PTt histories and exhumation of HP rocks in the Alps

F.M. Brouwer, R.L.M. Vissers, W.M. Lamb* & M.J.R. Wortel

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

Oral presentation at EGS 1999, The Hague, Netherlands, April 1999
Abstract published as: Geoph. Res. Abstr. 1 (1), 64.

Abstract

We reconstruct the retrograde metamorphic history of three units in the Alps: the country rocks of the Alpe Arami peridotite (Central Alps), the Gran Paradiso nappe (Western Alps) and a tectonic mŽlange in the Voltri massif (Ligurian Alps). All three areas underwent high-pressure metamorphism followed by rapid exhumation. The three areas show a conspicuous difference in their late thermal evolution. The thermal overprint has a complex geometry. Its peak lies in the Central Alps (the classical Lepontine dome), but a slightly earlier and less pronounced thermal peak affected rocks in the Western Alps. In the Ligurian Alps no significant late thermal peak was recorded. If the heat source was essentially the same for all three areas, it must have evolved through time to be hotter and focussed further to the east.

Any geodynamic model for the Alpine orogeny should account for the above orogen scale differences and similarities in PTt history. High-pressure metamorphism of lower crustal rocks suggests the subduction of continental lithosphere. An interplay of erosion and tectonic exhumation is needed to explain subsequent fast exhumation rates. Several tectonic scenarios are currently under investigation and we envisage break-off of the subducted lithospheric slab as a possible cause.

This research is supported by The Netherlands Geosciences Foundation (GOA). Microprobe analyses were carried out at the EU Geochemical Facility at Bristol University (UK) with funding from TMR (contract ERBFMGECT980128).