Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

Abstract

Deformation within orogenic plateaus functions to establish a dynamic equilibrium between tectonic boundary stresses and plateau gravitational potential energy. Temporal changes in deformation kinematics record perturbations to boundary stresses or internal plateau processes, such as lithospheric foundering. We integrate new mapping, field observations, and geochronologic ages with published data to document complex late Cenozoic upper crustal deformation in the region of the Antofalla depression, a ∼125 km long, sublinear basin within the southern Puna orogenic plateau, Argentina. The juxtaposition of stratal ages across the depression requires >900 m vertical offset on a surface-breaking fault. Regional geologic structure, basin geomorphology, and our observation of a breccia paralleling the depression margin suggest formation of the depression by normal faulting. We interpret published stratigraphic logs to suggest that the depression formed between ca. 16 and 11 Ma following Andean shortening. Folded Late Miocene to Quaternary strata on the eastern depression margin indicate that extension ended and shortening resumed before present, revealing toggling between extensional and contractional kinematic regimes. The kinematic evolution of the Antofalla depression contrasts with the rest of the southern Puna plateau, which underwent shortening until latest Miocene to Quaternary time, followed by extension and strike-slip deformation. Taken together, the spatial and temporal variations in late Cenozoic deformation of the southern Puna plateau are inconsistent with mechanisms that would affect the entire orogen, such as slowing convergence, but are compatible with lithospheric foundering.

Key Points

• The Antofalla depression is inferred to have formed by extension within the southern Puna orogenic plateau

• Inferred Antofalla extension was preceded and followed by thrusting, suggesting toggling between kinematic regimes

• Spatial and temporal variation in late Cenozoic southern Puna deformation is consistent with causation by lithospheric foundering

Plain Language Summary

Deformation of orogenic plateaus is thought to establish a balance between tectonic forces on plateau margins and the gravitational potential energy of plateau lithosphere. Normal faulting, resulting in extensional deformation, thins the lithosphere and can reduce surface elevations, potentially reflecting an excess of gravitational potential energy relative to tectonic boundary forces. We combine new and published data to investigate the formation of the Antofalla depression, a prominent sublinear basin within the southern Puna orogenic plateau, and implications for orogenic plateau evolution. Contrasting ages and elevations of geologic strata on either side of the depression imply vertical offset on a fault system within the depression, which our field observations and structural reconstructions suggest is a normal fault system. New and published data suggest that inferred normal faulting in the Antofalla area was temporary and that the kinematic regime of the Antofalla area contrasted with deformation elsewhere in the southern Puna plateau. The inferred localized, transient nature of the extensional deformation in the Antofalla area is consistent with a localized, transient perturbation to plateau gravitational potential energy that may have resulted from removal of a portion of dense lithosphere underlying the plateau. Such removal is independently suggested by geochemical and geophysical studies.