Weathering geochronology permits delimiting the ages of weathering profiles, determining rates of weathering and landscape evolution and inferring palaeoclimatic and environmental conditions that control the surficial evolution of continents. It also provides insights into the timing and rates of supergene enrichment of metal and precious mineral deposits. Until recently, weathering geochronology was primarily based on K-Ar and 40Ar/39Ar dating of supergene minerals. Recent advances by U-Th series dating of pedogenic carbonate (Sharp et aI., 2003), in situ U-Th series dating of iron hydroxides (Bernal et aI., 2006), U-Pb SHRIMP dating of opals (Nemchin et aI., 2006), and (U-Th)/He and 4HePHe analysis of supergene goethite (Shuster et aI., 2005) expand the number of minerals and the time range where weathering geochronology can be applied. Weathering profiles blanket more than one third of the Australian continent and are purportedly among the oldest weathering profiles on earth. Unravelling the history of weathering and landscape evolution in these areas requires techniques capable of dating supergene minerals at all time scales, but particularly at the > I Ma scale. Currently, the K-Ar, U-Pb, and U-Th/He methods are the only techniques suitable to directly date supergene minerals on such time-scales. In this study, I show how the combination of 4°ArP9Ar and (U-Th)/He geochronology can be used to unravel the weathering and landscape evolution history of continents. The application of radiogenic isotope-dating techniques and the proper interpretation of geochronological results require understanding of the dated mineralogy and the presence of potentially contaminating phases. The successful application of 40ArP9Ar weathering geochronology was made possible through detailed understanding of the hollandite-group Mn oxide and alunite-group sulfate crystal chemistry and mineral physics. Similarly, in this study, I have refined the application of (U-Th)/He dating of goethite. The successful application of this technique relies on the identification and selection of suitable goethitebearing samples in the field; the characterization of mineral paragenesis by optical microscopy; the determination of fine-scale mineral chemistry and paragenesis through electron microscopy and microprobe analysis; the determination of physico-chemical properties (mineral structure, crystallinity) of the various types of supergene goethite through x-ray (bench-top and synchrotron) diffraction techniques; and application to noble gas diffusion experiments in ultra-high vacuum techniques to quantify the crystallochemical controls on the helium (natural radiogenic 4He and artificial spallogenic 3He) diffusion properties and retentivity of goethite. Finally, to test the reliability of the (U-Th)/He dating method, I apply the methodology to weathering profiles in a range of geological environments where stratigraphic and paragenetic relationships provide tight constraints on the possible history of mineral precipitation. For the applied component of this study, I (with a group of collaborators) chose the Hamersley Province as the key study site for the application of combined 40ArP9Ar and (UTh)/ He geochronology, for several reasons: the Hamersley Province is one of the longest lived landscapes on Earth and is extensively blanketed by thick weathering profiles that contain a plethora of crystalline goethite suitable for (U-Th)/He dating; the goethite in these weathering profiles coexists with K-bearing Mn oxides suitable for 4°Ar/39Ar dating; and mining operations provide exposure and access to near complete weathering profiles, enabling the sampling of goethite and Mn oxides for geochronology. Deep (50 to 100 metres on average and up to 400 metres) lateritic weathering profiles in the Hamersley Province outcrop over 80, 000 km2 on ridges and plateaus, ranging in height from II 00 m to 400 m. These lateritic weathering profiles are developed on Archaean banded-iron formation and host some of the world's largest iron ore deposits. Some authors have proposed that the lateritic weathering profiles represent the remnants of a continuous Mesozoic land surface now partially eroded. Surrounding the plateaus and ridges, ferruginized detrital sediments on valley slopes and floors and ferruginized detritus in paleochannel deposits (channel iron deposits or CID), also hosting high-grade iron ore, reveal evidence of widespread erosion and re-deposition of former weathering profiles. They also display evidence of post-depositional weathering and ferruginization, suggesting a complex interplay between weathering and erosion during landscape evolution in the region. 4°ArP9Ar dating of 204 grains of supergene Mn oxides (mostly cryptomelane and hollandite) extracted from 70 samples from seven distinct weathering profiles at 7 field sites, up to 300 kilometres apart, yield precipitation ages ranging from 63.4 ± 0.9 to 1.5 ± 0.2 Ma. When combined with previous unpublished 40ArP9Ar results, ranging from 81.1 ± 0.4 to 11.6 ± 0.3 Ma, the geochronology indicates a prolonged (Late Cretaceous to Recent) and episodic weathering record for the Hamersley Province, where periods of intense dissolution-reprecipitation of Mn oxides (51-41, 24-16 Ma) alternate with periods of relatively subdued mineral precipitation. The intense periods of mineral dissolution-precipitation correlate with maj or global climatic events. The goethite precipitation record confirms the longevity of weathering processes identified in the Mn oxide record. (U-Th)/He dating of 85 grains of goethite from 39 samples (20 hand specimens) from six sampling sites (5 sites sampled for 4°Ar/39Ar geochronology) yield reliable precipitation ages, ranging from 84.3 ± 12.2 to 3.0 ± 0.2 Ma. The deep (~100 m) lateritic weathering profile overlying banded iron-formation in the Hamersley Province record weathering process already ongoing in Late Cretaceous and spanning the Paleogene and Neogene. The geochronological results also reveal that the lateritic profiles in the Metawandy Valley (50-2 Ma), Mt Wall (60-30 Ma), Mt Tom Price (81-12 Ma) and Marandoo (52-12 Ma) regions had already reached great depths (70-100 m on average and up to 220 m below present land surfaces) by at least the Late Cretaceous or Early Paleogene. The results also show that weathering has been less effective at promoting the advancement of the weathering front during the Late Paleogene and Neogene. The geochronological results for authigenic supergene Mn-oxides and goethite III ferruginized detrital deposits (canga) in the Rhodes Ridge (41 to 7 Ma) area indicate that former land surfaces blanketing the Hamersley Province plateaus and ridges had been partially or nearly completely eroded by at least the Eocene. Geochronological results for the channel iron deposits reveal a similar scenario. 40ArP9Ar dating of Mn oxides (ranging from 32 to 17 Ma at the Lynn Peak Cm) and (U-Th)/He dating of goethite (ranging from 18 to 5 Ma at the Yandicoogina cm, and from 43 to 28 Ma at the Lynn Peak Cm) in late-stage authigenic cements indicate that the channel iron deposits had completely aggraded with ironrich sediments and were undergoing goethite cementation (ferruginization) by, at least, the Late Middle Eocene. Age versus depth distributions in channel iron deposits indicate that ferruginization of the channel sediments becomes progressively younger with depth in the profile, strongly suggesting that ferruginization occurred at the groundwater-atmosphere interface and the process moved downwards through progressive deepening of the water table. I interpret that this process was driven by the overall transition towards aridity of northwestern Australia in the Neogene. Excursion towards more humid climates in the Early-Middle Miocene has promoted the partial dissolution and secondary precipitation of channel cements in the upper parts of the profiles or near surface environments. Correlation between the weathering record and independent climatic and environmental indicators suggests that the formation of lateritic weathering profiles on banded iron-formation can be linked to warm and humid climatic conditions in the Late Cretaceous and Early Paleogene, when Australia lay with Gondwana at low latitudes. Climate change at the end of Paleogene, a consequence of the break-up of Gondwana and Australia's accelerated drift away from Antarctica, is identified at this stage as the causal event that promoted the erosion and deposition of former weathering profiles and the formation of extensive detrital and paleochannel deposits. Amid the Neogene aridification of northwestern Australia, a brief excursion towards more humid climatic conditions at the Early Miocene has promoted extensive re-crystallization of supergene minerals in weathering profiles throughout the region. The Neogene aridification of northwestern Australia may also explain the decelerating rates of weathering and weathering front propagation in lateritic profiles of this regIOn. A comparison of the weathering record obtained for the Hamersley Province with the results of similar studies from the Carajas and Quadrilatero Ferrifero Regions, Brazil, and Burkina Faso and Gabon, West Africa, reveals that intense weathering and enrichment ofMn oxides within the weathering profiles occurred at the 50-40 Ma interval, but particularly at 47-45 Ma. The remarkably similar weathering history obtained for the three southern Hemisphere continents suggests that weathering in these ancient landscapes may be controlled by global (greenhouse) climatic conditions.

No description available.

L’analyse géomorphologique d'une zone d’étude permet d’identifier et de comprendre le rôle des facteurs de contrôle tectonique et climatique sur l’évolution passée, récente et future de la surface topographique. Ce travail de recherche porte sur l’analyse géomorphologique des paysages du secteur de l’état de Sarawak, localisé au nord de l’île de Bornéo en contexte tropical. À travers l'analyse morphotectonique des deux plus grands bassins versants : le bassin versant du Rajang et du Baram, il a été possible de dresser une première évaluation du cadre morpho-tectonique de la région et des conséquences topographiques. Les bassins versants étudiés et situés au centre et au Nord de Sarawak drainent une superficie totale d'environ 75 000 km². L'île de Bornéo présente une couverture végétale dense spécifique aux régions tropicales, et demeurant souvent difficile d’accès. Cette île témoigne de terrains montagneux et accidentés, découpés par de vallées profondes aux flancs abruptes, entraînant des taux de dénudation notables depuis le Miocène. La première contribution est d’identifier les principales failles mineures et majeures ayant par réactivation participées au rajeunissement de la surface topographie à l’échelle régionale. À l’échelle des deux bassins versants, les facteurs géomorphologiques suivants comme l’intégrale hypsométrique, les facteurs d’asymétrie, les anomalies de pente identifiées le long des rivières ont été cartographiées à l’aide des techniques d'autocorrélation spatiale. Les principaux mouvements verticaux identifiés sont accommodés le long des accidents structuraux majeurs et des chevauchements spécifiques de la zone Nord de Bornéo. Parmi les autres résultats, il est également observé des surfaces planes reliques, à haute altitude, n’ayant pas encore réajustées leur surface d’équilibre depuis les 5 derniers Ma malgré de phases rapides de soulèvement connues. Enfin, à l’échelle de la zone d’étude, où les contrastes lithologiques sont absents, la présence de nombreuses ruptures de pente ou knickpoints sont observés le long des principaux profils longitudinaux des rivières. Les ruptures de pente fortes sont dans la majorité corrélables aux principaux accidents structuraux. Des observations de terrain viennent renforcer nos hypothèses par la présence de terrasses fluviatiles soulevées. Ce travail d’analyse d’indices géomorphologiques complétés par des observations de terrain permet alors de proposer un modèle synthétique des principaux facteurs de contrôle responsables du rajeunissement de la surface topographique de l’état de Sarawak jusqu’alors sous-estimé et méconnu. / Geomorphic assessment of a region is considered to be crucial in understanding the present day landscapeand forces that have acted and is currently acting on the ever evolving topography. This thesis explores the geomorphology of the tropical landscape of Sarawak, north Borneo through morphotectonic analysis of two of the largest drainage basins of the entire Borneo Island: the Rajang and Baram basin; making this work the first systematic tectono-geomorphic evaluation of the region. The island of Borneo is enveloped by thick rainforests, hostile rugged mountainous terrain with deep and steep valleys, and is characterized by high denudation rates since Miocene. The studied drainage basins flow across entire central and north Sarawak and drain a total combined area of ca. 75, 000 km². The first contribution to the field is by conducting a study on the presence of active tectonic forces that modify the topography through rejuvenation of major and minor faults. The analysis using basin-scale hypsometry, asymmetry factor, normalized channel steepness index and spatial autocorrelation techniques showed that the landscape has been rejuvenated and experiences tectonic deformation to present-day in the form of active folding of the fold-thrust orogenic belts of the Interior Highlands which form the backbone of Borneo. From the results, we highlighted the presence of relict surfaces of landscapes which were isolated at high elevations unable to balance a rapid uplift phase experienced after 5 Ma. We extended the study in order to identify the current stage of landscape development by conducting stream profile analysis which displayed an array of knick-zones and knick points devoid of lithological and climatic controls. Deep V-shaped valleys formed in the zones that demonstrated active folding of the highlands also revealed relief anomalies highlighted through topographic analysis. We showed that enhanced orographic precipitation following the rapid creation of relief has supported adjustment of the topography to a state of transience. In the next part of this work, we conducted swath profile analysis, minimum bulk erosion and channel steepness anomaly maps in order to identify the role of rapid incision in exacerbating erosion rates as a response to tectonic and climatic forcing. We show that there exists a coupling between incision rates, precipitation and channel steepness which shows a relation of direct proportionality. Extensive geomorphic and sedimentological field campaigns were carried out in order to substantiate our results and conclusions. The field work revealed the presence of uplifted fluvial terraces, waterfalls and cataracts corresponding to knick-points identified by us. Finally, we combine our results from the geomorphic analysis and stratigraphic field work in order to construct a conceptual model showing the geomorphic evolution of Sarawak, north Borneo.

Thesis advisor: Noah P. Snyder / Land cover and climate changes, attributed to natural and anthropogenic forcings, cause deviations in geomorphic processes that act to deliver sediment from watersheds to lakes. In New England, contradictory evidence exists as to the influence of deforestation associated with EuroAmerican settlement and major flood events on watershed erosion rates over the past ~250 years. Through combining sediment core analysis from Ossipee Lake, New Hampshire with geomorphic analysis of the Ossipee Lake watershed, this study quantifies Holocene through Anthropocene watershed erosion rates, and assesses variations in rates in relation to short-term historic events such as major storm events or deforestation, and long-term variations related to natural climate variability and post-glacial landscape evolution. An 8.63 m core was collected and spans the entire period from deglaciation to present. Bulk composition and age-depth modeling, utilizing both short-lived radioisotopes and radiocarbon dating, are used to quantify changes in deposition and inferred erosion rates over time. Additional insight on sedimentary processes is provided by measurements of magnetic susceptibility and bulk geochemistry. Lake-sediment data suggests clastic sediment mass accumulation rates vary between 0.0032 to 0.5870 g/cm2/yr, with deposits of increased terrestrially derived sediment focused between ~8500 to 7800, ~6500 to 2500, and 1600 cal yr BP to present. Geomorphic analysis is used to identify regions within the watershed that act to deliver sediment to Ossipee Lake. Potential sources of sediment supply include loose, unconsolidated proglacial deposits near Ossipee Lake that transition to primarily till in upland areas. Calculated bed shear stress along rivers highlights areas in the watershed capable of transporting sediment and areas that can serve as traps thus limiting sediment delivery to Ossipee Lake. / Thesis (MS) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Geomorphologists have long observed the influence of lithology on landscape form and evolution. However, the specific mechanisms by which this is accomplished are not well characterized. Here, I investigate the role of lithology in landscape evolution processes across spatial and temporal scales and geomorphic domains, to progress our understanding of the basic controls on the processes which shape Earth's surface. These investigations were carried out within the Valley and Ridge province of the Appalachian Mountains, where contrasts in strength of underlying lithologies (juxtaposed by Alleghanian deformation) exert a clear, dominant control on the fabric of the landscape, providing an excellent opportunity to study the influence of lithology on a variety of landscape evolution processes. First, I assess the geomorphic function of boulders found on hillslopes and channels in the Valley and Ridge province of the Appalachians, which are sourced from resistant lithologies capping ridgelines. High-resolution UAV surveys and field mapping of boulder distributions and characteristics reveal that boulders are abundant on hillslopes and highly concentrated in channels, often trap sediment upslope, and appear to be long-lived. These observations suggest that boulders act as armor for hillslopes and channels, shielding weaker underlying units from erosion and inhibiting fluvial incision, and therefore play an important role in preserving topography in the Valley and Ridge landscape, highlighting a specific mechanism by which lithology exerts an influence on topography in this setting. Second, I investigate the relative importance of rock strength and discontinuity spacing in setting fluvial bedrock erodibility by comparing knickpoint and non-knickpoint bedrock, which correspond to end-member erodibility cases, and assess how lithology impacts knickpoint expression. Detailed field surveys of 21 lithologic knickpoints, surrounding non-knickpoint reaches, and corresponding bedrock properties reveal three key outcomes: 1) discontinuity spacing is a stronger predictor of knickpoint occurrence, and therefore more significant in setting bedrock erodibility in this setting, confirming quantitatively the hypothesis that discontinuities exert a dominant control on fluvial erodibility, 2) knickpoint expression is a function of the unique combination of characteristics within a given stratigraphic interval, and therefore highly complex and specific to local conditions, implying that knickpoint morphology should be interpreted with extreme caution, and 3) because all 21 study knickpoints occur within the same unit, inter-unit heterogeneity must be accounted for before lithologic influence on channel profile convexities can be ruled out, rather than comparing to geologic map contacts. These findings represent an important contribution towards a more functional understanding of the influence of lithology on fluvial bedrock incision processes. / Doctor of Philosophy / It has long been observed that underlying geology has a strong impact on the shape of the surrounding landscape and influences the erosional processes that act within that landscape. However, though the importance of rock type in shaping landscapes is recognized, the specific mechanisms by which this is accomplished are not well understood. The work presented here investigates the role of rock type and rock properties in landscape evolution processes in both hillslope and river environments within the Valley and Ridge Province of the Appalachian Mountains. This setting is ideally suited for investigating the role of rock type on landscape evolution processes because of the wide variation in rock types present in this setting, which exert a strong influence on local topography (e.g., strong rocks form ridges while weak rocks underlie valleys). First, I mapped the distribution of large boulders on local Valley and Ridge slopes and mountain streams to assess the potential for these boulders to play a role in preserving local topography. Results show that boulders are sourced from resistant rock types found along ridgelines, and are abundant on hillslopes and highly concentrated in channels. Boulders also trap sediment upslope and appear to remain in place for long periods of time. These observations suggest boulders play an important role in slowing erosion of weaker rock types underlying hillslopes and channels, and therefore aid in preserving topography in this setting. Second, I conducted detailed surveys of local small-scale waterfalls and surrounding flat river reaches and compared properties of the bedrock between these locations to better understand how bedrock properties influence erodibility. In this setting, waterfalls often signify strong underlying bedrock relative to the rock beneath flat river reaches, so comparing bedrock properties between these areas should give insight into how properties like rock strength and bed thickness impact how erodible the bedrock is. Results show that bed thickness is the most important variable impacting bedrock erodibility in this setting, and that waterfall appearance is a product of the unique combination of bedrock properties within a given area. These results are important for improving our ability to model natural landscapes and erosional processes, and for developing a more complete understanding for the relationships between rock type and river morphology.

The Snake River flows across the dynamically uplifting hotspot plume of the Yellowstone region, cuts through the Snake River Range and ultimately enters the lowlying eastern Snake River Plain. Thermal and mantle-dynamic uplift around Yellowstone has been recorded by short-term geodesy and modeled by geophysicists, but measurements over Quaternary timescales and an understanding of how that uplift influences regional incision are absent. The Snake River is the only regional river that crosses the uplifting Yellowstone Plateau and flows into the subsiding eastern Snake River Plain (SRP), and provides an opportunity to investigate both ends of the phenomenon on the tailing margin of the Yellowstone region. This thesis consists of two related studies conducted in Alpine Canyon of the Snake River. The first is a study of fluvial terraces and steepness patterns along the Snake River considering the spatial distribution of bedrock or varying hardness and resistance to erosion and in the context of regional tectonics. This study uses surficial mapping, optically stimulated luminescence (OSL) dating, bedrock strength measurements, and steepness analyses of the mainstem Snake River and tributary drainages. Results include the first incision rate estimates for the southwestern part of the Yellowstone hotspot region and a discussion of the possible sources of baselevel fall along the Snake River. The second study documents the transitions between bedrock and alluvial channels in the study area and evaluates morphometric and transport capacity thresholds between these reaches. Alluvial bed-cover mapping with a side-scan sonar along with channel morphometric data, clast-counts, and sediment transport estimates allow us to explore what controls these two fundamental channel types. Results confirm that the Snake River has relatively fast incision rates for the interior western U.S. and that the Snake River is adjusting to an actively deforming landscape. Additionally, our dataset provides field documentation of the magnitude of bedrock-alluvial transitions and may be valuable for parameterizing landscape evolution models or assisting in the restoration of reaches that are in disequilibrium due to changes in land use or climate. This study will hopefully inspire future studies of tectonism and landscape evolution of the Yellowstone hotspot region.

Arcing eastward from the deep gorge of Cataract Canyon on the Colorado River is a series of aligned valleys (graben) and ridges (horst). This unusual landscape has formed as subsurface salt deforms toward the river and dissolves away, causing the overlying rocks to fault, slide, and subside. Geologists have long been interested in this actively evolving area they call the Needles fault zone, because understanding its mechanics and origin may shed light on how faults work in general and similar, yet inaccessible places like offshore rift zones or even the surface of the Moon. Despite this interest, the timing and long-term patterns of deformation here and are poorly constrained. This study uses analysis of digital landscape models to better delineate these patterns and provide better age constraints on the development of the Needles fault zone. We find that the Colorado River incision that led to deformation here began as recently as 1 million years ago, and that faulting due to subsurface salt movement initiated between 700 and 200 thousand years ago. The first part of this study takes advantage of how the development of graben valleys has changed the path of many of the streams in the study area, resulting in numerous captured streams terminating into a type of sinkhole, called a swallow hole, that develops above opening faults. These fissures are so named because, by ongoingopening, they are “swallowing” material that is flushed into them by local drainages. By recording and numerically dating the exposed upper 6-14 m of basin-fill strata, we determined that sediment was deposited to an alluvial fan and to ponded water. We also compared calculated sediment yields over time to paleoclimate records for the region to test extant hypotheses about how drylands respond to changing climate of the same scale as modern climate change. Against expectations, our results suggest that the greatest sediment yield and storage in these upper basins occurred during the relatively warm and dry time from 9 to 5 thousand years ago, when overland flow to transport sediment was weak. This implies that we are actually measuring sediment storage, as the faults that form swallow holes were relatively less active, allowing sediment to accumulate, rather than be flushed out of the basins.

Landscapes evolve through a number of processes in response to a wide range of forcing mechanisms. Many of the processes that drive landscape evolution occur at the interface between fluid and sediment. Sediment transport leads to changes in topography that, in turn, influence fluid flow. Feedback mechanisms between topography and fluid flow can lead to the formation of patterns, such as sand ripples, dune fields, parallel channel networks, and periodically spaced valleys. In many cases, the development and evolution of patterns within landscapes are heavily influenced by environmental conditions. Therefore, given relationships between landform features and the underlying processes, present-day landscapes have the potential to be used to infer a record of climatic conditions over the course of their development. An inability to make direct observations over geologically relevant timescales makes it difficult to study the processes that influence landscape evolution. Mathematical models provide a means of quantitatively linking natural patterns and landscape features with physical processes. Patterns in landscapes also provide a simple means of testing quantitative representations of geomorphic processes. In this work, we develop landscape evolution models to study the development of debris-flow-dominated hillslopes, rill networks, and cinder cones. Through a combination of theoretical modeling, analysis of experimental data, and remote sensing data, we attempt to better understand each of these three systems. While each system is interesting in isolation, these and similar studies add to our knowledge of the mathematical representations of processes that are used more generally within the study of landscape evolution.

The magnitude of global sediment flux from streams to the oceans over the last 5 Ma is poorly quantified, yet important for predicting future fluxes and deciphering the relative control of tectonic uplift, climate change, vegetation, and related feedback mechanisms on landscape evolution. Despite numerous proxy studies on global sediment delivery to the oceans, it remains uncertain whether bulk sedimentation increased, decreased, or remained approximately constant across one of the most significant global climate changes: the Plio-Pleistocene transition. New developments and strategies in the application of cosmic-ray-produced isotopes, in part developed by this thesis, provide records of pre-historic denudation of confined fluvial catchments in Texas and Yukon. Non-glaciated, tectonically passive regions were targeted in contrast to other studies on modern sedimentation rates in order to isolate the climate influence from glacial and tectonic controls. The results suggest that average catchment temperature, and surficial processes and other factors such as vegetation cover associated with temperature, are the primary controls on the variation in landscape denudation in regions lacking tectonics and direct glacial cover. Specifically, warmer temperatures yield higher denudation rates, both at the scale of glacial-interglacial climate change and over the Plio-Pleistocene transition. The implication is that stream sediment flux to the ocean from tropical and temperate regions was higher during the Pliocene than in the Quaternary. However, this may have been balanced by an increase in sediment flux from regions covered by warm-based glaciers during glacial periods, or by increased temporary continental storage during interglacial periods.

Landscapes respond to external perturbations over a variety of timescales, including million-year tectonic forcing, millennial to decadal climate fluctuations, and minutes-long high intensity storms or large magnitude earthquakes. In mountainous regions, understanding the role of landslides in driving the hillslope response to these perturbations is paramount for understanding landscape evolution over geologic timescales and hazards over human timescales. Here I analyze the landslide-driven hillslope response over millennial to decadal timescales using a variety of tools and techniques (e.g. tephrochronology, lidar and air photo analysis, field and subsurface investigations, and seismic refraction) in the Waipaoa Basin (New Zealand) and Oregon Coast Range (USA). For the Waipaoa study catchment, pervasive landslides have been sculpting >99% of the hillslopes in response to >50 m of fluvial incision following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (~18 ka). Then, starting in the late 1800s, European settlement resulted in deforestation and conversion of >90% of the landscape to pastureland—spurring a rapid increase in landslide-driven erosion. To quantify the landscape response, I first reconstruct LGM and younger paleosurfaces using tephrochronology and lidar-derived surface roughness to estimate the volume, timing, and distribution of hillslope destabilization. From these reconstructions, I calculate the post-LGM catchment-averaged erosion rate (1.6 mm/yr) and determine that the timing of the initial hillslope adjustment was rapid and occurred by ~10 ka. Second, I quantify the rate and volume of historic hillslope degradation using a 1956-2010 sequence of aerial photographs, lidar, and field reconnaissance to map the spatial extent of active landslides, create a ‘turf index’ based on the extent and style of pastoral ground disruption, correlate that with downslope velocity, and calculate the average annual sediment flux. From the sediment flux, I calculate an erosion rate over the past ~50 years (~20 mm/yr) that is 10x greater than post-LGM. Lastly, in Western Oregon, I confirm that seismic refraction can determine the size (e.g. depth) and failure style of landslides in western Oregon—data needed to incorporate these poorly studied landslides into future landscape evolution or hazard models. This dissertation includes both previously published and unpublished co-authored material.