PTt histories in the Alps: implications for exhumation mechanisms

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 EUG 10, Strasbourg, France , March 1999
Abstract published as: J. Conf. Abstr. 4 (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. Recorded peak pressures are 1.6 GPa in the Alpe Arami country rocks, 1.3 ö 1.5 GPa in the Gran Paradiso and 1.5 GPa in eclogitic blocks of the Voltri mŽlange. Structures and mineral assemblages in the matrix of that mŽlange were formed under greenschist facies conditions, indicating that the eclogitic blocks were incorporated after peak metamorphism. The three areas show a conspicuous difference in their thermal evolution during exhumation. The Alpe Arami country rocks underwent cooling and were then reheated to about 600 ¼C. The Gran Paradiso massif cooled to below 425 ¼C and was subsequently reheated to about 550 ¼C. The rocks of the Voltri mŽlange show no evidence of any late-stage thermal overprint.

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.

The late 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. Several tectonic scenarios are currently under investigation and we envisage break-off of the subducted lithospheric slab as a possible cause. This would enhance exhumation and cause a flux of hot asthenospheric material into the orogenic root over a large area. Break-off is likely to migrate laterally along the slab rather than occurring at once over the entire along strike length of the slab, which could explain lateral migration of the heat pulse.

This research is (in part) supported by The Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organisation for Scientific Research (NWO). Microprobe analyses were carried out at the EU Geochemical Facility at Bristol University (UK) with funding from TMR (contract ERBFMGECT980128).