[1] Bedrock stresses induced through exhumation and tectonic processes play a key role in the topographic evolution of alpine valleys. Using a finite difference model combining the effects of tectonics, erosion, and long-term bedrock strength, we assess the development of near-surface in situ stresses and predict bedrock behavior in response to glacial erosion in an Alpine Valley (the Matter Valley, southern Switzerland). Initial stresses are derived from the regional tectonic history, which is characterized by ongoing transtensional or extensional strain throughout exhumation of the brittle crust. We find that bedrock stresses beneath glacial ice in an initial V-shaped topography are sufficient to induce localized extensional fracturing in a zone extending laterally 600 m from the valley axis. The limit of this zone is reflected in the landscape today by a valley "shoulder," separating linear upper mountain slopes from the deep U-shaped inner valley. We propose that this extensional fracture development enhanced glacial quarrying between the valley shoulder and axis and identify a positive feedback where enhanced quarrying promoted valley incision, which in turn increased in situ stress concentrations near the valley floor, assisting erosion and further driving rapid U-shaped valley development. During interglacial periods, these stresses were relieved through brittle strain or topographic modification, and without significant erosion to reach more highly stressed bedrock, subsequent glaciation caused a reduction in differential stress and suppressed extensional fracturing. A combination of stress relief during interglacial periods, and increased ice accumulation rates in highly incised valleys, will reduce the likelihood of repeat enhanced erosion events.

Crustal stresses beneath evolving alpine landscapes result from a combination of tectonic strain, bedrock exhumation, and topography. The stress field is regulated by elastic material properties and the brittle strength of critically stressed elements, particularly those within preexisting faults and intact rock. Combining Byerlee's law for crustal stresses with a recently developed trilinear fracture envelope, we propose an extension of the critically stressed crust concept to constrain in situ stresses through microcrack generation and extensional fracture propagation. A compilation of 814 global in situ stress measurements suggests microcrack development will limit long‐term rock strength, and maximum differential stresses in the upper ~1 km of the crust can be controlled by cohesive bedrock behavior. Using an elastoplastic, 2‐D finite‐difference model, we approximate in situ stress development within landscapes undergoing high rates of exhumation in both normal and reverse tectonic regimes. Critical near‐surface stresses in these environments are defined by the microcrack initiation threshold, estimated to be roughly one third of the unconfined compressive strength of intact rock, while stresses deeper in the crust adhere to Byerlee's law. Our models indicate that exhumation‐induced stresses limited by long‐term rock strength are the primary contributor to the near‐surface stress field, while topographic relief reduces stress near valley axes. Simulating glacial erosion then allows us to illustrate a path‐dependent relationship between critical stress development, fracture formation, and geomorphic processes. We find that glacial unloading can generate new microcracks in near‐surface bedrock, resulting in unstable macroscopic extensional (or “exfoliation”) fracture propagation during incision of U‐shaped alpine valleys.

Our concept of progressive rock slope failures is on the one hand embedded in aggregated subcritical crack growth mechanisms and on the other sensitive to environmental conditions, especially water. To anticipate failure dynamics in rock slopes, it is a key requirement to reveal the influence of water on subcritical crack growth mechanisms and material properties. We present experimental data on the time-dependent deformation of an exemplary rock, Carrara marble. We employed inverted single-edge notch bending creep tests on large Carrara marble samples to mimic an open joint system with controlled water supply. Constant stress was applied in two steps approaching 22-85% of a previously determined critical baseline stress. Introducing calcite-saturated water to subcritical stressed samples caused an immediate increase in strain by up to an order of magnitude. Time-dependent accumulation of inelastic damage at the notch tip occurred in wet and dry samples at all load levels. Subcritical crack growth and the evolution of localized intergranular fractures are enhanced if water is present and readily approach tertiary creep when loaded above 80%. The immediate strain response is attributed to the reduction of surface energy and diffusion of the water into the rock. The resultant more compliant and weaker rheology can even turn the subcritical stress into a critical state. Over time, subcritical and chemically enhanced mechanisms progressively alter especially grain boundaries, which become the key controls of progressive failure in Carrara marble. 1986; Withers, 2015; Figure 1b). These intermittent stress-dependent dynamics lead to a local competition VOIGTLÄNDER ET AL. 3780

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To investigate the mechanism of frost wedging in fractured low‐porosity bedrock, we monitored the opening of an artificial 4 mm wide and 80 mm deep crack, cut 20 mm from the end of a rectangular granite block. Two freezing protocols were employed – top‐down and bottom‐up, the former consisting of short‐ and long‐term variants, lasting 1 and 53 days, respectively. Our results demonstrate that (i) in 1‐day experiments, maximum crack widening during top‐down freezing is around 0.11 mm, while bottom‐up freezing produces only 0.02 mm of deformation; (ii) neither ice nor water pressure causes measurable irreversible crack widening during 1‐day tests; (iii) irreversible crack widening is only observed following the 53‐day experiment under top‐down freezing. Based on these results, we suggest (i) freezing direction plays a key role in determining the magnitude of crack widening; and (ii) freezing duration could be essential for crack propagation. The fracturing is both time‐dependent and subcritical; thus, persistent freezing in winter could actually be the active period of crack propagation. This allows us to propose a simplified method to calculate ice pressure according to crack widening. Here we show how freezing direction and duration in ice‐filled cracks control the path‐dependent efficacy of frost‐wedging. Copyright © 2017 John Wiley & Sons, Ltd.

Rock avalanches are low frequency natural hazards that can alter landscape morphology, and constraining the timing, volume and emplacement dynamics of prehistoric rock avalanches is crucial for understanding the hazards posed by these events. Here we perform cosmogenic nuclide dating, topographic reconstruction and runout modeling of the Molveno rock avalanche, located north of Lake Garda in the Province of Trento, Italy. The unique morphology of the deposits, which features numerous large scarps and prominent lineaments, have previously led researchers to interpret the Molveno rock avalanche as being the result of multiple events. Our results show that the Molveno rock avalanche had a volume of approximately 600 Mm 3 , and failed from a prominent niche located on Monte Soran. 36 Cl cosmogenic nuclide dating results shows that the deposits were emplaced as a single event approximately 4.8 ± 0.5 ka, and suggests that the unique deposit morphology is due to the emplacement processes acting during and soon after failure. Numerical runout modeling shows that this morphology could have resulted from a combination of runup and extensional spreading of the debris along the complex valley floor topography. The ages we determined for this event are coincident with the nearby Marocca Principale rock avalanche (5.3 ± 0.9 ka), which may suggest a common trigger. Our results have important implications for interpreting the morphology of rock avalanche deposits, and contribute to the evolving understanding of rock avalanche processes in the Alps.

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