Laurent Ciancaleoni^* 1, Bernhard Fügenschuh ², Didier Marquer ¹

While numerous studies place emphasis on the tectonic importance of ductile deformations during nappe emplacement, only very few have been carried out on brittle and brittle-ductile deformations in the internal part of a collision belt. One point of importance during these post-nappe deformations is the partitioning of deformation between late extension and strike-slip faulting at the scale of the internal domain of a mountain belt, or in other words, the distribution of the deformation field related to vertical and lateral extrusion (e.g. Tapponnier et al., 1977; Ratschbacher et al.,1991). The Periadriatic Line and its related secondary faults is the most important fault system in the Alps. A detailed understanding of the kinematics, timing and magnitude of movements on this fault system is critical to any tectonic model of Alpine collision.

These considerations are critical in the Bergell and Insubric areas in the eastern Central Alps, where late Oligocene-early Miocene movements have a great impact on the actual nappe pile geometry. In order to better understand the late deformation field in the internal domain of the Eastern Central Alps, we used a multi-scale approach running from the regional scale to the meso-scale. The bulk kinematics during the continental collision could be inferred from the analysis of shear-zone patterns in the ductile field (Gapais et al., 1987) and from the study of fault populations in the brittle field, by use of the classical methods of fault slip analysis (Angelier, 1984; Marrett and Allmendinger, 1990). To avoid problems due to previous deformations and strong mechanical anisotropy, studies are focused on Oligocene intrusions in the Bergell and Insubric areas.

At map scale, we define the Bergell block as delimited by four main discontinuities of first order, with kinematics compatible with its late Oligocene-early Miocene northeast-directed lateral escape. The NW-SE oriented Forcola and Muretto normal faults, at the western and eastern block boundary respectively, are bounded in the north and in the south by two major strike-slip faults, the NE-SW oriented, sinistral-reverse Engadine Line and the dextral E-W oriented Insubric Line. Complex brittle faulting at the Muretto fault suggests a continuous transition from normal faulting to dextral strike-slip tectonic regime, the extension axis remaining sub-horizontal and consistently NE-SW to ENE-WSW oriented.

The internal deformation of the Bergell block, deduced from the geometry and fault-slip analysis of the second and third order fault populations, is compared with the deformation at its boundaries, defined by major discontinuities.

In the Bergell block, the overall fault pattern includes strike-slip, oblique normal and normal faults. Because most sites revealed polyhase deformations in our field area, two successive paleostress tensors were computed, based on the observation of consistent relative chronology criteria, such as successive striations on fault surfaces or cross-cutting relationships between fault sets. These stress tensors correspond to an older extension regime and younger compressional strike-slip regime. During both these events, the long axis of the strain ellipsoid remained sub-horizontal and NE-SW to ENE-WSW directed.

Thus the internal deformation of the Bergell block is quite homogeneous and points out the compatibility of the late Oligocene-early Miocene deformation field at all scales. This overall fault pattern kinematically allowed the northeast-directed lateral escape of the Bergell block, the major discontinuities bounding the system accommodated most of the displacements. From their geometry and kinematics, both the Forcola and Muretto normal faults have to be seen in the context of late Oligocene-early Miocene dextral transpression, associated with an orogen-parallel stretch and coeval with conjugate dextral and sinistral strike-slip along the Insubric and Engadine Lines, respectively (Schmid and Froitzheim, 1993). Moreover NE-SW to ENE-WSW extension is compatible with simultaneous NW-SE to NNW-SSE bulk compression, well documented inside the Bergell block. However, such lateral escape doesn't imply a strong component of gravity spreading during the indentation.

Timing constraints of the Forcola Line are given indirectly by cooling ages from this well-studied part of Central Alps. The Forcola Line must have been active at a time when the southern Tambo nappe cooled down to below about 350°C, that is between 18 and 25 Ma according to biotite Rb-Sr and K-Ar ages (Jäger et al., 1967; Purdy and Jäger, 1976; Marquer et al, 1994). The beginning of the Forcola fault activity can not be placed before 25 Ma because the 25 Ma old Novate granite (Gulson, 1973) was also deformed by this normal faulting phase. During its last stage, the Forcola phase seems to be coeval with the beginning of displacements on the Simplon normal fault, its symmetrical equivalent structure starting around 20 Ma at the western part of the structural Lepontine dome (Mancktelow, 1985).

Published apatite fission track ages from the Bergell area, bracketed in the time interval 15 to 8 Ma, indicate that the Bergell block must have been uplifted earlier and cooled down to 120°C more rapidly than the surrounding area from which it is separated by the 4 main discontinuities (Wagner et al., 1979). Break points along the slope of cooling curves for the Bergell area reveals both a decreasing cooling rate and decreasing rate of uplift of the Bergell area from 25 Ma to present time (Wagner et al., 1979). The decreasing rate of uplift could be coeval with a period of tectonic assisted exhumation followed by a period of erosion assisted exhumation through time. New zircon and apatite fission track ages from several cross-sections through the border first order faults better constrain these cooling curves for the Bergell area and the time evolution of the Periadriatic fault system during the early Neogene times.