Numerical models of Andean orogeny: Deformation induced by slab dynamics

Abstract:

The central Andes orogen (15˚-26˚S) is the second largest mountain belt in the world. It formed through Cenozoic shortening of western South America during continual subduction of the Nazca plate. In contrast to other large orogens, there is no evidence for continental collision or terrane accretion in the last 200 Ma, and thus the formation mechanism of the central Andes orogen is unclear. Here, we use 2D thermal-mechanical models to study Andean orogenesis. The models have a 80 km thick oceanic plate and a continental plate that includes a 1200 km wide region of Phanerozoic margin lithosphere (100 km thick) and a craton (200 km thick) farther inboard. Convergence is imposed through side boundary conditions, and the subduction geometry evolves dynamically. The models extend to a depth of 1200 km and include a viscosity increase at the top of the lower mantle (670 km depth). In the reference model, the lower mantle viscosity is 30 times higher than that of the upper mantle; the oceanic plate moves at 7.5 cm/yr toward the subduction zone and the continental plate moves trenchward at 2.5 cm/yr. The slab does not readily enter the lower mantle and instead undergoes folding in the deep upper mantle which causes a decrease in slab dip at <200 km depth. This results in an increase in stress on the plate boundary that triggers shortening of the continent at a rate of 1.47 cm/yr. Model tests show that shortening is enhanced if the lower mantle viscosity is higher or if the margin lithosphere is relatively weak. Conversely, shortening is reduced as the trenchward continental motion decreases, and deformation ceases with no continental motion and a margin lithosphere that is ten times stronger than that of the reference model. Our models demonstrate that the lower mantle viscosity, continental trenchward motion and continental structure are key factors that govern Andean-style shortening. Future work will explore the effects of variations in continental structure on the details of deformation and its topographic expression.