No abstract

A series of outcrops, m long and m high, were exposed during construction of a factory at Sajima, Yokosuka City, Kanagawa Prefecture, central Japan. Before these outcrops were covered with concrete, we described the stratigraphy and structures in detail. The upper part of the Misaki Formation (Aburatsubo Member) of the Miura Group (roughly Ma in age) dips gently northeastward and is m thick. The sequence consists mainly of marine pyroclastic sediments, including mafic scoria, lithic fragments, tuffaceous sandstones, and felsic pumice ash beds, which are centimeters to several meters thick. Several characteristic tephra layers are determined as key beds and numbered from Sj to Sj , in stratigraphically ascending order, among which the tephra key bed Sj is correlated with the previously known key bed So. Several thrust-duplex-like structures were recognized in parts of the outcrop, and we discuss here the formation processes of four representative duplex-like structures. One group of duplex structures developed along thrust faults that are orientated oblique to the bedding and are spatially associated with cataclasites. These duplex structures were tectonically induced and of late-stage origin. The other three duplexlike structures developed along the bedding surfaces and were associated with submarine landslides. These gravitationally induced structures are associated with smaller duplex-like structures and show slump-like folds. The directions of thrusting and submarine sliding are different from the modern attitude of depositional beds in the Aburatsubo Member, and suggest an original transport direction from NW to SE.

丹沢 大磯地域に分布する火山岩類の K Ar 年代と 南部フォッサマグナ地域における鮮新世火山フロントの西方移動 K Ar ages of volcanic rocks in the Tanzawa Oiso area, central Japan: Implication for westward migration of the Pliocene volcanic front in the southern Fossa Magna region 森 慎一(Shin'ichi MORI) 山下 浩之(Hiroyuki YAMASHITA)  有馬 眞(Makoto ARIMA)   藤岡換太郎(Kantaro FUJIOKA)    We determined K Ar ages of the late Miocene to Pliocene andesitic to basaltic dykes and lavas distributed in the Tanzawa Oiso area in the southern Fossa Magna region, central Japan. Our data indicate two distinct periods of volcanic activity in this region; one took place during the middle Miocene (about 15 Ma) and another during the late Miocene to Pliocene (7.5 4.2 Ma). Combined the present results with the age data reported previously from volcanic rocks in the southern Fossa Magna region, the present study suggests that the volcanic front in this region during 8 to 4 Ma was located at about 40 km east of the present-day volcanic front. The volcanic front began to migrate westward to the present-day location at about 4 Ma. The westward migration of volcanic front was probably associated with an abrupt change of lateral motion of the Philippine Sea Plate from north-northwestward to northwestward occurred between 4 and 2 Ma.