Drylands (arid and semi-arid grassland ecosystems) cover about 40% of the Earth's surface and support over 40% of the human population, most of which is in emerging economies. Human development of drylands leads to topsoil loss, and over the last 160 years, woody plants have encroached on drylands, both of which have implications for maintaining soil viability. Understanding the spatial variability in erosion and soil organic carbon and total nitrogen under varying geomorphic and biotic forcing in drylands is therefore of paramount importance. This study focuses on how two plants, palo verde (Parkinsonia microphylla, nitrogen-fixing) and jojoba (Simmondsia chinensis, non-nitrogen fixing), affect sediment transport and soil organic carbon and total nitrogen pools in a dryland environment north of Phoenix, Arizona. Bulk samples were systematically collected from the top 10 cm of soil in twelve catenae to control for the existence and type of plants, location to canopy (sub- or intercanopy, up- or downslope), aspect, and distance from the divide. Samples were measured for soil organic carbon and total nitrogen and an unmanned aerial system-derived digital elevation map of the field site was created for spatial analysis. A subset of the samples was measured for the short-lived isotopes 137Cs and 210Pbex, which serve as proxy erosion rates. Erosional soils were found to have less organic carbon and total nitrogen than depositional soils. There were clear differences in the data between the two plant types: jojoba catenae had higher short-lived isotope activity, lower carbon and nitrogen, and smaller canopies than those of palo verde, suggesting lower erosion rates and nutrient contributions from jojoba plants. This research quantifies the importance of biota on influencing hillslope and soil dynamics in a semi-arid field site in central AZ and finishes with a discussion on the global implications for soil sustainability.

The collision of India and Eurasia constructed the Himalayan Mountains. Questions remain regarding how subsequent exhumation by climatic and tectonic processes shaped the landscape throughout the Late Cenozoic to create the complex architecture observed today. The Mount Everest region underwent tectonic denudation by extension and bestrides one of the world’s most significant rain shadows. Also, glacial and fluvial processes eroded the Everest massif over shorter timescales. In this work, I review new bedrock and detrital thermochronological and geochronological data and both one- and two-dimensional thermal-mechanical modeling that provides insights on the age range and rates of tectonic and erosional processes in this region.

A strand of the South Tibetan detachment system (STDS), a series of prominent normal-sense structures that dip to the north and strike along the Himalayan spine, is exposed in the Rongbuk valley near Everest. Using thermochronometric techniques, thermal-kinematic modeling, and published (U-Th)/Pb geochronology, I show exhumation rates were high (~3-4 mm/a) from at least 20 to 13 Ma because of slip on the STDS. Subsequently, exhumation rates dropped drastically to ≤ 0.5 mm/a and remain low today. However, thermochronometric datasets and thermal-kinematic modeling results from Nepal south of Everest reveal a sharp transition in cooling ages and exhumation rates across a major knickpoint in the river profile, corresponding to the modern-day Himalayan rainfall transition. To the north of this transition, exhumation histories are similar to those in Tibet. Conversely, < 3 km south of the transition, exhumation rates were relatively low until the Pliocene, when they increased to ~4 mm/a before slowing at ~3 Ma. Such contrasting exhumation histories over a short distance suggest that bedrock exhumation rates correlate with modern precipitation patterns in deep time, however, there are competing interpretations regarding this correlation.

My work also provides insights regarding how processes of glacial erosion act in a glacio-fluvial valley north of Everest. Integrated laser ablation U/Pb and (U-Th)/He dating of detrital zircon from fluvial and moraine sediments reveal sourcing from distinctive areas of the catchment. In general, the glacial advances eroded material from lower elevations, while the glacial outwash system carries material from higher elevations.

Sedimentary basins are defined by extensional tectonics. Rugged mountain ranges stand in stark relief adjacent to muted structural basins filled with sediment. In simplest terms, this topography is the result of ranges uplifted along normal faults, and this uplift drives erosion within upland drainages, shedding sediment into subsiding basins. In southeastern Arizona's Basin and Range province extensional tectonics waned at approximately 3-5 Myr, and the region's structural basins began transitioning from internal to external drainage, forming the modern Gila River fluvial network. In the Atacama Desert of northern Chile, some basins of the Central Depression remain internally drained while others have integrated to the Pacific Ocean. In northern Chile, rates of landscape evolution are some of the slowest on Earth due to the region's hyperarid climate. While the magnitude of upland erosion driven by extensional tectonics is largely recorded in the stratigraphy of the structural basins, the landscape's response to post-tectonic forcings is unknown.

I employ the full suite of modern geomorphic tools provided by terrestrial cosmogenic nuclides - surface exposure dating, conventional burial dating, isochron burial dating, quantifying millennial-scale upland erosion rates using detrital TCN, quantifying paleo-erosion rates using multiple TCN such as Ne-21/Be-10 and Al-26l/Be-10, and assessing sediment recycling and complex exposure using multiple TCN - to quantify the rates of landscape evolution in southeastern Arizona and northern Chile during the Late Cenozoic. In Arizona, I also use modern remnants of the pre-incision landscape and digital terrain analyses to reconstruct the landscape, allowing the quantification of incision and erosion rates that supplement detrital TCN-derived erosion rates. A new chronology for key basin high stand remnants (Frye Mesa) and a flight of Gila River terraces in Safford basin provides a record of incision rates from the Pliocene through the Quaternary, and I assess how significantly regional incision is driving erosion rates. Paired nuclide analyses in the Atacama Desert of northern Chile reveal complex exposure histories resulting from several rounds of transport and burial by fluvial systems. These results support a growing understanding that geomorphic processes in the Atacama Desert are more active than previously thought despite the region's hyperarid climate.

Climate and its influence on hydrology and weathering is a key driver of surface processes on Earth. Despite its clear importance to hazard generation, fluvial sediment transport and erosion, the drawdown of atmospheric CO₂ via the rock cycle, and feedbacks between climate and tectonics, quantifying climatic controls on long-term erosion rates has proven to be one of the grand problems in geomorphology. In fact, recent attempts addressing this problem using cosmogenic radionuclide (CRN) derived erosion rates suggest very weak climatic controls on millennial-scale erosion rates contrary to expectations. In this work, two challenges are addressed that may be impeding progress on this problem.

The first challenge is choosing appropriate climate metrics that are closely tied to erosional processes. For example, in fluvial landscapes, most runoff events do little to no geomorphic work due to erosion thresholds, and event-scale variability dictates how frequently these thresholds are exceeded. By analyzing dense hydroclimatic datasets in the contiguous U.S. and Puerto Rico, we show that event-scale runoff variability is only loosely related to event-scale rainfall variability. Instead, aridity and fractional evapotranspiration (ET) losses are much better predictors of runoff variability. Importantly, simple hillslope-scale soil water balance models capture major aspects of the observed relation between runoff variability and fractional ET losses. Together, these results point to the role of vegetation water use as a potential key to relating mean hydrologic partitioning with runoff variability.

The second challenge is that long-term erosion rates are expected to balance rock uplift rates as landscapes approach topographic steady state, regardless of hydroclimatic setting. This is illustrated with new data along the Main Gulf Escarpment, Baja, Mexico. Under this conceptual framework, climate is not expected to set the erosion rate, but rather the erosional efficiency of the system, or the steady-state relief required for erosion to keep up with tectonically driven uplift rates. To assess differences in erosional efficiency across landscapes experiencing different climatic regimes, we contrast new CRN data from tectonically active landscapes in Baja, Mexico and southern California (arid) with northern Honduras (very humid) alongside other published global data from similar hydroclimatic settings. This analysis shows how climate does, in fact, set functional relationships between topographic metrics like channel steepness and long-term erosion rates. However, we also show that relatively small differences in rock erodibility and incision thresholds can easily overprint hydroclimatic controls on erosional efficiency motivating the need for more field based constraints on these important variables.

Quantifying the temporal and spatial evolution of active continental rifts contributes to our understanding of fault system evolution and seismic hazards. Rift systems also preserve robust paleoenvironmental records and are often characterized by strong climatic gradients that can be used to examine feedbacks between climate and tectonics. In this thesis, I quantify the spatial and temporal history of rift flank uplift by analyzing bedrock river channel profiles along footwall escarpments in the Malawi segment of the East Africa Rift. This work addresses questions that are widely applicable to continental rift settings: (1) Is rift-flank uplift sufficiently described by theoretical elliptical along-fault displacement patterns? (2) Do orographic climate patterns induced by rift topography affect rift-flank uplift or morphology? (3) How do uplift patterns along rift flanks vary over geologic timescales? In Malawi, 100-km-long border faults of alternating polarity bound half-graben sedimentary basins containing up to 4km of basin fill and water depths up to 700m. Orographically driven precipitation produces climatic gradients along footwall escarpments resulting in mean annual rainfall that varies spatially from 800 to 2500 mm. Temporal oscillations in climate have also resulted in lake lowstands 500 m below the modern shoreline. I examine bedrock river profiles crossing the Livingstone and Usisya Border Faults in northern Malawi using the channel steepness index (Ksn) to assess importance of these conditions on rift flank evolution. River profiles reveal a consistent transient pattern that likely preserves a temporal record of slip and erosion along the entire border fault system. These profiles and other topographic observations, along with known modern and paleoenvironmental conditions, can be used to interpret a complete history of rift flank development from the onset of rifting to present. I interpret the morphology of the upland landscape to preserve the onset of extensional faulting across a relict erosion surface. The linkages of individual faults and acceleration of slip during the development of a continuous border fault is suggested by an analysis of knickpoint elevations and Ksn. Finally, these results suggest that the modern observed climate gradient only began to significantly affect denudation patterns once a high relief rift flank was established.

The Himalaya are the archetypal example of a continental collision belt, formed by the ongoing convergence between India and Eurasia. Boasting some of the highest and most rugged topography on Earth, there is currently no consensus on how climatic and tectonic processes have combined to shape its topographic evolution. The Kingdom of Bhutan in the eastern Himalaya provides a unique opportunity to study the interconnections among Himalayan climate, topography, erosion, and tectonics. The eastern Himalaya are remarkably different from the rest of the orogen, most strikingly due to the presence of the Shillong Plateau to the south of the Himalayan rangefront. The tectonic structures associated with the Shillong Plateau have accommodated convergence between India and Eurasia and created a natural experiment to test the possible response of the Himalaya to a reduction in local shortening. In addition, the position and orientation of the plateau topography has intercepted moisture once bound for the Himalaya and created a natural experiment to test the possible response of the range to a reduction in rainfall. I focused this study around the gently rolling landscapes found in the middle of the otherwise extremely rugged Bhutan Himalaya, with the understanding that these landscapes likely record a recent change in the evolution of the range. I have used geochronometric, thermochronometric, and cosmogenic nuclide techniques, combined with thermal-kinematic and landscape evolution models to draw three primary conclusions. 1) The cooling histories of bedrock samples from the hinterland of the Bhutan Himalaya show a protracted decrease in erosion rate from the Middle Miocene toward the Pliocene. I have attributed this change to a reduction in shortening rates across the Himalayan mountain belt, due to increased accommodation of shortening across the Shillong Plateau. 2) The low-relief landscapes of Bhutan were likely created by backtilting and surface uplift produced by an active, blind, hinterland duplex. These landscapes were formed during surface uplift, which initiated ca. 1.5 Ma and has totaled 800 m. 3) Millennial-scale erosion rates are coupled with modern rainfall rates. Non-linear relationships between topographic metrics and erosion rates, suggest a fundamental difference in the mode of river incision within the drier interior of Bhutan and the wetter foothills.

Sedimentary basins in the Afar Depression, Ethiopia archive the progression of continental breakup, record regional changes in east African climate and volcanism, and host what are arguably the most important fossiliferous strata for studying early human evolution and innovation. Significant changes in rift tectonics, climate, and faunal assemblages occur between 3-2.5 million years ago (Ma), but sediments spanning this time period are sparse. In this dissertation, I present the results of a geologic investigation targeting sediments between 3-2.5 Ma in the central and eastern Ledi Geraru (CLG and ELG) field areas in the lower Awash Valley, using a combination of geologic mapping, stratigraphy, and tephra chemistry and dating. At Gulfaytu in CLG, I mapped the northern-most outcrops of the hominin-bearing Hadar Formation (3.8-2.9 Ma), a 20 m-thick section of flat-lying lacustrine sediments containing 8 new tephras that directly overlie the widespread BKT-2 marker beds (2.95 Ma). Paleolake Hadar persisted after 2.95 Ma, and the presence and characteristics of the Busidima Formation (2.7-0.016 Ma) indicates Gulfaytu was affected by a reversal in depositional basin polarity. Combined with regional and geophysical data, I show the Hadar Formation underlying CLG is >300 m thick, supporting the hypothesis that it was the lower Awash Pliocene depocenter. At ELG, I mapped >300 m of sediments spanning 3.0-2.45 Ma. These sediments coarsen upward and show a progression from fluctuating lake conditions to fluvial landscapes and widespread soil development. This is consistent with the temporal change in depositional environments observed elsewhere in the lower Awash Valley, and suggests that these strata are correlative with the Hadar Formation. Furthermore, the strata and basalts at ELG are highly faulted, and overprinted by shifting extension directions attributed to the northern migration of the Afar triple junction. The presence of fossiliferous beds and stone tools makes ELG a high-priority target for anthropological and archaeological research. This study provides a new temporally-calibrated and high-resolution record of deposition, volcanism, and faulting patterns during a period of significant change in the Afar.

The San Andreas Fault (SAF) is the primary structure within a system of faults accommodating motion between the North American and Pacific plates. Physical models of faulting and characterizations of seismic hazard are informed by investigations of paleoseismology, slip distribution, and slip rate. The impact of earthquakes on people is due in large part to social vulnerability. This dissertation contributes an analysis about the relationships between earthquake hazard and social vulnerability in Los Angeles, CA and investigations of paleoseismology and fault scarp array complexity on the central SAF. Analysis of fault scarp array geometry and morphology using 0.5 m digital elevation models along 122 km of the central SAF reveals significant variation in the complexity of SAF structure. Scarp trace complexity is measured by scarp separation, changes in strike, fault trace gaps, and scarp length per SAF kilometer. Geometrical complexity in fault scarp arrays indicates that the central SAF can be grouped into seven segments. Segment boundaries are controlled by interactions with subsidiary faults. Investigation of an offset channel at Parkfield, CA yields a late Holocene slip rate of 26.2 +6.4/- 4.3 mm/yr. This rate is lower than geologic measurements on the Carrizo section of the SAF and rates implied by far-field geodesy. However, it is consistent with historical observations of slip at Parkfield. Paleoseismology at Parkfield indicates that large earthquakes are absent from the stratigraphic record for at least a millennia. Together these observations imply that the amount of plate boundary slip accommodated by the main SAF varies along strike. Contrary to most environmental justice analyses showing that vulnerable populations are spatially-tied to environmental hazards, geospatial analyses relating social vulnerability and earthquake hazard in southern California show that these groups are not disproportionately exposed to the areas of greatest hazard. Instead, park and green space is linked to earthquake hazard through fault zone regulation. In Los Angeles, a parks poor city, the distribution of social vulnerability is strongly tied to a lack of park space. Thus, people with access to financial and political resources strive to live in neighborhoods with parks, even in the face of forewarned risk.