Deformation of Gondwana margin turbidites during the Pampean orogeny, north-central Argentina

Abstract

Basement uplifts in the Eastern Sierras Pampeanas of north-central Argentina expose sedimentary rocks that originated on the Neoproterozoic to early Cambrian margin of Gondwana for over 1000 km along strike. A series of transects through unmetamorphosed sections of the Vendian-aged Puncoviscana Formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage has been folded into chevron structures, with a concomitant axial planar tectonic cleavage, prior to erosion and deposition of the overlying Middle to Late Cambrian Meson Group. Changes in structural style are shown to be gradational; all mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Late-stage Andean brittle folds and thrusts are more noticeable in the northern sections, but do not significantly alter the overall structural style or geometry of the basement rocks. A static metamorphic overprint in the Ordovician with associated granitic intrusions only affects fabrics and mesoscale structures adjacent to plutons. Published age constraints indicate an overlap between the timing of sedimentary deposition and that of structural and metamorphic events at depth. We develop a tectonic model for the Pampean orogeny that involves the buttressing of off-scraped sedimentary rocks above a subducting slab beneath the Gondwana margin. The collision of outboard terranes did not occur in this area until at least the mid-Ordovician Famatinian orogeny.