Reconstruction cinématique de la limite de plaque diffuse Europe-Afrique pour étudier l'evolution de la Ceinture Orogénique Alpine en Méditerranée Occidentale

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Resolving timing of early convergence from low-temperature thermochronology applied to Alpine-type systems

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Oblique extension favours propagation pulses during continental break-up

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Three-dimensional Qp- and Qs-tomography beneath Taiwan orogenic belt: implications for tectonic and thermal structure

International audienceWe determined the 3-D Qp- and Qs- structure of the Taiwan orogenic belt to enhance understanding of the related tectonic and thermal structure beneath the collision zone. The inversion used t* values measured from the spectra of P and S waves from the dense Taiwan strong motion network for moderate size earthquakes (ML 4.5-5.5) to avoid source complexity. The time period, 1991-2007, includes the aftershock sequence of the 1999 Chi-Chi earthquake that provides good ray coverage in central Taiwan. Over 18 000 velocity spectra from 883 earthquakes were analysed. A non-linear least square technique is applied to the spectra for t* determination by assuming a ω-2 source model for the frequency band of 1-30 Hz. A frequency-independent Q was assumed in this study. The corner frequency of a specific event was fixed for the corresponding stations, and a quality index was defined to assure good quality data for the inversion. The results reveal the sharp variation of Qp and Qs across the recently ruptured Chelungpu Fault, and the Kaoping and Chaochou Faults in Pingtung Plain. The Q values in the hangingwall are smaller by about 85 and 110 for Qp and Qs, respectively, relative to the footwall. The fault geometry is distinctly delineated by the contour of Qp/Qs of 1.2 that extends to the depth of the geologically identified décollement structure. Beneath the Central Range, the low Qp, low Qs and high Qp/Qs features coincide well with the aseismic zone. Comparison to the recent thermomechanical numerical models of Taiwan shows that the low Q zone corresponds to the exhumation of the lower crust. The low Qs regime (high attenuation) beneath the Central Ranges at the depth of 5-22 km coincides with predicted temperatures of 400-600 °C. The Qs comparison with the major tectonic and thermal mechanical models of Taiwan reveals that the shear wave attenuation model contains comprehensive rheological and thermal information of relevance to understanding mountain building processes. This technique appears particularly useful for distinguishing strong and weak crustal regions in the absence of other constraints

Formation de la zone de necking du rift pyrénéen par localisation de la déformation ductile (Massif de l'Agly)

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Investigating possible gravity change rates expected from long-term deep crustal processes in Taiwan

We propose to test if gravimetry can prove useful\ua0in\ua0discriminating different models of\ua0long-term\ua0deep\ua0crustal\ua0processes\ua0in\ua0the case of the\ua0Taiwan\ua0mountain belt. We discuss two existing tectonic models that differ\ua0in\ua0the\ua0deep\ua0processes\ua0proposed to sustain the\ua0long-term\ua0growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above\ua0crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal\ua0gravity\ua0changes caused only by\ua0long-term\ua0rock mass transfers at depth for each of them. We show that the underplating model implies a\ua0rate\ua0of\ua0gravity\ua0change\ua0of -6 7 10-2\ua0μGal yr-1, a value that increases to 2 7 10-2\ua0μGal yr-1\ua0if\ua0crustal\ua0subduction is neglected. If the accretion of the whole Eurasian crust occurs, a\ua0rate\ua0of 7 7 10-2\ua0μGal yr-1\ua0is obtained. The two models tested differ both\ua0in\ua0signal amplitude and spatial distribution. The yearly\ua0gravity\ua0changes\ua0expected\ua0by\ua0long-term\ua0deep\ua0crustal\ua0mass\ua0processes\ua0in\ua0Taiwan\ua0are two orders of magnitude below the present-day uncertainty of land-based\ua0gravity\ua0measurements. Assuming that these annually averaged\ua0long-term\ua0gravity\ua0changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable\ua0rates\ua0of\ua0gravity\ua0change. However, as\ua0gravity\ua0is sensitive to any mass redistribution, effects of short-term\ua0processes\ua0such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any\ua0long-term\ua0deep\ua0signal. This study indicates that temporal\ua0gravity\ua0is not appropriate for deciphering the\ua0long-term\ua0deep\ua0crustal\ua0processes\ua0involved\ua0in\ua0the\ua0Taiwan\ua0mountain belt

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

International audienceEarly phases of mountain building are usually poorly constrained although they may provide insights into both the wedge rheology and the role of inheritance. In the central Pyrenean Axial Zone, a main exhumation peak at 35-30 Ma is particularly well constrained but the previous exhumation stages are much less constrained. In this study, we present new low temperature thermochronological zircon fission track and (U Th)/He data and structural observations to constrain the thermal history of the Axial Zone and its shortening sequence during the whole Alpine collision. The dataset collected along the ECORS profile suggests two main collisional cooling phases. An early stage of Alpine shortening (from 70 to 40 Ma) is characterized by low rates of cooling/exhumation in the whole Axial Zone around 5-10°C/Myr and 200 ± 50 m/Myr. During this phase, deformation was distributed within the whole Axial Zone. This result questions the occurrence of a quiescence phase in the Axial Zone. Between 40 and 20 Ma, a more localized second phase with cooling rates around 30°C/Myr and high exhumation rates >800 m/Myr is only recorded in the Maladeta massif. During this second phase, much lower denudation rates are recorded further north and south, thus suggesting rather symmetrical exhumation in the Axial Zone, controlled by underplating of the youngest crustal ramp, the Rialp thrust. This sequence of shortening, from distributed to localized deformation, appears to be characteristic in other collisional wedges as well

Oblique rifting triggered by slab tearing and back-arc extension: the case of the Alboran rift in the eastern Betics

Abstract. The tectonic evolution of highly oblique continental margins that result from back-arc extension above lithospheric STEP faults is poorly understood. Here, we investigate the case of the Alboran margin in the eastern Betics characterized by crustal thinning of 15–10 km, oblique to the direction of slab retreat. The current deformation patterns indicate that oblique back-arc rifting is underway. However, it is unclear whether these conditions are those that prevailed during the formation of the metamorphic domes and intramontane basins. We review the temporal and spatial evolution of Neogene sedimentary basins and brittle deformation in the eastern Betics, and exploit offshore seismic reflection lines to propose a crustal-scale section across the oblique margin. The history of sediment infill and rates of subsidence combined with the analyses of fault slip data, confirm that brittle extension oriented from N20° E to EW occurred during an interval spanning from the Serravallian-early Tortonian to the late Tortonian (14–8 Ma). This extension is found associated with both normal and strike-slip regimes and the evolution of the strike-slip corridors flanking the metamorphic domes. The transtensional model forms a coherent scheme linking the ductile deformation associated with metamorphic domes and the formation of EW- and NW-SE/NNW-SSE-directed sedimentary basins in the brittle upper crust during the Tortonian. The oblique extension, which is closely associated with STEP faulting, occurred during the regional convergence between Africa and Iberia since the Miocene. Only recently, around 8 Ma, the slab detached, leading to local tectonic inversion. Such a type of narrow oblique rifted margin associated with transform-like plate boundaries is not unique but is expected to be hardly preserved in the geological record due to the transient nature of retreating subduction systems

Revisiting the Wilson cycle(s) across the Africa-Iberia-Europe diffuse plate boundary: How much an orogenic lifecycle is predestined?

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Revisiting the Wilson cycle(s) across the Africa-Iberia-Europe diffuse plate boundary: How much an orogenic lifecycle is predestined?

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