Southern Alps Research Articles

AN INSIGHT INTO THE SYSTEMATICS OF PLICATOSTYLIDAE (BIVALVIA), WITH A DESCRIPTION OF <em>PACHYGERVILLIA ANGUILLAENSIS</em> N. GEN. N. SP. FROM THE <em>LITHIOTIS</em> FACIES (LOWER JURASSIC) OF ITALY

The Lithiotis facies represents an Early Jurassic global bioevent characterized by a remarkable spread of gregarious bivalves, which produced large sedimentary bodies in tropical shallow-water marine environments. The most peculiar and common genera Lithiotis, Cochlearites and Lithioperna, with aberrant and extremely elongated or strongly flattened shells, have been studied since the second half of the nineteenth century. Despite numerous systematic studies, their phylogenetic relationship with the other bivalve families is still uncertain. The Lithiotis facies yields other bivalve genera, among which a large multivincular mytiloid, provisionally determined as Isognomon (Mytiloperna) sp. ind. or Mytiloperna sp., is recorded in the literature. This taxon is here studied from a systematic point of view to clarify its taxonomic position and solve the open nomenclature adopted in the past. Here, we propose a new genus Pachygervillia and a new species Pachygervillia anguillaensis. The stratotype is located in the lower part of the Rotzo Formation (Calcari Grigi Group, Lower Jurassic), while the type locality is in the Lessini Mountains (Verona Province, Trento Platform, Southern Alps). This new species is characterized by a thick aragonitic inner shell layer with a fibrous, irregular, spherulitic, prismatic microstructure combined with a nacreous middle layer, both also occurring in species of the genera Lithiotis and Cochlearites of the family Plicatostylidae. This microstructural layering is here proposed as the main taxonomic character of the family, which is here emended and divided into the following two subfamilies: Plicatostylinae, yielding Lithiotis and Cochlearites with stick-like shells, and Pachygervilliinae nov. subfam., yielding Gervilleioperna, Lithioperna, Pachygervillia n. gen., and Pachyperna, previously placed within the subfamily Isognomoninae.

S Hmax orientation in the Alpine region from observations of stress-induced anisotropy of nonlinear elasticity

SUMMARY The orientation of SHmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.

Paleokarst coastal caves at Torricelle Hills (Lessini Mountains, Venetian Prealps, Italy)

This paper describes a set of paleokarst caves at Torricelle Hills near Verona (Southern Alps, Italy.) At this locality, erosional surfaces and paleokarst cavities show that sedimentation of late Paleogene neritic limestones was interrupted by subaerial exposure. Karst features developed during a phase of marine regression that started after the early Oligocene and ended in the mid Miocene. These caves were originally completely filled by iron oxides- and hydrated oxides-rich paleosol sediments (ochre) that, for centuries, have been mined for pigments. Mining activity emptied the caves, leaving the voids and related shapes mostly intact; as a result, the original morphologies have been exhumed, making these caves a rare example of explorable paleokarst. These “ochre caves” were mapped in a series of surveys over a few years. The exploration of overall 4.5 km of accessible passages in four caves yielded a wealth of information on speleological features, stratigraphy, paleontology, and paleogeography, and here we exploit this information to infer the genesis of these unusual caves. Their evolution started in phreatic conditions, characterized by very slowly moving or still waters that led to the formation of solution facets. A vadose phase of development ensued, followed by infilling by reworked soil-derived sediment and associated paragenetic modifications. Sediment accumulation ended with the complete fossilization of the caves under epiphreatic conditions. Siliciclastic and carbonate sediments containing littoral fossils indicate that the caves developed in the vicinity of a coast, and that they were subject to marine ingression. Overall, these paleokarst coastal caves seem to be a fossilized example, well preserved and explorable, of the Carbonate Island Karst Model on larger islands. We interpret these caves as conduits that drained the freshwater lens in a spatially limited carbonate peninsula that existed in this part of the Lessini paleocoastline between the Oligocene and the Miocene.

A wealth of P–T–t information from metasediments in the HP–UHP terrane of the Pohorje Mountains, Slovenia, elucidates the evolution of the Eastern Alps

AbstractContrasting views exist in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh‐pressure metamorphism in the Late Cretaceous (Eo‐Alpine) and whether they were continuously exhumed or experienced a multiple subduction–exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modelling, conventional geothermometry, and monazite in‐situ U‐Th‐Pb dating using laser‐ablation inductively coupled plasma (ICP) mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak pressure of 1.31 ± 0.14 GPa at 603 ± 26°C in Eo‐Alpine times: 90.62 ± 2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure–temperature conditions of 0.66 ± 0.10 GPa and 577 ± 23°C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33 ± 0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sample 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo‐Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high‐pressure event in the Eocene (Stage II) was the result of intracontinental subduction because of transpression by the Periadriatic fault system that separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity.

A 10Be chronology of the Esmark Moraine and Lysefjorden region, southwestern Norway: Evidence for coeval glacier resurgence in both polar hemispheres during the Antarctic Cold Reversal

From 10Be surface-exposure dating we make the case that classic late-glacial moraines in southwestern Norway, including the iconic Esmark moraine, and in the Southern Hemisphere middle latitudes, including the iconic Birch Hill moraine of the Southern Alps of New Zealand, were constructed coevally within dating uncertainties. This finding of simultaneous glacier resurgence in the two polar hemispheres carries the implication that the millennial-scale climate reversal of the last deglaciation was global and required a change in the energy budget of the earth's climate system.

Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation

Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a post-Variscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cm-scale layered rocks showing compositional variation consisting of alternating ‘amphibole-rich’ (i.e., amphibolites) and ‘clinopyroxene/plagioclase-rich’ domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs.

Microfossils from Middle Triassic beds near Mišji Dol, central Slovenia

Middle Triassic beds exposed along the road between Mišji Dol and Poljane pri Primskovem (Posavje Hills) comprise marlstone, tuff, volcaniclastic sandstone, and thin- to medium-bedded limestone and dolostone. The succession was logged and sampled for conodonts. A relatively rich conodont assemblage was determined, consisting of Budurovignathus gabrielae Kozur, Budurovignathus sp., Cratognathodus kochi (Huckriede), Gladigondolella malayensis Nogami, Gladigondolella tethydis Huckriede, Gladigondolella sp., Neogondolella balkanica Budurov & Stefanov, Neogondolella cf. excentrica Budurov & Stefanov, Neogondolella constricta (Mosher & Clark), Neogondolella cornuta Budurov & Stefanov, Neogondolella sp., Paragondolella excelsa Mosher, Paragondolella liebermani (Kovacs & Kozur), Paragondolella trammeri (Kozur), Paragondolella cf. alpina (Kozur & Mostler), and Paragondolella sp. The assemblage correlates with the upper Anisian and lowermost Ladinian assemblages from the Global Boundary Stratotype Section and Point (GSSP) of the Ladinian at Bagolino in the Southern Alps in northern Italy. Along with conodonts, numerous specimens of benthic foraminifera Nodobacularia? vujisici Urošević & Gaździcki were recovered from the lowermost part of the succession. Previous research on this taxon is critically evaluated.

Gone‐wild grapevines in forests may act as a potential habitat for ‘Flavescence dorée’ phytoplasma vectors and inoculum

Abstract‘Flavescence dorée’ (FD) is a quarantine grapevine disease associated with FD phytoplasmas (FDp). In Switzerland, FD was first reported in 2004 in the southern part of the Canton Ticino, from where, within a few years, it spread to nearly the entire vineyard area of the Southern Alps, despite the mandatory control measures. The aim of this study was to assess the possible role of gone‐wild grapevines (GWGVs) in forests resulting from early abandoned vineyards as a habitat for the main FDp vector, Scaphoideus titanus, and for the best alternative vector candidate, Orientus ishidae, as well as an FDp inoculum reservoir in Southern Switzerland. Leaf samples were collected in 20 plots and tested for the presence of FDp. Moreover, fifteen of these plots were monitored with yellow sticky traps to determine the presence and infection status of insect vectors. Finally, a hatching experiment under controlled conditions was conducted to investigate the possible oviposition activity by FDp vectors, using wood collected from GWGVs present in 11 forest sites in the surroundings of cultivated vineyards. GWGVs in forests were confirmed to act as an FDp reservoir and to represent a suitable habitat for FDp vectors (in particular, for S. titanus). Abandoned vineyards should be included in the FD management strategy and their systematic rogueing should be applied as soon as possible and in particular before the transition from vineyard to forest occurs, in order to avoid the survival of S. titanus and the establishment of inoculum in uncontrolled landscape compartments.

Inversion of extensional basins parallel and oblique to their boundaries: inferences from analogue models and field observations from the Dolomites Indenter, European eastern Southern Alps

Abstract. Polyphase deformation of continental crust is analysed through physical analogue models for settings wherein platform–basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle–ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength difference between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel and oblique (10 to 20∘) to the basin axes has been applied, leading to the inversion of basins formed earlier. The experiments show that strength contrasts across platform–basin transitions control the localisation and overall style of compressional deformation, irrespective of the nature of the basal décollement (frictional versus viscous), the rheology of the basin fill, or changing platform–basin thickness ratios. Orientations of thrust faults change laterally across inherited platform–basin transitions throughout all experiments; higher obliquity of basin inversion leads to stronger alignment of thrust curvature with the orientation of pre-existing rift axes. At individual thrust faults, variations in the strike of thrust fronts are accompanied by changes in the shortening direction during incremental phases of deformation. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform–basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Our experiments are relevant for natural cases such as the Dolomites Indenter of the eastern Southern Alps, underlining the importance of inherited geologic features for the subsequent shortening geometries. Field structural data from the western segment of the Belluno thrust of the Valsugana fault system support predicted variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along-strike) along one single fault. Based on our modelling results, we suggest that this variability of thrust fault orientation and shortening directions, controlled by inherited structures, is consistent with strain partitioning during a single phase of deformation and does not necessarily reflect different deformation phases.

Reconstructing a 51-year record of catchment scale dissolved load for the Haast/Awarua River in the Southern Alps, New Zealand

Mountains dominate global denudation fluxes of solutes to the oceans despite being only a modest portion of the Earth's surface. Determining the flux of material exhumed from mountains as dissolved load, however, requires careful measurement of ionic concentrations and discharge that may be hard to obtain. We reconstruct the dissolved load and total dissolved solids yield from pre-existing discharge measurements and 32-years on monthly water quality sampling in a headwater catchment in the Southern Alps/Ka Tiritiri o te Moana, New Zealand. The dissolved weathering yield of the Haast/Awarua River for 1971–2020 was 218 ± 48 t km−2 a−1. The flux of dissolved material from the mountain catchment has a seasonal signal, with higher TDS concentrations during winter, but greater chemical weathering yields during summer. Hydrographic behaviour is an important control of weathering yields with 70 % of dissolved load being transported during event flow. Variations in chemical weathering yields are sensitive to changes in stream flow associated with broader climate oscillations, which will have a direct effect on estimations of chemical weathering rates since dissolved flux is strongly correlated to changes in stream flow.

Cordaites and pteridosperm-like foliage from the Kungurian (early Permian) flora of Tregiovo (Trento, NE Italy)

The Tregiovo flora is one of the best documented and well-dated Kungurian floras of Euramerica. It yielded a rich and diverse plant fossil assemblage including voltzian and walchian conifers, peltasperm seed ferns, sphenopterids, putative ginkgophytes, ferns and horsetails. Here two taxa are described and discussed, Tregiovia furcata gen. et sp. nov. and Cordaites sp. The former, characterized by a bifurcating lamina with a distinct midrib, is of difficult botanical affinity, with the closest resemblance to Auritifolia anomala. If this similarity is confirmed, it would represent the first comioid record in the Southern Alps and Europe. The two new taxa underline the bias of the fossil record for elements adapted to seasonally dry environments. A comparison with similar taxa of gymnosperm affinity show that the Cisuralian fossil record is still very scares in certain areas of Pangea such as the low and middle latitudes of Gondwana, eastern Gondwana and the high latitudes of Euramerica.

Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps)

The Ivrea-Verbano Zone, in the Italian Southern Alps, samples a complete section of middle to lower continental crust, which records the transition between the Variscan and the Alpine Tethys rift-related tectonics. We present a structural-metamorphic characterization of one of the most important structures, the Forno-Rosarolo shear zone which drove the post-Variscan tectonic evolution of the Ivrea-Verbano Zone. The Forno-Rosarolo shear zone is a NE-SW-oriented, subvertical shear zone, mainly made of metapelites, amphibolites, calc-silicates and granulites involved in anastomosed proto- to ultra-mylonite layers enveloping weakly deformed lenses. Mylonitic deformation developed on an already folded and sheared metamorphic (Variscan) complex but predates brittle fracturing and faulting, locally associated with pseudotachylites.In present day orientation, the kinematic indicators point to a sinistral sense of shear. Removing the Alpine ∼90° tilt of the IVZ crustal section, this kinematic results in a former extensional shear zone. Investigations on the kinematic of the flow in the mylonites revealed a non-coaxial deformation with a major component of pure shear acting together with simple shear. Metamorphic conditions of the wall rocks vary from upper amphibolite (SE, footwall) to granulite facies (NW, hanging wall). Estimations from the mineral assemblages within the mylonites point to amphibolite facies conditions during deformation (∼650 °C and ∼5.5 kbar).In absence of precise geochronological constraints, we discuss the role of the Forno-Rosarolo shear zone within the Tethyan rifting system in comparison with recent reconstructions. Kinematic data and metamorphic conditions allow to constrain the development of the Forno-Rosarolo shear zone mylonitic deformation during the intermediate phase of the Tethyan rift known as “thinning mode”. This stage was characterized by general shear conditions (pure shear between 70% and 50%) suggesting a phase of transition from a symmetric to an asymmetric configuration of rift.

Facies character and evolution of a mixed carbonate–siliciclastic shelf: Upper Triassic–Lower Jurassic succession in the eastern Northern Calcareous Alps (Stumpfmauer, Austria)

Western Tethys sedimentary successions constitute fundamental archives of Late Triassic–Early Jurassic environmental, carbonate production and tectonic changes. During the Late Triassic, the Northern Calcareous Alps (Austria) belonged to the Western Tethys passive margin, characterised by the deposition of the early-dolomitized peritidal Hauptdolomit (Norian) adjacent basinward to the Dachstein carbonate shelf and passing upward to the mixed carbonate–siliciclastic Kössen Formation (Upper Norian–Rhaetian). The Kössen Fm. was subdivided into the lower shallow-water Hochalm Member and the upper Eiberg Member, accumulated in an intraplatform basin coeval to shallow-water carbonates (Upper Rhaetian Limestone). The Eiberg Mb. and overlying Jurassic strata were extensively studied as a continuous marine record across the Triassic/Jurassic boundary. In contrast, shallow-water successions, time-equivalent to the Eiberg Mb. and Upper Rhaetian Limestone, located North of the Eiberg Basin, are poorly investigated. This study focusses on the approximately 350 m thick Norian to Lower Jurassic succession cropping out in the eastern Northern Calcareous Alps (Stumpfmauer). The 32 distinguished lithofacies compose seven, vertically superimposed, sedimentary units (A–G), belonging to the Hochalm Mb. of the Kössen Fm. (Unit A peritidal cyclothems, Unit B claystone/marlstone with fossiliferous beds, Unit C coral boundstone to floatstone), Upper Rhaetian Limestone (Unit D subtidal cyclothems with claystone), shallow-water carbonate strata transitional to Lower Jurassic (Unit E ooidal coated grain peloidal grainstone with basal transgressive lag, Unit F bivalve-rich, microbialite and oncoidal lithofacies, previously attributed to the Upper Rhaetian Limestone) and Hettangian–Sinemurian Kalksburg Fm. (Unit G cross-laminated coated grain peloidal grainstone with quartz and chert). The detailed lithofacies characterisation presented in this study contributes to the knowledge on the Northern Calcareous Alps stratigraphy and depositional environments across the Triassic/Jurassic boundary. The identified sedimentary units can be framed in the evolution of Western Tethys and share similarities with depositional systems from the Western Carpathians, Transdanubian Range, Southern Alps and Dinarides suggesting coherent sedimentary response and environmental, climate and tectonic controls in different palaeogeographic domains.

Alpine transpression in the Passo Rolle area (Dolomites, Italy): new structural and paleostress constraints

The Dolomites geologically belong to the south-verging eastern Southern Alps of Italy. They record the effects of multiple deformation episodes from the Permian onward and thus offer the possibility to study the role of earlier inherited structures upon the Cretaceous-Neogene Alpine deformation history. In the Passo Rolle area, Permian extension was followed by Late Triassic-Early Jurassic extensional tectonics during which the NNW-SSE striking Passo Rolle Fault (PRF) contributed to the development of horst and graben structures. The Alpine orogenesis in the Passo Rolle area was responsible for the build-up of the Pale di San Martino mountain range during ~ NNW- SSE compression. During this stage, the NNW-verging Cimon della Pala backthrust formed concomitant with the activation of minor thrusts and strike-slip faults, and the reactivation of Late Triassic-Early Jurassic extensional faults. The genetic relationships between the broad suite of tectonic structures in the Passo Rolle area have never been investigated in detail. To this end we performed mesostructural analysis of key representative exposures to add geometric and kinematic constraints on the tectonic structures associated with the different tectonic phases registered in the Passo Rolle area. We focused on collecting fault-slip data that we used for paleostress analysis aiming at reconstructing the local paleostress evolution through time. We show that (i) the PRF accommodated significant Alpine transpression and (ii) the Permian-to-Jurassic tectono-stratigraphic framework promoted the localization of Alpine deformation to the east of the PRF, where a thick carbonate-siliciclastic multilayer deformed through first-order folds and thrusts. To the west of the PRF, where the Permian ignimbrite formed instead a relatively rigid and thus less deformable block, only minor thrusts and strike-slip faults formed, commonly exploiting suitably oriented inherited structures.

Soil centipedes (Chilopoda, Geophilomorpha) in the Val Camonica forests (Southern Alps): species composition and richness.

Soil centipedes (Chilopoda, Geophilomorpha) are a widespread group of predators in the forest soils of the European Alps. While in the eastern and western parts of the Southern Prealps, larger efforts were devoted to sample and study the geophilomorph fauna, little is known about species richness and composition of geophilomorph communities in the central part of Southern Prealps. In this work, five sites located in the Val Camonica were surveyed by hand searching, between November 2021 and July 2022 and their species richness was estimated applying non-parametrical statistical methods (Chao-1 and Abundance-based Coverage Estimator) to account for incomplete detection. A total of 18 species were found amongst the five sites. A maximum of 12 species were recorded in each single site, while estimates suggest that another 1-3 species were likely undetected. Species composition were found highly variable also between sites with similar species richness.

Three alien bark and ambrosia beetles (Coleoptera, Curculionidae, Scolytinae) new to Switzerland

Identifying alien species is important to ensure the early detection of biological invasions and survey shifts in species distributions in the context of global change. Here, we report on three alien bark and ambrosia beetles newly detected in Switzerland: Cyclorhipidion distinguendum (Eggers, 1930), C. pelliculosum (Eichhoff, 1878), and Hypothenemus eruditus (Westwood, 1834). These species were recorded for the first time during a comprehensive survey of saproxylic beetles accross major forest types and along an altitudinal gradient during the entire growing season in the southern Alps, in the canton of Ticino. Their local abundance and number of occurrences accross different lowland forest habitats, including alluvial forests of national importance, indicates that all three species are already naturalized. Given their polyphagy, it is likely that all three species will become more extensively distributed across Switzerland, with a yet unknown environmental impact.

The Meran-Mauls Fault: Tectonic switching from compression to transpression along a restraining bend of the Periadriatic Fault

The WSW-ENE Meran-Mauls Fault (MMF) represents a break in the broadly E-W trending Periadriatic Fault (PAF), separating the Southern Alps from the N-verging wedge of the European Alps. Understanding the MMF evolution is mandatory to reconstruct the role of the entire PAF during the evolution of the belt since the late Cenozoic.Structural and microstructural analyses and paleostress calculations based on fault-slip data suggest the occurrence of four evolutionary stages for the MMF: 1) top-to-the-SE mylonites; 2) top-to-the-SE brittle faulting; 3) dextral faulting with re-activation of previous structures; 4) N–S compression associated with conjugate sets of mainly strike-slip fault systems. Paleostress reconstructions point to a σ2-σ3 permutation from stage 2 to stage 3, resulting in the switch from pure thrusting to strike-slip, followed by an anticlockwise rotation of the principal stress axes during stage 4, in a strike-slip regime related to N–S compression. Geochronological and thermochronological data point to 39-22 Ma age for stage 1, 22-17 Ma for stage 2, less than 17 Ma for stage 3. Our reconstruction strongly supports the interpretation of the MMF as a restraining bend of the PAF, which was weakly reactivated along its eastern portion as a dextral fault and later displaced by NNE-SSW left-lateral faults.

Structure and topology of a brittle-ductile fault swarm at Crawford Knob, Franz Josef, New Zealand

ABSTRACT We present surface and structural models of a swarm of dm-scale subparallel faults exposed in a ∼2000 m2 glaciated outcrop near Franz Josef Glacier, in the Southern Alps of New Zealand. These structures are inferred to have slipped at ∼20 km depth in the hanging-wall Alpine Schist of the Alpine Fault under conditions that were variably brittle to ductile as the Pacific Plate was tilted and uplifted. Using field mapping, real-time kinematic GPS and digital images from a remotely piloted aircraft system, we have created a digital surface model of the outcrop and orthophotographs at a ground resolution of ∼1 cm to map compositional layering in the metasediments and the array of brittle-ductile faults displacing them. In order of decreasing relative age (and average thickness), displaced markers in the schist include primary psammite and pelite beds, quartz veins, and a deformational foliation. The surface models have been used to create 2-D transects and a 3-D model where faults are projected down-dip, to determine the connectivity, topology and intersection types of the fault swarm. Lithological variations, particularly the interface between pelitic and psammitic schist, were a primary control on the topology of the fault network and the spacing between faults.

The age and potential causes of the giant Green Lake Landslide, Fiordland, New Zealand

Landslide deposits preserved in the geological record afford opportunities to better inform hillslope and seismic hazard and risk models, particularly in regions where observational records are short. In the Southern Alps of New Zealand, small coseismic landslides are frequent, but the geological record preserves several instances of more substantial (> 1 km3) but infrequent mass failures. With an estimated volume of 27 km3, the giant Green Lake Landslide represents one of the largest subaerial landslides on Earth. Previous work has suggested this deep-seated mass movement was most likely triggered by high-intensity seismic shaking, but that local structural weakness and/or glacial debuttressing may help to explain the anomalously large failure volume. Resolving the potential contribution of the latter is important given predictions concerning alpine deglaciation in the coming decades to centuries. Key to resolution are secure chronologies of landslide emplacement and past glacier change. Here we present in situ cosmogenic 10Be exposure ages from the Green Lake Landslide that suggest an emplacement age of 15.5 ± 0.7 ka. Recent work shows that glacial retreat in the region was underway by 19 ka, indicating that the Green Lake Landslide was emplaced 3–4 kyr after the onset of glacier retreat. Given the geometry of the former confining valley glacier, we expect that the deglaciation-landslide age gap is closer to the upper end of this estimate. If correct, this conclusion places greater weight on the roles of local geological structure and/or a great earthquake as factors contributing to the exceptionally large volume of this event.

Late-Quaternary Evolution of the Semi-Confined Alluvial Megafan of Isonzo River (Northern Adriatic): Where the Fluvial System of the Southern Alps Meets the Karst

The evolution of alluvial megafans has mainly been investigated in unconfined settings; however, at the boundary of these large depositional systems, the development of fluvial channels can be affected by structural constraints with regional extent. Here we present the study of the eastern sector of the megafan of Isonzo River, in the Gulf of Trieste, where this system fed through the southern Alps is constrained by the Karst and Istria cliffs. Although this area is now submerged under the northern Adriatic Sea, stratigraphy from seismo-acoustic profiles, drill cores and multi-beam bathymetry allows us to reconstruct the paleochannel system of the Isonzo River in detail, which was likely active within the period of 21–17.5 ka cal BP, at the end of LGM. This was reconstructed for over 50 km and currently represents the longest abandoned fluvial channel in the Mediterranean seabed. The occurrence of the mountain fringe and competition with nearby alluvial systems forced the paleochannel to follow the present coastline and conditioned the slope of its thalweg to decrease almost to null, resulting the transformation from the megafan to the undifferentiated alluvial plain.