Post-orogenic extension in southern Tuscany: distinct extensional episodes or progressive exhumation?

A detailed structural analysis of the Pseudoverrucano Unit at Punta delle Rocchette, southwestern Tuscany, Italy, reveals a complex kinematic evolution for well-ex- posed marly limestones and shales of the the Punta delle Rocchette Fm. of Middle Liassic age. These rocks, defor- med under very-low metamorphic conditions, are affected by W-verging, asymmetrical chevron folds that were first recognized in the '60s. The folds are characterized by horizontal to moderately ESE-dipping axial surfaces and sub-horizontal, NNE trending axes. These structures are intimately associated to W-dipping, low-angle normal faults cutting across the Punta delle Rocchette Fm. and small stepped faults cutting across the fold reverse limbs. The latter are mainly isolated, but occasionally they are linked to form extensional duplex structures with an overall top-to-the- west sense of shear. Low-angle normal faults trend parallel to fold axes, and are cut by a set of high-angle normal faults. Both normal fault sets are parallel in trend and dip, and are kinematically consistent to all previously de- scribed structures. The NNE trending high-angle normal faults separate the Punta delle Rocchette Fm. from the conformably underlying Upper Triassic Clastic basal Fm. (Pseudoverrucano s.s.) to the east, and from Upper Oligo- cene Macigno costiero Fm. and other terrains of the un- metamorphosed Falda Toscana Unit, to the west. Because of the close geometric and kinematic consistency between folds, low-angle and high-angle normal faults, it seems reasonable that folding of the Punta delle Rocchette Fm. was due to vertical shortening (i.e. flattening) accompanied by top-to-the-west shear. Detailed examination of the deformation fabrics associated with each recognised class of structures reveals that folding was dominantly ductile, whereas low-angle and high-angle normal faults were developed under brittle-ductile and brittle deformation conditions, respectively. Folds and normal faults postdate the development of top-to-the east duplexes hence an episode of horizontal shortening.