[Back]

T. Öberseder, M. Behm, I. Kovacs, G. Falus:
"A seismic discontinuity in the upper mantle between the Eastern Alps and the Western Carpathians: Constraints from wide angle reflections and geological implications";
Tectonophysics, 504 (2011), 122 - 134.

Seismic investigation of the lithosphere by means of active source experiments is mostly confined to the crust
and the Moho. Structures in the upper mantle are more likely to be discovered by analyses of teleseismic data,
although these methods are restricted in their resolution capabilities. The relatively rare evidence for upper
mantle refractors or reflectors in active source data enables challenging and interesting studies of the lower
and not so well known part of the lithosphere. We present such an example from the tectonically complex
region between the Eastern Alps and the Western Carpathians. This area was covered by several extensive 3D
wide-angle reflection/refraction experiments within the last decade, and their layout was designed to
illuminate the crustal structure and in particular the Moho discontinuity. In some areas, reflections from
below the Moho are also recorded. These reflections occur at recording offsets between 200 and 500 km, and
they are particularly strong in cross line recordings. We derive a set of travel times from the data and perform
a tomographic inversion for the depth and shape of the reflecting interface. The inversion makes use of an
existing 3D crustal model which also includes the Moho topography. Since the upper mantle velocities are
poorly constrained and the azimuthal distribution of the rays is biassed, several tests are applied to investigate
the reliability of possible solutions. The results from the tomographic inversion indicate an overall horizontal
and radially dipping reflector. The average depth of the reflector is 55 km, which is about 25 km below the
crust-mantle transition, and amplitude modelling suggests that the reflecting interface represents a velocity
increase. The investigated area is further characterised by deep sedimentary basins, high heat flow, high
velocities in the lower crust, diffuse Moho signature and a strong positive Bouguer anomaly. Nearby xenolith
outcrops exhibit a pronounced change in anisotropy and indicate the presence of two distinct layers in the
lithospheric mantle, whereas the deeper layer is thought to present more juvenile lithosphere derived from
thermal relaxation in the post-extension phase. Most likely the upper mantle reflector also represents this
change in anisotropy, though other scenarios are also possible. We conclude that the entire lithosphere is
significantly shaped by extensional processes which affect the area since the late Oligocene/early Miocene.

Project Head Ewald Brückl:
ALP 2002 - Ein seismisches 3D-Modell der Ostalpen

Project Head Ewald Brückl:
ALPASS - Seismisches Monitoring der Lithosphäre und des oberen Mantels in den Alpen 2. bis incl. 4. Projektjahr