Evolution of microstructures in Precambrian shear zones: An example from eastern India

Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.     

Natural deformation related to serpentinisation of an ultramafic inclusion within a continental shear zone: The key role of fluids

The role of fluids in the deformation of continental serpentinites is investigated from structural, microstructural and petrographic analyses applied to a km-scale porphyroclast mantled in a viscous matrix of amphibolites. The clast is sited within a shear zone of the Palaeozoic Maures massif, France. Syntectonic fluid–rock interactions occurred from km to mm scales, at first on the clast borders (along the main rheological boundaries) then within the clast. They are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack-seal veins, typifying an intermediate, brittle–ductile behaviour of serpentinites. Three main stages of deformation–serpentinisation processes occurred in relation with the left-lateral movement of the hosted shear zone. They developed under metamorphic conditions evolving from amphibolites to green-schists facies conditions ( 400 MPa/550 °C to  200 MPa/< 300 °C), as inferred from the surrounding sheared amphibolites. Deformation and serpentinisation increase through time although fluid pressure decreases. If the shape of the inclusion and its orientation relative to the shear zone mainly controlled the deformation pattern though time (P then R' shears), fluid pressure is required for starting deformation–serpentinisation processes along inherited anisotropy planes. Whatever the origin of fluids, they play a key role all along the deformation processes by influencing stress states within the shear zone at the onset of deformation and by changing at various scales and through time behaviour of the rock, depending of the intensity of serpentinisation and the rate of deformation.     

Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif – Western Alps)

The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean–continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta‐ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P–T (estimated in eclogitized metagabbros at 2–2.5 GPa and 550–620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo‐Moho.     

Supra-subduction affinity in the Neoproterozoic serpentinites in the Eastern Desert, Egypt: Evidence from mineral composition

Serpentinites in the Eastern Desert (ED) of Egypt represent integral components of the ophiolites. Metamorphic textures of the serpentinites preserve the complex mineralogical evolution from primary peridotite through metamorphism, and late-stage hydrothermal alteration. Two textural types are distinguished in the olivines of the present serpentinized peridotites, namely (a) highly-strained olivine grains with kink bands, as in the deformed mantle tectonites from ophiolites, and (b) non-strained grains. The latter may represent recrystallized crystals during later thermal metamorphic events due to the intrusion of granite. On the basis of X-ray diffraction analysis, antigorite is the main serpentine minerals with lesser chrysotile and lizardite which indicates that serpentinites were formed under prograde metamorphism. Relict primary minerals of the serpentinites are Cr-spinel, olivine and pyroxene. Chrome spinel relicts have high Cr# (0.60–0.80), whereas primary olivines are Mg-rich nature (Fo = 89–96). Geochemical compositions of serpentinites indicate that they formed not at mid-ocean ridges but at spreading centers associated with subduction zones and this could have happened in a supra-subduction zone either in the fore-arc or back-arc environments. Mineral compositions of primary chrome spinels and olivines are similar to those of modern fore-arcs. High Cr# in the relict chrome spinels and Fo in the primary olivines of serpentinites indicate that they are residual after extensive partial melting and originated by sea-floor spreading during subduction initiation.     

Magnetic fabric study across the Ailao Shan–Red River shear zone

The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. P_J value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes (K_max) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis (K_min) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.     

Geometry of the fault zone of the 2003 Ms = 7.5 Chuya earthquake and associated stress fields, Gorny Altai

The co-seismic deformations produced during the September 27, 2003 Chuya earthquake (Ms = 7.5) that affected the Gorny Altai, Russia, are described and discussed along a 30 km long segment. The co-seismic deformations have manifested themselves both in unconsolidated sediments as R- and R′-shears, extension fractures and contraction structures, and in bedrock as the reactivation of preexisting schistosity zones and individual fractures, as well as development of new ruptures and coarse crushing zones. It has been established that the pattern of earthquake ruptures represents a typical fault zone trending NW–SE with a width reaching 4–5 km and a dextral strike–slip kinematics. The initial stress field that produced the whole structural pattern of co-seismic deformations during the Chuya earthquake, is associated with a transcurrent regime with a NNW–SSE, almost N–S, trending of compressional stress axis (σ₁), and a ENE–WSW, almost E–W, trending of tensional stress axis (σ₃). The state of stress in the newly-formed fault zone is relatively uniform. The local stress variations are expressed in insignificant deviation of σ₁ from N–S to NW–SE or NE–SW, in short-term fluctuations of relative stress values in keeping their spatial orientations, or in a local increase of the plunge angle of the σ₁. The geometry of the fault zone associated with the Chuya earthquake has been compared with the mechanical model of fracturing in large continental fault zones with dextral strike–slip kinematics. It is apparent that the observed fracture pattern corresponds to the late disjunctive stage of faulting when the master fault is not fully developed but its segments are already clearly defined. It has been shown that fracturing in widely different rocks follows the common laws of the deformation of solid bodies, even close to the Earth surface, and with high rates of movements.     

Characterisation of deformation and flow mechanics around porphyroclasts in a calcite marble ultramylonite by means of EBSD analysis

This paper studies the flow heterogeneity around porphyroclasts associated with greenschist facies deformation of a calcite marble shear zone. Microstructural data from electron backscatter diffraction analyses (EBSD) are used to constrain the flow mechanics of this dominantly non-coaxial type of deformation. The microstructure of the undisturbed ultramylonite (grain-size range 5–100 μm, mean 40 μm) is interpreted to represent steady-state (time-independent) flow conditions with flow planes parallel to the shear zone boundary. Single calcite porphyroclasts (grain-size 1–3 mm) caused flow perturbation in the fine-grained marble ultramylonite. It is the shape, in particular, of these rigid porphyroclasts that controls their rotational behaviour during deformation and, therefore, the development of specific flow fabrics. The flow planes around elongated-rhomboidal, stable porphyroclasts change the orientation to become roughly parallel to the porphyroclast margin, whereas the geometry of flow planes around nearly equant, rotating porphyroclasts describes a δ-type flow pattern. We infer that to some extent decoupling at the clast–matrix interface has occurred to guarantee a stable orientation of elongated porphyroclasts, but was not sufficient to reduce the rotation rate of equant clasts to zero. According to the flow deflection, the general crystallographic preferred orientation (CPO) with its single c-axis maximum perpendicular to the flow plane is rotated about an axis which is (sub)parallel to the kinematic rotation axis of the shear zone. Ultramylonite microstructures, CPOs and misorientation data are best explained by the dual operation of grain-size-insensitive (dislocation creep with recovery and recrystallization) and grain-size-sensitive (diffusion creep) mechanisms. The limited grain-size reduction around porphyroclasts suggests that the grain-size-insensitive mechanisms controlled rheology.     

Dehydration reactions and micro/nanostructures in experimentally-deformed serpentinites

High-T torsion experiments on lizardite + chrysotile serpentinites produced mineralogical and micro/nanostructural changes, with important implications in rheological properties. High-resolution TEM showed that specimens underwent ductile [by microkinking and (001) interlayer glide] and brittle deformation (by microfracturing), together with dehydration and break-down reactions. Lizardite is affected by polytypic disorder and microkinking [kink axial planes at high angle with respect to (001) planes], that were not present in the initial ordered 1T-lizardite. Chrysotile fibres are deformed, resulting in elliptical cross-sections, with strong loss of interlayer cohesion. Both lizardite and chrysotile break down to a fine intergrowth of olivine (up to 200 nm), talc (up to 30 nm) and poorly-crystalline material. Lizardite-out reaction preferentially occurs at kink axial planes, representing sites of preferential strain and enhanced reactivity; conversely, chrysotile break-down is a bulk process, resulting in large healed olivine aggregates, up to micrometric in size. Overall observations suggest that dehydration and break-down reactions are more advanced in chrysotile than in lizardite.     

Ar/Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at 120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five ⁴⁰Ar/³⁹Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between 160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and 101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and 106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the 110–90 Ma period.     

陕西陈耳金矿床不同构造－岩相带微量元素地球化学特征及构造控矿机理

通过对陈耳金矿床两类不同构造-岩相带微量元素分布特征分析,发现元素特别是成矿元素Au的含量从片理化带→破碎蚀变带→石英脉逐渐增强。构造-岩相带内变形岩石的石英Rf/φ法有限应变测量表明,片理化带应变强度最大,破碎蚀变带应变强度小,应变集中在韧性剪切带边部的片理化带内。R型聚类分析表明Au与Ag、Cu、Pb及Zn的相关系数远大于Au与Ni、Co元素的相关系数,甚至接近于1。研究认为:破碎蚀变带内由于应变软化导致脆性裂隙大量发育,易于成矿流体的迁入与淀积而强富集;片理化带由于应变集中,造成成矿流体的迁出而弱富集。韧性剪切带脆-韧性变形转换不仅控制了矿体就位空间,同时也决定着元素迁移的方向。     

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.     

Principal fault zone width and permeability of the active Neodani fault, Nobi fault system, Southwest Japan

The internal structure and permeability of the Neodani fault, which was last activated at the time of the 1891 Nobi earthquake (M8.0), were examined through field survey and experiments. A new exposure of the fault at a road construction site reveals a highly localized feature of the past fault deformation within a narrow fault core zone. The fault of the area consists of three zone units towards the fault core: (a) protolith rocks; (b) 15 to 30 m of fault breccia, and (c) 200 mm green to black fault gouge. Within the fault breccia zone, cataclastic foliation oblique to the fault has developed in a fine-grained 2-m-wide zone adjacent to the fault. Foliation is defined by subparallel alignment of intact lozenge shaped clasts, or by elongated aggregates of fine-grained chert fragments. The mean angle of 20°, between the foliation and the fault plane suggests that the foliated breccia accommodated a shear strain of γ<5 assuming simple shear for the rotation of the cataclastic foliation. Previous trench surveys have revealed that the fault has undergone at least 70 m of fault displacement within the last 20,000 years in this locality. The observed fault geometry suggests that past fault displacements have been localized into the 200-mm-wide gouge zone. Gas permeability analysis of the gouges gives low values of the order of 10⁻²⁰ m². Water permeability as low as 10⁻²⁰ m² is therefore expected for the fault gouge zone, which is two orders of magnitude lower than the critical permeability suggested for a fault to cause thermal pressurization during a fault slip.     

Deformation mechanisms and flow regimes in limestones from the Helvetic zone of the Swiss Alps

This paper is based on a combined field, transmission-electron (TEM) and transmission-optical (TOM) microscope study of limestones from the Helvetic zone (Swiss Alps) and discusses the deformation mechanisms and flow regimes that governed the deformation of these rocks.During pre-metamorphic regional ductile deformations the limestones deformed by power-law dislocation creep with differential stresses probably not exceeding 1 kbar. Dynamic recrystallization with grain-boundary sliding and grain-boundary migration allowed the grains to be less elliptical than the strain ellipse. A characteristic of the structure is the existence of dislocation-free subgrains. In the footwall of and approaching the Lochseiten calc-mylonite along the Glarus overthrust, grain-boundary sliding becomes more important (shift to diffusional creep or superplastic flow).During a syn- and post-metamorphic deformation, dynamic recovery seems to have become less competitive (no dislocation-free sub-grains), and along thrust faults twinning indicates a shift to higher differential stresses at the close of the deformation.It was not possible to separate these deformation phases on the basis of the dislocation debris. Sub-grain sizes as observed in TEM and TOM were identical.In limestones that underwent cataclastic deformation the rocks seem to have started breaking up along the grain boundaries. The new grain fragments are very small (0.1–0.3 μm) and are heavily twinned. In TEM the old large grains show very long straight glide dislocations, cleavage and, when shattered, ring patterns in diffraction.     

The Bentong–Raub Suture Zone

It is proposed that the Bentong–Raub Suture Zone represents a segment of the main Devonian to Middle Triassic Palaeo-Tethys ocean, and forms the boundary between the Gondwana-derived Sibumasu and Indochina terranes. Palaeo-Tethyan oceanic ribbon-bedded cherts preserved in the suture zone range in age from Middle Devonian to Middle Permian, and mélange includes chert and limestone clasts that range in age from Lower Carboniferous to Lower Permian. This indicates that the Palaeo-Tethys opened in the Devonian, when Indochina and other Chinese blocks separated from Gondwana, and closed in the Late Triassic (Peninsular Malaysia segment). The suture zone is the result of northwards subduction of the Palaeo-Tethys ocean beneath Indochina in the Late Palaeozoic and the Triassic collision of the Sibumasu terrane with, and the underthrusting of, Indochina. Tectonostratigraphic, palaeobiogeographic and palaeomagnetic data indicate that the Sibumasu Terrane separated from Gondwana in the late Sakmarian, and then drifted rapidly northwards during the Permian–Triassic. During the Permian subduction phase, the East Malaya volcano-plutonic arc, with I-Type granitoids and intermediate to acidic volcanism, was developed on the margin of Indochina. The main structural discontinuity in Peninsular Malaysia occurs between Palaeozoic and Triassic rocks, and orogenic deformation appears to have been initiated in the Upper Permian to Lower Triassic, when Sibumasu began to collide with Indochina. During the Early to Middle Triassic, A-Type subduction and crustal thickening generated the Main Range syn- to post-orogenic granites, which were emplaced in the Late Triassic–Early Jurassic. A foredeep basin developed on the depressed margin of Sibumasu in front of the uplifted accretionary complex in which the Semanggol “Formation” rocks accumulated. The suture zone is covered by a latest Triassic, Jurassic and Cretaceous, mainly continental, red bed overlap sequence.     

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

Micaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO₂ contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.     

Chrome-spinels in serpentinites and talc carbonates of the El Ideid-El Sodmein District, central Eastern Desert, Egypt: their metamorphism and petrogenetic implications

Serpentinites and talc-carbonate rocks of El Ideid-El Sodmein District (ISD), central Eastern Desert, Egypt, contain variably altered chrome-spinels. Back-scattered electron images and electron microprobe analyses of chrome-spinels and associated silicates are made to evaluate their textural and compositional variations with metamorphism. In most cases the chrome-spinel crystals are concentrically zoned with unaltered cores through transitional zone of ferritchromit to Cr-magnetite toward the rims. In talc-carbonate rocks chrome-spinels are extensively altered to Cr-magnetite. Compared to cores, the metamorphic rims are enriched in Cr# (0.83–1.0 vs. 0.58–0.63 for rims and cores, respectively) and impoverished in Mg# (0.05–0.29 vs. 0.57–0.63), due to Mg–Fe and Al(Cr)–Fe³⁺ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. Textural and compositional features of the chrome-spinels suggest transitional greenschist-amphibolite up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the country rocks. The common preservation of unaltered chrome-spinel cores in the serpentinites, contrary to talc-carbonate rocks, implies that full equilibration has not been attained due to small metamorphic fluid–rock ratio. Microprobe analyses profile across a concentrically zoned grain confirms the presence of two compositional (miscibility?) gaps; one between chrome-spinel core and ferritchromit zone; and another one between ferritchromit zone and Cr-magnetite outer rim.Chrome-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, indicating relic magmatic composition not affected by metamorphism. Core compositions suggest an ophiolitic origin and derivation by high degrees of melting of reduced, depleted harzburgite to dunite mantle peridotites in an oceanic supra-subduction zone (marginal-basin) tectonic environment.     

Asbestos fibre identification vs. evaluation of asbestos hazard in ophiolitic rock mélanges,a case study from the Ligurian Alps (Italy)

In recent years, the high incidence of harmful health effects through inhalation of airborne asbestos from amphibole-bearing rock mélanges has been thoroughly documented. Here, we present a field-based, multi-scale geological approach aimed at illustrating the occurrence of amphibole fibrous mineralisation in an ophiolitic suite from the Ligurian Alps (Italy) and discussing the implication on in situ determination of the asbestos hazard. The rock mélange is composed of plurimetre-sized blocks of different lithotypes (metagabbro, serpentinite, chloritoschist) juxtaposed by the meaning of tectonic structures. The geological-structural survey revealed that the fibrous mineralisation is localised in specific structural sites of the rock volume, including veins and schistosity. Both micro-chemical and crystal structure analyses on selected fibrous samples revealed that actinolite fibres grow in veins within the metagabbro and in chloritoschists, while fibrous tremolite occurs in serpentinite schistosity. The morphological features of these amphibole fibres have been analysed in TEM images and used for classifying them as “asbestiform” or “non-asbestiform”. The results show that the asbestos hazard determination is not unequivocally identified when different procedures for asbestos fibre identification and classification are applied. This may have impact on normatives and regulations in defining environmental hazards due to asbestos occurrence.     

A shear-diffusion model of till genesis based on the dispersal pattern of indicator rocks in the Grand-Volume Till of central Gaspésie, Québec, Canada

Dispersal patterns of indicator rocks in central Gaspésie reveal that glacial debris is entrained in a basal debris-rich zone of shearing where clast diffusion takes place. The Grand-Volume Till forms a thin till sheet over the high plateaus of Gaspésie Peninsula and resulted from a succession of two Wisconsinan ice flows of distinct orientations (SSE and NE). The lithological composition of this till determined by pebble counts and the three-dimensional dispersal patterns of indicator rocks in it suggest that debris transport occurred principally by simple shear deformation of glacial debris. In addition, the intermixing of clasts at the intersection of two lithologically distinct dispersal trains of SSE and NE orientations, respectively, suggests that extensive mixing takes place during shearing. Physical interactions among the clasts lead to both upward and downward movements which cause the clasts to diffuse across the zone of shearing. This process of shear-diffusion results in continuous incorporation and mixing of the newly encountered rock types during glacial transport.     

Interaction between the Nazca and South American plates and formation of the Altiplano–Puna plateau: Review of a flexural analysis along the Andean margin (15°–34°S)

The results of a two-dimensional flexural analysis applied to the Andean margin, which is based on the correlation between topography and Bouguer anomaly, are here reviewed in order to characterize rigidity variations across and along the forearc–arc transition of the Central Andes and to understand the role of the forearc in the formation of the Altiplano Plateau. The forearc has maximum rigidities between 15° and 23°S. Forearc rigidity decreases gradually southward and sharply toward the plateau. The main orogen (elevations higher than 3000 m) is very weak along the entire Central Andes. A semi-quantitative interpretation of these trends, based on the relationship between flexural rigidity and the thermo-mechanically- and compositionally-controlled strength of the lithosphere, allows the following conclusions to be made: (1) across-strike rigidity variations are dominated by the thermal structure derived from the subduction process; (2) the forearc constitutes a strong, cold and rigid geotectonic element; (3) southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab; (4) very low rigidities along the main orogen are caused by the existence of a thick, quartz-rich crust with a low strain rate-to-heat flow ratio; (5) the strength of the plateau lithosphere is localized in an upper-crustal layer whose base at 15 km could be correlated with a P-to-S seismic wave converter (TRAC1 of Yuan et al., 2000 [Yuan, X., Sobolev, S., Kind, R., Oncken, O. et al. 2000. Subduction and collision processes in the Central Andes constrained by converted seismic phases. Nature, V 408, 21/28 Diciembre, p. 958–961]); (6) the forearc–plateau rigidity boundary corresponds to a zone of changing thermal conditions, eastward-increasing crustal thickness and felsic component in the crust, and low strain-rate deformation, which correlates with a west-verging structural system at the surface. These conclusions suggest that the rigid forearc acts as a pseudo-indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland. This pseudo-indenter is geometrically represented by a crustal-scale triangular zone rooted at TRAC1. This model allows the integration of existing contradictory ideas on the dynamics of forearc–plateau interaction that are related to the relative importance of upper-crustal compressive structures and lower crustal accumulation below the forearc.     

Metamorphic path constrained by metapelitic rocks from the inner Aiuruoca-Andrelândia nappe, south of the São Francisco craton, SE Brazil

A Nappe system south to southwest of the São Francisco Craton represents the southern extension of the Brasília belt and describes an inverted metamorphic pile of greenschist facies toward amphibolite facies. The Aiuruoca-Andrelândia nappe is one of the nappes of this system. The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain transported toward the E-NE. There is a metamorphic transition, from the kyanite zone to kyanite and sillimanite coexistence, until the sillimanite zone. Metapelitic rocks preserve high-pressure parageneses (Rt–Ky–Grt–Ms–Bt–Pl–Qtz) and contain retrograde eclogitic rocks. Sil–Pl–Qtz coronitic intergrowths around garnets are common decompressive textures. Kyanite schists register the P_max of 11 kbar at 660 °C and define a decompressive path until 6–7.5 kbar at 650 °C. These P–T conditions represent the equilibrium in S₂ schistosity (amphibolite facies) and the beginning of the cooling path in the Ky–Sil transition. The decompressive path suggests an extrusional process, immediately after burying at about 60 km. Exhumation controlled by convergent events, related to the São Francisco Plate subduction and tectonic erosion, took these units, isothermally, to higher levels (20–33 km). Later, the metamorphic path shifted toward near-isobaric cooling.