SHAPE OF THE NAZCA SLAB ALONG THE CENTRAL ANDEAN MARGIN IS PARTIALLY CONTROLLED BY THE COMPOSITIONAL STRUCTURE OF THE CONTINENTAL CRUST

It has been accepted that the shape of the subducted Nazca slab below the Andean margin, particularly the occurrence of sub-horizontal, flat-slab segments at 100-150 km depths, is controlled by the positive buoyancy of subducting oceanic ridges. However, published results of numerical and analogue modelling, which were explicitly designed to investigate this problem, demonstrate that the subduction of a buoyant oceanic ridge, like those characteristic of the Nazca plate, is at most a necessary but surely an insufficient condition for flattening the surrounding slab, and that the westward overriding of the continent over the slab should play a significant role. Into this context, variations of the slab shape (flat-steep-flat) along the Central Andes could be, at least partially, controlled by variations in the amount of convergence being absorbed by crustal shortening, and therefore by the mechanical strength of the crust resisting deformation. This contribution makes use of a three-dimensional density model, which describes the continental-scale structure of the Andean margin and was constrained by seismic and heat flow data. The model: 1) shows that the Iquique ridge is as thick and continuous as the Juan Fernandez ridge but it is not associated with a flat slab segment at depth, supporting the general conclusion of numerical and analogue modelling; 2) allows to quantify the crustal volume along the Central Andes to demonstrate that the east-west extension of flat-slab segments is anticorrelated with the crustal thickness in excess with respect to a normal crust; 3) demonstrates that these spatial tendencies are strongly correlated with the internal crustal structure, with the steep-subducting segment along the Central Andean plateau coinciding with a huge over-thickened crust which is itself comparatively much felsic, and hence weaker, than the one characterizing the crust above flat-slab segments. These results let to infer that the compositional structure of the continental crust, being an old heritage left by the geological history of the margin, is an important factor controlling the integrated strength of the lithosphere, its deformation capabilities in response to convergence and thus not only the shape of the subducted slab but also the long-term (10⁷-10⁸ yr) segmentation of the margin