Controls in Andean volume between 20 and 48º s.

SPAGNUOLO, M. G; POSE, F.; FOLGUERA, A; RAMOS, V.A.

Barcelona, España.

Simposio; 6th International Symposium on Andean Geodynamics; 2005

The Andes show important variations in mean elevation related to variable orogenic volume, flexural behaviour of lower lithosphere as a function of thermal structure, and isostatic rebounds. Different morphostructural settings appear along its extension between 20 and 48 ºS, reflecting specific gradients in shortening, with only minor and localized anomalies along strike (Kley et al., 1999; Ramos  et al., 2004). All these tectonic variations are not only a consequence of Mesozoic pre-Andean geological history of the western margin of South America, which has influenced differential shortening through weakening of continental lithosphere during extensional back-arc processes as indicated by widespread Jurassic-Cretaceous shallow seas. This non-isotropic medium, which differentially yielded under compression has also been determined since Late Paleozoic rifting episodes, Proterozoic to Paleozoic amalgamation of western Gondwana, and a heterogeneous distribution of thick sedimentary prisms that are more easily detachable than the rest of the craton (Allmendinger and Gubbels, 1996; Ramos et al., 2004). However, latitudinal variations in orogenic volume seem also to be the result of one to multiple Present tectonic controls.  Strain partitioning, age and thickness of the subducted oceanic slab, coupling between plates associated with ocean buoyancy, velocity and convergence vector, and changes in the Benioff-zone geometry through time have been also invoked as controls in orogenic morphology of the Andean range (Chemenda et al., 2000; Liu et al., 1995). Recently, Lamb and Davis (2003) have proposed that high mountains are restricted to regions where climatic conditions are right, concluding that climate may help to focus local shear stress by inhibiting sediment thickness inside the trench. The trench sediment fill would control the dynamic of subduction by acting as lubricant between subducted plates and lowering shear stress. The higher volumes of the Andes would then be related to starved sections of the trench, that reaches as much as 8,055 m deep at 23ºS, where shear stress reaches the highest values. The sediment-starved nature of the trench would be a direct consequence of the arid climate along the coast.   In this work we establish and discuss the different controls that are involved in generation of the Andean volume, separating those that are orogenic from those related with isostatic accommodations. This evaluation determined latitudinal bands in which more than one single factor would be operating, and anomalies to the general trend, which fit in the present frame of Andean dynamics.