Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

This study reviews and synthesizes the present knowledge on the Sesia–Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia–Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia–Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia–Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte–Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia–Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia–Dent Blanche nappes during the earliest Miocene.

Multiple metamorphic stages within an eclogite-facies terrane (Sesia Zone, Western Alps) revealed by Th-U-Pb petrochronology

Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels, but the details of such dynamics are controversial.To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internalWestern Alps.This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples.Two independent tectono-metamorphic ‘slices’ showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex).The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ~85 and 60 Ma, with evidence of intermittent decompression (∆P~0.5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ~85 and 75Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel.

Structure and dynamics of hybrid zones at different stages of speciation in the common vole (Microtus arvalis)

The genetic structure and dynamics of hybrid zones provide crucial information for understanding the processes and mechanisms of evolutionary divergence and speciation. In general, higher levels of evolutionary divergence between taxa are more likely to be associated with reproductive isolation and may result in suppressed or strongly restricted hybridization. In this study, we examined two secondary contact zones between three deep evolutionary lineages in the common vole (Microtus arvalis). Differences in divergence times between the lineages can shed light on different stages of reproductive isolation and thus provide information on the ongoing speciation process in M. arvalis. We examined more than 800 individuals for mitochondrial (mtDNA), Y-chromosome and autosomal markers and used assignment and cline analysis methods to characterize the extent and direction of gene flow in the contact zones. Introgression of both autosomal and mtDNA markers in a relatively broad area of admixture indicates selectively neutral hybridization between the least-divergent lineages (Central and Eastern) without evidence for partial reproductive isolation. In contrast, a very narrow area of hybridization, shifts in marker clines and the quasi-absence of Y-chromosome introgression support a moving hybrid zone and unidirectional selection against male hybrids between the lineages with older divergence (Central and Western). Data from a replicate transect further support non-neutral processes in this hybrid zone and also suggest a role for landscape history in the movement and shaping of geneflow profiles.

Diagnostics de base, mise en oeuvre et appréciation du potentiel des NTIC (radio, internet, concertation) dans les zones enclavées de la région de Daraina (sous-préfécture de Vohémar, Nord-Est Madagascar)

Rapport d'avancement - février 2005

Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy)

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to ~80 km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U-Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain δ18O values of 4.5–6 ‰, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low δ18O values between 0.2 and 3.8 ‰, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by δ18O values of 3.0 and 3.6 ‰ in antigorite from surrounding serpentinites. In metasomatised eclogites within the Lower Shear Zone, garnet rim formed at the metamorphic peak shows a shift to higher δ18O up to 6‰. The age of zircons in high-pressure veins and metasomatised eclogites constrains the timing of fluid flow at high pressure at around 45–46 Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the Lower Shear Zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals.

Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis

Keywords High-pressure fluids · Whiteschists · U–Pb dating · Oxygen isotopes · Ion microprobe · Metasomatism Introduction The subduction of crustal material to mantle depths and its chemical modification during burial and exhumation contribute to element recycling in the mantle and the formation of new crust through arc magmatism. Crustal rocks that Abstract The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahighpressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time– fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium Δ18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluidassisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction.

Contrasting PT paths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust

The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models.

Special Tax Treatment as Trade Policy: A Database on Export Processing and Special Economic Zones

Many countries treat income generated via exports favourably, especially when production takes places in special zones known as export processing zones (EPZs). EPZs can be defined as specific, geographically defined zones or areas that are subject to special administration and that generally offer tax incentives, such as duty‐free imports when producing for export, exemption from other regulatory constraints linked to import for the domestic market, sometimes favourable treatment in terms of industrial regulation, and the streamlining of border clearing procedures. We describe a database of WTO Members that employ special economic zones as part of their industrial policy mix. This is based on WTO notification and monitoring through the WTO’s trade policy review mechanism (TPRM), supplemented with information from the ILO, World Bank, and primary sources. We also provide some rough analysis of the relationship between use of EPZs and the carbon intensity of exports, and relative levels of investment across countries with and without special zones.