Fragmented Paleogene foreland basin in the Valles Calchaquíes, NW of Argentina

HONGN, F., DEL PAPA, C., POWELL, J., PAYROLA, P., PETRINOVIC, I., AND MON, R

Salfity, J.A., and Marquillas, R.A., eds., Cenozoic geology of the Central Andes of Argentina

SCS Publishers

Año: 2011; p. 189 - 209

ABSTRACT Sedimentologic-stratigraphic, paleontologic and structural detailed surveys on the well-exposed Cenozoic successions of the Valles Calchaquíes have revealed key points for understanding the Paleogene Central Andes evolution in NW-Argentina. Our work has documented several Eocene fossiliferous sections showing unconformable relationships with the underlying Salta Group and evidence of syndeformational deposition such as progressive unconformities (e.g. La Poma-Saladillo, Cerro Tin Tin, Luracatao). Our data along with those offered by other research-groups have increased the Eocene foreland record, which displays some particular characteristics: among them, most of the Paleogene faults were west- or bivergent, affected the Neoproterozoic- Lower Paleozoic basement defining a thick-skin deformation and followed a rather unsteady propagation path. Evidence of Eocene deformation and sedimentation indicate that the Cenozoic shortening in NW-Argentina is older than assumed in classical models on Andean evolution proposing that crustal shortening was delayed until the late Oligocene-Miocene due to southward deformation migration from the Peruvian and Bolivian Andes. Our new data allow discussion of the Paleogene (Eocene) basin evolution, which is generally analyzed in the context of the classic foreland basin system. Basement blocks with different grades of exhumation compartmentalized the Eocene foreland. Thus it evolved as a broken foreland from its initial stages. Tectonic inversion and reactivation of both Lower Paleozoic basement heterogeneities and normal faults related to Cretaceous rift enhanced the definition of the broken foreland and retrovergent structures. The easternmost recognized Eocene structures (La Poma- Saladillo and Tin Tin) are west-vergent. This back-vergence advances the hypothesis of an Eocene deformation front located even further eastward. Precise definition of this deformation front is necessary to determine the eastern limit of the Eocene deformation and whether the syntectonic deposits along the Puna-Eastern Cordillera transition fit in the classical model of flexural foreland basin related to tectonic load or whether alternative models have to be considered. Basement compartmentalization and lack of well-developed Eocene thrust belts westward (Puna and north Chilean Andes) suggest that alternative models should be explored.