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Paper No. 84-5
Presentation Time: 9:05 AM
THE KINEMATIC EVOLUTION OF THE ANDEAN THRUST BELT AT CA. 23 °S, NORTHWESTERN ARGENTINA
Two end-member models have been proposed for the formation of the Central Andean Plateau. North of 22°S, surface uplift is proposed to be rapid due to the removal of lithosphere and/or crustal flow. South of 22°S, surface uplift is proposed to be slow and coupled with crustal shortening and thickening. This contrasting behavior raises the question: has enough shortening occurred to account for crustal thickening and surface uplift south of 22°S? The amount of shortening in the plateau north of 22°S is well constrained; however, the amount of shortening south is less well-defined, but generally interpreted to be notably less. The goal of this work is to test the hypothesis that significant shortening happened during the late Eocene in the retroarc at ca. 23°S, and that shortening can account for crustal thickening and plateau formation. To constrain the kinematic evolution of the thrust belt we combined observations about the deformation mechanisms operating at different times and scales, geologic mapping and three analytical techniques: zircon U-Pb, apatite fission track and apatite (U-Th)/He. These data were used to build a ca. 250 km long structural cross-section that includes the Puna Plateau, Eastern Cordillera and Interandean Range and restore the deformation to the pre-Andean state. We estimate ca. 220 km of shortening in the retroarc thrust belt at ca. 23 °S. The kinematic evolution of the thrust belt supports an east vergent propagation of the orogenic front since middle Eocene and includes periods of internal out-of-sequence deformation during the Oligocene and Miocene times. Pre-Eocene structures remain largely unconstrained and seem to have been formed only west of the Puna Plateau. However, the new shortening estimate of 220 km in the retroarc, together with a rough constraint of up to ca. 30 km of shortening in the forearc, suggests that tectonic shortening accounts for most of the modern crustal thickness at this latitude.