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1. Variations in the P-T-t of deformation in a crustal-scale shear zone in metagranite

   https://doi.org/https://doi.org/10.1029/2020GC009384  

   Cawood, T.K.; Platt, J.P. (November 2020, Geochemistry geophysics geosystems)

   null (Ed.)

   Deformation in crustal-scale shear zones occurs over a range of pressure-temperature-time (P-T-t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P-T-t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn-kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal-sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high-T shearing preserved far into the footwall, to low-T shearing adjacent to the hanging wall. The Ti-in-quartz and Si-in-phengite thermobarometers yield deformation P-T conditions, as both were reset syn-kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite-facies conditions (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist-facies conditions (∼305°C, 1.5 kbar) at ∼11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P-T-t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P-T. 

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2. Early Jurassic subduction initiation recorded in the Easton metamorphic suite, Northwest Cascades (Washington, USA)

   https://doi.org/10.1130/B38167.1  

   Cunetta, Nicholas S; Mulcahy, Sean R; Schermer, Elizabeth R; Smit, Matthijs A (May 2025, Geological Society of America Bulletin)

   The Easton metamorphic suite is a Mesozoic era subduction complex in northwest and central Washington, USA, which contains amphibolite-facies units structurally overlying separate high- and low-grade blueschist units. New structural, petrographic, and geochronologic data record a complex history related to Early Jurassic subduction initiation. Two types of amphibolite occur as (1) meter-scale coarse garnet amphibolite blocks with an upper amphibolite- to granulite-facies assemblage and (2) continuous layers of ≤10-m-thick foliated garnet amphibolite. The amphibolite blocks are encased in the foliated amphibolite, quartzose schist, and serpentinite. Garnet Lu-Hf geochronology records prograde garnet growth at 203 Ma in the amphibolite blocks and 183 Ma in the foliated amphibolite unit. In situ titanite U-Pb ages on amphibolite blocks, quartzose schist, and foliated amphibolite cluster at 168−163 Ma, with minor inherited components of up to 200 Ma. Amphibole 40Ar/39Ar cooling ages from the amphibolite blocks are 160−158 Ma, which is slightly younger than previously published 40Ar/39Ar cooling ages of 167−165 Ma from the foliated amphibolite. The deformation-temperature-time history of foliated amphibolite records subduction initiation in the Easton metamorphic suite at ca. 183 Ma, followed by cooling to high-grade blueschist facies, ∼500−600 °C, at ca. 165 Ma, and <400 °C by ca. 160 Ma. The 203 Ma coarse amphibolite blocks may have formed in an earlier metamorphic belt before being incorporated into the newly initiated subduction zone at 183 Ma, though an older age of subduction initiation is possible. Combined with existing data from the lower-grade regional blueschists that lie structurally beneath the high-grade rocks, the Easton metamorphic suite preserves >70 m.y. of subduction metamorphism and deformation. Early Jurassic subduction initiation in both the Easton metamorphic suite and the Franciscan Complex of California, USA, reflects broadly synchronous initiation of east-dipping subduction along portions of the Cordilleran margin by 183−176 Ma. 

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3. Amphibolization of the Tso Morari UHP eclogites: a record of fluid infiltration at amphibolite-facies during uplift in the subduction channel

   https://doi.org/10.1002/essoar.10509538.1  

   Pan, R.; Macris, C. A.; Menold, C. A. (January 2021, Transactions American Geophysical Union)

   Ultra-high pressure (UHP) metamorphism of the Tso Morari coesite-eclogite during burial in NW Himalaya has been intensively studied over the past several decades. However, amphibolite-facies metamorphism and accompanying metasomatism occurring at lower-crustal depths in the Tso Morari terrane are less well-constrained. In this study, we characterize the eclogite amphibolization and related metasomatic fluids by systematically sampling and analyzing the eclogites at the core of an eclogite boudin and the amphiblolized eclogite (amphibolite) at the rim. Integrated techniques including modal mineralogy, mineral chemistry, whole-rock geochemistry, Mössbauer spectroscopy, and thermodynamic modelling are used to constrain the fluid-induced eclogite amphibolization and associated fluid behaviors. Petrographic observations show that infiltration of an external fluid caused complete amphibolite-facies overprinting of the eclogites at the boudin rim. This is recorded petrographically as increased modal proportions of amphibole, biotite, epidote, plagioclase, and calcite in the amphibolites. The infiltrating fluid caused increased K2O and CO2 concentrations and higher bulk-rock Fe3+/ΣFe ratio for the amphibolites, as well as increased LILE (e.g., K, Rb, Cs, Sr, Ba) and ratios of Ba/Rb and Cs/Rb. Phase equilibria modelling using P–T–M(H2O) pseudosections on the amphibolite and the surrounding gneiss indicate that the fluid infiltration occurred at 9.0–12.5 kbar and ~608 °C with >2.6–3.1 mol % H2O infiltration. The abrupt increase of bulk-rock Fe3+/ΣFe ratio from 0.192 to 0.395 near the boudin rim indicate that this phase of fluid most likely derived from the mixing of dehydrated host orthogneiss and/or metasediments during uplift at the amphibolite-facies zone in the subduction channel. This study also demonstrates the need for using careful petrographic observations and geochemical analysis in parallel with thermodynamic modelling to achieve realistic results. 

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4. Mesozoic Fabric Influence on the Formation and Geometry of the Quaternary Cucamonga Fault

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (October 2022, Meeting of the Southern California Earthquake Center (SCEC), LAX, Sept., 2022)

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (Ed.)

   Ancient basement rocks in Southern California contain mechanical anisotropies that may influence the architecture of Quaternary faulting. We study exposed basement rocks found within the southeastern San Gabriel lithotectonic block with the intention of reconciling the relationship between inherited ductile fabrics and the geometry of Quaternary faults that are part of the San Andreas Fault system. By focusing our study on the southeastern corner of the San Gabriel block we can study the exposed lower- to middle crustal shear zone fabrics near where the Cucamonga Fault and the San Jacinto Fault intersect. The brittle Quaternary Cucamonga Thrust Fault strikes E-W and dips to the north-northeast (35-25°) and is localized at the range front and cuts these older fabrics, however there is also brittle deformation distal from the fault that also affects the sequence of lower- to middle crustal (6-8 kbar) granulite- to upper amphibolite facies mylonite and granulite-facies metasedimentary rocks. Near the Cucamonga Fault, mylonitic fabrics strike E-W and dip northeast (40-50°). Quaternary brittle faults that strike E-W and dip northeast (30-40°) reactivate the mvlonites and slickenlines and record a sinistral, top-to-the-west sense of shear. Investigation of host rocks indicates that they formed in the roots of a continental arc which was active from the Middle Jurassic to Late Cretaceous (172-86 Ma) at 740-800°C. Ductile deformation was associated with granulite-facies metamorphism at approximately 30 km depth during the Late Cretaceous (88-74 Ma) at 730-800 °C. Our work shows that the exhumed Late Cretaceous mylonitic fabrics may have operated as stress guides during Quaternary faulting in the Cucamonga Fault zone. We conclude that these lower crustal fabrics influence the geometry and kinematics of late Cenozoic faulting of the Cucamonga and San Jacinto fault zones. 

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5. Distinct P-T histories in a subduction mélange reveal underplating/mixing processes at the plate interface (North Motagua Mélange, Guatemala)

   Bonnet, G; Flores, K; Martin, C; Harlow, G (December 2020, American Geophysical Union, Fall Meeting 2019)

   Relicts of subducted and exhumed ocean floor preserved in suture zones record the events occurring at the plate interface. In particular, underplating and exhumation are the two main processes required to recover rocks from mantle depths. High-grade blocks exposed in serpentinite mélanges of the Motagua Valley record evidence of past subduction events between the North American Plate and the Caribbean Plate. Previous works suggest the existence of two subduction zones during Cretaceous, with cold metamorphism (lawsonite eclogite and blueschists) in the South (South Motagua Mélange), and warmer eclogites and amphibolites in the North (North Motagua Mélange, NMM). Although little work as been done so far to characterize the P-T paths and variability of the metabasite blocks embedded within serpentinite matrix in the NMM. Here we present new thermobarometric estimates using conventional thermobarometry, pseudosection modeling and thermometry of carbonaceous matter on a set of metabasites of different grades. There a minimum of four kinds of P-T paths: (1) (lawsonite-bearing) garnet-blueschists with peak P-T around 2.1 GPa and 480°C, (2) "cold eclogites" at ~2.2 GPa and 550°C experiencing isothermal decompression and epidote-amphibolite overprints, (3) "warm eclogites" at ~2.3 GPa and 600°C exhumed in cold environments and affected by blueschist-facies overprints, and (4) garnet-bearing epidote-amphibolites that may represent either retrogression of some eclogites, or prograde metamorphism under warm conditions. We find that garnet fractionation has a limited impact on isopleth-derived P-T estimates and that lawsonite breakdown may drive retrograde metamorphism and rheological switches at the plate interface. These new P-T estimates suggest that high-pressure rocks of the NMM may be recovered from different depths of a unique subduction zone, between 65 and 80 km, and exhumed in a relatively cold (and serpentinized) environment. This suggests a more complicated story than previously described, and calls for additional geochronological evaluation (in process). 

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