Variable late Neogene exhumation of the central European Alps: Low‐temperature thermochronology from the Aar Massif, Switzerland, and the Lepontine Dome, Italy

Abstract

[1] Several recent studies proposed an important increase in exhumation rate in the western European Alps since circa 5-4 Ma. In order to assess potential spatial differences in exhumation histories, we present new apatite fission track (AFT) and apatite (U-Th)/He (AHe) ages from the central Aar Massif (Guttannen area, Switzerland) and the western Lepontine Dome (Formazza area, Italy). Internal U/Th zoning in apatites explains alpha-ejection-corrected AHe ages that are older than the corresponding AFT ages in this study. A qualitative interpretation of AFT and AHe ageelevation relationships suggests a two-phase (9-7 and 5-3 Ma) exhumation scenario affecting the central Alps, with a stronger expression of the Pliocene signal in the Formazza area. However, a quantitative evaluation of exhumation scenarios using the 3-D heat equation solver Pecube highlights the existence of several other likely scenarios, casting doubt on the validity of a qualitative interpretation of the age-elevation relationships. In Formazza, scenarios suggested by quantitative modeling include continuous denudation at a rate of $750 m/Ma and a one-step exhumation rate change from 300 to 1000 m/Ma at 5 Ma. In Guttannen, they include continuous denudation at a rate of $400 m/Ma with valley deepening and two periods of higher exhumation rate (increasing from 300 to 700 m/Ma repeatedly at 9-7 and at 5-3 Ma). Contingent upon further flexural isostatic modeling, the magnitude of exhumation recorded in the axial region of the Alps since circa 5 Ma does not appear sufficient to solely explain the denudation recorded in the North Alpine Foreland Basin. Citation:

Late Neogene Erosion of the Alps: A Climate Driver?

“…Subsequent cooling rates to the present day are not well resolved in time but must have been slower, on average, with rates of ∼0.3 to 0.5 km Ma −1 . Vernon et al (2009b) obtained a similar result for the central Aar Massif. They also found evidence of rapid cooling, but earlier, with a rate of ∼1.0 km Ma −1 between 9 and 7 Mya as inferred from the elevation distribution of apatite fission-track ages.…”

“…measured, and even then, resolving such signals may depend on new techniques such as 4 He/ 3 He measurement in apatite (Shuster et al 2005). Some thermochronometry studies claim to have detected a signal, or at least to have found a better explanation for their data with a recent increase in relief , Vernon et al 2009b), but these studies have not demonstrated that an increase in relief is required to explain thermochronometric ages.…”

“…The Aar Massif, the largest of the external massifs, has been the subject of a number of fissiontrack (Michalski & Soom 1990, Seward & Mancktelow 1994, Wagner et al 1977 or combined fission-track and (U-Th)/He thermochronometry studies (Aramowicz et al 2007, Glotzbach et al 2010, Vernon et al 2009b. These latter studies have focused on the most recent exhumation history.…”

“…Colored lines are specific regions isolated by Vernon et al (2008). Colored bars are times of fast exhumation identified by 1, Vernon et al (2009b); 2, Aramowicz et al (2007); 3, Glotzbach et al (2008);4, Bigot-Cormier et al (2006). Although there is a general increase in exhumation ∼5 Mya, the variation in timing is large.…”

A Fourier approach for estimating and correcting the topographic perturbation of low-temperature thermochronological data

“…U/Th and cosmogenic nuclide dating of cave deposits, however, revealed an increased valley incision since~0.9 Ma for this region (Haeuselmann et al, 2007). (2) Another promising site is the Reuss Valley in the NW Aar Massif, for which AHe data is already available (Vernon et al, 2009). Analytical complications (U-Th zoning of apatites), however, prevent a reliable estimation of palaeo-relief from that data.…”