This work investigates factors controlling the segmentation of the Andean margin. It is formed by three chapters, whose methods, results and conclusions can be summarized as follow:
The elastic thickness of the Andean forearc is maximum between 15° and 23°S, decreasing southward and toward the very weak orogen. Interpreting these trends suggests that: the subduction-related thermal structure dominates rigidity variations; southward weakening of the forearc is caused by decreasing age of the slab; the forearc is a rigid geotectonic element; thick, quartz-rich crust and low strain rate-to-heat flow ratio cause low cordilleran rigidity; strength below the Altiplano localizes in an upper-crustal layer whose base correlates with a geophysical discontinuity; the forearc-plateau boundary is a zone of changing thermal conditions, eastward-increasing crustal thickness and felsic component in the crust, and low strain-rate deformation, correlating at the surface with a west-verging structural system. These conclusions suggest that the forearc acts as a pseudo-indenter against the weak plateau allowing accumulation of ductile crust that moves westward from the foreland. This model integrates contradicting ideas on the relative importance of upper-crustal structures and lower crustal accumulation.
The design of a gravity-based Earth model for the Andes require knowledge of the effect exerted on density by several potential factors. This motivates petrophysical modelling for 55 major element analyses characterizing active continental margins. Mineral assemblages and densities were computed using two thermodynamic approaches along conductive geotherms for arcs and shields. Under dry conditions density is inversely correlated with SiO2 for all PT conditions. Empirical relationships with correlation factors > 0.9 allow density to be estimated from silica content at critical conditions. These relationships also hold for wet, melt-containing crustal columns of acidic to intermediate composition but cannot be applied for basic compositions: Mafic rocks absorb significant amounts of water in amphiboles, strongly reducing their density with respect to dry garnet-pyroxene granulites. This suggests that hydrated and partially molten lower-crustal zones along magmatic arcs thinner than 50 km could contain large amounts of basic material in a gravitationally stable situation. The results also suggest restricted conditions for the removal of crust into the mantle.
Forward modelling of Bouguer anomalies produced a three-dimensional representation of the continental-scale density structure for the oceanic Nazca plate, the subducted slab and the Andean margin. These major units are formed by a number of bodies, whose density was predefined in accordance to petrological considerations. Independent information constrains the geometry of the slab, locally the oceanic and continental Moho, and indirectly the lithosphere-asthenosphere boundary. The intracrustal density discontinuity was not constrained: It results by fitting observed and calculated Bouguer anomalies. The model is presented with the aim to serve as a tool for further interpretations and some results are discussed in order to show the potential application of the model to the study of a wide range of Andean geodynamic processes.
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