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Berger A.,University of Bern | Berger A.,Copenhagen University | Gnos E.,Natural History Museum | Janots E.,ISTerre | And 4 more authors.
Tectonics | Year: 2013

Two millimeter-sized hydrothermal monazites from an open fissure (cleft) that developed late during a dextral transpressional deformation event in the Aar Massif, Switzerland, have been investigated using electron microprobe and ion probe. The monazites are characterized by high Th/U ratios typical of other hydrothermal monazites. Deformation events in the area have been subdivided into three phases: (D1) main thrusting including formation of a new schistosity, (D2) dextral transpression, and (D3) local crenulation including development of a new schistosity. The two younger deformational structures are related to a subvertically oriented intermediate stress axis, which is characteristic for strike slip deformation. The inferred stress environment is consistent with observed kinematics and the opening of such clefts. Therefore, the investigated monazite-bearing cleft formed at the end of D2 and/or D3, and during dextral movements along NNW dipping planes. Interaction of cleft-filling hydrothermal fluid with wall rock results in rare earth element (REE) mineral formation and alteration of the wall rock. The main newly formed REE minerals are Y-Si, Y-Nb-Ti minerals, and monazite. Despite these mineralogical changes, the bulk chemistry of the system remains constant and thus these mineralogical changes require redistribution of elements via a fluid over short distances (centimeter). Low-grade alteration enables local redistribution of REE, related to the stability of the accessory phases. This allows high precision isotope dating of cleft monazite. 232Th/208Pb ages are not affected by excess Pb and yield growth domain ages between 8.03 ± 0.22 and 6.25 ± 0.60 Ma. Monazite crystallization in brittle structures is coeval or younger than 8 Ma zircon fission track data and hence occurred below 280°C. Key Points Isotope dating of cleft monazite allow dating of brittle structures Monazite crystallization is coeval or younger than zircon fission track data ©2013. American Geophysical Union. All Rights Reserved. Source

Yan Y.,LISTIC | Doin M.-P.,CNRS ENS Geology Laboratory | Lopez-Quiroz P.,National Autonomous University of Mexico | Tupin F.,Telecom ParisTech | And 3 more authors.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2012

In multi-temporal InSAR processing, both the Permanent Scatterer (PS) and Small BAseline Subset (SBAS) approaches are optimized to obtain ground displacement rates with a nominal accuracy of millimeters per year. In this paper, we investigate how applying both approaches to Mexico City subsidence validates the InSAR time series results and brings complementary information to the subsidence pattern. We apply the PS approach (Gamma-IPTA chain) and an ad-hoc SBAS approach on 38 ENVISAT images from November 2002 to March 2007 to map the Mexico City subsidence. The subsidence rate maps obtained by both approaches are compared quantitatively and analyzed at different steps of the PS processing. The inter-comparison is done separately for low-pass (LP) and high-pass (HP) filtered difference maps to take the complementarity of both approaches at different scales into account. The inter-comparison shows that the differential subsidence map obtained by the SBAS approach describes the local features associated with urban constructions and infrastructures, while the PS approach quantitatively characterizes the motion of individual targets. The latter information, once related to the type of building foundations, should be essential to quantify the relative importance of surface loads, surface drying and drying due to aquifer over-exploitation, in subsoil compaction. © 2012 IEEE. Source

Regnier J.,CETE Mediterranee | Cadet H.,ISTerre | Cadet H.,Pierre Mendes-France University | Fabian Bonilla L.,University Paris Est Creteil | And 2 more authors.
Bulletin of the Seismological Society of America | Year: 2013

This article investigates the effects of the nonlinear behavior of soils on site response, through various earthquake recordings from the KiK-net database in Japan. This network is composed of more than 688 surface-borehole instruments, from which a characterization of the shear- and compressive-wave velocity profiles down to the borehole depth is available. We selected events with a peak ground acceleration (PGA) at the downhole station of < 10 cm=s2 in order to characterize the linear soil behavior by computing the surface to downhole spectral ratios at each site. Modifications of site-response curves computed with strong events (PGA > 50 cm=s2) compared to the linear characterization are supposed to be caused by nonlinear soil behavior. To describe the effects of soil nonlinear behavior on site response per event, we propose the percentage of modification (either amplification or deamplification) of the site-response curve compared to the linear evaluation (PNLev, percentage of nonlinearity) and the associated shift frequency (Shev). These parameters are used to estimate the probability that nonlinear site response is significantly different that the linear counterpart. We find that, regardless the site, this probability is important even for low input-motion PGA (values equal to or larger than 30 cm=s2 at downhole sensor). This indicates that nonlinear soil behavior must be taken into account in site-response evaluation for moderate to strong motion. In addition, for 54 sites of the KiK-net database that have recorded at least two strong events (PGA at the downhole station > 50 cm=s2), we define four additional parameters that characterize the effects of soil nonlinear behavior on site responses for each site: (1) a PGA threshold (PGAth), defined as the PGA value for which PNLev is higher than 10%; (2) a site-specific PNL for a PGA of 50 cm=s2 (PNLsite); (3) a site-specific shift of the predominant frequency for a PGA of 50 cm=s2 (Shsite); and (4) a frequency from which we observe deamplification between nonlinear and linear site responses (fNL). We observe that nonlinear soil behavior can increase the amplification at frequencies below fNL. We find that fNL lies in between the fundamental and the predominant resonance frequencies of the site response and that sites having VS contrast close to the surface trigger nonlinear behavior at a lower input-motion PGA threshold. These results suggest that nonlinear behavior occurs mostly in the superficial soil layers. Furthermore, by investigating the nonlinear behavior of soils on earthquake horizontal-to-vertical spectral ratios at the surface, we find that they can give satisfactory results (equivalent to the analysis of borehole site responses) for the evaluation of the fNL frequency and shift frequency (Shsite), which indicates that part of the results obtained in this study can be extended to other databases without downhole sensors. Source

Mordret A.,CNRS Paris Institute of Global Physics | Shapiro N.M.,CNRS Paris Institute of Global Physics | Singh S.S.,CNRS Paris Institute of Global Physics | Roux P.,ISTerre | Barkved O.I.,British Petroleum
Geophysics | Year: 2013

We applied the Helmholtz tomography technique to 6.5 hours of continuous seismic noise record data set of the Valhall Life of Field network. This network, that has 2320 receivers, allows us to perform a multifrequency, high-resolution, ambient-noise Scholte wave phase velocity tomography at Valhall. First, we computed crosscorrelations between all possible pairs of receivers to convert every station into a virtual source recorded by all other receivers. Our next step was to measure phase traveltimes and spectral amplitudes at different periods from crosscorrelations between stations separated by distances between two and six wavelengths. This is done in a straightforward fashion in the Fourier domain. Then, we interpolated these measurements onto a regular grid and computed local gradients of traveltimes and local Laplacians of the amplitude to infer local phase velocities using a frequency dependent Eikonal equation. This procedure was repeated for all 2320 virtual sources and final phase velocities were estimated as statistical average from all these measurements at each grid points. The resulting phase velocities for periods between 0.65 and 1.6 s demonstrate a significant dispersion with an increase of the phase velocities at longer periods. Their lateral distribution is found in very good agreement with previous ambient noise tomography done at Valhall as well as with a full waveform inversion P-wave model computed from an active seismic data set. We put effort into assessing the spatial resolution of our tomography with checkerboard tests, and we discuss the influence of the interpolation methods on the quality of our final models. © 2013 Society of Exploration Geophysicists. Source

Lupker M.,French National Center for Scientific Research | France-Lanord C.,French National Center for Scientific Research | Lave J.,French National Center for Scientific Research | Bouchez J.,IPG Inc | And 6 more authors.
Journal of Geophysical Research: Earth Surface | Year: 2011

The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments. The model shows that ca. 390 (30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D 50 and D84 are 27 (±4) and 123 (±9) μm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large-scale geochemical budgets. Copyright 2011 by the American Geophysical Union. Source

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