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This paper discusses continuum models for simulating earthquake sequences on faults governed by rate-and-state dependent friction. Through detailed numerical analysis of a conventional strike-slip fault, new observations regarding the use of various continuum earthquake models are presented. We update a recently proposed plasticity-based model using a consistently linearized formulation, show its agreement with discrete fault models for fault thicknesses of hundreds of meters, and demonstrate mesh objectivity for slip-related variables. To obtain a fully regularized fault width description with an internal length scale, we study the performance and mesh convergence of a plasticity-based model complemented by a Kelvin viscosity term and the phase-field approach to cohesive fracture. The Kelvin viscoplasticity-based model can introduce an internal length scale and a mesh-objective response. However, on grid sizes down to meters, this only holds for very high Kelvin viscosities that inhibit seismic slip rates, which renders this approach impractical for simulating earthquake sequences. On the other hand, our phase-field implementation for earthquake sequences provides a numerically robust framework that agrees with a discrete reference solution, is mesh objective, and reaches seismic slip rates. The model, unsurprisingly, requires highly refined grids around the fault zones to reproduce results close to a discrete model. Following this line, the effect of an internal length scale parameter on the phase-field predictions and mesh convergence are discussed. [ABSTRACT FROM AUTHOR]

Venus today is inhospitable at the surface, its average temperature of 750 K being incompatible to the existence of life as we know it. However, the potential for past surface habitability and upper atmosphere (cloud) habitability at the present day is hotly debated, as the ongoing discussion regarding a possible phosphine signature coming from the clouds shows. We review current understanding about the evolution of Venus with special attention to scenarios where the planet may have been capable of hosting microbial life. We compare the possibility of past habitability on Venus to the case of Earth by reviewing the various hypotheses put forth concerning the origin of habitable conditions and the emergence and evolution of plate tectonics on both planets. Life emerged on Earth during the Hadean when the planet was dominated by higher mantle temperatures (by about 200 ∘ C ), an uncertain tectonic regime that likely included squishy lid/plume-lid and plate tectonics, and proto continents. Despite the lack of well-preserved crust dating from the Hadean and Paleoarchean, we attempt to review current understanding of the environmental conditions during this critical period based on zircon crystals and geochemical signatures from this period, as well as studies of younger, relatively well-preserved rocks from the Paleoarchean. For these early, primitive life forms, the tectonic regime was not critical but it became an important means of nutrient recycling, with possible consequences on the global environment in the long-term, that was essential to the continuation of habitability and the evolution of life. For early Venus, the question of stable surface water is closely related to tectonics. We discuss potential transitions between stagnant lid and (episodic) tectonics with crustal recycling, as well as consequences for volatile cycling between Venus' interior and atmosphere. In particular, we review insights into Venus' early climate and examine critical questions about early rotation speed, reflective clouds, and silicate weathering, and summarize implications for Venus' long-term habitability. Finally, the state of knowledge of the Venusian clouds and the proposed detection of phosphine is covered. [ABSTRACT FROM AUTHOR]

The subsidence history of forearc and back‐arc basins reflects the relationship between subduction kinematics, mantle dynamics, magmatism, crustal tectonics, and surface processes. The distinct contributions of these processes to the topography variations of active margins during subduction initiation, oceanic subduction, and collision are less understood. We ran 2D elasto‐visco‐plastic numerical models including surface and hydration processes. The models show the evolution of wedge‐top and retro‐forearc basins on the continental overriding plate, separated by a forearc high. They are affected by repeated compression and extension phases. Compression‐induced subsidence is recorded in the syncline structure of the retro‐forearc basin from the onset of subduction. The 2–4 km upper plate negative residual topography is produced by the gradually steepening slab, which drags down the upper plate. Trench retreat leads to slab unbending and decreasing slab dip angle that leads to upper plate trench‐ward tilting. Back‐arc basins are either formed along inherited weak zones at a large distance from the arc or are created above the hydrated mantle wedge originating from arc rifting. Back‐arc subsidence is primarily governed by crustal thinning that is controlled by slab roll‐back and supported by the underlying mantle convection. High subduction and mantle convection velocities result in large wavelength negative dynamic topography. Collision and continental subduction are linked to the uplift of the forearc basins; however, the back‐arc records ongoing extension during a soft collision. During the hard collision, both the forearc and back‐arc basins are ultimately affected by the compression. Our modeling results provide insights into the evolution of Mediterranean subduction zones. Plain Language Summary: In this study, we assess the physical and thermal parameters of the interior of our planet Earth by comparing the topographic variations, that is, subsidence and uplift patterns of high‐resolution numerical models and observations from the Mediterranean region of Europe. We particularly focus on subduction zones. These noteworthy regions experience the sinking of dense oceanic tectonic plates beneath less dense upper plates and are characterized by distinct uplift and subsidence patterns. Understanding these processes is important for evaluating seismic risk, but these regions also host important geo‐resources, including geothermal energy. Key Points: Evolution of forearc basins controlled by slab advance and steepeningSubsidence of back‐arc basins controlled by slab roll‐back and mantle flowInsights into the evolution of Mediterranean forearc and back‐arc basin systems [ABSTRACT FROM AUTHOR]

Subduction is the major plate driving force, and the strength of the subducting plate controls many aspects of the thermochemical evolution of Earth. Each subducting plate experiences intense normal faulting1–9 during bending that accommodates the transition from horizontal to downwards motion at the outer rise at trenches. Here we investigate the consequences of this bending-induced plate damage using numerical subduction models in which both brittle and ductile deformation, including grain damage, are tracked and coupled self-consistently. Pervasive slab weakening and pronounced segmentation can occur at the outer-rise region owing to the strong feedback between brittle and ductile damage localization. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs10, the development of large-offset normal faults6,7 near trenches, the occurrence of segmented seismic velocity anomalies11 and distinct interfaces imaged within subducted slabs12,13, and the appearance of deep, localized intraplate areas of reduced effective viscosity14 observed at trenches. Furthermore, brittle–viscously damaged slabs show a tendency for detachment at elevated mantle temperatures. Given Earth’s planetary cooling history15, this implies that intermittent subduction with frequent slab break-off episodes16 may have been characteristic for Earth until more recent times than previously suggested17.Numerical subduction models used to determine the consequences of bending-induced plate damage show that slab weakening and segmentation can occur at the outer-rise region of the subducting plate. [ABSTRACT FROM AUTHOR]

Partial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2–3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300–1550 °C, ∆T = T − Tmodern = 0–250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite–Trondhjemite–Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200–250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100–200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels. [ABSTRACT FROM AUTHOR]

The ancient cores of continents (cratons) are underlain by mantle keels—volumes of melt-depleted, mechanically resistant, buoyant and diamondiferous mantle up to 350 kilometres thick, which have remained isolated from the hotter and denser convecting mantle for more than two billion years. Mantle keels formed only in the Early Earth (approximately 1.5 to 3.5 billion years ago in the Precambrian eon); they have no modern analogues1–4. Many keels show layering in terms of degree of melt depletion5–7. The origin of such layered lithosphere remains unknown and may be indicative of a global tectonics mode (plate rather than plume tectonics) operating in the Early Earth. Here we investigate the possible origin of mantle keels using models of oceanic subduction followed by arc-continent collision at increased mantle temperatures (150–250 degrees Celsius higher than the present-day values). We demonstrate that after Archaean plate tectonics began, the hot, ductile, positively buoyant, melt-depleted sublithospheric mantle layer located under subducting oceanic plates was unable to subduct together with the slab. The moving slab left behind craton-scale emplacements of viscous protokeel beneath adjacent continental domains. Estimates of the thickness of this sublithospheric depleted mantle show that this mechanism was efficient at the time of the major statistical maxima of cratonic lithosphere ages. Subsequent conductive cooling of these protokeels would produce mantle keels with their low modern temperatures, which are suitable for diamond formation. Precambrian subduction of oceanic plates with highly depleted mantle is thus a prerequisite for the formation of thick layered lithosphere under the continents, which permitted their longevity and survival in subsequent plate tectonic processes. Modelling reveals how thick diamondiferous continental mantle 'keels' were formed only at increased mantle temperatures when the melt-depleted, hot, ductile mantle located under subducting oceanic plates flowed backwards, underplating the continents. [ABSTRACT FROM AUTHOR]

Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading. Key Points: We document the initiation of a proto‐transform fault during late‐stage continental rifting in Afar, EthiopiaSurface faulting, seismicity, and geodetic observations reveal extensional transfer between magmatic segmentsNumerical modeling predicts that the region between the offset magmatic segments will evolve to a stable oceanic transform fault [ABSTRACT FROM AUTHOR]

Extensional basins often show along‐strike variability in terms of fault geometries, basement structures, subsidence, and thermal evolution. This is particularly pronounced when extension reactivates preexisting suture zones with opposing dip directions, that is, opposite polarities, which creates wide strike‐slip transfer zones. We have studied this extensional variability by means of thermomechanical lithospheric‐scale 3‐D numerical modeling. We conducted a series of experiments to model the extension of a thick lithosphere simulating a young orogenic area containing segmented suture zones inherited from former opposing subduction polarities, implemented as rheologically weak inclined layers with opposite dips. Numerical experiments demonstrate that the initial subduction sutures are reactivated by two long‐lived lithospheric‐scale detachment faults, which remain active until the onset of oceanic spreading. The opposite polarity of these faults causes their migration toward each other by asymmetric mantle lithosphere extension and thermal accretion. Crustal kinematics shows the formation of extensional detachment faults and high‐offset listric normal faults. These structures undergo gradual tilting during their footwall exhumation, while the rheological weak suture layers are redistributed beneath the crust. The results demonstrate an active interaction along the strike of the system between the two opposite dipping weak zones, where fault segments mechanically interact. During extension, the maximum horizontal fault offset switches the apparent sense of shear in the separating strike‐slip transfer zone. This kinematics explain the rapid change in the sense of shear in major strike‐slip transfer zones or transform faults, such as observed during the extension of the Pannonian back‐arc basin. Key Points: We have conducted 3‐D lithospheric‐scale numerical modeling of rift interactionExtensional reactivation of lithospheric‐scale suture zones has been analyzedStrike‐slip transfer faults evolve and change their sense of shear with time [ABSTRACT FROM AUTHOR]

Geophysical imaging of the Yellowstone supervolcano shows a broad zone of partial melt interrupted by an a magmatic gap at depths of 15-20 km. We reproduce this structure through a series of regional-scale magmatic-thermomechanical forward models which assume that magmatic dikes stall at rheologic discontinuities in the crust. We find that basaltic magmas accumulate at the Moho and at the brittle-ductile transition, which naturally forms at depths of 5-10 km. This leads to the development of a 10- to 15-km thick midcrustal sill complex with a top at a depth of approximately 10 km, consistent with geophysical observations of the pre-Yellowstone hot spot track. We show a linear relationship between melting rates in the mantle and rhyolite eruption rates along the hot spot track. Finally, melt production rates from our models suggest that the Yellowstone plume is ~175°C hotter than the surrounding mantle and that the thickness of the overlying lithosphere is ~80 km. [ABSTRACT FROM AUTHOR]

During continental collision, a part of the lower‐plate material can be subducted, emplaced at the base of the upper plate, and eventually incorporated into its crust. This mechanism of continental‐crust transformation is called relamination, and it has been invoked to explain occurrences of high‐pressure felsic rocks in different structural positions of several orogenic systems. In the present study we reproduced relamination during continental collision in a thermomechanical numerical model. We performed a parametric study and distinguished three main types of evolution regarding the fate of the subducted continental crust: (i) return along the plate interface in a subduction channel or wedge, (ii) flow at the bottom of the upper‐plate lithosphere and subsequent translithospheric exhumation near the arc or in the back‐arc region (“sublithospheric relamination”), and (iii) nearly horizontal flow directly into the upper‐plate crust (“intracrustal relamination”). Sublithospheric relamination is preferred for relatively quick convergence of thin continental plates. An important factor for the development of sublithospheric relamination is melting of the subducted material, which weakens the lithosphere and opens a path for the exhumation of the relaminant. In contrast, a thick and strong overriding plate typically leads to exhumation near the plate interface. If the overriding plate is too thin or weak, intracrustal relamination occurs. We show that each of these evolution types has its counterpart in nature: (i) the Alps and the Caledonides, (ii) the Himalayan‐Tibetan system and the European Variscides, and (iii) pre‐Cambrian ultrahot orogens. [ABSTRACT FROM AUTHOR]

The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the 'Plutonic squishy lid' tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime. [ABSTRACT FROM AUTHOR]

Scientific theories of how subduction and plate tectonics began on Earth-and what the tectonic structure of Earth was before this-remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics. [ABSTRACT FROM AUTHOR]

The lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes. Here, various aspects of non-lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature. The main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones. They are, in particular, responsible for some geological processes, which are otherwise difficult to explain, such as downward water suction into the interior of subducting slabs. Magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to ±100% of the lithostatic pressure and up to a GPa-level. These magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process. Rheological heterogeneity of deforming rock units, which is common in nature, has a tendency to enhance overpressure and underpressures. Large overpressure can typically be expected in rheologically strong (dry) bending rock units, in particular in the mantle lithosphere. However, rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks. Therefore, the influence of significant tectonic overpressure and/or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono-metamorphic scenarios. Many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface. Some controversy exists concerning overpressure magnitudes for the ultrahigh-pressure ( UHP) rocks and several conflicting hypotheses are proposed, which need to be thoroughly tested in the future. In this respect, the Alpine region may offer a unique opportunity for the testing of geological-scale overpressures in (U) HP rocks by combining structural-geological and petrological data with realistic lithospheric-scale numerical modelling. [ABSTRACT FROM AUTHOR]

Although many continental rifts and passive margins are magmatic, some are not. This observation prompted endmember views of the mechanisms driving continental rifting, where magma-rich or active rifts would be caused by deep mantle plumes, whereas magma-poor or passive rifts would result from the stretching of the lithosphere under far-field plate forces. The Central East African Rift provides a unique setting to investigate the mechanisms of continental rifting because it juxtaposes a magma-rich (eastern) branch and magma-poor (western) branch on either side of the 250-km-thick Tanzanian craton. Here we investigate this contrasted behavior using a high-resolution rheologically consistent three-dimensional thermo-mechanical numerical model. The model reproduces the rise of a mantle plume beneath a craton experiencing tensional far-field stress. In our numerical experiments the plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of magma-rich and magma-poor rifts along opposite craton sides, fed by melt from a single mantle source. Our numerical experiments show strong similarities to the observed evolution of the Central East African Rift, reconcile the passive and active rift models, and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments. [ABSTRACT FROM AUTHOR]

Many aspects of Precambrian tectonics are still unclear due to the undetermined influence of several key physical parameters (the temperature of the mantle, the thickness of the lithosphere, etc.) on the geodynamic processes. The values of these parameters in the Precambrian are known to have been significantly different from current conditions. This work presents the results of two-dimensional (2D) numerical petrological-thermomechanical experiments that simulate the process of plate convergence at a velocity of 5 cm/year depending on the thickness of the continental lithosphere. The model continental-lithosphere thickness ranged from 100 to 200 km, the modern mantle temperature exceeded 150°C, while radiogenic heat generation in the continental crust was 1.5 times higher then that at present. The numerical simulations showed that if the lithosphere is 100-160 km thick, the subduction process (closure of an ocean) is terminated by detachment of the oceanic plate from the continental plate (slab break-off) followed by the formation of a large igneous province (an oceanic plateau) between the continents instead of orogeny. The thinner the lithosphere is, the earlier and closer to the surface the slab break-off occurs. Thus, for a model with a continental lithosphere thickness of 150 km, the slab was detached in 10.3 Ma at a depth of 150 km, whereas for a lithosphere of 100 km thick it occurred in just 5.1 Ma almost at the surface. In the latter case, the magma generation area becomes much larger due to the formation of igneous provinces on both sides of the oceanic slab instead of one side, as is proposed in other models. Collision of continents with a very thick lithosphere (200 km or more) is not accompanied by slab break-off and significant volcanic activity. Thus, the results of our modeling show that the SLAB PULL mechanism in a subduction zone contributes to the processes at a plate convergent boundary. [ABSTRACT FROM AUTHOR]

This work presents high-resolution 3D numerical model of transform fault initiation at rifted continental margins. Our petrological-thermomechanical visco-plastic model allows for spontaneous nucleation of oceanic spreading process in a continental rift zone and takes into account new oceanic crust growth driven by decompression melting of the asthenospheric mantle. Numerical model predicts that ridge-transform spreading pattern initiate in several subsequent stages: crustal rifting (0-1.5 Myr), spreading centers nucleation and propagation (1.5-3 Myr), proto-transform fault initiation and rotation (3-5 Myr) and mature ridge-transform spreading (> 5 Myr). Comparison of modeling results with the natural data from the Woodlark Basin suggests that the development of this region closely matches numerical predictions. Similarly to the model, the Moresby (proto-) Transform terminates in the oceanic rather than in the continental crust. This fault associates with a notable topographic depression and formed within 0.5-2 Myr while linking two offset overlapping spreading segments. Model reproduces well characteristic 'rounded' contours of the spreading centers as well as the presence of a remnant of the broken continental crustal bridge observed in the Woodlark Basin. Proto-transform fault traces and truncated tip of one spreading center present in the model are also documented in nature. Numerical results are in good agreement with the concept of Taylor et al. (2009) which suggests that spreading segments nucleate en echelon in overlapping rift basins and that transform faults develop as or after spreading nucleates. Our experiments also allow to refine this concept in that (proto)-transform faults may also initiate as oblique rather than only spreading-parallel tectonic features. Subsequent rotation of these faults toward the extension-parallel direction is governed by space accommodation during continued oceanic crust growth within offset ridge-transform intersections. [ABSTRACT FROM AUTHOR]

In the last decades, the development of the surface and satellite geodetic and geophysical observations brought a new insights into the seismic cycle, documenting new features of inter-, co-, and post-seismic processes. In particular since 2002 satellite mission GRACE provides monthly models of the global gravity field with unprecedented accuracy showing temporal variations of the Earth's gravity field, including those caused by mass redistribution associated with earthquake processes. When combined with GPS measurements, these new data have allowed to assess the relative importance of afterslip and viscoelastic relaxation after the Sumatra 26.12.2004 earthquake. Indeed the observed post-seismic crustal displacements were fitted well by a viscoelastic relaxation model assuming Burgers body rheology for the asthenosphere (60–220 km deep) with a transient viscosity as low as 4 × 1017 Pas and constant ∼1019 Pas steady state viscosity in the 60–660-km depth range. However, even the low-viscosity asthenosphere provides the amplitude of strain which gravity effect does not exceed 50 per cent of the GRACE gravity variations, thus additional localized slip of about 1 m was suggested at downdip extension of the coseismic rupture. Post-seismic slip at coseismic rupture or its downdip extension has been suggested by several authors but the mechanism of the post-seismic fault propagation has never been investigated numerically. Depth and size of localized slip area as well as rate and time decay during the post-seismic stage were either assigned a priory or estimated by fitting real geodesy or gravity data. In this paper we investigate post-seismic rupture propagation by modelling two consequent stages. First, we run a long-term, geodynamic simulation to self-consistently produce the initial stress and temperature distribution. At the second stage, we simulate a seismic cycle using results of the first step as initial conditions. The second short-term simulation involves three substeps, including additional stress accumulation after part of the subduction channel was locked; spontaneous coseismic slip; formation and development of damage zones producing afterslip. During the last substep post-seismic stress leads to gradual ∼1 m slip localized at three faults around ∼100-km downdip extension of the coseismic rupture. We used the displacement field caused by the slip to calculate pressure and density variations and to simulate gravity field variations. Wavelength of calculated gravity anomaly fits well to that of the real data and its amplitude provides about 60 per cent of the observed GRACE anomaly. Importantly, the surface displacements caused by the estimated afterslip are much smaller than those registered by GPS networks. As a result cumulative effect of Burgers rheology viscoelastic relaxation (which explains measured GPS displacements and about a half of gravity variations) plus post-seismic slip predicted by damage rheology model (which causes much smaller surface displacements but provides another half of the GRACE gravity variations) fits well to both sets of the real data. Hence, the presented numerical modelling based on damage rheology supports the process of post-seismic downdip rupture propagation previously hypothesized from the GRACE gravity data. [ABSTRACT FROM AUTHOR]

Geodynamic modeling often involves challenging computations involving solution of Stokes and continuity equations under condition of highly variable viscosity. Based on new analytical approach we developed generalized analytical solutions for 2-D and 3- D incompressible Stokes flows with both linearly and exponentially variable viscosity. We demonstrated how these generalized solutions can be converted into 2-D and 3-D test problems suitable for benchmarking numerical codes aimed at modeling various mantle convection and lithospheric dynamics problems. Main advantage of this new generalized approach is that large variety of benchmark solutions can be generated including relatively complex cases with open model boundaries, non-vertical gravity and variable gradients of viscosity and density fields, which are not parallel to Cartesian axes. Examples of respective 2-D and 3-D MatLab codes are provided with this paper. [ABSTRACT FROM AUTHOR]

A series of 2D petrological-thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high-temperature-ultrahigh pressure metamorphic (HT-UHPM) rocks. To this end, the models include fluid- and melt-induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large-scale crustal stacking, shallow crustal delamination, trans-lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid- and melt-induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid- and melt-related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large-scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high-temperature range of P-T conditions documented from UHPM rocks, including those retrieved from HT-UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100-1200 °C, are still to be achieved in experiments using numerical models. [ABSTRACT FROM AUTHOR]

SUMMARY Calculating surface topography in geodynamic models is a common numerical problem. Besides other approaches, the so-called 'sticky air' approach has gained interest as a free-surface proxy at the top boundary. The often used free slip condition is thereby vertically extended by introducing a low density, low viscosity fluid layer. This allows the air/crust interface to behave in a similar manner to a true free surface. We present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface. Two cases are evaluated that characterize the evolution of topography on different timescales: (1) isostatic relaxation of a cosine perturbation and (2) topography changes above a rising plume. We quantitatively compare topographies calculated by six different numerical codes (using finite difference and finite element techniques) using three different topography calculation methods: (i) direct calculation of topography from normal stress, (ii) body-fitting methods allowing for meshing the topography and (iii) Lagrangian tracking of the topography on an Eulerian grid. It is found that the sticky air approach works well as long as the term (ηst/ηch)/( hst/ L)3 is sufficiently small, where ηst and hst are the viscosity and thickness of the sticky air layer, and ηch and L are the characteristic viscosity and length scale of the model, respectively. Spurious lateral fluctuations of topography, as observed in some marker-based sticky air approaches, may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction. [ABSTRACT FROM AUTHOR]

The dynamics of subduction under an active margin are analysed by using a 2D-coupled petrological-thermomechanical numerical model of an oceanic-continental subduction system. The model includes spontaneous slab bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth. Based on our models, the following five geodynamic regimes of subduction, which may potentially form at active margins, are identified: (i) stable subduction with no backarc spreading centre and without plumes in the mantle wedge; (ii) retreating subduction with a focused backarc spreading centre and without plumes; (iii) retreating subduction with distributed intra-arc extension and trans-lithospheric sedimentary plumes; (iv) advancing subduction with underplating (laterally extending) sub-lithospheric plumes; and (v) stable to advancing subduction with stationary (laterally limited) sub-lithospheric plumes. Transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of (i) aqueous fluids percolating from the subducting slab into the mantle wedge and (ii) melts propagating from the partially molten areas formed in the mantle wedge towards the surface. The aqueous fluids mainly affect the forearc region. Strong fluid-related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism. In contrast, reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision-like subduction with enhanced subduction erosion. Thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences. On the other hand, melts, extracted from the hot regions above the slab, rheologically weaken the lithosphere below the arc, which thus controls overriding plate extension and shortening. Strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre. Also, weakening of the continental lithosphere by melts extracted from trans-lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust. If there is insufficient melt-related weakening, plumes cannot ascend but extend horizontally and thus underplate the lithosphere. [ABSTRACT FROM AUTHOR]

New experimental data on the solubility of NaCl in gaseous CO2 were obtained at pressures ( P) of 30–70 MPa and temperatures of 623 and 673 K on experimental equipment making possible to sample a portion of the gas in the course of the experiment. The new measures have demonstrated that the NaCl solubility increases with increasing temperature ( T) and pressure and is approximately four to five orders of magnitude higher than the saturated vapor pressure of NaCl at the corresponding temperature. The paper also reports newly obtained experimental data on the equilibrium conditions of the reaction of talc decomposition into enstatite and quartz at a variable H2O/NaCl ratio in the fluid. The results of the experiments validate the empirical equations previously suggested for H2O and NaCl activities in concentrated aqueous salt solutions that can be used in describing silica-saturated fluids at high T-P parameters. A new empirical equation is suggested for the Gibbs free mixing energy in the H2O-CO2-NaCl ternary system, with the parameters of the equation calibrated against experimental data on phase equilibria in marginal binary systems and on the location of the boundary of the region of homogeneous three-component fluid according to data on synthetic fluid inclusions in quartz. [ABSTRACT FROM AUTHOR]

The principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P–T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P–T paths of HP–UHP rocks is located in the bottom corner of the wedge-like confined channel with the characteristic magnitude of pressure deviation on the order of ∼0.3 GPa and 10–20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP-rocks subjected to significant (≥0.3 GPa) overpressure at intermediate subduction depth (50–70 km, P = 1.5–2.5 GPa) then underpressured at depth ≥100 km ( P ≥ 3 GPa); (ii) HP-rocks subjected to ≥0.3 GPa overpressure at peak P–T conditions reached at 50–70 km depth in the bottom corner of the wedge-like confined subduction channel ( P = 1.5–2.5 GPa); (iii) lower-pressure rocks formed at shallower depths (≤40 km depth, P ≤ 1 GPa), which are not subjected to significant overpressure and/or underpressure. [ABSTRACT FROM AUTHOR]

We investigate crustal growth processes on the basis of a 2D coupled geochemical—petrological-thermomechanical numerical model of retreating intraoceanic subduction. The model includes spontaneous slab retreat and bending, subducted crust dehydration, aqueous fluid transport, mantle wedge melting, and melt extraction resulting in crustal growth. Our numerical experiments show that (1) the rate of plate retreat influences both the rate of crust formation and composition of newly formed crust; (2) both the intensity of melt extraction and the age of subducted plate affect the volume of newly formed crust. In the present paper we describe possible scenarios of magmatic and geodynamic evolution of the arc system and then compare results with magmatic addition rates of Western Pacific arcs. [ABSTRACT FROM AUTHOR]

We demonstrate the possibility of studying several diffusion couples in a single run, i.e. under almost similar P– T– t– $$ f_{{{\text{O}}_{ 2} }} $$ conditions, allowing direct comparison of the diffusion rates in different diffusion couples. Thus the duration of experimental study and the risk of failure of expensive experimental equipment can be decreased considerably. The diffusion experiments were carried out in piston-cylinder apparatus. Gem-quality garnets of almandine, spessartine and grossular compositions together with inclusion-rich eclogitic garnets were embedded in a powder of natural pyrope and annealed together under dry conditions at P = 1.9–3.2 GPa and T = 1,070–1,400°C. Diffusion profiles were measured by electron microprobe and fitted numerically on the basis of multicomponent diffusion theory. The datasets derived from different diffusion couples yields parameters of the Arrhenius equation for Ca, Mg and Fe in natural eclogitic garnets and Mg, Mn and Fe in gem-quality garnets. We have also studied the effect of grain-boundary diffusion in the sintered pyrope matrix on interdiffusion on the basis of 2D modeling. Under conditions analogous to those of our experimental runs, we show that observed irregularities in some measured diffusion profiles (not applied for the diffusion modeling) can be directly related to the superposition of local grain-boundary diffusion on dominant volume diffusion. [ABSTRACT FROM AUTHOR]

Based on a consistent set of empirical interatomic potentials, static structure energy calculations of various Al/Si configurations in the supercell of Mg-cordierite and Monte Carlo simulations the phase transition between the orthorhombic and hexagonal modifications of cordierite ( Crd) is predicted at 1623 K. The temperature dependences of the enthalpy, entropy, and free energy of the Al/Si disorder were calculated using the method of thermodynamic integration. The simulations suggest that the commonly observed crystallization of cordierite in the disordered hexagonal form could be related to a tendency of Al to occupy T1 site, which is driven by local charge balance. The increase in the Al fraction in the T1 site over the ratio of 2/3(T1): 1/3(T2), that characterizes the ordered state, precludes formation of the domains of the orthorhombic phase. This intrinsic tendency to the crystallization of the metastable hexagonal phase could have significantly postponed the formation of the association of orthorhombic cordierite and orthopyroxene over the association of quartz and garnet in metapelites subjected to granulite facies metamorphism. The textures of local metasomatic replacement (the formation of Crd + Opx Or Spr + Crd symplectites between the grains of garnet and quartz) indicate the thermodynamic instability of the association of Qtz + Grt at the moment of the metasomatic reaction. This instability could have been caused by the difficulty of equilibrium nucleation of orthorhombic cordierite. [ABSTRACT FROM AUTHOR]

This paper continues the study of Perchuk et al. (2004), who reported a detailed petrographic description of peridotites and host gneisses composing the ultrahigh-pressure Samerberg complex. Part 2 presents the calculation of the P-T parameters of local equilibria and derivation of P-T paths: counterclockwise for spinel-gamet metaultrabasic rocks and decompression cooling for the host two-mica gneisses and migmatites. A possible clinoenstatite-orthoenstatite transition (characteristic twins in orthopyroxene) suggests high-temperature but shallow-depth, almost near-surface crystallization (quenching) of the ultrabasic protolith. During the subsequent burial of the already quenched rocks to great depths, metamorphic temperature increased but remained below the quenching temperature, which must be reflected in a clockwise P-T loop. Since the subsidence rate was higher than the rates of chemical reactions, complete reequilibration of the rocks upon heating was reached at maximum depths. Using these data, a geodynamic model was proposed for the formation and exhumation of the complex. It was found, in particular, that the P-T path of the gneisses closely reproduces the P-T path of the Erzgebirge gneiss dome in the western Bohemian Massif. This suggests that the thermal and dynamic Hercynian evolution of the entire Bohemian Massif can be adequately described by the model of lower crustal delamination. [ABSTRACT FROM AUTHOR]

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the Tmin of the older P-T trajectory corresponded to Tmax of the younger trajectory, and often Tmax– Tmin > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating. [ABSTRACT FROM AUTHOR]

Metapelites, migmatites and granites from the c. 2 Ga Mahalapye Complex have been studied for determining the P– T–fluid influence on mineral assemblages and local equilibrium compositions in the rocks from the extreme southwestern part of the Central Zone of the Limpopo high-grade terrane in Botswana. It was found that fluid infiltration played a leading role in the formation of the rocks. This conclusion is based on both well-developed textures inferred to record metasomatic reactions, such as Bt ⇒ And + Qtz + (K2O) and Bt ± Qtz ⇒ Sil + Kfs + Ms ± Pl, and zonation of Ms | Bt + Qtz | And + Qtz and Grt | Crd | Pl | Kfs + Qtz reflecting a perfect mobility (Korzhinskii terminology) of some chemical components. The conclusion is also supported by the results of a fluid inclusion study. CO2 and H2O ( = 0.6) are the major components of the fluid. The fluid has been trapped synchronously along the retrograde P– T path. The P– T path was derived using mineral thermobarometry and a combination of mineral thermometry and fluid inclusion density data. The Mahalapye Complex experienced low-pressure granulite facies metamorphism with a retrograde evolution from 770 °C and 5.5 kbar to 560 °C and 2 kbar, presumably at c. 2 Ga. [ABSTRACT FROM AUTHOR]

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen-scale viscous heat production of 0.1 to>1 μW m−3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time-span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation-generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions. [ABSTRACT FROM AUTHOR]

The Central Zone (CZ) of the Limpopo Complex of southern Africa is characterized by a complex deformational pattern dominated by two types of fold geometries: large sheath folds and cross folds. The sheath folds are steeply SSW-plunging closed structures whereas the cross folds are north–south-oriented with near-horizontal fold axes. In the area south of Messina this complexly folded terrain grades continuously towards the south into a crustal-scale ENE–WSW-trending ductile shear zone with moderate dip towards the WSW. All sheath folds document consistent top-to-the-NE thrust movement of high-grade material. The timing of this shear deformational event (D2) and thus of the gneissic fabric (S2) is constrained (at ∼2·6 Ga) by the syntectonic emplacement throughout the CZ of precursors to quartzo-feldspathic gneisses (Singelele-type gneisses). Cross folds deform the S2 fabric and are characterized by a near-vertical axial planar cleavage (S3). Recent single-phase Pb–Pb dating of garnet from a metapelitic gneiss with an S3 fabric from one of the largest cross folds in the CZ constrains the timing of the deformational (D3) and metamorphic (M3) event at ∼2·0 Ga. Mineral chemistry for metapelites from this cross fold shows a single peak on an NMg histogram for garnet reflecting a single phase of mineral growth. Metapelites from this cross fold also preserve evidence for only one well-developed reaction texture, Grt + Sil + Qtz → Crd. This reaction is accompanied by the simultaneously operating reaction Grt + Fsp + H2O = Bt + Sil + Qtz. Both these divariant reactions belong to the univariant KFMASH equilibrium Crd + Grt + Fsp + H2O → Bt + Sil + Qtz. The progress of the two divariant retrograde reactions leads to the consumption of Grt and Fsp: K-feldspar (Or94–100) never occurs with both cordierite and garnet. Microprobe profiling coupled with calculated isopleths for Bt, Grt and Crd in divariant equilibria define a decompression-cooling P–T path that reflects a single (M3) high-grade metamorphic event during the evolution of the cross fold. This decompression-cooling P–T path traverses from 780°C, 5·7 kbar to 600°C, 3·3 kbar. [ABSTRACT FROM AUTHOR]

Sheared and unsheared xenoliths of garnet-harzburgite from the K1 Venetia kimberlite pipe (South Africa) are composed primarily of olivine and orthopyroxene (partially replaced by serpentine, magnetite and chlorite) and abundant porphyroblasts of garnet with kelyphitic rims of orthopyroxene, clinopyroxene and spinel. Mylonitized, orthopyroxene defines a mineral elongation lineation whereas bent orthopyroxene and sigmoidal garnet with wings of orthopyroxene define consistent senses of motion within individual xenoliths. The kelyphite surrounding both deformed and undeformed garnet grains is not deformed and appears to have resulted from the inversion of garnet and olivine to orthopyroxene, clinopyroxene and spinel, corresponding to the isograd separating garnet peridotite from spinel peridotite. Subsequent hydration around rims and along fractures within xenoliths resulted in the formation of serpentine, magnetite and chlorite from olivine and orthopyroxene. This hydration apparently resulted from reaction with the entraining kimberlite magma. The preservation of mylonite rather than its annealing and recrystallization to coarse-grained rocks at high temperature strain free mantle conditions requires quenching shortly after mylonite formation. This preservation taken with the presence of undeformed kelyphite around garnet grains implies that mylonitization and entrainment of garnet-harzburgite into kimberlite magma and kelyphite formation occurred during very rapid magma ascent. [ABSTRACT FROM AUTHOR]

Abstract The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high-grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low-strain gneisses, bound by an imbricate system of kilometre-wide deep crustal shear zones characterized by the presence of high-strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high-grade terrane. Low- and high-strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low-strain gneisses as a uniform orientation of relict Al-rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg-rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high-strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid-shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg-rich garnet porphyroblasts, and syn-decompression M2 pencil-shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near-isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil-shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low-strain gneisses from thrust sheets show similar M2 decompression cooling and near-isobaric cooling reaction textures that formed within the same P–T range, but under low-strain conditions, as shown by their pseudo-idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low-strain conditions in gneisses from thrust sheets and high-strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post-decompression shearing under epidote–amphibolite facies conditions. [ABSTRACT FROM AUTHOR]

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