Proximal–distal fall deposit correlation of VEI-5 tephra (Towada-Chuseri) from Towada volcano,northeast Japan

A tephra layer offers an isochronous surface in sediments, thus serving as a key bed and/or an age marker. Recent high-resolution sediment research (e.g. varved sediments) has revealed optically invisible tephra fingerprints and provided high-precision tephra ages. However, a tephra-based correlation cannot succeed without detailed knowledge of the tephra characteristics in a proximal area to correlate with tephra in high-resolution sediments in remote areas. Here we documented the detailed characteristics of Towada-Chuseri (To-Cu) tephra, which is associated with the Middle Holocene volcanic explosivity index 5 eruption of Towada volcano, northeast Japan. We used To-Cu tephra samples to achieve the proximal–distal correlation of three members: Chuseri pumice (Cu), Kanegasawa pumice (Kn) and Utarube ash (Ut). These distal occurrences correlate with proximal To-Cu tephra based on volcanic glass morphology and refractive index, as well as on major element composition of volcanic glass shards. Refractive indices allow the preliminary correlation of each member, and major element composition helps in distinguishing Ut from the other members. Glass morphology provides additional support. These correlations reveal that To-Cu, especially Cu, covered central to northeast Japan while confirming that To-Cu is the representative tephra in the Middle Holocene of the Tohoku region.     

Orthopyroxene–cordierite mafic gneiss from the Nomamisaki metamorphic rocks,Southern Kyushu,Japan: Implication for western continuation of the Usuki–Yatsushiro Tectonic Line

We describe an orthopyroxene–cordierite mafic gneiss from the Nomamisaki metamorphic rocks in the Noma Peninsula, southern Kyushu, Japan. The mineral assemblage of the gneiss is orthopyroxene, cordierite, biotite, plagioclase, and ilmenite. Thermometry based on the Fe–Mg exchange reaction between orthopyroxene and biotite yields a peak metamorphic temperature of 680°C. The stability of cordierite relative to garnet, quartz, and sillimanite defines the upper limit of the peak metamorphic pressure as 4.4 kbar. These features indicate that the Nomamisaki metamorphic rocks underwent low‐pressure high‐temperature type metamorphism. Although a chronological problem still remains, the Nomamisaki metamorphic rocks can be regarded as a western continuation of the Higo Belt. The Usuki–Yatsushiro Tectonic Line, which delineates the southern border of the Higo Belt, is therefore located on the east of the Nomamisaki metamorphic rocks in southern Kyushu.