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Thermochronological approach to the late Neogene exhumation of the European Alps

Abstract : Sediment flux from the Alps shows a sharp increase around the Mio-Pliocene boundary (~5 Ma). This observation, linked to the exhumation of the Swiss Molasse basin since ca. 5-4 Ma has led to the suggestion that the Alps experienced accelerated exhumation and isostatic uplift at the orogen scale since this time. The core objectives of this thesis are to assess whether we can document post 5 Ma exhumation of the Alps and its spatial and temporal development, and to review the different potential (tectonic or climatic) factors controlling this denudation. I have developed a novel technique that uses isoage contours associated with age-elevation relationships to exploit the unique density of fission-track ages in the western European Alps, reconstruct cooling isoage surfaces and estimate exhumation rates on the orogen scale between 13.5 and 2.5 Ma. The exhumation histories reconstructed for eight areas of the Western Alps display strong similarities in timing and rate with orogen-wide average denudation rates inferred from sediment volumes. Exhumation rates increased more than twofold since Late Miocene times, and may have been locally modulated by the distinct response of different tectonic units. I then searched for correlation between the spatial pattern of long-term exhumation rates, from the apatite fission-track record, and potential controlling parameters. In the Western Alps, long-term exhumation rates correlate strongly with present-day rates of rock uplift, implying that the rock uplift pattern observed today is ancient. I also observed that the spatial pattern of released seismic energy does not correlate with rock uplift or exhumation, which suggests that exhumation is controlled by isostatic rebound rather than by active tectonic uplift. The lack of correlation between exhumation rates and the present-day distribution of precipitation suggests that the present-day pattern is either non representative of the long-term trend or that factors other than precipitation rate dominate the intensity of exhumation. In order to study the exhumation history in more detail, I sampled two elevation profiles in the central Aar massif (Switzerland) and the western Lepontine Alps (Italy) for AFT and AHe dating which are characterised by steep age-elevation relationships around 8 and 4 Ma. I used the Pecube model to predict AFT and AHe ages according to several tens of exhumation scenarios and compared modeled and measured ages. The results of numerical modeling do not reject the hypothesis of two exhumation pulses at 9-7 and 5-3 Ma in the Aar massif. However, this signal is not detected in the Lepontine Alps, and contingent upon further flexural modeling, the exhumation recorded in the Aar massif since 5 Ma does not match the amount required to explain the denudation in the Swiss Molasse basin by flexural isostatic rebound. Rather, the data hint at an additional mechanism of rock uplift, such as the delamination of lithospheric mantle.
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Submitted on : Monday, November 17, 2008 - 11:04:37 PM
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Antoine Vernon. Thermochronological approach to the late Neogene exhumation of the European Alps. Applied geology. Université Joseph-Fourier - Grenoble I; Edinburgh University, 2008. English. ⟨tel-00339424⟩

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