Abstract

Substantial Neogene deformation occurred in the part of the Andes Mountains and the Sierras Pampeanas that overlie a subhorizontal segment of the subducting Nazca plate in northern Argentina. The history of the volcanic arc suggests that the Nazca plate attained the present low-angle inclination between about 17 and 10 Ma, with the region underlain by the flat Nazca plate widening eastward thereafter. We compare the history of deformation, provided by magnetostratigraphy of Miocene strata in foreland and intermontane basins, to the history of the subducting plate.

At Las Juntas, in the Precordillera thin-skinned thrust belt, 4,780 m of Neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin. Magnetostratigraphic data, complemented by fission-track dating of an interbedded tuff, indicate that the strata span from 18 to ∼9 Ma. The earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the Frontal Cordillera. Subsequently, the deformation front migrated eastward, generating the Precordillera thrust belt, and finally the Sierras Pampeanas basement uplifts. The rate of strata accumulation varied systematically as new uplifts arose, with a maximum rate exceeding 1.0 mm/a between ∼15 and 12 Ma.

A correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed. Although large-scale factors affecting the plate margin beyond the flat-slab segment may have partly induced the compressional deformation, this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate.

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