FAULT SYSTEM ARCHITECTURE AND KINEMATIC EVOLUTION IN THE FLUMENDOSA AREA OF THE VARISCAN NAPPE ZONE, SARDINIA, ITALY: INSIGHTS INTO MODES OF INTERACTION BETWEEN CONTRACTION AND EXTENSION IN THE UPPER CRUST OF A COLLISIONAL OROGEN

Many collisional mountain belts contain complex structural features that acommodated an evolving balance between contractional and extensional processes. The Variscan Nappe Zone of Sardinia, Italy, provides an example in which early contraction may have transitioned into orogen-parallel movement, expressed in at least local extension, followed by later orogen-orthogonal extension.

In the Flumendosa area of east-central Sardinia, an early (D1) top-SSE thrust placed Cambro-Ordovician sandstone on Silurian phyllite. The lower sheet contains competent meta-arkosic sandstone stratigraphically beneath less-competent phyllite. The D1 thrust, and associated penetrative foliation, is folded by the upright, E-plunging Flumendosa antiform (D2). Within part of the lower plate in the core of this antiform are N-striking high-angle faults that have consistent W side down offset. These faults are clearly expressed in the meta-arkose, but their offset was absorbed by penetrative deformation in the overlying phyllite. The N-striking faults do not continue structurally upward and cut the D1 thrust; rather, they are limited to the lower plate. Deformational features in the phyllite suggests top-E (orogen-parallel) bulk shear based on asymmetric boudinage and S-C fabric geometry.

The geometry and kinematics of the fault system within the lower sheet suggest top-E shear in the phyllite and domino-style rotation of competent blocks of arkose bounded by high-angle faults. In this scenario, the folded D1 thrust may have acted as the roof fault in an extensional duplex. Following thrust imbrication, the composite Variscan Nappe stack may have undergone orogen-parallel extension during the later stages of its evolution, perhaps as a transitional phase from early contraction and vertical thickening (D1-D2), to late, widespread orogen-orthogonal extension and subvertical attenuation (D3).