The Exhumation history of the European Alps inferred from linear inversion of thermochronometric data

Abstract

ABSTRACT. Thermochronometric data collected across the Alps over the last three decades allows for investigation of the evolution of this orogen, which is subject to changes in climate and geodynamics. Exhumation rates are inferred from the thermochronometric ages using a statistical inversion method based on the fact that the distance a sample traveled since closure is equal to the integral of the exhumation rate from the present day to the age of the sample. Exhumation rates are assumed to be spatially correlated but are free to vary through time. This results in the quantification of exhumation rates across the Alps, since 32 Ma, along with assessments of the quality of these inferences. We find that exhumation rates are initially fast in the internal arc of the Western Alps at rates up to 0.8 km/Myr at 30 Ma, decreasing at 20 Ma to 0.3 km/Myr to remain slow to the present. At the same time, around 20 Ma, rates across the External Crystalline Massifs of Western Alps increase to 0.6 km/Myr. We also find that the onset of high exhumation rates in the Tauern Window and the Lepontine Dome occurs at around 20 Ma, a time characterized by major reorganizations in the Alpine chain. A general increase in exhumation rates at around 5 Ma over the entire Alps is not confirmed. Instead we find that the Western Alps exhibit a 2 to 3 fold increase in exhumation rate over the last 2 Ma, during a recent event not seen further east, in spite of very similar topographic characteristics. We attribute this strong signal to detachment of the European slab in the Western Alps, combined with efficient glacial erosion.

Thermochronometry Across the Austroalpine‐Pennine Boundary, Central Alps, Switzerland: Orogen‐Perpendicular Normal Fault Slip on a Major “Overthrust” and Its Implications for Orogenesis

“…In the past two decades, there have been numerous attempts to use thermochronometric systems to determine uplift and exhumation rates across the Alps (e.g., Bernet et al, , , ; Fox et al, ; Hurford, ; Hurford et al, ), the accuracy of which depends upon the initial assumption that the Alps have behaved (mostly) as a steady state, erosionally dominated, tectonically inert system since at least middle Miocene, circa 15 Ma (Bernet et al, , ) to possibly early Oligocene time, circa 30 Ma (Fox et al, ), the latter time limit determined approximately by the intrusion age of the Bergell pluton. In order to model steady state behavior, some representative or “a priori” exhumation rate must be applied and modulated across the entire orogen without regard for any bounding structures that may control exhumation rate.…”

“…To the extent that the models (e.g., Fig. 6 in Fox et al, ) go far enough back in time to overlap with the differential unroofing rates of the Pennine and Austroalpine domains in eastern Switzerland, they are not an accurate description of Alpine unroofing.…”

“…For instance, there is very little difference in apatite ages moving northward from the central Lepontine region across the Infrahelvetic complex to the Aar massif, suggesting that these rocks record the postkinematic descent of the 110°C and 70°C isotherms in late Miocene to Pliocene time (Michalski & Soom, ; Rahn, ; Wagner et al, ). The postkinematic character of these low‐temperature systems is indeed evident in the fact that a number of studies have shown that post‐20 Ma cooling ages are regionally contourable and reflect patterns of broad warping across the entire Alpine orogen (e.g., Bernet et al, , ; Fox et al, ; Luth & Willingshofer, ; Mahéo et al, ; Schlunegger & Willett, ).…”

“…Despite the large scatter in cooling ages within the Austroalpine and Pennine domains, and a scarcity of ZFT and ZHe cooling ages near the boundary between them, existing data nonetheless define a marked regional contrast in cooling histories. It is clear that a general westward younging of all ages in the Pennine domain is fairly continuous and is the result of structural doming and rapid erosion of the Lepontine region after 20 Ma (e.g., Bernet et al, ; Fox et al, ; Schlunegger & Willett, ; Wiederkehr et al, ). It is less clear, however, whether the contrast in pre‐20 Ma ages between the Austroalpine and Pennine domains in the study area is, (1) a continuation of the observed younging pattern, such that progressively older ages to the east simply reflect a higher structural position along the margins of the Neogene dome (Bernet et al, ; Evans, ); or (2) the result of tectonic exhumation of the Pennine zone on faults that are along or near the main Austroalpine overthrust, at some time prior to Neogene doming (e.g., Augenstein, ; Beltrando et al, ; Nievergelt et al, ).…”

“…It is less clear, however, whether the contrast in pre‐20 Ma ages between the Austroalpine and Pennine domains in the study area is, (1) a continuation of the observed younging pattern, such that progressively older ages to the east simply reflect a higher structural position along the margins of the Neogene dome (Bernet et al, ; Evans, ); or (2) the result of tectonic exhumation of the Pennine zone on faults that are along or near the main Austroalpine overthrust, at some time prior to Neogene doming (e.g., Augenstein, ; Beltrando et al, ; Nievergelt et al, ). Unfortunately, the current en vogue smooth‐contouring approach of thermochronometric ages across the orogen (e.g., Bernet et al, , , ; Fox et al, ) cannot discriminate between the simple doming and exhumation‐by‐faulting hypotheses.…”