Structural analysis, clay mineralogy and K–Ar dating of fault gouges from Centovalli Line (Central Alps) for reconstruction of their recent activity

Abstract

Between the cities of Domodossola and Locarno, the complex “Centovalli Line” tectonic zone of the Central Alps outlines deformation phases over a long period of time (probably starting ~ 30 Ma ago) and under variable P–T conditions. The last deformation phases developed gouge-bearing faults with a general E–W trend that crosscuts the roots of the Alpine Canavese zone and the Finero ultramafic body. Kinematic indicators show that the general motion was mainly dextral associated with back thrusting towards the S. The < 2 μm clay fractions of fault gouges from Centovalli Line consist mainly of illite, smectite and chlorite with varied illite–smectite, chlorite–smectite and chlorite–serpentine mixed-layers. Constrained with the illite crystallinity index, the thermal conditions induced by the tectonic activity show a gradual trend from anchizonal to diagenetic conditions. The < 2 and < 0.2 μm clay fractions, and hydrothermal K-feldspar separates all provide K–Ar ages between 14.2 ± 2.9 Ma and roughly 0 Ma, with major episodes at about 12, 8, 6 and close to 0 Ma. These ages set the recurrent tectonic activity and the associated fluid circulations between Upper Miocene and Recent. On the basis of the K–Ar ages and with a thermal gradient of 25–30 °C/km, the studied fault zones were located at a depth of 4–7 km. If they were active until now as observed in field, the exhumation was approximately 2.5–3.0 km for the last 12 Ma with a mean velocity of 0.4 mm/y. Comparison with available models on the recent Alpine evolution shows that the tectonic activity in the area relates to a continuum of the back-thrusting movements of the Canavese Line, and/or to several late-extensional phases of the Rhône–Simplon Line. The Centovalli–Val Vigezzo zone therefore represents a major tectonic zone of the Central–Western Alps resulting from different interacting tectonic events.