conditions in crustal rocks involve minerals with extremely strong mechanical properties, challenging strain localization theories.Field studies reveal that mineral reactions are ubiquitous in viscous shear zones, while undeformed rocks can remain largely metastable despite significant changes in P–T and/or fluid conditions. Local dissolution and precipitation processes under deviatoric stresses have long been recognized to promote brittle and viscous strain localization by complex chemo–mechanical processes including pressure solution, diffusive mass transfer, fluid flowand nucleation of fine-grained aggregates. Yet, quantifying the nature and relative contribution of these processes remains hindered by the general lack of experimental investigations on crustal rheology at high – to very high – pressure conditions and thermodynamic disequilibrium.Drawing on novel deformation experiments performed at eclogite-facies conditions and a compilation of characteristics of exhumed materials from fossil subduction zones worldwide, this presentation demonstrates that inception and progression of crustal shear zones are predominantly steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC). Strain accommodation and mass transfer are further accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Because intergranular fluid-assisted mass transfer is orders of magnitude faster than solid-state diffusion, DPC can indeed explain strain accommodation at relatively high strain rates and low magnitude of differential stress, regardless of the mineral plastic strength. Yet, DPC remains a transient process because both fluid depletion and completion of mineral reactions favor grain growth, reducing in turn the efficiency of intergranular mass transfer.

The Lyngen Magmatic Complex (Northern Norway) marks a major discontinuity in the metamorphic gradient of the North Norwegian Caledonides nappe sequence. The Lyngen Magmatic Complex preserves pre- and syn-collisional structures and parageneses of the Caledonian orogeny. This study differentiates the two deformation events and offers a tectono-metamorphic model for the region, using a combination of field-based data, geochemical analysis, radiochronometric dating, and thermodynamic modelling. It is demonstrated that the precollisional event (D1) developed in a dextral strike-slip transpressional regime, generating NS-trending subvertical shear zones with subhorizontal shear displacement and steeply inclined isoclinal folding. The D1 metamorphic fabrics developed at 486 +/- 9 Ma on a retrograde temperature-path, from amphibolite (680-800 degrees C, 0.5-0.9 GPa) to greenschist facies (300-450 degrees C, 0.3-0.75 GPa) conditions. The subsequent syn-collisional event (D2) produced a subhorizontal foliation, top-to-SE-directed thrust deformation at the base of the Lyngen Magmatic Complex. D2 rock fabrics represent typical nappe stacking structures during the Scandian collisional stage (similar to 430 Ma). The vertical D1 structures are overprinted by D2 fabrics in the thrust contact region only. Mineral assemblages crystallizing during D2 indicate a transition from lower amphibolite to greenschist conditions. Thermobarometry suggests significant re-heating after D1, with D2 maximum conditions similar to 650 degrees C and 1.1 GPa at the base of the Lyngen Magmatic Complex and similar to 558-610 degrees C and 0.8-1.3 GPa for the underlying graphite-bearing metasediments. Argon dating and the temperature difference between the Lyngen Magmatic Complex and the underlying units testify to out-of-sequence thrusting at 426 +/- 7.5 Ma at the base of the Lyngen Magmatic Complex. The present results constrain the localization of the oceanic units of the North Norwegian Caledonides outboard and to the North of their present position. Consequently, stacking of uppermost units (likely of Laurentian origin) on top of the Lyngen Magmatic Complex occurred before their final emplacement onto the Reisa Nappe Complex (probably mostly Baltica-derived).

Deep crustal shear zones, fundamental to the dynamics of terrestrial plate tectonics, exhibit complex processes of initiation and evolution that are yet to be comprehensively quantified across both long and short temporal scales. Conventionally, thermo-mechanical models posit that crustal rock behaviour is dominated by monomineralic aggregates undergoing processes like intracrystalline plastic deformation by dislocation creep. However, high-pressure and temperature conditions in crustal rocks involve minerals with extremely strong mechanical properties, challenging strain localization theories. Drawing on deformation experiments performed at eclogite-facies conditions, our research reveals that strain is efficiently localized through dissolution-precipitation creep, operating at notably lower stresses than dislocation creep. Strain accommodation and mass transfer are episodically accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Our results illuminate the interconnected thermo-hydro-mechanical-chemical processes underpinning crustal shear zone development, regardless of the plastic strength of mineral phases. We advocate that the inception and progression of subduction plate interfaces are predominantly steered by local transient changes of rheology beyond the seismogenic zone. Such changes are rooted in the chemical disequilibrium and fluid concentration of the slab materials, including sediments and mafic to ultramafic rocks.

The Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well-preserved example of a coherently-exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro-structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at similar to 166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at similar to 118 Ma, and (c) was subsequently subducted to a depth of similar to 20 km (within the seismogenic zone), as shown by local blueschist-facies assemblages formed at 0.6 GPa, 240 degrees C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction-related deformation was mainly accommodated along kilometer-scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter-thick crack-seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure-temperature-time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in-sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large-offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare-ups in the arc. The Snow Mountain Volcanic complex is a former hotspot volcano subducted to the seismogenic zone High-pressure fabrics and veins capture mixed brittle-ductile behavior of rocks and episodic fluid pulses in the seismogenic zone Thermal maturation of the subduction is achieved in similar to 10 My, subducting plate characteristics governs later accretion and arc activity

The Bay of Islands complex, Newfoundland, Canada, represents a fossil subduction system that was obducted onto the Laurentian margin during the Taconic Orogeny. We present whole-rock geochemistry and geochronology for the Bay of Islands metamorphic sole from the Table Mountain and North Arm Mountain massifs. The Bay of Islands metamorphic sole can be subdivided into three units: a high metamorphic-grade Grt-Cpx amphibolite and Grt-Cpx granulite unit, a medium metamorphic-grade common amphibolite unit, and a medium metamorphic-grade metasedimentary-dominated unit. Based on their major and trace element contents, two types of protoliths can be inferred for the Bay of Islands sole metabasites: cumulate gabbroic protoliths with high Mg numbers and depleted trace element contents, and basaltic protoliths with trace element contents defining a normal mid-oceanic-ridge basalt (N-MORB) affinity. Titanite in situ U-Pb analyses of two common amphibolite samples yield dates of 486 ± 7 Ma and 484 ± 4 Ma, which are interpreted as a metamorphic age (ca. 485 Ma). One sample from the metasedimentary unit yields detrital zircon dates that range from Archean to Ordovician, with the youngest dates defining the most dominant peak at ca. 490 Ma. This youngest peak is consistent with an arc-related source that may be linked with arc magmatism of the upper plate during the Taconic Orogeny. These observations suggest that (1) the subduction responsible for the formation of the Bay of Islands complex nucleated in an oceanic domain before ca. 485 Ma; and (2) the sediments that form the metasedimentary unit were partly sourced from the upper plate and were deposited on the lower plate while subduction was active.

Fragments of ancient oceanic lithosphere preserved in mountain belts, though volumetrically subordinate, provide essential insights into past geodynamics and formation and destruction of oceanic lithosphere. This contribution shows how the two types of oceanic fragments, blueschists and eclogites, on one hand, and ophiolites on the other, preserve crucial information on the dynamics of oceanic convergence, that is, subduction and obduction. Their mutual relationships, as well as the similarities and differences in the mechanisms leading to their preservation, allow tracking the evolution of the subduction process through time, from the onset of intraoceanic subduction to the cessation of continental subduction, and, in some cases, to the obduction of ophiolites. Fragments located at the base and immediately below unmetamorphosed (true) ophiolites represent witnesses of intraoceanic subduction initiation and reveal, in particular, initial mechanical resistance to subduction, subsequent cooling, and gradual strain localization. Subducted fragments of oceanic lithosphere metamorphosed as blueschists and eclogites, scraped off the downgoing slab episodically, at shallow or great depths, provide direct access to the composition, structure, and rheology of rocks at the plate interface. Both types reflect the mechanical behavior and "hiccups" of the subduction plate boundary, during subduction initiation and mature subduction, respectively.

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.

Fragments of ancient oceanic lithosphere preserved in mountain belts, though volumetrically subordinate, provide essential insights into past geodynamics and formation and destruction of oceanic lithosphere. This contribution shows how the two types of oceanic fragments, blueschists and eclogites, on one hand, and ophiolites on the other, preserve crucial information on the dynamics of oceanic convergence, i.e. subduction and obduction.Their mutual relationships, as well as the similarities and differences in the mechanisms leading to their preservation, allow tracking the evolution of the subduction process through time, from the onset of intra-oceanic subduction to the cessation of continental subduction – and, in some cases, to the obduction of ophiolites.Fragments located at the base and immediately below unmetamorphosed (true) ophiolites represent witnesses of intra-oceanic subduction initiation and reveal, in particular, initial mechanical resistance to subduction, subsequent cooling and gradual strain localization. Subducted fragments of oceanic lithosphere metamorphosed as blueschists and eclogites, scraped off the downgoing slab episodically, at shallow or great depths, provide direct access to the composition, structure and rheology of rocks at the plate interface.Both types reflect the mechanical behavior and 'hiccups' of the subduction plate boundary, during subduction initiation and mature subduction respectively.

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.

The Wager shear zone is an ∼450-km long zone of high strain hosted within Proterozoic and Archean rocks of the Rae domain in northwestern Hudson Bay, Nunavut. New field mapping and microstructural analyzes, combined with titanite and apatite geochronology, define the style, kinematics, and timing of Proterozoic ductile deformation. The results indicate that the Wager shear zone accommodated high-temperature deformation between ca. 1.75 and 1.74 Ga with post-kinematic cooling through apatite U–Pb closure (425–530 °C) at ca. 1705 Ma. These new data show that the Wager shear zone was last active later than inferred in previous work, at a time when the western Churchill Province was undergoing lithospheric delamination and exhumation following the terminal collisional events in the western portion of the Trans-Hudson Orogen.

Fragments of former oceans are commonly observed in mountain belts: blueschists and eclogites, on the one hand, and ophiolites, on the other hand, are all that remains of ancient oceanic lithosphere.Though volumetrically subordinate, they provide essential insights into past geodynamics and into the processes involved in the formation and destruction of oceanic lithosphere.This contribution apprehends these two types of oceanic fragments jointly and shows the advantage of doing so for understanding the dynamics of oceanic convergence, i.e. subduction and obduction.We examine the intimate relationships between blueschists/eclogites and ophiolites, as well as the similarities and differences in the mechanisms leading to their preservation.While the extensive, unmetamorphosed true ophiolites markedly differ from fragments of oceanic lithosphere offscraped from the slab during subduction, at shallow or great depths, both types record the mechanical behavior and 'hiccups' of the subduction plate boundary.Their preservation also highlights the importance of the evolution of the subduction regime through time, from the onset of intra-oceanic subduction to the cessation of continental subduction.

A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4 ​± ​0.3 ​Ma. Phase equilibria modelling, combined with Ti-in-biotite and quartz c-axis thermometry, outline a tight-to-hairpin pressure-temperature (P-T) path extending from ~515 ​°C and 5.5 ​kbar to peak conditions at ~575 ​°C and 7 ​kbar followed by deformation during the retrograde phase at 480–515 ​°C and 6–7 ​kbar. The new geochronology data place an upper bound on the evolution of metamorphism and deformation in the frontal-most part of the Himalaya, which lasted until 17.5 ​Ma, as indicated by previously published 40Ar/39Ar data. The P-T-time data from this part of the Himalaya, as well as that from more hinterland-ward portions of the orogen, outline a progressive, stepwise, commonly out-of-sequence evolution. Further data from along the orogen indicates that this evolution is not a local phenomenon, but instead characterizes the tectonics of this system as a whole.

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.

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.