Melting features along the Ryukyu slab tear, beneath the southwestern Okinawa Trough

Abstract

The present‐day active volcanic front associated with the Ryukyu subduction zone extends from Japan to the Ilan plain (northern Taiwan) and is located within the Okinawa Trough, 80–100 km above the Ryukyu slab. An abnormal amount of arc volcanism, which consists of basalt, andesite and rhyolite occurs within the southwestern Okinawa Trough, above a slab tear of the Ryukyu subduction zone (CBVT). The power spectrum analysis of magnetic data shows the occurrence of a thin crust above the slab tear and a thick crust beneath this volcanic area. We suggest that an excess of H2O‐rich fluid might occur at the slab tear and might increase the melt flux. Both are conveyed obliquely to the uppermost mantle and lower crust CBVT magmas. After interactions, basaltic magmas would rise up, accounting for the contrast of magnetization between this volcanic body and the adjacent OT crust.

Some first-order observations on magma transfer from mantle wedge to upper crust at volcanic arcs

“…Thus, the following arcs are classified as irregular on basis of slab discontinuities: CAS, where adakite-type volcanism and high surface heat flux are observed in the south at Lassen and Mount Shasta, east of the Mendocino triple junction and above the southern edge of the subducting Juan de Fuca plate (Baker et al 1994;Borg et al 1997); MEX, where slab detachment is evidenced by a lack of seismicity directly beneath the arc, and where eastward propagating late Miocene OIB-type and adakitic volcanism occurred due to asthenospheric mantle upwelling (Ferrari 2004), consistent with the present-day surface heat flux increase towards the west; NCH, where ongoing work indicates slab tearing as far south at 218S (Rietbrock et al 2006); NEJ, where a slab window in the Philippine Sea plate widens toward the backarc north of Mount Fuji (Ishida 1992;Mazzotti et al 1999), and where an extensive slab crack beneath the Hokkaido corner (Katsumata et al 2003) widens towards the backarc (Lundgren & Giardini 1990), consistent with elevated surface heat flux in both the central Honshu backarc and northern Honshu; and SWJ, where the slab ruptures between Honshu and Kyoshu (Zhao et al 2002) with slab melting proposed to result in adakitetype volcanism in SW Honshu (Morris 1995;Kimura et al 2005) and in NE Kyushu (Sugimoto et al 2006). Slab windows and tears have also been identified in the westernmost Aleutians (Yogodzinski et al 2001), south of the Central American volcanic front in southeastern Costa Rica and Panama (Johnston & Thorkelson 1997), beneath Tierra del Fuego north and east of the Austral Volcanic Zone (Gorring & Kay 2001), and in the southern Ryukyu arc (Lin et al 2004). However, due to the low number of Holocene effusive eruptions, these arc segments are not part of this study; although Shiveluch in northernmost KAM erupts adakitetype lavas related to the western Aleutian slab window (Yogodzinski et al 2001), this does not affect the average character of this arc with about 50 Holocene volcanoes, and KAM is therefore defined here as a regular arc.…”

Hydrothermal Activity in the Okinawa Trough Back-Arc Basin: Geological Background and Hydrothermal Mineralization

“…Chronological variations of the volcanism suggest complex backgrounds related to the collision in Taiwan (Shinjo 1999;Shinjo et al 1999). Lin et al (2004) inferred that a slab window originated from the subducting Gagua Ridge on the Philippine Sea Plate was the cause of such volcanism. Possible signs of magma chambers at the lower crust beneath the CBVT have been observed ).…”

Origin of the southern Okinawa Trough volcanism from detailed seismic tomography

“…In addition, a feeding channel starting from the slab border and bending in direction of the andesitic Kueishantao Island was imaged. On the basis of the inversion of magnetic data and the geochemistry of dredged rocks [ Shinjo et al , 2003a, 2003b] in the CBVT area, a similar mechanism was proposed for the emplacement of the CBVT with a feeding origin coming from the slab tear located along the 123.3°E meridian [ Lin et al , 2004a]. The goal of this study is to image the crust and mantle beneath the southern OT by using a new set of earthquake data in order to better understand the relationship between the different types of volcanism and the underlying slab and slab tear as well as to understand the driving forces acting in this region.…”