Behavior of Hybrid Thick- and Thin-Skinned Orogenic Wedges in the Western Interior USA and South-Central Andes

Abstract

The western interiors of North and South America host two of the largest fold and thrust belts on Earth, recording upper-plate processes active during ocean-continent convergence and the construction of Cordilleran orogenic systems. These systems are at dramatically different stages in their tectonic evolution, enabling a comparison of the actively forming Andes with their ancient counterpart in North America. Despite this advantageous geologic situation, fundamental questions remain concerning the long-term structural, thermal, and kinematic behavior of thrust belts. Cordilleran systems are inherently linked; therefore, answering questions related to the mechanics of thrust belts is critical to illuminating their feedback with other components such as the orogenic plateau, magmatic arc, and foreland basin system. This dissertation addresses the tectono-thermal history of relatively neglected segments of the thrust belt in the northern US Cordillera and south-central Andes using field mapping, cross section construction, structural analysis, low-temperature thermochronology, thermal history modeling, and regional space-time tectono-magmatic syntheses. Appendix A provides the first low-temperature thermochronologic data from the Little Belt Mountains of west-central Montana, showing that exhumation of the northernmost intraforeland uplifts occurred in the Late Cretaceous. These data, considered in the context of a regional synthesis from an adjacent salient in the Sevier thrust belt, are used to highlight abundant field evidence for Cretaceous out-of-sequence deformation and propose a new model linking thin- and thick-skinned structures in the region. In Appendix B, the highest thermochronology transect from the South American continent constrains uplift and exhumation of the Frontal Cordillera to the Early-Middle Miocene (ca. 20 Ma), contemporaneous with surface uplift and crustal thickening in this segment of the Andes. Detailed and regional mapping, an orogen scale cross section, and low-temperature thermochronology results in Appendix C provide insights into the regional stratigraphy, structural geology, and thermal behavior of the south-central Andean thrust belt at 34°S. Special focus is placed on preexisting stratigraphy and inherited structures, highlighting their essential role on the development of cordilleran thrust belts and identifies them as a contributing factor to the decrease in crustal shortening magnitude along strike of the Andes. Appendix D contains double-dated apatite fission track and (U-Th)/He thermochronology data from the San Rafael Block, a low-relief basement-cored structure that interrupts the modern Andean foreland basin system at 35°S. New data and associated thermal history modeling constrain important aspects of the SRB’s low-temperature thermal history, including Late Jurassic intraplate deformation, uplift and exhumation within the nascent Late Cretaceous (ca. 100-90 Ma) foreland basin system, limited Cenozoic burial, and ca. 10 Ma rapid uplift and exhumation of the SRB during the most recent phase of Andean mountain building.