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Dating specific pressure–temperature–time–deformation–fluid (P–T–t–d–f) events is a major petrological issue, particularly for polymetamorphic assemblages. In order to better assess such events using mica populations, this study uses a combined in situ Rb–Sr and Ar–Ar dating approach coupled with chemical mapping, with application to an exhumed subduction complex (Schistes Lustrés, Western Alps). Geochronological investigation of the most Si-rich (Tschermak substitution) mica population allows us to investigate the (near-)peak burial ages of the various tectonometamorphic units of this high-pressure/low-temperature subduction complex. In the blueschist-facies units, a wide range of ages between ∼36 (younger than previously obtained peak ages) and ~ 52 Ma suggests a diachronous slicing of units with different initial paleogeography and/or a long residence time at near-peak conditions. The combined use of Ar–Ar and Rb–Sr isochron ages reveals the existence of some excess argon in the studied metamorphic rocks, interpreted as the result of (i) heterogeneous contamination from an argon-rich fluid in the higher grade eclogite-facies units and (ii) partial removal, during pseudomorphic recrystallization, of argon inherited from earlier stages of metamorphism in the lower grade blueschist-facies units. [ABSTRACT FROM AUTHOR]

The largest ophiolite on Earth, in western Turkey, is a key place to study obduction and early subduction dynamics. Ophiolite remnants derived from the same Neotethyan branch (preserved as a result of long-lived Late Cretaceous continental subduction and later obduction) are underlain by hundred-metre-thick extensive metamorphic soles. These soles formed synchronously, at c. 93 Ma, and were welded to the base of the ophiolite, thereby dating the start of intra-oceanic subduction. This contribution focuses on the structure, petrology and pressure– temperature evolution of the soles and other subductionderived units. Peak pressure-temperature conditions were estimated at 10.5±2 kbar and 800±50 °C for the sole by means of pseudosection calculations using Theriak/Domino and at 12 kbar and 425 °C for the unique, enigmatic blueschist facies overprint of the sole. This study provides clues to the mechanisms of sole underplating during early subduction, later cooling, and the nature of the western Turkey ophiolite. [ABSTRACT FROM AUTHOR]

• Coupled in-situ Rb/Sr and U/Pb dating allows to constrain metamorphic terranes evolution. • Iran was affected by a Jurassic collisional metamorphism related to the Paleotethys closure. • Metamorphic terranes are exhumed from ∼170 Ma onward in a context of upper plate extension. The pre-Alpine evolution of the Tethyan domains between Gondwana and Laurasia, and in particular that of the Cimmerian continental blocks, remains poorly constrained. Central Iran is a key area to constrain the closure of the Paleotethys and the collision of Laurasia with the Cimmerian blocks drifted from Gondwana. The present study provides a combined metamorphic and geochronologic approach focused on two areas of Central Iran: the Kashmar-Kerman Tectonic Zone (KKTZ) and the Jandaq area, whose tectonometamorphic units are affected low- to middle-pressure high-temperature metamorphism. We performed in-situ texturally constrained U/Pb dating on titanite and Rb/Sr dating on mica to quantify the timing and intensity of burial and exhumation of these metamorphic rocks. Results show that metamorphism in the central and eastern KKTZ (7–9 kbar; ∼700 °C) is synchronous or slightly postdates the Cimmerian orogeny (∼190-180 Ma) and relates to the collision following the Paleotethys closure, whereas metamorphism in the Jandaq complex (12–13 kbar; ∼450 °C) relates to the Paleotethys subduction. Based on paleogeographic reconstructions, the KKTZ lied hundreds of kilometers south of the Paleotethys suture zone. As such, we propose that metamorphism along the KKTZ results from the closure and shortening of a rheologically weak domain located outboard of the main suture. In-situ Rb/Sr dating of biotite yields precise and accurate cooling ages (±2-5 Ma) ranging from 170 to 140 Ma. These cooling ages document the exhumation of these terranes from the Mid-Cimmerian event onward (∼170 Ma), coeval with the widespread extension distributed across Iran and thought to reflect upper plate extension above the Neotethyan subduction system. [ABSTRACT FROM AUTHOR]

Abstract: The tectono-metamorphic evolution of the high-pressure low-temperature (HP–LT) Tavşanlı zone (Western Anatolia, İzmir–Ankara suture zone) is herein reappraised to highlight processes occurring along a fossil subduction interface, from initial obduction stages to continental subduction. Structural and petrological data allow in particular to constrain the nature, internal structure and PT conditions of the oceanic accretionary complex sandwiched between the subducted continental margin of the Anatolide–Tauride Block (Orhaneli unit) and the non-metamorphic obducted ophiolite on top. Two distinct oceanic units (termed complexes 1 and 2) are recognised on top of one another, with metamorphic conditions ranging from incipient HP-LT imprint (complex 1) to blueschist facies conditions (complex 2). Based on the first occurrence of Fe–Mg carpholite and on pseudosection calculations, PT estimates of 250–350°C and 11–13kbar are inferred for complex 2. The internal structure of the accretionary complex points to the underplating of kilometre-scale units at different depths along the plate interface and to contrasting dynamics with respect to both the underlying continental unit and the ophiolite. Inter-plate mechanical coupling within the Tavşanlı zone is compared to the Oman case-study. The variable HP–LT overprint of the metamorphic sole places further constraints on regional scale tectonics, the accretionary dynamics and on the rapid thermal reequilibration of the subduction interface. [Copyright &y& Elsevier]

Abstract: Unlike the thermal structures of present-day subduction or collision zones, the variety of possible thermal evolutions (or “thermal regimes”) accompanying the transient stage of continental subduction (i.e., the shift from oceanic subduction to continental collision) remains poorly known. We herein show that the thermal regime of continental subduction can be confidently retrieved from three well-documented fossil settings (i.e., from high-pressure low temperature continental material from Oman, New Caledonia, Corsica) that were not modified by later collision or a later metamorphic imprint. Based on recently published data, we show the striking similarity of the overall thermal and tectonic patterns of these three HP belts derived from the subduction of a continental margin. These natural examples suggest that the thermal regime of continental subduction is largely invariant through time (hence suggesting a steady-state thermal structure) and independent from the initial geodynamic setting (i.e., from the initial thermal structure of oceanic subduction, the nature of the incoming plate or of the upper plate). We propose that continental cover units subducted over a short time period (i.e., ~10My) represent cold underplated material that buffers the subduction thermal regime, whatever the exact structure, nature, or thermal structure of incoming material. Similarities in the type, size and P–T conditions of the various tectonic units and in the overall tectonic organization point to specific accretionary-type subduction dynamics, yet to differences in long-term mechanical coupling between the three case studies. Our study thereby provides constraints on exhumation dynamics and models of continental subduction. [Copyright &y& Elsevier]

Abstract: Metapelitic rocks of the Schistes Lustrés in the Cottian Alps, Italy (peak metamorphic conditions of 350–500°C, 1.2–2.0GPa) and at the UHP Lago di Cignana locality (Valtournenche, Italy; ~550°C, 2.5–3.0GPa) preserve records of prograde devolatilization in their mineral modes and chemistry, contents of volatiles and fluid-mobile elements (elements relatively mobile in aqueous fluids), and B and N isotope compositions. This suite allows study of prograde devolatilization history, across a wide range in metamorphic grade, in metasedimentary rocks that experienced high-P/T prograde paths similar to those experienced in most modern subduction zones. Across grade, whole-rock samples are in general uniform in their concentrations of relatively fluid-mobile elements N, B, Li, Cs, Ba, and Rb, normalized to the concentrations of the less mobile K2O and Al2O3, showing only hints of loss in several of the highest-grade samples. With increasing grade, ion microprobe analyses of phengites show subtle decrease in B concentration, uniformity in Ba and Cs concentrations, and increase in Li concentrations, the latter likely due to release from chlorite during its breakdown. In one Cignana sample, phengite inclusions in garnets are enriched in B relative to matrix phengite, consistent with either whole-rock B loss after garnet growth or, more likely, closed-system behavior and partitioning of B into paragonite or tourmaline stabilized after garnet growth. In samples with both paragonite and phengite, paragonite shows relative enrichment in B and Sr, and phengite is enriched in Cs, Ba, and presumably also N and Rb (the latter showing strong whole-rock correlations with K2O). Whole-rock δ15N shows a hint of shift to higher values in the highest grade rocks (Cignana) and, accordingly, calculated prograde dehydration histories for appropriate bulk compositions, using the Perple-X database, indicate that significant (~20%) dehydration would for some rocks occur over the temperature interval of 450 to 550°C, largely related to the breakdown of chlorite (and to a lesser extent carpholite). Small amounts of loss of N into these fluids could have resulted in minor shift in δ15N, with decrease in whole-rock N concentration masked by heterogeneity inherent with the sedimentary protoliths. Partitioning of Cs and Li (possibly also Rb and Ba) from white micas into H2O-rich fluids largely produced by chlorite breakdown could similarly have produced the subtle decreases in the concentrations in these elements noted in several high-grade samples. Neoblastic tourmaline in higher-grade rocks likely sequestered some fraction of the B lost from micas, resulting in a lack of obvious whole-rock B loss to accompany the up-grade trend of decreasing B concentrations in phengite. This tourmaline shows core-to-rim decrease in δ11B consistent with growth during small amounts of progressive B loss from phengites. Taken together, the whole-rock and SIMS data presented here, and the whole-rock dataset of Busigny et al. (2003), demonstrate impressive retention, during prograde forearc devolatilization, of elements thought to be relatively fluid-mobile (particularly H, N, B, Li, Ba, and Cs). Retention of these elements in metasedimentary rocks subducted to depths overlapping those beneath arc volcanic fronts (~90km estimated for subsolidus, peak Cignana metamorphism) implies their availability for transfer into arc source regions, in aqueous fluids or silicate melts, or into the mantle to depths beyond subarc regions. [Copyright &y& Elsevier]

Sediment burial to depths beyond 60 km is documented by rare exposures and inferred from mass-balance calculations, element recycling, and numerical modeling. Although thermomechanical and geochemical models have suggested potential diapirism of subducted metasediments, natural evidence of this remains scarce, especially for large volumes. The southern Tianshan metamorphic belt (northwest China) represents a unique example of a deeply buried, ~4-5-kmthick, coesite-bearing sequence dominated by trench infill metasediments, with kilometer-scale variations in volcanoclastic and pelitic components. We demonstrate that these rocks were subducted down to ~80 km, as shown by evenly distributed pressure-temperature conditions of 2.5 ± 0.2 GPa and 536 ± 11 °C. Considering sedimentary budgets and geophysical constraints on subduction channel thickness (<2-5 km), this sequence likely detached as several 0.5-1.0-km-thick individual slices, and later accreted to the upper plate within >2 m.y. Results show that temperatures were buffered and cold (<550 °C), suggesting continuous refrigeration (by incoming cold material from the slab, juxtaposition to a mantle wedge "cold nose", presence of a thick overriding plate, and/or fluid circulation along the plate interface). The absence of evidence for diapirism in this buoyant metasedimentary package, at ~540 °C and 80 km depth, suggests that metasedimentary diapirs may occur only at greater depth and/or farther away from the plate interface. [ABSTRACT FROM AUTHOR]

Abstract: The Corinth Rift is superimposed on the Hellenic nappe stack that formed at the expense of the Apulian continental crust above the subducting African slab. Extension started in the Pliocene and the major steep normal faults that control the geometry of the present-day rift were born very recently, some 600kyr ago only. They root into a shallow-dipping zone of microseismicity recorded near the base of the upper crust. The significance of this seismogenic zone is debated. Considering the northward dip of the zone of microseismicity, the depth of microearthquakes and their focal mechanisms, we observe a strong similarity with the northern Cycladic detachments in terms of expected pressure, temperature conditions and kinematics. We herein show (1) that the formation of the Corinth Rift can be considered a part of a continuum of extension that started some 30–35Ma in the Aegean and that was recently localised in a more restricted area, (2) that the present-day structure and kinematics of the Corinth Rift can be explained with a series of decollements relayed by steeper ramps that altogether formed a mechanically weak, crustal-scale detachment, and (3) that the deformation, fluid behaviour and metamorphic features seen in the northern Cycladic metamorphic core complexes can be good analogues of the processes at work below the Corinth Rift. [Copyright &y& Elsevier]

Progressive closure of the Neotethys ocean led to the formation of numerous mountain belts from the Alps to the Himalayas. At the crossroads, the Sistan belt stretches ~700 km N-S at a high angle to the adjacent Iranian major structures of Zagros, Makran or Alborz. It is characterized by extensive Mesozoic ophiolites, large-scale Mezo-Cenozoic sedimentary basins, profuse Cenozoic magmatism, and varied metamorphic rocks, including high-pressure low-temperature relicts (blueschists and eclogites). Currently, four main geometries and timing of the oceanic subduction have been proposed to explain the Sistan geodynamics and tectonic evolution. In order to discriminate between existing hypotheses, we conducted field, petrological and geochemical analyses of the oldest post-spreading magmatic rocks in the region. These Upper Cretaceous magmatic rocks are always found in the Eastern part of the Sistan belt, within and on the edge of the Afghan Block. They are intrusive into the Campanian to Maastrichtian flysch and always below the unconformity of the shallow-water, Paleocene reef limestone. Basalts to rhyolites form two coexisting groups: 1) a typical low K calc-alkaline series and 2) high-silica adakites (HSA) as indicated by major elements, trace elements and isotopic data. These results allow us to conclude that a magmatic arc developed during the Upper Cretaceous (probably around 80 Ma) on the Afghan Block. This confirms the existence of a northeast dipping subduction of the Sistan ocean. HSA are produced by partial melting of the downgoing oceanic crust. As this magmatism is younger than the dated eclogites, we envision that the thermal regime of this subduction must have been warmed by slab break-off and/or subduction of an oceanic ridge. [ABSTRACT FROM AUTHOR]

The metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic section of the sole is provided in this publication: these rocks record valuable information in terms of petrological processes and conditions of accretion, with limited retrogression. The petrology of the metamorphic sole shows that it originates from ocean floor and that it is the equivalent of the Shick-Shock volcanics group, metamorphosed to granulite/higher amphibolite facies. Presence of aluminosilicate bearing metapelites allows constraining pressure conditions in a more robust way than in the case of the famous Semail ophiolite (Oman). Thermobarometric estimates for peak metamorphic conditions for the metamorphic sole of the Mont Albert ophiolitic complex indicate temperatures above 800 °C close to the contact with the overlying peridotite, decreasing to ~650 °C within <30 m, at pressure around 1 GPa. Structurally lower subunits of the sole metamorphosed in the amphibolite/higher greenschist facies retain evidence of basaltic protolith accreted to lower pressures. Evidence of partial melting in high-grade subunits, contradictory with the apparent lack of melting products, may be linked to preferential accretion of melt-poor lithologies to the overlying plate, melt-rich lithologies being too weak to allow coupling and accretion to non-serpentinised peridotite. The reconstructed petrological evolution is consistent with stepwise accretion of metamorphic soles in nascent obduction/subduction systems as proposed for the Semail example and other metamorphic soles worldwide: the subducted oceanic plate is metamorphosed up to granulite facies by heat transfer from the still hot overlying peridotite, and accreted to the base of the future ophiolite when the rheological contrast between the two is minimum. Thermobarometric estimates indicate clock-wise pressure-temperature paths. The inferred thermal evolution requires burial of the metamorphic sole below warm, young (or rejuvenated) oceanic lithosphere, consistent with subduction initiation close to the ridge followed by rapid accretion and exhumation of the metamorphic sole. In this context, exhumation along higher geothermal gradient than burial shows that cooling of the mantle wedge above the plate interface takes place slowly compared to exhumation of the metamorphic sole, even though the plate interface itself has been cooling between burial and exhumation of the rocks. • Mineralogy of the metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé) • Kyanite and sillimanite-beaing metapelites constrain thermobarometry • Clockwise pressure-temperature paths and melting with elusive melt remnants • Petrological model with implications for thermal evolution along plate interface [ABSTRACT FROM AUTHOR]

Exposures of low-grade metabasalts and ophicarbonates in the Northern Apennines, and their high- and ultrahigh-pressure metamorphic equivalents in the Western and Ligurian Alps and Tianshan (representing an overall peak P – T range of ~ 0.2–3.0 GPa, 200–610 °C), allow investigation of the effects of prograde metamorphic devolatilization, and other fluid–rock interactions, on degrees of retention and isotopic evolution of C in subducting oceanic crust and associated mantle rocks. Such work can inform models of C cycling at convergent margins, helping to constrain the efficiency of return of initially subducted C via arc volcanism and the fraction of this subducted C entering the deeper mantle beyond arcs. In the metabasaltic rocks, the preservation of finely disseminated carbonate with δ 13 C overlapping that of seafloor-altered protoliths, and the minimal mineralogical evidence of decarbonation, indicates large degrees of carbonate retention in this suite extending to UHP conditions similar to those beneath modern volcanic fronts. For many of the metabasalts, the δ 18 O of this carbonate can be explained by closed-system equilibration with silicate phases (e.g., garnet, clinopyroxene) during HP/UHP metamorphism. Larger volumes of carbonate preserved in interpillow regions and as breccia-filling largely escaped decarbonation, showing little or no evidence for reaction with adjacent metabasalt. Calculated devolatilization histories demonstrate that, in a closed-system model, carbonate in metabasaltic rocks can largely be preserved to depths approaching those beneath volcanic fronts (80–90 km). Modeling of open-system behavior indicates that episodic infiltration of such rocks by H 2 O-rich fluids would have greatly enhanced decarbonation. Trends in O–C isotope composition of carbonate in some metabasaltic suites likely reflect effects of infiltration by externally-derived fluid with or without resulting decarbonation. Most carbonated ultramafic rocks similarly show little mineralogical evidence for decarbonation, consistent with calculated reaction histories, and have δ 13 C largely overlapping that of seafloor equivalents. However, the high-grade ophicarbonates show more restricted ranges in δ 18 O consistent with some control by infiltrating fluids, likely during subduction. This combination of field, petrographic, and isotopic evidence, together with calculated decarbonation histories, is consistent with minimal loss of CO 2 from these rocks via decarbonation during forearc metamorphism. Combining our results with those of Cook-Kollars et al. (2014; Chemical Geology ) for associated W. Alps metasedimentary rocks, we suggest that the majority of the CO 2 (perhaps 80–90%, considering the full range of rock types) could be retained through forearcs in more intact volumes of subducting sediment, basalt, and ophicarbonate experiencing closed- or limited open-system conditions. Deep in forearcs and beneath arcs, decarbonation (and also carbonate dissolution) could be enhanced in shear zones and highly fractured volumes experiencing larger fluid flux in part from dehydrating sub-crustal ultramafic rocks in slabs. Degrees of C loss by decarbonation, carbonate dissolution, and partial melting should be particularly significant as the subducting sections experience heating to > 600 °C at depths of 80–120 km (i.e., approximately at depths beneath arcs). [ABSTRACT FROM AUTHOR]

Metamorphosed Jurassic oceanic sediment in the Italian Alps experienced peak P – T conditions (1.5–3.0 GPa; 330–550 °C) similar to those experienced by sediments subducting through forearcs in most modern subduction zones. Integrated field, petrologic, and geochemical study of the devolatilization history of these rocks provides evidence regarding the extents of loss and mobility of oxidized and reduced C during subduction of sediments to depths beneath volcanic fronts, thus constraining models of the release of initially subducted C into the atmosphere via arc volcanism or contributing to the C budget of the deeper mantle. In this suite, occurrences of lawsonite and grossular-rich garnet in higher grade calcschists indicate some decarbonation reaction along a P – T gradient of ~ 8 °C/km. Across grade, carbonates largely retain δ 13 C VPDB values typical of marine carbonates (mostly ranging from − 1.5 to + 1.5‰), with some shifts to lower values in low-carbonate samples possibly in part reflecting decarbonation but likely largely due to isotopic exchange with abundant reduced C. Carbonaceous matter in some carbonate-poor samples shows increase in δ 13 C at higher grades, from values of − 25 to − 22‰ typical of marine organic matter, to values as high as − 10‰, consistent with the expected effects of devolatilization involving release of varying fractions of the original reduced C into fluids as CH 4 . In more carbonate-rich samples, shifts in the carbonaceous matter to higher values, by up to 15‰, reflect varying degrees of exchange with the carbonate in these samples. Carbonate across grade has δ 18 O VSMOW of + 17 to + 22‰ significantly lower than values typical of marine carbonates (the latter + 28 to + 30‰), likely in part reflecting exchange with silicate phases. Such shifts in the more carbonate-rich samples cannot be explained by closed-system exchange with silicates and require exchange with an external reservoir. The observed extents of decarbonation in these carbonate-bearing sediments subducted to depths of up to 90–100 km are consistent with limited open-system behavior, hence intermediate between models of Rayleigh-like CO 2 loss and recent models supporting open-system flushing of carbonate-rich rocks by H 2 O-rich fluid from underlying subducting oceanic lithosphere. Based on a separate study of devolatilization history in the metapelites intercalated with the metacarbonates, and consideration of the O isotope compositions of the two lithologies, the metapelites appear to be sources for the H 2 O-rich fluid that drove decarbonation reactions and shifted the δ 18 O values of the carbonates to their present values. Calculated decarbonation histories of the calcschists, using the Perple_X software, predict significant release of CO 2 related to the breakdown of CO 2 -bearing phases (dolomite, aragonite) and the growth of Ca-rich silicate phases (lawsonite, grossular-rich garnet). This decarbonation occurred at very low X CO 2 (< 0.01), for the P – T range considered here, perhaps attained during pulses of infiltration by H 2 O-rich fluids from the nearby metapelites. These results demonstrate that a large fraction of the initially subducted oxidized and reduced C in pelitic–carbonate sediment can be retained during forearc metamorphism and thus be available for loss beneath arcs or addition to the deeper mantle. In forearcs, the C inventory in these sediments experience minimal isotopic shift due to devolatilization, and isotopic reequilibration of the oxidized and reduced reservoirs during heating likely results in little net change in the bulk δ 13 C of the subducting section. Further attention should be paid to understanding the extent of C release, and possible isotope fractionation, during decarbonation, partial melting, and carbonate dissolution at the 80–100 k [ABSTRACT FROM AUTHOR]

Abstract: Syn-orogenic detachments in accretionary wedges make the exhumation of high-pressure and low-temperature metamorphic rocks possible with little erosion. The velocity of exhumation within the subduction channel or the accretionary complex, and thus the shape of P–T paths, depend upon the kinematic boundary conditions. A component of slab retreat tends to open the channel and facilitates the exhumation. We document the effect of slab retreat on the shape of P–T paths using the example of the Phyllite–Quartzite Nappe that has been exhumed below the Cretan syn-orogenic detachment during the Miocene in Crete and the Peloponnese. Data show a clear tendency toward colder conditions at peak pressure and during exhumation where the intensity of slab retreat is larger. This spatial evolution of P–T gradient is accompanied with an evolution from a partly coaxial regime below the Peloponnese section of the detachment toward a clearly non-coaxial regime in Crete. [Copyright &y& Elsevier]

Using the peri-Mediterranean blueschist belts as a case study we discuss the mechanisms of syn-orogenic exhumation of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The Mediterranean examples, within an overall convergent zone, show variations in the rates of convergence, rates of slab retreat, available space, and various stages of maturation of accretionary complexes. After a compilation of the deformation history and the kinematic boundary conditions and their evolution through time, we discuss P-T-t paths. Most of the structures found in the field today relate to the exhumation stage and often to quite superficial events related to syn-orogenic detachments. However a significant vertical motion occurred from the depth of eclogites (or UHP eclogites) to the depth of the blueschist or greenschist facies along cold P-T paths. Two types of mountain belts are described: those where a single thermal gradient was recorded throughout its' history, like the Franco-Italian Alps, which suggest a steady-state evolution; and those where the thermal gradient has changed through time (and space), such as the Aegean region, which suggest a non-steady-state evolution. Slab retreat within a subduction complex does not lead to the exhumation of UHP rocks because the open subduction channel allows for fast circulation and detachment of sediments from the subducting basement. Subducted sediments lubricate the subduction channel and the basement is thus not involved in the return flow. Early exhumation is fast and the thermal regime in the subduction channel is partly controlled by the velocity of convergence and the velocity of slab retreat as well as by the nature of the subducted material. Final exhumation occurs within the accretionary complex at a much slower rate below extensional detachments. The removal of the overburden is achieved primarily by extension in the upper part of the accretionary complex. Extensional faults and shear zones are rooted in the... [ABSTRACT FROM AUTHOR]