U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

U\u2013Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 \ub1 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons (c. 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples

Deformation and metamorphism associated with crustal rifting : Permian to Liassic evolution of the Lake Lugano-Lake Como area (Southern Alps)

In connection with movements between Europe and Adria, the Southern Alps underwent extension leading to the formation of the Middle Jurassic South-Alpine passive continental margin. As a result of mainly S-vergent Alpine thrusting and folding, a substantially preserved Mesozoic crustal section, ranging from the surface to a paleo-depth of ca. 15 km is exposed in the Lugano-Lake Como area. Tectonic processes associated with Permo-Mesozoic extension can thus be investigated at different crustal levels. Rifting began with a thermal anomaly during which rocks at middle-crustal levels were sheared at ca. 650-750\ub0C. Deformation was distributed on a several km wide zone and late-kinematic pegmatites were emplaced at this stage. Temperatures then decreased and beginning in the Norian, extension was accommodated by a major, E-dipping normal fault, the Lugano-Val Grande normal fault. The fault which can be followed down to a paleo-depth of ca. 12 km, was listric and flattened (to 20\ub0) at ca. 7-9 km depth. In the upper 8-10 km, deformation was only brittle, whereas at deeper crustal levels greenschist mylonites were formed. With continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower-temperature ultramylonites and cataclasites. The thickness of the fault zone varies from some tens of meters in the upper 5-6 km to several hundred meters at deeper levels: deformation was thus discrete at a crustal scale

Superposed syn-metamorphic structures of the Alpine and pre-Alpine convergent cycles in the Southalpine basement of the Orobic Alps (Northern Italy)

New detailed maps of two key areas of the Southalpine basement unravel the superposition of pre-Alpine deformation stages associated with different metamorphic imprints, and distinguish Alpine thrust and fold structures involving basement and Permian-Mesozoic sedimentary cover. Basement rocks consist of metasediments (micaschists, quartzites and paragneisses) and metagranitoids and cover rocks comprise Permian volcanics and sediments. The boundaries between two tectono-metamorphic units have been distinguished at the map scale, thanks to the integrated use of structural and petrological analyses, on the basis of foliation trajectories supported by metamorphic assemblages. Two different pre-Alpine metamorphic evolutions characterise a basement portion that seemingly appears as a monotonous sequence dominantly affected by a greenschist-facies imprint. The volcano-sedimentary sequence of Permian age has been used as the time marker that separates Alpine from pre-Alpine superposed structures; Alpine deformation consists of two superposed groups of structures, the first of which is locally associated with a very low-grade metamorphic imprint and related at the regional scale to a south-verging thrust system

The role of structural and metamorphic memory in the distinction of tectono-metamorphic units : the basement of the Como Lake in the Southern Alps

The concept of 'metamorphic field gradient' applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso-Cortafo, Dervio-Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the T(max)-P(Tmax) recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the 'metamorphic field gradient' concept cannot therefore be utilised to discriminate tectono-metamorphic units. (C) 2000 Elsevier Science Ltd. All rights reserved

Campagne di misura nell'ambito del Progetto HELI-DEM: livellazioni geometriche di precisione e rilievi GNSS

Nell'ambito del Progetto HELI-DEM sono state commissionate ed eseguite linee di livellazioni sia in Regione Piemonte che in Regione Lombardia. Lo scopo di tali campagne era triplice: connettere il sistema altimetrico italiano a quello svizzero, chiudere anelli di livellazione italiani che IGM non aveva ancora completato, valutare l'ondulazione del geoide attraverso misure congiunte GNSS su alcuni capisaldi livellati e così confrontare le differenze di ondulazione con quelle disponibili nei tre modelli di geoide gravimetrico disponibili per il Progetto. Le linee di livellazione sono state misurate e collaudate in entrambe le Regioni italiane. Il presente articolo propone un approfondimento dei controlli eseguiti sulla rete di livellazione del Piemont

The role of structural and metamorphic memory in the distinction of tectono-metamorphic units : the basement of the Como Lake in the Southern Alps

The concept of 'metamorphic field gradient' applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso-Cortafo, Dervio-Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the T(max)-P(Tmax) recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the 'metamorphic field gradient' concept cannot therefore be utilised to discriminate tectono-metamorphic units. (C) 2000 Elsevier Science Ltd. All rights reserved