Building the central Andes through axial lower crustal flow
Resource
Tectonics, 29, TC3010
Journal
Tectonics
Journal Issue
29
Pages
-
Date Issued
2010
Date
2010
Author(s)
Ouimet, William B.
Cook, Kristen L.
Abstract
The orogen‐scale morphology of the central Andes, which are longitudinally symmetric at ∼19°S, correlates with the modern geometry of the subducting Nazca slab but is largely independent of local bedrock geology or precipitation. Crustal thickness, as measured by the cross‐strike width of the orogen above 3000 m and the cross‐sectional area of the crust, is not well correlated with observed magnitudes of upper crustal shortening. In our interpretation, the large‐scale morphology is instead correlated with and controlled by lithospheric‐scale processes and subduction dynamics. Using a semianalytic, three‐dimensional Newtonian viscous flow model, we produce Andean‐like topography and distribution of upper crustal shortening during subduction of an oceanic lithosphere eastward beneath the Andes provided that (1) the more steeply dipping slab segment beneath the central Andes is overlain by the weak lower or middle crust, (2) the flat slab subduction segments to the north and south of this zone are overlain by the strong middle and lower crust, and (3) the steeply dipping central slab segment is overlain by a narrow, centrally localized zone of increased crustal shortening. The resulting model orogen displays substantial orogen‐parallel flow in the weak lower crust above the steep slab zone. Orogen‐parallel flow does not penetrate into the regions of stronger lower crust above the flat slab segments, resulting in a broad, plateau‐topped orogen above the central slab segment and narrow but high orogenic segments above the flat slab region. Redistribution of material through lower crust flow can account for the observed mismatch between crustal volume and upper crustal shortening in the central Andes and may explain young (<10 Ma) crustal thickening and surface uplift that has been argued for in some regions.
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Type
journal article
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