Active deformation at the boundary between the Precordillera and Sierras Pampeanas, Argentina, and comparison with ancient Rocky Mountain deformation

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Abstract
The Precordillera and Sierras Pampeanas of western Argentina are active thin-skinned and thick-skinned deformational provinces in the foreland of the Andean orogenic system and occur above a subhorizontal segment of the subducting plate. The two morphostructural provinces are analogous to the Idaho-Wyoming thrust belt and Rocky Mountain foreland provinces, respectively, that were formed in the western United States in Late Cretaceous to early Cenozoic time. Comparisons of the younger structures at the boundary between the Precordillera and Sierras Pampeanas with analogous, more eroded, structures in North America aid in the interpretation of the subsurface structural geometry of both orogenic belts and shed some light on the factors that control the style of deformation at the contact between thin- and thick-skinned deformational provinces. The Eastern Precordillera subprovince is bordered on the east by the Sierras Pampeanas basement block-uplift province. It has been deformed in late Cenozoic to Recent time into a set of folds and thrusts that verge westward, as opposed to the eastward vergence of the structures in the adjoining Central Precordillera subprovince. While few subsurface data are available, we have interpreted this subprovince to have been deformed by a combination of basement-involved and thin-skinned structures, in a manner analogous to that of the Moxa Arch in the zone of overlap between the thin-skinned and thick-skinned thrust belts in North America. We suggest that the change in structural style between the Precordillera and the Sierras Pampeanas provinces is most likely related to paleogeographic contrasts, and that the change in vergence between the Eastern and Central Precordillera subprovinces may be due to the presence of a reactivated Paleozoic fault zone.

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