FLAT SLAB SUBDUCTION, TECTONIC INHERITANCE, AND BROKEN FORELAND BASINS IN THE ANDES

Retroarc orogenic systems along convergent plate boundaries show sharp variations in total orogenic width, extent of intraplate deformation, and structural disruption of foreland basins. Analysis of late Cenozoic contractional deformation along the Andean mountain front and adjacent foreland highlights the contrasting degrees of deformation advance toward the plate interior. The retroarc positions of the Andean topographic front (marked by frontal thrust-belt structures) and foreland deformation front (defined by isolated basement block uplifts) range from 300 to 900 km inboard of the trench. Over the ~8000 km arcuate length of the Andes (10°N–55°S), four discrete maxima of inboard deformation advance are spatially co-located with: (a) the Peruvian (5–14°S) and Pampean (27–33°S) zones of flat slab subduction and associated broken foreland basins; (b) the Nazca-Antarctica ridge collision (45–48°S) and resulting Patagonia slab window; and (c) the anomalously thick Paleozoic stratigraphic wedge of Bolivia (17–23°S). The spatial correspondence of retroarc shortening with specific geodynamic configurations demonstrates the role of flat slab subduction, slab window development, and combined structural and stratigraphic geometries in shaping Andean orogenic architecture. Although this analysis shows a clear spatial correlation between flat slab subduction and zones of distributed intraplate shortening and broken foreland conditions, the underlying mechanisms are poorly understood. We suggest that deformation advance, while clearly sensitive to geodynamic configuration, is also regulated by mechanical processes related to crustal/lithospheric strengthening, weakening, and/or tectonic inheritance. Finally, as is the case for the Laramide broken foreland, flat slab subduction is neither necessary nor sufficient to uniquely explain all examples of inboard deformation advance within continental plate interiors.