The Middle to Upper Jurassic and Lower Cretaceous silicic volcanic and sedimentary rocks of Isla de los Estados, Argentina, at the southern extremity of the Andean Cordillera, have been interpreted to lie on the rear (continental) side of an Early Cretaceous marginal basin. The basin floor was uplifted, together with the magmatic arc on its Pacific side, in mid-Cretaceous time. Uplift was accompanied by inhomogeneous deformation of the rocks of the magmatic arc, of the basin floor, and particularly of those on the continental side of the basin.
Detailed structural analysis shows that the rocks of Isla de los Estados are disposed in a major asymmetric, noncylindrical syncline. The island, nearly 1,000 m high, lies in the core of this fold. A strong slaty cleavage developed prior to the large-scale folding. Both bedding and early slaty cleavage were reoriented by this folding. A new foliation axial planar to the large syncline is present only on the inverted limb in the more strongly compressed eastern area. Late, flat-lying cleavages are also most strongly developed in the east, but in the lower limb of the fold.
The structures are all interpreted to be the result of progressive deformation of the rocks during a single period of regional compression. The sequence of strains correlates closely with that deduced from theoretical and experimental studies of buckling. The early cleavage is thought to have developed during initial shortening of the layering of the rocks. Subsequent buckling of the thick multilayer of competent silicic volcanics produced the syncline. The late, flat-lying cleavages are considered to have resulted from gravitationally induced vertical shortening produced by the greatly increased overburden of the tectonically thickened rock pile.
The dominant northward vergence of the structures undoubtedly reflects the uplift of the Pacific margin of the continent, but the initial layer-parallel shortening indicates the operation of a stress system with maximum compressive principal stress subhorizontal and at right angles to the Pacific margin. The crustal thickening resulting from the horizontal shortening and the buckling seems to have played a major role in the mid-Cretaceous orogenic uplift of the southernmost part of the Andean Cordillera.