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1. The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

   https://doi.org/10.2113/2020/8824736  

   Lusk, Alexander D. J. ; Platt, John P. ; Roeske, ed., Sarah M. ( September 2020 , Lithosphere)

   Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed inmore »naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

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2. Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

   https://doi.org/10.1029/2019TC005914  

   Grujic, Djordje ; Ashley, Kyle T. ; Coble, Matthew A. ; Coutand, Isabelle ; Kellett, Dawn A. ; Larson, Kyle P. ; Whipp, Jr., David M. ; Gao, Min ; Whynot, Nicholas ( March 2020 , Tectonics)

   We postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti‐in‐quartz thermobarometry and quartzcaxis textures. These data were combined with thermochronology, including new and published⁴⁰Ar/³⁹Ar ages of muscovite and published apatite fission track, and apatite and zircon (U‐Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland‐dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear tomore »be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until ~11 Ma.

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3. Competing Influence of Compositional Variability and Metamorphism on Strain Localization in a Lower-Crustal, Transpressional Shear Zone, Fiordland, New Zealand

   Lindquist, P. ; Klepeis, K. ; Miranda, E. ; Schwartz, J. ; Stowell, H. ( October 2019 , American Geophysical Union Abstracts with Programs)

   Over 500 km2 of rock exposure in Fiordland, New Zealand records strain localization processes accompanying the formation of a steep, transpressional shear zone within the root of a Cretaceous continental magmatic arc. Here, we pair field observations with microstructural and petrographic analyses of the George Sound shear zone (GSSZ) to investigate how metamorphism and compositional variability influenced shear zone evolution in the lower continental crust. The northern portion of the 50 km-long GSSZ deforms a monzodioritic pluton where superposed mineral fabrics record a narrowing of the shear zone width over time. Early stage deformation was accommodated mostly by dynamic recrystallization of pyroxene and plagioclase, forming a steep zone of coarse, gneissic foliations over 10 km wide. Subsequent deformation created a 2 km-thick zone of mylonite containing fine-grained plagioclase, hornblende, biotite, and quartz. The latter three minerals formed during the hydration of older minerals, including igneous pyroxene. The change in mineralogy and grain size also produced thin (< 1 mm), weak layers that localized deformation in shear bands in the highest strain zones. The southern ~35 km of the GSSZ deforms a heterogeneous section of granite, diorite, and metasedimentary rock. In this area, the hydration of igneous assemblages also is pervasivemore »but is not restricted to high-strain zones. Instead, the shear zone branches into four ≤1 km-wide strands that closely follow lithologic contacts. The thinnest branch occurs at the contact of a coarse-grained, dioritic pluton and a fine-grained granitic pluton. These patterns suggest that the factors that controlled strain localization in the GSSZ vary along its length. In the north, where its host rock is homogeneous, retrograde metamorphism helped localized strain into shear bands at the micro scale, mirroring a narrowing at the km scale. In the south, lithologic contacts created weak zones that appear to have superseded the effects of metamorphism, creating a series of thin, branching high-strain zones. These results suggest that the rheology of lower-crustal shear zones also varies significantly along their length and over time. Both of these factors can be used to generate improved models of continental deformation.« less
4. Alpine Fault‐Related Microstructures and Anisotropy of the Mantle Beneath the Southern Alps, New Zealand

   https://doi.org/10.1029/2022JB024950  

   Shao, Yilun ; Prior, David J. ; Scott, James M. ; Kidder, Steven B. ; Negrini, Marianne ( November 2022 , Journal of Geophysical Research: Solid Earth)

   Mantle xenoliths from the Southern Alps, New Zealand, provide insight into the origin of mantle seismic anisotropy related to the Australian‐Pacific plate boundary. Most xenoliths from within 100 km lateral distance of the Alpine Fault are coarse grained, but a small number are finer grained protomylonites. The protomylonites contain connected networks of fine grains with a different crystallographic preferred orientation (CPO) to coarse porphyroclasts in the same xenolith, suggesting that protomylonites and coarse‐grained samples record different deformation kinematics. The CPOs of fine grains in protomylonites have monoclinic symmetry, with the 2‐fold rotation axis normal to a plane that contains olivine [010] and orthopyroxene [100] maxima, suggesting that the protomylonite deformation involved significant simple shear. Some coarse‐grained samples contain unconnected lenses and layers of fine grains with the same CPO as the coarse grains. Microstructures suggest that these fine grains formed by subgrain rotation recrystallization and that protomylonites may represent an up‐strain progression of this microstructure, where the connectivity of fine grains has allowed them to localize shear and develop a new Alpine Fault CPO. The samples tell us about the state of the mantle at 25 Ma, in the early history of the plate boundary. If this suite of samples ismore »representative of the mantle beneath the Alpine Fault in the present day, then we can interpret the complex seismic anisotropy patterns in the lithospheric mantle as representative of blocks containing variably rotated older CPOs juxtaposed by narrow shear zones associated with Alpine Fault deformation.

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5. Quartz Vein Formation and Deformation during Porphyry Cu Deposit Formation: A Microstructural and Geochemical Analysis of the Butte, Montana, Ore Deposit

   https://doi.org/10.2113/2022/3196601  

   Acosta, Marisa D. ; Reed, Mark H. ; Watkins, James M. ; Cheng, ed., Feng ( August 2022 , Lithosphere)

   Hydrothermal quartz veins from the Butte deposit display euhedral and mottled cathodoluminescent (CL) textures that reflect the growth and deformation history of quartz crystals. A CL-euhedral texture consists of oscillatory dark-light zonations that record primary precipitation from an aqueous fluid. The origin of a CL-mottled texture, which consists of irregularly distributed dark and light portions, is less clear. Previous work showed that in some veins, CL-euhedral and CL-mottled crystals coexist, but the processes leading to their formation and coexistence were unknown. We find that CL-mottled crystals occur predominantly along the wall rock fracture surface and in vein centers and that CL-euhedral cockscomb quartz protrudes from the mottled layers along the wall rock. We infer that the mottled crystals formed by strain-induced recrystallization that was preferentially accommodated by the rheologically weaker layers of noncockscomb quartz because cockscomb crystals are in hard glide orientations relative to adjacent noncockscomb layers. During strain, crystals in noncockscomb layers that are not initially susceptible to slip can rotate in their deforming matrix until they deform plastically. Some of the CL-mottled crystals exhibit a relict CL-euhedral texture (“ghost bands”) whereby bright bands have been blurred and deformed owing to Ti redistribution facilitated by grain boundary migration.more »The edges of some CL-euhedral crystals become CL-mottled by localized grain boundary migration along adjacent crystals that do not align perfectly. Throughout the veins, CL-mottled crystals are randomly oriented, indicating that small deviatoric stresses were sufficient to drive recrystallization and mobilization of trace elements. Ti concentrations in CL-mottled crystals (23-47 ppm Ti; mean of 31 ppm) overlap those of CL-euhedral dark growth bands (16-40 ppm Ti; mean of 25 ppm Ti) in neighboring CL-euhedral crystals. Average Ti concentrations in CL-mottled quartz and CL-euhedral dark growth bands correspond to temperature estimates of 600°C (31 ppm Ti; CL-mottled) and 619°C (25 ppm Ti; dark bands), which are in good agreement with previous quartz precipitation temperature estimates based on independent thermobarometers. We conclude that recrystallization resets CL-mottled Ti concentrations close to the equilibrium value for the conditions of deformation and that CL-dark growth bands record near-equilibrium Ti concentrations. Recognition of widespread quartz recrystallization in porphyry Cu deposits underscores the significant role that strain plays in deposit formation. Individual veins host crystals that preserve conditions of primary growth and other crystals that preserve conditions of deformation and thermal overprint. Textural information is key to accurately interpreting trace element data and identifying different stages of vein formation. Our suggestion that CL-dark bands are the best candidates for near-equilibrium growth will aid the interpretation of trace element zoning in other hydrothermal systems.

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