From the Colorado Front Range to global topography: evaluating the roles of tectonics and climate on long term landscape evolution
Marder, Eyal, author
Gallen, Sean, advisor
Pazzaglia, Frank, committee member
Wohl, Ellen, committee member
Schutt, Derek, committee member
Kampf, Stephanie, committee member
Landscapes 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.
Includes bibliographical references.
Includes bibliographical references.