Exhumation of a Variscan orogenic complex: insights into the composite granulitic–amphibolitic metamorphic basement of south-east Corsica (France)

A structural, petrological and geochronological (U‐Th‐Pb of zircon and monazite) study reveals that the lower crust sequences of the Variscan high‐grade basement cropping out between Solenzara and Porto Vecchio, south‐east Corsica (France) have been tectonically juxtaposed along with middle crustal rocks during the extrusion of the orogenic root of the Variscan chain. We propose that a system of high‐temperature, orogen‐parallel shear zones that developed under a transpressive dextral tectonic regime caused the exhumation of the entire sequence. This tectonic complex is thus made up of rocks having undergone different P–T conditions (eclogite‐?, high‐pressure granulite facies and amphibolite facies) at different times, reflecting the progressive foreland migration of the orogenic front. The Solenzara granulites were derived from burial of continental crust to high‐pressure (1.8–1.4 GPa) and high‐ to ultrahigh‐temperature conditions (900–1000 °C) during the Variscan convergence: U–Pb ELA‐ICPMS zircon dating constrained the timing of this metamorphism at c. 360 Ma. The gneisses cropping out at Porto Vecchio are middle crustal‐level rocks that reached their peak temperature conditions (700–750 °C at <1.0 GPa) at c. 340 Ma. The diachronism of the metamorphic events, the foliation patterns and their geometry suggest that the granulites were exhumed to middle crustal levels through channel flow tectonics under continuous compression. The amphibolite facies gneisses of Porto Vecchio and the granulites of Solenzara were accreted through the development of a major dextral mylonitic zone forming under amphibolite facies conditions: in situ monazite isotope dating (ELA‐ICPMS) revealed that this deformation occurred at c. 320 Ma and was accompanied by the emplacement of syntectonic high‐K melts. A final HTLP static overprint, constrained at 312–308 Ma by monazite U‐Th‐Pb isotope dating, is related to the emplacement of the igneous products of the Sardinia‐Corsica batholith and marks the transition from the Variscan orogenic event to the Permian extension.     

All-sky Data Assimilation of MWTS-2 and MWHS-2 in the Met Office Global NWP System.

Microwave radiances from passive polar-orbiting radiometers have been, until recently, assimilated in the Met Office global numerical weather prediction system after the scenes significantly affected by atmospheric scattering are discarded.Recent system upgrades have seen the introduction of a scattering-permitting observation operator and the development of a variable observation error using both liquid and ice water paths as proxies of scattering-induced bias. Applied to the Fengyun 3 Microwave Temperature Sounder 2(MWTS-2) and the Microwave Humidity Sounder 2(MWHS-2), this methodology increases the data usage by up to 8% at 183 GHz. It also allows for the investigation into the assimilation of MWHS-2 118 GHz channels, sensitive to temperature and lower tropospheric humidity, but whose large sensitivity to ice cloud have prevented their use thus far. While the impact on the forecast is mostly neutral with small but significant shortrange improvements, 0.3% in terms of root mean square error, for southern winds and low-level temperature, balanced by 0.2% degradations of short-range northern and tropical low-level temperature, benefits are observed in the background fit of independent instruments used in the system. The lower tropospheric temperature sounding Infrared Atmospheric Sounding Interferometer(IASI) channels see a reduction of the standard deviation in the background departure of up to 1.2%. The Advanced Microwave Sounding Unit A(AMSU-A) stratospheric sounding channels improve by up to 0.5% and the Microwave Humidity Sounder(MHS) humidity sounding channels improve by up to 0.4%.     

Assessment and Assimilation of FY-3 Humidity Sounders and Imager in the UK Met Office Global Model

China's Feng Yun 3(FY-3) polar orbiting satellites are set to become an important source of observational data for numerical weather prediction(NWP),atmospheric reanalyses,and climate monitoring studies over the next two decades.As part of the Climate Science for Service Partnership China(CSSP China) program,FY-3 B Microwave Humidity Sounder 1(MWHS-1) and FY-3 C MWHS-2 observations have been thoroughly assessed and prepared for operational assimilation.This represents the first time observations from China's polar orbiting satellites have been used in the UK's global NWP model.Since 2016,continuous data quality monitoring has shown occasional bias changes found to be correlated to changes in the energy supply scheme regulating the platform heating system and other transient anomalies.Nonetheless,MWHS-1 and MWHS-2 significantly contribute to the 24-h forecast error reduction by 0.3% and 0.6%,respectively,and the combination of both instruments is shown to improve the fit to the model background of independent sounders by up to 1%.The observations from the Microwave Radiation Imager(MWRI) also are a potentially significant source of benefits for NWP models,but a solar-dependent bias observed in the instrument half-orbits has prevented their assimilation.This paper presents the bases of a correction scheme developed at the Met Office for the purpose of a future assimilation of MWRI data.     

Insights into the Microwave Instruments Onboard the Fengyun 3DSatellite: Data Quality and Assimilation in theMet Office NWP System

正1Met Office, Exeter EX1 3PB, UK2National Satellite Meteorological Center, China Meteorological Administration, Beijing 100081, China     

Influence of deformation partitioning and metamorphic re-equilibration on P–T path reconstruction in the pre-Alpine basement of central Southern Alps (Northern Italy)

The dominant foliation (S2) in the metapelites of the Southalpine basement, near the western side of the Tertiary Adamello intrusive stock, is a Variscan greenschist facies planar fabric, slightly reworked during thick-skin Alpine tectonics. S2 is defined by muscovite and chlorite and was formed by decrenulation of pre-existing foliations, which are confined to metre-size, less-deformed domains and defined by biotite and white mica. The pre-S2 fabric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld+Bt_I+Grt+Ms_I+Pl+Qtz and St+Bt_II+Grt+Ms_II+Pl+Qtz assemblages mark D1a and D1b fabrics respectively; these developed during successive steps of a single, temperature-prograde polyphase event, rather than during separate tectonometamorphic imprints affecting different tectonic units, later coupled during a D2 greenschist facies stage. Thermobarometric estimates of assemblages formed during D1a, D1b and D2 show a transition from T  =480–540  °C (during D1a) to T  =570–660  °C (during D1b), corresponding to a slight pressure-increase from 0.75–0.95  GPa to 0.85–1.15  GPa. D2 greenschist retrogression corresponds to a pressure and temperature decrease ( T  <400–550  °C and P <0.3–0.4  GPa). This P–T– deformation–time path is inferred to be the result of uplift from a depth of c. 35  km, after Palaeozoic subduction and continental collision; it is consistent with models postulated for other metamorphic units of the Variscan Belt in Europe. This is the first documented example in the Southern Alps of temperature-prograde metamorphism before Palaeozoic collision.