Late Alpine brittle extension above the Frontal Pennine Thrust near Briancon, Western Alps

The internal HP-LT metamorphic nappe pile SE of the Pelvoux massif, built d uring the Eocene, was subsequently thrust onto the external zone during the Oligocene. This resulted in the shearing of the Subbrianconnais domain, re duced to same tectonic lenses along the Frontal Pennine Thrust. We propose that this major thrust zone was subsequently reactivated as a normal fault, controlling the collapse of the Brianconnais zone with respect to the exte rnal zone. The collapsed zones underwent important synchronous NNW-SSE and ENE-WSW normal faulting and strike-slip faulting. A major associated struct ure is the High Durance fault zone, inferred to branch onto the inverted Fr ontal Pennine Thrust at depth. Brittle extension is particularly well devel oped between this fault zone and the Frontal Pennine Thrust (FPT), while st rike-slip reactivation of normal faults is more important to the east. This "late" brittle deformation everywhere postdates schistosities associated w ith Alpine folding and thrusting. Its onset cannot be dated but the corresp onding tectonic regime seems to be still active. To discuss its Alpine sign ificance new geophysical data are needed.