Apatite ®ssion-track analysis of Neogene exhumation in northern Corsica (France) W. Cavazza, 1 * M. Zattin, 2 B. Ventura 2 and G. G. Zuffa 2 1 Dipartimento di Scienze Geologiche, Universita Á della Basilicata, 85100 Potenza, Italy; 2 Dipartimento di Scienze della Terra e Geologico- ambientali, Universita Á di Bologna, 40126, Bologna, Italy Introduction The island of Corsica is divided into two distinct geological terranes (Durand-Delga, 1978) (Fig. 1). The western portion is characterized by Carboniferous±Permian granitoid rocks and acid volcanic rocks related to the Hercynian orogeny, with Precambrian±middle Palaeozoic meta- morphic host rocks and scattered outcrops of Palaeozoic sedimentary rocks. The eastern portion, Alpine Corsica, is a nappe stack dominated by Jurassic oceanic crust and its sedi- mentary cover that was metamor- phosed during the Alpine orogeny (see Gibbons et al., 1986, for a review). Major thrusting in the late Eocene is responsible for much of the now visible nappe stacking and metamorphism (Caron, 1994). Relat- ively small remnants (e.g. the auto- chthonous and para-autochthonous clastic units of the Balagne region of Fig. 2) of an Eocene foreland basin are present between the Hercynian crystalline basement complex and Alpine Corsica. This paper reports the results of apatite ®ssion-track analysis along a W±E-orientated traverse from the Balagne region to the town of Bastia (Figs 1, 2), and provides new con- straints on the exhumation history of the Alpine orogenic wedge and its adjacent foredeep and foreland. This traverse has been the object of much research and probably constitutes the best-studied part of Corsica. The fol- lowing is a concise description of the major geological domains present along the traverse from west to east (Fig. 2). 1 The autochthonous, relatively undeformed basement complex of the European plate (Hercynian Corsica). 2 Remnants of the Balagne fore- land basin with mid-Eocene sedimen- tary deposits, overthrust by W-verging nappes comprising Jurassic ophiolites and their Cretaceous±Eocene, unmeta- morphosed sedimentary cover. 3 The highly deformed parts of the European continental margin, inclu- ding the dome-shaped Tenda massif and slices of crystalline basement of the Oletta-Serra di Pigno-Farinole crystalline unit interspersed in the `Schistes LustreÂs' nappe (see below). The Tenda massif is mostly made of Hercynian granitoid rocks and Palaeozoic volcanics, recrystallized during westward Alpine thrusting under medium-pressure conditions and later overprinted by ductile exten- sion in greenschist facies along its eastern border during the Oligocene (Waters, 1990; Fournier et al., 1991; Brunet et al., 2000). 4 The Nebbio nappe, composed of rock units similar to those of the Balagne nappe and similarly unmeta- morphosed. The Nebbio nappe over- lies the previously deformed and metamorphosed Schistes Lustre s nappe and is unconformably overlain by the Miocene Saint Florent calcar- enites. The Nebbio, Balagne and Macinaggio (NE Corsica; not shown in Figs 1 and 2) nappes are the uppermost, unmetamorphosed tec- tonic units of the Alpine orogenic wedge. The basal thrust faults of these nappes, emplaced during the Eocene (e.g. Durand-Delga, 1984), were tec- tonically inverted as low-angle detach- ment faults during the Oligocene (Daniel et al., 1996). 5 The Schistes LustreÂs nappe com- posed of ophiolitic sequences covered by metamorphosed marine sedimen- tary rocks. It comprises eclogitic associations (Caron et al., 1981; LahondeÁ re, 1988; Fournier et al., 1991) of Late Cretaceous age (Laho- ndeÁre and Guerrot, 1997) which were retrogressed to the blueschist facies (Gibbons et al., 1986; LahondeÁ re, 1988; Waters, 1989) during Eocene time (Brunet et al., 2000). Westward emplacement of the Schistes LustreÂs nappe onto the Hercynian basement and its Eocene sedimentary cover ended in late Eocene time (Durand- Delga, 1978). Methods and samples The apatite ®ssion-track method is based on measurements of the areal density and length distribution of etched linear tracks produced by spontaneous ®ssion of trace amounts of 238 U in apatite (see Wagner and Van den Haute, 1992, for a sum- mary). Complete annealing of apatite ABSTRACT Apatite ®ssion-track analyses along a W±E-orientated transect across northern Corsica indicate an important episode of crustal exhumation in late early Miocene time. Samples taken from the Alpine orogenic wedge, from the adjacent foreland basin and from the crystalline basement complex ¯ooring the basin are completely reset. This implies that a ³ 2.0±2.3-km-thick crustal section made of thrust sheets and/or autochthonous foreland deposits has been removed by erosion since early Miocene time. A geometric projection of this lost cover towards the west indicates that all of northern Corsica was covered either by Alpine nappes or middle Eocene foreland deposits. Fission-track ages are the same across the main boundary fault system separating the Alpine orogenic wedge and the foreland, indicating the absence of signi®cant differential vertical dis- placement between upper and lower plates during Neogene unroo®ng. Terra Nova, 13, 51±57, 2001 *Correspondence: W. Cavazza, Dipartimento di Scienze Geologiche, UniversitaÁ della Basil- icata, 85100 Potenza, Italy. Tel.: +39/ 0971206178; fax: +39/0971206077; e-mail: cavazza@unibas.it Ó 2001 Blackwell Science Ltd 51