Rock structures formed during back-arc basin deformation in the Andes of Tierra del Fuego

Abstract

A back-arc basin formed along the Pacific margin of South America south of 50°S in the latest Jurassic and was uplifted and deformed beginning in the middle Cretaceous. Mapping in Tierra del Fuego between 67°W and 69°W revealed major lateral and vertical variations in the intensity and style of deformation. Penetrative rock structures, resulting dominantly from heterogeneously distributed progressive simple shear deformation, were formed along the rear part of the back-arc basin in the transition zone from the basin to the continental shelf. In this zone, the deformation affected the ophiolitic basin floor rocks, continental basement, and upper Mesozoic silicic to mafic volcanic and sedimentary rocks. In contrast, deformation in the central part of the basin was less intense and the sedimentary infill and ophiolitic crust were, in general, not penetratively deformed. Along the rear edge of the basin, the main-phase slaty cleavage dips southward, toward the original center of the basin. An extension lineation pitches south to southwest in the slaty cleavage plane and defines the direction of maximum finite extension. In the upper part of the upper Mesozoic rock section, the main-phase fold axes trend east to southeast, and the folds overturn to the north. Strain at this level was a flattening type. The intensity of deformation increased downward and is marked by the increased structural transposition of bedding, the tightening of folds, and the rotation of fold axes toward the direction of maximum finite extension. At the lowest stratigraphic levels, the fold axes parallel the extension lineation and are oriented perpendicular to the fold axes at the higher levels. Second-phase, asymmetric folds formed along the basement-cover contact during progressive deformation. In the central part of the basin, the sedimentary infill was folded into large flexural folds, but the ophiolitic crust was deformed only in narrow shear zones.