Late Eocene shoreline volcanism along the continental margin:the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

Abstract

The Oga Peninsula is situated on the Japan Sea side of the NE Japan Arc (Fig. 1). A Cenozoic succession is well exposed along the coast and has been referred to as a type of Cenozoic succession of northern Japan since Huzioka (1959) established the stratigraphic framework of the succession. Volcanic rocks are extensively exposed in the western part of the peninsula and occupy the lower part of the Cenozoic succession (Huzioka, 1959, 1973). They are believed to represent the initial process of the Japan Sea opening that prevailed during the Early Miocene time, but their volcanological and tectonic aspects remain poorly understood (e.g. Kano et al., 2007a,b, 2008; Kobayashi et al., 2008). This paper describes the occurrence of the volcanic rocks which are exposed at Kabuki Iwa (Rock) near the northern tip of the peninsula (Fig.1), in order to reveal volcanic and associated tectonic activities at that time. The lower volcanic succession in the western Oga Peninsula is divided into the Akashima Formation, Monzen Formation, and Nomuragawa Formation in ascending stratigraphic order and is unconformably overlain by the Middle to Late Miocene marine sediments of the Nishikurosawa and Onnagawa Formations (Figs. 1 and 2). The Nomuragawa Formation is a stratigraphic unit newly identified by Kobayashi et al. (2004). This formation comprises mainly dacite pyroclastic flows, basaltic andesite lava flows and scoriaceous deposits and is correlative with the main part of the Daijima Formation distributed on the immediate eastern side (Fig. 2). The Daijima Formation is accompanied by non-marine clastic sediments at its uppermost part, but radiometric dates indicate both the Nomuragawa and Daijima Formations are Early Miocene in age and mutually correlative (Kobayashi et al., 2004). The Monzen Formation is a mafic to felsic volcanic complex emplaced in a transition area from land to shallow water (Kobayashi et al., 2008) and can be divided into eight units (Fig. 2). Radiometric ages reported for this volcanic succession have been diverse but could span a few million years of Late Eocene to Early Oligocene time (Kano et al., 2007b, 2008). There are many contemporaneous dikes extending mostly in a NE-SW direction, as shown by the geological maps (Huzioka, 1959, 1973), perhaps reflecting an NW-SE extensional stress field of that time (Yamamoto, 1991). The upper most subaqueous rhyolite lava dome of Shinzan Rhyolite is over hundreds of meters thick and suggests the increase of crustal rifting. On the other hand, the overlying Nomuragawa Formation and Daijima Formation are characterized by bimodal volcanism and on-land eruptions. Shallow-water sediments of the uppermost Daijima Formation are likely to record the succeeding subsidence of this area in association with Late Eocene shoreline volcanism along the continental margin: the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan