Andean reactivation of Cretaceous border faults of the Eastern Cordillera of Colombia

The Eastern Cordillera of Colombia absorbed a considerable amount of the Neogene plate convergence at the northern Andes by a penetrative shortening. Fold styles vary according to the underlying slab configuration, as evidenced by a first-order relay above a vertical slab tear. Along the flat-slab segment to the north of this tear the Cordillera displays neat deformation fronts against the uniformly inclined western flank of the Central Cordillera on its western side and the rigid Guayana Shield to its east. It thus displays a "vise" setting of a weak core buttressed between strong crustal parts. We link this constellation to the long-lived evolution of a forebulge which controlled the sedimentation pattern within a Cretaceous retro-arc basin. An initial sagging of its peripheral borders was accomplished by normal faults with both foreland and hinterland dipping attitudes. Fault reactivation near the tear zone is invariably tied to the formation of folds, affecting both the pre-Mesozoic basement as the Cretaceous cover. During their reactivation inward dipping normal faults acted as buttresses with respect to the lateral escape of the Cretaceous cover and prompted the formation of marginal folds of amplitudes exceeding 10 km. During the Pliocene, the growth of these marginal folds was blocked and gave way to a subsequent gravitational collapse, as out-of-sequence shortcuts linked ductiley deformed lower crustal parts to the foreland basin. Outward dipping normal faults, on the other hand, formed "negative" buttresses and localized the formation of tight fault-related folds with inverted forelimbs during the Pliocene deformation climax. Fault arrays in conjunction with changing fold style, as determined by a metamorphic overprint near the relay of the slab tear, give rise to a characteristic segmentation of the deformation fronts.