Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography

The objective of the TRANSALP project is an investigation of the Eastern Alps with regard to their deep structure and dynamic evolution. The core of the project is a 340-km-long seismic profile at 12°E between Munich and Venice. This paper deals with the P-wave velocity distribution as derived from active source travel time tomography. Our database consists of Vibroseis and explosion seismic travel times recorded at up to 100 seismological stations distributed in a 30-km-wide corridor along the profile. In order to derive a velocity and reflector model, we simultaneously inverted refractions and reflections using a derivative of a damped least squares approach for local earthquake tomography. 8000 travel time picks from dense Vibroseis recordings provide the basis for high resolution in the upper crust. Explosion seismic wide-angle reflection travel times constrain both deeper crustal velocities and structure of the crust–mantle boundary with low resolution. In the resulting model, the Adriatic crust shows significantly higher P-wave velocities than the European crust. The European Moho is dipping south at an angle of 7°. The Adriatic Moho dips north with a gentle inclination at shallower depths. This geometry suggests S-directed subduction. Azimuthal variations of the first-break velocities as well as observations of shear wave splitting reveal strong anisotropy in the Tauern Window. We explain this finding by foliations and laminations generated by lateral extrusion. Based on the P-wave model we also localized almost 100 local earthquakes recorded during the 2-month acquisition campaign in 1999. Seismicity patterns in the North seem related to the Inn valley shear zone, and to thrusting of Austroalpine units over European basement. The alignment of deep seismicity in the Trento-Vicenza region with the top of the Adriatic lower crust corroborates the suggestion of a deep thrust fault in the Southern Alps.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

Experimental study of the seismic properties of the Eastern Alps (Italy) along the Aurina–Tures–Badia Valleys transect

In order to better constrain the interpretation and the nature of the seismic reflectors, experimental measurements at high confining pressure (up to 300 MPa) and room temperature of the compressional wave velocity (Vp) on 10 samples representative of the most common lithologies along the Aurina (Ahrntal), Tures (Tauferer Tal), and Badia (Abtei Tal) Valleys profile (Eastern Alps, Italy) have been performed. For each sample, the speed of ultrasonic waves was measured in three mutually perpendicular directions, parallel and normal to the rock foliation and lineation.The main results are:(a) Good agreement between the calculated vs. measured modal compositions of the considered rocks, indicating that they were presumably equilibrated at the estimated P–T conditions; therefore, the seismic properties are representative of the crustal level indicated by the thermobarometry.(b) Measured and calculated average Vp are in good agreement, and are typical of mid-crustal level (6.0–6.5 km/s). Only the amphibolites show Vp typical of the lower crust (7.2 km/s).(c) The seismic anisotropy of metapelites is very high (12–27%), both with orthorhombic and transverse isotropy symmetry; amphibolites are transversely isotropic with an anisotropy of 8%; orthogneisses and granitoids are isotropic or weakly anisotropic.(d) The contacts between amphibolites and all other rock types may generate good reflections, provided they are not steeply inclined. Although the metamorphic foliation remains steeply inclined, discordant buried sub-horizontal igneous contacts may be detected.     

Late Neogene–Quaternary evolution of the intermontane Clusone Basin (Southern Alps,Italy): integration of seismic and geological data

In the Clusone Basin (a large intermontane basin filled by thick late Neogene–Quaternary sediments in the Middle Val Seriana, Southern Alps), two high‐resolution seismic profiles have been acquired in order to reconstruct the geometries of the sediments that fill the depression, with a maximum thickness of more than 200 m as documented by available well data, and to define their relationships with the bedrock, consisting of Late Triassic carbonates. In addition to standard seismic reflection processing, a seismic refraction inversion technique has been applied. The integration of geological (both surface and well data) and seismic data indicates a complex history of the drainage patterns of the Clusone Basin, documenting a shift of the Serio River from a palaeodrainage toward the southeast (Val Borlezza) to the present situation, toward the south (Val Seriana): between the older and the present‐day drainages an important depositional stage occurred, as documented by the thick sediments that fill the Clusone Basin, controlling the capture of the Serio River along the Val Seriana. Copyright © 2004 John Wiley & Sons, Ltd.