Mylonitic Shear Zones Research Articles

Late Mesoproterozoic Deformation of SW Amazonia (Rondônia, Brazil): Geochronological and Structural Evidence for Collision with Southern Laurentia

Proposed assembly of the Rodinia supercontinent in the late Mesoproterozoic involved the collision of the Amazon craton with some portion of the southern or eastern margin of cratonic North America. Previously reported paleomagnetic data from the SW Amazon craton suggest a paleogeographic link between “Grenvillian” deformation of the SW Amazon craton and late Mesoproterozoic tectonometamorphism in southern Laurentia. A structural, geochronological, and petrological investigation of the western Amazon basement rocks (Rondônia, Brazil) was carried out in order to document evidence of a Grenvillian collision connecting the Amazon to Laurentia. Integration of 40Ar/39Ar data and feldspar thermometry data from regionally extensive strike‐slip mylonitic shear zones (Ji‐Paraná shear zone network) indicates that deformation took place at 450°–550°C between 1.18 and 1.15 Ga. An older, ca. 1.35‐Ga event found exclusively in less‐deformed basement rocks is interpreted as recording cooling from an earlier metamorphic episode (650°–800°C indicated by feldspar thermometry) unrelated to the Grenville collision. The style of deformation in the SW Amazon craton contrasts with that observed in southern Laurentia, where extensive crustal thickening accommodated by deep‐seated thrust sheets resulted in widespread thermal resetting of isotopic systems during exhumation and postorogenic cooling. In contrast, the predominantly strike‐slip activity observed in the Amazon resulted in age resetting through strain‐induced recrystallization, not regional‐scale thermal resetting. Consequently, the ages recorded by hornblende in the SW Amazon craton are slightly older than the cooling ages preserved in southern Laurentia. Differences in structural style and geochronological record are interpreted as indicative of an exhumed, asymmetric crustal structure similar to that of modern orogens.

Internal dynamics of a paleoaccretionary wedge: insights from combined isotope tectonochronology and sandbox modelling of the South-Central Chilean forearc

Forearc accretionary wedges are cyclic systems in which material is frontally and/or basally accreted. Material cycling involves underthrusting, subduction, underplating, exhumation, erosion, transfer to the trench and underthrusting again. In this study we present a novel, tectonochronologic approach to constrain long-term exhumation rates of basally accreted wedge complexes, based on isotopic dating of structural features, on petrological data and sandbox analogue simulations. Congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics. For the present-day surface material of the paleoaccretionary wedge of South-Central Chile (Valdivia area, 40°S), published U–Pb ages of detrital zircon place a maximum age of ∼278 Ma for subduction. Prograde metamorphism at transitional greenschist to blueschist facies conditions (420 °C, 8–9 kbar) was immediately followed by progressive penetrative deformation associated with basal accretion, dated at ∼250–245 Ma using Rb/Sr internal mineral isochrons. The accretion process involved duplex tectonics and antiformal stacking, with formation of near-horizontal mylonitic shear zones at around 241 Ma. Continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels. Both semi-ductile, small-scale extensional shear zones and post-kinematic vein mineralizations yield Rb/Sr ages of ∼235 Ma. Tension gashes, representing the latest isotopically dateable stage of structural evolution, were formed at ∼210 Ma, at conditions of ∼230 °C at 1.5–3 kbar, as constrained by fluid inclusion data. Zircon fission track data indicate final cooling to below ∼200 °C at 186±24 Ma. The results suggest continuous basal accretion for at least 50 Ma, with long-term average exhumation rates of 0.6±0.2 mm/a, most probably outbalanced by similar long-term average erosion rates. Changing plate boundary conditions at about 210–200 Ma terminated the accretion process, as evident from a dramatic decrease of exhumation rates at that time. Since then, the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the Andean active margin. The tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high-temperature increments of P– T trajectories.

TEXTURES, GEOCHEMISTRY AND SIGNIFICANCE OF SHEAR ZONE IN THE PERIDOTITIC MASSIF OF LANZO (ALPS-ITALY)

The localization of deformation in the lithosphere is well known from the study of seismic reflexion profiles (e.g. Flack et al., 1990; Baird et al., 1995), peridotite mylonite in mantle xenoliths (e.g. Nielson and Schwarzman, 1977; Harte, 1983) and peridotite massifs (e.g. Drury et al., 1990; Newman et al., 1999; Dijkstra et al., 2002). The deformation conditions of such mylonites indicate the strain localization in the lithosphere mantle peridotite because the basal part of subducted or obducted ophiolite is often characterized by strongly deformed peridotite (e.g., Vissers et al., 1991; Suhr, 1993; Boudier and Nicolas, 1995). The Lanzo peridotite massif in NW-Italy is characterized by fresh peridotite, well preserved high-temperature shear zones (Boudier, 1978) and many features indicating pervasive magma flow (Bodinier et al., 1986, Muntener and Piccardo, 2003). The presence of melt is known to lower the resistance to stress in solid state flow (Hirth & Kohlstedt, 1995a, 1995b). To determine constraints on the rheological weakening of mantle lithosphere, we investigate the mutual relationships between high-temperature deformation, meltrock reaction and emplacement of plutonic rocks in the plagioclase peridotite massif of Lanzo. Mapping of the Lanzo massif, in the northern and central parts, has revealed the exposed dimension of the peridotite mantle shear zone, which is at least 250 meters wide with a sub-vertical foliation generally dipping to the NE. Fieldwork, microscopic observation and preliminary grain size analyses highlight 4 types of microstructures from “coarse grained secondary protogranular texture (CGSPT)” (according to Mercier and Nicolas 1975) to “mylonite”. CGSPT is characterized by weakly deformed porphyroclastic zones and clear domains of igneous recrystallization with olivine (ol), orthopyroxene (opx), plagioclase (pl), ± clinopyroxene (cpx) and ± spinel (sp). The mylonite is characterized by porphyroclastic opx extremely stretched (aspect ratios up to 10:1) and a fine grained matrix (grain size: to 2 km) and an anastomosing texture in accordance to the high temperature foliation. Fieldwork allowed to localize a series of discordant gabbroic dikes ranging from ol-gabbro to kaersutite-bearing diorites, which are abundant on the southern side of the mylonite and are generally weakly deformed. Some cpx porphyroclasts show signs of previous reaction textures with a melt (cpx1 +liq -> opx + plg ± ol), a texture, which is common in the southern Lanzo massif (Muntener and Piccardo, 2003). This indicates that the mylonite formation postdates cpx-corroding melt/rock reactions. Thermobarometry of porphyroclastic mineral assemblages in the peridotite indicate high temperatures (1100°C-1150°C and two pyroxene thermometry of neoblast indicate final equilibration temperatures close to 850°C. Compositionalprofiles in porphyroclastic cpx from a CGSPT sample infiltrated by melt are heterogeneous and zoned. The core of some cpx contains high values of Al (0.3 Al per formula unit) and low values in Ti (0.015 p.f.u.), which corresponds to relic spinel peridotite. The rim is zoned with a decrease in Al (0.13 p.f.u), an increase in Ti (0.027 p.f.u.) and a Cr increase from the core to the rim. This variation corresponds to the equilibration in plagioclase facies. The heterogeneity of the profile and the increase of Ti towards cpx rims suggest melt/rock reaction during the transition from spinel to plagioclase facies conditions. Cr# (molar Cr/Cr+Al) and TiO2 concentrations in spinel from strongly deformed plagioclase peridotites show an extreme variability and do not seem to be microstructurally controlled. This indicates that once the plagioclase peridotites completely crystallized, exhumation to shallower depth must have been rapid, in order to preserve disequilibrium chemical compositions. Our results indicate that melt migration and high temperature deformation are juxtaposed both in time and space. Melt-rock reaction may cause grain size reduction, which in turn led to localization of deformation. Observations indicate that actively deforming peridotite mylonite might suppress brittle failure and that ascending gabbros might terminate and crystallize along actively deforming shear zones. This is supported by the observation that gabbros are asymmetrically distributed with respect to the shear zone. If true, a ‘gabbro-rich zone’ might be the footwall of high-temperature mylonitic shear zone.

STRUCTURES AND MICROSTRUCTURES OF MANTLE ROCKS: KEYS TO THE RHEOLOGY OF THE LITHOSPHERE

Upper mantle peridotites from ophiolitic and alpine-type massifs, as well as those brought up as basalt-borne xenoliths, show a spectrum of structures and microstructures that potentially reflect the mechanical behaviour of the upper mantle during flow. It is uncertain, however, if microstructures are unambiguous in this respect. Based on experimental studies and materials science know-how, we use microstructure in its broadest sense to infer the nature of the microphysical processes once operating in the upper mantle rocks in question. Aside the many uncertainties involved in such extrapolation of experimentally determined mechanical data via the microstructure, there is the even more difficult question if the flow and fine structure of a natural rock, deformed at depth and high temperature, really reflects the flow of interest. Inherent to the fact that these rocks are now found at the Earth’s surface, the microstructures seen in naturally deformed mantle rocks may to a certain extent represent the waning stages of deformation and recrystallization in the mantle, i.e., the microstructure frozen in the rock may not appropriately reflect the main flow. The question thus arises in how far the deformation microstructures in mantle rocks really represent a significant part of the ductile flow history. In this context it seems appropriate to use the concept of a steady-state microstructure, i.e., a microstructure that continuously evolves during flow without changing its principal characteristics such as average grain size, grain shape and grain orientation, and that likely reflects the microphysical processes during flow. These concepts are particularly important at high homologous temperatures where deformation-induced rotation recrystallization (development of new grains from subgrains during dislocation creep) may or may not be balanced by migration recrystallization (recrystallization due to the migration of grain boundaries). It may be envisaged that high-temperature flow in the asthenosphere or lower lithosphere thus leads to seemingly undeformed granular rocks, even protogranular in the sense of commonly used peridotite microstructure classifications, or that the samples recovered preserve more clearly deformed, porphyroclastic microstructures including coarse to fine-grained tectonites. A distinct class of microstructures in peridotites concerns very fine to ultrafine-grained deformation microstructures in sometimes spectacular mylonitic shear zones. The mechanical aspects of the initiation and growth of these structures is surrounded with uncertainties, but there is growing consensus regarding their mechanical significance once these structures have developed: they are mechanically extremely weak due to the fact that flow in such ultrafine-grained rocks will be dominated by grainsize-sensitive diffusion creep mechanisms. Mylonites in upper mantle rocks often overprint earlier flow structures, probably because they tend develop at lower temperatures and higher stresses inherent in the later stages of a mantle fragment’s history in the Earth’s upper mantle, e.g., related to emplacement at crustal levels in rifts, or near the ocean floor in ophiolites.

EXHUMATION HISTORY OF SUBCONTINENTAL GARNET PYROXENITE-BEARING MANTLE FROM THE EXTERNAL LIGURIDE OPHIOLITES (NORTHERN APENNINE, ITALY): IMPLICATIONS FOR RIFTING AND MANTLE EXHUMATION PROCESSES AT OCEAN-CONTINENT TRANSITION

The External Liguride Jurassic ophiolites derive from an ocean-continent transition similar to the West Iberian margin. The mantle sequence consists of spinel and spinel-plagioclase lherzolites recording a lithospheric thinning history culminated into mantle exhumation at the ocean-floor. They represent unroofed subcontinental litosphere of the former Adria-Europe system largely affected by thermochemical erosion, heating and refertilization by asthenospheric melts during the Ligurian Tethys formation (Rampone et al., 1995; Piccardo et al., 2004). Peridotite bodies which escaped melt contamination and intrusion preserve relics of a deep-seated litospheric origin, i.e. occurrence of graphitebearing garnet clinopyroxenite and websterite layers. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites therefore represents a unique tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. Garnet clinopyroxenites are mafic rocks with a primary assemblage of pyrope-rich garnet + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~12.5 wt%), with minor amounts of orthopyroxene, graphite, Fe-Ni sulphides and rutile. Decompression yielded Na-rich plagioclase (An50-45) exsolutions in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An85-72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under synkinematic conditions, at the expenses of two-pyroxene + Alspinel. Mg-rich garnet has been also found in websterite layers, which are commonly characterised by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu-Hf and Sm-Nd mineral isochrons (220 ± 13 Ma and 186 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. The early decompression was associated with significant cooling, corresponding to T = 900-1050°C, and development of the spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase-olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution was characterized by a widespread polyphase brittle deformation under decreasing temperature conditions, coupled with hydration. In particular, the brittle evolution includes an early amphibolite-facies veining stage, followed by low-temperature hydrothermal alteration and serpentinisation associated with polyphase cataclasis. The exhumation was likely accomplished as a two-step process starting during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, possibly including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence was involved into an extensional detachment structure up to sea-floor exhumation, which was associated with development of serpentinisation-related hydrothermal activity. Integrated values of cooling and uplift rates for the Jurassic rifting evolution are ~35°C/Ma and 0.6-1.4 mm yr-1.

Kinematic history of western Marie Byrd Land, West Antarctica: direct evidence from Cretaceous mafic dykes

Abstract Intracontinental deformation occurred in West Antarctica during the final stages of plate convergence along the Cretaceous Gondwana margin. In western Marie Byrd Land, 115 Ma to 95 Ma A-type granitoids and mafic dykes record a change in plate kinematics. The magmatism typically is viewed as a record of extension leading to orthogonal break-up between New Zealand and Marie Byrd Land by c. 67 Ma. This paper presents new kinmatic and 40 Ar/ 39 Ar age data for a mafic dyke array in the Ford Ranges, a region >1000 km 2 dominated by plutonic and metamorphic bedrock. The mean dyke trend of N16W corresponds to a maximum finite strain axis orientated N74E, highly oblique to the N58E-trending margin and to on-land crustal structures defined from airborne geophysics. 40 Ar/ 39 Ar emplacement ages for most dykes fall between 114 Ma and 97 Ma, coeval with emplacement of a gneiss dome at 101-96 Ma and with development of mylonitic shear zones at 100–95 Ma in coastal western Marie Byrd Land. The oblique orientation of maximum finite strain with respect to large faults, geophysical lineaments and the rifted margin of western Marie Byrd Land is consistent with transcurrent tectonics along this segment of the Gondwana margin at c. 100 Ma.

Contrasting structural histories of the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho

The Salmon River belt is composed of metamorphic tectonites with poorly dated Mesozoic and Paleozoic protoliths that lie between volcanic arc assemblages of northeastern Oregon and the western margin of the North American continent in west-central Idaho. To the west, the Salmon River belt overlies late Paleozoic and Mesozoic volcanic arc rocks of the Wallowa terrane on a shallowly east-dipping fault; to the east the belt is separated from North American rocks by the western Idaho shear zone. The Salmon River belt is divided into western and eastern assemblages composed of greenschist- and amphibolit-facies metamorphic tectonites, respectively, and is deformed in three and locally four generations of penetrative structures. First-generation structures are synmetamorphic, west-vergent, tight to isoclinal folds with a penetrative axial-planar cleavage and downdip stretching lineation. Second- and third-phase structures are postmetamorphic and consist of open to close, upright to slightly overturned folds with north-south and east-west trending penetrative axial-planar cleavages, respectively. Low-angle faults truncate metamorphic structures within the Salmon River belt and juxtapose rocks with different metamorphic grades. The faults formed together with second-generation structures and are deformed by third-generation folds. The age of metamorphic and postmetamorphic structures is constrained by deformed carbonate rocks of Late Triassic age and clastic rocks of late Early Jurassic age and by the ca. 145 Ma emplacement of an undeformed tonalitic pluton. Along the eastern margin of the Salmon River belt, structures are overprinted and transposed by a younger, steeply east-dipping mylonitic shear zone, the mid-Cretaceous western Idaho shear zone. Synmetamorphic structures within the Salmon River belt formed before amalgamation of the Wallowa terrane and the North American fringing-arc complex and developed as part of a regional belt of intra-arc metamorphic tectonites. The pr-Cretaceous docking of the Wallowa terrane is recorded within the Salmon River belt by development of postmetamorphic structures and predates formation of the western Idaho shear zone. The western Idaho shear zone formed after the amalgamation of the arc terranes and records the intraplate deformation associated with the collapse of the fringing-arc and back-arc-basin system.

40Ar– 39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam

The Truong Son Belt forms the eastern rim of the Indochina Block in Southeast Asia. The age of the metamorphism, mainly along NW–SE mylonitic shear zones that affects this belt, has been formerly determined at about 240–250 Ma. This age corresponds to the Indosinian tectonometamorphic episode. The Kon Tum Massif, situated to the south of this belt, comprises high-temperature rocks, the Kan Nack Complex, including charnockites and granulites. The main charnockitic outcrops, restricted to the Song Ba Valley, establish the intrusive nature of these magmatic rocks within granulite facies material. Basic charnockitic rocks are mainly quartz enderbites to norites and hornblende–pyroxene granulite facies rocks. The 40Ar– 39Ar age of intrusion-cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 Ma. In the east of the Kan Nack Complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses, whereas others show evidence of overprinting by a retrogressive low-grade metamorphism. Ar–Ar dating confirm this evolution, giving ages of 400 Ma for primary relict granulite facies phases and 260–270 Ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism. Granulites intruded by charnockites in the Song Ba Valley yield ages of about 250 Ma, equivalent to the ages of the charnockites, and have evidently been completely reset by these high temperature intrusions. Therefore, the Kan Nack Complex of the Kon Tum Massif is not an independent unit with respect to the Indosinian orogen, but represents the deep-crustal part of this belt.

A wide ocean-continent transition along the south-west Australian margin: first results of the MARGAU/MD110 cruise

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

High-temperature crustal scale shear zone at the western margin of the Eastern Ghats granulite belt, India: implications for rapid exhumation

A mylonitic shear zone occurs along the western margin of the Eastern Ghats Granulite belt, India. In the northern sector, it separates metapelitic and mafic granulites from the granite gneisses of the Bastar craton, while in the southern sector, it separates charnockitic gneisses and mafic granulites from the granite gneisses of the Bastar craton. This boundary shear zone is characterized by a mylonitic foliation and a stretching lineation. Microstructures characterize high-grade conditions and quartz c-axis patterns show strong point maxima close to the long axes of the strain ellipsoids, indicating dominant prism [c] slip in quartz, commonly interpreted in terms of ductile deformation at high-temperatures. Quartz c-axis patterns in the recrystallized and unrecrystallized domains also imply grain boundary migration as the principal mechanism of dynamic recrystallization at high temperature conditions. A rapid exhumation during shearing in this marginal part of the convergent Eastern Ghats orogen is indicated by decompression reaction textures and retrograde hydration reactions, presumably due to hydrous fluid ingress, in the shear zone rocks.

Shear zones in the upper mantle: evidence from alpine- and ophiolite-type peridotite massifs

Abstract There is abundant field and microstructural evidence for localization of deformation in alpine- and ophiolite-type mantle massifs. On the basis of field relationships and microstructures we recognize two types of tectonite shear zones (medium- to coarse- and fine-grained), as well as two types of mylonitic shear zones (anhydrous and hydrous peridotite mylonites). In tectonite shear zones, softening processes responsible for localization are probably melt-related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine-grained tectonites. In peridotite mylonites, the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep. Microstructural and petrological study of mylonite rocks reveals that reactions, either continuous net-transfer reactions (anhydrous and hydrous) or melt-rock reactions, play a key role in the formation of fine-grained material that promotes grain size sensitive creep. These reactions occur over a broad range of pressure-temperature conditions encompassing a large part of the lithospheric upper mantle. We conclude that mantle shear zones are widespread and that they reduce the (bulk) strength of the lithosphere significantly.

Modeling shear zones in geological and planetary sciences: solid- and fluid-thermal–mechanical approaches

Shear zones are the most ubiquitous features observed in planetary surfaces. They appear as a jagged network of faults at the observable brittle surface of planets and, in geological exposures of deeper rocks, they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of “mylonitized”, reduced grain sizes behind. The overall size of the entire shear network rarely exceeds kilometer scale at depth. Although mylonitic shear zones are only visible to the observer, when uplifted and exposed at the surface, they govern the mechanical behavior of the strongest part of the lithosphere below 10–15 km depth. Mylonitic shear zones dissect plates, thus allowing plate tectonics to develop on the Earth. We review the basic multiscale physics underlying mylonitic, ductile shear zone nucleation, growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization. We also discuss a framework for analytic and numerical modeling including the effects of thermal–mechanical couplings, thermal-elasticity, the influence of water and void-volatile feedback. The physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium. Positive feedback can amplify strength heterogeneities by cascading through different scales. We define basic, intrinsic length scales of strength heterogeneity such as those associated with plasticity, grain size, fluid-inclusion and thermal diffusion length scale. For an understanding ductile shear zones we need to consider the energetics of deformation. Shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes. Additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates. For the long time scale, deformational energy stored inside the shear zone through plastic dilation or crystallographic- and shape-preferred orientation consumes only a maximum of 10% of energy dissipated in the shear zones but creates structural anisotropy. Shear zones become long-living features with a long-term memory. A special role is attributed to the presence of water in nominally anhydrous minerals. We show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale. We have shown that fully coupled finite element calculations, using mechanical data from the laboratory, can reproduce the basic mode of deformation of an entire mylonitic shear zone. The next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure, without losing the large-scale constraint. Numerical methods capable of fulfilling the goal are emerging. These are adaptive wavelet techniques, and hybrid particle–finite element codes, which can be run over a computational GRID across the net.

The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps

Research Article| May 01, 2003 The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps Giulio Viola; Giulio Viola 1Department of Geological Sciences, University of Cape Town, 7701 Rondebosch, South Africa Search for other works by this author on: GSW Google Scholar Neil S. Mancktelow; Neil S. Mancktelow 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Diane Seward; Diane Seward 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Andreas Meier; Andreas Meier 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Silvana Martin Silvana Martin 3Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Giulio Viola 1Department of Geological Sciences, University of Cape Town, 7701 Rondebosch, South Africa Neil S. Mancktelow 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Diane Seward 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Andreas Meier 2Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland Silvana Martin 3Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy Publisher: Geological Society of America Received: 13 Dec 2001 Revision Received: 16 Oct 2002 Accepted: 04 Nov 2002 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2003) 115 (5): 515–532. https://doi.org/10.1130/0016-7606(2003)115<0515:TPFSAE>2.0.CO;2 Article history Received: 13 Dec 2001 Revision Received: 16 Oct 2002 Accepted: 04 Nov 2002 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Giulio Viola, Neil S. Mancktelow, Diane Seward, Andreas Meier, Silvana Martin; The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps. GSA Bulletin 2003;; 115 (5): 515–532. doi: https://doi.org/10.1130/0016-7606(2003)115<0515:TPFSAE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Pejo fault in the Italian Eastern Alps is a major sinistral transtensional fault. It marks the boundary between basement units displaying contrasting thermal histories, with Alpine (i.e., Mesozoic–Cenozoic) cooling ages preserved in the footwall juxtaposed against Variscan (i.e., Carboniferous– Permian) age in the hanging wall. Structural investigations, together with fission-track analysis, confirm a Late Cretaceous age for the Pejo fault, which excludes any direct kinematic contribution of the Pejo fault to the late Oligocene–Neogene evolution of the central-eastern segment of the Periadriatic fault. However, our results establish the importance of a major early Oligocene north-south to north-northwest–south-southeast shortening phase in the Central-Eastern Alps, which resulted in the development of new reverse shear zones, in the reactivation of the Pejo fault with a reverse motion, and in regionally important folding. The Pejo mylonites are folded on a kilometer scale around an east-northeast–trending axis. Field observations and fission-track analysis suggest a post-Oligocene age for the folding phase. Apatite fission-track data in the Pejo valley area reveal the base of a fossil apatite partial annealing zone exhumed to the surface. This finding argues for >4 km of exhumation since the Miocene, which was related to a major pulse of exhumation that began at ca. 15 Ma. This study suggests that the simple distinction between largely pre-Alpine fabrics of Variscan age in the hanging wall of the Pejo fault (Tonale nappe) and Alpine fabrics (Cretaceous) in the footwall (Campo-Ortler nappe) is not universally valid. Alpine overprinting is confined to the mylonitic shear zone itself. Deeper into the footwall, pre-Alpine structures are still well preserved. Earlier maps and interpretations based on a clear distinction between Tonale and Campo should be viewed with caution. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural geology and kinematic history of rocks formed along low-angle normal faults, Death Valley, California

Several late Cenozoic low-angle normal, or detachment, faults on the western flank of the Black Mountains are each characterized by the following, in descending structural order: (1) a hanging wall of upper Tertiary to Quaternary sedimentary and volcanic strata that displays little evidence for fault-related damage other than widely spaced planar or listric normal faults; (2) a sharp, planar, and locally striated principal slip plane forming the lower boundary of the hanging wall; (3) an upper zone—zone I—of very fine grained, fault-generated rocks composed dominantly of gouge; (4) a lower zone—zone II—of coarser-grained fault rocks consisting chiefly of foliated breccia; and (5) a variably damaged footwall, consisting of partly mylonitic Precambrian units or Tertiary plutonic rocks, that has been exhumed from depths of 10–12 km since late Miocene time. The fault rocks, which were mostly derived from the footwalls, preserve evidence for cataclastic and particulate flow. Fault rocks contain authigenic minerals but lack the cyclically deformed, mineral-filled syntectonic veins that are abundant in some other late Cenozoic high-angle and strike-slip faults. Meso- and microscale fabrics in zones I and II indicate that finite shear strains increase progressively upward, toward the principal slip plane. In a conceptual kinematic model of a shear box, displacements of the hanging wall produced a shearing flow in the fault rocks below. Some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones I and II. Since 770 ka, during the latest stages of incremental deformation in the brittle shear zones, distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes. In this respect, the detachment faults differ from inactive segments of the San Andreas system, along which displacements were progressively and irreversibly localized onto a single principal slip surface. The strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry. Strain gradients in the Black Mountains are notably asymmetric: they are present only below the principal slip planes, not above.

Reavaliação da evolução geológica em terrenos Pré- cambrianos brasileiros com base em novos dados U-Pb shrimp, parte iii: Províncias Borborema, Mantiqueira Meridional e Rio Negro-Juruena(*)

This paper discusses new SHRIMP U-Pb data for 17 key-exposures (mostly granites and orthogneisses) from the Borborema, Southern Mantiqueira (Pelotas Orogen) and Rio Negro-Juruena provinces. In the Borborema Province (Ceara state) two samples from the Cruzeta Complex TTG orthogneisses, ascribed to the Paleoproterozoic basement, were studied. One revealed Paleoarchean crystallization minimum age of ca. 3270 Ma. Accordingly, the gneiss is interpreted as the oldest continental crustal remnant already recognised in Ceara. The other sample, from the Saboeiro-Aiuaba Granite gave a crystallization age of ca. 625 Ma, suggesting the correlation of this syn-orogenic pluton with the Brasiliano II orogenic system (climax at 630 Ma). In the Paraiba state the granodioritic gneiss pluton ascribed to the Mesoproterozoic Sume Complex showed a crystallization age of ca . 640 Ma, also indicating that its evolution is associated with the Brasiliano II orogenic system. In the Pernambuco state one widespread orthogneissic unit within the Pernambuco-Alagoas Massif (Belem do Sao Francisco Complex), mapped as a component of the Mesoproterozoic Cariris Velho Orogen, yielded a crystallization age of ca. 2079 Ma and metamorphic overprinting at ca. 655 Ma (1 s ), without evidence of a Mesoproterozoic (Cariris Velhos) reworking. In the southern part of the province, near the northern margin of the Sao Francisco Craton, the Santa Maria da Boa Vista (S-type) orthogneiss yielded a crystallisation age of ca. 3070 Ma. In the southern Mantiqueira Province/Pelotas Orogen a foliated granitic pluton (mylonitic) from the Florianopolis Batholith showed Paleoproterozoic protolithic age of ca . 2175 Ma and imprecise Brasiliano age on reprecipitated overgroths. Both results match previous ages obtained on the orthogneisses protoliths from the Aguas Mornas complex, the main exposure of reworked basement within the batholith. The large, zoned calc-alkaline pluton of the Maruim Suite, confirmed its complex post-collisional history relatively to the ca. 650-630 Ma Pelotas Orogen. Two tonalitic rocks yielded crystallization ages of ca . 611 Ma and ca . 608 Ma, whereas a granitic end member a crystallisation age of ca . 580 Ma. An attempt to determine the age of rifting that originated the post-colisional foreland Itajai Basin, a resedimented volcaniclastic felsic layer was investigated. The CL imagery revealed a very heterogeneous zircon population with a dominant detrital group ranging in age between ca. 1730-1800 Ma. One euhedral volcanic crystal, yielded an apparent age of ca . 608 Ma, interpreted as the best estimate age for the onset of the Itajai volcanic-sedimentary basin, and a minimum age for the volcanogenic episode. In the SW domain of the Rio Negro- Juruena Province in Mato Grosso state, a pluton related to the Aripuana Granitic Suite, revealed a crystallization age of ca . 1540 Ma. As the granite is related to Au and Cu-Zn hydrothermal mineralization, this precise geochronologic constraint on its emplacement age is also an important clue for exploratory purposes. In the central domain of the province, in Rondonia state, two orthogneissic units exposed in the vicinities of Ariquemes and Mutum-Parana were dated at ca . 1660 Ma and ca . 1728 Ma respectively, and attributed to the Jamari Complex. In the same domain, two mylonitic leucogranites exposed close to Cacoal and Espigao do Oeste, belonging to the Serra da Providencia Intrusive Suite yielded crystallisation ages of ca . 1522 Ma and ca . 1545 Ma, respectively. The former, showed also solid-state external overgrowths dated at ca . 1400 Ma, owing to recrystallizing processes at the roots of deep seated mylonitic shear zones. Finally, two orthogneissic units with crystallization ages of ca . 1555 Ma and ca . 1545 Ma - coeval with the crytallization age of the Serra da Providencia Intrusive Suite - showed metamorphic overgrowths aged at ca . 1325 Ma, suggestive of overprinting by a regional metamorphic event, not reported in previous works on the suite, but already recognised in other associations in the region.

Implications of middle Eocene epizonal plutonism for the unroofing history of the Bitterroot metamorphic core complex, Idaho-Montana

Petrologic and geochronologic limits on the progression of east-directed extensional unroofing of the Bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation. U-Pb and 4 0 Ar/ 3 9 Ar data from epizonal plutons in the northwestern and southwestern Bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels (1-5 km) by ca. 49 Ma, while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression. U-Pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of Belt Supergroup metasedimentary rocks in the western footwall no later than 56 Ma and emplacement of the adjacent epizonal Lolo Hot Springs batholith at ca. 49 Ma in the northwestern Bitterroot Range. Tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks, suggesting that the initial stages of unroofing may have been accommodated by a planar, moderately east-dipping (30°-50°) shear zone. 4 0 Ar/ 3 9 Ar cooling data show that cooling in the western footwall was synchronous with unroofing. In contrast, we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage. Exposure of a west-dipping mylonitic shear zone along the western limb of the Bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex. If this shear zone can be correlated to the Bitterroot mylonite zone to the east, then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges.

Structural evolution of an antiformal window: the Scheiblingkirchen Window (Eastern Alps, Austria)

The Scheiblingkirchen window, a Lower Austroalpine tectonic window at the eastern margin of the Eastern Alps (Austria) was formed during Late Cretaceous continent–continent collision. Structural investigations including structural mapping, microstructural studies and texture analysis revealed a decompression-related three-stage tectonic history of Lower Austroalpine units during the formation of the Scheiblingkirchen window. (1) Intra-Lower Austroalpine nappe stacking was by top-to-the-N out-of-sequence thrusting of the Kirchberg fold nappe over the Wechsel nappe under lower greenschist facies metamorphic conditions. The structural expression of the stacking event (D 1) comprises a penetrative foliation containing a N–S trending stretching lineation, isoclinal recumbent folds trending subparallel to the stretching lineation and ultramylonites. Quartz and calcite microstructures indicate that dynamic recrystallization processes accompanied deformation. Their commonly moderately developed lattice preferred orientation record dominant slip on the prism and rhomb planes parallel to 〈 a〉. (2) Subsequent exhumation of previously stacked rocks is related to the formation of foliation-parallel mylonitic shear zones within an E–W extensional regime (D 2). Microstructures and textures suggest similar deformation temperatures during thrusting and extension. (3) A superimposed phase of NW–SE oriented horizontal shortening (D 3) was accommodated by large- and small-scale upright folding of the area around NE–SW trending axes and by backthrusting leading to the antiformal doming of the Scheiblingkirchen Window. Subsequent subvertical flattening resulting from the shortening phase led to the formation of NE–SW trending, outcrop-scale open recumbent folds. Low temperature deformation conditions as inferred from the low degree of recrystallization of quartz and calcite aggregates and the dominance of glide on the basal planes point to a cooling-related deformation event.

On the role of melt-rock reaction in mantle shear zone formation in the Othris Peridotite Massif (Greece)

A 1-km-wide peridotite mylonite shear zone is exposed in the Othris peridotite massif in central Greece. The mylonites contain lenses of relatively coarse olivine crystals, which are interpreted as remnants of the tectonite microstructure in the adjacent wall rocks. Microstructure and texture analysis using light and SEM microscopy suggests that the dominant deformation mechanism in the tectonites was dislocation creep, whereas the deformation in the mylonites was probably controlled by grain-size sensitive (GSS) creep in fine-grained (<50μm) bands consisting of a mixture of olivine and orthopyroxene. The development of the fine-grained material in the mylonites can be explained by a melt-present reaction taking place in the tectonite protolith. This reaction led to the replacement of orthopyroxene porphyroclasts by fine-grained olivine and orthopyroxene. Tectonites adjacent to the mylonite zone preserve evidence for this reaction in the form of rims of fine-grained olivine and orthopyroxene around orthopyroxene porphyroclasts. This study illustrates the significance of rheological weakening of oceanic mantle lithosphere as a result of a change from dislocation to GSS creep.

O–H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O–H isotope signatures of Alpine ultramafic rocks and eclogitic metagabbros of the Erro–Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction-related deformation along serpentinite mylonite shear zones favoured preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and mineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5–8‰) are slightly enriched in 18O compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4–7.6‰). The lowest values occur in high-pressure veins (3.5–5.7‰) and eclogitic metagabbros (3.1–5.3‰). These variations are comparable to variations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic δ18O signatures of the Erro–Tobbio rocks and a lack of oxygen isotope re-equilibration between different shear zones imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generations show a bimodal distribution in δD values: pre-eclogitic lizardite and chrysotile range from –102 to –77‰; high-pressure antigorite in the mylonites and in low strain metaperidotites range from –71 to –57‰ and –83 to –60‰, respectively. Comparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptake in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The 18O-enriched and H-depleted compositions of chrysotile and lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at the early stages of burial in a forearc setting, where slabs undergo large-scale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. The Erro–Tobbio unit thus represents an example of cycling of internally-derived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure metamorphism.

The Louzidian Normal Fault near Chifeng, Inner Mongolia: Master Fault of a Quasi-Metamorphic Core Complex

A special metamorphic core complex underlain by a low-angle strike-slip ductile shear zone is present near Chifeng in eastern Inner Mongolia, northern China. The geology of the study area is similar to that of several Cordilleran metamorphic core complexes, but contrasts in significant ways as well. A major ESE-dipping normal fault, the Louzidian Range frontal fault, formed during Late Cretaceous extension. This fault separates a crystalline footwall locally containing mylonitic basement gneisses and granitic rocks (0 to >3 km thick) from a non-metamorphic hanging wall that is distended by normal faults. However, the shear sense of the underlying mylonitic shear zone, a low-angle strike-slip zone, is not compatible with the Louzidian fault. It may be related to a pre-Cretaceous regional sinistral strike-slip event rather than the Late Cretaceous regional crustal extension common throughout eastern China. Pre-existing mylonitic fabric anisotropy appears to have controlled the development of the Louzidian normal fault. Chloritic breccias locally developed along the fault indicate that it cut deeply into the crust of northern China.