Abstract
The Oiso Hill is an emerged northwestern tip of extensional blocks called the Okinoyama Bank Chain on the landward slope of the Sagami Trough. The Neogene marine clastics in this area developed in two distinctive depositional stages separated by hiatus between Middle and Late Miocene. Each stage is represented by specific combination of depositional systems under an extensional (transtensional) tectonic stress field.
Middle Miocene (Komayama Group): Minor alkali-olivine basalt effused on the oceanic basin-slope under the ESE-WNW extensional (transtensional) tectonic stress field. Abundant dacitic pyroclastic flows were simultaneously debouched from submarine volcanoes. General lack of the terrigenous sediments indicates a far distal oceanic position from the subduction zone or low relief of the northern source area.
Late Miocene (Miura Group): Doming intrusion and eruption of andesitic magma took place and gave rise to two small volcanic cones on the landward slope under the E-W extensional (transtensional) tectonic stress field after a jump of the subduction zone to the south of this area during the time represented by the hiatus. A submarine canyon developed along the NE-SW trending normal faulting. The Middle Miocene oceanic basin-slope deposits were then uplifted along the southeastern wall of the fault to be a submarine swell-like horst. Recessions around the submarine volcanoes and the submarine canyon were eventually burried by gravelly submarine fan channel deposits prograded out from the rising northern source area.
The structural framework for evolution of the Upper Miocene Miura Group is largely similar to that of the present Okinoyama Bank Chain. The Okinoyama Bank Chain, therefore,can be interpreted as a relic of landward slope extensional structure initiated during Late Miocene which, however, has not yet been burried by abundant terrigenous clastics from the northern highlands.
