Abstract
The mid-Cretaceous Oku-Niikappu Dam Complex (ONDC: Fig.1) occurs in the Niikappu River area of the Idonnappu Zone (Ueda and Miyashita, 2002, 2003). The ONDC contains a tectonic block with a stratigraphic succession from basal volcanic rocks through alternating conglomerate and chert (middle unit) to an upper chert unit (Figs.2-4). The conglomerate clasts consist dominantly of basaltic to rhyolitic volcanic rocks, with subordinate of ultramafic to felsic plutonic clasts (Figs.3 and 5). The basal volcanics and the volcanic clasts in the conglomerate have chemical characteristics of island arc origin. This succession represents an island arc in which activity ceased, followed by dissection in a pelagic environment, and is suggestive of an intraoceanic remnant arc isolated by back-arc spreading. Similar strata and clastic compositions have been reported from the remnant arcs of the Daito and Kyusyu-Palau Ridges in the Philippine Sea Plate. The occurrence of accreted remnant arcs is significant for considering subduction zone tectonics, because they indicate subduction of oceanic crust associated with island arcs and back-arc basins similar to those of the present-day Western Pacific.
