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oai:www.ideals.illinois.edu:2142/86521 2023-07-11 col_2142_5131 col_2142_16322 com_2142_5130 com_2142_16321 com_2142_13459 com_2142_8903

Chen, Wang-Ping Brudzinski, Michael Robert 2015-09-28T15:12:27Z 2015-09-28T15:12:27Z 10000-01-01 2002 2002 Using high-resolution, triplicate body waveforms, I investigate strong lateral variations in P- and SH-wave speeds (VP and VS) beneath the Tonga subduction zone where cold lithosphere is subducting rapidly at 200 mn/yr. Under these conditions, the lack of prominent anomalies of high VP and VS in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle. This inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to one thousand kilometers away from the Wadati-Benioff zone. Beneath the back-arc, I find a sharp, east-west trending demarcation north of which outboard earthquakes occur and VP and VS abruptly drop by about 3%. Thus the effect of cold temperature to raise VP and VS, as indicated by the presence of outboard earthquakes, must be counteracted by petrologic variations. I interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes. Considering the density of these candidates and the strong effect of partial melt on VS, volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in V P and VS. Meanwhile, metastable olivine remains a viable candidate. As a cold slab passes below the 410-km discontinuity, olivine becomes metastable and makes the tip of the slab buoyant. Such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone. Made available in DSpace on 2015-09-28T15:12:27Z (GMT). No. of bitstreams: 2 license.txt: 4848 bytes, checksum: 96035ab3f5e1c23cc7138a224ce498bd (MD5) 3070267.pdf: 6082404 bytes, checksum: d6be963914356c1b4a4d6bfc846fa392 (MD5) Previous issue date: 2002 Embargo set by: Seth Robbins for item 87802 Lift date: Forever Reason: Restricted to the U of I community idenfinitely during batch ingest of legacy ETDs Restricted to the U of I community idenfinitely during batch ingest of legacy ETDs U of I Only 119 p. Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2002. http://hdl.handle.net/2142/86521 (MiAaPQ)AAI3070267 eng Geophysics Seismic Studies of Subducted Lithosphere Beneath Fiji: Evidence for a Petrologic Anomaly text Geology Geology University of Illinois at Urbana-Champaign Dissertation Ph.D.