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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, temperature and strain rate are crucial factors governing the development of crystallographic preferred orientations (CPO) in ice. To better understand how CPO patterns change in response to these variables, we performed quantitative analyses on neutron diffraction data between 2010 and 2019, collected in situ during uniaxial compression experiments on deuterium ice. At strains >10% and temperatures <−10°C, the c -axis pattern switches from a single maximum ('cluster') to small circle ('cone'), both oriented parallel to shortening. The diameter and mean width of the cone pattern decrease as strain and/or strain rate increases. Prismatic axis (a and m) patterns are characterised by great circles parallel to the pole figure margin and may be distinguishable from the patterns in ice deformed under simple shear. While strain has the main influence on the degree of preferred orientation (or CPO 'strength'), both temperature and strain rate have minor influences, which limits the extent to which CPOs can be used to measure strain. As cluster patterns can be observed in the c -axes of ice deformed under both pure and simple shear settings, this may complicate interpretations of flow geometry in terrestrial ice unless the prismatic axis patterns are also considered. [ABSTRACT FROM AUTHOR]

In frozen cylinders composed of deuterium ice (Tm+3.8 °C) and 10% water ice (Tm 0 °C) it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 °C produces water (H2O) which combines with the D2O ice to form mixtures of HDO. As a consequence of deformation, HDO and H2O 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 provides instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers. [ABSTRACT FROM AUTHOR]

Byline: Edward S. Grew (1), Martin G. Yates (1), Christopher J. L. Wilson (2) Abstract: Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80--150 um across and are anhedral two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm--1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO.sub.4) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70--80 um band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% [PbAl.sub.2][Si.sub.2]O.sub.8 component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7--1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% [PbAl.sub.2][Si.sub.2]O.sub.8 component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb.sup.2+ + 2(K,Na)[AlSi.sub.3]O.sub.8 [right arrow] [PbAl.sub.2][Si.sub.2]O.sub.8 + 4SiO.sub.2 + 2(K,Na).sup.+ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700degC, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating. Author Affiliation: (1) Department of Earth Sciences, University of Maine, Orono, ME, 04469, USA (2) School of Earth Sciences, The University of Melbourne, Melbourne, VIC, 3010, Australia Article History: Registration Date: 07/08/2007 Received Date: 29/03/2006 Accepted Date: 07/08/2007 Online Date: 25/08/2007 Article note: Communicated by T.L. Grove.

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]

Gold precipitation at Bendigo postdates cleavage development and was initiated during folding, increased during a reverse faulting stage in bedding-parallel quartz veins, and culminated in quartz reefs associated with a strike-slip faulting event. Some gold was remobilized along carbonate-rich cataclasites associated with later brittle faults. Understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis, from source to sink. In this study, a petrographie approach, closely tied to macroscopic structural observations, along with stable isotope and fluid inclusion analyses, places constraints on the relative timing of gold mineralization with respect to deformation. Early sulfide assemblages in bedding- and cleavage-parallel veins are dominated by pyrite-pyrrhotite-siderite, and are free of visible gold. Later assemblages in reactivated bedding-parallel veins and other fold-related veins are characterized by the presence of arsenopyrite-ankerite-gold. Fault-related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold, but lack pyrrhotite. Small amounts of late-stage antimony-bearing minerals occur in many vein types and postdate the precipitation of gold. Decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins. Stable oxygen isotope data show that quartz from all vein types (δ18O = 15.9-19.0) homogenized with the host rocks, whereas carbon (δ13C = -14.0 to 4.1) and oxygen (δ18O = 4.5-24.0) in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature. This corresponds with early siderite, pyrrhotite, and anhedral pyrite, and with later ankerite and ferroan dolomite associated with arsenopyrite, galena, and euhedral pyrite that are crosscut by calcite veins. Fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide, with smaller proportions of nitrogen and methane, particularly in the later strike-slip faults. The sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks. Although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the Bendigo orogenic gold deposits, we believe there is little direct or deposit-scale evidence for a relationship between original metal content in the host-rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites. It is suggested that deep-seated faults act as conduits for fluid flow and the source for the gold at Bendigo; the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites. [ABSTRACT FROM AUTHOR]

Strain localization and fluid flow patterns adjacent to the massive quartz-liosted reefs of the Bendigo gold- field have been investigated using new structural observations, three-dimensional geometry of fold-fault relationships, and gold distribution. Quartz reef and vein formation in the Bendigo goldfield is associated with an initial E-W compression event that produced a penetrative upright cleavage and N-S-trending fold hinges. In the immediate hanging wall of quartz reefs with high gold grades, the S1 cleavage is locally deformed by an E-W-trending S2 cleavage that is attributed to a low-strain N-S-oriented compressive event. Further evidence of this low-strain event is provided by gently S plunging quartz-carbonate slickenline lineations on S1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef. The reefs are commonly situated on or immediately to the south of a left-flex (sinistral) change in strike of the axial surface of the folds, and the flexure occurs close to an anticlinal culmination defined by the double-plunging N-S-striking folds. The folds become deformed with meter to decimeter wavelengths, which generally affect the stiff units, in a disharmonic style. Accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition. Numerical models that best simulate the observed distribution of strain, based upon the location of quartz reefs, involve NW-SE-oriented contraction with a significant sinistral strike-slip component. This contraction event has an overall N-S-sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs. The models also demonstrate a geometrical dependence on the localization of strain and, thus, the potential localization of fluid flow. Volume and shear strain increase with the flex angle, and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure. This analysis highlights that the modification of preexisting fold structures and quartz veins by a low-strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins. Such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems. [ABSTRACT FROM AUTHOR]

The article presents a study on the deformation of ice resulting from compression and simple shear. It relates the experimental procedure in which test samples have been deformed and subjected to constant load or compressive stress and to constant shear stress conditions at -20 to -2° Celsius (C). Results show that temperature, associated with the localization of the deformation, affects the initiation of recrystallization, strength and deformational behaviour of ice. Findings suggest that fine-grained polycrystalline ice can exhibit a more rapid approach towards steady-state tertiary creep as compared to coarser-grained ice.

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]

The ability to effectively target ore shoots requires a clear understanding of the numerous parameters that control their formation. However, identification of the nature and timing of these parameters is difficult, particularly in polydeformed deposits with complex stratigraphic and hydrothermal histories. For example, ore shoots at Stawell gold mine's Magdala gold deposits in southeastern Australia are hosted by rocks that experienced at least three ductile and two brittle deformation events in the 70 m.y. prior to the principal mineralization event, and have a close spatial relationship to strongly chlorite ± stilpnomelane--altered sedimentary units near the apex of numerous >5-m-thick reentrant basalt lobes. Whereas the ore shoots at Stawell are located only on the southwestern flank of the 1-km-thick Magdala basalt, about 15 km along strike at the Wildwood prospect the ore shoots are predominantly restricted to the northeastern flank of the basalt. Here, the important issue of controls on ore shoot formation and distribution at Stawell is addressed by presenting the results of a multidisciplinary review of the characteristics and architecture of the volcanic and sedimentary facies, the spatial distribution of alteration assemblages, and the variations in intensity and vergence relationships of the cleavages associated with ductile deformation. This investigation, undertaken at both a regional and shoot scale, shows that the structures hosting the ore shoots at Magdala formed during preferential and localized reactivation of preexisting faults following rotation of the stress field. Preexisting structures, such as Central Lode fault, host many of the ore shoots and were initially generated during east-northeast-west-southwest compression at ca. 51.0 to 488 Ma. The geometries of these faults were also influenced by the irregular paleotopography of the basalt lobes, which represent flows of pillow and massive basalt separated by interflow sedimentary milts. Coincident with this deformation, locally intense chlorite ± stilpnomelane alteration indurated the mud-rich units between the basalt flow-lobes and the Central Lode-type structures. Those thick, mud-rich sedimentary units are present on the southwestern flank of the basalt at Stawell, but predominantly near the top and on the northeastern flank at Wildwood. The switch to east-west compression at ca. 440 Ma caused localized failure and focusing of mineralized fluids in the indurated sedimentary rocks near the apex of the northwest-plunging portions of the basalt lobes. A second, subordinate mineralizing event at ca. 425 to 420 Ma utilized the same architecture under a different stress regime. During both mineralizing events, the high-grade ore shoots formed in zones of high fluid flow defined by the complex preexisting architecture. [ABSTRACT FROM AUTHOR]

The Rauer Group, in Prydz Bay, contains reworked Archaean-Proterozoic crust in high-strain zones that formed during a pervasive high-temperature ductile deformation event related to intracratonic mechanisms. The effects of this event extend southwards from Prydz Bay into the southern Prince Charles Mountains. The associated structural evolution involved development of ductile and brittle structures that formed during an approximately north-south directed transpressional deformation event that is confined to high-grade (>800deg;C) shear zones in the Rauer Group. Minerals from the Rauer Group, yield 40Ar/39Ar cooling ages ranging from 560 to 460 Ma. Thermal histories derived from hornblende, biotite and feldspar suggest that the onset of rapid cooling began sometime prior to 510 Ma with cooling rates of c. 42 to 33°C myr-1 from c. 510 Ma to c. 500 Ma. Whereas, 40Ar/39Ar data obtained from plagioclase and K-feldspar suggest a slower cooling from c. 500 Ma to c. 460 Ma with cooling rates from 5 to 2°C myr-1. These results demonstrate that the early Palaeozoic cooling history and comparable palaeostress regimes are regionally extensive, which has important implications for the tectonothermal and stress-field variability across Gondwana. The elevated thermal conditions would induce lithospheric weakening and promote the early Palaeozoic intraplate orogeny observed in eastern Antarctica with the development of a large intracratonic shear system. [ABSTRACT FROM AUTHOR]

Analysis of five basement samples from the Vest fold Hills (East Antarctica) reveals pooled apatite fission track (FT) ages ranging from 188 to 264 Ma and mean lengths of 13.7 to 14.9 gm. Quantitative thermal histories derived from these data give consistent results indicating onset of cooling/denudation began sometime prior to 240 Ma, with final cooling below 105° 125°C occurring between 240 and 220 Ma (Triassic). A Cretaceous denudation phase can be inferred from the sedimentary' record of the Prydz Bay offshore the Vestfold Hills. The two denudational episodes are likely associated with Palaeozoic large-scale rifting processes that led to the formation of the adjacent Lambert Graben, and to the Cretaceous Gondwana break-up between Antarctica and India. Subsequent evolution of the East Antarctic passive continental margin likely occurred throughout the Cenozoic based on tile depositional record in Prydz Bay and constraints (though tentative) from FT data. [ABSTRACT FROM AUTHOR]

The Wonga gold lodes (∼300,000 oz Au) in the Victorian gold fields of eastern Australia formed at a depth of less than 7 km within the seismogenic zone. The Wonga gold deposit is hosted by mudstone-dominant lithofacies that define the core of the regional synform. These are underlain by sandstone-dominant lithofacies that are interlayered with mafic schist. The lodes occur within the contact aureole of a late-stage granitic body that intrudes an earlier phase of the fractionated pluton. In contrast to the nearby 440 Ma Magdala orogenic gold lodes (∼5 Moz Au), this smaller deposit formed at ca. 400 Ma and has been classified as intrusion related. The orogenic and intrusion-related gold deposits formed at different stages in the structural and magmatic evolution of the fold belt, and at different crustal levels. Compared to the Magdala lodes, the Wonga lodes have a close temporal and spatial relationship with the intrusive rocks, and a distinct structural style and mineralogy, including acicular arsenopyrite, stibnite, gold telluride alloys, bismuth, silver, and late-stage molybdenite. The Wonga lodes formed between two distinct phases of regional faulting, and the kinematics of the mineralized structures cannot be correlated with any preserved, far-field regional paleostresses. Instead, they are inferred to reflect the local state of stress at the point that magmatic-retated fluid overpressure occurred. Pervasive fluid overpressure drove deformation across a broad zone, producing massive, mineralized quartz-vein structures that included angular clasts of wall rock. A discrete overpressure event is suggested, rather than repeated crack-seal behavior. The lodes represent an early stage in the development of a fault zone characterized by a series of discontinuous, discrete shear failures. The geometry of the lodes is strongly dependent on structural position and rheology. Northeast-dipping, preexisting weaknesses, such as dike margins and variably developed... [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]