Abstract
The Mogi area, northeast of the Nomo Peninsula, Nagasaki Prefecture, exposes the Nagasaki metamorphic rocks of high P/T type. It is divided into two geologic units by the newly detected Mogi thrust. The thrust trends N 22° W and dips 32° W. Its hanging wall, which is named Unit I, consists mostly of pelitic and basic phyllites grading into schists, accompanied by small amounts of metagabbroic and metagranitic rocks. The footwall, which is named Unit II, is typified by a dominance of pelitic and basic spotted schists containing porphyroblastic albite. Mineral assemblages in Unit I indicate the pumpellyiteactinolite facies passing into the chlorite zone of the greenschist facies and/or glaucophane schist facies, whereas those in Unit II indicate biotite zone of the greenschist facies and/or glaucophane schist facies. Carbonaceous material in pelitic metamorphic rocks gives broad spireshaped X-ray powder diffraction peaks with d002 = 3.550d—3.414 Å in Unit I, and sharp peaks with d002 = 3.3619—3.3569 Å indicating fully ordered graphite in Unit II. Remarkably large gaps are revealed between the two units in the mineral assemblages as well as in carbon d002 data. Phengite K-Ar ages from pelitic metamorphic rocks also indicate a large gap between the two units; the ages for 6 separates from Unit I give 214—162 Ma (Late Triassic to Middle Jurassic), and those of 4 separates from Unit II cluster at 87—86 Ma (Late Cretaceous). It is concluded that the Mogi thrust is a northeast extension of the Wakimisaki—Fukabori thrust, and hence Units I and II are assigned to the Suo belt in the Inner Zone and the Sanbagawa belt in the Outer Zone of Southwest Japan respectively. The Mogi and Wakimisaki—Fukabori thrusts are collectively named the Nomo tectonic line. This probably corresponds to a part of the paleo-Median Tectonic Line.
