Along-strike segmentation of the Andean foreland: causes and consequences

Abstract

Thrust belts of Tertiary age characterize the east flank of the Andes and part of the adjacent foreland from Venezuela to the southern tip of South America. This foreland deformation comprises three basically different structural styles: (1) thin-skinned thrust belts detached within the sedimentary cover, (2) thick-skinned thrust belts with an inferred basal detachment in the basement, at 10–20 km depth and (3) foreland basement thrusts which possibly affect the entire crust. Alternating foreland structural styles along the orogen, often with sharp boundaries, produce a segmentation of the Andean foreland. The changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the South American plate, for instance: thin-skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic/crystalline basement unaffected by Mesozoic extension. In areas of Mesozoic rifting, normal faults have been reactivated during Tertiary contraction, resulting in the formation of thick-skinned thrust belts. The basement becomes involved even if potential décollement levels are present. Deep-seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover. The three basic structural styles are unequally efficient in terms of shortening: Thin-skinned belts are typically shortened by 40–70%, thick-skinned belts by 20–35%, and areas of foreland thrusts by less than 10%. The structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening. Available estimates of shortening along the Central Andes actually suggest that shortening does not vary steadily, but has several maxima and minima. Strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments (`flat slabs'), but closely with lithospheric structure: The most strongly shortened segment coincides with thick mantle lithosphere, suggesting that shortening of the edge of the South American plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the Andes. This underthrusting of thick lithosphere may have caused steepening of a formerly `flat' slab in the Arica bend region to the `normal' 25–30° dip presently observed.