22. SEDIMENTOLOGY SUMMARY: CLUES TO ARC VOLCANISM, ARC SUNDERING, AND BACK-ARC SPREADING IN THE SEDIMENTARY SEQUENCES OF DEEP SEA DRILLING PROJECT LEG 59

and other published works, the Leg 59 staff concedes that arc volcanism and sundering along with back-arc spreading have occurred in distinct pulses in the Philip-pine Sea region, migrating eastward relative to the Eura-sian plate. Igneous basement becomes younger eastward ranging from pre-middle Oligocene at Site 447 in the West Philippine Basin, lower to middle Oligocene at Site 448 on the Palau-Kyushu Ridge, upper Oligocene a