Abstract
Miocene fromations widely overlie the Mesozoic formation, ultramafic rocks and Cretaceous granite in the Minakami area, northern part of Gunma Prefecture. This series is divided into the following six formations, in ascending order: Okura Formation (300 m in thickness), Awazawa Formation (550 m), Gokan Formation (260-2600 m), AkayaFormation (200 m+), Mitsumineyama Formation (350 m) and Tone Welded Tuff (200 m). The Gokan Formation unconformably overlies the underlying formations. The Mitsumineyama Formation and Tone Welded Tuff unconformably cover the Gokan and the Akaya Formations. Late Cenozoic tectonic evolution in this area is summarized as follows. In the Early Miocene, a large amount of basalt erupted on land. Many dykes of basalt show NE-SW trendingσ_<H-max>. In the earliest Middle Miocene, felsic pyroclastics deposited in shallow seas. The feeder dykes of the andestie-rhyolite show NE-SW trending σ_<H-max>. In the early Middle Miocene (15-14 Ma), mud deposited in lower middle bathyal environment. In the latest Middle Miocene (11 Ma), dacitic pyroclastics deposited on land and no more major geologic deformation have not occurred since then.
