Miocene surface uplift and moisture-source evolution resolved by stable-isotope proxies in the southern Central Andean foreland basin ( ∼ 32∘S)

How surface uplift is coupled to crustal shortening and thickening remains a central question in orogenic systems research. Existing stable-isotope records in the southern Central Andes show increasing δ 18 O and δ D values during Miocene retroarc shortening, inconsistent with expected enhanced orographic distillation. However, the role of changing climate and moisture source is underexplored in this region, where Pacific- and Atlantic-derived precipitation regimes meet. We pair pedogenic carbonate δ 18 O/ δ 13 C and volcanic glass δ D from the Manantiales foreland basin ( ∼ 32 ∘ S) with constraints from regional sedimentology to assess topographic and climatic evolution. Carbonate δ 18 O and δ 13 C values are high between ∼ 37 and 18.6 Ma ( δ 18 O means −8.5 and −7.9 ‰), across a 33–21 Ma depositional hiatus, recording low paleoelevation in an arid foredeep. These values then decrease by ∼ 4.5 ‰ between 18.6 and 18.3 Ma, interpreted to reflect surface uplift as the basin entered the wedge-top depozone. Glass δ D values are consistently low after 17.9 Ma (−95.3 to −104.9 ‰), whereas carbonate δ 18 O values increase by ∼ 1.7 ‰ after ∼ 16.5 Ma. We interpret this to record increased monsoonal moisture delivery during the Miocene Climatic Optimum and/or enhanced dry-season evaporation. Using isotope–elevation models accounting for ancient temperature and moisture-source changes, we calculate 1,886 m (68% CI: 1,446–2,255 m) of syndepositional surface uplift from carbonate δ 18 O, directionally consistent with glass δ D. Surface uplift coincided with rapid crustal shortening and thickening, indicating coupled orogenic growth. These findings support model predictions that Andean mountain building alters isotopic signals by enhancing orographic distillation and shifting moisture-source regime.