Seismotectonics of the Philippine Archipelago

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[Geochemistry]In this study, K-Ar ages of 37 samples from eleven volcanoes in Bicolpenmsula of Luzon were measured in order to constrain the timing of the initiation of the subduction along the Philippine Trench. Major and trace element composition was also measured by XRF. Dating results show that the volcanism started at latest 7 Ma in Bicol, suggesting earlier initiation of volcanism and subduction in Bicol than in Leyte and Mindanao. Although volcanic activitiesin Bicol may be divided into three groups by age gaps, major and trace element chemistry did not show significant difference between these groups. Dating results suggest that the subduction along the Philippine Trench initiated at latest 8-9 Ma. This age is consistent with the timing of the collision of the Palawan block with the Philippine archipelago at 8-9 Ma which is considered to have triggered the subduction along the Philippine Trench.[GPS]1. For determining complicated tectonics in the triple junction region between … More the Philippine Sea, the Australian and the Sundaland plates, we have implemented GPS campaign measurementsevery year since 1997. To estimate subduction style in the Moluca Sea, we used seismicity and gravity anomaly data in addition to the GPS velocity field. While a westward dipping slab exists in the northern part of the Moluca. Sea, two main slabs that dips to the east and the west exist in the southern part. Thesubduction style in the Moluca Sea shows large variability from north to south.2. From September 1998 through September 2001, five GPS campaigns were made inand aroung the Macolod Corridor (MC), in the Luzon island, Philippines. After analyzing data using Bernese GPS software Ver. 4.0, a left lateral motion at 2-3 cm/yr with a large amount of counterclockwise motion was detected in the southern part of the study area. Furthermore, a large amount of extension, directing NNW-SSE or N-S was found in the MC. These observations are consistent with the block motion after 2Ma determined through a recent paleomagnetic research and a K-Ar dating study of volcanic rocks in andaround the MC. It is possible that these crustal movements detected by GPS observations are closely related to the formation process of the MC. Less