DEPTH-DEPENDENT CHANGES IN DEFORMATION AND MAGMATISM IN THE FAMATINIAN ARC, ARGENTINA: IMPLICATIONS FOR ISOSTATIC RESPONSE AND MOUNTAIN BUILDING

Tectonic regimes in magmatic arcs may influence magma transfer, crustal rheology, seismic structure, mountain building and erosion. The Ordovician Famatinian arc in the Sierras Pampeanas of Argentina exposes crustal depths from volcanics to ~ 8 kbar in foreland, main arc and rear-arc sections and preserves the temporal history of arc magmatism, metamorphism and deformation. This allows investigating deformation accommodation and intensity at different crustal levels during arc magmatism. We use constraints on the duration of magmatism, magma addition rates (MARs) and deformation intensities at different crustal levels to model the predicted isostatic behavior of the Famatinian orogeny.

Intrusive rocks dominate (up to 90 %) in the mid to deep crust of the main arc section and make up ~ 40 % of the upper crust, whereas in the foreland and rear-arc sections, igneous rocks comprise ≤ 30 % of the exposed rocks. MARs into the ~ 30 km of vertical exposure increases with depth and are 8.8 km3/Ma/arc-km on average.

Tectonic shortening synchronous with magmatism at all crustal levels is recorded by tilted and folded sedimentary and volcanic strata and at deeper levels by magmatic fabrics and ductile host rock deformation. 10% shortening is determined in the upper crust, whereas the mid- to deep-mid crustal levels record shortening amounts of ~ 60 % in host rocks. An increase in magmatic fabric intensity with depth, related to strain intensities through a study of magmatic folding, is used to determine depth dependent shortening rates via thermal modeling. Results show regional, east-west shortening and crustal thickening with stronger intensities in the mid to deep-mid crust (2 to 5 kbar) compared to the upper crust. These shortening amounts reflect a vertical deformation gradient, partly accommodated by changing deformation mechanisms with depth, the development of large shear zones in deeper parts and the development of greater shortening at depth due to emplacement of larger volumes of magmatism.

Isostatic mass balance modeling of this hot orogen predicts that tectonic shortening and magmatic thickening during arc magmatism drives an increase in surface elevation and mountain building from a shallow marine environment at 490 Ma to ~ 2 km at 450 Ma with non-coaxial, west-vergent shortening continuing after magmatism ceased.