IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2127
New constraints on structure and dynamics of the lithosphere and
asthenospherefrom global upper-mantle tomography
Dr. Sergei Lebedev
Earth Sciences Utrecht University IASPEI
Rob D Van Der Hilst
We derive structural constraints from our new shear-speed model of the upper mantle. The model is a
result of the application of the Automated Multimode Inversion (AMI) (Lebedev et al. 2005) to a large
global dataset of over 60000 vertical-component seismograms. AMI extracts structural information from
both surface waves and regional S and multiple-S waves and provides resolution of a few hundred km
(varying with data sampling) everywhere the upper mantle (0--660 km). An elaborate filtering,
windowing, and weighting procedure enables highly complete and balanced use of the information
contained in the seismogram regarding Earth structure. Unlike other methods for tomographic mantle
imaging, AMI has also been benchmarked with numerical wave-propagation modelling, both the method
and its underlying assumptions thus validated. We observe that low-Sv-velocity anomalies beneath midocean
ridges and back-arc basins extend down to 100 km depth only; this agrees with estimates of
primary melt production depth ranges there. Seismic lithosphere beneath cratons bottoms at depths up
to 200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South
America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of
these low-velocity zones may be showing hot material - possibly of mantle-plume origin - trapped or
spreading beneath the thick cratonic lithosphere. We suggest that sub-horizontal flow of asthenosphere
from beneath cratons is the immediate cause of the intraplate, hotspot-like volcanism observed near
cratonic boundaries. High-velocity lithosphere is observed beneath the southern part of the Tibetan
Plateau, but beneath its northern part the lithosphere is seismically slow. This is consistent with a large
part of Tibetan lithosphere being weak, suggesting that previously proposed scenarios involving the
dominance (or even formation) of rigid, possibly cratonic lithosphere beneath Tibet are unlikely.
Keywords: craton, hotspot, tibet