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1. Understanding fault-fluid interaction through stable isotope analysis of tourmaline-coated brittle faults of the West Antarctic Rift System

   Pollatsek, Emory; Siddoway, Christine; and Gevedon, Michelle (October 2022, Geological Society of America Abstracts with Program)

   Brittle faults in the southern Ford Ranges of Marie Byrd Land, West Antarctica, provide unique opportunity to study fluid-rock interactions in the West Antarctic Rift System and the role of crustal fluids during regional-scale faulting. This fault array contains steep, NNW-striking, normal-oblique slip faults and sub-vertical WNW-ESE strike-oblique faults. \ Faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak display strongly aligned tourmaline, indicating syntectonic mineralization; surfaces in one location feature distinctive mirror surfaces, suggestive of formation during seismic slip. Tourmaline has been demonstrated to resist chemical and isotopic re-equilibration during even high-temperature metamorphism, and to maintain a record of conditions during formation, therefore oxygen isotope compositions of tourmaline and quartz pairs may elucidate crustal conditions (e.g. temperatures and fluid-rock ratios) and fluids sources. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces; host rocks are tourmaline-free. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰; ∆Qtz-Trm values between 1.3 and 2.0 may reflect an inability of quartz to equilibrate during tourmaline crystallization. Equilibrium between quartz and tourmaline would suggest temperatures of formation in excess of 550°C. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. Geometric and kinematic relationships suggest a link to deeper level shears hosting muscovite, and sillimanite with garnet. On-going investigation includes O isotope analyses of these shears, as well as analysis of H and B isotopes in tourmaline, which will better characterize the relationship between the deeper crustal shears with the brittle fault array, and the fluid sources and metasomatic processes of regional fault systems. Furthermore, temporal constraints on tourmaline mineralization will establish whether faulting is associated with Cretaceous intracontinental extension of the West Antarctic rift system (Siddoway 2008) or a crustal response to Neogene mantle delamination beneath the South Pole region (Shen et al 2018). 

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2. Investigation of tourmaline-mineralized mirrored brittle faults from West Antarctica using ∂O (qz-tur), tourmaline Ar/Ar thermochronology, and brittle kinematic analysis, GSA Abstracts with Program, 56(4), doi: 10.1130/abs/2024CD-399623.

   Siddoway, Christine S; Breyak, A; Gevedon, M; Pollatsek, E; Hemming, S; Cox, SE (May 2024, Geological Society of America)

   Brittle faults are widespread but rarely exposed in Marie Byrd Land, a part of the West Antarctic rift system, owing to enhanced erosion of zones of cataclasis by the regional ice sheet. Tourmaline-mineralized faults discovered at three locations in the Ford Ranges constitute a new record of fluid-rock interactions in this region of extended crust. Tourmaline resists re-equilibration, even during metamorphism, thus strongly aligned tourmaline from high-angle faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak likely contain direct records of fault-hosted fluids and timing of fault movements. The faults form an array oriented NNW-SSE and WNW-ESE, which displays brittle kinematic criteria indicating normal-oblique and strike-oblique displacement. Mirrored fault surfaces suggest formation during seismic slip. Tourmaline is concentrated within a 2 to 4 mm zone bordering the fault planes. Petrography and EMPA analyses show unzoned tourmaline , with the dravite variety at Lewissohn Nunatak and schorl at the other two sites. Fluid inclusions in dravite are tubular (A-axis-parallel), 10 to 15 um, and up to 25 um, in length, containing gas and fluid phases. Fluid inclusions in schorl are C-axis-parallel and breached. Tourmaline ∂18O ratios (n=4) range from 9.2 to 10.4 ± 0.1 ‰ VSMOW (average 9.7‰, s.dev. = 0.7). Paired quartz yield ∂18O values of 11.1 to 10.3 ± 0.1 ‰, and ∆Qtz-Trm values between 1.3 and 2.0. Brittle microfractures in parallel arrays, evident in thin section, indicate tensile opening along ENE- WSW axes, in accordance with outcrop evidence. The strong preferred orientation and uniform mineral composition of tourmaline indicate syntectonic growth of tourmaline along fault planes. ∆Qtz-Trm values suggest equilibration between host-rock quartz and tourmaline was not achieved, likely due to rapid tourmaline precipitation. Relative isotopic homogeneity between sites suggests similar fluid conditions across the region, for crust underlying a minimum area of 2000 km2. Preliminary results of tourmaline 40Ar/39Ar dating indicate broadly Cretaceous timing for fault-related fluid flow. Ongoing work seeks to determine the temperature of mineralizing fluids and evaluate whether the brittle array localizes geothermal heat beneath the contemporary icesheet. 

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3. Transport of mid-crustal fluids to the upper crust through brittle fault networks as revealed by stable isotope ratios of tourmaline-mineralized fault surfaces, West Antarctica

   Gevedon, Michelle; Siddoway, C; Pollatsek, E; Breyak, A; Cox, SE; Hemming, S; Rasbury, ET (July 2024, Geological Society of America, https://gsa.confex.com/gsa/2024AM/meetingapp.cgi/Paper/404810)

   Stable isotope (δ18O, δD, δ11B) ratios of fault surface and shear zone minerals sampled from Marie Byrd Land in the West Antarctic rift system (WARS) provide opportunity to monitor potential fluid transport across multiple levels of the crust during active rifting. In the upper crust, high-angle brittle faults in the southern Ford Ranges display tourmaline-mineralized surfaces at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak. Tourmaline are strongly aligned with fault striae indicating mineralization during normal-oblique and strike-oblique displacement, with dilatancy allowing fluid infiltration of fault surfaces. Tourmaline’s refractory nature preserves isotopic compositions, which serve as a proxy for fluid sources and water-rock ratios. We compare tourmaline isotopic ratios with those of muscovite and quartz that occupy progressively deeper, kinematically linked fault-shear networks, and high-grade sillimanite-garnet-quartz±biotite associations, with the objective of characterizing potential fluid sources, relative depths of fluid interactions, and eventual estimation of volume of migrated fluids. Tourmaline δ18O values range from 9.1 and 10.4 ± 0.2 ‰ VSMOW (avg.= 9.8 ‰; st.dev. = 0.6), with intrasample reproducibility from 0.9 ‰ to 1.2‰, either as the result of variation in fluid sources or minor fluctuations in temperature during tourmaline formation. Quartz δ18O ratios range from 11.1 to 10.3 ± 0.2 ‰ (avg. =11.0‰; st.dev. = 0.64), with paired ∆Qtz-Tur values lower than quartz calculated to be in equilibrium with tourmaline at 450°C. Calculated qtz-tur temperatures exceed values reasonable for brittle crust (>700°C), indicating tourmaline grew rapidly or quartz has undergone subsolidus reequilibration. Fluids calculated to be in equilibrium with tourmaline at 450°C range from 8.2 to 9.5‰. Tourmaline 40Ar/39Ar geochronology in progress yields Early Cretaceous dates, indicating mineralization coincided with rifting onset. Very rapid development of the WARS and high thermal gradients during ENE- WSW transtension promoted upward movement of fluids in equilibrium with magmatic bodies or dehydrating metamorphic or sedimentary protolith. Tourmaline of Mt. Douglass and Mt. Dolber yield δD values of –60 and –64‰; these values confirm the role of fluids derived from mid crustal sources transported to the upper crust through fault-shear network. 

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4. Signatures of Fluid-Rock Interactions in Shallow Parts of the San Andreas and San Gabriel Faults, Southern California

   https://doi.org/10.55575/tektonika2024.2.2.47  

   Evans, James; Crouch, Kaitlyn; Studnicky, Caroline; Bone, Sharon; Edwards, Nicholas; Webb, Samuel (January 2024, Tektonika)

   We examine deformed crystalline bedrock in the upper parts of the active San Andreas and ancient San Gabriel Faults, southern California, to 1) determine the nature and origin of micro-scale composition and geochemistry of fault-related rocks, 2) constrain the extent of fluid-rock interactions, and 3) determine the interactions between alteration, mineralization, and deformation. We used drill cores from a 470 m long inclined borehole through the steep-dipping San Gabriel Fault and from seven inclined northeast-plunging boreholes across the San Andreas Fault zone to 150 m deep to show that narrow fault cores 10 cm to 5 m wide lie within 100s m wide damage zones. Petrographic, mineralogic, whole-rock geochemical analyses and synchrotron-based X-ray fluorescence mapping of drill core and thin sections of rocks from the damage zone and narrow principal slip surfaces reveal evidence for the development of early fracture networks, with iron and other transition element mineralization and alteration along the fractures. Alteration includes clay $$\pm$$ chlorite development, carbonate, and zeolite mineralization in matrix and fractures and the mobility of trace and transition elements. Carbonate-zeolite mineralization filled fractures and are associated with element mobility through the crystalline rocks. Textural evidence for repeated shearing, alteration, vein formation, brittle deformation, fault slip, pressure solution, and faulted rock re-lithification indicates significant hydrothermal alteration occurred during shallow-level deformation in the fault zones. The rock assemblages show that hydrothermal conditions in active faults develop at very shallow levels where seismic energy, heat, and fluids are focused. 

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5. The Role of Quartz Cementation in the Seismic Cycle: A Critical Review

   https://doi.org/10.1029/2021RG000768  

   Williams, Randolph T.; Fagereng, Åke (March 2022, Reviews of Geophysics)

   Abstract Because quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture. 

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