PALEOELEVATION HISTORY OF THE NORTHERNMOST ALTIPLANO PLATEAU: IMPLICATIONS FOR GEODYNAMIC EVOLUTION OF THE ANDES

The mode of the geodynamic evolution of the Altiplano plateau has been a topic of significant recent debate. Two end-member models argue between a slow and steady uplift from ~45 to 0 Ma as a result of distributed shortening and crustal thickening versus pulses of rapid surface uplift through lower lithospheric removal. Previous paleoelevation studies suggest surface uplift of ≥2 km or more in the Eastern Cordillera between ~25 and 17 Ma (Leier et al., 2013), followed by a similar-scale event in Altiplano between ~10 and 6 Ma (Garzione et al., 2008). Both were inferred to reflect lower lithospheric removal events. We present new Miocene – Pliocene paleoelevation data from the northernmost Altiplano plateau (Descanso – Yauri basin in Peru, 14 – 15°C) with the goal to better understand along-strike variations in the surface uplift history and its implication for geodynamic processes that drive surface uplift. New data comes from multiple proxies: meteoric water compositions from the d¹⁸O values of carbonates, paleotemperature estimates derived from clumped isotope (D₄₇) of carbonates and brGDGT’s (branched glycerol dialkyl glycerol tetraether) extracted from organic rich mudstones. The basin deposits are divided into members A, B and C (Cerpa and Meza, 2001). An angular unconformity separates MB from MC in some locations. Both previous and new age constraints from ⁴⁰Ar/³⁹Ar biotite and U/Pb zircon indicate that basin deposits span a minimum age range of 18.74±0.19Ma to 4.8±1.1Ma. The d¹⁸O_mw and paleotemperature from D₄₇ and the brGDGT data show a consistent pattern throughout MB, with a significant shift towards more negative values and ~10°C cooler surface temperature in MC. This climate change event suggests that significant surface uplift (≥1 km) took place between the deposition of MB and MC. Such rapid surface uplift, coupled with an active deformational phase during deposition of MA and MB and waning of deformation during deposition of MC, points to a possible lower crustal removal event before the onset of deposition of MC at ~12Ma. The earlier onset of surface uplift in the northernmost Altiplano, in comparison to the central part, supports previous inferences of piecemeal removal of the lower lithosphere. Some component of the along-strike differences in the timing of surface uplift may be associated with crustal flow.