Petrogenetic Evolution During Cordilleran Orogenesis: Constraints from the North and South American Cordilleras

Abstract

Subduction-related mountain building along the margins of continents is often characterized by alternating periods of predominant compressional, extensional, and neutral stress conditions. Shifts may arise from changes in plate configuration or dip, or cyclical processes and feedbacks within the upper plate. In the North and South America cordilleras, orogen-wide connections between diverse kinematic, magmatic, and geodynamic processes are proposed to shape the formation and eventual demise of large mountainous regions. The mountainous interior, retroarc, regions of mountain belts are characterized by crustal thickening and surface uplift but are also prone to erosion and subsequent collapse. The primary goal of this study is to gain a deeper understanding of crustal and mantle dynamics in Cordilleran retroarcs to refine tectonic models of orogenesis.

Chapter I presents a combined study of zircon Hf and trace element geochemistry in combination with apatite and zircon U–Pb geochronology of retroarc plutons in the North American Cordillera to reconstruct spatiotemporal changes in melt sources and evaluate proposed links between magmatism and deformation. We compile new and existing data to assign the first reliable ages and geochemical assessments for most plutons in the northern Great Basin. Discrete changes in melt geochemistry and emplacement style indicate a close association with geodynamic processes and support proposed orogen-wide linkages in Cordilleran orogenic systems.

Chapter II provides a reconstruction of the Jurassic–Oligocene exhumation history of the same suite of retroarc plutons using zircon and apatite (U-Th)/He thermochronology. Thermal history modeling reveals a prolonged history of upper-crustal cooling and exhumation that is punctuated by a rapid and high-magnitude late Eocene–Oligocene exhumation event triggered by the arrival of regional magmatism. We attribute this event to the dynamic thermal and isostatic effects of Farallon slab removal and possible lithospheric delamination.

Chapter III presents results of a study of crustal evolution of western South America using U–Pb geochronology and Hf isotope geochemistry of detrital zircon extracted from sedimentary samples that span a broad stratigraphic range in the southern central Andes. Time-calibrated Hf isotopic values indicate several dynamic periods of crustal generation and recycling that correlate to assembly of the South American continent.