Date of Award

Summer 8-27-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Hoke, Gregory D

Subject Categories

Earth Sciences | Geology | Geomorphology | Physical Sciences and Mathematics

Abstract

Understanding how tectonic and climatic forces influence erosion and shape mountains is important to understand the evolution of the landscapes through earth's history. This dissertation aims to determine how do tectonic processes and climate variability interact to shape Earth's tropical mountains. In chapter 1, I studied the Eastern Cordillera of the Northern Andes, a mountain range located in Colombia in the tropics of South America. To obtain exhumation rates, I used thermochronology, which is a method that records—over timescales of millions of years—the rate at which rocks located at great depths within the Earth cool as they are transported to the surface through the erosion of overlying rock from the surface. The results show that the rates and timing of exhumation are spatially variable along the Eastern Cordillera and the highest erosion rates occur near the places that have the most tectonically active faults. These zones of highest exhumation are associated with inherited structures. This finding shows that higher precipitation rates are not always associated with higher erosion rates and, by extension, that precipitation rates along the Eastern Cordillera are not a major factor controlling erosion rates. This study underscores the importance of a thorough characterization of the location and recent activity of faults in a region to understand erosion patterns and natural hazards in tropical mountains. In chapter 2, I explore how the timing of incision of kilometer-scale canyons into high topography can inform us about the surface uplift history of mountain ranges by tectonic and geodynamic processes or via changes in paleoclimate. The main goal of the project was to decipher the timing and, ultimately, the role of tectonic versus climatic mechanisms that led to incision of the Cauca River Canyon in the northern Central Cordillera of the Northern Andes. To do so, I used thermochronology, because the cooling history of rocks on the walls of canyons, as a response to locally focused exhumation, can be used to constrain the rate and timing of canyon incision. Previously published U-Th/He data from other canyons in the Central Cordillera revealed old cooling ages ranging from 26-45 Ma. The absence of younger ages in these canyons could be explained if these canyons were incised recently (< 10 Ma) but the magnitude of incision was insufficient to reveal the younger cooling ages below a Partial Retention Zone. By studying the much deeper Cauca River Canyon, where incision has exposed lower structural levels, I was able to reveal younger U-Th/He ages at the valley bottom which helped to constrain the timing of incision at 6-7 Ma. The Cauca Canyon was carved because of major rock uplift in the northern Central Cordillera and propagation of an erosion wave into the mountain range starting in the latest Miocene. In chapter 3, I explored how the topography of the Northern Andes has responded to Neogene variations in slab geometry, climate, and drainage reorganization. I also discuss how the Neogene topographic changes may be one of the drivers that make the tropical Northern Andes of Colombia one the world's most biodiverse places on earth. I used a geomorphic analysis to characterize the topography of the Western and Central Cordilleras of the Northern Andes. The topographic analysis was supplemented with erosion rate estimates based on gauged suspended sediment loads and river incision rates from volcanic sequences. There are several geomorphic differences from south to north in the Central Cordillera, which coincide with the proposed location of a slab tear and flat slab subduction under the northern Central Cordillera, as well as with a major transition in the channel slope of the Cauca River. Slab flattening appears to be the most likely cause of strong and recent uplift in the Northern Andes leading to ~2 km of surface uplift since 8–4 Ma. Large scale drainage reorganization of major rivers is probably mainly driven by changes in upper plate deformation in relation to development of the flat slab subduction geometry. Instead, to the south of the slab tear, other factors such as the emplacement of volcanic rocks likely play important roles in driving drainage reorganization. Several isolated biologic observations above the area of slab flattening suggest that surface uplift of the Central Cordillera isolated former lowland species on the high elevation plateaus, and drainage reorganization may have driven diversification of aquatic species.

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