LITHOSPHERIC FOUNDERING RECORDED BY THE COMPLEX MIOCENE TO QUATERNARY TECTONIC EVOLUTION OF THE SOUTHERN PUNA PLATEAU, ARGENTINA

Convective removal (foundering) of lower crust and/or mantle lithosphere is proposed to have affected many continental regions, but its detection in the geologic record remains controversial. Recent numerical modeling studies have documented complex dynamic effects of foundering, including subsidence, uplift, shortening, and extension, providing predicted deformation histories that can be tested against geological data. We present constraints on the tectonic evolution of the southern Puna plateau, Argentina, where late Cenozoic foundering is hypothesized from volcanic geochemistry and geophysical imaging, as well as new numerical modeling results, to test the connection between lithospheric foundering and upper crustal deformation.

Our work and previous studies together reveal a complex Neogene to Quaternary evolution of deformation in the southern Puna plateau, which forms the southern extent of the Andean high orogenic plateau. During Miocene time, regional WNW-ESE Andean shortening was disrupted by extensional formation of the Antofalla depression, a ~125 km long, NNE-trending, linear basin within the plateau. Syntectonic sedimentary strata record the initiation of Antofalla extension between ca. 16 Ma and 11 Ma. Deformed strata and earthquake focal mechanisms show that shortening resumed locally on the margin of the Antofalla depression before Recent time. In contrast, prior studies show that other regions of the southern Puna plateau transitioned from shortening to strike-slip and extension, which are ongoing, after ca. 5 Ma. Thus, the Antofalla depression and the broader southern Puna plateau have had contrasting senses of deformation since Middle Miocene time. The heterogeneous evolution of deformation within the orogen is difficult to explain via a process that would likely affect the whole orogen in a uniform way, such as slowing convergence. However, our new numerical modeling reveals that foundering of a lithospheric drip can induce crustal deformation that matches well with the observations from the southern Puna plateau. Lithospheric foundering in the Central Andes suggests that gravitational instability may be fundamental to orogenic evolution.