6 Metamorphic Petrology I

Monday, 5 November 2001
8:00 AM - 12:00 PM

THERMAL EVOLUTION OF HIGH-PRESSURE ROCKS FROM THE ALPS: THE RESPONSE TO SLAB BREAKOFF?

Paper number: 6-13
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Pressure-Temperature-time 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 as high as > 7 mm/yr in the central Alps. Exhumation rates are much slower after re-heating: about 0.6 and 0.8 mm/yr respectively.

The late thermal overprint has a complex geometry. Its peak lies in the Central Alps (the classical Lepontine dome), but a less pronounced thermal peak affected rocks in the Western Alps. 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.

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 from away from the central Alpine rocks afterwards, due to oblique convergence, block rotations and out-of-section transport.