Title

Volcaniclastic sedimentation and volcanism during the rifting of western pacific backarc basins

Document Type

Article

Publication Date

1-1-1995

Abstract

© 1995 by the American Geophysical Union. The episodic rifting of both continental and oceanic volcanic arc complexes to form marginal basins has characterized the tectonic evolution of the western Pacific since the Eocene. The sedimentary sequences that have accumulated within the backarc and forearc basins provide a record of the nature, volume, and chemistry of volcanism in different parts of the arc system during the pre-rift, syn-rift, and spreading stages of basin opening. The location of rifting relative to the active volcanic front is- a significant factor in governing the nature of basin opening, which is therefore not uniform between different arcs. High silica volcanism in the Japan, Izu-Bonin, Mariana and Tonga arcs reached a peak of intensity during earliest rifting and was then followed by a brief (approximately 1 m.y.) hiatus during creation of the earliest backarc crust in the Sumisu Rift, Sulu Sea, Lau Basin, and South Fiji Basin. Elsewhere volcanism in the Japan and Ryuku arcs continued through the initial breakup period. Eruption of MORB-like basalts marked the early rifting stage in the Sumisu Rift, Japan Basin, Sulu Sea, and Lau Basin, while the adjacent arc produced voluminous acidic and andesitic volcanics. However, after rifting of the Parece Vela, Shikoku, and South Fiji Basins, arc volcanism declined markedly. In the Lau Basin the early rift stage was prolonged and volcanism on the remnant arc was modest. Volcanoes within the extending arc crust were the focus of explosive volcanism. A new arc, the Tofua arc, formed as propagating spreading centers developed during the transition to the spreading stage. However, in most cases the rift stage did not involve voluminous volcanism in the basin itself. In the Ryuku arc volcanism continued through the rifting of the Okinawa Trough, but retreated as propagating spreading ridges developed. Trace and rare earth element data from the Tonga and Izu-Bonin arcs show trends to increasing depletion of incompatible elements prior to rifting of the Lau and Shikoku Basins and a subsequent enrichment with time after activity resumed on the arc. These trends are principally the result of arc lithospheric thickness controlling the degree of partial melting. Volcanism in the basin is generally more enriched than that on the volcanic front, possibly a result of melt extraction along backarc spreading ridges. Pb, Sr, Nd isotopes from these basins also show the strong heterogeneity between early backarc basin and arc mantle sources, as well as temporal changes in the mantle source under the arc caused by roll back of the subducting plate.

Publication Source (Journal or Book title)

Geophysical Monograph Series

First Page

67

Last Page

96

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