Syn-extension leucogranite deformation during convergence in the Eastern Central Alps: example of the Novate intrusion Laurent Ciancaleoni 1 and Didier Marquer 2 1 Institut de Ge ´ologie, Universite ´ de Neucha ˆtel, Rue Emile Argand, 11, CH-2007 Neucha ˆtel, Switzerland; 2 EA2642 Ge ´osciences, Universite ´ de Franche-Comte ´ Ge ´osciences, 16 route de Gray, F25030 Besanc ¸ on Cedex, France Introduction Over the last decades pluton emplace- ment in the crust has been described in a wide range of tectonic settings, either contractional (D’lemos et al., 1992; Hutton and Ingram, 1992; Tik- off and De Saint-Blanquat, 1997; Brown and Solar, 1998; De Saint- Blanquat et al., 1998), transcurrent (Hutton and Reavy, 1992; Vauchez et al., 1997) or extensional (Hutton et al., 1990; Scaillet et al., 1995; Aco- cella and Rossetti, 2002). Common to these studies is the general spatial and temporal relationships between magmatic bodies and crustal shear zones and the resolution of the Ôspace problemÕ, i.e. space needed to accom- modate magma (e.g. Brown, 1994; Tikoff et al., 1999). According to some authors, regional tectonic struc- tures are believed to exert some control on the ascent and emplace- ment of plutons (e.g. Strong and Hanmer, 1981; Hutton et al., 1990; Paterson et al., 1990; D’lemos et al., 1992; Clemens et al., 1997; Roman- Berdiel et al., 1997; Vauchez et al., 1997). In the case of the calcalkaline mag- matism in the Alps, such a relation- ship between deformation and magma emplacement has been reviewed (Rosenberg, 2004). This magmatism is spatially and temporally associated with transpressive deformation along the Periadriatic Fault System (PFS) during convergence between the Adria and Europe (e.g. Schmid et al., 1989; Berger et al., 1996). In this paper we address the syn-tectonic emplacement of a Late Alpine leucogranite, the Novate intrusion. We propose that this granite was intruded along an extensional segment of the PFS, the Forcola Fault. Geological setting In the studied area, after the Eocene nappe emplacement and exhumation (D1 and D2 regional phases, Meyre et al., 1998 and references therein), the PFS is a major Alpine tectonic feature related to the collisional his- tory of the Alps (Schmid et al., 1989 and references therein). In the Bergell region of the Eastern Central Alps (Fig. 1a), the post-nappe deforma- tion history is constrained by the ages of three Oligocene intrusions (Schmid et al., 1996b): the Bergell tonalite and granodiorite (32 and 30 Ma; Von Blanckenburg, 1992), the Sondrio tonalite (Biotite K/Ar cooling ages of 30–32 Ma; Giger, 1991) and the Novate leucogranite (24–26 Ma; Liati et al., 2000 and references therein). The Late Oligo- cene post-collisional dextral trans- pression across the Insubric Mylonites belt (D3, Milnes, 1974; Schmid et al., 1996a) has led to: (1) back-folding and back-thrusting of the Central Alps over the Southern Alps, providing a rapid exhumation of the Bergell area (Giger and Hur- ford, 1989; Berger et al., 1996); and (2) a dextral offset of 30–100 km (Heitzmann, 1987; Schmid et al., 1996a; Viola et al., 2001). Late Oligocene–Early Miocene orogen-parallel extension along the brittle–ductile Forcola normal fault post-dates back-thrusting and is coe- val with purely dextral slip on the Insubric Line (D4, Fig. 1b, Schmid et al., 1989; Meyre et al., 1998). Exposed in the northwestern end of the Novate granite, this normal fault reaches Val Mera, where it is cov- ered by Quaternary deposits, and is thought to splay further south inside the Novate granite on the basis of foliation maps (Meyre et al., 1998). The fine-grained Novate granite is a garnet-bearing S-type two-mica leuco- granite derived from partial melting of crustal rocks during the Late Alpine decompression (e.g. Oschidari and Ziegler, 1992; Von Blanckenburg et al., 1992). The Novate granite and the associated dike swarms cross-cut the late back-folding structures and Bergell Pluton fabrics (Fig. 1b, Wenk, 1973; Berger et al., 1996). The struc- tural trends in the wall rocks are undisturbed by the granite intrusion (Fig. 1b). The southeastern contact ABSTRACT The Novate intrusion is a Late Alpine leucogranite that intruded the structures related to dextral back-thrusting along the Periadriatic Fault System in the Eastern Central Alps. The Novate granite was heterogeneously deformed from amphi- bolite to greenschist facies conditions during cooling of the intrusion. The deformation inside the granite is characterized by strongly localized and anastomosed ductile shear zones sur- rounding lenses of weakly deformed granite and by late faults formed at the brittle–ductile transition. The fault kinematic analysis of conjugated shear zones suggests that the Novate leucogranite was emplaced at 25 Ma in an extensional regime along the southern tip of the Forcola Fault. A model of extensional jog opening by vertical shearing along the Forcola Fault provided the space for magma accommodation. The Novate granite is the first evidence for orogen-parallel syn- extensional leucogranite emplacement during the Oligocene collision in the Alps. Terra Nova, 18, 170–180, 2006 Correspondence: Dr L. Ciancaleoni, Lab- oratoire de ge´odynamique des rifts et des marges passives, UFR Sciences et Tech- niques, Av. Olivier Messiaen, 72085 Le Mans Cedex 09, France. Tel.: 00 41 32 718 26 57; fax: 00 41 32 718 26 00; e-mail: laurent.ciancaleoni@unine.ch 170 Ó 2006 Blackwell Publishing Ltd doi: 10.1111/j.1365-3121.2006.00677.x