The relation between peak metamorphic temperatures and subsequent cooling during continent\u2013continent collision (western Central Alps, Switzerland)

Abstract

The maximum temperature (Tmax) and subsequent exhumation reflect the relations between advective and conductive heat transport, which in turn depend on the tectonic evolution. To unravel these relations in an orogen, precise Tmax data need to be combined with relative time information for the displacements of adjacent units. We present new Tmax data based on Raman spectroscopy of carbonaceous material (RSCM) and zircon fission track (FT) data, which are combined with previous data and then discussed jointly. We follow this approach in the Central Alps at the western edge of the Lepontine dome. Our analysis indicates two main tectono-metamorphic domains in this area: domain A comprises the Lower Helvetic units involving the Aar Massif; domain B is situated south of the Helvetic main thrust, in the footwall of the Simplon line. In domain A, thrusted Helvetic units were overprinted mainly by reverse faulting in the Aar Massif. The thermal evolution is related to the inversion of the former Doldenhorn basin. Tectonic transport during inversion brought into contact units with substantially different Tmax. Temperature gradients were then reduced by conductive heat transfer, but thermal overprinting during cooling involved subsequent vertical movements as well. Zircon FT data yield apparent ages between 12 and 18 Ma in the external part, but 8–9 Ma in the internal part of the Aar Massif. The youngest ages are taken as the cooling at a given temperature, whereas the other data are discussed as being only partially resetted along a temperature path in the partial annealing zone of the zircon FT. When combined with age data for Tmax and apatite FT data from the literature, the youngest group exhibits exhumation rates between 0.5 and 1.2 km/Ma in the time range between 20 Ma and today. In all of domain B, Tmax was significantly higher than in domain A. In domain B the estimated rates of exhumation are 0.8–1.0 km/Ma for the post-20 Ma time interval. Despite of different temperature evolution, the exhumation rates are similar in both domains. The study shows the necessity to combine detailed tectonic data to interpret the T–t evolution of such an area.