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1. Evidence and timing for multiple cooling mechanisms in a long-lived subduction zone from the Easton metamorphic suite in the northwest Cascades

   Mulcahy, S.R.; Schermer, E.L.; Cordova, J.; Gates, E. (January 2023, Geological Society of America Abstracts with Programs)

   Numerical models of subduction initiation and observations of exhumed subduction complexes indicate that the early stages of subduction are characterized by rapid cooling followed by a prolonged steady thermal state that can last tens of millions of years. Several mechanisms are proposed to drive cooling in subduction zones and include thermal relaxation, exhumation, and underplating, but determining the relative contribution of each mechanism in the history of an exhumed subduction complex can be difficult. The Easton metamorphic suite in the Northwest Cascades of Washington is a Jurassic-Cretaceous subduction complex that records the subduction and accretion of distinct units within a thermally maturing nascent subduction zone. The region preserves an inverted metamorphic sequence with metamorphic temperatures and ages that decrease structurally downward from an early accreted metamorphic sole to younger regionally extensive blueschist facies units. In the metamorphic sole Grt±Cpx amphibolite was metamorphosed at 750-800 C at 1.0 GPa prior to 167 Ma. The amphibolite is underlain by a high temperature Grt-Ab-Gln blueschist that was metamorphosed at ~530 C and 1.0 GPa at 165 Ma. The contact between the units is gradational and the general lack of deformation suggests initial cooling to lower temperatures may have been caused by cooling of the overall subduction zone. Retrograde Lws-Ep-Gln-Ms assemblages suggest that cooling of both the amphibolite and high-grade blueschist units to below 400-500 C was caused by exhumation to 0.7 GPa by 157 Ma. In the regionally extensive blueschist units, Ep-Ab-Chl-Ms±Grt±Lws phyllite was metamorphosed at 430-450 C and 0.7 GPa by 149 Ma. Retrograde fabrics in the phyllite record similar temperatures to peak metamorphic conditions in an underlying Ep-Ab-Gln/Act greenschist/blueschist unit that was accreted and metamorphosed at ≤350 C by ≤140 Ma. The contact between the phyllite and greenschist is marked by a high strain mylonite zone and the combined observations suggest that cooling of the phyllite was driven by underplating of the younger greenschist unit. The observed assemblages and fabrics within the Easton metamorphic suite record cooling as the result of thermal relaxation, underplating, and exhumation at distinctly different times in the subduction history. 

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2. Structural evolution of subduction initiation and early cooling: insights from the Easton metamorphic suite, northwest Washington

   Cunetta, N.; Schermer, E.L.; and Mulcahy, S.R. (January 2023, Geological Society of America Abstracts with Programs)

   Exhumed high pressure/low temperature metamorphic belts provide evidence of subduction processes at depth, but the mechanisms by which subduction zones initiate and evolve to steady thermal state remain contested due to a paucity of recovered subduction infancy rocks. The Easton metamorphic suite in northwest Washington is a Jurassic-Cretaceous subduction complex that preserves a high-grade metamorphic sole inferred to have formed during subduction initiation. Near Gee Point, WA, coherent garnet amphibolite, hornblende-bearing white mica quartzose schist, and underlying garnet blueschist are characterized by their proximity to overlying serpentinized peridotite and the presence of blocks and layers of foliated and unfoliated metasomatic rock. These high-grade rocks are exposed in a series of northeast vergent, steeply inclined, tight-isoclinal folds that post-date regional high pressure/low temperature metamorphism. Two shear zones dip steeply to the southwest, concordant to adjacent blueschist-greenschist, quartzose schist, and amphibolite fabrics. Pervasive retrogression from amphibolite to blueschist-greenschist facies from high to low structural levels is documented at the map, outcrop, and micro scale. Amphibolites contain a foliation defined by aligned hornblende, or in places are weakly foliated. Retrogression of amphibolite is associated with a well-developed glaucophane-white mica-chlorite fabric that wraps older garnet and hornblende, with pargasite cores rimmed by glaucophane and chlorite. Both garnet blueschist and quartzose schist preserve at least one younger foliation. Prior 40Ar/39Ar thermochronology and thermobarometry are consistent with a cooling subduction zone, from amphibolite facies conditions of 760C at >167 Ma to <400C at ~163 Ma. Paired garnet 176Lu-176Hf geochronology and 40Ar/39Ar thermochronology samples will elucidate the timing of (1) proposed subduction initiation, (2) early subduction zone cooling rates, and (3) two shear zones that deformed the Easton metamorphic suite. 

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3. 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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4. Investigating Post-Taconic Deformation in the Pine Hill Thrust, Southern Vermont

   Khadka, S; Webb, L; Karabinos, P; Long, M; Espinal, K (December 2024, Transactions American Geophysical Union)

   The Pine Hill thrust, a western frontal thrust of the Green Mountain massif in southern Vermont, is characterized by reverse faults that place Precambrian basement rocks over mid-Ordovician rocks. Based on cross-cutting relationships, it has been considered a late-stage Taconic thrust. However, recent investigations in the western front of the Sutton Mountains, Green Mountain massif, and Berkshire massif of southern Quebec, Vermont, and Massachusetts, respectively, suggest fault displacement at 420 Ma and younger. Therefore, motion on these faults may instead be associated with the late Salinic or early Acadian orogeny. This study investigates the hypothesis that the Pine Hill thrust records deformational events associated with the late Salinic and/or Acadian orogenies. Preliminary studies from fieldwork and microstructural analysis of slabbed samples from transects across the Pine Hill thrust, where the lower Cambrian Dalton Formation is mapped as thrust over the Upper Ordovician Ira Formation, reveal at least four generations of foliation. The oldest tectonic foliation, S1, is parallel to primary compositional layering (S0) and is associated with isoclinal F1 folds. Moving from the Dalton Formation in the hanging wall towards the fault zone, S1 becomes progressively transposed into S2, marked by metamorphic compositional layering. Closer to the fault, S2 is crenulated, and S3 emerges as the dominant foliation, becoming the only foliation exhibited by the phyllonites in the fault zone. Finally, the youngest foliation, S4, is a localized crenulation cleavage developed in more pelitic material. These preliminary results suggest a complex deformation history, possibly involving multiple phases of post-Taconic motion on the fault during subsequent orogeneses. Further microstructural analysis and geochronology of these deformation fabrics will help establish the timing of deformation and its tectonic significance, helping to correlate surface geology with results from New England Seismic Transect (NEST) imaging of crustal and mantle lithospheric structure in the northern New England Appalachians. 

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5. Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

   https://doi.org/10.5194/se-12-1335-2021  

   Cisneros, Miguel; Barnes, Jaime D.; Behr, Whitney M.; Kotowski, Alissa J.; Stockli, Daniel F.; Soukis, Konstantinos (January 2021, Solid Earth)

   Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes. 

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