No abstract

Constraining the relative roles of erosion and tectonics in the evolution of mountain belts is a challenging scientific goal. In this review article on the Western Alps, we show how it becomes possible to “bridge the gap” between the long–term (>Ma) orogenic evolution controlled by tectonics and exhumation processes and the recent geomorphological evolution that is accessible on an annual–decadal basis. Advances in mineral dating that have grown in relation to deformation in the ductile and brittle crustal fields have allowed us to constrain the evolution of deformation through time and depth. A drastic change from early collision, dominated by rapid underthrusting of the European plate, to a more stagnant syn–collisional tectonic context is documented since about 26–20 Ma by syn–kinematic phengites and vein–hosted monazites along the Alpine arc. The overall dextral kinematic context is accompanied by local extensional domains in the Simplon and High Durance Valley. Activation of the Simplon ductile fault is documented from 20 Ma, whereas the High Durance extensional system commenced after 10 Ma. The application of cosmogenic nuclide dating of incised river gorges demonstrates that the erosion pattern of the Western Alps follows a different evolution within the valleys dominated by upstream glacial erosion than in peripheral watersheds devoid of glaciers. The very low peripheral incision is found to be similar to the vertical GPS signal, suggesting equilibrium of tectonic uplift and incision, whereas the glacial–dominated valleys exhibit significantly increased and transient river incision during interglacials and a constant ongoing tectonic regime.

<p>In South-Western European Alps, although scarce, evidences of recent vertical motions suggest a slow (~0.1 mm/yr) uplift of the northern Ligurian margin, which increases towards to East from the Var river mouth to the gulf of Genova. Whether this uplift is due to active compressional tectonics, to isostatic rebound or to a combination of both is still unclear. In addition, because of the large topographic gradient, rivers have carved deep gorges in the bedrock of the SW subalpine chains. However, neither the role of vertical motion nor that of climatic changes since the LGM on river incision rates is well established.</p><p>Over the last 10 years, a dataset of <sup>10</sup>Be and <sup>36</sup>Cl based cosmic ray exposure (CRE) ages obtained on river and glacier polished surfaces in the SW French Alps has been gathered. This dataset covers several areas located in the Argentera crystalline massif, in the Nice and Castellane subalpine chains, and in the Provence domain.</p><p>We will present a compilation of these data in an attempt to answer the following questions: - what is the influence of the last glaciation on river incision rates? - Is there any evidence of a W-E gradient in incision rates that could reflect increasing uplift rates of the SW Alps and North Ligurian margin? First results tend to indicate that all river incision rates are remarkably similar since the Holocene glacial optimum, whereas two different tendencies arise before that time: catchments within the influence of Alpine glaciers tend to have larger incision rates during the last deglaciation, while at the same time catchments out of any glacial influence have slightly lower incision rates. This suggests that, at first order, the release of glacier meltwaters enhanced river incision rates downstream during the ~20-12 ka period.</p>