38. PALEOMAGNETIC STUDY OF DEEP-SEA SEDIMENTS FROM THE CAGAYAN RIDGE IN THE SULU SEA: RESULTS OF LEG 1241

Discrete samples from ODP Sites 769 and 771, located on the Cagayan Ridge in the Sulu Sea, are subjected to paleomagnetic study. The general results indicate that the coarse-grained and chaotic tuff and lapillistone below the soft sediment cover are not suitable for paleomagnetic study and that sediment layers cored by XCB and RCB techniques produce poor data due to rotary disturbance. The top 220 m of the sediments penetrated by APC cores at Site 769, however, produced very high-quality data. The polarities of the discrete samples agree very well with the magnetic reversal sequences that we established for Site 769 during the shipboard paleomagnetic study using archive halves of APC cores. Hole 769A includes the Brunhes-Matuyama Epoch boundary and Hole 769B contains 36 reversals including most of the reversals after the boundary between Chrons C4a and C5 about 8.92 Ma. Rapid change in both declinations and inclinations occurs at about 135 m below seafloor. This change can be shown to be due to normal faulting rather than tectonic movements. The resulting lack of convincing evidence for rotation or north-south migration for the Cagayan Ridge site is remarkable because the nearby Philippine Islands and the Celebes Sea to the southeast have had counter-clockwise rotation in their tectonic evolution history. The result suggests that the spreading of the Sulu Sea as a back-arc basin did not cause rotation or north-south migration of the Cagayan Ridge and that the tectonic activities within the Philippine Mobile Belt have been decoupled from the Cagayan Ridge since middle Miocene time