Lithospheric Dripping Under a Weak Crust: Geodynamic Modelling of the Southern Puna Plateau, Argentina

Lithospheric foundering is hypothesized to affect many of Earth's orogenic regions, including the Central Andean Puna Plateau. In the southern Puna Plateau, crustal thickening, lithospheric thinning, volcanism, upper-crustal strain, and Cenozoic hinterland basin development have all been ascribed to deep-seated geodynamic processes. Here, we develop the hypothesis that lithospheric foundering occurred during the Late Miocene to Early Pliocene in the form of a Rayleigh-Taylor drip, specifically as a "weak-crust" drip. In contrast to strong-crust drips, which yield syn-drip upper-crustal contraction and surface subsidence (i.e., a transient "bobber" basin), weak-crust drips yield syn-drip extension and surface uplift, and they occur when the crust has an effective viscosity less than or similar to that of the mantle lithosphere. We explore the weak-crust dripping hypothesis using numerical models. These models show that drip growth is accompanied by upper-crustal horizontal extension above the center of the drip, which is accommodated by horizontal compression in the peripheral regions of the drip. Instead of flexural subsidence and the formation of a bobber basin, the models predict that subsidence above the center of the drip would be associated with extensional structures such as normal faults. Basin sedimentation would likely be isolated, confined to elongated depocenters, such as hanging-wall basins, bounded by footwall uplifts. Existing geologic evidence is consistent with this model of lithospheric dripping, and we identify the Late Miocene to present Antofalla depression as a drip-related extensional basin. The hypothesis of a weak-crust drip for the Antofalla depression does not necessarily conflict with the strong-crust drip previously inferred for the Arizaro basin ~150 km to the north, but it implies that lithosphere rheology differs between the two regions. This work highlights the diverse nature of lithospheric foundering and its varying effects on upper crustal strain and surface topography.