Date of Award

5-10-2026

Date Published

June 2026

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth & Environmental Sciences

Advisor(s)

Gregory Hoke

Keywords

Andes;Burial dating;Cosmogenic nuclides;Geomorphology;Quaternary;Tectonics

Subject Categories

Earth Sciences | Geology | Physical Sciences and Mathematics

Abstract

Landscape evolution in the tropical Northern Andes is governed by complex interactions between tectonic deformation, climatic shifts, and bedrock lithology. Standard geomorphic models often struggle to capture the full spectrum of these processes, particularly in active orogens where topographic steady state is frequently challenged by variable sediment dynamics and deep weathering. Understanding how these factors dictate landscape responses over different timescales is critical for interpreting the relationship between landscape evolution and surface processes of tropical mountain belts. This dissertation utilizes a combination of cosmogenic nuclide dating, numerical modeling, and subsurface borehole analysis to examine the structural, climatic, and lithological controls on landscape evolution, sediment routing, and Critical Zone (CZ) development across the Colombian Andes. For my first chapter, I determined the timing and climatic drivers of Quaternary sediment dynamics by dating the Bucaramanga Alluvial Fill (BAF). Accurately dating such sequences requires accounting for complex burial histories, so I evaluated four 26Al/10Be burial dating models on quartz samples from the BAF. I demonstrated that a complex burial model incorporating progressive sediment accumulation and a two-exponential muon parameterization provides the most robust age estimates, yielding ages roughly 10% older than simple burial models. Applied to the BAF, these well-constrained ages indicate that peak aggradation occurred between ~1.2 and 0.5 Ma, aligning with the Mid-Pleistocene Transition (MPT). These results suggest that early Pleistocene 41-kyr orbital cycles favored sediment accumulation, while the MPT's shift to high-amplitude 100-kyr cycles transitioned the system toward net incision, catalyzing major regional drainage reorganizations and extending the known pattern of MPT-driven landscape shifts approximately 1,500 km northward into the tropical Andes. Shifting focus from depositional basins to the high-elevation source areas, my second chapter investigates the anomalously long-lived topography of the Eastern Cordillera’s axial plateau to understand the precise mechanisms governing its preservation. I integrated paired 10Be and 26Al catchment-wide denudation rates, residence time analysis, Schmidt-hammer erodibility assessments, and a novel 3D talus production model across the Oiba and Arcabuco anticlines. Our results reveal exceptionally low regional erosion rates (13.1±8.6 m/Myr) and extended residence times for excess topography (~2.9 Myr), far exceeding those of highly active orogens. I demonstrate that a "soft-over-hard" stratigraphic sequence imposes rigid local base levels that halt upstream incision. Furthermore, intense structural deformation in resistant units triggers a negative feedback loop: highly fractured cliffs shed massive blocks that overwhelm fluvial transport capacity, armoring channel beds and stalling vertical incision. Ultimately, scaling these localized litho-structural controls indicates that the axial plateau is a "buffered" landscape, actively maintained by resistant sandstone rims that shield the interior from headward capture by transverse rivers. My third and final chapter moves to the eastern flank of the tropical Western Cordillera to examine how these coupled structural and lithological controls influence deep subsurface weathering and the CZ. To circumvent the limitations of surface-level studies, I analyzed 18 boreholes to constrain the complexities of the CZ. The data show that deep (>48 m), multilayered weathering profiles dominate high-elevation ridges, while saprolite is absent at lower elevations. Estimates of fracture density (P10) indicate that fracturing exhibits dual controls on weathering profile development: at mid-to-low elevations, high fracture density enhances erosion and inhibits saprolite formation, whereas at high elevations, fracturing combines with vegetation and active meteoric water percolation to promote deep weathering. By showing that fractured mafic rocks can develop weathering profiles comparable to granitic systems, this chapter underscores the profound, coupled role of lithology and structure in sustaining thick CZ and shaping the broader geomorphic evolution of the Andes.

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