30. SUBSIDENCE AND SEDIMENTATION ANALYSIS OF MARGINAL BASINS: CELEBES SEA AND SULU SEA, LEG 124, SITES 767 AND 7681

Based on the chronostratigraphy and porosity of the sediments recovered from ODP Sites 767 and 768 drilled in the Celebes Sea and Sulu Sea, respectively, we examine the decompacted sedimentation and basement subsidence rates using a quantitative method. The results of this study help us to recognize several important tectonic events in these two marginal basins which improve understanding of the regional tectonic development. Among the recognized events are: (1) a pronounced increase in decompacted sedimentation rate in the Celebes Sea at 23.4 Ma, indicating basin transformation from a larger, more open oceanic setting to a marginal basin; (2) an increased basement subsidence rate in both basins between 10.8 and 8.9 Ma associated with the initiation of the Sulu Trough and the Sulawesi Trough; (3) appearance of a decompacted sedimentation rate peak between 10.0 and 8.9 Ma in both basins, indicating the combined effect of tectonic uplift in the source area and a sea-level drop; (4) uplift of the basement since 2.0 Ma, first in the Celebes Sea and then in the Sulu Sea, probably occurring as the result of collision events.