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A discussion on “The 410‐km‐depth discontinuity: A sharpness estimate from near‐critical reflections” by Vidale et al.
Author(s) -
Krishna V. G.,
Ramesh D. S.
Publication year - 1996
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/96gl02189
Subject(s) - classification of discontinuities , geology , discontinuity (linguistics) , transition zone , core–mantle boundary , mantle (geology) , low velocity zone , seismology , seismic wave , p wave , geophysics , seismic velocity , lithosphere , medicine , cardiology , atrial fibrillation , mathematical analysis , mathematics , tectonics
Vidale et al. [1995] presented a convincing evidence of a fairly sharp ‘410‐km’ seismic discontinuity uncjer the western United States by examining clear noise‐free reflected arrivals from this discontinuity in the near‐critical distance range of 11.5° to 14° (Fig. Ia). They reconstructed the P velocity‐depth model from the arrival times of the 410‐km reflection and initial P waves across the distance range 8°–14°. The prominent features of the upper mantle P velocity model given by them include; a low velocity layer (LVL) in the depth range of about 110–200 km with a broad transitional bottom, a fairly large velocity gradient (0.4 km/s velocity increase from 200 to 410 km depth) above the 410‐km discontinuity which is sharp and associated with a 5.5% velocity increase. Seismic estimates of sharpness as well as velocity contrast at the ‘410‐km’ and ‘660‐km’ discontinuities in the mantle transition zone provide important constraints to plausible models of composition, temperature and convective processes in the deep earth. The rapid increases of seismic wave velocities inferred in the transition zone, at depths of 410‐Km and 660‐km, are classically associated [Ringwood, 1975] with the pressure‐induced phase transitions in Olivine which is considered to be the predominant mineral in the upper mantle. Recent observations of high quality coherent seismic reflections from the transition zone [Benz and Vidale, 1993; Vidale et al., 1995] conclusively reveal that the 410‐km discontinuity is fairly sharp. Contrastingly the experimental results from mineral physics [Akaogi et al., 1989; Bina and Wood, 1987; Katsura and Ito, 1989] predict a substantially larger transition width, on the order of 6–19 km for the olivine‐beta spinel phase transition at pressures corresponding to 410‐km‐depth discontinuity. Similar inconsistencies in the magnitude of velocity contrast at the 410‐km discontinuity are also found [Duffy et al., 1995; Jeanloz, 1995] by comparison of results from modern seismic measurements and the new experimental models of the olivine‐beta spinel phase transition, the latter revealing a relatively large contrast. In view of these apparently contradicting results it is certainly critical to evaluate the high quality seismic data [Vidale et al., 1995] for reliable estimates of both sharpness and velocity contrast of the 410‐km discontinuity in order to consider its traditional association with the simple phase transition in the olivine dominated compositional model.