ABSTRACT: Rifting of the Izu-Bonin Arc in the Quaternary and Mid-Oligocene
Eruption of middle Eocene-lower Oligocene boninites and island arc tholeiites created the 200 km wide Izu-Bonin arc massif following the initiation of subduction ^sim50 Ma. Mid-Oligocene rifting formed a 40-70 km wide forearc basin between the Eocene outer-arc high and the Eo-Oligocene arc (now the frontal arc high), with maximum extension at the latitudes of the Bonin Ridge and Trough. The Oligocene forearc basin was rapidly (100-300 m/m.y.) filled with turbidite and debris flow deposits produced by concurrent volcanism and erosion of the surrounding highs. Contemporaneous stretching in the backarc region produced dominantly east-dipping, NNE-trending, normal faults and culminated in backarc spreading in the Shikoku basin (25-15 Ma), isolating the Palau-Kyushu remnant ar . The forearc and remnant arc sediments record a dearth of volcanogenic input between 23 and 17 Ma; evidence that an arc volcanic minimum accompanied backarc spreading. The middle Miocene to Recent volcanic front formed 0(S)-50(N) km west of the Oligocene arc and has loaded and flexed the forearc. Further west, chains of submarine volcanoes erupted along the extension of Shikoku basin fracture zones. Basaltic sills were emplaced in the forearc. Explosive volcanism from rhyolitic calderas has increased dramatically since the late Pliocene, especially in the last 0.2 Ma. Since ^sim2 Ma the arc has been stretched again, producing rapidly subsiding (300-2250 m/m.y.) graben, immediately west of the volcanic front, which are segmented along strike by oblique transfer zones, The Sumisu Rift is artially filled with (<1.5 Ma) volcaniclastic turbidites and hemipelagic sediments and is intruded by backarc basin basalts. About 1 km of syn-rift uplift of the arc margin footwall has produced an unconformity, beneath surficial pumice, that extends back to pumiceous sediments >2.35 Ma.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990