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1. Deciphering late Devonian–early Carboniferous P–T–t path of mylonitized garnet‐mica schists from Prins Karls Forland, Svalbard

   https://doi.org/10.1111/jmg.12529  

   Kośmińska, Karolina; Spear, Frank S.; Majka, Jarosław; Faehnrich, Karol; Manecki, Maciej; Piepjohn, Karsten; Dallmann, Winfried K. (April 2020, Journal of Metamorphic Geology)

   Abstract Quartz‐in‐garnet inclusion barometry integrated with trace element thermometry and calculated phase relations is applied to mylonitized schists of the Pinkie unit cropping out on the island of Prins Karls Forland, western part of the Svalbard Archipelago. This approach combines conventional and novel techniques and allows deciphering of the pressure–temperature (P–T) evolution of mylonitic rocks, for which theP–Tconditions could not have been easily deciphered using traditional methods. The results obtained suggest that rocks of the Pinkie unit were metamorphosed under amphibolite facies conditions at 8–10 kbar and 560–630°C and mylonitized at ~500 to 550°C and 9–11 kbar. TheP–Tresults are coupled with in‐situ Th–U‐total Pb monazite dating, which records amphibolite facies metamorphism atc.359–355 Ma. This is the very first evidence of late Devonian–early Carboniferous metamorphism in Svalbard and it implies that the Ellesmerian Orogeny on Svalbard was associated with metamorphism up to amphibolite facies conditions. Thus, it can be concluded that the Ellesmerian collision between the Franklinian margin of Laurentia and Pearya and Svalbard caused not only commonly accepted brittle deformation and weak greenschist facies metamorphism, but also a burial and deformation of rock complexes at much greater depths at elevated temperatures. 

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2. Intra-arc transpression and thrusting in the Southern California Batholith document moderate ‘Sierra-BC’ translation and collision with the conjugate Hess oceanic plateau

   Schwartz, J; Klepeis, K; Miranda, E; Mora_Klepeis, G; Bixler, J; Robles, F (April 2025, Geological Society of America)

   The Late Cretaceous paleogeography of Southern California potentially plays a central role in resolving conflicting models for postulated large-magnitude dextral translations along the western margin of North America (the Baja-BC hypothesis) and the beginning of the Laramide orogeny. The Mt. Pinos sector of the Southern California Batholith provides a unique window into this time because it preserves evidence for a kinematically and temporally partitioned fault system that includes a ductile shear zone (the Tumamait shear zone) and a ductile-to-brittle thrust fault (the Sawmill thrust). These two structures accommodated intra-arc strain during the Late Cretaceous to Paleocene during three phases of deformation (D3-D5) that are superimposed on older (D1 and D2) structures. D1 structures only occur in Pre-Mesozoic rocks and provide a reference frame for understanding subsequent deformation phases. D2 structures form part of a previously unmapped dextral-normal shear zone that predates the Tumamait shear zone. The initiation of displacements within the Tumamait shear zone is recorded by the formation of D3 mylonites which everywhere record reverse-sinistral movement. Petrochronology of syn- D3 titanites give lower-intercept 206Pb/238U dates ranging from 77.0 to 74.0 Ma and upper amphibolite-facies temperatures ranging from 699 to 718°C. Subsequent folding of the D3 mylonites during D4 was synchronous with late-stage, peraluminous magmatism at ca. 70 Ma. Near the Sawmill thrust, the D4 event resulted in a S4 crenulation cleavage and asymmetric, overturned folds that record top-to-the-NE tectonic displacements. NE-directed thrusting along the Sawmill thrust occurred at 67-66 Ma is interpreted to have been kinematically linked to D4 deformation. This thrust placed upper plate rocks of the Southern California Batholith above the Late Cretaceous Pelona schist. We interpret deformational fabrics in the Mt. Pinos area to record a kinematically partitioned, transpressional system that involved sinistral-reverse shearing (D3) closely followed by folding and arc-directed thrusting (D4-D5). We speculate that D3 structures developed in response to opening of the Kula-Farallon plate boundary and we hypothesize that the Kula-Farallon-North American plate triple junction was located at the present-day location of the Garlock Fault at ca. 85 Ma thereby segmenting the arc at this location. This geometry resulted in in dextral shearing in the Sierra Nevada Batholith (and northward) and sinistral shearing in the Southern California Batholith and Baja California. Continued subduction of the Farallon plate beneath the Southern California Batholith led to a major arc flare-up event from 90-70 Ma which was associated with D3 sinistral transpression. We interpret D3-D5 structures to record oblique convergence and the underthrusting of the Hess oceanic plateau beneath the Southern California Batholith at ca. 70-66 Ma. Our model for the segmentation of the California arc is compatible with a moderate (1000-1600 km), ‘Sierra-BC’ translation model in which the Insular superterrane was located north of the Southern California Batholith in the Late Cretaceous. 

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3. Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

   https://doi.org/10.1029/2020TC006528  

   Kotowski, Alissa J.; Cisneros, Miguel; Behr, Whitney M.; Stockli, Daniel F.; Soukis, Konstantinos; Barnes, Jaime D.; Ortega‐Arroyo, Daniel (June 2022, Tectonics)

   Abstract Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex. 

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4. Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith, 2022, Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith

   Schwartz, J.; Lackey, J.S.; Miranda, E.; Klepeis, K.; Mora-Klepeis, G.; Robles, F.; Bixler, J. (October 2022, Geological Society of America Abstracts with Programs)

   We present >90 new igneous and metamorphic zircon and titanite petrochronology ages from the eastern Transverse Ranges of the Southern California Batholith (SCB) to investigate magmatic and tectonic processes in the frontal arc during postulated initiation of Late Cretaceous shallow-slab subduction. Our data cover >4000 km2 in the eastern Transverse Ranges and include data from Mesozoic plutons in the Mt. Pinos, Alamo Mountain, San Gabriel Mountain blocks, and the Eastern Peninsular mylonite zone. Igneous zircon data reveal 4 discrete pulses of magmatism at 258-220 Ma, 160-142 Ma, 120-118 Ma, and 90-66 Ma. The latter pulse involved a widespread magmatic surge in the SCB and coincided with garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust (Cucamonga terrane, eastern San Gabriel Mountains). In this region, metamorphic zircons in gneisses, migmatites and calc-silicates record high-temperature metamorphism from 91 to 74 Ma at 9–7 kbars and 800–730°C. The Late Cretaceous arc flare-up was temporally and spatially associated with the development of a regionally extensive oblique sinistral-reverse shear system that includes from north to south (present-day) the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. Syn-kinematic, metamorphic titanite ages in the Tumamait shear zone range from 77–74 Ma at 720–700°C, titanites in the Black Belt mylonite zone give an age of 83 Ma, and those in the eastern Peninsular Ranges mylonite zone give ages of 89–86 Ma at 680–670°C. These data suggest a progressive northward younging of ductile shearing at amphibolite- to upper-amphibolite-facies conditions from 88 to 74 Ma, which overlaps with the timing of the Late Cretaceous arc flare-up event. Collectively, these data indicate that arc magmatism, high-temperature metamorphism, and intra-arc contraction were active in the SCB throughout the Late Cretaceous. These observations appear to contradict existing models for the termination of magmatism and refrigeration of the arc due to underthrusting of the conjugate Shatsky rise starting at ca. 88 Ma. We suggest that shallow-slab subduction likely postdates ca. 74 Ma when high-temperature metamorphism ceased in the SCB. 

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5. Effect of Water‐Rock Ratio on the Stable Isotope Record of Fluid‐Rock‐Deformation Interactions in Detachment Shear Zone

   https://doi.org/10.1029/2023GC011340  

   Gottardi, Raphaël; Mire, Camron; Davis, Niya; Casale, Gabriele (May 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Oxygen and hydrogen stable isotope analyses of quartz and muscovite veins from the footwall of the Raft River detachment shear zone (Utah) provide insight into the hydrology and fluid‐rock interactions during ductile deformation. Samples were collected from veins containing 90%–100% quartz with orientations either at a high angle or sub‐parallel to the surrounding quartzite mylonite foliation. Stable isotope analysis was performed on 10 samples and compared with previous quartzite mylonite isotope data sets. The results indicate that the fluid present during deformation of the shear zone was meteoric in origin, with a δ²H value of approximately −100‰ and a δ¹⁸O value of approximately −13.7‰. Oxygen stable isotope O¹⁸O depletion correlates with the muscovite content of the analyzed rocks. Many of the analyzed samples in this and other studies show an apparent lack of equilibrium between the oxygen and hydrogen isotope systems, which can be explained by hydrogen and oxygen isotope exchange at varying fluid‐rock ratios. Our results suggest that the Raft River detachment shear zone had a low static fluid‐rock ratio (<0.1), yet experienced episodic influxes of fluids through semi‐brittle structures. This fluid was then expelled out into the surrounding mylonite following progressive shearing, causing further¹⁸O‐depletion and fluid‐related embrittlement. 

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