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 similar to 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 mu m 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 mu M 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
degrees 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 Rhone-Simplon line. The Centovalli-Val
Vigezzo zone therefore represents a major tectonic zone of the
Central-Western Alps resulting from different interacting tectonic
events. (C) 2011 Elsevier B.V. All rights reserved.