Exhumation and Thermal Evolution of High-Pressure Rocks From the Alps: the Response to Slab Breakoff?

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

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

Poster presented at the VMSG symposium, November 2000.

Abstract

PTt trajectories were reconstructed for high-pressure metamorphic rocks from the western and central Alps. The rocks underwent high-pressure metamorphism, followed by cooling during initial decompression, a stage of re-heating and final exhumation to the Earth's surface. The exhumation rates are initially fast in both areas, but get much slower after re-heating.

Results from thermomechanical modelling of radiogenic heating and slab breakoff suggest that the observed re-heating in the Alps could not have been achieved by radiogenic heat. Slab breakoff, however, is a possible explanation for the observed PTt paths. We suggest that slab breakoff occurred in the central Alps, and that the rocks that are now exposed in the western Alps moved from away from the central Alpine rocks afterwards.

Slab breakoff is likely to have a mechanical, as well as a thermal impact. A mechanical response to slab breakoff would precede the thermal effects, which implies that slab breakoff would affect exhumation rates prior to re-heating.

The exhumation of high-pressure rock in the Alps likely involves buoyancy-driven exhumation from sub-crustal depth. At shallower levels exhumation was probably achieved by corner flow within the orogenic wedge. Both mechanisms were assisted by erosion, and possibly rebound forces related to slab breakoff.

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).