Dissolution-precipitation Creep Research Articles

The structure of reactive grain-boundaries under stress containing confined fluids

We present numerical experiments on structure development in grain-boundaries during dissolution–precipitation creep. Two solids that are represented by an elastic spring configuration are pressed together with a compressible fluid in the grain-boundary. The solid can dissolve or precipitate depending on elastic and surface energy as well as fluid pressure and concentration of dissolved material in the fluid. We perform a number of numerical experiments with different starting configurations that represent a large-scale island-channel interface with solid–solid contacts across the islands, a rough grain-boundary interface with a fluid along the whole interface and a smooth thin-film interface. The simulations suggest that the solid–solid islands become unstable by necking and anti-cracking so that the island-channel interface will develop into a grain-boundary interface. The rough interface of the grain-boundary will coarsen with time so that in the extreme case a smooth thin-film interface will emerge. However the thin-film interface is also not necessarily stable but may develop into a rough grain-boundary where the roughness is initiated by heterogeneous dissolution of the solid. Our numerical experiments suggest that interface structures during dissolution–precipitation creep are transient and that the dominance of one interface structure depends on material properties, the displacement rate, purity of the reacting solid and the scale of observation.

Retrieval of P–T information from shear zones: thermobarometric consequences of changes in plagioclase deformation mechanisms

Changes in deformation mechanism coupled with spatial and temporal variations in reaction rates can result in preservation of disequilibrium mineral compositions in rocks affected by synmetamorphic shearing. Thermobarometric calculations on such rocks may thus yield meaningless results. We use Garbenschiefer samples from a shear zone in the Eastern Alps to study the effects of different deformational processes on calculated pressures and temperatures in samples that experienced the same overall PTt history. We focus on plagioclase, which accommodates strain by a variety of deformation mechanisms and is a key mineral in many thermobarometers. Plagioclase that deformed largely via dislocation creep mechanisms shows concentric chemical zoning, whereas plagioclase that experienced dissolution-precipitation creep preserves complex zoning. Rim compositions in the latter domains are not necessarily the youngest compositions, nor did they typically equilibrate with other phases in the assemblage. The timing of hornblende breakdown reactions relative to changes in plagioclase deformation mechanism also affected chemical zoning. Samples that escaped shear strain while near the thermal maximum yield internally consistent thermobarometric results, whereas samples that experienced shearing near the thermal maximum yield scattered results. Some of the variability in the results likely represents real differences in the P–T conditions at which equilibration occurred during deformation. However, much of the variability represents spurious results obtained by pairing mineral compositions that were never in equilibrium with one another. Extraction of useful P–T information from samples that experienced synmetamorphic deformation requires careful documentation of the relationships between deformation mechanisms and chemical zoning in order to select appropriate mineral compositions for thermobarometric calculations.

Dating of subduction and differential exhumation of UHP rocks from the Central Dabie Complex (E-China): Constraints from microfabrics, Rb–Sr and U–Pb isotope systems

The correlation of deformation fabrics and metamorphic reactions with geochronologic data of UHP metamorphic rocks demonstrate that the multistage subduction and exhumation history of the Central Dabie Complex requires rapid subduction and rapid initial exhumation. Moreover, these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems. Thus, our age data do not simply reflect a thermal/cooling history. In the investigated section, the maximum age for UHP is given by the 244 ± 3 Ma (2 σ) U–Pb age of a pre-UHP titanite phenocryst that survived UHP metamorphism and subsequent tectonometamorphic events. A minimum age for UHP is set by the 238 ± 1 Ma (2 σ) 238U– 206Pb mineral isochron age of titanite and cogenetic epidote. These minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust. In the period of ca. 238–218 Ma, the UHP terrain records HT metamorphism, local partial melting, and extensive pervasive strain below the eclogite (jd + grt) stability field. Exhumation was polyphase with a first phase of fast exhumation, succeeded by episodes of HT metamorphism and concomitant deformation at deep/mid crustal level between 238 and 218 Ma. Slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca. 209–204 Ma. The resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep, which was the dominant deformation mechanism in quartz–feldspar rocks, in combination with fluid influx.

Deformation and fluid flow during orogeny at the palaeo‐Pacific active margin of Gondwana: the Early Palaeozoic Robertson Bay accretionary complex (north Victoria Land, Antarctica)

AbstractStructural investigations, integrated with X‐ray diffraction, fluid inclusion microthermometry and oxygen‐stable isotope analyses are used to reconstruct the deformation history and the palaeo‐fluid circulation during formation of the low‐grade, turbidite‐dominated Early Palaeozoic Robertson Bay accretionary complex of north Victoria Land (Antarctica). Evidence for progressive deformation is elucidated by analysing the textural fabric of chronologically distinct, thrust‐related quartz vein generations, incrementally developed during progressive shortening and thickening of the Robertson Bay accretionary complex. Our data attest that orogenic deformation was mainly controlled by dissolution–precipitation creep, modulated by stress‐ and strain‐rate‐dependent fluid pressure cycling, associated with local and regional permeability variations induced by the distribution and evolution of the fracture network during regional thrusting. Fracture‐related fluid pathways constituted efficient conduits for episodic fluid flow. The dominant migrating fluid was pre‐to‐syn‐folding and associated with the migration of warm (160–200 °C) nitrogen‐ and carbonic (CO2 and CH4)‐bearing fluids. Both fluid advection and diffusive mass transfer are recognized as operative mechanisms for fluid–rock interaction and vein formation during continuous shortening. In particular, fluid–rock interaction was the consequence of dissolution–precipitation creep assisted by tectonically driven cooling fluids moving through the rock section as a result of seismic pumping. The most likely source of the migrating fluids would be the frontal part of the growing accretionary complex, where fluids from the deep levels in the hinterland are driven trough channelization operated by the thrust‐related fracture (fault) systems.

Introduction: linked processes of metamorphism and deformation

The interaction of deformation and metamorphism within the crust results in textures, structures and mineral growth that provide a record of tectonic activity as well as the progressive metamorphic and structural development of the crust. Analysis of these textures and structures is central to deciphering and evaluating the progressive nature of their development. Such rocks are commonly interpreted from the viewpoint of either structural or metamorphic processes; however, integration of these two perspectives has proved to be a powerful tool in enhancing our understanding of the structural development of metamorphic rocks at all scales. In recent decades, research into the linkages between metamorphism and deformation has increasingly involved the integration of textural, chemical and structural data. The present volume arose from a topical session titled Understanding Coupled Metamorphic and Deformational Processes: Advances in Integrated Textural, Chemical and Microstructural Analysis held at the 2003 annual meeting of the Geological Society of America, Seattle, USA. We were pleased that the session attracted 21 contributions from 48 geologists representing seven countries. The breadth of topics covered by the presentations, and the interest expressed by both participants and attendees encouraged us to assemble a set of thematic papers related to the topical session. The result is the present special issue of the Journal of Metamorphic Geology. Whitmeyer & Wintsch document the linkages between stress, metamorphic reactions, grain shapepreferred orientation (SPO) and reactivation. They describe the reactivation of an amphibolite facies ultramylonite along discrete greenschist facies mylonitic shear bands. A random SPO within the ultramylonite created a texturally hardened rock that displaced strain into adjacent weaker foliated gneiss. Later replacement reactions within the ultramylonite were concentrated at areas of high stress and resulted in a chemical and mechanical softening of the rock via the development of a SPO of muscovite, chlorite and quartz. Reactivation of the ultramylonite was localized along discrete mylonitic shear bands at the sites of these reactions and accompanying SPO development. The relationship between strain and SPO development is also explored by Stallard et al. who measured the 3D shape, size and orientation of quartz grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand. Quartz SPO was initiated by dissolution–precipitation creep at sub-greenschist facies conditions, and was triggered by strain accompanying fold development. Detrital grains became blade-shaped, and were an important component of the metamorphic foliation. A second fold event, accompanied by crenulation cleavage development and resulting segregation of the rock mass, initiated a change in both the texture of the quartz SPO and deformation mechanisms. Previously discrete and aligned detrital quartz grains were replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and alignment. Pressure solution then affected the margins of quartz-rich layers rather than individual grains. The authors emphasize the contribution of quartz SPO to foliation development and the significance of fold events as a control on SPO formation and development. In a companion paper that analyses the 3D shape, size and orientation of white mica grains from the same samples, Stallard & Shelley describe the initiation and development of white mica SPO with increasing metamorphic grade. The character of white mica SPO is determined early in the deformation and metamorphic history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic micas in favour of numerous new smaller metamorphic grains of contrasting composition, higher aspect ratio and a strong SPO. The SPO results from the progressive competitive anisotropic growth of blade-shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher-grade samples. Selverstone describes how changes in fluid composition correlate with changes in mechanical behaviour of graphitic schists during decompression. Interlayered graphitic and non-graphitic schists from the Tauern Window, Eastern Alps, record different mechanical behaviour during extensional exhumation. Graphitic schists contain brittle extensional structures oriented J. metamorphic Geol., 2005, 23, 409–410 doi:10.1111/j.1525-1314.2005.00593.x

The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain‐shape data

AbstractShape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.

Investigation of loaded halite-SiO2 interfaces undergoing dissolution-precipitation processes

Deformation assisted by transport in aqueous solutions is important for the rheology of earth materials, and models commonly used to describe deformation by dissolution-precipitation creep assume a steady state process in a closed system which is driven by the potential energy of the system. Here, we studied the effect of uni-axial stress on the convergence of halite crystals in contact to SiO2 at dry conditions and in presence of saturated solution by optical interferometry. Adding saturated solution caused the convergence to increase strongly. Simultaneously, Linnik-based phase shift interference microscopy was used to in situ monitor the nano-scale morphology of the loaded surface of the crystal. These investigations revealed that the measured crystal convergence is not dominated by dissolution within the loaded halite-SiO2 interface but rather by an increased plasticity of the halite crystal due to the addition of saturated solution. Temporarily reducing the stress reduces the elastic strain energy of the crystal and potential energy of the system as driving force for dissolution, but increases the potentiality for material transport from the interface into the bulk fluid. Thus, mechanisms which cause a remanent excess energy contribute to increased solubility and material flux during periods of reduced stress. Local plastic deformation satisfies this condition. We expect from our results that episodic cycling of tectonic stress, pore pressure, or solute activity significantly enhances deformation rates in rocks.

Rheology of fine-grained siliciclastic rocks in the middle crust—evidence from structural and numerical analysis

Constitutive models to describe the rheology of the crust are based on a combination of indirect methods, and many aspects are still unknown and controversial. Here we present a new method to quantify the rheology of fine-grained siliciclastic rocks, which are common in the middle crust and deform by solution-precipitation processes. We use a combination of structural analysis at different scales and a geomechanical model, to develop a parameter estimation scheme to calculate rheological parameters at 350–400 °C and geologic strain rates. The results of this study demonstrate that fine-grained siliciclastic rocks in the middle crust have a Newtonian viscous rheology, approximately 10 times weaker than wet quartz. This is in agreement with observed microstructures. Our results imply that the strength of polyphase quartz-rich rocks located in the middle crust is much lower than predicted by conventional models based on flow laws for dislocation creep. Because fine-grained siliciclastic rocks control the rheology of the middle crust in many sedimentary basins, our results provide new quantitative parameters for geodynamic modelling in settings where dissolution-precipitation creep is important.

Development of single-crystal σ-shaped quartz porphyroclasts by dissolution–precipitation creep in a calcite marble shear zone

Within a greenschist facies calcite marble shear zone, isolated quartz grains change shape across the shear zone profile. Whereas quartz grains have a spherical to elongated shape in the coarse grained marble protolith and protomylonite, they are asymmetric σ-shaped porphyroclasts with wedge shaped appendages (wings) in ultramylonites. In all cases quartz grains are single-crystals. They are sometimes twinned but never recrystallized. Stress-induced dissolution–precipitation creep is the favourable shape-controlling process during deformation providing a relatively undeformed core, truncation of pre-existing cathodoluminescence (CL) patterns and syntaxial precipitation of wings. The necessary fluids may have been released from fluid inclusions during dynamic recrystallization of the calcite matrix. The development of σ-shape is not related to the crystallographic orientation of the quartz porphyroclasts. Crystallographic orientation analysis by electron backscatter diffraction (EBSD) and CL analyses exclude crystal-plasticity as a shape-controlling mechanism of quartz grains during mylonitisation. However, in ultramylonites quartz clasts have a strong crystallographic preferred orientation (CPO), with c-axes (sub)parallel to the shear direction. This fabric is uncommon for quartz under greenschist facies conditions. It might be explained by a strengthening of a pre-existing weak CPO during mylonitisation by rigid particle rotation of elongated quartz grains.

Instabilities in Stress Corrosion and the Transition to Brittle Failure

Studies on the dissolution of stressed crystals have shown that stress corrosion can lead to roughening of interfaces. The corrosion pattern develops due to gradients in elastic energy along a free crystal surface with an initial roughness. Dissolution will cause further increase in elastic energy, which in turn will speed up dissolution, the so-called Asaro-Tiller-Grinfeld instability. We present a numerical model in order to study the effect of stress corrosion that is observed in physical experiments of interface roughening of brittle salt crystals. In the simulations a salt crystal that is immersed in saturated brine is stressed and dissolution patterns develop. Stress corrosion can lead to an Asaro-Tiller-Grinfeld instability that develops into cusp instabilities and crack-like structures or anti-cracks. The system breaks its symmetry and produces a secondary instability where the number of growing anti-cracks is reduced. This coarsening of the surface roughness is due to stress-shielding effects of growing anti-cracks. Finally, one single anti-crack remains on each side of an initial hole as a “superstructure”. The large anti-crack grows at an almost constant speed and eventually leads to brittle failure of the crystal. This mechanism may be important not only for dissolution-precipitation creep but also other phase-transitions and may lead to large scale brittle failure that can be associated with earthquakes.

Phase shift interference microscope study of dissolution-precipitation processes of nonhydrostatically stressed halite crystals in solution

Halite single crystals in saturated solution were used to study dissolution precipitation creep (DPC) at conditions where plastic deformation is negligible. Specifically, the free unloaded surfaces of these crystals were investigated by a novel Linnik-based phase shift interference microscope. The method allows observations of the crystal surface in-situ and with an axial resolution in the nanometer scale. Transport phenomena in open systems, temperature gradients, and gradients in strain energy density were found to cause morphological changes on the free crystal surface by dissolution/reprecipitation. We did not find evidence for DPC by applying a homogeneous stress field to the crystal as long as plastic deformation was avoided. These findings suggest that deformation of rocks by DPC in situations where dislocation creep is not activated, but is rather promoted by fluid transport through the rock or by episodic changes of extensive parameters affecting solubility than by homogeneous stress alone.

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈 a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.

Exhumation rates of high pressure metamorphic rocks in subduction channels: The effect of Rheology

Exhumation of high‐pressure metamorphic rocks can take place with typical plate velocities of cm/year. This is consistent with a model of forced flow in a subduction channel. The (micro)structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep, according to a power‐law rheology. Instead deformation appears to be localized in low‐strength shear zones, and is dominated by dissolution precipitation creep or fluid assisted granular flow, implying a Newtonian rheology. 1D modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation. When the subduction flux either equals or exceeds the return flux, the maximum exhumation rate for Newtonian behavior of the material is at least twice as high (∼1/3 of the subduction burial rate) compared to that for power‐law creep (∼1/6 of the subduction burial rate).

Stress and deformation in subduction zones: insight from the record of exhumed metamorphic rocks

Abstract High pressure (HP) and ultrahigh (UHP) metamorphic rocks are exhumed from subduction zones at high rates on the order of plate velocity (cm/year). Their structural and microstructural record provides insight into conditions and physical state along the plate interface in subduction zones to depths of >100 km. Amazingly, many identified (U)HP metamorphic rocks appear not to be significantly deformed at (U)HP conditions, despite their history within a high strain rate mega-shearzone. Other (U)HP metamorphic rocks seem to be deformed exclusively by dissolution-precipitation creep. Indications of deformation by dislocation creep are lacking, apart from omphacite in some eclogites. Available flow laws for dislocation creep (extrapolated to low natural strain rates, which is equivalent to no deformation on the time scales of subduction and exhumation, i.e., 1 to 10 Ma) pose an upper bound to the magnitude of stress as a function of temperature along the trajectory followed by the rock. Although the record of exhumed (U)HP metamorphic rocks may only be representative of specific types or evolutionary stages of subduction zones, for such cases it implies: (1) strongly localized deformation; (2) predominance of dissolution-precipitation creep and fluid-assisted granular flow in the shear zones, suggesting Newtonian behaviour; (3) low magnitude of differential stress; which (4) is on the order of the stress drop inferred for earthquakes; and (5) negligible shear heating. These findings are easily reconciled with exhumation by forced flow in a low viscosity subduction channel prior to collision, implying effective decoupling between the plates.

Crystallographic preferred orientation development by dissolution–precipitation creep

Crystallographic preferred orientations (CPOs) in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms, where deformation is achieved by the movement of dislocations. In this paper we investigate the possibility of CPO-development by dissolution–precipitation creep or pressure solution. A numerical model is presented, which simulates the development of a grain aggregate that deforms by reaction-controlled dissolution–precipitation creep. Grains are simulated as rectangular boxes that change their shape by growth, or dissolution of their surfaces, depending on the normal stresses acting on the individual surfaces. Grains can also rotate due to an applied vorticity (for non-coaxial deformation) and if they have a non-equidimensional shape. For each strain increment, stress that is applied to the grains is the same for all grains, while individual grains deform and rotate by different amounts. A variety of CPOs develop at moderate strains, depending on the reaction rates of the different crystal-surfaces and type of deformation (uni-axial shortening, plane strain pure shear and simple shear). The modelling results confirm that dissolution–precipitation creep may play a role in CPO-development in rocks.

Low effective viscosity during high pressure metamorphism due to dissolution precipitation creep: the record of HP–LT metamorphic carbonates and siliciclastic rocks from Crete

The (micro)structural record of high pressure–low temperature (HP–LT) metamorphic rocks ( T=400±50°C, P=10±2 kbar) of the Phyllite-Quartzite Unit in western Crete, Greece, is interpreted in terms of deformation mechanisms and flow stress. Phyllites were deformed at low stress by dissolution precipitation creep, governed by strongly enhanced dissolution along quartz–mica (001) phase boundaries. Quartzites and quartz veins were deformed by dislocation creep at higher flow stress. The contrasting effective viscosities caused stress concentration in the quartzites and quartz veins. Also, microstructures from aragonite marbles indicate that dissolution precipitation creep was the dominant deformation mechanism and that dislocation creep has not been activated in these rocks. Pervasive ductile deformation was restricted to HP–LT metamorphic conditions and the microstructural record of deformation at maximum depth has been preserved, with all subsequent deformation localized and confined to the brittle field. Constraints on the timing of deformation allow an estimation of strain rates. Experimentally determined flow laws for dislocation creep are used to pose upper bounds on flow stress and bulk viscosities of rocks deformed by dissolution precipitation creep. For the phyllites, a conservative estimate is about 10 19 Pa s, or below, in contrast to 10 20 Pa s derived for the quartzites. This compares well to the viscosity contrast of 1–2 orders of magnitude reflected by the mesoscopic structures. Since phyllites and carbonate rocks form large portions of the subducted sedimentary pile, the low flow stress during rapid deformation of these rocks at HP–LT metamorphic conditions, and the lack of deformation along the burial and exhumation path, imply very low strength of the plate boundary shear zones and negligible shear heating.

Experimental simulation of the formation of fibrous veins by localised dissolution-precipitation creep

AbstractFibrous veins are generally interpreted in terms of the crack-seal mechanism. Several aspects of fibrous veins (fibrous structure, curved fibres, symmetry of antitaxial veins) are however better explained by vein formation without fracturing. Mass transfer to such veins would be by diffusional transport rather than by fluid flow through the veins. Deformation by dissolution-precipitation creep can provide the driving force for the necessary mass transfer. Veins form when mass transfer is heterogeneous and precipitation is localised.Experiments were performed which enforced a chemical potential gradient, acting as the driving force for diffusional mass transfer. These experiments resulted in fibrous growths in aggregates of soluble salts (NaCl and KCl) saturated with brine. The experimental results support the theory that fibrous veins may form without fracturing and that rather than providing evidence for major fluid pathways, fibrous veins may instead represent localised precipitation during diffusional material transfer.

Pressure solution in siliciclastic HP-LT metamorphic rocks — constraints on the state of stress in deep levels of accretionary complexes

The magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured. The only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high-pressure/low-temperature metamorphic rocks recorded during their crustal evolution. Microfabrics in HP-LT metamorphic ( T = 400 ± 50°C, P = 10 ± 2 kbar) phyllites and quartzites of the Phyllite-Quartzite Unit on the island of Crete, southern Aegean, reveal (1) that progressive deformation was by pressure solution (dissolution precipitation creep), (2) that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation, (3) that the unilaterally rational (001) mica quartz phase boundaries were sites of strongly enhanced dissolution, and (4) dislocation creep was restricted to a minor role in quartzites poor in mica. Consequently, the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial, to a depth of more than 30 km, and subsequent exhumation. Currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 MPa at temperatures around 400°C at a depth of 30 km. This implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep.