SURFACE RESPONSE TO LITHOSPHERIC DELAMINATION: AN EXAMPLE FROM THE PUNA PLATEAU OF NW ARGENTINA

Lithospheric foundering has been inferred to be responsible for magmatic activity and surface uplift during the last ca. 8 Ma in the Central Andes. Geophysical evidence documents the loss of mantle lithosphere and lower crust, but intriguing questions remain: by what mechanism and when was lithosphere removed, and what was the response at the surface to such process? Numerical modeling of lithospheric delamination predicts that in case of convective removal through formation of a Rayleigh-Taylor instability, shortening and basin accomodation space occur during formation of the drip, whereas extension and uplift occur after drip detachment.

Preliminary data from the southern Puna Plateau (Pasto Ventura) suggest that shortening and basin accomodation space were followed by a rapid transition to extension, meeting model predictions. Furthermore, several basins (e.g. Arizaro, Antofalla) within the Plateau preserve >2 km of Miocene-Pliocene lacustrine-fluvial deposits. The creation of accommodation space for these thick sequences is inconsistent with tectonic loading by surrounding ranges, whose deformation and exhumation pre-dates sediment deposition. This would suggest an alternative mechanism for subsidence, such as lithospheric foundering. Interestingly the thickest sedimentary section is preserved in the Arizaro Basin, in the central Puna Plateau, which overlies directly the area of thinnest crust and lithosphere. Thus, delamination in the Arizaro Basin may have occurred sometime in the recent past and what we are observing today is the post-removal state. This would also imply that the scale of the basin can provide a proxy for the scale of lithospheric removal, which would be on the order of 100 km, and argue against wholesale plateau delamination.

Preliminary stable isotope data show a shift towards more positive δ¹⁸O values consistent with increasing aridity since 11 Ma. Pilot clumped isotope data show temperatures of ca. 17-20^oC and no significant change in temperature between 11 Ma and 4.7 Ma, which, if borne out, may suggest little plateau uplift during this time and significant elevation by 11 Ma. Our dataset suggests that small scale foundering events were responsible for late Cenozoic deformation, basin formation and low magnitude surface elevation changes within the Plateau.