Geochemical evolution of Andean arc volcanism and the uplift of the Andes

Abstract

The Andean Plateau is the only modern-day orogenic plateau formed by subduction and not continental collision. The geodynamic mechanisms responsible for initiating crustal shortening and thickening in the Andes at ~70 Ma, after subduction had been ongoing for over 200 Ma, are still debated. The timing of Andean surface uplift is also controversial, and currently two models remain prevalent: (1) slow and steady uplift synchronous with crustal shortening from ~70-50 Ma, and (2) rapid surface uplift of up to 2 km in the last 20-16 Ma, resulting from large scale lower lithosphere removal or lower crustal flow.
We find a very strong correlation between Nazca plate age entering the trench and Andean crustal thickness (r = 0.939; r2 = 0.882; p value < 0.01). Thrust earthquakes at the Nazca-South America plate interface occur only where the slab age at the trench is older than 30 Myr, except where oceanic ridges are colliding with the trench. Based on these observations a new hypothesis is proposed, that long-term subduction of intermediate aged oceanic lithosphere (~30-80 Myr) can provide an explanation for the initiation and distribution of crustal shortening in the Andean plate. Intermediate aged slabs have a greater mechanical strength than weak and young oceanic lithosphere (<30 Myr), but a greater positive buoyancy compared to older slabs. These slab properties combined with trench-ward overriding plate motion transmit higher compressional stress into the overriding South American plate than during other subduction zone configurations.
A new approach is used to constrain regional Andean surface uplift. Cross correlations are tested between several physical parameters relating to the Andean subduction zone and several geochemical parameters from Andean arc volcanoes. 87Sr/86Sr and 143Nd/144Nd ratios have the strongest correlations with both crustal thickness and, due to isostasy, elevation. Very strong linear correlations exist between smoothed elevation and 87Sr/86Sr (R2=0.858, n=17) and 143Nd/144Nd (R2=0.919, n=16) ratios of non-plateau arc lavas. 87Sr/86Sr and 143Nd/144Nd ratios of lavas erupted through the Andean Plateau are distinctly crustal (>0.705 and <0.5125, respectively) and can be used as a plateau discriminant. These relationships are used to constrain 200 Myr of surface uplift history for the Western Cordillera (present elevation 4200±516 m). Jurassic-Paleogene isotope compositions (including new Paleogene analyses) became gradually more ‘crust-like’ over time, yet the distinct isotope signatures relating to the Andean Plateau were not achieved until the Miocene (23 Ma).
Results support the slow and steady surface uplift model, synchronous with gradual crustal shortening and thickening. The isotope paleo-elevation model predicts that current elevations were attained in the Western Cordillera by 23 Ma. From 23-10 Ma, both deformation and surface uplift gradually propagated southwards by ~400 km.