Accreted Kula plate fragment at 94Ma in the Yokonami-melange, Shimanto-belt, Shikoku, Japan

The Cretaceous Shimanto belt that is distributed along the Pacific side of southwest Japan contains one of the best records of accretionary processes and history of oceanic plate subduction in the circum-Pacific region. Yokonami melange, Shimanto belt, Shikoku, SW Japan is composed of pillowed basalt that erupted in the equatorial region, about 200. m-thick bedded radiolarian chert sequence that contains radiolarians with age determinations spanning ca. 135. Ma (Upper Valanginian) to 93.9. Ma (Cenomanian), younging southward with repetition, and alternated sandstone and mudstone. Although the chronology of the bedded chert sequence is well studied using the radiolarian, the chronology of the clastic units was not reported sufficiently. We performed a detailed geologic mapping investigation of rock outcrops along the Goshiki-no-Hama Coast, the Yokonami melange. We have determined U-Pb ages of zircon grains extracted from tuffs and sandstones to determine the depositional age of the turbidite and hemipelagic units. The youngest depositional age of these clastic sediments is interpreted to be the arrival time at the trench. We reconstructed the ocean plate stratigraphy with our new chronological data, which includes mid-oceanic basalt, bedded chert, hemipelagic sediments, and trench-turbidite deposits. We recognize that the turbidite sequence has been repeated by layer-parallel thrust in the study area. We have delineated the travel trajectory of the oceanic lithosphere based on (1) the relative plate motion model of the Paleo-Pacific Ocean, (2) the arrival time of the subducted lithosphere at the trench (94.32. Ma) and (3) paleomagnetic measurement of the basalt and turbidite. The trajectory suggests that the MORB in the Goshiki-no-Hama Coast, Yokonami melange, erupted at the Kula-Pacific Ridge in the Paleo-Pacific Ocean, traveled northward, and eventually arrived at the continental margin of East Asia at 94.32. Ma.