Analytical model for the prediction of building deflections induced by ground movements

The objective of this study is to develop an analytical model that can predict the building‐relevant deflections induced by tunnelling or mining subsidence. The model takes into account soil–structure interactions due to differences in stiffness between the ground and the building. The ground is modelled by the Winkler model with an initial ground curvature equivalent to the free‐field ground movements. The building is modelled by a horizontal beam with uniform loading. The static and cinematic equilibrium of both the ground and the building are then calculated to assess the final building and ground shape, and the building deflection is derived. The resulting analytical model is used to investigate the influence of the ground and the building's mechanical properties, the building load and the initial value of the free‐field ground curvature (hogging or sagging). The model appears to be more comprehensive than those reported elsewhere that address the problem with numerical models. In particular, the analytical model makes it possible to distinguish two different final situations—with continuous or discontinuous contact between the ground and the building. The model is compared with numerical results and used to analyse a case study. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of frictional forces acting on sidewalls of buried box culverts

The effects of frictional forces acting on the sidewalls of buried box culverts are presented as determined with finite element method (FEM) and detailed soil modelling. The possibility of reducing earth pressure on deeply buried concrete box culverts by the imperfect trench installation (ITI) method has been contemplated during the last several decades. There have been limited research results published primarily regarding the qualitative aspect of load reduction in ITIs. It was found during the course of this study that significant frictional forces develop along the sidewalls of box culverts and adjacent sidefills in ITIs. Current American Association of State Highway and Transportation Officials provisions do not consider these frictional forces, but they cannot be neglected in ITIs, as their effect is dominant. An optimum geometry for the soft zone in ITIs is presented to maximize earth load reductions. The soil–structure interaction at the box culvert–soil interface was found to have a significant effect on total earth pressure acting on the bottom slab. Predictor equations for earth load reduction rates were formulated for ITIs incorporating the optimum soft zone geometry based on the FEM. Copyright © 2007 John Wiley & Sons, Ltd.     

Trenching studies of active faults in Kamchatka, eastern Russia: Palaeoseismic, tectonic and hazard implications

The central part of the Kamchatka Peninsula is characterized by a well defined depression associated with active volcanism, aligned NE–SW. On the east, the depression is bounded by a prominent system of active faults known as the East Kamchatka Fault Zone (EKFZ). In order to improve understanding of the behaviour and kinematic role of this fault zone a fieldwork programme, including study of trenches, was conducted in the north-central part of this system. Aerial photograph analysis, ground-truthed, indicates a westward fault dip with predominantly normal slip, while lateral offsets of river terraces and stream channels demonstrate a combined dextral component. Over 20 excavated pits and natural exposures were examined to confirm a detailed tephra succession extending from the early Holocene to recent historic eruptions. This chronological framework then provided age control on five past faulting events recognised in three trenches. These events took place at about 10.5, 6.0, 4.5 and, in a two-event succession within a short time span, at 3.3–3.2 ka BP. Event clustering may be characteristic and fault length–displacement values suggest earthquakes of M6.5, thus representing a significant new element in regional seismic hazard evaluations; additional to events generated at the subduction interface. The relatively long gap in faulting since the two most recent events may also be significant for hazard scenarios and there is a possible link between the faulting and volcanic activity in the depression. Overall, the EKFZ, together with the Nachiki Transverse Zone farther south, is thought to define a regional-scale block that is extending eastwards independently from the rest of Kamchatka.     

Elastoplastic medium for foundation settlements and monotonic soil–structure interaction under combined loadings

Foundation settlements and soil–structure interaction are important problems to structural and geotechnical engineers. This study introduces a novel elastoplastic three‐degree‐of‐freedom medium which models foundations settlements under combined loadings. A soil–structure interaction problem can then be solved by replacing the soil mass with this three‐degree‐of‐freedom elastoplastic medium, thus reducing significantly the size of the problem. The model was developed by extending the classical plasticity concepts to the force‐deformation level. Its ability to predict foundation deformations was evaluated using finite element solutions of a typical shallow foundation problem and was found reasonably accurate while producing significant time savings. Copyright © 2006 John Wiley & Sons, Ltd.     

The field and microstructural signatures of deformation‐assisted melt transfer: Insights from magmatic arc lower crust,New Zealand

Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as <10° and up to <60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions.     

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral,NW Pakistan): Insights from phase equilibria modelling,and garnet–monazite petrochronology

The Karakoram–Hindu Kush–Pamir and adjacent Tibetan plateau belt comprise a series of Gondwana‐derived crustal fragments that successively accreted to the Eurasian margin in the Mesozoic as the result of the progressive Tethys ocean closure. These domains provide unique insights into the thermal and structural history of the Mesozoic to Cenozoic Eurasian plate margin, which are critical to inform the initial boundary conditions (e.g. crustal thickness, structure and thermo‐mechanical properties) for the subsequent development of the large and hot Tibetan–Himalaya orogen, and the associated crustal deformation processes. Using a combination of microstructural analyses, thermobarometry modelling and U–Th–Pb monazite and Lu–Hf garnet geochronology, the study reappraises the metamorphic history of exposed mid‐crustal metapelites in the Chitral region of the South Pamir–Hindu Kush (NW Pakistan). This study also demonstrates that trace elements in monazite (especially Y and Dy), combined with thermodynamical modelling and Lu–Hf garnet dating, provides a powerful integrated toolbox for constraining long‐lived and polyphased tectono‐metamorphic histories in all their spatial and temporal complexity. Rocks from the Chitral region were progressively deformed and metamorphosed at sub‐ and supra‐solidus conditions through at least four distinct episodes from the Mesozoic to the Cenozoic. Rocks were first metamorphosed at ~400–500°C and ~0.3 GPa in the Late Triassic–Early Jurassic (210–185 Ma), likely in response to the accretion of the Karakoram during the Cimmerian orogeny. Pressure and temperature subsequently increased by ~0.3 GPa and 100°C in the Early‐ to Mid Cretaceous (140–80 Ma), coinciding with the intrusion of calcalkaline granitic plutons across the Karakoram and Pamir regions. This event is interpreted as the record of crustal thickening and the development of a proto‐plateau within the Eurasian margin due to a long‐lived episode of slab flattening in an Andean‐type margin. Peak metamorphism was reached in the Late Eocene–Early Oligocene (40–30 Ma) at conditions of 580–600°C and ~0.6 GPa and 700–750°C and 0.7–0.8 GPa for the investigated staurolite schists and sillimanite migmatites respectively. This crustal heating up to moderate anatexis likely resulted in the underthrusting of the Indian plate after a NeoTethyan slab‐break off or to the Tethyan Himalaya–Lhasa microcontinent collision and subsequent oceanic slab flattening. Near‐isothermal decompression/exhumation followed in the Late Oligocene (28–23 Ma) as marked by a pressure decrease in excess of ~0.1 GPa. This event was coeval with the intrusion of the 24 Ma Garam Chasma leucogranite. This rapid exhumation is interpreted to be related to the reactivation of the South Pamir–Karakoram suture zone during the ongoing collision with India. The findings of this study confirm that significant crustal shortening and thickening of the south Eurasian margin occurred during the Mesozoic in an accretionary‐type tectonic setting through successive episodes of terrane accretions and probably slab flattening, transiently increasing the coupling at the plate interface. Moreover, they indicate that the south Eurasian margin was already hot and thickened prior to Cenozoic collision with India, which has important implications for orogen‐scale strain‐accommodation mechanisms.     

Geochemistry and stable isotopic composition of tufa waters and precipitates from the Interlake Region, Manitoba, Canada: Constraints on groundwater origin, calcitization, and tufa formation

New major, trace and isotopic geochemical results from a regional study of springs discharging from the major carbonate rock aquifer in the Interlake Region of Manitoba, Canada, are used to understand water–rock reactions, timing of recharge/discharge, tufa formation processes, and as baseline data. Spring waters are fresh with total dissolved solids (TDS) concentrations ranging from 150 to 880 mg/L. Waters discharging in the northern part of the study area have lower TDS, are dominantly Ca–Mg–HCO₃ waters with low SO₄ concentrations (<< 50 mg/L), and appear to have interacted primarily with Silurian carbonate lithologies. In contrast, waters in the southeastern part of the study area have higher TDS and have elevated SO₄ concentrations (up to 210 mg/L). Spring waters have elevated Mg/Ca_molar (1.23 ± 0.23), typically greater than congruent dissolution of dolomite. Ca and Mg concentrations and Mg/Ca_molar indicate that groundwater residence times were sufficient to allow equilibration with bedrock dolomite lithologies; elevated tritium in northern waters indicates a significant recharge component in the 1960's and 1970's. Tufa precipitates that have formed from many of the spring waters are low-Mg calcite (MgO = 1.70 to 5.80 wt.%). Sr concentrations are variable (57 to 657 ppm) and tufa Sr/Ca_molar ratios appear to be entirely controlled by spring water Sr/Ca_molar. Empirically determined Sr distribution coefficients (D_Sr = 0.389 ± 0.083) indicate rapid crystallization following CO₂ degassing, consistent with heavier δ¹³C_VPDB compared to spring waters. Sulfate concentrations are generally too low for calcitization (dedolomitization) reactions driven by anhydrite dissolution to be the dominant control on the elevated groundwater Mg/Ca_molar, implying either extensive sulfate reduction along the flow paths (however, δ¹³C_DIC suggests the elevated SO₄ is more consistent with Fe-sulfide oxidation), or that other processes are involved. Major ion ratios suggest that the waters in the southern part of the study area are more consistent with interaction with siliciclastic rocks than with anhydrite dissolution. We suggest that calcitization (dedolomitization) reactions driven by anhydrite dissolution may not dominate all carbonate aquifers and that mixing of waters in karst conduits combined with ion exchange reactions are important controls on water chemistry in these systems.     

Behavior of the fresh- and saline-water phases in an urban area in Western Buenos Aires Province, Argentina

Given the particular hydrogeological setting at Salliqueló County, Western Buenos Aires Province, Argentina, this paper places attention on the lens that supplies fresh water to its capital, Salliqueló. This lens constitutes the most important fresh-water reservoir in the county. It overlies saline waters, and is recharged through high-permeability sandy sediments at a rate that exceeds by far the actual water demand. Restrictions in exploitation are not limited by the total requirement but rather by the flows extracted from individual wells that may generate up-coning of saline waters, with a resulting overall deterioration in quality of the pumped water. The variations in the position of the interface between saline and fresh waters, as related to the change in extraction rates, were obtained by means of a simulation of the flow of two fluids of different densities. Finally, based on the general hydrogeological characterization and on the simulation results, various methods for the management of these fresh-water resources are proposed, such as the deactivation of urban supply wells, the monitoring of the saline interface position, and the establishment of a protection perimeter for the water-supply wells.

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Resumen Enmarcado dentro de las características hidrogeológicas generales del partido de Salliqueló, oeste de la provincia de Buenos Aires, Argentina, este trabajo se centra en la lente que abastece de agua dulce a Salliqueló, la capital del partido. Esta lente constituye la reserva de agua dulce más importante del partido, se ubica sobre aguas salinas y es recargada a través de sedimentos arenosos de alta permeabilidad a una tasa que excede grandemente los requerimientos de la población. Sin embargo, las restricciones en la explotación no están dadas por las demandas totales sino por las extracciones en perforaciones individuales, que pueden generar ascensos de agua salina con el consiguiente deterioro en la calidad del agua extraída. Las variaciones de la posición de la interfase respecto de los cambios en la explotación fueron obtenidas por medio de la simulación del flujo de dos fluidos de diferente densidad. Finalmente, con base en la caracterización hidrogeológica general y en los resultados de la simulación, se proponen varias medidas para la protección del recurso hídrico, tales como: la desactivación de los pozos de abastecimiento en el ámbito urbano, el control de la posición de la interfase salina y la definición de perímetros de protección de los pozos de abastecimiento, entre otras.

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Résumé Etant donné les particularités hydrogéologiques de la région de Salliqueló, ouest de la province de Buenos Aires, Argentine, cet article traite de la lentille deau douce alimentant en eau potable la capitale, Salliqueló. Cette lentille constitue la réserve la plus importante deau douce de la région. Elle se superpose aux eaux salées et est rechargée via les sédiments sableux et perméables, à un taux dépassant largement la demande actuelle en eau. Les restrictions dans lexploitation ne sont pas limitées par la demande totale mais par les débits extraits des forages privés, qui pourraient générer des rabattements induisant des arrivées deaux salées, arrivées qui peuvent dégrader la ressource pompée. Les variations de la position de linterface eau douce–eau salée, en relation avec les taux dexploitation, ont été obtenues en simulant lécoulement de deux fluides de différentes densités. Finalement, sur base dune caractérisation hydrogéologique et des résultats de la simulation, des méthodes variées pour la gestion de la ressource en eau douce sont proposées, entre autre, la désactivation de pompages urbains, suivi de linterface eau douce–eau salée, et la mise en place de zones de protections des captages deau souterraine.

     

Metasomatism of garnet peridotite from Jiangzhuang,southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh‐pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse‐grained garnet porphyroblasts and a fine‐grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti‐clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO₂ and higher Cr₂O₃ and REE contents. Diopside displays LREE enrichment, and has low but variable large‐ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti‐clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al₂O₃, CaO and TiO₂, and enriched in SiO₂, K₂O, REE and LILE. In contrast to phlogopite‐free peridotites, the phlogopite‐bearing peridotites have higher K₂O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite‐bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite‐free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite‐bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile‐bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.     

Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province,Russia

Although eclogites in the Belomorian Province have been regarded as Archean in age and among the oldest in the world, there are also multiple studies that have proposed a Paleoproterozoic age. Here, we present new data for the Gridino‐type eclogites, which occur as boudins and metamorphosed dykes within tonalite–trondhjemite–granodiorite gneisses. Zircon from these eclogites has core and rim structures. The cores display high Th/U ratios (0.18–0.45), negative Eu anomalies and strong enrichment in HREE, and have Neoarchean U–Pb ages of c. 2.70 Ga; they are interpreted to be magmatic in origin. Zircon cores have δ¹⁸O of 5.64–6.07‰ suggesting the possibility of crystallization from evolved mantle‐derived magmas. In contrast, the rims, which include the eclogite facies minerals omphacite and garnet, are characterized by low Th/U ratios (<0.035) and flat HREE patterns, and yield U–Pb ages of c. 1.90 Ga; they are interpreted to be metamorphic in origin. Zircon rims have elevated δ¹⁸O of 6.23–6.80‰, which was acquired during eclogite facies metamorphism. Based on petrography and phase equilibria modelling, we recognize a prograde epidote amphibolite facies mineral assemblage, the peak eclogite facies mineral assemblage and a retrograde high‐P amphibolite facies mineral assemblage. The peak metamorphic conditions of 695–755°C at >18 kbar for the Gridino‐type eclogites suggest an apparent thermal gradient of <39–42°C/kbar for the Lapland–Kola collisional orogeny.     

Subsurface thermal and hydrothermal characterization based on geothermal resources map, drill hole thermal gradients, Curie point depths and hypocenter distribution — Examples of Tohoku district, Japan

Subsurface thermal structure in Tohoku district are characterized by existing data such as geothermal resources maps, drill hole thermal gradients, Curie point depths and hypocenters distribution maps. The collected data are registered in a database system, then, compared in plan view, cross-section and bird's-eye pictures. The comparison indicates that subsurface temperatures extrapolated from drill hole thermal gradients are generally concordant to the Curie point depth, assumed to be 650 °C. Tohoku district is generally divided into 5 type areas; fore arc lowland, fore arc mountain country, Quaternary volcanic terrain, back arc lowland and back arc mountain country. The surface thermal manifestations in Quaternary volcanic terrain are mainly controlled by the magma chambers as heat sources, while, surface thermal features such as hot springs in non-volcanic areas are controlled by degrees of heat flows, and hydrothermal flows in permeable Cenozoic formations and along permeable fault zones.     

Cyclic macro‐element for soil–structure interaction: material and geometrical non‐linearities

This paper presents a non‐linear soil–structure interaction (SSI) macro‐element for shallow foundation on cohesive soil. The element describes the behaviour in the near field of the foundation under cyclic loading, reproducing the material non‐linearities of the soil under the foundation (yielding) as well as the geometrical non‐linearities (uplift) at the soil–structure interface. The overall behaviour in the soil and at the interface is reduced to its action on the foundation. The macro‐element consists of a non‐linear joint element, expressed in generalised variables, i.e. in forces applied to the foundation and in the corresponding displacements. Failure is described by the interaction diagram of the ultimate bearing capacity of the foundation under combined loads. Mechanisms of yielding and uplift are modelled through a global, coupled plasticity–uplift model. The cyclic model is dedicated to modelling the dynamic response of structures subjected to seismic action. Thus, it is especially suited to combined loading developed during this kind of motion. Comparisons of cyclic results obtained from the macro‐element and from a FE modelization are shown in order to demonstrate the relevance of the proposed model and its predictive ability. Copyright © 2001 John Wiley & Sons, Ltd.     

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks,central Tibet

The P–T–t path of high‐P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post‐P_max heating–decompression and subsequent isothermal decompression following the attainment of peak high‐P and low‐T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.     

Slow Senonian and fast Palaeocene–Early Eocene uplift of the granitoids in the Central Eastern Pontides, Turkey: apatite fission-track results

Dating and forward modelling of the fission-track data of apatite samples from the Dereli– ebinkarahisar region, south of Giresun in the Eastern Turkish Pontides, provides quantitative data on the regional tectonics resulting from the closure of neo-Thetys and the collision of Eurasia and Gondwana. The age vs. elevation profiles identified Senonian (80.7±3.2 to 62.4±2.5 Ma) slow uplift and denudation, interpreted as the result of the diapiric ascent of subduction-related plutons above the neo-Tethyan subduction zone beneath the Eurasian continent. This was followed by rapid differential uplift during the Palaeocene–Early Eocene (57.4±2.4 to 47.8±2.4 Ma), which juxtaposed granitoid units of different ages, compositions, and emplacement levels in the crust, and is thought to be related to the collision between the Pontide (Eurasian) and Anatolide (Gondwana) basements. The modelling results must be interpreted with caution, but appear to indicate a period of Mio-Pliocene (ca. 5 Ma) reheating related to volcanism associated with the westward escape of the Anatolian plate and uplift from the Pliocene (ca. 3.5 Ma) up to the present.     

Metamorphic evolution,partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist,southern California

The San Emigdio and related Pelona, Orocopia, Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent southern California batholith along a shallow segment of the subducting Farallon plate in Late Cretaceous to early Tertiary time. These subduction accretion assemblages represent a regional, deeply exhumed, shallowly dipping domain from an ancient slab segmentation system and record the complete life cycle of the segmentation process from initial flattening and compression to final extensional collapse. An important unresolved question regarding shallow subduction zones concerns how the thermal structure evolves during the slab flattening process. New field relationships, thermobarometry, thermodynamic modelling and garnet diffusion modelling are presented that speak to this issue and elucidate the tectonics of underplating and exhumation of the San Emigdio Schist. We document an upsection increase in peak temperature (i.e. inverted metamorphism), from 590 to 700 °C, peak pressures ranging from 8.5 to 11.1 kbar, limited partial melting, microstructural evidence for large seismic events, rapid cooling (825–380 °C Myr^?1) from peak conditions and an ‘out and back’P–T path. While inverted metamorphism is a characteristic feature of southern California schists, the presence of partial melt and high temperatures (>650 °C) are restricted to exposures with maximum depositional ages between 80 and 90 Ma. Progressive cooling and tectonic underplating beneath an initially hot upper plate following the onset of shallow subduction provide a working hypothesis explaining high temperatures and partial melting in San Emigdio and Sierra de Salinas schists, inverted metamorphism in the schist as a whole, and the observed P–T trajectory calculated from the San Emigdio body. Lower temperatures in Pelona, Orocopia and Rand schists are likewise explained in the context of this overarching model. These results are consistent with an inferred tectonic evolution from shallow subduction beneath the then recently active Late Cretaceous arc to exhumation by rapid trench‐directed channelized extrusion in the subducted schist.     

Crustal structure of the Lofoten–Vesterålen continental margin, off Norway

By compiling wide-angle seismic velocity profiles along the 400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, 25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of 26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels.     

Oxygen isotope composition and geothermometry of granulite to greenschist facies metamorphic rocks: a study from the Neoproterozoic collision-related nappe system, south of São Francisco Craton, SE Brazil

Oxygen isotope studies were carried out across units of a Neoproterozoic nappe system, south of São Francisco Craton. A temperature decrease toward the base of the system is found, consistent with a previously recognized inverted metamorphic pattern. The tectonic contact of the basal unit and the reworked southern São Francisco craton show a steep temperature gradient, suggesting that low temperature thrusting acted as the dominant tectonic process. The contrasts between the δ¹⁸O values of the Três Pontas-Varginha and Carmo da Cachoeira nappes and the differences among the samples and minerals are consistent with the preservation of sedimentary isotopic composition during metamorphism. The small differences in the δ¹⁸O values between the undeformed and the deformed calc-silicate samples (1.6‰) suggest that the δ¹⁸O value of mylonitization fluids was close to that which equilibrated with the metamorphic assemblage. The distinct δ¹⁸O values of metapelitic and calc-silicate samples and the great temperature difference from one type to the other indicate that no large-scale fluid interaction processes occurred during metamorphism. Oxygen isotopic estimations of both Três Pontas-Varginha undeformed rocks and Carmo da Cachoeira unaltered equivalents indicate δ¹⁸O values of up to 18‰. Comparison between these values and those from the ‘basement’ orthogneisses (8.3–8.5‰) indicates the latter are not sources for the metapelites.     

Analytical and numerical analyses of the load‐bearing capacity of retaining walls laterally supported at both ends

This paper investigates the load‐bearing capacity of a perfectly smooth retaining wall laterally supported at both ends assuming that the wall fails by the development of three plastic hinges. The study considers the case of a cohesionless elastic–perfectly plastic backfill with a Mohr–Coulomb yield criterion and an associative flow rule in drained conditions. A kinematically admissible soil–structure failure mechanism is proposed and compared with the conventional solutions and with results from a numerical finite element modelling. The study shows that the proposed solution and the numerical solution are in good agreement. These solutions are found to be much more favourable for the wall than the conventional solutions. Copyright © 2010 John Wiley & Sons, Ltd.     

Postglacial fire, vegetation, and climate history in the Clearwater Range, Northern Idaho, USA

The environmental history of the Northern Rocky Mountains was reconstructed using lake sediments from Burnt Knob Lake, Idaho, and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large-scale climate changes during the Holocene. Vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold-dry conditions. From 14,000 to 12,000 cal yr B.P., abundant Pinus pollen suggests warmer, moister conditions than the previous period. Most sites record the development of a forest with Pseudotsuga ca. 9500 cal yr B.P. indicating warm dry climate coincident with the summer insolation maximum. As the amplification of the seasonal cycle of insolation waned during the middle Holocene, Pseudotsuga was replaced by Pinus and Abies suggesting cool, moist conditions. The fire reconstructions show less synchroneity. In general, the sites west of the continental divide display a fire-frequency maximum around 12,000–8000 cal yr B.P., which coincides with the interval of high summer insolation and stronger-than-present subtropical high. The sites on the east side of the continental divide have the highest fire frequency ca. 6000–3500 cal yr B.P. and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late Holocene. This study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher-than-present fire frequency occurred in the past.     

Diachronous Palaeoproterozoic deformation and metamorphism in the Committee Bay belt,Rae Province,Nunavut: insights from 40Ar–39Ar cooling ages and thermal modelling

Regional‐scale ⁴⁰Ar–³⁹Ar data presented in this paper reveal significant across‐strike and along‐strike age differences in the Committee Bay belt (CBb), Rae Province, Nunavut, Canada, that complement variations in observed monazite ages. ⁴⁰Ar–³⁹Ar hornblende ages are c. 1795, 1775, and 1750 Ma in the western, eastern and central parts of the Prince Albert Group (PAG) domain respectively. The migmatite domain and Walker Lake intrusive complex are characterized by c. 1750–1730 ⁴⁰Ar–³⁹Ar hornblende ages without significant along‐strike variation. The ⁴⁰Ar–³⁹Ar data provide important constraints on the cooling history and on thermal modelling that elucidates the controls on diachroneity and metamorphic patterns within the belt. In the western CBb, prograde monazite growth occurred 26 ± 10 Myr earlier in the migmatite domain (1864 ± 9 Ma; peak P–T = 5 kbar?700 °C) than in the PAG domain (1838 ± 5 Ma; peak P–T = 5 kbar?580 °C). Calculations indicate that this earlier monazite growth results from tectonic thickening of higher heat productivity Archean lithologies in the migmatite domain, which undergoes more rapid prograde heating than the less radiogenetic and lower grade rocks of the PAG domain. Granite generation via biotite dehydration melting at 800 °C and 20 km depth is predicted to occur c. 1835 Ma, in agreement with geochronological constraints. The tectonic burial of crustal domains with contrasting radiogenic properties also explains the general congruence of lower to upper amphibolite facies metamorphic zones generated during the two main orogenic cycles (i.e. M₂–D₁ and M₃–D₂). The modelled timing of prograde monazite growth in the migmatite domain suggests that D₂ tectonic thickening began at 1872 ± 9 Ma, some 8 ± 3 Myr before monzazite growth, coeval with the inferred time of collision of the Meta Incognita terrane with the southern Rae Province. Along‐strike diachroneity, reflected in 25 Myr younger monazite and ⁴⁰Ar–³⁹Ar hornblende ages in the eastern relative to the western PAG domain, cannot be accounted for by heat productivity contrasts along the belt. Instead the younger deformation and metamorphism in the eastern CBb was driven by its proximity to the eastern promontory of the Superior Province which collided with the Rae Province at c. 1820 Ma. The ⁴⁰Ar–³⁹Ar data presented here support the interpretation that the youngest monazite in the CBb crystallized at c. 1790 Ma in the central CBb when this part of the belt was downfolded into a gentle synformal structure while the western part of the belt cooled through ⁴⁰Ar–³⁹Ar hornblende closure. The results of this study illustrate the important influence of contrasting rock properties on the thermal evolution of orogenic belts and on the temporal record of this evolution.