Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France

Ion-microprobe U–Pb zircon dating of lower-crust metasedimentary granulite are reported on samples from two localities in Europe in order to determine (a) how this environment recorded the Variscan and eo-Alpine events, and (b) whether the transition between the two orogenic cycles was continuous or separated by a gap. The samples come from enclaves hosted by Miocene volcanoes at Bournac in the French Massif Central, and from the granulitic metasedimentary basement of the Alpine Santa Lucia nappe in Corsica, on the South European paleomargin of the Ligurian branch of the Tethys Sea. The zircon ages from Bournac range between 630 and 430 Ma and between 380 and 150 Ma with a major frequency peak at 285 Ma; the zircons older than 430 Ma are interpreted as detrital, whereas those younger than 380 Ma are considered to have formed by metamorphic processes after burial in the lower crust. Zircon ages from Santa Lucia range from to 356 to 157 Ma, with exception of one inherited Archean grain, and are interpreted like the younger Bournac zircons as having been formed by metamorphic processes.

In a granulite metamorphic environment, as opposed to an anatectic environment, new zircon growth can occur in the solid state. Once Zr has been incorporated into zircon, however, it is difficult to remobilize without dissolution; thus Zr available for new zircon growth must result from the breakdown of Zr-bearing minerals during prograde and/or retrograde events. In this light, the U–Pb zircon-age probability curves are interpreted as markers for major tectonometamorphic events, as suggested by the close correspondence between peaks in the curve and geological events recorded in the upper-crust, such as magma emplacement and basin subsidence.

Evidence of a tectonometamorphic gap between the Variscan and Alpine orogeneses is provided by the Santa Lucia zircon-age probability curve, which reveals a probable interlude during the Variscan–Alpine transition between 240 and 210 Ma. Here, the peak at 240 Ma is interpreted as the very beginning of crustal extension and the low at 210 Ma as a period of quiescence prior to the formation of an active margin and oceanization.     

Record of enhanced late Pliocene tectono‐volcanic activity in the Lesser Antilles forearc basin: Insights into the long‐term behaviour of the Lesser Antilles subduction zone

In this study, we investigate the Lesser Antilles forearc basin, focusing on the late Pliocene to Pleistocene sedimentary archives in order to track the occurrence of extreme events triggered by enhanced subduction‐related tectono‐volcanic activity. We identify late Piacenzian deposits covering a major regional erosional surface, displaying sedimentary dykes and large marine boulders embedded in a mixed continental–marine matrix, characteristic of tsunamites. We interpret this episode of platform emersion and the successive cataclysmic deposits as resulting from enhanced tectonic activity at the interface of the subduction zone, synchronous with the initiation of the Lesser Antilles volcanic arc. We then discuss the implications in terms of the mechanical behaviour of the Lesser Antilles subduction zone.     

A low-dimensional subsurface model for saturated and unsaturated flow processes: ability to address heterogeneity

A low-dimensional model that describes both saturated and unsaturated flow processes in a single equation is presented. Subsurface flow processes in the groundwater, the vadose zone, and the capillary fringe are accounted for through the computation of aggregated hydrodynamic parameters that result from the integration of the governing flow equations from the bedrock to the land surface. The three-dimensional subsurface flow dynamics are thus described by a two-dimensional equation, allowing for a drastic reduction of model unknowns and simplification of the model parameterizations. This approach is compared with a full resolution of the Richards equation in different synthetic test cases. Because the model reduction stems from the vertical integration of the flow equations, the test cases all use different configurations of heterogeneity for vertical cross-sections of a soil-aquifer system. The low-dimensional flow model shows strong consistency with results from a complete resolution of the Richards equation for both the water table and fluxes. The proposed approach is therefore well suited to the accurate reproduction of complex subsurface flow processes.     

Characterization of a major refractory component of marine dissolved organic matter

Refractory carboxyl-rich alicyclic molecules (CRAM) are characterized in marine dissolved organic matter (DOM) using nuclear magnetic resonance spectroscopy and ultrahigh resolution mass spectrometry. CRAM are distributed throughout the water column and are the most abundant components of deep ocean DOM ever characterized. CRAM are comprised of a complex mixture of carboxylated and fused alicyclic structures with a carboxyl-C:aliphatic-C ratio of 1:2 to 1:7. CRAM are expected to constitute a strong ligand for metal binding, and multiple coordination across cations could promote aggregation and marine gel formation thereby affecting CRAM reactivity and the bioavailability of nutrients and trace metals. It appears CRAM are ultimately derived from biomolecules with structural similarities to sterols and hopanoids. The occurrence of CRAM in freshwater and terrestrial environments seems likely, considering the global distribution of biomolecules and the similarities of biogeochemical processes among environments.     

himu-em: The French Polynesian connection

himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.     

Eurocode 8-compatible synthetic time-series as input to dynamic analysis

Nonlinear dynamic analysis of existing or planned structures often requires the use of accelerograms that match a target design spectrum. Here, our main concern is to generate a set of motions with a good level of fit to the Eurocode 8 design spectra for France. Synthetic time series are generated by means of a non-stationary stochastic method. To calibrate the input parameters in the stochastic approach, we select a reference set of accelerograms for a Eurocode 8 type B site category from the PEER Ground-Motion Database, which are then adjusted to the target spectrum through wavelet addition. Then, we compute nonlinear seismic responses of a soil column, including pore pressure effects, and brittle and ductile structures to the stochastic time-series, the natural accelerograms and time-series generated using stationary stochastic approaches. The results of these calculations reveal considerable variability in response despite the similarities in terms of spectral acceleration.     

Adaptation of prestack migration to multi-offset ground-penetrating radar (GPR) data

In adapting the prestack migration technique used in seismic imaging to the inversion of ground‐penetrating radar (GPR) from time‐ to depth‐sections, we show that the theoretical integral formulation of the inversion can be applied to electromagnetic problems, albeit with three assumptions. The first two assumptions concern the electromagnetic characteristics of the medium, primarily that the medium must be perfectly resistive and non‐dispersive, and the third concerns the antennae radiation pattern, which is taken to be 2D. The application of this adaptation of the inversion method is confirmed by migrating actual GPR measurements acquired on the test site of the Laboratoire Central des Ponts et Chaussées. The results show good agreement with the geometry of the structures in the medium and confirm that the possible departure from the assumption of a purely resistive medium has no visible effect on the information concerning the geometry of scattering and reflecting structures. The field experiments also show that prestack migration processing is sufficiently robust with regard to the assumption of a non‐dispersive medium. The assumption of a 2D antennae radiation pattern, however, produces artefacts that could be significant for laterally heterogeneous media. Nevertheless, where the medium is not highly laterally heterogeneous, the migration gives a clear image of the scattering potential due to the geometry of structural contrasts in the medium; the scatterers are well focused from diffraction hyperbolae and well localized. Spatial geometry has limited dimensional accuracy and positions are located with a maximum error equal to the minimum wavelength of the signal bandpass. Objects smaller than one wavelength can nevertheless be detected and well focused if their dielectric contrasts are sufficiently high, as in the case of iron or water in gneiss gravels. Furthermore, the suitability of multi‐offset protocols to estimate the electromagnetic propagating velocity and to decrease the non‐coherent noise level of measurements is confirmed. Our velocity estimation is based on the semblance calculation of multi‐offset migrated images, and we confirmed the relevance of this quantification method using numerical data. The signal‐to‐noise ratio is improved by summing multi‐offset results after the addition of random noise on measurements. Thus the adaptation of prestack migration to multi‐offset radar measurements significantly improves the resolution of the scattering potential of the medium. Limitations associated with the methods used here suggest that 3D algorithms should be applied to strongly laterally heterogeneous media and further studies concerning the waveform inversion are necessary to obtain information about the electric nature of the medium.