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 Fall AGU meeting, San Francisco, December 2000.
Abstract published as Eos Trans. AGU, 81 (48), Fall Meet. Suppl., Abstract T51C-13, p. F1071 (2000).

Abstract

PTt trajectories were reconstructed for high-pressure metamorphic rocks from the European Alps. Trajectories of Gran Paradiso in the Italian western Alps, and the Lepontine gneisses in the Swiss central Alps reflect high-pressure metamorphism (P > 13 kbar and 21 kbar respectively) during Eocene times. This high-pressure stage was followed by cooling during initial exhumation and then re-heating at about 20 km depth. Gran Paradiso was heated by a few tens of degrees to about 550 C, while the Lepontine gneisses were heated by approximately 110 C, to a peak temperature around 660 C. Re-heating occurred around 33 million years ago, after which the rocks were exhumed to the Earth's surface. The exhumation rates are initially fast in both areas, about 3.3 mm/yr in Gran Paradiso, and > 7 mm/yr in the central Alps. Exhumation rates are much slower after re-heating: about 0.6 and 0.8 mm/yr respectively.

Results from 2D 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, because insufficient time is available between high-pressure metamorphism and re-heating. Results show that slab breakoff, however, is a possible explanation for the observed PTt paths. We therefore suggest that slab breakoff occurred in the central Alps, and that the rocks that are now exposed in the western Alps moved away from the central Alpine rocks afterwards, due to oblique convergence, block rotations and out-of-section transport.

Slab breakoff is likely to have a mechanical, as well as a thermal impact on the developing orogen, because the removal of subducted slab affects the vertical stresses working on the subducting plate. A mechanical response to slab breakoff would precede the thermal effects, which implies that slab breakoff would affect exhumation rates prior to re-heating.

Our results point to a scenario for the exhumation of high-pressure rock in the Alps involving buoyancy-driven exhumation from sub-crustal depth. Exhumation within the crust is likely to have been achieved by corner flow within the orogenic wedge. Both mechanisms have been assisted by erosion, and possibly by 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).