DOMINO-FAULT DUPLEX IN THE CORE OF A LATE-OROGENIC ANTIFORM, VARISCAN NAPPE ZONE, SARDINIA, ITALY: A PRODUCT OF OROGEN-PARALLEL EXTENSION?

The structural architecture of collisional mountain belts reflects an evolving interplay between contractional structures that produce crustal thickening, and extensional structures that modify or attenuate the orogen and are driven by gravity acting on lateral gradients in crustal thickness or topography. The structure and kinematic history in part of the Variscan Nappe Zone of Sardinia, Italy, may provide insight into one mode of this type of interaction. In the Flumendosa area of east-central Sardinia, an early (D1) top-SSE thrust places Cambro-Ordovician sandstone and volcanic rocks on Silurian phyllite. The stratigraphy of the lower plate includes a competent meta-arkosic sandstone beneath the penetratively deformed, less-competent phyllite. The thrust and associated penetrative foliation are folded by the upright, E-plunging, orogen-parallel Flumendosa antiform (D2). Within the core of this antiform and restricted to the lower plate are a series of faults that strike north, dip steeply E or W, and have a consistent W-side-down sense of offset. These high-angle faults are well-developed in the meta-arkose, but their offset is absorbed by the penetrative deformation in the overlying phyllite. The N-striking faults do not cut the D1 thrust and propagate into the upper plate; rather, their deformation is contained solely within the lower plate. Analysis of the penetrative strain in the phyllite suggests top-E bulk shear based on S-C fabric geometry and asymmetric boudinage. Collectively, these structural features are consistent with top-E shear and domino-style rotation of competent blocks bounded by high-angle faults in the lower plate, with the folded D1 thrust acting as the roof fault of an extensional duplex. This pattern of deformation suggests that the composite Variscan Allochthon may have undergone orogen-parallel extension during the later stages of its evolution, possibly as part of the transition from early contraction and vertical thickening (D1-D2), to late, widespread orogen-orthogonal extension and subvertical attenuation (D3).