HR: 0800h
AN: DI51A-0286 INVITED    [Abstracts]
TI: Aseismic anomalies in the mantle transition zone: subduction versus continental collision
AU: Tseng, T
EM: tseng1@uiuc.edu
AF: University of Illinois, Department of Geology, 1301 W Green St, Urbana, IL 61801, United States
AU: * Chen, W
EM: wpchen@uiuc.edu
AF: University of Illinois, Department of Geology, 1301 W Green St, Urbana, IL 61801, United States
AU: Green, H W
EM: hgreen@ucrac1.ucr.edu
AF: University of California, Institute of Geophysics and Planetary Physics, Riverside, CA 92521, United States
AB: Deep earthquakes are always associated with zones of recent convergence where cold temperature is expected in the mantle. Since low temperatures will raise both P- and S-wave speeds ( V_P and V_S), aseismic anomalies of high V in the transition zone of the mantle (TZ) provide unique insights into the origin of deep seismicity: Low temperature is only one necessary factor in causing deep earthquakes. Along the Tonga subduction zone, where over 1/3 of all deep earthquakes occur, a collection of complementary evidence indicates that a petrologic anomaly must be present to counteract the effect of very low temperature associated with extremely fast subduction of cold lithosphere: High V_P and V_S are conspicuously absent in the source zone of outboard earthquakes that extends about 1000 km farther to the west of the Wadati-Benioff zone. Meanwhile, preferred alignment of highly anisotropic material such as metastable olivine must be present in the source zone to account for over 1% of polarization anisotropy in the TZ. Moreover, the occurrence of large earthquakes in sub-horizontal slabs where a regional pattern of strain is generally absent calls for a localized source of stress, such as stress concentration associated with transformation-induced faulting, in the nucleation of deep earthquakes. Over time, petrologic anomaly due to metastable olivine would gradually dissipate with rising temperature and only a pure thermal anomaly of modest amplitude is expected to remain. This is precisely the case as an aureole of isotropic anomaly, characterized by modest-raised high V_P and V_S, is observed just outside of outboard earthquakes beneath Tonga-Fiji. A similar case is observed in the TZ beneath the Northern Philippine Sea, apparently the western extension of the Izu-Bonin subduction zone. A contrasting example is the absence of high V_S where a large-scale anomaly of high V_P was recently recognized in the 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 collision, the Tibetan lithospheric mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. The aseismic nature of the Tibetan anomaly is consistent with the fact that a small amount of water is likely to eliminate any metastable olivine. Furthermore, convective removal of thickened sub-continental lithospheric mantle is potentially a new pathway for water to enter the TZ.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8108 Continental tectonics: compressional
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting