The tectonism, volcanism, and sedimentation along the East African Rift System (EARS) produced a series of rift basins with a rich paleoanthropological record, including a Late Miocene–present record of hominin evolution. To better understand the relationship between Earth system history and human evolution within the EARS, the Hominin Sites and Paleolakes Drilling Project (HSPDP) collected paleolake sediments near key paleoanthropological sites in Ethiopia and Kenya, compiling a multi-proxy, high-resolution geological and environmental record. As part of the HSPDP, I studied the detrital mineral record of the basins and evaluated tectonic and climatic controls on East African landscapes during the Plio-Pleistocene using samples from three of the drill sites, Chew Bahir: (CHB, ~620–present; Ethiopia), Northern Awash (NA, ~3.3–2.9 Ma; Ethiopia,), and West Turkana (WTK, ~1.9–1.4 Ma; Kenya). I employed laser ablation U/Pb and (U-Th)/He double dating (LADD) of detrital zircons, which yields paired U/Pb and (U-Th)/He dates, and (U-Th)/He dating of detrital apatites to evaluate sediment provenance and the cooling history of the source rocks. In addition, I used in situ 10Be cosmogenic radionuclide analyses to determine paleoerosion rates. Two chapters of this dissertation focus on results from the NA and WTK drill sites. Source units for the NA and WTK drill sites are largely Cenozoic volcanic rocks, and the detrital zircon record yields an extensive record of the timing of various phases of volcanism within the EARS. Exceptionally young zircon (U-Th)/He dates reflect partial resetting associated with late mafic volcanism and/or hydrothermal activity. Erosion rates are consistent and relatively low across the Plio-Pleistocene, despite significant tectonic and geomorphic shifts in the landscape. Two other chapters of this dissertation cover results from the CHB drill site. The Chew Bahir basin has significant exposures of Neoproterozoic and Early Paleozoic crystalline basement units, and the detrital zircon record yields one singular phase of volcanism in the EARS. The CHB erosion rates show an overall decreasing trend over time, consistent with an aridifying climate, and increased environmental variability after ~200 ka.

I investigate the Black Canyon City landslide (BCC landslide), a prominent deep-seated landslide located northeast of Black Canyon City, Arizona. Although the landslide does not appear to pose a significant hazard to structures, its prominent features and high topographic relief make it an excellent site to study the geologic setting under which such features develop. This study has the potential to contribute toward understanding the landscape evolution in similar geologic and topographic settings, and for characterizing the underlying structural processes of this deep-seated feature. We use field and remotely-based surface geology and geomorphological mapping to characterize the landslide geometry and its surface displacement. We use the Structure from Motion (SfM) method to generate a 0.2 m resolution digital elevation model and rectified ortho-photo imagery from unmanned aerial vehicle (UAV) - and balloon-based images and used them as the base map for our mapping. The ~0.6 km2 landslide is easily identified through remotely-sensed imagery and in the field because of the prominent east-west trending fractures defining its upper extensional portion. The landslide displaces a series of Early and Middle Miocene volcanic and sedimentary rocks. The main head scarp is ~600 m long and oriented E-W with some NW-SE oriented minor scarps. Numerous fractures varying from millimeters to meters in opening were identified throughout the landslide body (mostly with longitudinal orientation). The occurrence of a distinctive layer of dark reddish basalt presents a key displaced marker to estimate the long-term deformation of the slide mass. Using this marker, the total vertical displacement is estimated to be ~70 m, with maximum movement of ~95 m to the SE. This study indicates that the landslide motion is translational with a slight rotational character. We estimate the rate of the slide motion by resurvey of monuments on and off the slide, and examination of disturbed vegetation located along the fractures. The analysis indicates a slow integrated average landslide velocity of 10-60 mm/yr. The slide motion is probably driven during annual wet periods when increased saturation of the slide mass weakens the basal slip surface and the overall mass of the slide is increased. Results from our study suggest that the slide is stable and does not pose significant hazard for the surrounding area given no extreme changes in the environmental condition. Although the landslide is categorized as very slow (according to Cruden and Varnes, 1996), monitoring the landslide is still necessary.

Shallow earthquakes in the upper part of the overriding plate of subduction zones can be devastating due to their proximity to population centers despite the smaller rupture extents than commonly occur on subduction megathrusts that produce the largest earthquakes. Damaging effects can be greater in volcanic arcs like Java because ground shaking is amplified by surficial deposits of uncompacted volcaniclastic sediments. Identifying the upper-plate structures and their potential hazards is key for minimizing the dangers they pose. In particular, the knowledge of the regional stress field and deformation pattern in this region will help us to better understand how subduction and collision affects deformation in this part of the overriding plate. The majority of the upper plate deformation studies have been focused on the deformation in the main thrusts of the fore-arc region. Study of deformation within volcanic arc is limited despite the associated earthquake hazards. In this study, I use maps of active upper-plate structures, earthquake moment tensor data and stress orientation deduced from volcano morphology analysis to characterize the strain field of Java arc. In addition, I use sandbox analog modeling to evaluate the mechanical factors that may be important in controlling deformation. My field- and remotely-based mapping of active faults and folds, supplemented by results from my paleoseismic studies and physical models of the system, suggest that Java’s deformation is distributed over broad areas along small-scale structures. Java is segmented into three main zones based on their distinctive structural patterns and stress orientation. East Java is characterized by NW-SE normal and strike-slip faults, Central Java has E-W folds and thrust faults, and NE-SW strike-slip faults dominate West Java. The sandbox analog models indicate that the strain in response to collision is partitioned into thrusting and strike-slip faulting, with the dominance of margin-normal thrust faulting. My models test the effects of convergence obliquity, geometry, preexisting weaknesses, asperities, and lateral strength contrast. The result suggest that slight variations in convergence obliquity do not affect the deformation pattern significantly, while the margin shape, lateral strength contrast, and perturbation of deformation from asperities each have a greater impact on deformation.

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.

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 Gabriel Mountains (SGM) of southern California provide the opportunity to study the topographic controls on erosion rate in a mountain range where climate and lithology are relatively constant. I use a combination of digital elevation model data, detailed channel survey data, decadal climate records, and catchment-averaged erosion rates quantified from 10Be concentrations in stream sands to investigate the style and rates of hillslope and channel processes across the transition from soil-mantled to rocky landscapes in the SGM. Specifically, I investigate (1) the interrelations among different topographic metrics and their variation with erosion rate, (2) how hillslopes respond to tectonic forcing in "threshold" landscapes, (3) the role of discharge variability and erosion thresholds in controlling the relationship between relief and erosion rate, and (4) the style and pace of transient adjustment in the western SGM to a recent increase in uplift rate. Millennial erosion rates in the SGM range from 0.03-1.1 mm/a, generally increasing from west to east. For low erosion rates (< 0.3 mm/a), hillslopes tend to be soil-mantled, and catchment-averaged erosion rates are positively correlated with catchment-averaged slope, channel steepness, and local relief. For erosion rates greater than 0.3 mm/a, hillslopes become increasingly rocky, catchment-mean hillslope angle becomes much less sensitive to erosion rate, and channels continue to steepen. I find that a non-linear relationship observed between channel steepness and erosion rate can be explained by a simple bedrock incision model that combines a threshold for erosion with a probability distribution of discharge events where large floods follow an inverse power-law. I also find that the timing of a two-staged increase in uplift rate in the western SGM based on stream profile analysis agrees with independent estimates. Field observations in the same region suggest that the relict topography that allows for this calculation has persisted for more than 7 Ma due to the stalling of migrating knickpoints by locally stronger bedrock and a lack of coarse sediment cover.