EVOLUTION OF THE MULARGIA LAKE AREA OF THE VARISCAN NAPPE ZONE, SARDINIA ITALY: THRUSTING, NORMAL FAULTING, OR BOTH?

Many collisional orogens exhibit a progression from contraction to extension in later stages of their tectonic development, and the Variscan orogen of Sardinia, Italy, provides an example. Early Carboniferous SSW-directed thrusting in the Variscan Nappe Zone of central Sardinia produced a composite allochthon composed of four nappes (lowest to highest): the Rio Gruppa, Gerrei, Meano Sardo, and Barbagia. The SSW-directed contraction was followed by W-directed emplacement of the Sarrabus nappe over the southern portion of the existing nappe stack. Analysis of the structure and kinematic history or the Nappe Zone suggests that both contractional and extensional processes contributed to the final architecture of the nappe stack. In the Mulargia Lake study area, the Rio Gruppa, Gerrei, and Meano Sardo nappes display a fault geometry that may have resulted from three different scenarios of fault interaction, each of which predicts a different kinematic pattern on the faults bounding the Gerrei nappe. Hypothesis one: both the upper and lower bounding faults of the Gerrei are SSW-directed thrust faults, implying that the Meano Sardo unit was emplaced over the Gerrei unit by an out-of-sequence, foreland-directed thrust which truncated the trailing edge of the Gerrei. Hypothesis two: the upper bounding fault of the Gerrei unit is a NNE-directed normal fault and the lower bounding fault is a SSW-directed thrust fault, suggesting that initial imbrication of the composite allochthon was followed by hinterland-directed normal faulting. Hypothesis three: both the upper and lower bounding faults of the Gerrei unit display NNW-directed movement, consistent with normal-sense reactivation of early thrusts followed by hinterland-directed normal faulting which cut through portions of the nappe stack. Kinematic analysis of the faults and shear zones will be used to test these ideas and explore the interaction of contractional and extensional processes in the evolution of the Variscan Nappe Zone of Sardinia.