Browsing by Author "Gallen, Sean, advisor"
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Item Open Access Exploring new approaches to understanding channel width and erosion rates in bedrock rivers, Puerto Rico, USA(Colorado State University. Libraries, 2022) Eidmann, Johanna Sophie, author; Gallen, Sean, advisor; Rathburn, Sara, committee member; Hughes, Kenneth Stephen, committee member; Ham, Jay, committee memberEarth system dynamics produce constant adjustments to sea level, tectonics, and climate. Bedrock rivers communicate these changes throughout mountains by driving landscape and erosional responses that facilitate topographic change. It follows that an improved understanding of bedrock rivers can help us better model and reconstruct the interplay of changes to base level, uplift, and climate from landscapes. Although bedrock channel width plays a first-order role in river stream power and stream power-based landscape evolution models, because of the physical challenges associated with acquiring these data, channel width is often estimated and introduces uncertainty. In addition, the lack of bedrock channel width data has limited our understanding of what factors control channel width. In this dissertation (Chapter 2), I leverage high-resolution topographic data, Mean Annual Precipitation information, and use the HEC-RAS river modeling software to remotely derive bedrock channel width at desired flow scenarios. The accuracy of modeling results is verified for rivers in Puerto Rico using USGS gauging station field measurements, as well as my own channel width field measurements associated with 1-year recurrence interval discharges. As a next step, (Chapter 3) I implement the bedrock width modeling method derived in Chapter 2 to obtain >4,000 channel width measurements from reaches across Puerto Rico. I then compare these bedrock river width values to various factors (e.g. rock type and rock strength, drainage area, Ecozone, and grain size) that have been identified in the literature to scale with or influence channel width. My analyses indicate that, in Puerto Rico, rock type is a dominant control of bedrock channel width in small (≤6-10 km2) drainage areas. Contrary to patterns of rock strength and bedrock width documented in the literature (e.g. Montgomery and Gran, 2001), I find that width doesn't appear to correlate with proxies for bedrock channel strength. Strong granodiorites have the widest low-order channels and the strong volcaniclastics and weak serpentinites have comparably narrow low-order channels. Analysis of limited grain size measurements shows a discernable difference in the coarse grain size distribution between the three rock types, with the volcaniclastic and serpentinite draining rivers having coarser sediment than granodiorite draining streams. These findings suggest that bedrock channel width may be influenced by unmeasured lithological parameters that impact the size of grains delivered to river channels from adjacent hillslopes (i.e. rock fracture density and spacing, as well as weathering). Lastly, (Chapter 4) I spatially analyze in-situ cosmogenic nuclide (10Be in quartz and 36Cl in magnetite) concentrations and find that bedrock erosion rates are higher in the central part of Puerto Rico than toward the east. Analysis of erosion rates compared to other parameters reveals that channel steepness, rather than precipitation or rock type, is positively associated with erosion rates. I further apply these erosion rate data to test the accuracy of four incision models of varying complexity. Model comparisons reveal that drainage area is a better predictor of incision rates in Puerto Rico than a precipitation-weighted drainage area parameter. In addition, whereas an increase in model complexity slightly improves model performance, the model only explains ~35% of the variability in erosion rates. It follows that current incision models are still missing many controlling factors of river incision rates in Puerto Rico.Item Open Access From the Colorado Front Range to global topography: evaluating the roles of tectonics and climate on long term landscape evolution(Colorado State University. Libraries, 2022) Marder, Eyal, author; Gallen, Sean, advisor; Pazzaglia, Frank, committee member; Wohl, Ellen, committee member; Schutt, Derek, committee member; Kampf, Stephanie, committee memberLandscapes are primarily shaped by the interactions between tectonics and climate, and their interplay and relative roles in landscape evolution over thousands to millions of years have a significant impact on global erosion and nutrient and sediment productions. Thus, understanding and quantifying the impact of tectonics and climate on short- to long-term landscape evolution has large implications on natural global cycles (e.g., climate change, atmospheric and terrestrial carbon circulations), biodiversity and ecological sustainability, hazard management (e.g., earthquakes, landslides), infrastructure planning, and decision making. In the last decades, significant progress has been made in the field of tectonic geomorphology to try and resolve the relative roles of tectonics and climate in landscape evolution. Yet, many questions remained unresolved, for instance: - What drives landscape evolution in post-orogenic settings? - What is the relative role of climate in landscape evolution at the global scale? In my PhD, I address these questions by investigating the impact of tectonics and climate on fluvial topography and geomorphology at different spatiotemporal scales. In my first chapter, I present a local study in the southern Colorado Front Range to explore the relative roles of tectonics and climate on observed landscape unsteadiness that affected the area during the late Cenozoic. In the second chapter, I extend this study and address this question to the scale of the entire Colorado Front Range. In my third chapter, I explore the impact of climate on fluvial topography at the global scale. For all these studies, I integrate field data, digital topographic analysis, geochronology, and modeling to compare new and existing predictions for the roles of tectonics and climate at the local (chapter I), regional (chapter II), and global (chapter III) scales to empirical observations. Results from these studies shed light on some ongoing controversies (e.g., what drives topographic rejuvenation in the Colorado Front Range) and resolve misunderstood concepts (e.g., how climate is recorded in fluvially-dominated landscapes). The first and third chapters in this dissertation were submitted to peer-reviewed journals and are under review, while the second chapter is in its final stage as a third manuscript for a peer-reviewed journal. FIRST CHAPTER: LATE CENOZOIC DEFORMATION IN THE SOUTHERN COLORADO FRONT RANGE REVEALED BY RIVER PROFILE ANALYSIS AND FLUVIAL TERRACES Post-orogenic landscapes are important sources of sediment and nutrients relevant to many natural global cycles and ecological sustainability. Many of these settings exhibit evidence of recent landscape unsteadiness, but their driving mechanisms are poorly understood. The Colorado Front Range (FR), a post-orogenic setting that maintains high relief, elevated topography, and evidence of ongoing unsteadiness, is a good example of this enigma. Two prevailing hypotheses have been proposed to explain the geologically-recent landscape unsteadiness in the FR: (1) mantle dynamics and active tectonics during the late Cenozoic; (2) enhanced erosional efficiency associated with a Quaternary climate change. Here we evaluate these end-member hypotheses through a case study of tectonic geomorphology of the Upper Arkansas River basin in southern Colorado. We perform river profile analysis on bedrock channels in the eastern Rockies and map and analyze fluvial terraces in the western High Plains. We find that knickpoints in the eastern Rockies record a one- to two-stage increase in base level fall rate downstream of the FR mountain front and an eastward increase in the magnitude of incision. Similarly, terraces in the western High Plains record an eastward increase in the magnitude of incision. Collectively, and supported by flexural and supplemental geomorphic analyses, these results suggest a previously undetected regional-scale, west-directed back tilting signal associated with differential rock uplift. Based on existing geodynamic models, we interpret these deformation patterns and related landscape response as a result of a migrating dynamic topography that swept the southern FR from west to east during the late Cenozoic. SECOND CHAPTER: TECTONIC AND GEODYNAMIC CONTROL ON REJUVENATION IN THE COLORADO ROCKY MOUNTAINS The Colorado Rocky Mountains (CRM) ancient foreland basin, currently known as the High Plains, shows a steeper long-wavelength tilt away from its hinterland relative to other active mountain range foreland basins worldwide. Further, studies showed that the High Plains experienced a transition from a system of net deposition to one characterized by net erosion at ~5 Ma. However, the mechanisms proposed to explain these observations are the center of ongoing debate. Some argue that the tilting and the transition from deposition to erosion were facilitated by tectonically- or geodynamically-driven changes in rock uplift rate, while others argue that these records are simply the result of an increase in erosional efficiency driving river incision and relaxation with some amount of isostatic rebound. One of the main reasons this controversy continues is that empirical studies trying to address this question were conducted mostly in the High Plains, where landscape geomorphic signatures used to distinguish between these two hypotheses are ambiguous. Here, we conduct a geomorphic analysis of the Colorado Rockies, which lies upstream of the High Plains province and is characterized by a harder crystalline basement, where bedrock rivers might still achieve a record of the transient landscape of the CRM and help clarify potential drivers. We combine river profile analysis with a compilation of new and existing basin average erosion rates from cosmogenic 10Be and channel incision rates from luminescence dating on fluvial terraces to differentiate two geomorphic zones in the Colorado Rockies: 1. an upper, relict topography upstream of convex upward knickpoints that is consistent with lower long-term background erosion rates of ~0.03 mm/yr and lower channel steepness of ~80-100 m0.9; 2. a transient landscape downstream of these knickpoints that is consistent with higher channel incision rates of ~0.3 mm/yr and higher channel steepness that increases systematically from ~150 m0.9 in the northern CRM to 300 m0.9 in the southern CRM. These results and their spatial patterns across the CRM are inconsistent with existing predictions from a climate-induced increased erosional efficacy during the last Cenozoic. Rather, they imply a long-wavelength deformation and a sustained tectonic uplift rate associated with active tectonics and geodynamics that impacted the CRM in the last 5 Ma. THIRD CHAPTER: CLIMATE CONTROLS ON FLUVIAL TOPOGRAPHY Conceptual and theoretical models for landscape evolution suggest that fluvial topography is sensitive to climate. However, it has remained challenging to demonstrate a compelling link between fluvial topography and climate state in natural landscapes. One possible reason is that many studies compare erosion rates to climate data, although theoretical studies note that, at steady-state, climate is encoded in topography rather than in erosion rates. Here, we use an existing global compilation of 10Be basin average erosion rates to isolate the climate signal in topography for fluvially-dominated catchments underlain by crystalline bedrock that appear to be in morphological steady state. Our results show that the nonlinearity between erosion rates and the normalized river channel steepness index, which is a proxy for fluvial relief, systematically increases with increasing mean annual precipitation and decreasing aridity. When interpreted in the context of detachment-limited bedrock incision models that account for incision thresholds and stochastic distribution of floods, this systematic pattern can be explained by a decrease in discharge variability in landscapes that are wetter and less arid, assuming incision thresholds are important. Our results imply a climate control on topography at a global scale and highlight new research directions that can improve understanding of climate’s impact on landscape evolution.Item Open Access Neotectonic effects of glacial erosion and deglaciation on the Sangre de Cristo Mountains, southern Colorado(Colorado State University. Libraries, 2023) Hurtado, Cecilia, author; Gallen, Sean, advisor; Singleton, John, committee member; McGrath, Daniel, committee member; Denning, Scott, committee memberInterrelations between climate and tectonics are important to the development of active mountain belts, but rarely are there natural examples that lend themselves to studying the effects of climate on tectonics. The Sangre de Cristo Mountains in southern Colorado provide an optimal natural laboratory to explore the effects of alpine valley glaciation on surface uplift of the footwall and on the active extensional normal fault system in the northern Rio Grande rift. This region has experienced changes in surface loads associated with long-term glacial erosion and sedimentation over the course of the Quaternary, as well as shorter-term deglaciation after the Last Glacial Maximum. These changing loads correspond with stress changes that affect the flexural isostatic response of the lithosphere, and further act as clamping or unclamping stresses on the Sangre de Cristo fault that bounds the western margin of the mountain range. This work quantifies the masses and spatial distributions of these various loads and models the associated flexural isostatic response to estimate potential uplift and subsidence patterns in the study area that could be attributed to climate-driven mechanisms. The glacially-scoured footwall material was estimated by using remnants of the fluvial reaches downstream of glaciated drainage basins, reconstructing the paleofluvial topography, and subtracting it from the modern topography. The quantification of the deposited sediment in the San Luis Basin was measured from an interpolated surface tethered by existing drill cores, geophysical data, and geologic maps. Lastly, the glacial extents and thicknesses were constructed using a simple numerical modeling tool, GlaRe, constrained by preserved depositional and erosional evidence of glaciers. Isostatic responses were calculated using a flexure model with two effective elastic thickness (Te) values, 2 km and 5 km, and stress changes on faults at depth were calculated using an analytical line load model. The results estimate ~29 m of footwall uplift and ~47 m of subsidence in the hanging wall for a realistic Te of 5 km, and footwall uplift of ~48 m and a hanging wall subsidence of ~80 m for an independently calibrated Te of 2 km. Importantly, while topographic reconstructions indicate an ~50 m reduction in mean footwall elevations, isostatic rebound pushes mountain peaks upward by tens of meters. Footwall uplift due to deglaciation has a response of 4 m and 6 m, for a Te of 5 km and a Te of 2 km, respectively. The Sangre de Cristo fault trace was mapped to quantify the offset of fault scarps on Quaternary alluvial fans to determine the spatial and temporal patterns of offset along-strike of the fault. Fault offset magnitudes correlate with glacial domains, and fault slip rates correlate with the post-glacial spatial pattern of isostatic uplift, indicating an unambiguous link between deglaciation and elevated fault activity. This work demonstrates that (1) differential glacial erosion reduces mean footwall elevations, but the associated isostatic response drives surface uplift of mountain peaks, and (2) seismicity along normal faults could be amplified by load changes associated with climate-driven mechanisms, which will become increasingly important as we continue in a period of anthropogenic warming and deglaciation.Item Open Access Quaternary alluvial lineaments in the Atacama Desert, northern Chile: morphology, relationships to bedrock structures, and link to the seismic cycle of the Andean subduction margin(Colorado State University. Libraries, 2023) Perman, Emily A., author; Singleton, John, advisor; Gallen, Sean, advisor; Bhaskar, Aditi, committee memberLocated in the upper plate of the modern Nazca-South American subduction zone, the hyperarid Atacama Desert is an ideal place to study forearc deformation through surface geomorphology. We studied neotectonic lineaments in alluvium between ~25.5° and 26° S in the Coastal Cordillera with the goal of understanding modern forearc strain. Visible in satellite imagery, these lineaments are defined by linear to curvilinear structures consisting of subparallel ridges that range from tens of meters to kilometers in length. Field observations and 10 cm-resolution digital elevation models (DEMs) derived from drone imagery record a consistent, asymmetrical "ridge-trough-ridge" morphology that commonly traces into ~1–2 m-wide bedrock fissures containing gypsum and calcite. Most lineaments in this region trend ~N-S to NW-SE, parallel to the dominant bedrock structural grain and the Cretaceous Atacama and Taltal fault systems. Small-scale faults found in the lineament ridges have cm- to mm-scale apparent normal-sense displacement and consistently dip moderately to steeply (50–70°) towards the lineament troughs, defining a graben-like structure. In outcrop, these normal fault zones are enhanced by differential erosion, and in thin section faulted material is distinguishable by increased cementation within fractures penetrating grain boundaries. A tuff deposit within an alluvial fan containing several lineaments yields a zircon U-Pb age of 2.2 ± 0.1 Ma, indicating that lineaments in this fan are Quaternary in age and likely related to upper plate strain due to modern subduction along the Nazca-South American plate boundary. Older alluvial surfaces tend to have lineaments with broader and taller ridges than those formed on relatively younger alluvial surfaces, indicating that these structures formed progressively through time. In addition, profile data gathered from DEMs show a weak linear correlation between lineament trough width and ridge height, meaning that wider lineaments tend to have taller ridges. Along the flanks of two trenched ridges, we observed shallowly-dipping planes that resemble thrust faults, suggesting the ridges may have formed in response to contractional deformation. We propose the lineaments record alternating forearc shortening and extension related to interseismic and coseismic phases of the earthquake cycle, with the development of ridges and thrust faults recording interseismic shortening and normal faults and fissures which form the central troughs recording coseismic extension.