40 research outputs found
Evolution de la perception de la complexité chez des adolescents à la suite de la pratique d'un jeu épistémique: le jeu Pearl Arbor, Musée de la nature, Sion
Present-day geodynamics in the bend of the western and central Alps as constrained by earthquake analysis
The contrasted tectonics of the western/central Alps is examined using a synthesis of 389 reliable focal mechanisms. The present-day strain regime is mapped and interpolated for the entire Alpine belt based on a newly developed method of regionalization. The most striking feature is a continuous area of extension which closely follows the large-scale topographic crest line of the Alpine arc. Thrusting is observed locally, limited to areas near the border of the Alpine chain. A majority of earthquakes within the Alps and its forelands are in strike-slip mode. Stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation. The stress state is confirmed to be orogen-perpendicular both for s3 in the inner extensional zones and s1 in the outer transcurrent/transpressional zones. Extensional areas are well correlated with the part of the belt which presents the thickest crust, as shown by the comparison with the Bouguer anomaly and the average topography of the belt. In the northwestern Swiss Alps, extension is also correlated with currently uplifting zones. These observations and our strain/stress analyses support a geodynamic model for the western Alps in which the current activity is mostly a result of gravitational ‘body' forces. Earthquakes do not provide any direct evidence for ongoing convergence in the Alpine system, but a relationship with ongoing activity of complex block rotations of the Apulian microplate cannot be ruled ou
Flexural isostatic response of the Alps to increased Quaternary erosion recorded by foreland basin remnants, SE France
International audienceWe test the hypothesis that flexural isostatic compensation of the mass removed by enhanced Quaternary erosion is responsible for uplift of the Western European Alps and their forelands. We use two well-preserved and well-dated (1.8 Ma) abandonment surfaces of foreland basin remnants in SE France (the Chambaran and Valensole plateaux) as passive benchmarks for tilting of the foreland. Estimating their initial slope from morphometric scaling relationships, we determine bulk post-depositional tilting of 0.5–0.8% for these surfaces. The calculated isostatic response of the Alpine lithosphere to erosional unloading, using the method recently proposed by Champagnac et al. [Geology 35 (2007) 195–198], yields a predicted tilting of 0.3–0.4% in the considered areas, explaining approximately half of the determined post-depositional tilting. Such long-term deformation being insensitive to cyclic loading/unloading because of glaciations, we suspect the other half to be related to as yet undetermined long-wavelength and long-lived tectonic process(es)
Extensional neotectonics around the bend of the Western/Central Alps: an overview
The Western Alps' active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive/compressive tectonics along its borders. We examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics, numerical modeling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening, in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the Alpine realm and in the foreland. The stress-orientation pattern is radial for σ3 in the inner, extensional zones, and for σ1 in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10-20% of the geodesy-documented deformation can be explained by the Alpine seismicity. We propose that, Alpine active tectonics are ruled by isostasy/buoyancy forces rather than the ongoing shortening along the Alpine Europe/Adria collision zone. This interpretation is corroborated by numerical modeling. The Neogene extensional structures in the Alps formed under increasingly brittle conditions. A synthesis of paleostress tensors for the internal parts of the West-Alpine Arc documents major orogen-parallel extension with a continuous change in σ3 directions from ENE-WSW in the Simplon area, to N-S in the Vanoise area and to NNW-SSE in the Briançon area. Minor orogen-perpendicular extension increases from N to S. This second signal correlates with the present-day geodynamics as revealed by focal-plane mechanisms analysis. The orogen-parallel extension could be related to the opening of the Ligurian Sea during the Early-Middle Miocene and to compression/rotation of the Adriatic indenter inducing lateral extrusio
Exploring IRSL 50 fading variability in bedrock feldspars and implications for OSL thermochronometry
International audienceOptically Stimulated Luminescence (OSL) is a well-established Quaternary dating method, which has recently been adapted to application in low-temperature thermochronometry. The Infra-Red Stimulated Luminescence (IRSL) of feldspar, which so far is the most promising target signal in thermochronometry, is unfortunately prone to anomalous fading. The fading of feldspar IRSL is at times not only challenging to measure, but also laborious to incorporate within luminescence growth models. Quantification of IRSL fading is therefore a crucial step in OSL thermochronometry, raising questions regarding (i) reproducibility and reliability of laboratory measurements of fading, as well as (ii) the applicability of existing fading models to quantitatively predict the level of IRSL field saturation in nature. Here we investigate the natural luminescence signal and anomalous fading of IRSL measured at 50 °C (IRSL50) in 32 bedrock samples collected from a variety of lithologies and exhumation settings (Alaska and Norway). We report a large span of IRSL50 fading rates between samples (g2days ranging from ∼0.5 to ∼45%/decade), which further demonstrates (i) a good reproducibility between two common fading measurement protocols, and (ii) the ability of tunnelling models to predict the level of feldspar IRSL50 field saturation in nature. We observe higher IRSL50 fading in feldspar with increasing Ca content, although other factors cannot be dismissed at present. Finally, our dataset confirms that the applicability of feldspar IRSL50 in OSL thermochronometry is limited to rapidly-exhuming settings or warm subsurface environments
Active strike-slip faulting in the Chablais area (NW Alps) from earthquake focal mechanisms and relative locations
Abstract.: The Chablais area is characterized by a complex geological setting, resulting from the transport of nappes of various internal origins (the Prealpine nappes), thrusted in Oligocene times onto the Helvetic cover of the external zones of the Alps. While the structural setting and timing of nappe emplacement are well understood, current tectonics and associated faulting remain unclear. The detailed analysis of the Bonnevaux and Samoëns earthquakes, presented in this study, constitutes a significant contribution to the active tectonics of the Chablais area. The associated seismotectonic regime appears to be constant with depth, both focal mechanisms yielding a strike-slip regime, one in the crystalline basement at around 17km depth and the other probably cross-cutting the cover/basement interface at around 5km depth. Relative location techniques, applied in this study to the seismic sequence associated to the Samoëns earthquake, represents the best way to identify active faults in a region where neotectonic evidence is scarce and controversial. The resulting seismic alignment corresponds to the E-W oriented nodal plane inferred from the Samoëns main shock focal mechanism, thus defining an active near vertical E-W dextral fault. This strike-slip regime, compared to the current regional stress field, corresponds to the one observed in the Jura/Molasse basin area but contrasts with the exclusively dextral and NE-SW-oriented transcurrent regime of the Wildhorn/Martigny regio
Deglaciation and glacial erosion: A joint control on magma productivity by continental unloading
Glacial-interglacial cycles affect the processes through which water and rocks are redistributed across the Earth's surface, thereby linking the solid Earth and climate dynamics. Regional and global scale studies suggest that continental lithospheric unloading due to ice melting during the transition to interglacials leads to increased continental magmatic, volcanic, and degassing activity. Such a climatic forcing on the melting of the Earth's interior, however, has always been evaluated regardless of continental unloading by glacial erosion, albeit the density of rock exceeds that of ice by approximately 3 times. Here we present and discuss numerical results involving synthetic but realistic topographies, ice caps, and glacial erosion rates suggesting that erosion may be as important as deglaciation in affecting continental unloading. Our study represents an additional step toward a more general understanding of the links between a changing climate, glacial processes, and the melting of the solid Earth
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Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau
Pleistocene drainage basin integration led to progressive excavation of
Tertiary-Quaternary sedimentary basins along the Yellow River in the northeastern Tibetan
Plateau. Cosmogenic burial dating of ancestral river deposits and basin fill from two key
watershed divides confirms a fluvial connection between basins at 0.5–1.2 Ma, prior to excavation
by the Yellow River. Preservation of the relict depositional surface that represents the maximum
height of basin fill allows reconstruction of the volume of eroded material across a broad region.
We quantify the isostatic response to this erosional unloading using a two-dimensional (not
one-dimensional) flexural model. Calculated maximum vertical displacements for different
effective elastic thicknesses vary from ~160 m to ~260 m near the Pleistocene spillway from the
Qinghai paleo-lake. We suggest that the isostatic response to fluvial excavation along the Yellow
River defeated local tributaries, isolated Lake Qinghai, and led to the development of an internally
drained basin in the past 0.5–1.2 Ma
The growth of northeastern Tibet and its relevance to large-scale continental geodynamics: A review of recent studies
Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10–20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation