Testing geodynamic models for exhumation of high-pressure metamorphic rocks in the Alps

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

Poster presented at the VMSG symposium, November 1999 and the 5th Dutch Earth Scientific Conference (NAC V), Veldhoven, Netherlands, April 2000.

Abstract

Four groups of large-scale mechanisms have been proposed to explain the exhumation of high-pressure metamorphic rocks. Here, we investigate which of these large-scale processes may be capable of producing the observed retrograde history of three Alpine high-pressure units.

The main features of the retrograde PTt path deduced for the rocks surrounding the Alpe Arami peridotite (Central Alps) are fast decompression coeval with slowly decreasing temperature, interrupted by a sudden temperature increase during exhumation. Microstructures and metamorphic mineral assemblages in the Gran Paradiso massif (Western Alps) point to a similar PT-history but the late temperature increase is smaller. In the Voltri Massif (Ligurian Alps) the HP rocks underwent fast decompression at continuously decreasing temperatures. There is no evidence for a late stage of heating.

For the case of the Alps the only mechanism that fits our observations seems squeezing slices back up the subduction zone. The sudden increase in temperature is likely to be caused by the breakoff or detachment of the subducting slab allowing for the inflow of hot asthenospheric material close to the base of the thickened crust. We will test the hypothesis using 2D thermomechanical models to improve our understanding of exhumation processes 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).