Regional brittle extension in Quaternary sediments of Lanslebourg (Haute-Maurienne valley, western Alps)

Brittle tectonics of the internal zones of the Western Alps is characterized by important normal faulting and minor transcurent faulting. Two different directions of extension occur through time, but the age of transition remains unconstrained. In order to fill this gap, and to provide new neotectonic data, we studied faulted Quaternary peri-glacial deposits in the core of the W-Alpine belt (Contamines Quarry, Lanslebourg, Haute-Maurienne/Vanoise area). These glacio-lacustrine and fluvio-glacial sediments are capped by a ground moraine, which is related to a small glacier advance that occurred during the latest deglaciation of the Arc Valley (Pleistocene-Holocene transition). The faults cross-cut the moraine and are younger than this latest glacier advance. They are not affected by ice-related loading and movement. These faults show normal components with moderate to high dips (50 o to 80o) and small to medium vertical offsets (mm- to m- scale). Offsets are visible due to contrasted lithologies of the sediments. Conjugated dihedra with two fault orientations are recognized, namely N-S and NW-SE. These fault orientations are consistent with an E-W to ENE-WSW direction of extension, and an unknown (but possibly minor) part of transcurrence. This direction of extension is parallel to the Arc Valley, and to the downstream direction of the paleo-glacier, as well as to the shear direction of the loadcast structures created by the ice weight and flow. The different possible origins of the faults are discussed, but their orientation is likely to be tectonically driven. The directions of faulting are consistent with N-S and NW-SE regional faulting within the Cretaceous Schistes lustres inferred by in situ analyses and remote sensing measurements. The distribution of faults in the quarry near a km-scale fault, as well as their cross-cutting relationships to the moraine and the shear figures, suggest that the faulting of the Quaternary sediments is guided by a regional (i.e. tectonic) stress field. Since the faulting is younger than the latest glacier advance (ca. 10 to 15 ky) and is consistent with the current orogen-perpendicular extension, we propose that the seismogenic extension occurred from (at least) the latest Pleistocene onward.