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In frozen cylinders composed of deuterium ice (Tm+3.8 ∘ C) and 10 % water ice (Tm0 ∘ C), it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 ∘ C produces water (H 2 O) which combines with the D 2 O ice to form mixtures of HDO. As a consequence of deformation, HDO and H 2 O meltwater are expelled along conjugate shear bands and as compactional melt segregations. Melt segregations are also associated with high-porosity networks related to the location of transient reaction fronts where the passage of melt-enriched fluids is controlled by the localized ductile yielding and lowering of the effective viscosity. Accompanying the softening, the meltwater also changes and weakens the crystallographic fabric development of the ice. Our observations suggest meltwater-enriched compaction and shear band initiation provide instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers. [ABSTRACT FROM AUTHOR]

Strain rate histories and strain magnitude are two crucial factors governing the evolution of dynamic recrystallized grain size and crystallographic preferred orientation (CPO) in rocks and ice masses. To understand the effect of cyclic variations in strain rate or non-steadystate deformation, we conducted two-dimensional, coaxial plane strain experiments with time-lapse observations from a fabric analyzer. There is a continuous reequilibration of microstructure and CPO development associated with constant and oscillating strain rate cycles. These can be correlated with c-axis small circle distributions, diagnostic of dynamic recrystallization involving new grain nucleation and grain boundary migration (GBM) as observed in the high-temperature deformation of ice and quartz. Inhomogeneous stress distribution can lead to grain size reduction for relatively slower strain rates and GBM for relatively faster rates within a long-interval cycling event, a behavior that contradicts the classic view for dynamic recrystallization processes. Where there is a rapid short-term cycling of strain rate, GBM is hampered and nucleation dominates, accompanied by a marked reduction of grain size and no new CPO development. During such short-term cycling, GBM and crystallographic change is impeded not by impurities, but by an inability of newly nucleated grains to grow. [ABSTRACT FROM AUTHOR]

East Tibet preserves broad areas of low relief that occur at elevations >3000 m and are dissected by major strike-slip faults that have been repeatedly reactivated since the late Mesozoic. Apatite fission-track samples from these low-relief, high-elevation surfaces indicate Cretaceous cooling (ca. 90 Ma). Fast rock uplift of the 110 °C isotherm and a fossilized partial annealing zone began in the plateau interior of east Tibet in the Miocene. Low denudation rates in the plateau interior during the Cenozoic, which are constrained by thermal history modeling and a consistency in ages from apatite fission tracks, suggest that dip-slip displacement on faults has been minor. Some localized areas have experienced the effects of structurally enhanced rock uplift and denudation or fault-related hydrothermal reheating. Cooling ages in Eocene sediments deposited on the older plateau surface suggest that rapid denudation occurred from the Miocene to Holocene. Other evidence of significant Ceno zoic denudation comes from river valleys, in particular, the Yalong River valley, where incision was initiated in the Oligocene to Miocene (28--12 Ma). Estimated mean denudation in east Tibet during the Cenozoic was ~1--2 km in the low-relief, high-elevation plateau interior and at least 5 km at the highrelief plateau margin in the Longmen Shan. The region south of the Longmen Shan and the Sichuan Basin was not uplifted with the main Tibetan Plateau in the Miocene, but it has undergone enhanced denudation since the Eocene. Due to an orographic effect, most of the denudation in the east Tibetan Plateau interior has occurred in major river valleys. Incision in the major rivers of the Longman Shan occurred simultaneously and has kept pace with surface uplift that began in the middle--late Miocene (12 Ma), with acceleration of denudation rates in the late Miocene (>5 Ma). [ABSTRACT FROM AUTHOR]

Microstructures inside exhumed crustal shear zones provide a window into the mechanics of Earth's interior. However, difficulties remain in quantifying their physical properties due to the numerous macro-, meso-, and grain-scale factors controlling their development and modification. The complex interplay between these factors has meant that inconsistencies remain between the knowledge derived from controlled laboratory deformation experiments and that from empirical observations in nature. We present a quantitative toolbox for identifying variations in the deformation characteristics of shear zones, using the microstructure and crystallographic preferred orientation (CPO) of quartz. The Main Central Thrust in the Alaknanda Valley (NW Himalaya) is an archetypal major ductile thrust zone that contains abundant quartzites and quartz-mica schists that permit microstructural characterization across gradients in strain and temperature. Using a combination of detailed microstructural analysis and neutron diffraction, we show that quartzites across the Main Central Thrust sequence are characterized by: (1) systematic increases in grain lobateness and coarseness, indicating the transition from subgrain rotation (SGR) to dominant grain boundary migration (GBM) dynamic recrystallization; (2) a concomitant shift in CPO from oblique (top-to-the-SSW) girdles to a strong single y maximum, indicating increasing activity of prismatic slip; and (3) high CPO intensities coinciding with the switch to GBM and prismatic slip activity. Our analyses suggest suggest the Main Central Thrust may be characterized as an ~3-km-wide zone of dominantly non-coaxial deformation coinciding with a sharp change in peak metamorphic temperatures, from ~450 °C in the footwall to 750-800 °C in the hanging wall. We suggest that the observed systematic microstructural and CPO transitions may typify the deformational behavior of quartz in the Main Central Thrust across the wider Himalaya. [ABSTRACT FROM AUTHOR]