The Volcanic Periodicity in Central and Southern Luzon, the Philippines: as Inferred from Tephra Study in the Ocean Core

The completeness and continuous nature of marine sediment sequences makes marine tephrochronology an excellent geologic record of past volcanism from the general area. Total of 21 tephra layers in the giant piston core MD972142 raised from northern Palawan, South China Sea by the IMAGES 1997 cruise were identified for this purpose. The age constrains of tephra layers were based on the proxy of the previous reported foraminifer oxygen isotope stratigraphy. The tephrochronological results show two major periods of the tephra deposits in this area, 870 ka to 810 ka and 480 ka to present, respectively. It was notable that the volcanic activities between the above two periods existed an obvious about 330,000 yrs of apparent quiescence in this area. The fingerprints of mineral assemblages, chemical and isotopic compositions of glasses in tephra help to trace back to the potential on land sources. Seventeen layers of total 21 layers in the MD972142 in the younger deposit period dominantly consisted of the brown to light-brown sponge-type or pumice-type glass particles with some plagioclase, clinopyroxene and rare biotite. The SiO2 contents and 87Sr/86Sr values of glasses in these tephra layers revealed the range from 56 to 72 wt % and 0.7045 to 0.7053, respectively, those are indistinguishable with the previously reported characteristics of the volcanic rock from the Macolod Corridor segment (0.7043-0.7053). The rests of 4 tephra layers, three in the older deposit period and one at 83 ka, mostly constituted by white pumice, biotite and plagioclase. Characteristics of the calc-alkaline series of glasses, ranges from 73 to 79 wt % in SiO2 and 0.7039 to 0.7044 values in 87Sr/86Sr ratio, are well correlated to the reports from the Bataan Segment (0.7041-0.7045) in the central Luzon. The above results clearly illustrated that the volcanic activities in the Bataan segment with the more differentiation in the chemical compositions and less eruptive frequency was interrupted at around 800 ka, and after about 330,000 yrs of apparent quiescence, the volcanic activities in the Macolod Corridor segment dramatically increased the intensities and frequency from last 480 ka. It can be interpreted as corresponding with the changing stress field on the Philippine Fault in this area. In previous study, the tectonic setting of the Macolod Corridor segment has been considered as a pull-apart rifting zone or a shearing zone, mainly originated by the Philippine Fault. The volcanic activities in this region have been interpreted as related to the rifting environments. From above studies, in summery, overall fluctuations in the volcanic periodicity in central and southern Luzon should be correlated the variations in the regional tectonic (stress) regime field with time.