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Basin-wide erosion rates can be determined through the analysis of in situ-produced cosmogenic nuclides based on the idea that samples taken at the outlet of a catchment are representative of the entire upstream basin. However, this is only true when a certain number of assumptions are verified. Among the most important assumptions is that denudation rates are uniform through space and time; i.e. the catchment is in cosmogenic steady-state. If this is the case, then the in-going nuclide flux through production is equal to the out-going flux through decay and erosion, and the system is in isotopic equilibrium. When this condition is not matched, the calculation of basin-wide erosion rates through cosmogenic nuclides is subject to the well-mixing conditions of the alluvial sediment in the rivers. Here, we bring a new contribution towards the understanding of sediment mixing effects on in-situ cosmogenic nuclide concentrations in alluvial sediment. We proceed by following the methodology proposed by Binnie et al. (2006), and present the results from a sediment mixing model for a small ( 42 km2) catchment in the Alps. The model results show the importance of the sampling location and of the catchment’s size to have reliable erosion rates. However, the models also highlight the importance of knowing the sediment transfer mechanisms and the processes magnitude. When the transport of sediment mainly occurs through soil creep and shallow landslides, high concentration material is supplied into the fluvial system, increasing the 10Be concentration in the alluvial sediment. Likewise, the high-frequency of mass-wasting processes or the occurrence of sporadic but large-magnitude events, allow the supply of low-concentration sediment that result in a lower cosmogenic nuclide signal in the channels. A strong bias arises for the calculation of basin-wide erosion rates if mass-wasting processes dominate the mechanisms of sediment transfer in the catchment. In this situation the use of cosmogenic nuclides signal results in an over- or underestimation of the erosion rates. Therefore, our results suggest that when using cosmogenic nuclides, the achievement of mixing conditions in alluvial sediment is of primary importance for the correct estimation of the erosion rates and that this latter has to be accurately interpreted when the mixing conditions are unknown or not achieved.

Landscape evolution in mountainous catchments is the result of multiple factors acting at different scales in time and space. The main components affecting basin morphologies are: uplift, glacial cycles, erosion (long timescale), mass movements and cycles of freeze-thaw (short timescale). In this study, we quantify the rates of sediment production and transfer from an alpine catchment in which debris flows are a dominant geomorphic agent. We use cosmogenic nuclides to determine the connectivity between source areas and sinks and to estimate the timescale of sediment production and the fluxes of sediment through the basin. The Zielbach basin consists of a ca. 30 km2-large central basin where cascade and step-pool channels have eroded into the highly fractured and foliated metamorphic bedrock, and a ca. 10 km2-large eastern tributary basin where a network of debris flow channels are perched on a deep-seated sackung. Whereas the central basin shows a poor connectivity between hillslopes and the channel network, the debris flow channels of the eastern tributary basin are closely connected with the bordering hillslopes. In the central basin, the geometry of the channel network is strongly affected by the litho-tectonic fabric as channels parallel the major faults and foliations. In the headwaters, rock glaciers and rock fall deposits tens of meters thick form transverse topographic ridges which retain the sediment in a semi-closed sedimentary traps. Moreover field work and GIS analysis reveal a strong relationship between structural setting and sediments production in which sediment production is high in weakened shear and fault zones. The eastern tributary basin has highly fractured bedrock, partially related to ice retreat after the LGM age. The whole catchment is characterize by a high production of sediments related both to tectonic structure present in the area and to the steep slopes left by the glacial retreat. The geomorphology shows a clear difference between the main basin and the debris flow tributary: the first one is composed by several lateral valley in which sediment can be stored before reaching the channel network, while the tributary is characterize by a long steep slope where the sediments produced can be easily moved down to the fan. 10Be based denudation rates confirm these differences: rates are nearly constant at between 0.3-0.6 mm/yr in the decoupled hillslope-channel segments. This central tributary system of the Zielbach basin can thus be understood as a supply-limited basin with a poor connectivity between sediment sources and drainage network. In contrast, basin averaged denudation rates are high and locally exceed 2.7 mm/yr in the eastern debris flow basin revealing a high connectivity between hillslopes and channel network. Mass balance calculations reveal that individual debris flows contribute up to 90% of the sediment budget of the entire Zielbach basin. These results reveal the importance of stochastic sediment transfer from a high-connectivity tributary on the sediment budget of an entire drainage basin.

Because of competing forces and variations through time the relative importance of geomorphic processes responsible for the long term topographic evolution of a mountain range is not always obvious. Here we perform a space for time substitution with the western escarpment of the Andes between 10 and 20°S to identify the mechanisms of plateau destruction over geological timescales. We use this setting to propose that variations in the precipitation rate play a primary role in setting hillslope relief in uplifted mountainous landscapes. We find that in dry climates local topographic relief grows with increasing precipitation independent of the underlying lithology and given an overall uniform rock uplift history. We proceed by differentiating Andean landscapes with generally low precipitation rates (80–500 mm a- 1 Peruvian Andes 10–20°S) where local relief correlates positively with precipitation from those with higher precipitation rates (400–1400 mm a- 1 Chilean Andes 35–40°S) where increases in precipitation lead to topographic decay. We suggest that these trends result from dominant bottom up control (channel incision is faster than hillslope response) giving way to an increasing top down control (hillslope lowering is faster than channel incision). With low precipitation relief growth is controlled by stream incision and knickzone retreat into a largely undissected plateau. With higher precipitation rates relief is set by the steepness of graded streams and the rates of sediment production and transport on hillslopes. Trends of topography can also be interpreted in temporal terms in which the higher precipitation results in shorter response times such that the Peruvian Andes between 10 and 20°S are still responding to Miocene uplift while in the Chilean Andes between 35 and 40°S these knickzones have already propagated through the entire fluvial network. We anticipate that such changes also operate during the formation and destruction of other mountainous plateau landscapes.

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. Copyright © 2012 John Wiley Sons Ltd.

This paper explores the mechanisms by which inner gorges in the Alps were formed. It focuses on the ca. 1.5 km long 80 m deep and a few hundred meter wide Lammschlucht located in the northern foothills of the central Alps. We reconstructed the glacial cover using lateral moraines and hanging talus cones that record the elevation of the ice surface at the deglaciation stage of the LGM (Last Glacial Maximum). We used the reconstructed ice thickness patterns to calculate the erosional potential of the subglacial meltwater. The applied model is based on the principle of energy conservation and yields the pattern of downstream changes of the dynamic pressure which is considered a measure for erosion potential. The model results suggest a maximum of the dynamic pressure at the end of the inner gorge. We interpret therefore that the subglacial meltwater scoured the reach toward the end of the Lammschlucht because of the enhanced dynamic pressure which was ultimately controlled by the ice overburden. Post glacial fluvial erosion then resulted in a readjustment through a regressive shift of the erosional front along the inner gorge farther upstream. The current location of this front lies almost in the middle of the Lammschlucht inner gorge where a step pool channel changes into a straight plane bed channel flowing on a deeply scoured bedrock.

The interior of Hellas Basin displays a complex landscape and a variety of geomorphological domains. One of these domains, the enigmatic banded terrain covers much of the northwestern part of the basin. We use high-resolution (Context Camera and High-Resolution Imaging Science Experiment) Digital Terrain Models to show that most of the complex viscous flowing behavior exhibited by the banded terrain is controlled by topography and flow-like interactions between neighboring banded terrain. Furthermore, the interior of the basin hosts several landforms suggestive of the presence of near-surface ice, which include polygonal patterns with elongated pits, scalloped depressions, isolated mounds, and collapse structures. We suggest that thermal contraction cracking and sublimation of near-surface ice are responsible for the formation and the development of most of the ice-related landforms documented in Hellas. The relatively pristine form, lack of superposed craters, and strong association with the banded terrain, suggest an Amazonian (

High resolution 32–20 Ma old stratigraphic records from the Molasse foreland basin situated north of the Alps and Gonfolite Lombarda conglomerates deposited on the southern Alpine margin document two consecutive sedimentary responses an immediate and delayed response to slab breakoff beneath the central Alps c. 32–30 Ma ago. The first signal which occurred due to rebound and surface uplift in the Alps was a regional and simultaneous switch from basin underfill to overfill at 30 Ma paired with shifts to coarse grained depositional environments in the foreland basin. The second signal however arrived several million years after slab breakoff and was marked by larger contributions of crystalline clasts in the conglomerates larger clast sizes larger sediment fluxes and shifts to more proximal facies. We propose that this secondary pulse reflects a delayed whiplash type erosional response to surface uplift where erosion and sediment flux became amplified through positive feedbacks once larger erosional thresholds of crystalline bedrock were exceeded.

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U-Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6-5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis.

A steep escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). Strong north–south and east–west precipitation gradients are exploited in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. Late Miocene tectonics uplifted the Meseta/Altiplano plateau (~3000 m a.s.l.), which is underlain by a series of Tertiary volcanic-volcanoclastic rocks. Streams on this plateau remain graded to the Late Miocene base level. Below the rim of the Meseta, streams have responded to this ramp uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean.\ud \ud It is found that the Tertiary units on the plateau function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky−1 on the Meseta/Altiplano, irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky−1 to 250 mm ky−1 and correlate positively with precipitation rates, but show no strong correlation with hillslope angles or channel steepness. These relationships are explained by the presence of a cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions.\ud \ud Since escarpment retreat and the precipitation pattern were established at least in the Miocene, it is speculated that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated hea

The evolution of mountain belts is a balance between forces that build topography and those that destroy topography. This study focuses on the destructive forces, weathering and mass wasting, fluvial or eolian erosion, glaciers, etc., that are inherently surfacial processes. In order to understand how orogens change through time, we need to identify and quantify the processes responsible for denudation. We apply a combination of novel and proven techniques to address the question of how these forces modify the landsurface. Morphometric analysis based on high resolution LIDAR data is used to identify transient landscapes. These geomorphic data are then combined with cosmogenic nuclides, 10Be from quartz in sediment specifically, in order to quantify process rates. In a case study in the Alpine Foreland, we identified two adjacent drainage basins which have responded differently to the same forcing event, a fall in stream base-level at 16,000±3,000 yrs BP. One of the watersheds has a coupled hillslope-channel system, while the other is decoupled. Denudation rates in the coupled system, 380±50 mm ky-1, are equal to those in the headwaters of the decoupled system, 380±50 mm ky-1. However denudation rates in the decoupled system increase downstream to 540±100 mm ky-1, with denudation in the incised region occurring at extremely high rates, ~1800 mm ky-1. The denudation rates show that within 16,000 yrs, only the channels themselves have responded, and spatially extensive erosion occurs only after a significantly longer lag time. The advantage of this approach is that the geomorphic response of a landscape to external forces can be identified by morphometric analysis, and the process rates that are measured with cosmogenic nuclides can be used to quantify landscape response times on the time scale of climate change or fault movement.

Knickpoints are a reliable indicator of stream channel incision. How these features impact a drainage basin as a whole as they migrate upstream however is less clear. To address this question, we have studied drainage basins in the Swiss Mitteland that have differing histories with respect to knickpoints and knickpoint migration. The Grosse Fontanne is a bedrock stream that drains the Molasse conglomerates of the Napf region. The system has been described as an actively incising rejuvenated landscape. Incised valleys and knickpoints are present on the trunk stream and many tributaries. Seven basinaveraged erosion rates varying from 330 to 640 mm/ky, derived from cosmogenic nuclides, were calculated for the watershed. The highest rates were recorded for the trunk stream near the confluence with the Kleine Emme River, while the lowest rates are found in small headwater basins. Of the samples, three were collected upstream of visible knickpoints, and two were collected downstream of visible knickpoints. The two remaining samples were collected on tributaries that had no visible knickpoints. The upstream erosion rates are lower, 350 mm/ky vs. 560 mm/ky, than the downstream erosion rates. The watersheds with no visible knickpoints are similar to the downstream samples, eroding at 500 mm/ky, suggesting that the knickpoint has already propagated through these catchments. The basins with active or presumed relict knickpoints also tend to have higher average relief than the basins which the knickpoints have not yet reached. The timing of initial formation, based on the volume of incision and a calculated erosion rate for the areas experiencing rapid downcutting, appears to be post-glacial. The cause of the knickpoint formation appears to be stream piracy by the Kleine Emme.