Cretaceous to Cenozoic evolution of the southern Central Andes and basins therein

Abstract

The Andean Cordillera exhibits a striking contrast between the high, broad Altiplano-Puna plateau in its central portion and the narrower, lower-elevation Patagonian Andes to the south. The transition between these end-members across the southern Central Andes (~28°–36°S) reflects fundamental, yet poorly understood, differences in crustal architecture and tectonic evolution. The four interconnected studies of this dissertation integrate synorogenic sedimentary, magmatic, and stable-isotope records to clarify these issues. The first study (Appendix A) analyzes the well-preserved Eocene–Miocene Manantiales foreland basin (~32°S), documenting evolution from distal foredeep to syndepositionally deformed wedge-top. Facies, detrital zircon U–Pb, and structural data indicate episodic wedge growth characterized by phases of eastward thrust belt propagation alternating with significant out-of-sequence deformation. This finding reconciles conflicting kinematic models: apparent westward-younging deformation is consistent with cyclic behavior predicted by Coulomb-wedge theory, rather than exotic west-vergent tectonics. The second study (Appendix B) expands on the preceding basin analysis by evaluating the isotopic stratigraphy of the Manantiales basin. Volcanic glass ?D and carbonate ?18O/?13C stable-isotope data are compared to seasonal Rayleigh fractionation models and an empirical isotope–elevation curve to estimate paleoelevation under differing moisture regimes. This analysis supports a two-step basin evolution: (1) ca. 18 Ma moisture-source reorganization during Cordillera del Tigre uplift; (2) post-16.5 Ma isotopic enrichment from enhanced evaporation and/or monsoonal influence during the Miocene Climatic Optimum. This work develops paired carbonate–glass proxies as a tool to deconvolve climate versus uplift signals in tectonically complex settings near moisture source transitions. The third study (Appendix C) uses whole-rock geochemistry and zircon petrochronology to assess the Late Cretaceous–Quaternary evolution of crustal thickness below the Andean arc at ~35°S. Results suggest minor Late Cretaceous crustal thinning, coinciding with normal faulting and Farallon plate subduction initiation below the region. Little change in crustal thickness is evident across Paleogene time, challenging popular models invoking widespread extension. Crustal thickening of at least 8.4 ± 4.7 km is found to have initiated after ca. 20 Ma, linking Neogene shortening to construction of the Andean crustal root. This work also evaluates zircon trace elements as crustal thickness proxies, finding that Sm/Yb, Gd/Yb, and Dy/Yb may most reliably track the depth of the sub-arc Moho. The fourth study (Appendix D) integrates sedimentology and geochronology results from Cretaceous–Miocene deposits across the High Andes (~31.5–32.5°S) with the broader basin record of the southern Central Andes. Newly recognized outcrops of the Diamante Formation are interpreted as the northernmost deposits of a mid-Cretaceous foreland basin. This basin was interrupted after ca. 84 Ma by low-silica alkaline magmatism, likely recording passage of the Aluk–Farallon slab window. By the Late Eocene, deposits of the Río de los Patos Formation mark establishment of a foreland basin in front of thrust belt structures near the international border. Growth strata and angular unconformities help constrain Early Miocene (ca. 21–17 Ma) fold-thrust belt propagation and later (ca. 13–9 Ma) out-of-sequence deformation. Through the framework of flexural wave migration, these results suggest that foreland basin development was more episodic in the southern Central Andes than in regions to the north. This dissertation improves understanding of the tectonic framework of the southern Central Andes, showing a fundamentally different orogenic development than sectors to the north. Thus, modern along-strike change in Andean topography are shown to have ancient origins in divergent Cretaceous–Paleogene tectonic histories. Mountain building in the region is seen as an unsteady process modulated by both internal dynamics—such as wedge mechanics and sedimentation feedbacks—and external forces like plate reorganizations and ridge subduction. This framework advances understanding of how upper- and lower-plate factors interact to produce along-strike variations in Cordilleran orogenic systems globally.