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Discordant Contrasts of P- and S-Wave Speeds Across the 660-km
Discontinuity beneath Tibet: A Case for Hydrous Remnant of Sub-
Continental Lithosphere
Tai-Lin Tseng (University of Illinois, Urbana-Champaign), Wang-Ping Chen (University of Illinois, Urbana-Champaign)
Using high-resolution, triplicate shear-waveforms recorded by broadband seismic arrays, we show that a corresponding
anomaly of high S-wave speed (VS) is absent where an anomaly of high P-wave speed (VP) was recently recognized in the transition
zone of the mantle (TZ) beneath central Tibet. A likely cause of the discrepancy between anomalies in VP and VS is a
minor amount of water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent-continent collision, the
Tibetan mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. Hydration of the subcontinental
lithospheric mantle (SCLM) provides a natural mechanism to facilitate ductile deformation, so rapid thickening of
the SCLM, which would have been hindered by advection of cold materials, can take place. Convective instability would then
lead to removal and sinking of thickened, cold SCLM, leaving a remnant in the TZ detectable only in VP. This interpretation not
only is consistent with previous reconstruction of the SCLM in Tibet based on several types of independent data, but also provides
a new pathway for water to enter and be stored in the TZ.
References
Chen, W.-P., and T.-L. Tseng, Small 660-km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere, J. Geophys.
Res., 112, B05309, doi:10.1029/2006JB004607, 2007.
Tseng, T.-L., and W.-P. Chen, Discordant contrasts of P- and S-wave speeds across the 660-km discontinuity beneath Tibet: A case for hydrous
remnant of sub-continental lithosphere, Earth Planet. Sci. Lett. , 268, 450-462, 2008.
Chen, W.-P., M. Martin, T.-L. Tseng, R. L. Nowack, S.-H. Hung, and B.-S. Huang, Shear-wave birefringence and current configuration of converging
lithosphere under Tibet, Earth Planet. Sci. Lett., 295(1-2), 297-304, doi:10.1016/j.epsl.2010.04.017, 2010.
Acknowledgements: Data centers of the IRIS (USA), the Institute of Earth Sciences, Academia Sinica (Taiwan, R. O. C.), and GEOSCOPE
(France) kindly provided seismograms to us. We benefited from discussions with C.-T. Lee, P. Kapp, S.-L. Chung, J. Wang, J. Bass, J.
Smyth, and S. Jacobsen; and an anonymous reviewer provided insightful comments. This work is supported by the U. S. National Science
Foundation Grants EAR9909362 (Project Hi-CLIMB: An Integrated Study of the Himalayan-Tibetan Continental Lithosphere during
Mountain Building, contribution I07) and EAR0551995. Any opinions, findings and conclusions or recommendations expressed in this
material are those of the authors and do not necessarily reflect those of the NSF.
A schematic cross-section (Chen
et al., 2010) showing the current
configuration of the sub-continental
lithospheric mantle,
including a large-scale anomaly
of high P-wave speed resting
on top of the lower mantle
and interpreted as the remnant
of thickened (and subsequently
detached due to Rayleigh-Taylor
instability) lithospheric mantle
(Chen and Tseng, 2007). This
anomaly is invisible to S-waves,
indicating the presence of water
deep beneath an active zone
of continental collision. The
dimension of the detached
lithosphere shown is a minimal
estimate, limited by current
coverage of seismic arrays
around Tibet. For reference, we
also show key geologic units/
boundaries and topography at
the present (vertically exaggerated
by a factor of 13.15).
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-223