Evidence for a Middle-Late Paleozoic Foreland Basin and Significant Paleolatitudinal Shift, Central Andes

Devonian-Permian data of western Bolivia and adjacent regions are used to construct a paleogeography of the central Andes. Four phases characterize the sedimentation history. (1) Shallow marine clastic deposition occurred through the Devonian (Lochkovian-Frasnian), with an increase in sedimentation in Emsian-Eifelian time. Lithofacies distribution and sediment thicknesses indicate primarily a western source. Endemic, high-latitude (>55° S) fauna with several megafaunal originations in Bolivia also contain organisms characteristic of North Africa and northeastern United States (Middle Devonian megafaunas and Late Devonian palynomorphs). (2) Latest Devonian-Early Carboniferous (Famennian-Visean) sedimentation is characterized by glaciomarine and fan-deltaic sedimentation. Clasts are derived from underlying sedimentary units and andesitic, granitic, and tuffaceous rocks. (3) A middle Carboniferous (Serpukhovian-Bashkirian) hiatus in sedimentation occurred, its age and duration varying across the region. (4) Siliciclastic and carbonate deposition occurred in Late Carboniferous-middle Permian time (Moscovian?-Artinskian). The clastics were derived from a western source. Carbonate rocks (Copacabana Formation) were deposited in situ, in low latitudes (25° S lat). Devonian sedimentation is inferred to have occurred on continental crust in a foreland setting, with a western magmatic arc. Restoration of the San Nicolas batholiths (U-Pb zircon, 425 and 394-388 Ma) relative to the Devonian basins suggests that they may have constituted the magmatic arc. Intra-arc basins may have existed near present-day coastal Peru. Following the middle Carboniferous hiatus, sedimentation continued in a back-arc region, although differentiation of distinct Carboniferous and Permian basins, the intrusion of plutons inboard of and within the basin along strike, and extensional faulting in the Late Permian indicate major changes in the tectonic setting, possibly including a reorientation of the subducting slab to a low angle. Uncertainties in the tectonic setting interpretation are introduced by the incomplete stratigraphic record, which is obscured in the Altiplano and Cordillera Occidental, and by the undefined history of plate boundary interactions, such as possible postdepositional strike-slip motion and tectonic erosion along the plate margin.