RECOGNITION OF ANDEAN FAULT-BEND FOLDS IN PAMPEAN BASEMENT ROCKS, N. AND E. SIERRAS PAMPEANAS, ARGENTINA

The N. and E. Sierras Pampeanas of north-central Argentina contain >1000 km by ca. 100 km of almost continuous exposure of deformed and metamorphosed turbidites (Puncoviscana Fm. and equivalent rocks to the south) that record major deformational events in the tectonic evolution of the western Gondwana margin. Turbiditic rocks underwent first a NE-SW oriented orthogonal shortening and chevron-folding event with associated development of a fanning cleavage that can be followed from shallower sections in the north to deeper, more ductily deformed sections in the south. A subsequent, almost coaxial, compression of the sequence “locked up” the chevron folds and lead to the development of a second fold set, very likely shortly after chevron folding; there are no related metamorphic changes and no further cleavage development during this event. Chevron folding and, presumably, the fold tightening event, took place before the deposition of the Upper Cambrian Meson Group and late Cambrian intrusions. Locally, Ordovician plutons cut the folded sequence producing static metamorphic mineral growth; Ordovician and Devonian deformation is restricted to major transpressional shear zones.

Younger, fault-bend folds and associated kinks display characteristically larger wave-lengths than the earlier chevron folds, but clearly reorient them. Fault-bend folds are related to a west-to-east overthrust event that affected the Puncoviscana Fm., the overlying Meson Group and younger sedimentary rocks. Fault detachment planes are mainly sub-parallel to S0 although ramp structures help identify duplication of section in the Meson Group.

We interpret the structural sequence as recording a mainly orthogonal contraction of the turbiditic sequence during the Cambrian Pampean Orogeny, followed by shear zones and plutonism of the Famatinian and Achalan Orogenies, and culminating in a major thrusting event, most likely related to the Upper Tertiary Andean Orogeny.