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Master of Science (MS)
DeBari, Susan M., 1962-
The focus of volcanic activity in the Izu-Bonin arc has migrated across the arc over time, creating distinct across-arc geochemical regions including the arc front, rift region, and the rear arc seamount chains (RASC). This study challenges the previously held assumptions that the rear arc was inactive after 2.8 Ma and that tephra deposited in the arc younger than 2.8 Ma with K2O > 1 wt.% and La/Yb > 2.2 was sourced from the SW Japan arc. I studied 1.1 - 2.7 Ma tephra retrieved from core from the rear arc at Site U1437, IODP Expedition 350, in order to clearly identify tephra provenance using geochemistry.
Our new dataset includes geochemical data from glass shards for 53 < 2.7 Ma tephra layers from Site U1437 combined with rhyolite dredge samples from the rift and rear arc regions. These data are compared with a previously published dredge sample dataset for the same latitude range (~32°N) and a SW Japan arc tephra dataset to distinguish between three possible tephra sources, 1) volcanic front/rift-related 2) rear arc seamount chain-type, or RASC type, and 3) SW Japan arc. Compared to the volcanic front and rift, the RASC-type signature is more continent-like with a medium to high K2O (> 1 wt.%), high La/Yb (> 2.2.), and lower eNdand eHf. Thorium concentration is distinctly lower in magmas from all regions of the Izu-Bonin arc (< 5 ppm) as compared to the SW Japan arc (> 5 ppm). Additionally, SW Japan arc tephra is distinctively lower in eNd and eHf compared to the RASC-type tephras.
We find that 8 of 130 consecutive tephra intervals in this 1.1 - 2.7 Ma age range are sourced from the SW Japan arc, 36 are RASC-type, interpreted to be sourced from the rear arc, and 86 are arc front or rift. Thus 28% of tephra intervals from 1.1 - 2.7 Ma are newly interpreted as rear-arc derived rather than from the SW Japan arc.
Younger RASC-type tephra show a trend to more rift-related signatures both in trace elements and isotope ratios. This trend, along with interpreted spatial patterns of tephra sources, suggests eastward migration of RASC-type tephra eruptions and concomitant changes in geochemical signature producing a smooth transition to rift-like characteristics (with some exceptions).
Because the rear arc is >1000 mbsl, an important outcome of this study is that these 36 newly identified RASC-type tephra intervals imply previously unrecognized deep-water (>1000 m) silicic explosive eruptions.
Western Washington University
Subject – LCSH
Plate tectonics--Micronesia; Volcanic ash, tuff, etc.--Research
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King, Cassandra, "Reidentifying Sources of Tephra in the Izu-Bonin Arc: Recognizing Recent Rear-arc Volcanism 1.1 – 2.7 Ma" (2021). WWU Graduate School Collection. 1059.