Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.     

Locally triggered seismicity in the central Swiss Alps following the large rainfall event of August 2005

In 2005 August, an unusual series of 47 earthquakes occurred over a 12-hr period in central Switzerland. The earthquakes occurred at the end of 3-d period of intensive rainfall, with over 300 mm of precipitation. The highest seismicity occurred as two distinct clusters in the region of Muotatal and Riemenstalden, Switzerland, a well-known Karst area that received a particularly large amount of rainfall. The large increase in seismicity, compared to the background, and the short time delay between the onset of the intense rainfall and the seismicity strongly suggest that earthquakes were triggered by rainfall. In our preferred model, an increase in fluid pressure at the surface due to a large amount of rain leads to a local increase in pore fluid pressure at depth. The increase in pore fluid pressure will reduce the shear strength of a porous medium by counteracting normal stress and, at the end, provoke failure. The series of triggered earthquakes in central Switzerland occurred in regions that have been seismically active in the past, showing similar hypocentre locations and magnitudes. This suggests that these earthquakes occurred on existing faults that were critically stressed. We modelled the intense rainfall as a step increase in fluid pressure at the surface that migrates to greater depths following the solution of the one-dimensional diffusion equation in a homogeneous half space. This allowed us to estimate the hydraulic diffusivity by plotting triggered seismicity in a time–depth plot. We found values of hydraulic diffusivity in the range from 0.01 to 0.5 m² s⁻¹ for our study area. These values are in good agreement with previous studies on earthquakes that were triggered by fluids, supporting the idea that the observed earthquake series was triggered by the large amount of rainfall.     

Structural evolution of the contact between two Penninic nappes (Zermatt-Saas zone and Combin zone,Western Alps) and implications for the exhumation mechanism and palaeogeography

The boundary zone between two Penninic nappes, the eclogite-facies to ultrahigh-pressure Zermatt-Saas zone in the footwall and the blueschist-facies Combin zone in the hanging wall, has been interpreted previously as a major normal fault reflecting synorogenic crustal extension. Quartz textures of mylonites from this fault were measured using neutron diffraction. Together with structural field observations, the data allow a refined reconstruction of the kinematic evolution of the Pennine nappes. The main results are: (1) the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast-directed normal faulting; (2) exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening; (3) the Sesia-Dent Blanche nappe system originated from a continental fragment (Cervinia) in the Alpine Tethys ocean, and the Combin zone ophiolites from the ocean basin southeast of Cervinia; (4) out-of-sequence thrusting played a major role in the tectonic evolution of the Penninic nappes. An erratum to this article can be found at     

Rockfall hazard in the Daisekkei Valley, the northern Japanese Alps, on 11 August 2005

This paper describes a rockfall event in the Daisekkei Valley of Mount Shirouma-dake (2,932 m), the northern Japanese Alps. The rockfall occurred on a steep cliff comprising well-jointed felsites and produced debris of ≥8,000 m³. Most debris was deposited on an elongated snowpatch located immediately beneath the cliff, and it caused casualties among people who were trekking along a trail on the snowpatch. Additionally, a large rock block slipped 1 km on the snowpatch. The rockfall could have been due to the differential retreat of the rockwall, which contains areas of high- and low-density joints. Seasonal and diurnal freeze–thaw activities and snow avalanches and wash appear to be important factors responsible for the retreat. Although some rock blocks that can collapse further remain on the rockwall, the position of the mountain trail in the Daisekkei Valley is fixed. Fundamental reform of tourism systems for climbers, including education on natural hazards, is required.     

The relief of the Swiss Alps and adjacent areas and its relation to lithology and structure: topographic analysis from a 250-m DEM

In this paper we discuss the large-scale geomorphological characteristics of the Swiss Alps based on numerical analysis of a digital elevation model and compare these to an erodibility map constructed from a geotechnical map of Switzerland and regional geomorphological studies. Comparing the erodibility map with the large-scale morphometry shows an intimate relationship between mountain-scale erodibility and topography. On average, higher mean elevations and steeper mean slopes correlate with regions where rocks of low erodibility prevail. Areas with high peaks as well as the main water divides are controlled by the presence of bedrock with low to very low detachability. The drainage network of the Swiss Alps shows a close relationship to the lithological differences as well. Major longitudinal valleys follow easily erodible units. In the eastern and western part of the Swiss Alps, the highest values of local relief are located to the south of the main water divide, whereas in the central part, local relief is higher to the north of the main water divide. The large-scale geomorphic characteristics regarded in the framework of the geological history of uplift and denudation suggest that low and very low erodibilities lead to the development of areas of high elevations which are likely to persist over periods of 10–15 Ma. As the analysis of the Lepontine area shows, 20 Ma after cessation of exhumation, such high elevations are likely to be worn down and to manifest themselves as high relief only.     

Carbonate-hosted siliceous crust type mineralization of Carnic Alps (Italy-Austria)

A carbonate-hosted stratabound siliceous crust type (SCT) mineralization (base metal sulphides, barite, fluorite) occurs over large areas of Carnic Alps and Karawanken in the Eastern Alps. It concerns a pervasively silicified lithological unit, up to some tens of metres thick, which caps the unconformity landscapes developed on epicontinental Devonian–Dinantian carbonates. The SCT mineralization is directly overlaid by different transgressive siliciclastic sediments, which range from Lower Carboniferous to Lower Permian. The presence of fragments of the SCT mineralization in the transgressive siliciclastic sediment bounds its whole lithological evolution within a short stratigraphic interval of Lower Carboniferous age. Selected features of the regional and lithostratigraphic setting are discussed. The chemical characterisation is based on the statistical evaluation of compositional data of 581 selected samples. Three significant groups of elements have been distinguished: (1) the hydrological and metasomatically active elements (Si, Ba, F), which show a strong negative correlation amongst themselves and characterise the silica-saturated aqueous solutions; (2) the terrigenous elements (Al₂O₃, K₂O, Fe_tot, TiO₂, B, Be, Ce, La, Nb, V, Y, Zr), which suggest a continental margin environment for silica deposition; (3) the sulphide metals (Cu, Pb, Zn, Ni, Sb, As, Hg, Cd), which define the metalliferous signature of the SCT mineralization.Some consistent, but still debatable genetic aspects of the SCT mineralization are as follow: (1) silica may be supplied by illitization of clay-rich basinal sediments during their diagenesis. δ¹⁸O of microcrystalline quartz ranges from +18.5‰ and +24.6‰ and is very similar to δ¹⁸O of authigenic quartz deriving from diagenetic processes of illitization of clay-rich basinal sequences. (2) The diagenetic evolution of these sediments may trigger off the movement of silica-rich marine pore waters. δ³⁴S of barite ranges from +15.5‰ to +19.3‰ with an average of +17.7‰ and are in good agreement with δ³⁴S of sulphate in ocean waters of Upper Devonian–Lower Carboniferous age. (3) A convective hydrological system, connected with sinsedimentary transtensive tectonics, active in the Carnic Alps since the Frasnian, may be the transport mechanism of aqueous solutions. (4) A weak drop in pH in the dominant carbonate environment represents the conditions for silica precipitation.SCT mineralization, showing persistent, independent and distinct characters, occurs over large areas also in other sites of the Alpine belt and outside Italy and Austria. Therefore, it points to important markers for some sedimentary sequences as well as to a worldwide significant cyclic metallogenic event. It represents a new ore deposit-type within the carbonate-hosted mineralization.     

Emplacement at granulite facies conditions of the Sesia–Lanzo metagabbros: an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust (P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions (P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤P≤0.35 GPa and T<550 °C). This retrograde P–T evolution suggests that the exhumation occurred in a high thermal gradient regime, such as that induced by upwelling of an asthenospheric plume during continental rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian times. The gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the Lanzo Massif. This lithologic association and the metamorphic evolution is similar to that of the Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely different from that observed in passive margins, where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidence for asymmetric rifting.     

Alpine Evidence for Atmospheric Circulation Patterns in Europe during the Last Glacial Maximum

The configuration of Alpine accumulation areas during the last glacial maximum (LGM) has been reconstructed using glacial–geological mapping. The results indicate that the LGM ice surface consisted of at least three major ice domes, all located south of the principal weather divide of the Alps. This implies that the buildup of the main Alpine ice cover during oxygen isotope stage (OIS) 2 was related to precipitation by dominant southerly atmospheric circulation, in contrast to today's prevalent westerly airflow. Such a reorganization of the atmospheric circulation is consistent with a southward displacement of the Oceanic Polar Front in the North Atlantic and of the associated storm track to the south of the Alps. These results, combined with additional paleoclimate records from western and southern Europe, allow an interpretation of the asynchronous evolution of the different European ice caps during the last glaciation. δ¹⁸O stages (OIS) 4 and 3 were characterized by location of the Polar Front north of 46°N (Gulf of Biscay). This affected prevailing westerly circulation and, thus, ice buildup in western Scandinavia, the Pyrénées, Vosges, and northern Alps. At the LGM, however, the Polar Front lay at 44°N, causing dominating southerly circulation and reduced precipitation in central and northern Europe.     

阿尔卑斯山区的危险区区划

通过介绍荒溪治理林业工程体系的基本原理,论述了阿尔卑斯山区危险区区划的重要性、产生背景、现状、原理和方法,并对其区划存在的问题和局限性进行了系统分析,以期为我国开展山地自然灾害防治工作提供借鉴资料．     

Regionalization of rainfall thresholds: an aid to landslide hazard evaluation

 Rainfall, soil properties, and morphology are major factors controlling shallow landsliding. A series of meteorological events that triggered soil slips in northern Italy were studied to define rainfall thresholds and to evaluate a possible regionalization. Soil properties, triggering rainfall, and local lithological and morphometrical settings of different sites were used as input to an infiltration model. The approach allows the recognition of several triggering conditions in the Piedmont, Pre-Alpine and Alpine regions. This suggests the need for different rainfall thresholds with respect to those derived with other methods. Intensity versus rainfall duration relationships become particularly important when related to soil permeability and thickness, and demonstrate the role of antecedent precipitation. Events with exceptional water discharge from obstructed road culverts reveal the role played by anthropic structures in triggering such phenomena. Different approaches to slope stability analysis are shown, taking into account bedrock lithology, topography, seepage, and local saturation conditions. Received: 23 October 1997 · Accepted: 25 June 1997