EXTENSIONAL SUBSIDENCE, CONTRACTIONAL FOLDING AND THRUST INVERSION OFTHE EASTERN CAMEROS BASIN, NORTHERN SPAIN

Tectonic analysis of the Cameros massif (northern Spain) demonstrates the need to integrate detailed structural analysis with stratigraphic studies in order to understand the development of inverted basins. The Cameros massif underwent three major tectonic events, each characteri zed by different mechanisms of deformation: The first event (Tithonian -Early Albian) manifested as continental rifting and formation of the Cameros basin and was attended by normal faulting and related folding of pre- and syn-rift sedimentary sequences (up to 9 km thick). It corr esponded to the beginning of rifting and opening of the Bay of Biscay to the north of the massif. The second event occurred at the end of th e Early Cretaceous. The basin underwent contraction, manifested as gen tle folds containing axial-plane cleavage. This contraction was due to the migration of the center of rifting away from the basin and toward the northeast, and correlates with the onset of oceanic spreading in the Bay of Biscay. Deformation happened during a low-grade metamorphis m period that reached its thermal peak after folding and cleavage form ation (90-100 Ma). The final stage of deformation (Eocene-Miocene), fo llowing a structurally quiescent period of approximately 40 Ma, occurr ed during the convergence/collision between Iberia and Europe. It form ed contractional folds related to large E/W-striking overthrusts. The Mesozoic basin was inverted and transported 25 km toward the north. Du ring this stage both unfolding and tightening of the former folds occu rred, as determined from analysis of cleavage orientation associated w ith the previous shortening.