EARLY CRETACEOUS ACCRETIONARY COMPLEX OF THE VALAISAN OCEAN, WESTERN ALPS?
Keywords:Valaisan ocean, Versoyen Complex, accretionary prism, tectonic mélange, geochemistry, U/Pb dating, Briançonnais, Petit-Saint-Bernard Pass.
Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor (“basalt” and “black schist” formation) and the other three units -grouped under the name of “schistes à blocs”- contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic mélange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Briançonnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic mélange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen “schistes à blocs” units and the absence of any Aroley limestones within the underlying tectonic mélange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former mélange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillère, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Briançonnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through compressional shearing tectonics, developing a “schistes à blocs” fabric and imbricating the former rift allochthons. During the Tertiary Alpine collision, Versoyen and Valaisan Trilogy units were deeply underthrusted, and then exhumed under the Subbriançonnais units - including the Petit-Saint Bernard and Arguerey Liassic calcschists, which underthrusted in turn the Briançonnais Front.