EPISODIC EXHUMATION IN THE EUROPEAN WESTERN ALPS

Oligocene to Miocene sediments of the Tertiary Piedmont Basin (TPB) in north-western Italy are derived from erosion of crystalline basement rocks belonging to the Internal Western Alps (including the Ligurian Alps). Detrital white micas from different Formations in the stratigraphy have been analysed through ⁴⁰Ar/³⁹Ar thermochronology. In addition, detrital micas from sands of three present-day rivers coming off the Internal Western Alps have been analysed in order to obtain information on the age range of crystalline rocks in the mountain belts in the present day. Mica ages from the TPB sediments are representative of the age of rocks exposed in the source mountains at the time of sediment deposition (crystallisation age, or cooling age depending on the maximum temperatures reached). Our dataset includes more than 500 single grain total fusion analyses, which allows us to derive new constraints on the Western Alpine cooling/exhumation history from Oligocene time until present time.

Our work documents the existence of two substantially different cooling events in the Internal Western Alps. The first event corresponds to a regional fast cooling/exhumation prior to ca. 38 Ma occurring in the Western Alpine retro-wedge in agreement with a regional mid-Tertiary metamorphism related to HP-UHP rocks exhumation within the Western Alpine Arc. The second event involves post Eocene to Present slow cooling/exhumation. This cooling pattern produces a constant youngest signal up section for over 25 Myr. The most likely scenario, proposed for the period following the fast exhumation event, involves gentle topography rejuvenating tectonics producing continuous upward movement of rock masses.

Our data produce strong implications for lag time models. Namely, they show that increasing lag time up section is representative of slow erosion of a crustal column with an isotopically indistinguishable signal produced during a period of fast exhumation.

Furthermore, our data show a lack of overprinting which would be expected to occur due to thermal relaxation caused by radiogenic heating following crustal thickening. A possible explanation for this would be that exhumation of crustal rocks took place close to the downgoing slab. In this way the subduction zone would provide the necessary cooling to avoid thermal overprinting during exhumation.