Gabrielli P.,Ohio State University |
Barbante C.,CNR Institute for the Dynamics of Environmental Processes |
Barbante C.,Centro B Segre |
Carturan L.,University of Padua |
And 14 more authors.
Geografia Fisica e Dinamica Quaternaria | Year: 2012
During autumn 2011 we extracted the first ice cores drilled to bedrock in the eastern European Alps from a new drilling site on the glacier Alto dell'Ortles (3859 m, South Tyrol, Italy). Direct ice core observations and englacial temperature measurements provide evidence of the concomitant presence of shallow temperate firn and deep cold ice layers (ice below the pressure melting point). To the best of our knowledge, this is the first cold ice observed within a glacier of the eastern European Alps. These ice layers probably represent a unique remnant from the colder climate occurring before ~1980 AD. We conclude that the glacier Alto dell'Ortles is now changing from a cold to a temperate state. The occurrence of cold ice layers in this glacier enhances the probability that a climatic and environmental record is fully preserved in the recovered ice cores.
Willcock M.A.W.,Monash University |
Cas R.A.F.,Monash University |
Giordano G.,Third University of Rome |
Morelli C.,Ufficio Geologia e prove materiali
Journal of Volcanology and Geothermal Research | Year: 2013
The Permian Ora Formation (277-274Ma) preserves the products of the Ora caldera 'super-eruption', Northern Italy. The stratigraphic architecture of the exceptionally well preserved intra-caldera succession provides evidence for caldera collapse at the onset of the eruption, a multiple discharge point, fissure eruption style, and progressive, incremental caldera in-filling by numerous pyroclastic flow pulses within the caldera. The ignimbrites of the Ora Formation are voluminous (>1290km3), crystal-rich (~25 to 55%), and ubiquitously welded. The Ora Formation has been divided into four members (a-d), which also define the principal eruption phases. The eruption proceeded in four main stages: (1) early caldera collapse and vent opening, producing locally distributed, basal co-ignimbrite lithic breccia (member a); (2) vent clearing, which produced the eutaxitic, lithic-rich ignimbrite and minor thin ground and ash-cloud surge deposits (member b); (3) waxing and steady eruption, which produced the dominant eutaxitic, coarse-crystal-rich ignimbrite, with local lithic-rich and fine-crystal-rich ignimbrite and minor surge deposits (member c); and (4) waning eruption, recorded by the eutaxitic, fine-crystal-rich ignimbrite, with local lithic-rich ignimbrite deposits (member d).The incremental filling and late-stage outpouring of pyroclastic material from the caldera is recorded by vertical and lateral lithofacies deposit variation and some correlation between stratigraphic sections. These findings reveal a structure to the outwardly monotonous, > 1300. m thick, intra-caldera fill and thinner (< 230 m) outflow successions. These data together with the gradational contacts between the main ignimbrite lithofacies, support the hypothesis that pyroclastic material was erupted from multiple source regions in various parts of the caldera, during quasi-steady, low eruption column collapse and pyroclastic flow forming events. Field study revealed the absence of a Plinian fallout deposit, suggesting a lack of a high, buoyant, Plinian precursor eruption phase. This caldera was initiated immediately by a low collapsing column phase, producing the main, thick ignimbrite succession. Simultaneously, catastrophic volcano-tectonic caldera collapse and decompression of the magma chamber occurred, facilitated by the regional extensional environment in the Permian and pre-existing crustal weaknesses. The Ora pyroclastic flow system is suggested as having been a hot and poorly expanded, high particle concentration, granular density current. The confined nature of the majority of the erupted products to the intra-caldera setting, reduced the formation of the full array of facies commonly expected in ignimbrites in extra-caldera settings.© 2013 Elsevier B.V.
Willcock M.A.W.,Monash University |
Mattei M.,Third University of Rome |
Hasalova P.,Monash University |
Hasalova P.,Czech Geological Survey |
And 3 more authors.
Geological Society Special Publication | Year: 2015
Abstract Anisotropy of magnetic susceptibility (AMS) data reveal heterogeneous pyroclastic flow processes and variable flow directions within the intra-caldera setting of the Permian rhyolitic welded Ora ignimbrite. Magnetic fabric is primary, orientated during the pyroclastic flow emplacement, and is controlled by paramagnetic and ferromagnetic mineral phases. The ignimbrite has typically weak mean magnetic susceptibilities (1.32-21.8×10-4 SI) but with a large spread and low anisotropy degrees (1.003-1.023), which vary in different parts of the caldera. The intra-caldera magnetic fabric provides significant information on the dynamics of the intra-caldera setting, relating to changing vertical and lateral flow emplacement processes. AMS shape ellipsoids range from oblate to prolate; these are interpreted to reflect the heterogeneous nature of the flow resulting from the influence of underlying topography, constraints of the caldera walls, primary welding and post-emplacement mineral growth. We have identified different depositional units and possible eruptive source regions, indicating that more than one source fissure vent was active during eruption within this caldera system. The lateral variations demonstrate a meandering of flow pulses. The caldera margin acts as an obstacle in preventing and rebuffing certain flows from scaling the caldera margin. © 2015 The Geological Society of London.
Zanchetta S.,University of Milan Bicocca |
Poli S.,University of Milan |
Rubatto D.,Australian National University |
Zanchi A.,University of Milan Bicocca |
Bove G.M.,Ufficio Geologia e Prove Materiali
Rendiconti Lincei | Year: 2013
We report here the discovery of UHP eclogites in the Ulfas Valley within the Texel Complex (Meran, NE Italy), which has important implications on the evolution of the Austroalpine nappe stack in the Eastern Alps. Exsolved K-feldspar and phengite lamellae within omphacite relics, together with an unusually high K-content of the host clinopyroxene, point to equilibrium pressures at least close to the quartz/coesite transition. The age of the high-pressure metamorphism is constrained by U-Pb dating of metamorphic zircon at 85 ± 4 Ma. These new data suggest pressure conditions for the Cretaceous eclogitic metamorphism of the Texel Complex higher than hitherto reported data, possibly indicating that several sectors of the Africa-Adria derived Austroalpine margin were subducted to mantle depths during the early stages of the Alpine orogeny. © 2013 Accademia Nazionale dei Lincei.