Localized Shear Zones Research Articles

Evaluation on the Minimum Principal Stress State and Potential Hydraulic Jacking from the Shotcrete-Lined Pressure Tunnel: A Case from Nepal

Reliable estimation of in situ stress state is very important in implementing unlined/shotcrete-lined pressure tunnels and shafts. The topography, local tectonic setting and geological environment greatly influence the magnitude of in situ stress level. This paper aims to evaluate in situ stress state at the Upper Tamakoshi Hydroelectric Project (UTHP), where unlined/shotcrete-lined headrace tunnel with considerable hydrostatic head is being implemented. Initially measured minimum principal stress indicated much lower values than the hydrostatic pressure at the downstream end of the headrace tunnel, which led to shift the alignment at the upper elevation with reduced hydrostatic pressure. In order to explore the reason behind much lower stress level as expected, a comprehensive assessment is carried out by developing a full rock stress model so that the minimum principal stress along the unlined pressure tunnel is evaluated. To address the complex geotectonic and topographic environment of the UTHP project area, a final rock stress model (FRSM) concept as suggested by Stephansson and Zang (2012) has been utilized. The FRSM concept considers stepwise evaluation of the in situ stress state analysis integrating the best estimate stress model (BESM), stress measurement methods (SMM) and integrated stress determination methods (ISD). The analysis carried out revealed that the in situ stress state at the project area has high degree of spatial variation even at the similar overburden due to the presence of complex topography and the presence of local shear and weakness zones. The analysis further demonstrates that a presence of local shear/weakness zone has considerable de-stressing effect, which leads to the reduction of in situ minimum principal stress magnitude. The reduction in the minimum principal stress along the pressure tunnel increases the risk for the potential hydraulic jacking and leakage if static water pressure is higher than the magnitude of minimum principal stress.

Crustal strain rates of southeastern Tibetan Plateau derived from GPS measurements and implications to lithospheric deformation of the Shan-Thai terrane

The link between the crustal deformation and mantle kinematics in the Tibetan Plateau has been well known thanks to dense GPS measurements and the relatively detailed anisotropy structure of the lithospheric mantle. However, whether the crust deforms coherently with the upper mantle in the Shan-Thai terrane (also known as the Shan-Thai block) remains unclear. In this study, we investigate the deformation patterns through strain rate tensors in the southeastern Tibetan Plateau derived from the latest GPS measurements and find that in the Shan-Thai terrane the upper crust may be coupled with the lower crust and the upper mantle. The GPS-derived strain rate tensors are in agreement with the slipping patterns and rates of major strike-slip faults in the region. The most prominent shear zone, whose shear strain rates are larger than 100×10–9 a–1, is about 1000-km-long in the west, trending northward along Sagaing fault to the Eastern Himalayan Syntaxis in the north, with maximum rate of compressive strain up to –240×10–9 a–1. A secondary shear zone along the Anninghe-Xiaojiang Fault in the east shows segmented shear zones near several conjunctions. While the strain rate along RRF is relatively low due to the low slip rate and low seismicity there, in Lijiang and Tengchong several local shear zones are present under an extensional dominated stress regime that is related to normal faulting earthquakes and volcanism, respectively. Furthermore, by comparing GPS-derived strain rate tensors with earthquake focal mechanisms, we find that 75.8% (100 out of 132) of the earthquake T-axes are consistent with the GPS-derived strain rates. Moreover, we find that the Fast Velocity Direction (FVDs) at three depths beneath the Shan-Thai terrane are consistent with extensional strain rate with gradually increasing angular differences, which are likely resulting from the basal shear forces induced by asthenospheric flow associated with the oblique subduction of the India plate beneath the Shan-Thai terrane. Therefore, in this region the upper crust deformation may be coherent with that of the lower crust and the lithospheric mantle.

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

AbstractLocalization processes in the viscous lower crust generate ductile shear zones over a broad range of scales affecting long‐term lithosphere deformation and the mechanical response of faults during the seismic cycle. Here we use centimeter‐scale numerical models in order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks. Our 2‐D Cartesian models are benchmarked to high‐temperature and high‐pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The models successfully reproduce bulk stress‐strain transients and final strain distributions observed in the experiments by applying a simple, first‐order softening law that mimics rheological weakening. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. At the tip of the propagating shear zone, weakening occurs within a process zone, which expands with time from the inclusion tips toward the matrix. Rheological weakening is a precondition for shear zone localization, and the width of this shear zone is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogous to the formation of ultramylonite bands in mylonites. These results elucidate the transient evolution of stress and strain rate during inception and maturation of ductile shear zones.

Evaluation of in-situ stress state along the shotcrete lined high-pressure headrace tunnel at a complex Himalayan geological condition

ABSTRACT The Himalayan region is renowned with its complex topography and active tectonic movement, which causes accumulation and sudden release of strain energy instigating changes in the in-situ stress state. This paper evaluates in-situ stress state at the Upper Tamakoshi Hydroelectric Project where shotcrete lined headrace tunnel with considerable hydrostatic head is being implemented. Start of the pressure tunnel at the downstream-end was proposed inside slope topography of about 300 m above valley bottom based on in-situ stresses measured by 3D overcoring at the test tunnel located at valley level. It was believed that pressure tunnel start location with a hydrostatic head of 420 m was inferred to have almost similar in-situ stress state measured at the test tunnel. However, minimum principal stresses measured by hydraulic fracturing were found insufficient to avoid hydraulic jacking. This manuscript evaluates in-situ stress state of the area and explores reasons behind much lower stresses than expected. Detailed assessment of the stress state using both measured data and 3D numerical analysis is performed, which revealed that the stress state has high degree of spatial variation even at similar overburden conditions. The analysis further demonstrates that presence of local shear zone has considerable de-stressing effect.

The dependence of planetary tectonics on mantle thermal state: applications to early Earth evolution.

For plate tectonics to operate on a planet, mantle convective forces must be capable of forming weak, localized shear zones in the lithosphere that act as plate boundaries. Otherwise, a planet's mantle will convect in a stagnant lid regime, where subduction and plate motions are absent. Thus, when and how plate tectonics initiated on the Earth is intrinsically tied to the ability of mantle convection to form plate boundaries; however, the physics behind this process are still uncertain. Most mantle convection models have employed a simple pseudoplastic model of the lithosphere, where the lithosphere 'fails' and develops a mobile lid when stresses in the lithosphere reach the prescribed yield stress. With pseudoplasticity high mantle temperatures and high rates of internal heating, conditions relevant for the early Earth, impede plate boundary formation by decreasing lithospheric stresses, and hence favour a stagnant lid for the early Earth. However, when a model for shear zone formation based on grain size reduction is used, early Earth thermal conditions do not favour a stagnant lid. While lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain size reduction, increases. Thus, the ability of convection to form weak plate boundaries is not impeded by early Earth thermal conditions. However, mantle thermal state does change the style of subduction and lithosphere mobility; high mantle temperatures lead to a more sluggish, drip-like style of subduction. This 'sluggish lid' convection may be able to explain many of the key observations of early Earth crust formation processes preserved in the geologic record. Moreover, this work highlights the importance of understanding the microphysics of plate boundary formation for assessing early Earth tectonics, as different plate boundary formation mechanisms are influenced by mantle thermal state in fundamentally different ways.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge

AbstractFault rocks can weaken dramatically with increasing slip rate, which results in localization of slip and earthquakes. Exhumed fault zones and fault rocks deformed at seismic rates in the laboratory both show that deformation can become extremely localized to zones less than or equal to millimeters thick. However, localization can occur during aseismic slip, so evidence of localization cannot necessarily be interpreted as having occurred coseismically. Dynamic weakening that occurs during earthquakes is the result of processes that are unique to seismic slip rates, and previous results from carbonates show that these processes produce unique microstructures. We evaluate whether coseismic deformation at low normal stress produces unique structures within the localized slip zones and adjacent gouge that develop in clay‐rich gouge from the Central Deforming Zone of the San Andreas fault. We measured the thickness and orientations of localized slip zones and their internal lamina, particle orientations, and particle size distributions of gouge sheared from 0.35 to 1.3 m/s velocity, up to 25‐m displacement, and 1‐MPa normal stress, under water‐wet and room‐dry conditions. We find that the thicknesses of localized slip zones and their internal laminae are consistent with numerical formulations for thermal pressurization in wet gouge and both thermal decomposition and cataclastic deformation in dry gouge. Localized zones form coincident with a zone of fluidized gouge that accommodates at most 10% of the shear strain. We conclude that the combined occurrence of foliated localized shear zones with a zone of fluidized gouge may provide a record of seismicity in clay‐rich gouges.

Intraplate Strike‐Slip Faulting, Stress Accumulation, and Shear Localization of a Crust‐Upper Mantle System With Nonlinear Viscoelastic Material

AbstractWe investigate the evolution of tectonic background stress and elastic as well as inelastic strain in a crust‐upper mantle system around an infinitely long vertical strike‐slip fault in a self‐consistent mechanical earthquake cycle model. In the early stage of the stress evolution, deformation of the crust and the upper mantle is dominated by a uniform simple shear. Shear localization in the lower crust starts when coseismic rupture extends to the entire brittle upper crust. Together with this transition, the earthquake recurrence intervals decrease by an order of magnitude due to a basal drag originated from a localized plastic flow of the lower crust. After the shear zone is fully developed in the lower crust, the fault slip rate catches up with the far‐field velocity and earthquakes starts to occur periodically. Such a steady state can be reached in several hundred thousand years from the beginning, which includes few hundreds of earthquake cycles. A shear zone with large cumulative strain needs a few million years to develop under an intraplate strike‐slip fault, which is much longer than the time for shear strain rate to be localized. The model successfully reproduced evolution of tectonic stress around an intraplate strike‐slip fault, interacting with the development of localized shear zone in the lower crust. The model demonstrates the importance of considering the whole mechanical system in which rheological structure and fault activities interacting with each other for the better understanding of the intraplate earthquakes.

On the dynamics of coupled grain size evolution and shear heating in lithospheric shear zones

The formation of weak, localized shear zones in the lithosphere via tectonic forces is a requirement for the operation of plate tectonics on Earth. However, how shear zones form is not well understood. Shear heating and grain size reduction are two commonly cited mechanisms, and both processes are likely to be active in lithospheric shear zones. However, shear heating and grain size reduction can potentially act against each other, as elevated temperatures increase grain growth rates, and fast grain growth can eliminate weak shear zones formed via grain size reduction. Here, an effectively one-dimensional simple shear model that considers shear heating and grain size evolution is used to determine how grain size reduction and shear heating interact. The results indicate that for constant velocity boundary conditions at typical tectonic strain rates and mid- to lower-lithosphere temperatures, grain size reduction suppresses shear heating by weakening the shear zone. As a result shear heating at these conditions is small (∼10 K or less), unless grain size reduction via damage is significantly less effective than the models assume. For constant stress boundary conditions, grain size reduction enhances shear heating and lowers the stress threshold required to trigger thermal runaway instabilities, as previous studies have found. Finally, combining shear heating and grain size reduction allows weak shear zones to form over a wider range of conditions than would result from either mechanism acting in isolation; coupling between shear heating and grain size reduction does not impede weak shear zone formation.

Tectonometamorphic evolution of the Lago della Vecchia metaintrusive and its country rocks, Sesia-Lanzo Zone, Western Alps

In the Sesia-Lanzo Zone different complexes are separated on the basis of rock types and dominant metamorphic imprints: one of these is the Eclogitic Micaschists Complex (EMC) that is distinguished by a dominant Alpine eclogite facies fabric. In this contribution, we present the results of a multiscale structural analysis devoted to reconstruct the tectonometamorphic evolution of the Lago della Vecchia metaintrusive and its country rocks (upper Cervo valley, Biella), which are part of the EMC. Seven groups of superposed structures have been detected. Igneous pre-Alpine M0 relicts are preserved in metaintrusive. Alpine structures consist of: S1 foliation (D1) developed under eclogite facies conditions; S2 foliation (D2) is the most pervasive fabric and developed under low-pressure blueschist facies conditions; localised shear zones and open folds (D3 and D4) are associated with the development of greenschist facies assemblages; a kilometre-long shear zones (D5) developed under greenschist facies conditions. Finally, andesitic dikes intrusion postdates D1-D5 structures. On the basis of deformation-metamorphism relationships geothermobarometric estimates were performed to constrain the PT conditions characterising subsequent deformation stages. The resulting Pressure-Temperature-relative time of deformation (P-Td-t) path shows a pre-Alpine magmatic emplacement at around 15 km depth at T=710±19°C that is followed by re-equilibration indicating the conditions of the Alpine eclogitic peak (D1: 2.23±0.18 GPa; 537±43°C), compatible with a cold-subduction environment. A near-isothermal exhumation postdates D1, down to 1.12±0.11 GPa at 477±39°C during D2 deformation stage. Afterwards, temperature and pressure decreased from D2 low-pressure blueschist to D3-D4 greenschist re-equilibrations (P=0.6±0.10 GPa and T=302±24°C). In contrast with the most part of the EMC, where the dominant fabric developed under eclogitic facies conditions, at Lago della Vecchia the dominant fabric developed under low-pressure blueschist facies conditions. This difference suggests that the entire EMC should be characterised by kilometre-scale structural heterogeneities in the dominant tectonometamorphic record.

Earthquake-induced transformation of the lower crust.

The structural and metamorphic evolution of the lower crust has first order effects on the lithospheric response to plate tectonic processes involved in orogeny, including subsidence of sedimentary basins, stability of deep mountain roots, and extension of high topography regions. Recent research shows that prior to orogeny most of the lower crust is dry, impermeable, and mechanically strong1. During an orogenic event, the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones. In the Bergen Arcs of Western Norway, shear zones initiate as faults generated by lower crustal earthquakes. Seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms. However, regular earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust2. Here, we show that the volume of lower crust affected by such aftershocks is very significant and that fluids driving associated metamorphic and structural transformations of the lower crust follow in the wake of these earthquakes. This provides a novel ‘top-down’ effect on crustal geodynamics and connects processes operating at very different time scales.

Three‐Dimensional Failure Patterns Around an Inflating Magmatic Chamber

AbstractBedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3‐D numerical models of elasto‐plastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized or diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber's crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest how natural observations may be used to constrain the mode of failure occurring underneath a volcano. We discuss several natural examples in this context.

Influence of pre-tectonic carbonate facies architecture on deformation patterns of syntectonic turbidites, an example from the central Mexican fold-thrust belt

The Mexican fold-thrust belt in central México excellently exposes relatively well preserved syntectonic deposits that overlay rocks with lateral lithostratigraphic changes across the belt. We consider the deformational effects of these changes by investigating the geometry, kinematics and strain distribution within syntectonic turbidites, which are deposited on top of Albian-Cenomanian shallow and deep water carbonate layers. Field observations and detailed structural analysis at different stratigraphic and structural levels of the Late Cretaceous syntectonic formation are compared with the deformation as a function of lithological and structural variations in the underlying carbonate units, to better understand the effect of these lithostratigraphic variations on deformation, kinematics, strain distribution and propagation of deformation. From our kinematic analyses, we conclude that the syntectonic strata are pervasively affected by folding in all areas and that deformation partitioning localized shear zones at the boundaries of this unit, particularly along the contact with massive carbonates. At the boundaries with massive platformal carbonates, the turbidites are strongly deformed by isoclinal folding with a pervasive sub-horizontal axial plane cleavage and 70-60% shortening. In contrast, contacts with thinly-bedded carbonate layers (basinal facies), do not show strain localization, and have horizontal shortening of 50–40% that is accommodated by buckle folds with a less pervasive, steeply dipping cleavage. The mechanical properties variations in the underlying pre-tectonic units as a function of changes in lithostratigraphy fundamentally control the deformation in the overlying syntectonic strata, which is an effect that could be expected to occur in any deformed sedimentary sequence with such variations.

Temperature dependent crack initiation of 42CrMo4 steel at high loading rates

Dynamic crack initiation with crack tip loading rates K˙ of approximately 2 ‧ 106 MPa√ms− in high-strength 42CrMo4 steel was investigated. To this end, a recently developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and precracked Charpy specimens were tested. The tests were performed at temperatures of –40 °C and 20 °C. The loading of the specimen was determined by analyzing the strain in the incident and transmission bars. Furthermore, strain gauges at the specimen’s surface were applied to measure the crack tip loading. High-speed photography complemented the analysis of the specimens loading and the detection of the crack initiation. Fracture surface analysis by means of scanning electron microscopy enabled the measurement of the fracture surface topography and, consequently, stretch zone height and width. Hence, the macroscopically measured dynamic crack initiation toughness was correlated with the toughness at microscopic scale. It was observed that the resistance against dynamic crack initiation decreased with decreasing temperature. Microscopically, a decrease in toughness was analogously observed. Non-metallic inclusions resulted in crack path deflection with localized shear zones. After a small stable crack extension, cleavage fracture was observed.

Microstructures in landslides in northwest China – Implications for creeping displacements?

Microstructures, mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the Gansu Province (China) were examined with optical microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD) and synchrotron x-ray diffraction measurements. Common sheet silicates are chlorite, illite, muscovite, kaolinite, pyrophyllite and dickite. Other minerals are quartz, calcite, dolomite and albite. In one sample, graphite and amorphous carbon were detected by TEM-EDX analyses and TEM high-angle annular dark-field images. The occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the Suoertou and Xieliupo landslides is particularly significant for reducing the frictional strength of the landslides. It is proposed that the landslides underwent comparable deformation processes as fault zones. The low friction coefficients provide strong evidence that slow-moving landsliding is controlled by the presence of weak minerals. In addition, TEM observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion. For calcite and quartz, grain size reduction was attributed to both pressure solution and cataclasis. Therefore, besides landslide composition, the occurrence of ultrafine-grained slip zone material may also contribute to weakening processes of landslides. TEM images of slip-zone samples show both locally aligned clay particles, as well as kinked and folded sheet silicates, which are widely disseminated in the whole matrix. Small, newly formed clay particles have random orientations. Based on synchrotron x-ray diffraction measurements, the degree of preferred orientation of constituent sheet silicates in local shear zones of the Suoertou and Duang-He-Ba landslide is strong. This work is the first reported observation of well-oriented clay fabrics in landslides.

Deformation of mantle pyroxenites provides clues to geodynamic processes in subduction zones: Case study of the Cabo Ortegal Complex, Spain

In the Herbeira massif, Cabo Ortegal Complex, Spain, a well exposed assemblage of deformed dunites and pyroxenites offers a unique opportunity to investigate key upper mantle tectonic processes. Four types of pyroxenites are recognized: clinopyroxenites with enclosed dunitic lenses (type-1), massive websterites (type-2), foliated and commonly highly amphibolitized clinopyroxenites (type-3) and orthopyroxenites (type-4). Field and petrological observations together with EBSD analysis provide new insights on the physical behavior of the pyroxenes and their conditions of deformation and reveal the unexpected journey of the Cabo Ortegal pyroxenites.We show that, during deformation, type-1 pyroxenites, due to their enclosed dunitic lenses, are more likely to localize the deformation than types-2 and -4 pyroxenites and may latter act as preferred pathway for fluid/melt percolation, eventually resulting in type-3 pyroxenites. All pyroxenite types display a similar response to deformation. Orthopyroxene deformed mostly by dislocation creep; it shows kink bands and undulose extinction and its fabric is dominated by [001](100). Clinopyroxene displays subgrain rotation, dynamic recrystallization and fabric with [010] axes clustering next to the foliation pole and [001] axes clustering next to the lineation suggesting activation of [001]{110} and [001](100) in some samples. These observations are in good agreement with deformation at temperatures greater than 1000 °C. Olivine in type-1 and type-4 pyroxenites shows [100](010) or [001](010) fabrics that are consistent with deformation at temperatures >1000 °C and may indicate deformation in a hydrous environment. The amphibole [001](100) fabric gives insights on a lower-temperature deformation episode (∼800 to 500 °C). Our results, interpreted in the light of published experimental data, together with the regional geological and geochemical studies are consistent with the following tectonic evolution of the Cabo Ortegal pyroxenites: (1) delamination from an arc root in a mantle-wedge setting at temperatures above 1000 °C and (2) introduction into a relatively softer subduction channel where deformation was accommodated by localized shear zones, thus preserving the high-temperature fabrics of pyroxenites. The Cabo Ortegal pyroxenites may therefore be seen as a rare exposure of deformed mantle-wedge material.

Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

Chromitite from the Harold’s Grave locality in the mantle section of the Shetland ophiolite complex is extremely enriched in Ru, Os and Ir, at µg/g concentrations. High-resolution X-ray computed tomography on micro-cores from these chromitites was used to determine the location, size, distribution and morphology of the platinum-group minerals (PGM). There are five generations of PGM in these chromitites. Small (average 5µm in equivalent sphere diameter, ESD) euhedral laurites, often with Os-Ir alloys, are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised. Also within the chromitite there are clusters of larger (50µm ESD) aligned elongate crystals of Pt-, Rh-, Ir-, Os- and Ru-bearing PGM that have different orientations in different chromite crystals. These may have formed either by exsolution, or by preferential nucleation of PGMs in boundary layers around particular growing chromite grains. Thirdly there is a generation of large (100µm ESD) composite Os-Ir-Ru-rich PGM that are all interstitial to the chromite grains and sometimes form in clusters. It is proposed that Os, Ir and Ru in this generation were concentrated in base metal sulfide droplets that were then re-dissolved into a later sulfide-undersaturated magma, leaving PGM interstitial to the chromite grains. Fourthly there is a group of almost spherical large (80µm ESD) laurites, hosting minor Os-Ir-Ru-rich PGM that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer. These may be hosted in an annealed late syn- or post magmatic fracture. Finally a few of the PGM have been deformed in localised shear zones through the chromitites.The vast majority of the PGM – including small PGM enclosed within chromite, larger interstitial PGM and elongate aligned PGM – have Os isotope compositions that give Re-depletion model ages approximately equal to the age of the ophiolite at ∼492Ma. A number of other PGM – not confined to a single textural group – fall to more or less radiogenic values, with four PGM giving anomalously unradiogenic Os corresponding to an older age of ∼1050Ma. The 187Os/188Os isotopic ratios for PGM from Cliff and Quoys, from the same ophiolite section, are somewhat more radiogenic than those at Harold’s Grave. This may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal Os.

Neoarchean arc magmatism and subsequent collisional orogenesis along the eastern Rae domain, western Churchill Province: Implications for the early growth of Laurentia

The Athabasca granulite terrane, on the eastern margin of the Rae domain of the western Churchill Province, contains a well exposed record of Neoarchean magmatism, deformation, and metamorphism. Circa 2.63–2.60Ga plutonic rocks range from 58.01 to 73.71 SiO2, and vary from ferroan to magnesian. They form shallow linear trends on modified alkali-lime index diagram, plotting largely in the calc-alkaline field. On tectonic discrimination diagrams, all samples plot in the volcanic arc field. Samples are LREE enriched, and contain relatively steep negative slopes, and flat, depleted, HREE profiles when normalized to chondrites. The entire Neoarchean plutonic complex, independent of lithology, contains strong enrichments in the LILEs and Pb, and depletions in HFSEs. Monazite U-Th-total Pb results from a migmatitic paragneiss suggest that partial melting occurred at ca. 2.57Ga within the Chipman subdomain. Monazite results from the adjacent northwestern subdomain suggest that deformation occurred at ca. 2.57Ga. IN-SIMS and ICP-MS zircon U-Pb geochronology corroborates these conclusions. Although the region is dominated by Paleoproterozoic localized shear zones, internal to each subdomain is evidence of a widespread tectonothermal event during the Neoarchean that involved arc plutonism, crustal thickening, lower crustal flow at pressures exceeding 1.0GPa, and extensive partial melting of the lower continental crust. We interpret this sequence of magmatic, deformational, and anatectic processes to reflect the transition from a continental arc to a collisional tectonic setting. The recognized series of events presented herein is likely correlative to the 2.60–2.50Ga MacQuoid Orogen, which has been recognized further to the northeast, and likely parallels the eastern margin of the Rae domain of the western Churchill Province. These data support an important component of modern style plate tectonics during continental magmatism and accretion in the Neoarchean.

The Paleoproterozoic Copper-Gold Deposits of the Gaoua District, Burkina Faso: Superposition of Orogenic Gold on a Porphyry Copper Occurrence?

The Gaoua mining district, underlain by Paleoproterozoic volcanic and intrusive rocks of the Boromo-Goren greenstone belt in southern Burkina Faso, is a rare case where economic copper and gold resources occur together in the Birimian terrane of West Africa. The main country rock lithologies consist of flows and volcaniclastic sediments of basaltic to andesitic compositions. These are intruded by subvolcanic diorite and quartz diorite stocks and dikes, granophyre dikes and, less commonly, gabbroic dikes. The stocks are porphyritic in texture, locally brecciated, and characterized by development of a carbonate, anhydrite, and quartz stockwork system. Structural observations, detailed petrography, and sulfide geochemistry reveal that copper and gold in the stockworks and surrounding altered rocks cannot be associated with the same mineralizing event. Copper mineralization, mainly in the form of chalcopyrite, is typically observed in hydrothermal breccia and is associated with early arsenic-poor and gold-free pyrite. The geochemistry of the plutonic rocks and associated basalts and andesites implies formation in an arc setting. Textural evidence indicates that gold mineralization is closely tied to a second generation of sulfides, with the later pyrite hosting visible gold and minor chalcopyrite that clearly postdates the copper ore. Analyses of these pyrites by LA-ICP-MS reveal compositions rich in Ag, As, Au, Bi, Cu, and Pb. Pyrite with this signature was only observed in local shear and fracture zones that crosscut the host rocks and copper-rich breccia. Therefore, our data indicate the strongly deformed and metamorphosed porphyry copper mineralization in the Gaoua district is overprinted by orogenic gold mineralization. Formation of the early porphyry copper deposit took place at 2165 ± 24 Ma based upon Re-Os dating of the early pyrite; the age of the younger gold event, which is similar in style to that described throughout the West African craton, is not known but may be related to one of the lode gold episodes that are widespread through the West African craton at ~2150 and ~2100 to 2040 Ma. The Gaoua copper deposit, one of very few Precambrian porphyry copper deposits known in West Africa, formed in a volcanic-arc setting that predated the Eburnean orogeny in the Boromo-Goren greenstone belt.

Polymetamorphism of the Chupa Sequence of the Belomorian mobile belt (Fennoscandia): Evidence from the isotope-geochemical (U-Pb, REE, O) study of zircon

U-Pb age and isotope-geochemical features were determined for zircon from kyanite gneisses and amphibolites of the Chupa Sequence of the Belomorian mobile belt (BMB) of the Fennoscandian shield. The cores of the zircon from the gneisses marks the Neoarchean events within 2700–2800 Ma known in the BMB, while those of the amphibolites correspond to the age of magmatic crystallization (2775 ± 12 Ma). The inner rims of zircon from the amphibolites and gneisses likely record two different Neoarchean metamorphic events (2650 ± 8 and 2599 ± 10 Ma, respectively). The outer rims record Paleoproterozoic metamorphism with an age of 1890 Ma, which formed the modern appearance and mineral assemblages of the rock association. The value of δ18O in the zircon from the gneiss is 8.6‰ in cores, slightly decreases to 8.0‰ in inner rims, and sharply decreases to 3.9‰ in outer rims. The value of δ18O in the zircon from the amphibolite is around 6.2‰ in cores, increases up to 8.6 in inner rims, and decreases to 5.2‰ in outer rims. A significant decrease of δ18O is likely related to the anomalous composition of Svecofennian metamorphic fluid restricted to local shear zones. The geochemical features of the zircons in combination with their morphology and anatomy make it possible to distinguish zircon generations of different age and change in metamorphic environments.

Instantaneous healing of micro-fractures during coseismic slip: Evidence from microstructure and Ti in quartz geochemistry within an exhumed pseudotachylyte-bearing fault in tonalite

Exhumed faults within the tonalitic Adamello pluton (Southern Alps) were seismic at depth as indicated by the presence of pseudotachylytes (solidified friction-induced melts). During cooling of tonalite, early-formed joints were first exploited by localized ductile shear zones associated with deposition of quartz veins (at ~500°C), and later by pseudotachylyte-bearing cataclastic faults (at ~250–300°C ambient temperature). Adjacent to pseudotachylytes, quartz of the host tonalite shows pervasive thin (1–10μm wide) healed micro-fractures and ultra-fine (1–2μm grain size) recrystallized aggregates along micro-shear zones. Under cathodoluminescence (CL) the healed micro-fractures have a darker gray shade than the host “magmatic” quartz that reflects a change in Ti concentrations ([Ti]) as indicated by NanoSIMS measurements. [Ti] vary from 35–55ppm in the CL-lighter host quartz to 10–13ppm along the CL-darker healed micro-fractures. These [Ti] were inherited by the ultra-fine recrystallized aggregates that overprinted both the magmatic quartz and the healed micro-fractures during the high temperature transient related to frictional seismic slip. Based on Ti-in-quartz thermometry, we infer that micro-fracture healing occurred at higher temperatures than the ambient temperatures of faulting (250–300°C at 0.2GPa), for which [Ti] <1ppm would be expected. Micro-fracture healing can be ascribed to the stage of seismic slip of faults on the basis of the observation that: (i) they are absent in the host rock surrounding high-T quartz veins un-exploited by faults; and (ii) they locally occur at the tip of pseudotachylyte injection veins filling new fractures developed during the propagation of the earthquake rupture. The relatively high [Ti] of micro-fractures are therefore interpreted to reflect quartz healing by a fluid overheated during the initial stages of frictional seismic slip and escaping from fault surface through the damage zone. This suggests that, in the presence of fluids, thermal pressurization of the fault did not occur and did not prevent frictional melting. The small-scale microstructures and geochemistry of quartz in the wall of the studied paleo-seismic fault record a complex deformational history, referable to the short-lived (on the order of 104s) thermal anomaly induced by frictional seismic slip, that includes both micro-fracture healing and recrystallization. This microstructural assemblage of the natural exhumed fault provides a key for understanding the mechanics of an earthquake source.