Italian Southern Alps Research Articles

Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)

Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature (∼ 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at ∼ 250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures (∼ 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar– 40Ar ages of ∼ 30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.

Live-trapping success of the mountain hare ( Lepus timidus ) in the southern Italian Alps

Within a study on the mountain hare (Lepus timidus) in Alpine habitat, in March-May 2005 ten live-traps were set in an area included in the Stelvio National Park, while captures with nets were conducted in May 2005 in a surrounding area (Vezzola val- ley). Trapping success with live-traps was three hares in 210 trap-nights (1.43 captures/100 trap-nights), while two speciemens were captured with nets. Trapping success of our live trapping trial was compared with other studies on alpine mountain hare. All had very low trapping success. We give some indications for improving trapping success of mountain hares occurring at low density population and discuss costs and benefits of the capture by nets.

Main determinants of forest soil respiration along an elevation/temperature gradient in the Italian Alps

AbstractThe main determinants of soil respiration were investigated in 11 forest types distributed along an altitudinal and thermal gradient in the southern Italian Alps (altitudinal range 1520 m, range in mean annual temperature 7.8°C). Soil respiration, soil carbon content and principal stand characteristics were measured with standardized methods. Soil CO2fluxes were measured at each site every 15–20 days with a closed dynamic system (LI‐COR 6400) using soil collars from spring 2000 to spring 2002. At the same time, soil temperature at a depth of 10 cm and soil water content (m3 m−3) were measured at each collar. Soil samples were collected to a depth of 30 cm and stones, root content and bulk density were determined in order to obtain reliable estimates of carbon content per unit area (kg C m−2). Soil respiration and temperature data were fitted with a simple logistic model separately for each site, so that base respiration rates and mean annual soil respiration were estimated. Then the same regression model was applied to all sites simultaneously, with each model parameter being expressed as a linear function of site variables. The general model explained about 86% of the intersite variability of soil respiration. In particular, soil mean annual temperature explained the most of the variance of the model (0.41), followed by soil temperature interquartlile range (0.24), soil carbon content (0.16) and soil water content (0.05).

Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)

The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates ∼ 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons ( faults 1–2–3 ). Fault displacement is in the range of few centimeters ( faults 3 ) to a maximum of a few tens of meters in major faults 1 . Faults 1–2 nucleated on pre-existing joints, whereas faults 3 are newly generated fractures produced during slip along faults 1–2 . Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe 3+ compared to host rock. We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios.

Scenario modelling of basin-scale, shallow landslide sediment yield, Valsassina, Italian Southern Alps

Abstract. The SHETRAN model for determining the sediment yield arising from shallow landsliding at the scale of a river catchment was applied to the 180-km2 Valsassina basin in the Italian Southern Alps, with the aim of demonstrating that the model can simulate long term patterns of landsliding and the associated sediment yields and that it can be used to explore the sensitivity of the landslide sediment supply system to changes in catchment characteristics. The model was found to reproduce the observed spatial distribution of landslides from a 50-year record very well but probably with an overestimate of the annual rate of landsliding. Simulated sediment yields were within the range observed in a wider region of northern Italy. However, the results suggest that the supply of shallow landslide material to the channel network contributes relatively little to the overall long term sediment yield compared with other sources. The model was applied for scenarios of possible future climate (drier and warmer) and land use (fully forested hillslopes). For both scenarios, there is a modest reduction in shallow landslide occurrence and the overall sediment yield. This suggests that any current schemes for mitigating sediment yield impact in Valsassina remain valid. The application highlights the need for further research in eliminating the large number of unconditionally unsafe landslide sites typically predicted by the model and in avoiding large overestimates of landslide occurrence.

Problems of the “Cordevolian” Limestone and Dolomite” in the Slovenian part of the Southern Alps

Stratigraphic position and age of the so called Cordevolian limestone and dolomite in the Slovenian part of the Southern Alps is a long lasting problem. Its name originates from the (somewhere abandoned) Cordevolian substage (Lower Carnian). Limestone and dolomite both correspond to the Cassian dolomite from the Italian Southern Alps. Platform type of limestone and dolomite, which could be genetic and time equivalent with the Ladinian Schlem (Scilliar) dolomite, is unknown in Slovenia. In the NE part of the Kamnik-Savinja Alps, the Korošica formation (Upper Ladinian) overlies platform limestone and dolomite of the Ojstrica Formation (Ladinian). Diplopora annulata is also present in the Ojstrica formation. Various Ladinian rocks in the Julian Alps and Southern Karavanke are mainly of the Lover Ladinian age, whereas the overlying massive limestone and dolomite (“Cordevolian limestone and dolomite”) are partially of the Ladinian and partially of the Carnian age. There are no Upper Ladinian “Wengen” beds in the carbonate massive, so it is impossible to draw a time line between the Ladinian and the Carnian stages. We can therefore conclude, that litological boundaries do not coincide with the chronostratigraphic ones.

Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps)

Pseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25–0.3 GPa and 250–300 °C. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly <2 cm) according to the relation: x=(0.29±0.12)a. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (Tmelt≅1450 °C). Melt temperatures estimated by the clast/matrix ratio (O'Hara, 2001) are considerably lower (316–577 °C). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes.

The Piànico tephra: an early Middle Pleistocene record of intraplate volcanism in the Mediterranean

ABSTRACT Distal tephra are a valuable record of the volcano‐tectonic evolution of an area under study. Here, we document the case of the Early Middle Pleistocene rhyolitic tephra of Piànico, discovered in the Southern Italian Alps. The geochemical characteristics of Piànico are unique among the distal tephra outcropping in Italy and indicate an intraplate volcanism at the source. The alkali composition and trace elements show a striking resemblance with the rhyolitic complex of the Euganean Hills, located 170 km from Piànico. However, these rhyolites are much older (Oligocene). Alternatively, the source of this intraplate volcanic episode could be located in the southern Tyrrhenian Sea.

Fracture networks on the Belluno syncline, a fault-propagation fold in the footwall of the Belluno thrust, Venetian Alps, NE Italy

Abstract The Belluno syncline is a structure located in the Venetian fold-and-thrust belt in the Italian Southern Alps. This SSE-vergent chain mainly developed during the Neogene as a back-thrust belt of the Alps deforming the northern margin of the Adria plate. The syncline is asymmetric in shape, with a steep backlimb, as the forelimb of the paired Monte Coppolo Anticline; it is the result of fault-propagation folding during the development of the underlying Belluno thrust. In plan view the Belluno thrust shows a prominent curvature interpreted as reactivation of inherited Mesozoic extensional structures. The hinge of the Belluno syncline presents a similar curvature. Rocks cropping out in the syncline core are Upper Chattian to Langhian fine-grained molassic sediments, mainly siltstones and marls. Most fractures on the Belluno syncline have formed at high angles to bedding. The dominant fracture trends are subparallel and orthogonal to the fold hinge. Sometimes these fractures show plumose structures revealing extension (Mode I) fracture mechanism and therefore they are referred to as longitudinal ( bc ) joints and cross ( ac ) joints respectively. Other fracture trends oblique to the hinge line are found in measurement stations located on limbs distant from the fold hinge. They are referred to as oblique (shear) fractures. Throughout the western part of the Belluno syncline the fracture network maintains a roughly symmetrical distribution with respect to the fold hinge, suggesting a development in association with folding.

Distribution patterns of four Orthoptera species in relation to microhabitat heterogeneity in an ecotonal area

Microhabitat heterogeneity is considered to be one of the main factors affecting the structure and diversity of natural communities. This study evaluated: (i) whether it is possible to associate the distribution of four orthopteran species with small-scale spatial microhabitat heterogeneity based on floristic composition; and (ii) whether interspecific differences exist in microhabitat was among the different orthopteran species over a gradient of vegetation succession induced by abandonment of meadows. Orthoptera and plant species were sampled on 72 plots across an ecotonal area on Monte Bondone in the Southern Italian Alps. Microhabitats were identified based on grassland and undergrowth vegetation composition and by classifying sample plots using cluster analysis. Eight microhabitats were identified, each corresponding to a separate successional stage, and microhabitat use by each species was assessed. The distribution of orthopteran species revealed a different use of microhabitats. Species also had differing patterns of distribution, and a shift in distribution occurred following a change in microhabitat structure caused by mowing. The importance of the maintenance of a mosaic of microhabitats, with differently managed adjacent areas is discussed.

Segmentation and linkage of the Lessini Mountains normal faults, Southern Alps, Italy

The Lessini Mountains on the southern margin of the Italian Southern Alps exhibit two Tertiary graben systems at an angle of about 40°. The NNW-trending system is orthogonal to the ENE-trending Tertiary extension direction. The NNE-trending system parallels the neighbouring Jurassic western margin of the Trento Platform and experienced sinistral transtension during the Paleogene. The interaction between the two normal fault trends defines a ca. 8 km long and 6 km wide rhomb-shaped structure containing igneous intrusions and dolomitized rock bodies, suggesting that the structure controlled syn-extensional fluid migration. At a smaller scale, fault segments of the NNE-trending set show two basic types of transfer zones, i.e. conjugate and synthetic. Due to the sinistral and normal oblique-slip on these faults, left steps of en échelon segments had a kinematics similar to that of pull-apart structures. The location of intrusion-centres within such transfer zones led to the collapse of the roof of the magma chambers and consequent deep basins development. In contrast, right-stepping synthetic en échelon normal-oblique fault segments developed relay ramps.

Extreme Tree Rings in Spruce (Picea abies [L.] Karst.) and Fir (Abies alba Mill.) Stands in Relation to Climate, Site, and Space in the Southern French and Italian Alps

The similarity over long distances of dendroecological pointer years (with extreme ring-widths) were studied at both regional and country scales in order to investigate the geographical extension of climate influences on tree-rings. Two common species, Norway spruce (Picea abies Karst.) and white fir (Abies alba Mill.) were compared. The regional study was carried out on 33 populations located in four alpine valleys along a climatic gradient of summer aridity (Tarentaise, Maurienne, and Briançonnais, in France, and Susa valley in Italy). For most of species and regions, several negative ring-width pointer years with abrupt growth reductions such as 1976, 1922, 1986, and 1944 were common (listed in order of decreasing importance). However, spruce growth was more reduced in 1948 than that of fir. At the country scale, some of the strongest positive (e.g., 1932, 1964, 1969) and negative (e.g., 1956, 1962, 1976, 1986) pointer years extended over the whole of France, whereas the geographic variability was explainable by the autoecology of species. At both studied scales, evident climatic interpretations such as severe winter frosts, unusual summer droughts, or excessive wet and cold springs can explain most of the negative pointer years. Conversely, most positive growth responses are caused by a local combination of favorable climatic factors rather than simple extreme events, and therefore are less efficient for wood dating.

Extreme Tree Rings in Spruce (Picea abies [L.] Karst.) and Fir (Abies alba Mill.) Stands in Relation to Climate, Site, and Space in the Southern French and Italian Alps

The similarity over long distances of dendroecological pointer years (with extreme ring-widths) were studied at both regional and country scales in order to investigate the geographical extension o...

Strain localization mechanisms in deep-seated layered rocks

More than 160 shear zones of the Norwegian Caledonides and Italian Southern Alps, formed under high-temperature or high-pressure conditions, are analysed with respect to mechanisms of strain localization. In metabasic rocks with a pre-existing (magmatic) layering strain localizes preferentially in mafic layers, although experiments in monomineralic rocks would predict a location in the weaker feldspar-rich layers. In addition, in some of these zones amphibole recrystallizes dynamically, whereas feldspars only show undulatory extinction. These are evidences for strength inversion between feldspars and mafic minerals. This inverse strength behaviour is caused by a reduced grain size of feldspars and sometimes amphiboles in the pre-existing mafic layers, i.e. prior to shearing. The localization of the shear zones is therefore a grain-size-sensitive process (grain-size-dependent softening), nearly independent of the composition of the deformed material. The numerous amphibole–amphibole contacts favour dynamical recrystallization by grain-boundary migration and the rare feldspar–feldspar contacts prevent or suppress the same process in feldspars. Therefore, the feigned inverse strength is mainly caused by the interaction of reduced grain size and grain-boundary effects in the original rock. This behaviour can lead to the preferential localization of shear zones in mafic layers, although they are not favourably oriented with respect to the stress system.

Measuring lithospheric stretching on Tethyan passive margins

Preliminary estimates of lithospheric stretching have been made by examining 63 stratigraphic sections, from the northern and southern Tethyan margins (southeast France to Bulgaria). The study was centred on the Italian Southern Alps (the Dolomites), a portion of the southern margin within the external zones of the Alpine orogen. Sections were decompacted and backstripped using the standard procedure, allowing for errors in palaeowater-depth estimates. Theoretical subsidence curves based on the finite-duration stretching model were then fitted to the resultant water-loaded subsidence curves in an attempt to clarify the pre-compressional tectonic history of the region. According to our results, stretching started in the early to mid-Triassic and lasted 30–40 Ma. This conclusion is broadly in agreement with some previously published estimates. The stretching factor, β, is generally small (1.1–1.3), especially in southeast France and northern Italy. Larger values of β occur in the Balkans ( c. 1.5). To our knowledge, the finite-duration stretching model has not yet been applied to water-loaded stratigraphic sections from the Tethyan belt. Theory and observation appear to agree well.

M-sequence reversals recorded in DSDP sediment cores from the western Mid-Pacific Mountains and Magellan Rise

New paleomagnetic and paleontologic data from Pacific DSDP Sites 463 and 167 define the magnetic reversals that predate the Cretaceous Normal Polarity Superchron (K-N). Data from Mid-Pacific Mountain Site 463 provide the first definition of polarity chron M0 in the Pacific deep-sea sedimentary record. Foraminiferal biostratigraphy suggests that polarity chron M0 is contained entirely within the lower Aptian Hedbergella similis Zone, in agreement with foraminiferal data from the Italian Southern Alps and Atlantic Ocean. Nannofossil assemblages also suggest an early Aptian age for polarity chron M0, contrary to results from the Italian Umbrian Apennines and Southern Alps, which place polarity chron M0 on the Barremian-Aptian boundary. Biostratigraphic dating discrepancies caused by the time-transgressive, preservational, or provincial nature of paleontological species might be reconciled by the use of magnetostratigraphy, specifically polarity chron M0 which lies close to the Barremian-Aptian boundary. At Magellan Rise Site 167, five reversed polarity zones are recorded in Hauterivian to Aptian sediments. Correlation with M-anomalies is complicated by synsedimentary and postsedimentary sliding about 25 m.y. after basement formation, producing gaps in, and duplications of, the stratigraphic sequence. The magnitude and timing of such sliding must be addressed when evaluating the stratigraphy of these oceanic-rise environments.