Mardi 14 Mars à 16h dans la salle des séminaires (1.10) : Daniela Rubatto (Institute of Geological Sciences, Université de Berne, Suisse)

Titre : Fast and furious or slow and steady: rates of high grade geological processes.

Metamorphism at high temperature and pressure can proceed at different rates, from several cm/year to fractions of mm/year. This talk will explore the relation between tectonic setting and rate of metamorphism during collisional orogenies. U-Th-bearing accessory minerals are ideal for determining rates and duration of high-grade metamorphism because they record and preserve overprinting stages of metamorphism, and their age can be linked to metamorphic conditions using textures, inclusions, thermometry and trace elements.

In-situ geochronology of zircon, titanite and allanite in high-pressure rocks of the Western Alps indicates short duration of metamorphic cycles. Relatively small slices of high-pressure rocks were exhumed fast, in the order of several cm/year. A double cycle of subduction-exhumation-subduction (yo-yo subduction) occurred in less than 20 million years and testifies to the dynamic nature of oblique convergence. These fast rates are in contrast with the slower rates determined for high-pressure metamorphism of larger crustal units, which were metamorphosed in a different tectonic setting.

Multiple zircon, allanite and monazite domains can form during regional anatexis of crustal rocks. Geochronology of samples from migmatites in central Australia, the central Alps and Sikkim Himalaya indicates protracted high temperature regimes lasting over 10 million years. The contrast between preservation of melt layers and the variation in absolute ages within and between samples give insight into the dynamics of anatexis in these long-lived systems. Short-lived anatexis is instead documented in the Chugach Metamorphic Complex, southern Alaska, where zircon and monazite crystallization occurred over a few million years on a ~200 km long section of the orogen. The fast Alaskan tectonic is the result of a different tectonic setting than the thick orogenic roots of the Alpine-Himalayan system.

Example of pressure-temperature-time paths of units that underwent metamorphism during subduction (Dora Maira, Alps, left) and collision (Sikkim, Himalaya, right).