Metamorphic rocks in the Alps: A thermal history and its geodynamic significance

F.M. Brouwer, R.L.M. Vissers, D.M.A. van de Zedde, M.J.R. Wortel & W.M. Lamb*

Utrecht University, Netherlands
* Texas A&M University, USA

Invited oral presentation at VMSG symposium, November 1999

Abstract

An integrated study combining field-oriented geological techniques with numerical simulations allows us to test hypotheses on mountain building processes in the Western and Central Alps. Our study focuses on pressure-temperature-time histories (PTt) of metamorphic rocks reflecting their structural and thermal evolution. Any scenario for the geodynamics of the Alps has to comply with these observables. They must also take into account that the Alps are a subduction-collision system. We illustrate our approach by discussing the metamorphic history of the Gran Paradiso nappe in the Italian Western Alps.

The history of Gran Paradiso is complicated. Continental rocks were subducted and underwent high-pressure metamorphism. The peak metamorphic conditions were about 1.4 GPa and 550 ¼C, 43 million years ago. The rocks cooled below 425 ¼C during initial decompression. After 34 million years ago they were re-heated to about 525 ¼C at pressures of 0.4 GPa. Final cooling took place during decompression. Fission track dating indicates that the rocks cooled below 250 ¼C by 30 million years ago. The most striking features of this history are the fast exhumation rate (3 mm/yr) and the stage of re-heating. This re-heating can not be explained by radiogenic heat production in view of the narrow time span available.

We propose that slab detachment took place in the Alps and that this may account for the stage of re-heating. We test this hypothesis by 2D-thermomechanical modelling. With our model we investigate the effect of various parameters on the temperature-time history (Tt) of material points within the model. The role of different breakoff mechanisms, the location of the points with respect to the subducting slab and radiogenic heating by accreted crustal material will be discussed. Thermal modelling results show that significant heating can take place at shallow levels in a subduction system.

We conclude that the stage of re-heating documented in the Gran Paradiso PTt-history may reflect slab detachment in the Alps.

The research was supported by ALW (NWO) and microprobe analyses were carried out at the EUGF at Bristol University, UK, supported by EU-TMR (contract ERBFMGECT980128).