The Valaisan controversy revisited: Multi-stage folding of a Mesozoic hyper-extended margin in the Petit St. Bernard pass area (Western Alps)

摘要

The Valaisan units, in the Alps, sample remnants of a Mesozoic basin located along the distal margin of the European plate. The extent and timing of crustal thinning and the presence/absence of rift-related magmatism in this basin have been investigated in the newly defined Punta Rossa unit, at the Petit St. Bernard pass (Western Alps). The Punta Rossa unit consists of laterally discontinuous slivers of Paleozoic basement, rare meta-pillow lavas and abundant metasedimentary breccias. These rock types are located along the multiply folded interface between serpentinized ultramafics and radiolaria-bearing garnet-chloritoid micaschists. Fault breccias pre-dating Alpine metamorphism are common at the top of the ultramafics and throughout the continental basement slivers, but they are absent in all other lithologies. These observations suggest that subcontinental mantle and Paleozoic basement were juxtaposed by brittle faulting before the overlying sediments were deposited. Rift-related thinning was probably accompanied by minor mafic magmatism, as indicated by pillow lavas directly in contact with garnet-chloritoid micaschists.The Punta Rossa unit preserves evidence of a multi-stage Alpine evolution. Post-high pressure isoclinal folding (F_(ctd)) is associated with a pervasive axial planar cleavage (S_(ctd)), defined by chloritoid and white mica. Following re-heating to ~400°C, Sctd was statically overgrown by garnet and chloritoid, prior to large-scale recumbent folding at greenschist facies conditions (F_(rec)). Interference between Fctd and Frec is responsible for the regional occurrence of basement rocks resting upon Mesozoic metasediments. Following Frec, shear zones with top-to-the-south kinematics dissected the tectonic pile, prior to the formation of upright folds with NNE-SSW trending fold axes.Therefore, the Punta Rossa unit preserves evidence of complete crustal excision in the Valaisan basin, with exhumation of ultramafics and minor mafic magmatism. Multi-stage deformation and a laterally discontinuous pre-Alpine architecture, typical of hyper extended rifted margins, are responsible for the complex outcrop pattern observed in the field.