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

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

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

Oral presentation at MSG meeting "Exhumation of metamorphic terranes", Rennes, France, August-September 1999

Abstract

At least four groups of large-scale mechanisms have been proposed to explain the exhumation of high-pressure metamorphic rocks. These mechanisms include: whole-lithosphere extension (e.g. McKenzie & Bickle, 1988), extensional collapse (e.g. Dewey, 1988) possibly driven by lithospheric root detachment (cf. Platt & Vissers, 1989), orogenic wedge tectonics (e.g. Platt, 1986), and the movement of HP rocks up the subduction zone by unspecified mechanisms involving squeezing of slices of lithosphere back up the subduction zone (e.g. Ernst, 1974). Here, we investigate which of these large-scale processes may be capable of producing the observed retrograde history of three Alpine high-pressure units in the Central, Western and Ligurian Alps.

The main features of the retrograde PTt path deduced for the country rocks surrounding the Alpe Arami peridotite body (Central Alps) are fast decompression coeval with slowly decreasing temperature, interrupted by a sudden temperature increase during ongoing exhumation. Microstructures and metamorphic mineral assemblages in the Gran Paradiso massif (Western Alps) point to a similar PT-history (also Borghi et al., 1996) but the late temperature increase is slightly 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 may allow for both decompression at decreasing temperatures and the late heat pulse seems squeezing slices back up the subduction zone (Ernst, 1974). 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 (Wortel & Spakman, 1992; Von Blanckenburg & Davies, 1995). This hypothesis needs to be rigorously tested against observations. Some basic work has been carried out (e.g. Davies & Von Blanckenburg, 1998; Grasemann et al., 1998). We will test the hypothesis using 2D thermomechanical models to improve our understanding of exhumation processes in the Alps. This study shows the importance of using a combination of tools (detailed petrological information and computer models) to validate geodynamic hypotheses.

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).

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