Linking orography, climate, and exhumation across the central Andes

Abstract

Quantifying interactions between uplift, climate, deformation, and exhumation remains difficult, in many cases due to a paucity of data relevant to all processes. We synthesize new and existing data to understand the orogen-scale orographic changes across the central Andes, Bolivia. We use a regional climate model and geo-thermochronologic data to identify the correlations between changes in precipitation due to surface uplift and spatiotemporal patterns of deformation and erosional exhumation. Mean orographic rainfall patterns do not reach near present-day gradients and values until the topography grows to >75% modern elevations. New fission-track data near the orocline apex indicate that rapid exhumation moved eastward, beginning in the Eastern Cordillera ca. 50–15 Ma, the Interandean zone ca. 18–6 Ma, and in the Subandes ca. 7–3 Ma. Throughout Bolivia, exhumation is consistent with deformation until ca. 15–11 Ma, after which the pattern corresponds better with the increased rainfall toward modern values. These linked observations suggest that ca. 15–11 Ma, regional elevations reached threshold values (>75% modern) necessary to generate near present-day, enhanced rainfall gradients. These gradients have resulted in variable exhumation implied by the structural level of rocks exposed across the thrust belt and confirmed by fission tracks in apatite. The main insight is that the climate-induced Middle Miocene–recent exhumation varies over scales of a few hundred kilometers across Bolivia and implies that high mean rainfall (>∼3 m/yr) and long time scales (∼10 m.y.) may be necessary for climate to induce orographically driven exhumation patterns recorded by fission tracks.