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Diapir Growth by Longitudinal Extension During the Development of a Thrust Salient in a Deepwater Fold and Thrust Belt: The Ainsa Oblique Zone in the Central Pyrenees


Diapirs in fold and thrust belts normally result from the reactivation of preexisting salt structures. In emergent contractional systems denudation by erosion play a significant role as a trigger mechanism. However, in deep water fold and thrust belts other mechanisms such as local extensional deformation may permit the development of diapirs. The studied example concerns with diapirs that evolved from salt-cored detachment folds by longitudinal extension in a deep to shallow water fold and thrust belt. Extension resulted from a divergent thrust transport direction related with a thrust salient developed by progressive curvature. The thrust salient and related gradient in thrust displacement was inherited from the previous Triassic salt basin configuration. The kinematic evolution of the thrust salient and related structures has been deduced from the integration of field data (structural, sedimentological and paleomagnetic data), subsurface data and analogue modeling. The N-S trending structures of the Ainsa oblique thrust and fold belt experienced a clockwise vertical axis rotation that varies from 700 in the east (Mediano anticline) to 550 in the west (Boltaña anticline). Rotation occurred from Early Lutetian to Priabonian, synchronously with thrusting and related folding and is the result of a gradient of about 50 km in the amount of thrust displacement. Such displacement gradient was produced by the distribution of Triassic evaporites, which showed a NE-SW trending pinch out. The significant clockwise vertical-axis rotations, as well as their along- and across-strike variations, recorded in the Ainsa Oblique Zone produced strike-parallel extension in the outer arcs of the two bends bounding the Ainsa Oblique Zone. In the northern bend, close to the edge of the Triassic salts, extensional faults developed, whereas in the southern bend, where the Triassic salt bed was significantly thicker, strike-parallel extension triggered transverse extensional faults and diapirs. These extensional features developed synchronously with the folds and related thrusts of the Ainsa Oblique Zone to resolve the space problems during the vertical-axis rotation. In addition, oblique to strike-slip displacements reactivated previously developed thrusts during the clockwise vertical-axis rotation. All these structures developed during the slope to deltaic and continental filling of the Ainsa basin and have a significant influence on the sedimentary dispersal patterns.