Constraints on the age of movement of the Montsech and Cotiella Thrusts, south central Pyrenees, Spain

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Abstract
Palaeocene to Oligocene thrusting in the Pyrenees was synchronous with the development of the South Pyrenean foreland basin (collectively the Tremp–Graus, Ainsa and Jaca basins). Sedimentological and stratigraphical evidence from the foreland basin is used to constrain the age of movement of the Montsech, Cotiella and External Sierras thrusts. Movement on the emergent imbricate fan at the Montsech front began in the Early Eocene (Illerdian) during an early phase of thrusting that detached in the Triassic evaporites in that region. This phase of thrusting defined the Tremp–Graus basin as a thrust-sheet-top basin. Following the early thrusting, Cuisian age tectonics in the Tremp–Graus and Ainsa basins was dominated by minor normal fault movements during flexural subsidence associated with thrust sheet loading. Growth of the Boltaña anticline to the west of the Ainsa basin indicates that the Ainsa basin had become detached as a thrust-sheet-top basin in the Lutetian. Movement on the Peña Montañesa thrust, and by inference on the Cotiella thrust, began in post-Lutetian times during the propagation of lower thrusts that involved pre-Triassic (Variscan) basement. Thrusting in the External Sierras caused folding and faulting of Oligocene sediments, indicating the continued southward propagation of basement-involved thrusts in the Upper Eocene and Oligocene. Many faults in the Southern Pyrenees have evidence of early normal and later reverse senses of movement indicating reactivation of normal faults during thrusting.