Abstract

The Puna-Altiplano, a 500,000-km2 area at an average elevation of 3.7 km in the central Andes, occurs in a non-collisional plate setting over a 30°-east–dipping segment of the Benioff zone and is bounded to the north and south by nearly flat segments of the subducted Nazca plate. A 120-km-long portion of the southeastern boundary of the Puna in northwestern Argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction. The structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays. The present southeastern margin of the Puna was uplifted between 5–10(?) and ∼2 Ma. Northwest-southeast horizontal shortening and vertical extension during uplift were produced by northeast-striking en echelon thrust faults dipping 17°–55° northwest. Four to seven en echelon structural blocks, each 20–30 km long, constitute the border of the Puna; their structural geometry precludes large-scale overthrusting as an important mechanism of plateau uplift along the studied segment. In the past 2 m.y., there has been a 45° change in the direction of shortening and, locally, a 90° change in the extension direction, indicating that the structures accompanying uplift are no longer active. This change could be due to a change either in the geometry of the subducted plate or in convergence directions, although neither of these hypotheses is testable given presently available data.

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