WHAT INDUCES THE COLLAPSE OF OROGENS? IS SOUTH AMERICA NEXT?

Comparisons between the North and South American Cordilleras are easy and natural. Both are archetypical non-collisional orogens, with 100's of millions of years of subduction history, a western arc and eastern fold thrust belt and genetic links to the opening of the Atlantic Ocean basin. The ~70 Myr difference in age of the two orogens has been linked to the ~70 Myr lag in the opening of the south Atlantic following the central Atlantic. The subsequent evolution of the Cordilleran orogens are also quite similar, including a period of flat slab subduction ~60-70 Myr after the development of a wide and potentially thick fold-thrust belt. Projecting ahead, will the Andes experience the orogenic collapse that has been documented in western North America? To answer this question it is necessary to understand the plate tectonic environment that facilitated the collapse. We integrate two new large GIS-based datasets to create palinspastic restorations of extension and volcanism. From this we readdress the relationship between plate-boundary deformation, intra-plate extension and magmatism in western North America. Using ArcGIS and custom software, we retrodeformed the NAVDat (North American Volcanic Database, navdat.geongrid.org) using the western North America reconstruction of McQuarrie and Wernicke (2005). We compare this data to strain rates calculated over a 50 km-grid forward-deformed from 36 Ma to present. These reconstructions illuminate southwestward migration of volcanism in the northern Basin and Range province in conjunction with westward expansion of the continental margin. This pattern strongly supports the contention that orogenic collapse was accommodated by slab rollback. Southern Arizona and the Colorado River extensional corridor show a remarkably similar westward migration of extension and volcanism. Together these patterns suggest that extension was not an intra plate phenomenon, solely driven by gravitational collapse, but rather a plate boundary phenomenon, i.e. a result of a change in plate boundary conditions (slowing of North America plate motion and foundering of the Farallon plate). This view of North America tectonics suggests that although the Central Andean plateau is high, underlain by weak crust and is the site of local extension, full-scale orogenic collapse is not immanent.