OXYGEN ISOTOPE EVIDENCE FOR RAPID LATE MIOCENE UPLIFT OF THE ALTIPLANO, BOLIVIA

The oxygen isotopic compositions of carbonates deposited in the northern Altiplano are used to estimate the late Miocene paleoelevation of the Altiplano. Oxygen isotope paleoaltimetry is based on the observed systematic decrease in the δ¹⁸O of meteoric water with increasing altitude. Paleoelevation is estimated by comparing paleo-meteoric water compositions, determined from authigenic minerals or fossils, to an empirically derived δ¹⁸O vs. altitude gradient. The Altiplano basin, with an average elevation of 3700 m, has been characterized as a large piggyback basin that has been internally drained since late Oligocene time. North of the oroclinal bend in the Andes, the Altiplano receives between ~0.7 and 1.2 m/yr of rainfall, whereas south of the bend receives <0.2 m/yr. Significant rainfall in the northern Altiplano makes this region ideal for oxygen isotope paleoaltimetry because it minimizes evaporative enrichment of ¹⁸O in rainfall and surface waters, which causes an underestimation of paleoelevation. Most of the rainfall in the northern Altiplano occurs during the summer and is derived from the Amazon basin. Rainfall sampled at 10 stations over an elevation transect adjacent to the northern Altiplano in 1984 displays a systematic decrease in mean annual δ¹⁸O values from –7.8 at 395 m (Sapecho) to –20.4 at 5200 m (Chacaltaya) (Gonfiantini et al., 2001). These rainfall data, as well as water samples collected during this study from small tributaries between ~800 and 4800 m in the Coroico drainage basin are used to determine the local δ¹⁸O vs. altitude gradient. This gradient is compared to paleo-meteoric water values determined from carbonates deposited in the northern part of the Altiplano basin to calculate paleoelevation. Paleoelevation estimates and age constraints from previously dated tuffs deposited in the northern Altiplano suggest that surface uplift of ~1 to 2 km occurred between ~9 and 7 Ma, raising the Altiplano to its current elevation. Uplift on this scale over such a short time period can only be generated by removal of a significant portion of the mantle lithosphere.