SYNOROGENIC STRATA IN COMPRESSIONAL SETTINGS: EXAMPLES FROM THE CENTRAL ANDES

Compressional orogenesis generates an array of sedimentary basins with diagnostic stratigraphic and structural signatures. Accurate identification of these signatures can provide insight into the kinematics and styles of fold-thrust shortening, modes of basin development, and paleoelevation histories of mountain belts. An idealized four-part classification of foreland basin systems (wedge-top, foredeep, forebulge, and backbulge depozones) successfully describes the growth of many modern and ancient systems at low elevations. However, additional basins may develop in orogenic interiors, commonly at high altitudes. Distinguishing low-elevation foreland basins from elevated intermontane basins is possible through structural and stratigraphic studies at local and regional scales, including analysis of basin-margin faults, growth strata, subcrop relationships, provenance, and lithofacies. These studies facilitate reconstructions of long-term shortening records, sediment accommodation mechanisms, and accumulation histories. In the central Andes, both foreland and intermontane basins recorded Cenozoic shortening, crustal thickening, and uplift. In the Eastern Cordillera of Bolivia, eastward emplacement (insertion) of a basement-involved tectonic wedge, rather than simple thrust-front advance by frontal imbrication, resulted in a pattern of dominantly late Paleogene foredeep evolution followed by early Neogene growth of intermontane basins. The foredeep deposits occur in regionally continuous synclines, are flanked by major structures, concordantly overlie lower Cenozoic strata, contain 1-2 km of upward coarsening fill, exhibit quartzose sandstone compositions, contain a mix of Paleozoic-Mesozoic clasts, display transverse paleocurrents, and include fluvial megafan facies. In contrast, the intermontane basins occur in spatially limited areas, lack basin-margin structures, unconformably overlie a range of deformed Paleozoic and Mesozoic rocks, contain <0.5 km of interbedded fine and coarse sediment, exhibit more lithic sandstone compositions, contain mostly lower Paleozoic clasts, display axial paleocurrents, and include dominantly lacustrine and local alluvial-fan facies. The two-phase history of basin development is consistent with major mid-Cenozoic shortening, thickening, and suface uplift in Bolivia.