Timing and Paleoclimatic Significance of Latest Pleistocene and Holocene Cirque Glaciation in the Enchantment Lakes Basin, North Cascades, WA

The Enchantment Lakes Basin in the Alpine Lakes Wilderness, Washington, preserves two sets of moraines that record distinct post-Wisconsin maximum advances of cirque glaciers in the eastern North Cascades. Cores collected from five lakes adjacent to the moraines indicate that there were two Neoglacial advances, culminating with the Little Ice Age, and one slightly larger advance that ended coincident with the termination of the North Atlantic Younger Dryas event. The cores show no evidence for an early Holocene advance, in contrast to some other studies in the North Cascades, (e.g., Heine, 1998; Thomas, 1997; Thomas et al., 2000). Upstream glacier activity, as indicated by rock-flour production, is recorded in the lake sediments as fluctuations in magnetic susceptibility, organic content, and sediment particle size. Tephra identification, AMS 14C dating, and paleomagnetic secular variation of the sediments provide detailed age constraints for the lake cores. The presence of the 475 cal yr B.P. Mount St. Helens Wn tephra within outwash associated with the inner (Brynhild) moraines indicates that they are Little Ice Age (LIA) equivalent. The age constraints on the lake sediments show that this advance began between ~ 1000-800 cal yr B.P. and culminated after the Wn tephra was deposited. The age of the outer (Brisingamen) moraines, previously reported as early Holocene (Waitt et al.,1982), are instead latest Pleistocene; close limiting 14C dates demonstrate that this advance ended shortly before ~11,300 cal yr B.P., suggesting temporal equivalence with the North Atlantic Younger Dryas climatic reversal (12,940 ± 260 - 11,640 ± 250 cal yr B.P; Alley et al., 1993). A ~500-yr interval of high rock-flour flux in the cores records an early Neoglacial advance between ~3300 and ~2800 cal yr B.P. that was less extensive than the subsequent LIA advance. Steady-state equilibrium-line altitudes (ELAs) for Brynhild and Brisingamen advances estimated with accumulation-area ratio and balance-ratio methods are distinct but nearly indistinguishable at ~2355 m, roughly 200 m below the modern ELA. Conditions required to form and sustain the Brisingamen and Brynhild paleoglaciers include a summer temperature depression of ~3° C, an increase of ~90 cm water-equivalent in winter precipitation, or, more likely, some lesser combination of the two. These constraints imply a local climate that could support only small-scale advances in both the latest Pleistocene and late Holocene, and warmer as well as drier conditions throughout the early Holocene

Hillslope response to climate-modulated river incision and the role of deep-seated landslides in post-glacial sediment flux: Waipaoa Sedimentary System, New Zealand

Quantifying how hillslopes respond to river incision and climate change is fundamental to understanding the geomorphic evolution of tectonically uplifting landscapes during glacial-interglacial cycles. Hillslope adjustment in the form of deep-seated bedrock landslides can account for a large proportion of the regional sediment yield and denudation rates for rapidly uplifting landscapes. However, the timing and magnitude of the response of hillslopes to climatic and tectonic forcing in moderate uplift temperate maritime catchments characteristic of many active margins worldwide is not well quantified. This study seeks to investigate how hillslopes respond to climate-modulated river incision and to quantify the magnitude of the sediment flux from this response in a typical active margin setting. The non-glacialWaipaoa Sedimentary System (WSS) on the East Coast of the North Island of New Zealand consists of river catchments, coastal foothills to uplifting mountain ranges, and terrestrial and marine sediment depocentres collectively underlain by relatively young (Cretaceous and younger) sedimentary rocks within a tectonically active setting and temperate maritime climate. These attributes make theWSS similar to many coastal catchments on oceanic-continental convergent margins worldwide. However, because of widespread destruction of primary forests for conversion to pasture lands by the mid 20th Century, theWSS is currently a globally significant source of sediment to the world’s oceans. Because of these factors, theWSS was selected as one of two global study sites for the international, NSF supported, MARGINS Source-to-Sink initiative designed to investigate the transfer of sediment from terrestrial source to marine sink. Previous studies on theWSS have shown a strong link between climate change and geomorphic response in the system. River incision since the last glacial coldest period has generated a significant amount of topography, leaving small remnants of the ca.18,000 cal. yr BP last glacial aggradation terrace scattered up to 120 m above modern rivers. In this study, the hillslope response to river incision is quantitatively examined using new high resolution topographic data sets (lidar and photogrammetry) in combination with 3 field mapping and tephrochronology. Hillslopes are found to be coupled to river incision and adjusted to rapid incision through the initiation and reactivation of deep-seated landslides. In the erodible marine sedimentary rocks of the terrestrialWSS, post-incision deep-seated landslides can occupy over 30% of the surface area. Many of these slides show evidence of multiple “nested” failures and landslide reactivation. The ages of tephra cover beds identified by electron microprobe analysis show that following an initial 4,000 to 5,000 year time lag after the initiation of river incision, widespread hillslope adjustment started between the deposition of the ca. 13,600 cal. yr BPWaiohau tephra and the ca. 9,500 cal. yr BP Rotoma tephra. Tephrochronology and geomorphic mapping analysis indicates that river incision and deep-seated landslide slope adjustment is synchronous between mainstem rivers and headwater tributaries. Tephrochronology further shows that many slopes have continued to adjust to channel incision into the late Holocene. Hillslope response in the catchment can involve the entire hillslope from river to ridgeline, with some interfluves between incising sub-catchments being dramatically modified through ridgeline retreat and/or lowering. Using the results of the landform tephrochronology and geomorphic mapping, a conceptual time series of hillslope response to uplift and climate change-induced river incision is derived for a timeframe encompassing the last glacial-interglacial cycle. Using the same high resolution topography datasets, in-depth field analysis, and tephrochronology, the 18,000 year sediment yield from terrestrial deep-seated landslides in theWSS is estimated in order to investigate the magnitude of hillslope response to climate-modulated, uplift driven river incision. This completes one of the first processbased millennial time-scale sediment budgets for this class of temperate maritime, active margin catchments. Fluvial and geomorphic modelling is applied to reconstruct pre 18,000 cal. yr BP topography in 141 km2 of detailed study area and the resulting volumetric estimates from 207 landslides are used to estimate deep-seated landslide sediment flux for the broader system. An estimated 10.2 km3 of deep-seated landslidederived sediment with a multiplicative uncertainty of 1.9 km3 (+9.2 km3, -4.8 km3) was delivered to terrestrial and marine sinks. This accounts for between 10 and 74% of the total mass of the terrestrialWSS budget of ca. 91,000 Mt (+37,000 Mt, -26,000 Mt). Combining the deep-seated landslide results with other studies of terrestrial sediment sources and terrestrial and shelf sinks, the estimated terrestrial source load ranges from 4 Abstract 1.2 to 3.7 times larger than the mass of sediment sequestered in terrestrial and shelf depocentres. This implies that off-shelf transport of sediment is important in this system over the last 18,000 cal. yr BP, as it is today for anthropogenic reasons. Based on the derived sediment budget, the denudation rate for the terrestrialWSS of 0.8 mm yr-1 (+0.3 mm yr-1, -0.2 mm yr-1) is indistinguishable from the average terrestrialWaipaoa late Quaternary uplift rate, indicating an approximate steady-state balance between denudation and uplift. This thesis provides a quantitative analysis of the role of deepseated landslides in an active margin catchment that is used to improve the understanding of landscape and terrestrial source-to-marine-sink sediment transfer dynamics

Fault kinematics and surface deformation across a releasing bend during the 2010 M_W 7.1 Darfield, New Zealand, earthquake revealed by differential LiDAR and cadastral surveying

Dextral slip at the western end of the east-west–striking Greendale fault during the 2010 M_W 7.1 Darfield earthquake transferred onto a northwest-trending segment, across an apparent transtensional zone, here named the Waterford releasing bend. We used detailed surface mapping, differential analysis of pre- and postearthquake light detection and ranging (LiDAR), and property boundary (cadastral) resurveying to produce high-resolution (centimeter-scale) estimates of coseismic ground-surface displacements across the Waterford releasing bend. Our results indicate that the change in orientation on the Greendale fault incorporates elements of a large-scale releasing bend (from the viewpoint of westward motion on the south side of the fault) as well as a smaller-scale restraining stepover (from the viewpoint of southeastward motion on the north side of the fault). These factors result in the Waterford releasing bend exhibiting a decrease in displacement to near zero at the change in strike, and the presence within the overall releasing bend of a nested, localized restraining stepover with contractional bulging. The exceptional detail of surface deformation and kinematics obtained from this contemporary surface-rupture event illustrates the value of multimethod investigations. Our data provide insights into strike-slip fault bend kinematics, and into the potentially subtle but important structures that may be present at bends on historic and prehistoric rupture traces

Catastrophic landscape modification from a massive landslide tsunami in Taan Fiord, Alaska

The October 17th, 2015 Taan Fiord landslide and tsunami generated a runup of 193 m, nearly an order of magnitude greater than most previously surveyed tsunamis. To date, most post-tsunami surveys are from earthquake-generated tsunamis and the geomorphic signatures of landslide tsunamis or their potential for preservation are largely uncharacterized. Additionally, clear modifications described during previous post-tsunami surveys are often ephemeral and unlikely to be preserved. Documented geomorphic modifications of several low gradient fan deltas within Taan Fiord make it an excellent laboratory for characterizing signatures of a landslide tsunami event. Geomorphic changes to fan deltas in Taan Fiord caused by the landslide-generated tsunami included complete vegetation loss over more than 0.6 km2 of fan surfaces, formation of steep fan front scarps up to 10 m high, extensive local alterations of fan topography, and formation of new tsunami return-flow channels. Two relatively stable fan deltas in Taan Fiord were heavily vegetated prior to the Taan event and may preserve features of tsunami modification for decades to centuries. If this is the case, fan deltas may be a previously unrecognized location for preservation of tsunami signatures in the recent past. Fans in poorly monitored regions, such as Greenland, could thus hold evidence of previously unidentified recent landslide tsunami events

The 2015 landslide and tsunami in Taan Fiord, Alaska

Abstract Glacial retreat in recent decades has exposed unstable slopes and allowed deep water to extend beneath some of those slopes. Slope failure at the terminus of Tyndall Glacier on 17 October 2015 sent 180 million tons of rock into Taan Fiord, Alaska. The resulting tsunami reached elevations as high as 193 m, one of the highest tsunami runups ever documented worldwide. Precursory deformation began decades before failure, and the event left a distinct sedimentary record, showing that geologic evidence can help understand past occurrences of similar events, and might provide forewarning. The event was detected within hours through automated seismological techniques, which also estimated the mass and direction of the slide - all of which were later confirmed by remote sensing. Our field observations provide a benchmark for modeling landslide and tsunami hazards. Inverse and forward modeling can provide the framework of a detailed understanding of the geologic and hazards implications of similar events. Our results call attention to an indirect effect of climate change that is increasing the frequency and magnitude of natural hazards near glaciated mountains