Evidence of synextension tilting and doming during final exhumation from analysis of multistage faults (Queyras Schistes lustre ´s, Western Alps) Pierre Tricart a, * , Ste ´phane Schwartz b , Christian Sue c , Jean-Marc Lardeaux d a Labo. de Ge ´odynamique des Chaı ˆnes Alpines, Observatoire des Sc. de l’Univers de Grenoble, Univ. J. Fourier, BP53, F-38041 Grenoble, Cedex 9, France b BRGM, BP 6009, F-45060 Orle ´ans, Cedex, France c Institut de Ge ´ologie, Univ., CP 2, CH-2007 Neucha ˆtel, Switzerland d Ge ´osciences Azur, Institut de Ge ´odynamique, 250 rue A. Einstein, Sophia-Antipolis, F-06560 Valbonne, France Received 31 March 2003; received in revised form 2 February 2004; accepted 4 February 2004 Available online 2 April 2004 Abstract During the Neogene, the internal arc of the Western Alps underwent extension behind its inverted Paleogene frontal thrust while shortening affected the external arc. In the core of the internal arc, doming of eclogite-bearing gneissic nappes has formed the Dora–Maira massif. The blueschist-bearing Schistes lustre ´s of Queyras, which overlie the western flank of the Dora–Maira dome, allow observation of how the Neogene extension developed in ductile to brittle conditions. During the evolution from ‘chocolate tablet’ boudinage to cross-trend normal faulting, the extension kept a tendency to be multidirectional. The analysis of variably oriented fault families allows considering them as globally associated with the same long-lived tectonic regime. Focusing on faults bearing several generations of slickenlines and using an adapted right dihedra analysis, we propose that faulting accompanied the westwards tilting of the eastern Queyras structure. This tilting is consistent with the doming of the Dora – Maira massif, itself associated with tectonic denudation along its western flank. Therefore, two processes operated simultaneously during final exhumation: regional thinning through widespread multitrend normal faulting and more localised tectonic denudation and doming. Both are consistent with a Neogene regime dominated by vertical compression. The role of an ascending deep indenter below the internal arc remains to be documented. q 2004 Elsevier Ltd. All rights reserved. Keywords: Ductile– brittle transition; Exhumation; Late orogenic extension; Right dihedra; Queyras Schistes lustre ´s; Western Alps 1. Introduction Along the southern edge of Europe, the building of the Alps from the Late Cretaceous onwards, has resulted from subduction and subsequent collision processes in front of Adria indenter (see Schmid and Kissling, 2000 for review). During the Neogene, the external arc of the western Alps (Fig. 1) underwent increasing fold – thrust development while most of the internal arc underwent widespread extension (see Sue and Tricart, 2002 for review). In the core of the internal arc, the uplift of deeply exhumed continental basement nappes from below the oceanic Schistes lustre ´s complex, has formed the ‘gneiss dome’ structures of the Gran Paradiso and Dora–Maira, Internal Crystalline Massifs (Rolland et al., 2000; Agard et al., 2001; Schwartz et al., 2004; Tricart et al., 2004a). At the western foot of the (or Viso) ophiolitic slab that bounds the Dora– Maira massif to the west, the Pie ´mont Schistes lustre ´s of Queyras (Figs. 2 and 3) have widely recorded the Neogene ductile to brittle extension. Thanks to a specific analysis using right dihedra, we show that normal faulting accom- panied a regional-scale westward tilting in the western flank of the Dora – Maira dome that achieved this form. Therefore we confirm the close association of two processes active during the final exhumation of the Alpine innermost HP– LT metamorphic units: regional-scale extension and more localised doming and tectonic denudation. 2. Alpine tectonic context The internal arc is derived from the Tethyan Ocean and its European margin (e.g. Tricart, 1984; Lemoine et al., 1986). The Brianc ¸onnais and Dora–Maira piles of nappes 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.02.002 Journal of Structural Geology 26 (2004) 1633–1645 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ33-4-76-51-40-72; fax þ33-4-76-51-40-58. E-mail address: ptricart@ujf-grenoble.fr (P. Tricart).