3-DIMENSIONAL GEOMETRY AND INTERFERENCE OF FAULT-BEND FOLDS - EXAMPLES FROM THE PONGA UNIT, VARISCAN BELT, NW SPAIN

Examples of the three-dimensional geometry and interference patterns b etween fault-bend folds occur in the Ponga Unit of the Variscan Cantab rian foreland thrust and fold belt of NW Spain. The Ponga Unit consist s of several E-directed thrust sheets with a cumulative displacement o f 93 km, which were emplaced during Upper Westphalian-Stephanian time. Two major lateral ramps developed in the basal thrust of the Ponga Un it, reaching 15 and 20 km in length, respectively, and some 3 km in he ight. Lateral structures are also common within the hangingwall thrust sheets. The three-dimensional staircase trajectories of thrust faults , together with the tectonic superposition of thrust sheets, has produ ced a complex interference pattern of fault-related folds. Most map-sc ale folds formed as fault-bend folds related to interference between f rontal and lateral ramps. Fold geometry and size varies depending on t he geometry and size of the associated thrust ramp and flat. The axial traces are grouped into lateral and frontal sets, which are sub-paral lel or sub-perpendicular to the tectonic transport direction, respecti vely. 'Corner folds' form with plunging axes at the intersections of f rontal and lateral ramps and generally link pairs of frontal-lateral f olds. They have relatively short axial traces that are oblique to the tectonic transport direction. Fold plunge varies due to the superposit ion of lateral and frontal structures of a thrust fault and also depen ds on the geometry of subsequent thrusts in the footwall. The final ge ometry of frontal folds in the Ponga Unit are not only the result of t he bending mechanism but have been modified by simple shear parallel t o the emplacement direction. Major lateral structures have been reacti vated as reverse faults and are locally overturned due to postemplacem ent north-south compression.