Alexander Logan Densmore

As mountain populations grow and exposure to hazard increases, understanding the dynamic hazard posed by landslides is essential. However, our ability to constrain landslide hazard is limited by relatively few good-quality datasets that are often collected across small geographical areas. These datasets typically focus on specific triggering events, such as earthquakes or rainstorms, further complicating assessments of landslide hazard through time. To better capture landslide hazard through space and time, long-term datasets over large extents are needed. Here, we analyse a 22-year record of landslide occurrence to decipher the dynamic nature of the hazard posed by landslides in Nepal. We find that recurrent or persistent landslides dominate national-scale patterns of landsliding, accounting for c. 80% of all visible landslides at any given point in time. Of new landslide areas, only half occurred in locations >30 m away from other landslides. These findings emphasise the importance of also considering pre-existing landslides when assessing hazard. Individual triggering events, including both earthquakes and rainfall, leave a distinctive footprint visible at the national scale in metrics such as landslide density, persistence, and recurrence. Such metrics add significant value to multi-temporal landslide inventories and provide a basis for mitigation efforts, particularly where resources are limited.

Fluvial valley networks in mountain ranges record the interactions between climate, tectonics, and lithology. While drainage network analysis has transformed our understanding of these interactions in subaerial settings, the landscape evolution of ice-covered orogens is poorly known. The Gamburtsev Subglacial Mountains are a ∼ 600 km-long mountain range situated beneath the East Antarctic Ice Sheet. These mountains were an important nucleation site for the ice sheet approximately 34 million years ago and are now buried beneath ∼ 2 km of ice. Airborne radar surveying has revealed that the Gamburtsevs are characterised by a rugged, incised landscape, but their geological structure and uplift history remain enigmatic. Here we use a compilation of radar survey data to extract and quantify valley longitudinal profiles from the Gamburtsevs and in turn infer details of their tectonic and geomorphic development. We use χ -mapping and stream power incision modelling to show that valley network morphology is consistent with a fluvial landscape that was locally overprinted by early mountain glaciation. The spatial distribution of channel steepness indices allows us to determine the position of major geological boundaries at the edges of the mountains. We also use independent estimates of denudation rates to evaluate competing scenarios for the timing of mountain uplift and valley incision, finding that uplift of the modern Gamburtsevs most likely commenced in the Mesozoic or early Cenozoic. Regional geomorphic analysis suggests that base level for some Gamburtsev fluvial catchments was set by enclosed interior basins associated with extensional faulting. These depocentres may retain detrital sedimentary material eroded from the Gamburtsevs prior to Antarctic glaciation and are potential targets for future sub-ice drilling campaigns.

Fluvial valley networks in mountain ranges record the interactions between climate, tectonics, and lithology. While drainage network analysis has transformed our understanding of these interactions in subaerial settings, the landscape evolution of ice-covered orogens is poorly known. The Gamburtsev Subglacial Mountains are a ~600 km-long mountain range situated beneath the East Antarctic Ice Sheet. These mountains were an important nucleation site for the ice sheet approximately 34 million years ago and are now buried beneath ~2 km of ice. Airborne radar surveying has revealed that the Gamburtsevs are characterised by a rugged, incised landscape, but their geological structure and uplift history remain enigmatic. Here we use a compilation of radar survey data to extract and quantify valley longitudinal profiles from the Gamburtsevs and in turn infer details of their tectonic and geomorphic development. We use 𝜒-mapping and stream power incision modelling to show that the morphology of the valley network is largely consistent with fluvial incision that occurred prior to glaciation. In addition, the spatial distribution of channel steepness indices allows us to confirm the position of major geological boundaries at the edges of the mountains. We also use independent estimates of denudation rates to evaluate competing scenarios for the timing of mountain uplift and valley incision, finding that uplift of the modern Gamburtsevs most likely commenced in the Mesozoic. Regional geomorphic analysis suggests that base level for some Gamburtsev fluvial catchments was set by enclosed interior basins associated with extensional faulting. These depocentres may retain detrital sedimentary material eroded from the Gamburtsevs prior to Antarctic glaciation and are potential targets for future sub-ice drilling campaigns.

Current debate exists on whether the depositional processes forming valley fill deposits in the Lesser Himalaya can be linked across multiple Himalayan river systems. Some studies have attributed the deposition of 100s of metres of sediment fill in the Lesser Himalaya to short-lived, catastrophic filling events. These events are thought to originate from large rock-slope failures and glacial lake outburst floods. Other studies have suggested that the deposition of fill on such large scales is a long-term process caused by fluctuations in the Indian Summer Monsoon (ISM), resulting in the oversupply of alluvial sediment into a transport-limited system. Cross-catchment patterns of terrace formation that might help to distinguish between these end-member driving mechanisms are limited by an incomplete record of fill terraces in the Lesser Himalaya, with multiple reaches having not yet seen detailed sedimentological study. Here, we address this gap by investigating a flight of fill terraces along the Middle Kali Gandaki River upstream of the town of Kushma, central Nepal. Terraces along this reach are preserved up to 400 m above the modern river channel. We build on previous studies of these terraces by identifying the stratigraphy of the terrace fill deposits and by recording clast lithology and morphology of the sediment preserved beneath each terrace level. We find the terraces to be predominantly composed of a coarse debris fill, with little to no visible stratigraphy, apart from occasional inverse grading from gravel to coarse boulder conglomerate. We identify five terrace levels and find differences in clast morphologies between T1, T2, T3, and T5, and a difference in clast lithology between T2 and T3. We therefore suggest that there have been at least four major filling and re-incision events along this reach, with the possibility of a fifth undocumented filling event forming T4. We also recreate approximate palaeo-valley floors for each terrace level to measure palaeo-valley widths. We interpret that multiple periods of extensive sediment aggradation and incision which led to the formation of five major terrace levels along the Middle Kali Gandaki River were driven by intensified monsoons leading to significant periods of sediment oversupply, contrasting the interpretations of catastrophic debris fill made along other rivers in the catchment. We attribute a decrease in valley width over time to high denudation rates near the MCT. We investigate terrace preservation along the study reach by calculating the percentage of preserved terrace area compared to the approximate area of the abandoned valley floor for each terrace level. We find that terrace levels T3 and T4 are poorly preserved compared to T1 and T2. We suggest post-depositional cementation to be a primary control, with indurated older terrace deposits acting as bedrock, rapidly reducing valley width so that the river can erode the entire floodplain more easily and thus limiting terrace preservation.

Debris-flow fans form by repeated deposition of debris-flow sediments. Catchment lithology affects debris-flow grain-size distribution, and thereby rheology, erosive potential, and depositional morphology. We can therefore expect that lithology also influences debris-flow fan characteristics. Here, we determine how catchment lithology affects the surface morphology and sedimentology of debris-flow fans, and by extension their spatio-temporal evolution. We study nine fans along the eastern margin of northern Owens Valley, California, USA, originating from catchments with contrasting lithologies, and similar climate, tectonics, and geological history.Results show that debris flows originating from catchments comprising magmatic rocks are rich in cobble- to boulder-sized grains. The coarse sediment along the flow fronts and margins minimizes lateral spreading of debris-flow lobes, forming distinct levees and thick depositional mounds. In contrast, debris flows originating from catchments dominated by sedimentary rocks are rich in relatively fine gravel. Their fine-grained levees and lobes lack strongly frictional margins, spread more easily, and form distinctly thinner and wider deposits. Debris flows originating from catchments with metamorphic lithologies show intermediate grain-size and depositional morphology.These contrasts in debris-flow characteristics guide the morphology and spatio-temporal development of debris-flow fans. Fine-grained debris flows spread laterally and tend to fill topographical lows, whereas lateral spreading of coarser-grained flows is hampered, instigating a low tendency to fill topographic lows. The more efficient topographic compensation on fans formed by fine-grained debris flows causes smaller elevation differences across a less rugged surface, and likely to higher avulsion frequencies. The limited mobility and spreading of coarse-grained debris flows promote frequent deposition on top of and directly adjacent to channel margins, forming well-defined channels bordered by thick composite levees, and raised fan sectors. These results illustrate how catchment lithology can affect the morphology, sedimentology, and evolution of debris-flow fans, providing guidelines for reading their depositional archives and avulsion hazard assessment.

Fluvial valley networks in mountain ranges record the interactions between climate, tectonics, and lithology. While drainage network analysis has transformed our understanding of these interactions in subaerial settings, the landscape evolution of ice-covered orogens is poorly known. The Gamburtsev Subglacial Mountains are a ~600 km-long mountain range situated beneath the East Antarctic Ice Sheet. These mountains were an important nucleation site for the ice sheet approximately 34 million years ago and are now buried beneath ~2 km of ice. Airborne radar surveying has revealed that the Gamburtsevs are characterised by a rugged, incised landscape, but their geological structure and uplift history remain enigmatic. Here we use a compilation of radar survey data to extract and quantify valley longitudinal profiles from the Gamburtsevs and in turn infer details of their tectonic and geomorphic development. We use 𝜒-mapping and stream power incision modelling to show that the morphology of the valley network is largely consistent with fluvial incision that occurred prior to glaciation. In addition, the spatial distribution of channel steepness indices allows us to confirm the position of major geological boundaries at the edges of the mountains. We also use independent estimates of denudation rates to evaluate competing scenarios for the timing of mountain uplift and valley incision, finding that uplift of the modern Gamburtsevs most likely commenced in the Mesozoic. Regional geomorphic analysis suggests that base level for some Gamburtsev fluvial catchments was set by enclosed interior basins associated with extensional faulting. These depocentres may retain detrital sedimentary material eroded from the Gamburtsevs prior to Antarctic glaciation and are potential targets for future sub-ice drilling campaigns.

Fluvial valley networks in mountain ranges record the interactions between climate, tectonics, and lithology. While drainage network analysis has transformed our understanding of these interactions in subaerial settings, the landscape evolution of ice-covered orogens is poorly known. The Gamburtsev Subglacial Mountains are a ~600 km-long mountain range situated beneath the East Antarctic Ice Sheet. These mountains were an important nucleation site for the ice sheet approximately 34 million years ago and are now buried beneath ~2 km of ice. Airborne radar surveying has revealed that the Gamburtsevs are characterised by a rugged, incised landscape, but their geological structure and uplift history remain enigmatic. Here we use a compilation of radar survey data to extract and quantify valley longitudinal profiles from the Gamburtsevs and in turn infer details of their tectonic and geomorphic development. We use 𝜒-mapping and stream power incision modelling to show that the morphology of the valley network is largely consistent with fluvial incision that occurred prior to glaciation. In addition, the spatial distribution of channel steepness indices allows us to confirm the position of major geological boundaries at the edges of the mountains. We also use independent estimates of denudation rates to evaluate competing scenarios for the timing of mountain uplift and valley incision, finding that uplift of the modern Gamburtsevs most likely commenced in the Mesozoic. Regional geomorphic analysis suggests that base level for some Gamburtsev fluvial catchments was set by enclosed interior basins associated with extensional faulting. These depocentres may retain detrital sedimentary material eroded from the Gamburtsevs prior to Antarctic glaciation and are potential targets for future sub-ice drilling campaigns.

Debris‐flow fans form by repeated deposition of debris‐flow sediments. Catchment lithology affects debris‐flow grain‐size distribution, and thereby rheology, erosive potential and depositional morphology. We can therefore expect that lithology also influences debris‐flow fan characteristics. Here, we determine how catchment lithology affects the surface morphology and sedimentology of debris‐flow fans and by extension their spatiotemporal evolution. We study nine fans along the eastern margin of northern Owens Valley, California, USA, originating from catchments with contrasting lithologies, and similar climate, tectonics and geological history. Results show that debris flows originating from catchments comprising magmatic rocks are rich in cobble‐ to boulder‐sized grains. The coarse sediment along the flow fronts and margins minimizes lateral spreading of debris‐flow lobes, forming distinct levees and thick depositional mounds. In contrast, debris flows originating from catchments dominated by sedimentary rocks are rich in relatively fine gravel. Their fine‐grained levees and lobes lack strongly frictional margins, spread more easily and form distinctly thinner and wider deposits. Debris flows originating from catchments with metamorphic lithologies show intermediate grain size and depositional morphology. These contrasts in debris‐flow characteristics guide the morphology and spatiotemporal development of debris‐flow fans. Fine‐grained debris flows spread laterally and tend to fill topographical lows, whereas lateral spreading of coarser‐grained flows is hampered, instigating a low tendency to fill topographic lows. The more efficient topographic compensation on fans formed by fine‐grained debris flows causes smaller elevation differences across a less rugged surface and is likely to lead to higher avulsion frequencies. The limited mobility and spreading of coarse‐grained debris flows promote frequent deposition on top of and directly adjacent to channel margins, forming well‐defined channels bordered by thick composite levees and raised fan sectors. These results illustrate how catchment lithology can affect the morphology, sedimentology and evolution of debris‐flow fans, providing guidelines for reading their depositional archives and avulsion hazard assessment.