Understanding Extension within a Convergent Orogen: Lithospheric Structure of the Pannonian Basin
SEIS-UK provided a loan of 51 broadband seismic systems (5 x CMG3T and 46 x CMG6TD) for periods of between 14 and 24 months, for deployment in an array that extended more than 500 km across Austria, Hungary and Serbia, from September 2005 until August 2007. These systems recorded continuously (in most cases for a period of 16 months at 100 samples per second), with relatively minor data losses due to equipment problems. The dataset thus collected has been archived with IRIS (Incorporated Research Institutions for Seismology) and has been the central element of a major NERC funded research project based at the University of Leeds: "Understanding Extension within a Convergent Orogen: Lithospheric Structure of the Pannonian Basin" (NE/C004574/1). Records of teleseisms have been processed to deliver P-wave and S-wave velocity models for the upper mantle beneath the Pannonian Basin. Receiver functions have also been calculated to reveal the structure of the upper mantle discontinuities, and fast anisotropy directions obtained from SKS wave arrivals to reveal the signature of lithospheric deformation. Further processing for surface wave tomography using ambient noise correlation techniques is in progress and was presented at Fall AGU 2010. A PhD by Ben Dando, based mainly on the teleseismic tomography, has recently been successfully examined (Jan. 2011). The results show clearly (and unexpectedly) a seismically fast structure reaching underneath the Pannonian Basin, extending out eastward from the present-day eastern Alps, and extending down into the Mantle Transition zone (between 410 and 660 km depth) where it has spread out to cover an area at least as big as the Pannonian Basin. This fast material is interpreted as the signature of mantle downwelling associated with Miocene extension of the Pannonian Basin lithosphere. Another important result from our seismic experiment is that the seismic discontinuity at 660 km depth is depressed by as much as 40 km under a large part of the Pannonian Basin. Such a widespread depression of the 660 km seismic discontinuity has not previously been seen beneath a major sedimentary basin, but the observation is fully consistent with the presence of the fast material revealed by the teleseismic tomography.