Investigating the Interplay Between Crustal Thickening, Magmatism, and Lithospheric Removal During the Evolution of Cordilleran Arcs

Abstract

Cordilleran arcs are high-elevation magmatically active mountain belts that have thick (up to 75 km) continental crust as a result of high-magnitude shortening in the retroarc thrust belt. These systems, which form where oceanic crust subducts beneath a continent, undergo cycles of deformation, magmatism, lithospheric foundering, and surface uplift. In this dissertation, which is composed of four manuscripts, I investigate the interplay between crustal thickening, magmatism, and lithospheric removal during the evolution of cordilleran arcs. In Appendix A, I use well constrained correlations between elevation and igneous geochemistry to reconstruct the crustal thickness and uplift history of the central Andean arc, the type locality of a modern cordilleran orogenic system, from the Jurassic-Present. Results from this study indicate prolonged crustal thickening throughout the Mesozoic, generation of thick (>40 km) crust along the central Andes by the Paleocene, and surges of surface uplift throughout the Cenozoic that are diachronous along strike of the orogen, such that the frontal arc along Puna plateau latitudes achieved modern elevations by the Eocene while the frontal arc at Altiplano plateau latitudes did not attain modern elevations until the Miocene. In Appendix B, I use Zn/Fe ratios of a global database of volcanic rocks to show that, at cordilleran arcs, pyroxenites rather than peridotites are the primary source of magmas in these settings. Since pyroxenites have lower melting temperatures and higher melt productivities compared to peridotites, models of cordilleran arcs that assume peridotite melting must be revised to invoke pyroxenite melting as the primary mechanism of magma generation. In Appendix C, I explore via thermodynamic modeling the factors that control the foundering ability of subarc lithospheric roots, which are typically composed of negatively buoyant residual garnet clinopyroxenites (“arclogites”). Results indicate that the presence of melt, as well as the protolith lithology of restitic arclogites, greatly impacts the density of the lithospheric root and its ability to founder into the mantle. Finally, in Appendix D, I conduct partial melting experiments on arclogites to determine the geochemical composition of arclogite-derived melts and to show that, rather than contributing significantly to the magmatic budget of cordilleran arcs, these lithologies are dominant sources of rare-earth element-hosting alkaline-silicate complexes that erupt along post-collisional rift zones. Altogether, this document yields new insights into the processes that control, and that occur as a result of, the cyclic behavior of cordilleran orogens.