A COMPARISON OF THE NORTH AMERICAN AND SOUTH AMERICAN RETROARC FORELAND BASIN SYSTEMS

The South American (SA) and North American (NA) retroarc orogenic belts are often cited as modern-ancient counterparts, but no detailed comparison of their foreland basin systems has been attempted. The NA Cordilleran foreland basin system evolved from Late Jurassic through Campanian time and was subsequently partitioned into local depocenters with intervening basement-cored Laramide-style uplifts during late Campanian-Eocene time. Shortening in the Cordilleran thrust belt continued throughout the Laramide interval, and eventually involved the Jurassic-Campanian foreland basin deposits. A complete foreland basin system is preserved in the NA Cordilleran thrust belt, consisting of (1) upper Jurassic fluvial and eolian back-bulge deposits; (2) a regional disconformity and paleosol condensation interval inferred to represent the passage of the forebulge through the region; (3) a >3 km thick eastward tapering foredeep prism of lower Cretaceous-Santonian alluvial, fluvial and marine clastic deposits; and (4) a variably thick (up to ~1 km) progressively deformed wedge-top depozone consisting of alluvial fan and fan delta deposits. Recent work in the Bolivian and northern Argentine Andes suggests a similar succession of back-bulge, forebulge, foredeep, and wedge-top depozones is preserved in the Paleogene-lower Miocene deposits of the Eastern Cordillera. Along with the eastward propagating central Andean thrust belt, this system migrated across westward underthrusting South American lithosphere to its present location in the modern Andean foreland. Still unknown is the southward extent of the Paleogene foreland basin system in the central Andes: possible complications owing to interference with the Cretaceous Salta rift and early (i.e., Paleogene) partitioning of the foreland by a developing Puna Plateau pose challenges to simple models of regional foreland basin subsidence. In both the NA and SA foreland basin systems, local complications resulted from vagaries of pre-orogenic stratigraphic architecture, reactivation of ancient structures, and relative plate motions. Nevertheless, remarkable first-order similarities between the NA and SA thrust belts and foreland basin systems encourage further comparisons of tectonic and magmatic processes in these two great orogenic systems.