Geophysical Investigation of Subsurface Three-Dimensional Architecture of Icy Debris Fans With Ground Penetrating Radar in the Wrangell Mountains, Alaska

Icy debris fans have are newly-described landforms (Kochel and Trop, 2008 and 2012) as landforms developed immediately after deglaciation on Earth and similar features have been observed on Mars. Subsurface characteristics of Icy debris fans have not been previously investigated. Ground penetrating radar (GPR) was used to non-invasively investigate the subsurface characteristics of icy debris fans near McCarthy, Alaska, USA. The three fans investigated in Alaska are the East, West, and Middle fans (Kochel and Trop, 2008 and 2012) which below the Nabesna ice cap and on top of the McCarthy Creek Glacier. Icy debris fans in general are a largely unexplored suite of paraglacial landforms and processes in alpine regions. Recent field studies focused on direct observations and depositional processes. Their results showed that the fan's composition is primarily influenced by the type and frequency of depositional processes that supply the fan. Photographic studies show that the East Fan receives far more ice and snow avalanches whereas the Middle and West Fans receive fewer mass wasting events but more clastic debris is deposited on the Middle and West fan from rock falls and icy debris flows. GPR profiles and Wide-angle reflection and refraction (WARR) surveys consisting of both, common mid-point (CMP), and common shot-point (CSP) surveys investigated the subsurface geometry of the fans and the McCarthy Creek Glacier. All GPR surveys were collected in July of 2013 with 100MHz bi-static antennas. Four axial profiles and three cross-fan profiles were done on the West and Middle fans as well as the McCarthy Creek Glacier in order to investigate the relationship between the three features. GPR profiles yielded reflectors that were continuous for 10+ m and hyperbolic reflections in the subsurface. The depth to these reflections in the subsurface requires knowledge of the velocity of the subsurface. To find the velocity of the subsurface eight WARR surveys collected on the fans and on the McCarthy Creek glacier to provide information on variability of subsurface velocities. The profiles of the Middle and West fan have more reflections in their profiles compared to profiles done on the McCarthy Creek Glacier. Based on the WARR surveys, we interpret the lower energy return in the glacier to be caused by two reasons. 1) The increased attenuation due to wet ice versus drier ice and on the fan with GPR velocities >0.15m/ns. 2) Lack of interfaces in the glacier compared to those in the fans which are inferred to be produced by the alternating layers of stratified ice and lithic-rich layers. The GPR profiles on the West and Middle Fans show the shallow subsurface being dominated by lenticular reflections interpreted to be consistent with the shape of surficial deposits. The West Fan is distinguished from the Middle Fan by the nature of its reflections patterns and thicknesses of reflection packages that clearly shows the Middle fan with a greater thickness. The changes in subsurface reflections between the Middle and West Fans as well as the McCarthy Creek Glacier are thought to reflect the type and frequency of depositional processes and surrounding bedrock and talus slopes.

The influence of bedrock orientation on the landscape evolution, surface morphology and denudation (10Be) at the Niesen, Switzerland

Landscape evolution and surface morphology in mountainous settings are a function of the relative importance between sediment transport processes acting on hillslopes and in channels, modulated by climate variables. The Niesen nappe in the Swiss Penninic Prealps presents a unique setting in which opposite facing flanks host basins underlain by identical lithologies, but contrasting litho-tectonic architectures where lithologies either dip parallel to the topographic slope or in the opposite direction (i.e. dip slope and non-dip slope). The north-western facing Diemtigen flank represents such a dip slope situation and is characterized by a gentle topography, low hillslope gradients, poorly dissected channels, and it hosts large landslides. In contrast, the south-eastern facing Frutigen side can be described as non-dip slope flank with deeply incised bedrock channels, high mean hillslope gradients and high relief topography. Results from morphometric analysis reveal that noticeable differences in morphometric parameters can be related to the contrasts in the relative importance of the internal hillslope-channel system between both valley flanks. While the contrasting dip-orientations of the underlying flysch bedrock has promoted hillslope and channelized processes to contrasting extents and particularly the occurrence of large landslides on the dip slope flank, the flank averaged beryllium-10 (10Be)-derived denudation rates are very similar and range between 0.20 and 0.26 mm yr−1. In addition, our denudation rates offer no direct relationship to basin's slope, area, steepness or concavity index, but reveal a positive correlation to mean basin elevation that we interpret as having been controlled by climatically driven factors such as frost-induced processes and orographic precipitation. Our findings illustrate that while the landscape properties in this part of the northern Alpine border can mainly be related to the tectonic architecture of the underlying bedrock, the denudation rates have a strong orographic control through elevation dependent mean annual temperature and precipitation.

Geomorphic coupling between hillslopes and channels in the Swiss Alps

The coupling relationships between hillslope and channel network are fundamental for the understanding of mountainous landscapes' evolution. Here, we applied dendrogeomorphic methods to identify the hillslope–channel relationship and the sediment transfer dynamics within an alpine catchment, at the highest possible resolution. The Schimbrig catchment is located in the central Swiss Alps and can be divided into two distinct geomorphic sectors. To the east, the Schimbrig earth flow is the largest sediment source of the basin, while to the west, the Rossloch channel network is affected by numerous shallow landslides responsible for the supply of sediment from hillslopes to channels. To understand the connectivity between hillslopes and channels and between sources and sink, trees were sampled along the main Rossloch stream, on the Schimbrig earth flow and on the Rossloch depositional area. Geomorphic observations and dendrogeomophic results indicate different mechanisms of sediment production, transfer and deposition between upper and lower segments of the channel network. In the source areas (upper part of the Rossloch channel system), sediment is delivered to the channel network through slow movements of the ground, typical of earth flow, shallow landslides and soil creep. Contrariwise, in the depositional area (lower part of the channel network), the mechanisms of sediment transfer are mainly due to torrential activity, floods and debris flows. Tree analysis allowed the reconstruction of periods of high activity during the last century for the entire catchment. The collected dataset presents a very high temporal resolution but we encountered some limitations in establishing the source-to-sink connectivity at the catchment-wide scale. Despite these uncertainties, for decennial timescales the results suggest a direct coupling between hillslopes and neighbouring channels in the Rossloch channel network, and a de-coupling between sediment sources and sink farther downstream, with connections possible only during extraordinary events.

Sediment dynamics in the subaquatic channel of the Rhone delta (Lake Geneva, France/Switzerland)

bstract With its smaller size, well-known boundary conditions, and the availability of detailed bathymetric data, Lake Geneva’s subaquatic canyon in the Rhone Delta is an excellent analogue to understand sedimentary pro- cesses in deep-water submarine channels. A multidisciplinary research effort was undertaken to unravel the sediment dynamics in the active canyon. This approach included innovative coring using the Russian MIR sub- mersibles, in situ geotechnical tests, and geophysical, sedimentological, geochemical and radiometric analysis techniques. The canyon floor/levee complex is character- ized by a classic turbiditic system with frequent spillover events. Sedimentary evolution in the active canyon is controlled by a complex interplay between erosion and sedimentation processes. In situ profiling of sediment strength in the upper layer was tested using a dynamic penetrometer and suggests that erosion is the governing mechanism in the proximal canyon floor while sedimen- tation dominates in the levee structure. Sedimentation rates progressively decrease down-channel along the levee structure, with accumulation exceeding 2.6 cm/year in the proximal levee. A decrease in the frequency of turbidites upwards along the canyon wall suggests a progressive confinement of the flow through time. The multi-proxy methodology has also enabled a qualitative slope-stability assessment in the levee structure. The rapid sediment loading, slope undercutting and over-steepening, and increased pore pressure due to high methane concentrations hint at a potential instability of the proximal levees. Fur- thermore, discrete sandy intervals show very high methane concentrations and low shear strength and thus could cor- respond to potentially weak layers prone to scarp failures.

Landscape Evolution of the Dry Valleys, Transantarctic Mountains: Tectonic Implications

There are different views about the amount and timing of surface uplift in the Transantarctic Mountains and the geophysical mechanisms involved. Our new interpretation of the landscape evolution and tectonic history of the Dry Valleys area of the Transantarctic Mountains is based on geomorphic mapping of an area of 10,000 km(2). The landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution. Following a lowering of base level about 55 m.y. ago, there was a phase of rapid denudation associated with planation and escarpment retreat, probably under semiarid conditions. Eventually, downcutting by rivers, aided in places by glaciers, graded valleys to near present sea level. The main valleys were flooded by the sea in the Miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast. There has been remarkably little landform change under the stable, cold, polar conditions of the last 15 m.y. It is difficult to explain the Sirius Group deposits, which occur at high elevations in the area, if they are Pliocene in age. Overall, denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland, which is in good agreement with the results of existing apatite fission track analyses. It is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy. Most crustal uplift (2-4 km) is inferred to have occurred in the early Cenozoic with 400 m of subsidence in the Miocene followed by 300 m of uplift in the Pliocene.