Late Cenozoic volcanic activity in the Chugoku area, southwest Japan arc during back‐arc basin opening and reinitiation of subduction

Abstract

Temporal–spatial variations in Late Cenozoic volcanic activity in the Chugoku area, southwest Japan, have been examined based on 108 newly obtained K–Ar ages. Lava samples were collected from eight Quaternary volcanic provinces (Daisen, Hiruzen, Yokota, Daikonjima, Sambe, Ooe–Takayama, Abu and Oki) and a Tertiary volcanic cluster (Kibi Province) to cover almost all geological units in the province. Including published age data, a total of 442 Cenozoic radiometric ages are now available. Across‐arc volcanic activity in an area approximately 500 km long and 150 km wide can be examined over 26 million years. The period corresponds to syn‐ and post‐back‐arc basin opening stages of the island arc. Volcanic activity began in the central part of the rear‐arc ca 26 Ma. This was followed by arc‐wide expansion at 20 Ma by eruption at two rear‐arc centers located at the eastern and western ends. Expansion to the fore‐arc occurred between 20 and 12 Ma. This Tertiary volcanic arc was maintained until 4 Ma with predominant alkali basalt centers. The foremost‐arc zone activity ceased at 4 Ma, followed by quiescence over the whole arc between 4 and 3 Ma. Volcanic activity resumed at 3 Ma, covering the entire rear‐arc area, and continued until the present to form a Quaternary volcanic arc. Adakitic dacite first occurred at 1.7 Ma in the middle of the arc, and spread out in the center part of the Quaternary volcanic arc. Alkali basalt activities ceased in the area where adakite volcanism occurred. Fore‐arc expansion of the volcanic arc could be related to the upwelling and expansion of the asthenosphere, which caused opening of the Japan Sea. Narrowing of the volcanic zone could have been caused by progressive Philippine Sea Plate subduction. Deeper penetration could have caused melting of the slab and resulted in adakites. Volcanic history in the Late Cenozoic was probably controlled by the history of evolution of the upper mantle structure, coinciding with back‐arc basin opening and subsequent reinitiation of subduction.

Marine tephra in the Japan Sea sediments as a tool for paleoceanography and paleoclimatology

“…Therefore, the spatiotemporal variability in tephra characteristics can reflect the magmatic evolution of volcanic islands and island arcs. For example, several researchers have inferred volcanic-tectonic relationships from the spatiotemporal changes in the geochemistry of the Neogene to the Quaternary volcanic rocks (Kimura et al 2003(Kimura et al , 2005Yoshida et al 2014). Moreover, because of the high degree of preservation of volcanic glass in deep-sea sediments, the marine tephras from long ODP/IODP cores are potentially amenable to similar analysis.…”

Electromagnetic View of the Seismogenic Zones Beneath Island Arcs

“…2. Not only the structure but also the source of volcanism seems 3-D with its peculiar distribution of volcanoes and multiple sources for the magma production (e.g., Iwamori, 1991;Kimura et al, 2003). 3.…”

“…Most of the focal depths there are shallower than approximately 10km and thus the earthquakes are occurring in the upper crust. In the seismic belt, several large earthquakes of M6.2-7.4 also occurred in 1943, 1983 and 2000, respectively. Furthermore, quaternary volcanoes, such as the Daisen and Oginosen Volcanoes are located in the seismic belt while the basalt that formed the Oki Islands in the back-arc region is much older (> 5 Myr) and of different composition (Kimura et al, 2003). Wide-band MT observations have been made along a number of north-south profiles on land since 1998 so as to reveal high conductivity regions beneath the seismic belt on each MT profile.…”

“…It stems from the deep mantle behind the mature island arc. On the contrary, that of southwest Japan cannot be presumed as simple as northeast Japan, if one studies basalt samples of this area (Iwamori, 1991;Kimura et al, 2003). The alkaline, sub-alkaline and adakite basalt magmas of southwest Japan imply multiple sources for its magma production in the mantle including slab-melting of the hot and young Philippine Sea plate.…”

Petrography and bulk rock composition of the Wakurayama Dacite from Matsue City, Shimane Prefecture, Southwest Japan

“…(b) Age compilation of Cenozoic alkali-basalts and adakites in the San-in zone, Southwest Japan (modified from Kimura et al, 2003).…”