Characterisation and Analysis of Catastrophic Landslides and Related Processes using Digital Topographic Data

This thesis represents a large body of work that seeks to describe, quantify, and simulate the behaviour of large rock slope failures (> 1 Mm³), in the form of landslides and rock avalanches, and their secondary processes, such as landslide-dammed lakes, utilizing remotely sensed data. Remotely sensed data includes aerial photography, high resolution satellite imagery from various platforms (e.g. LANDSAT, ASTER, EO-1, SPOT), and digital topographic elevation models of the Earth’s surface (e.g. SRTM-3, ASTER GDEM2, LiDAR). This thesis focused on regions in northwest North America (British Columbia, Yukon Territory, and Alaska), and on regions in the Himalaya and Pamirs Mountain chains (Tajikistan, Afghanistan, Pakistan, Tibet, and India). These study regions are each highly dynamic landscapes, where the occurrence of rock slope failures per area is higher than non-mountainous regions, and these events are aiding to the shape and profile of the landscapes and surfaces found today. This thesis focuses on: 1) the ability to accurately calculate geometrics (e.g. areas, volumes, runouts, debris depths) for large scale landslides and their associated landslide dammed lakes (e.g. areas, volumes, outbursts), utilizing data from remotely sensed sources; 2) the attempt to successfully simulate the observed dynamics for both landslide emplacement and their resulting debris deposits (DAN-W, DAN3D), and possible outburst flood scenarios (FLO2D); and, 3) attempt to quantify the kinetic and specific energy involved in rock avalanches, and how these energetics relate to fragmentation, as well as the lateral spreading and thinning of debris sheets. The river valleys of the northwest Himalayas (Pakistan and India) and the adjacent Pamirs Mountains of Afghanistan and Tajikistan contain in excess of two hundred known rockslide deposits of unknown age that have interrupted surface drainage and previously dammed major rivers in the region in recent and prehistoric time. Some prehistoric rockslide dams in the northwest Himalayas have impounded massive lakes with volumes in excess of 20 Gm³. The region contains: 1) the highest rockslide dam in the world (the 1911 Usoi rockslide, Tajikistan), which impounds the current largest rockslide-dammed lake (Lake Sarez) on Earth (est. volume 17 Gm³); 2) the largest documented outburst flood (6.5 Gm³) associated with a historical rockslide dam outburst (the 1841 Indus Flood, Pakistan); and, 3) the world’s most recent rockslide-dammed lake emergency, the 2010 Attabad rockslide dam on the Hunza River, in the Upper Indus basin, including the newly created Lake Gojal. By accurately quantifying the volume of an impoundment, and the downstream valley topography (DEM), floodwave scenarios can be created for various breaching situations, allowing for the delineation of downstream inundation areas, or the creation of hazard and risk scenarios. Two methods are used to attempt to quantify the volumes of landslide-dammed lakes: 1) a contour interpolation method, focusing on the creation of contours to represent lake levels in the DEM data; and, 2) a new technique using digitized shorelines and statistical methods to obtain lake elevations on specific dates. A new technique has also been developed to quantify the larger block fragmentation from rock avalanches in the glacial environment, and a credible grain-size curve for the largest blocks can be obtained, aiding in the creation of a more complete grain-size curve for a particular event. The combination of landslides and their associated landslide dammed lakes are an important geomorphic process to study, as these events have a direct relationship to the hazard and risk faced by local communities living and working in these regions. By understanding the emplacement and deposit dynamics of large landslides and/or the outburst flood scenarios from naturally impounded reservoirs, we can attempt to reduce the direct impacts these events have to local communities.4 month

Lost landslides: Rock-avalanche occurrence and fluvial censoring processes on South Island, New Zealand

Rock-avalanches (RAs) are a large (typically >10⁶ m3) and extremely rapid (30 - >100 m/s) type of landslide. RAs pose a significant hazard as they can runout over long distances and generate secondary hazards such as tsunami and unstable, cross-valley dams. Previous research on the distribution of rock-avalanche deposits (RADs) on the South Island, New Zealand has suggested that there are fewer deposits than would be expected for a seismically active, high-mountain region. This is due to their removal from the sedimentary record (censoring) by fluvial erosion, glacial entrainment, vegetation cover, sub-aqueous occlusion and deposit misidentification. Censoring of deposits skews magnitude-frequency relationships of RA occurrence and hinders hazard planning. This research examines processes acting to fluvially censor RADs on the South Island. 268 known, and 47 possible RADs were identified to provide the first RAD inventory for the entire South Island. The temporal distribution of RADs indicates censoring of the record over the Holocene. >500 year intervals exist between RA events from 12,000 to 2,000 years ago; a more complete record is shown for the last 1,000 to 100 years with intervals of >50 - <150 years. The last 100 years shows phases of co-seismic RAD generation, a period of RAD quiescence and a recent increase in aseismic RAD occurrence. The spatial distribution of RADs suggests that the West Coast, Fiordland and Nelson could have experienced fluvial censoring of deposits. The sediment routing characteristics of catchments in these regions, where the majority of rivers have direct pathways from RADs to the ocean, suggest that fluvially reworked RAD material could be stored within alluvial flats and braidplains. Agglomerate grains (microscopic grains which are diagnostic of RAs) were used to identify fluvially reworked RAD material. Grains were detected in dam-breach flood terraces up to 1km downstream of known RADs. Contemporary river sediment samples showed no agglomerate presence, this suggests that 1) agglomerates break down under extended fluvial transport, 2) they are not supplied to river systems outside of flood events, 3) agglomerates become diluted by other river sediment or 4) they become buried in discrete sedimentary layers. In order to investigate the redistribution of coarse RAD material within South Island rivers, a micro-scale flume model was developed. Using ultra-violet sand as a novel analogue for a RAD, the redistribution of material through an idealised South Island catchment could be examined. The model showed that RAD material is deposited in discrete aggradational layers in dam proximal locations. Downstream, the sedimentary signal is rapidly diluted by ordinary river sediment flux. The research shows that the RAD record for the South Island is incomplete and that fluvial censoring is prevalent within the West Coast, Nelson and Fiordland. The agglomerate tracing method can be used to identify the presence of RADs in fluvial systems proximal to RADs but the signal is undetectable after ~1km from the deposit. Both field sampling and flume modelling show that localised flood derived aggradational layers, close to deposit locations, will archive reworked RAD material. These results have important implications for understanding the magnitude and frequency of RADs within New Zealand and other similar high-mountain, tectonically active regions of the globe

Cenozoic evolution of the Yakutat-North American collision zone, southeast Alaska

A better understanding of orogenic syntaxes is necessary in order to improve our knowledge of continental deformation, which impacts human life immensely. Orogenic syntaxes are kinematic transition zones and potentially concentrate large amounts of stress and strain, which can lead to frequent and high-magnitude earthquakes and mass wasting processes. The St. Elias syntaxis in the St. Elias Mountains of southeast Alaska and adjacent western Canada recently gained attention for its high exhumation and erosion rates, and by comparison to the “tectonic aneurysm” model that was developed for the eastern and western Himalayan syntaxes. The St. Elias syntaxis is the area where transform motion along the Fairweather Fault segment of the Yakutat-North American plate boundary transitions into flat-slab subduction of the thick, oceanic Yakutat crust. Detailed studies of exhumation processes at the St. Elias syntaxis are hampered by its extensive glaciation. The approach in this study therefore comprises the use of detrital material from large and small glacio-fluvial catchments and the application of multiple thermo- and geochronometric dating techniques in order to reveal the long-term exhumation history of the syntaxial region and its relation to other parts of the orogen. Cooling age populations were extracted from sand-sized samples and complete cooling histories (500–65 °C) and provenance information (U-Pb dates, lithology) were ob-tained from glacially transported cobbles and interpreted in combination with previ-ously published and new bedrock thermo- and geochronologic ages. A total of 4905 new single-grain zircon fission-track (ZFT) ages (~430–0.2 Ma) of modern, sand-sized detrital samples from 47 different catchments covering an area of almost 45,000 km2, 1350 new single-grain apatite fission-track (AFT) ages (~433–1 Ma) of modern, sand-sized detrital samples from 15 of the 47 catchments, five ZFT bedrock ages (~154–9.4 Ma), three bedrock biotite 40Ar/39Ar ages (~42–5 Ma), as well as data of 27 cobble-sized detrital samples with 21 zircon U-Pb ages (~277–31 Ma), eight amphibole 40Ar/39Ar ages (~276–16 Ma), seven biotite 40Ar/39Ar ages (~50–42 Ma), four zircon (U-Th)/He ages (~35–4.8 Ma), four AFT ages (~17–1.6 Ma), and six apatite (U-Th)/He ages (~4.2–0.6 Ma) are presented. Two large-scale terrane subduction and accretion phases influenced the upper crustal cooling of the study area; the Jurassic–Cretaceous accretion of the Wrangellia Composite Terrane to the former North American margin and the ongoing flat-slab subduction and collision of the Yakutat microplate. The Fairweather plate boundary segment has been transpressional in nature since at least 30 Ma and collision of the Yakutat microplate with the North American Plate began ~15–12 Ma. Rapid exhumation in the St. Elias syntaxis area began ~10 Ma and was confined by an unmapped, ice covered, discrete structure northeast of the northern Fairweather Fault and possibly the Fairweather Fault itself, most likely forming a one-sided, positive flower structure. The locus of rapid exhumation shifted southwest into the central syntaxis area at ~5 Ma and exhumation rate and depth were increased, causing ~10 km of exhumation ~5–2 Ma. This occurred probably due to a combination of i) an increase in the compressional component of Yakutat-North American convergence, ii) the subduction of increasingly thicker oceanic crust of the wedge-shaped Yakutat microplate, and iii) a change in erosional patterns and rates due to 6–5 Ma onset of glaciation. Pliocene exhumation might have been accommodated by a two-sided, positive flower structure centered at the northern Fairweather Fault, but with deep exhumation focused on the North American Plate. After ~2 Ma, the focus of most rapid exhumation migrated farther south to the lower plate of the syntaxial region (Yakutat microplate). Overall, the results indicate that syntaxial regions should be treated as 4D-problems with spatio-temporally heterogeneously distributed deformation and exhumation. If process rates are high, as in the case of the St. Elias and Himalayan syntaxes, then, the dynamics of these regions are likely to respond very quickly (0.5–1.0 Myr) to changes in tectonic, rheologic, and climatic settings.Kontinentale Deformation beeinflusst die Gestalt der Erdoberfläche und hat dadurch einen großen Einfluss auf das Leben auf der Erde. Die Untersuchung von Gebirgssyntaxen, den Bereichen starker Krümmung in sonst geradlinig verlaufenden Gebirgszügen, ist ein wichtiger Schritt zu einem besseren Verständnis von Deformationsprozessen. Einige Syntaxen sind durch extreme Verformung gekennzeichnet, was zu starken Erdbeben und anderen Massenbewegungen, wie Hangrutschungen, führen kann. Die St. Elias Syntaxis im gleichnamigen Gebirge in Südostalaska und angrenzenden Gebieten Westkanadas weist sehr hohe Erosions- und Gesteinsexhumierungsraten auf. Vergleichbar hohe Werte sind aus der östlichen und westlichen Syntaxis des Himalayas bekannt und wurden dort unter anderem mit dem Modell des „tectonic aneurysm“ erklärt. Die St. Elias Syntaxis stellt einen kinematischen Übergangsbereich zwischen zwei Segmenten der Plattengrenze zwischen der Yakutat Mikroplatte und der Nordamerikanischen Platte dar. Dextrale Seitenverschiebung entlang der Fairweather-Störung geht innerhalb der St. Elias Syntaxis in flache Subduktion der ozeanischen Yakutat-Kruste über. Das gesamte St. Elias Gebirge ist vergletschert, was eine detaillierte Untersuchung von Exhumierungsprozessen erheblich erschwert. Diese Studie zeigt, dass dieses Problem durch Untersuchung von Detritus, der aktiven, fluvioglazialen Systemen entnommen wurde und daher hauptsächlich von Gletschern erodiertem Material entspricht, umgangen werden kann. Mithilfe verschiedener thermo- und geochronologischer Datierungsmethoden kann die Exhumierungsgeschichte der Syntaxis über einen Temperaturbereich von 500–65 °C sowie die Provenanz (U-Pb Datierung, Lithologie) von Sand- und Geröllproben rekonstruiert und zusammen mit publizierten thermochronologischen Daten von Sandproben und Festgesteinen interpretiert werden. Insgesamt werden in dieser Studie 4905 neue Zirkonspaltspur-Einzelkornalter (~430–0,2 Ma) von 47 Sandproben, die 47 verschiedenen Gletschereinzugsgebieten mit einer Gesamtfläche von fast 45000 km2 entsprechen, und 1350 neue Apaptitspaltspur-Einzelkornalter (~433–1 Ma) von Sandproben aus 15 der 47 Einzugsgebiete präsentiert. Des Weiteren wurden 27 Geröllproben anhand von 21 Zirkon U-Pb Analysen (~277–31 Ma), acht Amphibol 40Ar/39Ar Analysen (~276–16 Ma), sieben Biotit 40Ar/39Ar Analysen (~50–42 Ma), vier Zirkon (U-Th)/He Analysen (~35–4,8 Ma), vier Apatitspaltspur-Analysen (~17–1.6 Ma), sowie sechs Apatit (U-Th)/He Analysen (~4.2–0,6 Ma) datiert. An neun Festgesteinsproben wurden fünf Zirkonspaltspurdatierungen (~154–9,4 Ma) und vier Biotit 40Ar/39Ar Datierungen (~42–5 Ma) vorgenommen. Die Daten zeigen zwei Phasen von Subduktion und Akkretion von Terranen, die für das Abkühlen der oberen Kruste im Arbeitsgebiet verantwortlich waren; die jurassisch–kretazische Akkretion des Wrangellia Composite Terrane an den damaligen Rand der Nordamerikanischen Platte sowie die noch andauernde Subduktion und Kollision der Yakutat Mikroplatte. Das Fairweather-Segment der Plattengrenze ist seit etwa 30 Mio. Jahren von Transpression gekennzeichnet; die Kollision der Yakutat Mikroplatte mit der Nordamerikanischen Platte begann vor 15–12 Mio. Jahren. Die schnelle Gesteinsexhumierung in der St. Elias Syntaxis setzte vor etwa 10 Mio. Jahren ein und war von einer nicht kartierten, heute mit Eis bedeckten Störung nordöstlich der Fairweather-Störung und möglicherweise der Fairweather-Störung selbst begrenzt, was sich strukturgeologisch wahrscheinlich durch eine einseitige, positive „flower structure“ geäußert hat. Der Fokus der schnellen Exhumierung hat sich vor etwa 5 Mio. Jahren nach Südwesten verschoben, begleitet von einer Erhöhung der Exhumierungsrate und –tiefe. So wurden zwischen 5 Ma und 2 Ma ungefähr 10 km an Gestein exhumiert. Ausgelöst wurde diese konzentrierte Deformation vermutlich durch eine Kombination von drei, interagierenden Faktoren: i) der Erhöhung der Kompressionskomponente der Plattenkonvergenz, ii) der Subduktion von zunehmend mächtigerer, ozeanischer Kruste der keilförmigen Yakutat Mikroplatte, sowie iii) veränderter Erosionsmuster und -raten durch die vor 6–5 Mio. Jahren einsetzende Vergletscherung des Gebirges. Die schnelle, pliozäne Exhumierung, die sich um die Fairweather-Störung konzentrierte, wurde strukturell von einer zweiseitigen „flower structure“ getragen, wobei die tiefe Exhumierung auf die Nordamerikanische Platte beschränkt war. Vor etwa 2 Mio. Jahren wanderte der Fokus der stärksten Deformation weiter nach Süden auf die subduzierende Yakutat Mikroplatte. Zusammenfassend veranschaulichen die Ergebnisse, dass Syntaxen als vierdimensionales Problem behandelt werden müssen, da Deformation und Exhumierung zeitlich und räumlich variabel sind. In Syntaxen sind hohe Prozessraten, wie im Fall der St. Elias Syntaxis und der Syntaxen des Himalayas, möglich. Dadurch passen sich geodynamische Prozesse schnell (innerhalb von 0,5 bis 1 Mio. Jahren) an tektonische, rheologische und klimatische Änderungen an