Steven M. Reddy

Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere

Abstract The Palaeoproterozoic era was a time of profound change in Earth evolution and represented perhaps the first supercontinent cycle, from the amalgamation and dispersal of a possible Neoarchaean supercontinent to the formation of the 1.9–1.8 Ga supercontinent Nuna. This supercontinent cycle, although currently lacking in palaeogeographic detail, can in principle provide a contextual framework to investigate the relationships between deep-Earth and surface processes. In this article, we graphically summarize secular evolution from the Earths core to its atmosphere, from the Neoarchaean to the Mesoproterozoic eras (specifically 3.0–1.2 Ga), to reveal intriguing temporal relationships across the various ‘spheres’ of the Earth system. At the broadest level our compilation confirms an important deep-Earth event at c. 2.7 Ga that is manifested in an abrupt increase in geodynamo palaeointensity, a peak in the global record of large igneous provinces, and a broad maximum in several mantle-depletion proxies. Temporal coincidence with juvenile continental crust production and orogenic gold, massive-sulphide and porphyry copper deposits, indicate enhanced mantle convection linked to a series of mantle plumes and/or slab avalanches. The subsequent stabilization of cratonic lithosphere, the possible development of Earths first supercontinent and the emergence of the continents led to a changing surface environment in which voluminous banded iron-formations could accumulate on the continental margins and photosynthetic life could flourish. This in turn led to irreversible atmospheric oxidation at 2.4–2.3 Ga, extreme events in global carbon cycling, and the possible dissipation of a former methane greenhouse atmosphere that resulted in extensive Palaeoproterozoic ice ages. Following the great oxidation event, shallow marine sulphate levels rose, sediment-hosted and iron-oxide-rich metal deposits became abundant, and the transition to sulphide-stratified oceans provided the environment for early eukaryotic evolution. Recent advances in the geochronology of the global stratigraphic record have made these inferences possible. Frontiers for future research include more refined modelling of Earths thermal and geodynamic evolution, palaeomagnetic studies of geodynamo intensity and continental motions, further geochronology and tectonic syntheses at regional levels, development of new isotopic systems to constrain geochemical cycles, and continued innovation in the search for records of early life in relation to changing palaeoenvironments.

Crystal-plastic deformation of zircon: a defect in the assumption of chemical robustness

Orientation contrast imaging and quantitative electron backscatter diffraction analysis of a zircon collected from an Indian Ocean gabbro reveal intragrain crystallographic misorientations (up to 14°) and low-angle orientation boundaries concentrated in the zircon tips. These features represent the formation and migration of dislocations and provide the first evidence of crystal-plastic deformation of zircon under crustal conditions. Panchromatic and wavelength cathodoluminescence (CL), combined with quantitative rare earth element (REE) ion microprobe analyses, demonstrate modification of zircon REE chemistry within the areas of crystal plasticity. These data indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations that behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by several orders of magnitude. In addition, discrete ∼2 μm zones of reduced panchromatic CL correspond exactly to the position of low-angle orientation boundaries and demonstrate a defect dependence on CL signal at high dislocation densities. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for the interpretation of zircon trace-element and isotopic data.

Kinematic reworking and exhumation within the convergent Alpine Orogen

Abstract Kinematic data from the internal zones of the Western Alps indicate both top-to-SE and top-to-NW shearing during synkinematic greenschist facies recrystallisation. Rb/Sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 °C. The data indicate an initial phase of accretion and foreland-directed thrusting at ca. 60 Ma followed by almost complete reworking of thrust-related deformation by SE-directed shearing. This deformation is localised within oceanic units of the Combin Zone and the base of the overlying Austroalpine basement, and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike. The timing of deformation in this shear zone spans 9 Ma from 45 to 36 Ma. The SE-directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy, and such structures have been interpreted in the past as backthrusts in response to ongoing Alpine convergence. However, on a regional scale, the top-to-SE deformation is related to crustal extension, not shortening, and is coincident with exhumation of eclogites in its footwall. During this extension phase, deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking. Top-NW kinematics preserved within the Combin Zone show a range of ages. The oldest (48 Ma) may reflect the final stages of emplacement of Austroalpine Units above Piemonte oceanic rocks prior to the onset of extension. However, much of the top-to-NW deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone. 40Ar/39Ar data from the region are complicated due to the widespread occurrence of excess 40Ar in eclogite facies micas and partial Ar loss during Alpine heating. Reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 Ma. These ages are slightly younger than Rb/Sr deformation ages and suggest that cooling below ca. 350 °C occurred after juxtaposition of the units by SE-directed extensional deformation. Our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the Alpine Orogen. To constrain the palaeogeographic and geodynamic evolution of the Alps requires that these data be integrated with data from the more external zones of the orogen. Complexity such as that described is unlikely to be restricted to the Western Alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens. Recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens.

Naturally occurring gold nanoparticles and nanoplates

During the weathering of gold deposits, exceptionally pure, <200 nm diameter, nanoparticulate gold plates (6 nm thick) are formed. The particles display controlled growth of both size and shape and signs of assembly to form belts and sheets. The gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event. The size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids. This represents the fi rst direct observation of colloidal nanoparticulate gold in nature, confi rming this as an active mechanism of gold transport during the weathering of gold deposits.

The carbonate tectonic units of northern Calabria (Italy): a record of Apulian palaeomargin evolution and Miocene convergence, continental crust subduction, and exhumation of HP-LT rocks.

In northern Calabria (Italy), the metasedimentary succession of the Lungro–Verbicaro tectonic unit preserves mineral assemblages suggesting underthrusting to depths in excess of 40 km. Internal deformation of these rocks occurred continuously during the following decompression. Index mineral composition associated with progressively younger tectonic fabrics indicates that a substantial part of the structural evolution took place within the blueschist-facies P–T field. Despite their tectonic and metamorphic history, the rocks of the Lungro–Verbicaro Unit preserve significant sedimentary and palaeontological features allowing correlations with successions included in adjacent thrust sheets and the reconstruction of the Mesozoic continental margin architecture. The subduction–exhumation cycle recorded by the Lungro–Verbicaro Unit is entirely of Miocene age. This portion of the Apulia continental palaeomargin was involved in convergence-related deformation not earlier than the Aquitanian. The integration of our results with available constraints on the tectonic evolution of the Apennine–Calabrian Arc system suggests that subduction and most of the subsequent exhumation of the Lungro–Verbicaro Unit occurred, up to Langhian time, at maximum vertical rates in excess of 15 mm a−1. The exhumation process was then completed, at much slower rates (<2 mm a−1) in Late Miocene time, as indicated by both apatite fission-track data and stratigraphic information.

Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4°) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations. A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20–60 ppm, 30–110 ppm, and 14–36 ppm, respectively) and Th/U ratio (1.13 – 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. 207Pb/206Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent 207Pb/206Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.

Laser-probe 40Ar/39Ar investigation of a pseudotachylyte and its host rock from the Outer Isles thrust, Scotland

The formation of a friction melt or pseudotachylyte in the footwall of the Outer Isles thrust, Scotland, during thrust movement in the early Paleozoic Caledonian orogeny caused argon loss from biotites in the host gneiss. Mean 40 Ar/ 39 Ar ages for biotite grains decrease from 1450 to 923 Ma in a zone 730°C in the host gneiss; these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion. An 40 Ar/ 39 Ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin, reflecting the incorporation of partially outgassed host-rock clasts. Apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein. A weighted mean age of 430 ±6 Ma (2σ) obtained in the center corresponds closely to movement ages derived for the associated Moine thrust zone.

Complex high strain deformation in the Usagaran Orogen, Tanzania: structural setting of Palaeoproterozoic eclogites

Abstract The Palaeoproterozoic Usagaran Orogen of Tanzania contains the Earths oldest reported examples of subduction-related eclogite facies rocks. Detailed field mapping of gneisses exposed in the high-grade, eclogite-bearing part of the orogen (the Isimani Suite) indicates a complex deformation and thermal history. Deformation in the Isimani Suite can be broadly subdivided into five events. The first of these (D1), associated with formation of eclogite facies metamorphism, is strongly overprinted by a pervasive deformation (D2) at amphibolite facies conditions, which resulted in the accumulation of high strains throughout all of the exposed Isimani rocks. The geometry of foliations and lineations developed during D2 deformation are variable and have different shear directions that enable five D2 domains to be identified. Analysis of these domains indicates a geometrical and kinematic pattern that is interpreted to have formed by strain and kinematic partitioning during sinistral transpression. U–Pb SHRIMP zircon ages from a post-D2 granite and previously published geochronological data from the Usagaran eclogites indicate this deformation took place between 2000 ± 1 Ma and 1877 ± 7 Ma (at 1σ error). Subsequent greenschist facies deformation, localised as shear zones on boundaries separating D2 domains, have both contractional and extensional geometries that indicate post-1877 Ma reactivation of the Isimani Suite. This reactivation may have taken place during Palaeoproterozoic exhumation of the Usagaran Orogen or may be the result of deformation associated with the Neoproterozoic East African Orogen. U–Th–Pb SHRIMP zircon ages from an Isimani gneiss sample and xenocrysts in a “post-tectonic” granite yield ∼2.7 Ga ages and are similar to published Nd model ages from both the Tanzanian Craton and gneiss exposed east of the Usagaran belt in the East African Orogen. These age data indicate that the Isimani Suite of the Usagaran Orogen reflects reworking of Archaean continental crust. The extensive distribution of ∼2.7 Ga crust in both the footwall and hangingwall of the Usagaran Orogen can only be explained by the collision of two continents if the continents fortuitously had the same protolith ages. We propose that a more likely scenario is that the protoliths of the mafic eclogites were erupted in a marginal basin setting as either oceanic crust, or as limited extrusions along the rifted margin of the Tanzanian Craton. The Usagaran Orogen may therefore reflect the mid-Palaeoproterozoic reassembly of a continental ribbon partially or completely rifted off the craton and separated from it by a marginal basin.

Kinematic linkage between internal zone extension and shortening in more external units in the NW Alps

In the Alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c. 10 kbar, implying of the order of 60 km of relative vertical movement in the period 45–36 Ma. The horizontal component of divergence was of the order of 100 km. We use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites (the Briançonnais and/or External Zones). This thrust-related convergence during that time period was of the order of 44–56 km and therefore of the same order of magnitude as internal zone divergence. Extension may have been driven by internal density contrasts. Buoyant continental crust underneath the mafic eclogites, or dense subducted slab offset from the orogen, can induce extension. Any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen: relative plate motions in the period 45–36 Ma played a subsidiary role.

Structural evolution of the High Himalayan Gneiss sequence, Langtang Valley, Nepal

Abstract Within the High Himalayan Crystalline Sequence (HHCS) of the Langtang Valley of central Nepal, four different lithotectonic units have been identified in the field. The structurally lowest unit, immediately in the hanging wall of the Main Central Thrust (MCT) records an inverted metamorphic sequence with kyanite-grade rocks overlain by rocks containing sillimanite. Pressure-temperature-deformation histories of the different units since Himalayan collision 50 Ma ago, in combination with geochemical data, indicates that the ‘apparent’ metamorphic inversion is a result of the tectonic juxtaposition along high-temperature shear zones that accommodated top-to-southwest thrusting. It is also argued that the contacts between different sillimanite-grade units are also tectonic in origin and that the HHCS in the Langtang Valley does not represent a structurally undisturbed section through the crust. Internally the different units contain minor extensional and thrust faults which at the same structural level developed at similar metamorphic conditions. Deformation along the high temperature shear zones of the HHCS are coeval with the earliest of these minor fault zones at the lowest structural levels. With time, this deformation migrated upwards and minor faulting within the upper levels of the HHCS took place at chlorite-grade. These minor fault zones are interpreted to be the result of dynamic spreading of the developing orogenic wedge associated with changes in the geometry of this wedge due to underplating and accretion of material to the base of the metamorphic pile.