Dissolution-precipitation Creep Research Articles

Relict cataclasis in the high-pressure marbles of the Samaná complex, Northeast Dominican Republic

Cold-cathodoluminescence (CCL) images have unmasked relict cataclastic microstructures in marbles and calc-schists of the higher-pressure units of the Samaná complex. The grain size of the cataclastic microstructures always has a self-similar distribution of power law type, with a slope break at log(ri)=1.7. The large grain size fraction (1.7<log(ri)<2.4) is characterized by fractal dimension D2=2.43 in the calcite-microstructures of the Punta Balandra unit and D2=2.72 in the Santa Bárbara unit. Both D-values are within the usual range of the carbonate-cataclasites and they are significant with the linear correlation of R2=0.95 and 0.93, respectively. However, the finer grain size fraction (0.9<log(ri)<1.7) is characterized by D1<1 and this grain-size distribution is beyond the fractal-range for pure cataclastic-fabric, even though it has better fit power-law distribution. Microstructures such as dissolution-surfaces, interpenetrated particles, and the higher roundness and lower irregularity of the boundary grain from the fine fraction indicate that dissolution-precipitation creep was dominant after cataclasis. Clast size and stress reduction after cataclasis and the high fluid pressure provided favorable conditions to allow dissolution-precipitation creep. Transition from cataclastic flow to dissolution-precipitation creep was scale-dependent and it tended to homogenize and reduce the finer grain size fraction. Consequently, the slope break between D1 and D2 is interpreted as a record in the grain size distribution produced by a change in the dominant deformation mechanism.

Costerfield antimony-gold deposit, southeast Australia: Coupling between brittle deformation and dissolution-precipitation reactions in the Melbourne Zone

The antimony-gold deposits at Costerfield are a swarm of ‘dyke-like’ antimony-gold lodes within a brittle transpressional fault system. They are hosted in the Lower Silurian sedimentary rocks of the Melbourne Zone of the Lachlan Orogen. Individual lodes are dominated by stibnite or stibnite-gold and widths vary from 1cm to ∼200cm (averaging 30cm) and lengths can exceed 1km. Lodes occur as fault-parallel layers or lenses mantled by deformed quartz-rich margins or overgrowths of hydrothermal quartz and carbonate. Inclusions in the stibnite-rich lodes include vuggy randomly oriented prismatic crystals, crack-sealed and stretched quartz with sugary mosaics of equant quartz grains and quartz breccias. Randomly oriented quartz and stibnite textures, common in the centre of the lodes, indicate free growth from a fluid. Throughout the mineralized system, the hydrothermal stibnite has replaced and overgrown the earlier quartz. Deformed lode margins preserve sub-horizontal lineations and evidence of oblique crack-seal re-opening that reflect emplacement associated with late sinistral strike-slip movements in bounding faults. Crystallographic orientation data collected from the marginal quartz, using neutron diffraction and supported by microstructural observations, reveal that lode emplacement also involved dissolution-precipitation creep and pressure solution. Both the marginal quartz and the lodes are overprinted by later microfractures, within which there has been precipitation of stibnite, pyrite, arsenopyrite, sphalerite and minor gold. 3D neutron tomography revealed individual gold crystals, with gold voxel sizes ranging from 140µm to ∼580µm, were located in late arsenopyrite-gold domains that overprint the stibnite. The mechanical evolution of the lodes involved short-lived brittle processes in combination with dissolution-precipitation crystallization in active fault zones. These ore bodies provide insights about the dynamics of fluid flow and flow velocities when faults breach over-pressured reservoirs of hydrothermal fluid. Our results also suggest that the Costerfield deposit was formed directly from metal-rich metamorphic fluids produced during Tabberabberan (∼375Ma) prograde metamorphism of sedimentary rocks near the base of the Lower Paleozoic Melbourne Zone, which lie above the Proterozoic Selwyn Block. From a reinterpretation of seismic reflection data, the deformation in the Melbourne Zone sedimentary wedge is characterized by thrusts isolating relatively undeformed packages of sedimentary units, which were moved in a piggyback manner up west-dipping faults. The ore-bearing fluids migrated upwards, at least in part, along these footwall thrusts from the base of the Heathcote Fault Zone. In the upper levels of the sedimentary package, out-of-sequence strike-slip, sub-vertical splays from these second order faults carried the mineralizing fluids to the orebodies. East-west compression during the Tabberabberan Orogeny preceded a northwest-southeast-oriented stress field which reflects the collision of VanDieland against the Gondwanan margin. This produced the sinistral strike-slip faults, which in turn controlled the overall permeability of the hydrothermal system.

Fabric evolution of polydeformed orthogneisses and quartzites along the Curitiba Shear Zone, Curitiba Domain, Southern Brazil

In Southern Brazil there is a series of strike-slip shear zones that represent the late-collisional event of Western Gondwana in the Neoproterozoic-Eopaleozoic transition. The Curitiba Shear Zone (CSZ) is a part of this strike-slip regime separating two different domains: the supracrustal metasedimentary rocks and the basement composed of orthogneiss, migmatite and quartzite. The absence of detailed structural data for this discontinuity has motivated us to study this important part of the geology of southern of Brazil. The main purpose of this work is the characterization of meso and microstructures and determination of crystallographic fabric of quartz in orthogneisses and quartzites along the CSZ southern block. Structural data suggest that the CSZ consists of one ductile-brittle strike-slip deformation phase leading to the deformation of orthogneiss and quartzites of the Atuba Complex. The ductile regime was responsible for the formation of low-grade mylonite in greenschist facies. Crystallographic texture indicate a restricted recrystallization of quartz mainy via dislocation creep through activation ob basal <a> slip system. The brittle regime led to the formation of cataclasites, faults and joints, with a mean orientation of N50E/75SE and N33E/80SE, respectively. Extensive dissolution-precipitation creep affected the rocks and infilled veins with carbonate and quartz.

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone, New Hampshire, USA

AbstractWe describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na‐poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric‐forming Na‐rich muscovite is also progressively replaced by fabric‐forming Na‐poor muscovite. The mineralogy of the new phyllonitic fault‐rock produced is dominated by Na‐poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric‐forming Na‐rich muscovite is selectively replaced at high‐strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high‐grade porphyroblasts by weaker Na‐poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na‐rich muscovite‐defined foliation destroys not only the metastable high‐grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.

Deformation-induced silica redistribution in banded iron formation, Hamersley Province, Australia

The formation of banded iron formations (BIF) remains controversial despite their potential to provide key information on Precambrian atmospheres and hydrospheres. It is widely agreed that BIF are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases. Many models address the chemical transformation of such precursor iron oxide phases into BIF during compaction and diagenesis. However, the formation of chert and the influence of physical forces in this process have received less attention.Microstructural analysis of BIF from the Hamersley Province (Western Australia) reveals that significant amounts of silica were redistributed by dissolution-precipitation creep during both diagenesis and regional-scale deformation. This physicochemical process led to silica remobilisation and volume loss by stress-induced dissolution of microcrystalline quartz in an aqueous fluid. The dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low-pressure zones, leading to local volume increase. These processes were further enhanced by rheological contrasts between different minerals, resulting in significant variations of chert band thickness.Microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations (CPO) in quartz grains within chert layers. The CPO fabrics record strain regimes (e.g., pure and simple shear, extension and shortening) that modified quartz aggregates by dissolution-precipitation creep, providing new insights into the metamorphic and deformation history of BIF. We document microstructures that indicate that non-coaxial deformation was active during diagenesis and subsequent deformation of the Hamersley Province BIF. Further, relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution-precipitation creep, providing a possible explanation for the destruction of primary features.We show that different generations of chert alternate on a micro-scale, suggesting that the role of deformation has been underestimated in many aspects of BIF research. The discovery of dissolution-precipitation creep in low-grade metamorphic BIF provides a potential link between the physical and chemical processes that occur within these rocks after deposition. In the Hamersley Province, dissolution-precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events. We also suggest that chert layers without CPO record the least overprint by diagenesis, and as the most pristine form of BIF should be targeted for future paleo-environmental research.

Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle

We report microstructural evidence for fluid-assisted ductile strain localization in a ≤50m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain localization occurred at relatively low pressure (<0.8GPa) and moderate temperature (750–1000°C). Initial deformation by dislocation creep resulted in formation of mylonites. Focusing of aqueous fluids in the shear zone favored the activation of dissolution–precipitation creep, resulting in further strain localization. This process is recorded by two generations of ultramylonitic bands composed of fine-grained, well-mixed olivine–orthopyroxene aggregates. Microstructural observations in the ultramylonites suggest alternating dissolution and precipitation of olivine and orthopyroxene, which may be explained by local changes in silica molality of the percolating fluid (disequilibrium and mass transfer at scales > mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between strain localization and fluid transport.

Structural analysis and shape-preferred orientation determination of the mélange facies in the Chañaral mélange, Las Tórtolas Formation, Coastal Cordillera, northern Chile

This study sheds light on the tectonic and structural knowledge of the mélange facies located to the south of Chañaral city, Chile. The Chañaral mélange has been related to an accretionary prism at the western active continental margin of Gondwana. Based on the fossil content, the original turbidite sequence would have been deposited during Devonian to Carboniferous times.The Chañaral mélange is included in the Las Tórtolas Formation, which corresponds to the Paleozoic metasedimentary basement located in the Coastal Range in northern Chile. It consists of a monotonous sequence of more than 90% of interbedded sandstones and shales, with a few limestones, pelagic chert, conglomerates and basic volcanic rocks, metamorphosed to the greenschist facies. In the study area, the Las Tórtolas Formation is divided into two structural domains separated by a major reverse dextral structure, called here the Infieles fault. To the east, the Las Tórtolas Formation is characterized by a brittle–ductile deformation, defined by the original sedimentary contacts in the turbiditic sequence. Besides, thrust faults and associated thrust propagation folds promotes a penetrative axial plane foliation. Mélange facies are located to the west of the Infieles fault. Although lithologies comprising this domain are similar to the rest of the Las Tórtolas Formation, mélange facies (ductile domain) are characterized by the complete disruption of the original architecture of the turbidite succession. The most significant structures in the mélange are the ubiquitous boudinage and pinch and swell structures, asymmetric objects, S–C structures and tight to isoclinal folds. This deformation is partitioned in the Chañaral mélange between linear fabric domains (L), characterized by quartzite blocks with prolate shape in a phyllite matrix with pencil structures, and linear-planar fabric domains (L-S), where quartzite objects show oblate shape and phyllites present a penetrative foliation. The intensity of deformational process is reflected in the high aspect ratios yielded by the quartzite constrictive (L) and flattened (L-S) object axes.Meso-scale shape preferred orientation (SPO) has been compared with quartz microtextures in quartzite blocks, resulting aspect ratios considerably lower than those obtained from the mesoscopic fabric. Main deformation mechanism observed in quartz microtextures are bulging-subgrain rotation recrystallization and dissolution-precipitation creep for pure and impure quartzites respectively. The temperatures deduced from these microtextures are between 350 and 400 °C, which coincides with the greenschists facies metamorphism observed in the Las Tórtolas and the mélange facies. Extremely dissociation between micro- and meso-scale deformation could be generated by dissolution at high differential stress in the boundaries of the quartzite layers and precipitation at low differential stress parts, which would increase the aspect ratio of the lenses whereas internally, quartz would have remained virtually unstrained.We propose here a tectonic setting for the Chañaral mélange formation based on the geodynamic evolution of the western active margin of South America during Late Paleozoic to Early Jurassic. Thus, the study area is located in a LT-HP zone of an accretionary complex, where rocks from the subduction channel (mélange facies) and the basal domain of the prism (brittle–ductile domain of the Las Tórtolas Formation) are in contact through the Infieles fault.

A mechanical model for dissolution–precipitation creep based on the minimum principle of the dissipation potential

In contrast to previous approaches that consider dissolution–precipitation creep as a multi-stage process and only simulate its governing subprocess, the present model treats this phenomenon as a single continuous process. The applied strategy uses the minimum principle of the dissipation potential according to which a Lagrangian consisting of elastic power and dissipation is minimized. Here, the elastic part has a standard form while the assumption for dissipation stipulates the driving forces to be proportional to two kinds of velocities: the material-transport velocity and the boundary-motion velocity. A Lagrange term is included to impose mass conservation. Two ways of solution are proposed. The strong form of the problem is solved analytically for a simple case. The weak form of the problem is used for a finite-element implementation and for simulating more complex cases.

LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems

The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution–precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite.This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet–kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40–36Ma and from subsequent shearing through cooling until 36Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U–Pb data on rutile and Rb–Sr data of biotite.Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I–IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130–150Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution–precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced.Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and altered monazite of the three pre-mylonitic domains by having a significantly more pronounced negative Eu anomaly, a flatter HREE pattern, and high Th content. These compositional characteristics are linked with syn-mylonitic formation of plagioclase and resorption of garnet in the shear bands under amphibolite facies conditions. The absence of pre-mylonitic monazite in the shear zones, in contrast to the other domains, suggests complete dissolution of old and formation of new monazite. This probably results from an increased alkalinity and reactivity of the fluid that again is controlled by syn-mylonitic interaction with feldspar and apatite in the shear zones. There, the deformation was accommodated by dissolution precipitation creep at ca. 690±50°C and 6–7.5kbar. Growth of monazite at 55±1Ma dates this deformation, which precedes the regional migmatization of the Sidironero Complex, whereas rutile and biotite ages reflect these later stages. This new pressure–temperature–time constraint for a relictic deformation structure provides insight into the still missing parts of the overall metamorphic, deformation and exhumation processes of the UHP units in the Rhodope.

New indicators from bedding-parallel beef veins for the fault valve mechanism

Beef structures (bedding-parallel veins of fibrous calcite) are widespread within the Lower Triassic carbonate rocks in the Sichuan Basin of China, especially within clay-rich strata of low permeability. In the veins, fibrous calcite occurs in the outer zones, and coarse equant calcite in the inner zones. At least two generations of calcite crystallization took place during aqueous alteration, at the same time as deformation recorded by the calcite. The first-generation calcite fibers are at steep angles to the hydrocarbon-bearing host beds, and they grew vertically against the force of gravity at a time when the source rocks were maturing. Second-generation calcite occurs as coarse equant grains that sealed pores via localized fluid flow during horizontal tectonic compression, so that shear stresses acted at the fracture margins. Shearing of the host rock was accommodated in part by dissolution-precipitation creep (DPC), grain rotation, and grain slippage, recorded in crystallographic preferred orientations (CPOs) of the host calcite grains beside the crack walls. The beef veins formed during high pore-fluid overpressures along hydrofractures. We propose that the bedding-parallel veins with beef structures are evidence of a “crack-seal slip” fault valve process during hydrocarbon generation. The hydrocarbon-bearing calcite beef structure may be a good indicator of oil or gas migration, and of the flow direction of aqueous solutions.

Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

The deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8–1.0 GPa, 725–850 °C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4–0.6 GPa, 550–650 °C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolution-precipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

The Early-Middle Miocene subduction complex of the Louisiade Archipelago, southern margin of the Woodlark Rift

Field, microstructural, and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early-Middle Miocene N-dipping subduction of the Australian margin and transpression along the Australian-Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist-facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top-to-the-SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE-SSW shortening and ESE-WNW extension via dissolution-precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step-heating analyses of three white mica separates yield Middle Miocene plateau or plateau-like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ∼12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for crosscutting brittle strike-slip faults. These data are critical to constraining the subduction-exhumation history of the world's youngest high-ultrahigh-pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.

Possible control of subduction zone slow-earthquake periodicity by silica enrichment.

Seismic and geodetic observations in subduction zone forearcs indicate that slow earthquakes, including episodic tremor and slip, recur at intervals of less than six months to more than two years. In Cascadia, slow slip is segmented along strike and tremor data show a gradation from large, infrequent slip episodes to small, frequent slip events with increasing depth of the plate interface. Observations and models of slow slip and tremor require the presence of near-lithostatic pore-fluid pressures in slow-earthquake source regions; however, direct evidence of factors controlling the variability in recurrence times is elusive. Here we compile seismic data from subduction zone forearcs exhibiting recurring slow earthquakes and show that the average ratio of compressional (P)-wave velocity to shear (S)-wave velocity (vP/vS) of the overlying forearc crust ranges between 1.6 and 2.0 and is linearly related to the average recurrence time of slow earthquakes. In northern Cascadia, forearc vP/vS values decrease with increasing depth of the plate interface and with decreasing tremor-episode recurrence intervals. Low vP/vS values require a large addition of quartz in a mostly mafic forearc environment. We propose that silica enrichment varying from 5 per cent to 15 per cent by volume from slab-derived fluids and upward mineralization in quartz veins can explain the range of observed vP/vS values as well as the downdip decrease in vP/vS. The solubility of silica depends on temperature, and deposition prevails near the base of the forearc crust. We further propose that the strong temperature dependence of healing and permeability reduction in silica-rich fault gouge via dissolution-precipitation creep can explain the reduction in tremor recurrence time with progressive silica enrichment. Lower gouge permeability at higher temperatures leads to faster fluid overpressure development and low effective fault-normal stress, and therefore shorter recurrence times. Our results also agree with numerical models of slip stabilization under fault zone dilatancy strengthening caused by decreasing fluid pressure as pore space increases. This implies that temperature-dependent silica deposition, permeability reduction and fluid overpressure development control dilatancy and slow-earthquake behaviour.

Interplay between fluid flow and fault–fracture mesh generation within underthrust sediments: Geochemical evidence from the Chrystalls Beach Complex, New Zealand

The Chrystalls Beach Complex, in the Otago Schist on the South Island of New Zealand, is a melange comprising sheared trench-fill sediments and fragments of oceanic crust. It represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces. The melange is cross-cut by a fault–fracture mesh, comprising subvertical extension veins and subhorizontal slickenfibre-coated shear surfaces. Both shear and extension veins have a ‘crack–seal’ microstructure indicating episodic growth. Shear veins are associated with pressure solution selvages along the shear surface, whereas wall rock alteration is not observed adjacent to extension veins. Electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins, and therefore these slickenfibres probably grew by local dissolution–precipitation of silica. On the contrary, no depletion or addition of silica is detected around extension veins, indicating these veins grew by precipitation from advecting fluids. Oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 %° < δ18O < 10 %°, consistent with a fluid derived from low-grade metamorphic dehydration reactions. Fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable melange with increasing metamorphic grade and decreasing porosity. Fault–fracture mesh generation therefore involved localized shear assisted by dissolution–precipitation creep and concomitant extension fracturing. This led in turn to transient permeability associated with a fluid pressure drop, allowing episodic vein growth. This process may be analogous to geophysically observed episodic tremor and slow slip, which also involves a mixture of deformation styles that, put together, achieve shear slip along the subduction thrust interface.

Mid-crustal shear zone formation in granitic rocks: Constraints from quantitative textural and crystallographic preferred orientations analyses

Abstract This paper presents quantitative microstructural and crystallographic preferred orientation (CPO) analyses of an Alpine amphibolite facies shear zone developed in the Fibbia metagranite (Gotthard massif, Central Alps). The weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse-grained monomineralic aggregates, derived from quartz and K-feldspar porphyroclasts, and softer fine-grained plagioclase-bearing shear bands. A characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates. CPOs and microtextures suggest that quartz and K-feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid-assisted diffusive mass transfer and grain boundary sliding. In the mylonite and ultramylonite shear zone core, the porphyroclasts-derived quartz and K-feldspar layers are broken-down to produce a polyphased matrix that is characterized by a homogeneous micron-scale grain size and regular/random distribution. Here, the deformation of the whole aggregate occurs via a fluid-assisted dissolution–precipitation creep and grain boundary sliding, referred as a fluid-assisted granular flow. We propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain. The lateral broadening of the shear zone is driven by (1) the increase in fluid pressure in permeable albite–oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates, and (2) the thermodynamic re-equilibration via metasomatic reactions of the shear zone walls.

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution–precipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution–precipitation creep and low stresses during subduction and exhumation.

Rheology of the plate interface — Dissolution precipitation creep in high pressure metamorphic rocks

Subduction zone models invoke deformation to be concentrated along the plate interface, in a region of particularly low temperature. Geophysical observations do not provide constraints on temperature, stress and deformation patterns with desired resolution. In contrast, the record of high pressure metamorphic rocks exhumed from subduction zones provides details on P–T-history, deformation mechanisms, and stress state, albeit not readily correlated with the former dynamic situation on larger scale. Here we review available information on dissolution precipitation creep (DPC) in high pressure metamorphic rocks, which – if representative for subduction zones in general – can pose constraints on conditions, rheology, and flow patterns along the plate interface. The key observations and conclusions are that: (1) Deformation is typically highly inhomogeneous and localized into shear zones; (2) stresses are generally too low to drive crystal plastic deformation; (3) microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism; (4) an aqueous fluid at quasi-lithostatic pressure is available throughout, allowing for tensile fracturing and crack healing or sealing; (5) low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity; and (6) contribution of shear heating to the thermal budget of subduction zones should be moderate. The dominant deformation mechanism DPC is reviewed in some detail, including experimental and theoretical approaches. Various examples of DPC in high pressure metamorphic rocks are illustrated, emphasizing the role of interphase boundaries as sites of dissolution. Rheology governed by DPC is proposed to control interplate coupling and development of a subduction channel with return flow, being a likely candidate for rapid exhumation of high pressure metamorphic rocks.

Strain localization in homogeneous granite near the brittle–ductile transition: A case study of the Kellyland fault zone, Maine, USA

We present a detailed case study of a strike-slip shear zone that cut homogeneous granite near the brittle–ductile transition. This zone contains three distinct strain facies: (1) a 2–3-km-wide belt of foliated granite, (2) a 100–300-m-wide belt of small localized shear zones, and (3) a 200–400-m-wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites. The foliated granite formed via dislocation creep of quartz and minor dissolution–precipitation creep and dislocation creep of feldspar under amphibolite-facies conditions shortly after granite crystallization. The localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite, and they deformed by granular flow—grain-boundary sliding coupled with dissolution–precipitation creep. These deformation features reveal a three-phase rheologic evolution in granite near the brittle–ductile transition. Phase 1 is recorded by the foliated granite, and the rheology was governed by dislocation creep of quartz. Phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone. Phase 3 was a long-lived period of ductile deformation, and the rheology was governed by the granular flow mechanism. Strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation.

The effects of Dissolution–Precipitation Creep on quartz fabrics within the Purgatory Conglomerate, Rhode Island

Quartz Crystallographic Preferred Orientation (CPO) patterns are most commonly a result of deformation by dislocation creep. We investigated whether Dissolution–Precipitation Creep (DPC), a process that occurs at lower differential stresses and temperatures, may result in CPO in quartz. The Purgatory Conglomerate is part of the SE Narragansett basin where strain intensity increases from west to east and is associated with top-to-the-west transport and folding during the Alleghanian orogeny. Within the Purgatory Conglomerate, DPC led to quartz dissolution along cobble surfaces perpendicular to the shortening direction, and quartz precipitation in overgrowths at the ends of the cobbles (strain shadows), parallel to the maximum extension direction. Quartz c-axis orientations as revealed by Electron Backscatter Diffraction (EBSD) methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample. Quartz dissolution probably occurred exclusively along the cobbles' margins, leaving the remaining grains unaffected by DPC. The fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress.

Low stress deformation of garnet by incongruent dissolution precipitation creep

Microstructures indicating incongruent dissolution precipitation creep of garnet in eclogite-facies graphitic micaschist (Tauern window, Eastern Alps) are investigated. Garnet dissolution is observed where garnet poikiloblasts grown at eclogite facies metamorphism approached each other as a consequence of progressive deformation during exhumation, with estimated P-T-conditions between 570 °C, 1.7 GPa and 470 °C, 0.9 GPa. The poikiloblasts are separated by a dissolution seam and flanked by strain shadows filled with quartz, white mica, and chlorite; there is no evidence for crystal plastic deformation of garnet. Two cases are investigated: (A) stylolitic contact zone, (B) smooth contact zone. In both cases, internal fabrics of the poikiloblasts and concentric chemical zoning are truncated. Material previously forming inclusions in the garnet poikiloblasts is now passively enriched in a dissolution seam, the original microstructure of fine-grained mica–graphite aggregates remaining preserved. Though microstructures suggest that garnet dissolution was driven by local stress concentration, the level of differential stress remained too low for plastic deformation of the fine-grained white mica-graphite aggregates set free from the stress supporting garnet. Incongruent dissolution precipitation creep appears to be a particularly effective deformation mechanism at low stress in a subduction channel.