During the late Oligocene and early Miocene, the frontal thrust complex of one of the south Pyrenean thrust sheets (the Guarga or Gavarnie sheet) became emergent to form part of the northern margin of the Ebro Basin. The principal structures in the frontal thrust complex show a progressive development from a simple recumbent fold in the west to an emergent thrust in the east. Southward movement at the thrust front decreased from east to west and in the west the thrust tip line is buried. These structures influenced Miocene continental sedimentation on two scales. The emergent thrust front formed an orographic barrier to rivers draining the higher Pyrenees. These rivers passed around the barrier rather than across it, and the positions of the apices of fluvial distributary systems in the Ebro Basin were thus controlled by the low points in the structure of the thrust front. On a smaller scale, sedimentary sequences and syn-depositional structures in small alluvial fan bodies at the basin margin were controlled by late thrust movements in the adjacent frontal thrust complex. Syn-depositional faults, folds and unconformities in the alluvial fan conglomerate sequences can be related to movements on individual faults within the thrust complex.

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