A new type of intra‐plate volcanism; Young alkali‐basalts discovered from the subducting Pacific Plate, Northern Japan Trench

Abstract

Abstract. Alkali pillow basalts were collected from the toe of the oceanward slope of the northern Japan Trench. These alkali-basalts formed as a result of a low degree of partial melting of Pacific Ocean mantle and rapid rise of the magma (no fractionation in shallow magma chambers). Reconstructing Pacific Plate motion based on 40Ar-•9Ar age dates of 5.95_ 0.31 Ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the Japan Trench in the NW Pacific. We suggest that these alkali-basalts represent a new form of intra-plate volcanism, whereby magmatic activity occurs off the forebulge of the downgoing Pacific slab, perhaps using conduits related to fracturing of the slab during bending prior to subduction.

High heat flow anomalies on an old oceanic plate observed seaward of the Japan Trench

“…The initial temperature of the sphere is 1,000 K higher than the surroundings and the thermal diffusivity is 1.0 9 10 -6 m 2 /s Fig. 7 Schematic model of possible processes giving high heat flow on the seaward slope of the Japan Trench (modified from Hirano et al 2001). The crust and uppermost mantle are repeatedly heated by intraplate volcanism.…”

“…2). Hirano et al (2001Hirano et al ( , 2006 sampled alkali-basalt lavas from some of the knolls and obtained very young isotopic ages of 4-8 m.y. Such intra-plate volcanism may also have affected the thermal structure of the old Pacific plate.…”

“…Hirano et al (2001Hirano et al ( , 2006 collected basaltic rocks with radiometric ages ranging from 4.2 to 8.5 m.y. on the seaward slope of the Japan Trench about 70 km north of the 38°45 0 N survey line (box in Fig.…”

“…2. Stars are the sites where young volcanic rocks formed by intra-plate volcanism were sampled (Hirano et al 2001(Hirano et al , 2006. Topographic profile along the line A at 38°45 0 N (modified from Kobayashi 1991) (bottom).…”

Editorial Submarine geomorphology: new views on an ‘unseen’ landscape

“…How the Earth melts beneath the oceans is currently the subject of debate, not insignificantly contributed to by the petit-spot volcanoes first identified by Hirano et al [2001]. These monogenetic edifices seem to be a response to flexure of the lithospheric plate which, significantly, suggests that although volumetrically small sub-lithospheric melting may be ubiquitous.…”