Structure and kinematics of a foothills transect, Lago Viedma, southern Andes (49 degrees 30 ' S)

The western edge of Patagonia, south of 47 degrees S, experienced a major t ectonic reorganization during the Tertiary. The Chile ridge, separating Naz ca from Antarctica, collided obliquely with western Tierra del Fuego at abo ut 14 Ma and the triple point migrated northwards to its present position a t about 47 degrees S. Consequently, the southern tip of South America has p assed from a Miocene context of rapid oblique convergence (ENE-WSW at about 9 cm/yr) between Nazca and South America, to a Pliocene context of slow fr ontal convergence (EW at about 2 cm/yr) between Antarctica and South Americ a. The Andean foreland fold-and-thrust belt lies on the eastern side of the Patagonian Cordillera and is well exposed along the northern shore of Lago Viedma (49 degrees 30'S). Structural observations, digital mapping, subsur face data, balancing of a cross-section and kinematic analysis of fault pop ulations provide new information on the structure of the fold-and-thrust be lt, the timing and style of deformation and their relationship with Tertiar y plate tectonics. Along the studied transect, synsedimentary structures sh ow that compressional deformation began at least during the Late Cretaceous , was ongoing during the syntectonic emplacement of the Lower Miocene grani tic Monte Fitz Roy pluton and continued into the Pliocene. Folds and thrust s are thick-skinned in the west, and mostly thin-skinned above a decollemen t in Early Cretaceous black shales in the east. Analysis of fault populatio ns, measured within Jurassic basement and its Cretaceous cover, provides su bhorizontal principal directions of shortening, striking between E-W and EN E-WSW. Compressional deformation was associated with a major component of r ight-lateral wrenching parallel to the Cordillera. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.