Lithologic, geomorphic, and permafrost controls on recent landsliding in the Alaska Range

Abstract

Because landslide regimes are likely to change in response to climate change in upcoming decades, the need for mechanistic understanding of landslide initiation and up-to-date landslide inventory data is greater than ever. We conducted surficial geologic mapping and compiled a comprehensive landslide inventory of the Denali National Park road corridor to identify geologic and geomorphic controls on landslide initiation in the Alaska Range. The supplemental geologic map refines and improves the resolution of mapping in the study area and adds emphasis on surficial units, distinguishing multiple glacial deposits, hillslope deposits, landslides, and alluvial units that were previously grouped. Results indicate that slope angle, lithology, and thawing ice-rich permafrost exert first-order controls on landslide occurrence. The majority (84%) of inventoried landslides are <0.01 km2 in area and occur most frequently on slopes with a bimodal distribution of slope angles with peaks at 18° and 28°. Of the 85 mapped landslides, a disproportionate number occurred in unconsolidated sediments and in felsic volcanic rocks. Weathering of feldspar within volcanic rocks and subsequent interactions with groundwater produced clay minerals that promote landslide initiation by impeding subsurface conductivity and reducing shear strength. Landslides also preferentially initiated within permafrost, where modeled mean decadal ground temperature is −0.2 ± 0.04 °C on average, and active layer thickness is ~1 m. Landslides that initiated within permafrost occurred on slope angles ~7° lower than landslides on seasonally thawed hillslopes. The bimodal distribution of slope angles indicates that there are two primary drivers of landslide failure within discontinuous permafrost zones: (1) atmospheric events (snowmelt or rainfall) that saturate the subsurface, as is commonly observed in temperate settings, and (2) shallow-angle landslides (<20° slopes) in permafrost demonstrate that permafrost and ice thaw are also important triggering mechanisms in the study region. Melting permafrost reduces substrate shear strength by lowering cohesion and friction along ice boundaries. Increased permafrost deg­radation associated with climate change brings heightened focus to low-angle slopes regionally as well as in high-latitude areas worldwide. Areas normally considered of low landslide potential will be more susceptible to shallow-angle landslides in the future. Our landslide inventory and analyses also suggest that landslides throughout the Alaska Range and similar climatic zones are most likely to occur where low-cohesion unconsolidated material is available or where alteration of volcanic rocks produces sufficient clay content to reduce rock and/or sediment strength. Permafrost thaw is likely to exacerbate slope instability in these materials and expand areas impacted by landslides.

Ongoing Landslide Deformation in Thawing Permafrost

“…At all three landslides, we interpret initial failure on a thawing, shallow-angle permafrost plane at ∼1 m depth (Panda et al, 2014;Patton et al, 2020). Positive pore water pressures above the permafrost boundary after ice melt allow for slope failure even on shallow hillslopes (Harris & Lewkowicz, 2000;Lewkowicz & Harris, 2005a;Patton et al, 2020).…”

“…At all three landslides, we interpret initial failure on a thawing, shallow-angle permafrost plane at ∼1 m depth (Panda et al, 2014;Patton et al, 2020). Positive pore water pressures above the permafrost boundary after ice melt allow for slope failure even on shallow hillslopes (Harris & Lewkowicz, 2000;Lewkowicz & Harris, 2005a;Patton et al, 2020). Despite the similarity of slope angle (10.3-16.5°), lithology, and aspect (116-162°), the three landslides studied in DNP demonstrate diverse morphology, including compact (Stony Pass Slide); elongate (Ptarmigan ALD); and complex (Eielson ALD) ALDs as described by Lewkowicz and Harris (2005b).…”

“…Under these conditions, discontinuous permafrost is expected to degrade rapidly (Panda et al., 2014; Pastick et al., 2015), resulting in drastic changes to groundwater hydrology (Walvoord & Kurylyk, 2016) and facilitating the initiation of shallow‐angle landslides (Patton et al., 2019; Swanson, 2021). Shallow‐angle permafrost landslides have been documented elsewhere in the Denali Park Road corridor (Patton et al., 2020), Canada (e.g., Blais‐Stevens et al., 2015; Lewkowicz, 2007; Zwieback et al., 2019), Russia (Khak & Kozyreva, 2012; Zwieback et al., 2018), and other parts of Alaska (Bowden et al., 2008; Daanen et al., 2012; Gooseff et al., 2009; Swanson, 2021). Although increased evapotranspiration rates may result in lower soil moisture over the course of the year (Rupp & Loya, 2008), changes to storm intensity will continue to promote rain‐induced and thaw‐induced landslides.…”

“…The study area is geologically diverse, with regional-scale fault and fold structures and bedrock lithologies including Cretaceous-Paleogene sedimentary and volcanic rocks underlain by Triassic basalt and metabasalt (Gilbert, 1979;Wilson et al, 2015). Tectonic activity, Pleistocene glaciation, and fluvial and hillslope processes create a complex geomorphic setting (Csejtey et al, 1982;Gilbert, 1979;Nokleberg et al, 1994;Patton et al, 2020;Yeend, 1997).…”

“…Although individual ALDs do not displace large volumes of material, their collective frequency and broad spatial distribution results in a large cumulative impact. Landslide investigations in northern Canada (Lewkowicz & Harris, 2005a; Lewkowicz, 2007), along the Alaska‐Yukon Highway (Blais‐Stevens et al., 2015), central Alaska (Patton et al., 2020), and Russia (Khomutov & Leibman, 2014; Leibman et al., 2014) have documented frequent shallow‐angle permafrost landslides. These investigations found that ALDs accounted for 3%–93% of all documented slides, with an expected increase as permafrost thaws (Blais‐Stevens et al., 2015; Leibman, 1995).…”