HR: 14:35h
AN: T23G-04 INVITED    [Abstracts]
TI: Discordant anomalies of P- and S-wave speeds under Tibet: a case for hydrous remnant of sub-continental lithospheric mantle
AU: * Tseng, T
EM: tseng1@uiuc.edu
AF: University of Illinois, Dept. of Geology 245 Natural History Bldg. 1301 W Green Street, Urbana, IL 61801, United States
AB: Using high-resolution, triplicate shear-waveforms recorded by broadband seismic arrays, I show that a corresponding anomaly of high S-wave speed ( V_S) is absent where an anomaly of high P-wave speed ( V_P) was recently recognized in the transition zone of the mantle (TZ) beneath central Tibet. A likely cause of the discrepancy between anomalies in V_P and V_S is a minor amount of water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent-continent collision, the Tibetan lithospheric mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. Hydration of the sub-continental 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 more easily. Convective instability would then lead to removal and sinking of thickened, cold SCLM, leaving a remnant in the TZ detectable only in V_P. 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.
DE: 1038 Mantle processes (3621)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting