The Pacific-North America plate boundary in central and southern California has a complex tectonic history, and constraints are poor for inception of an extensional fault system linked to the southern San Andreas fault, a major tectonic element of this plate boundary. Furthermore, decades of research has shown relationships between climate, tectonics, and surface processes in most orogens across the globe (e.g. Alps, Himalaya, Andes, Alaska Ranges), however the role climate plays in modulating erosion and mass fluxes from extensional mountains blocks to sedimentary basins over 104-5 yr timescales is debated. In the eastern California-Walker Lane shear zone, exposures of sedimentary basin fill allow inversion of erosion- and sediment-flux rates from a linked catchment-fan system within an extensional block. In this dissertation, I present two field and geo-thermochronology based studies that explore research topics related by common tectonic setting and geography within the Pacific-North America plate boundary. First I present new low-temperature thermochronology (apatite U-Th-Sm/He) and thermal history modeling to document the kinematic evolution of the Santa Rosa mountains, where the cooling history constrains initiation timing of the west Salton Detachment fault, and the southern San Andreas fault system. I document an age of ca. 8 Ma for exhumation initiation of the Santa Rosa block, from paleodepths of ~4.5–3 km, at vertical rates of ~0.15–0.36 mm/yr, accelerating to ~1.3 km/Ma since ca. 1.2 Ma during initiation of the San Jacinto fault zone. Second, I present a new data set of cosmogenic radionuclide-derived burial ages and paleodenudation rates (26Al/10Be) from the Pleasant Canyon complex in the Panamint Range, and show that denudation rate and sediment flux have varied by a factor of ~2x since the middle Pleistocene. I conclude high frequency variability is driven by climate change, and not tectonic perturbations, as supported by published constraints for exhumation timing. The middle Pleistocene transition from 40–100 ka periodicity may drive the observed changes, a tentative conclusion that makes testable predictions for stratigraphic records of past climate in other locations. Empirical evidence for climate-modulated erosion and sediment flux provides valuable constraints for numerical models of landscape evolution and sedimentary basin architecture. / Ph. D. / Vertical motions along faults produce uplift of mountain blocks, often with steep high topography, which is accompanied by subsidence of adjacent sedimentary basins. Understanding cycles of fault initiation, uplift, and eventual degradation of mountainous fault blocks through erosion is a fundamental goal of the geoscience community, as is inversion of records of past environmental conditions preserved in sedimentary basins. The Pacific-North America plate boundary in California, USA, is composed of several major fault systems that provide an opportunity to study vertical uplift and erosion of mountains, and the sedimentary basins that preserve records of changes in erosion rates through time. In this context, I present a dissertation composed of two original research articles. In Chapter Two, I use thermochronometry in the Santa Rosa Mountains, Coachella Valley, to constrain initiation timing and vertical uplift rates for an extensional fault system called the west Salton detachment fault (WSDF). Localization of the plate boundary in Coachella Valley led to initiation of the WSDF and the southern San Andreas fault system at ca. 8 Myr ago, timing which may reflect a global plate-tectonic driver. Vertical uplift of the Santa Rosa Mountains via the WSDF was moderate during the time between ca. 8–1.2 Myr, then vertical uplift increased four-fold during the initiation of a new strike-slip fault within the southern San Andreas system. In Chapter Two, I use rare isotopes called cosmogenic radionuclides in sediment from basin stratigraphy to constrain the magnitude and variability of erosion in the Pleasant Canyon catchment of the Panamint Mountains since ca. 1.5 Myr ago. The mean erosion rate for Pleasant Canyon is 36 ± 8 mm/kyr, and individual samples vary by up to 2x, indicating erosion rates were not constant through time. The timescales of variability, and evidence from basin stratigraphy suggest that glacial-interglacial climate change produced the observed changes in erosion in this mountain block. This conclusion makes testable predictions for other unglaciated catchments in extensional fault blocks, while evidence of climate-induced changes in sediment fluxes from mountains to basins has potential implications to recovering information about past climate change from stratigraphy.

Thesis advisor: Jeremy D. Shakun / Glaciers in the tropics have undergone significant retreat in the past several decades, but the magnitude of this retreat in the long-term context of the Holocene has mostly been qualitatively assessed. This study produces a quantitative reconstruction of Holocene glacier extent relative to today from the Quelccaya Ice Cap, Peru, and the Rwenzori Mountains of east Africa. I use measurements of in situ 14C and 10Be from bedrock that was recently exposed by glacier retreat to constrain possible bedrock exposure and erosion histories at each site. The results are strikingly similar in both areas, and suggest that ice was generally smaller than today during the first half of the Holocene and larger than today for most, if not all, of the last several millennia. These findings give evidence toward a coherent Holocene expansion of glaciers across the tropics, and suggest that recent retreat is unusual in a multi-millennial context. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

The evolution of mountain landscapes is controlled by complex interactions between large-scale tectonic, surficial and climate conditions. Dominant processes are attributed to creating characteristic features of the landscape, but topographic features are the cumulative result of coupled surficial processes, each locally effective in a different climate or elevation regime. The focus of erosion by glacial, fluvial, or mass wasting processes is highly sensitive to small changes in boundary conditions, therefore spatial and temporal variability can be high when observed over short time scales. This work evaluated methods for dissecting the history of complex alpine landscapes to understand the role of individual processes influenced by changing climate and underlying bedrock. It also investigated how individual and combined mechanisms of surficial processes influenced the evolution of topography in the Teton Range in Wyoming. Detrital apatite (U-Th)/He thermochronology and cosmogenic radionuclide erosion rates were applied to determine spatial and temporal variability of erosion in the central catchments of the range. Spatial variability existed between the glacial and fluvial systems, indicating that sediment erosion and deposition by these processes was controlled by short-term variability in climate conditions. Effective glacial incision also controlled other processes, specifically enhancing rock fall activity and inhibiting fluvial incision. Short-term erosion rates were highly variable and were controlled by stochastic processes, particularly hillslope failures in response to slope oversteepening due to glacial incision and orientation and spacing of bedrock fractures. Erosion rates averaged over 10 ky time scales were comparable to long-term exhumation rates measured in the Teton Range. The similarity of spatial erosion patterns to predicted uniform erosion and the balance between intermediate and long-term erosion rates suggests the landscape of the Teton Range is approaching steady-state, but frequent stochastic processes, short-term erosional variability and coupled processes maintain rugged topographic relief. / Ph. D.