The deepsea cores recovered from about 50 drilling sites in the Philippine Sea, equally distributed in marginal basins, remnat arcs, present arcs and others, during the DSDP/IPOD/ODP offer significant geotectonic information. Of these cores of the drillsites, sediment accumulation rates, lithologic changes and frequency of tephras were reviewed in the light of the recent advanced nannofossil biostratigraphy of the sediment cores. Sediment accumulation rate curves of these sites were classified into two major types, A and B types, respectively. A type has rapid accumulation rates just above the arc basement and then decreasing pattern. In contrast, B type has rather constant accumulation rate throughout the cores. Rapid accumulation rates imply volcanogenic debris flow and volcaniclastic turbidite sequences derived from arcs which represent activity of magmatic arc consisting of tholeiitic and calc -alkalic volcanic rocks. On the contrary, low sediment accumulation rates imply biogenic materials instead of volcaniclastic rocks. This means the termination of intense are volcanism.
Frequency of volcanic ash layers deduced from these cores has maxima just after the rapid sediment accumulation stage of A type curves. As for the remnant arcs such as the Kyushu-Palau and the Daito Ridge, tephra maxima exist at late Eocene to early Oligo cene time and the present arc such as the Izu-Bonin Arc, there are two major maxima at Eorene-Oligocene and Pliocene-Pleistocene time, respectively.
Explosive volcanism may take place when oceanic arc develops as shallo w as the pressure compensation level (PCL). If this is the case, we may draw the volcanic history of oceanic island arc. At the incipient stage in Fig. 6, style of volcanism is quiet resultant formation of pillow lavas and hyaloclastite. On the contrary, volcanism takes place very intense with form ation marine tephras at the explosive stage, and at the subareial stage, large amount of tephras are exhausted from the summit of the volcanoes. No volcanisms were happened instead continuous rapid subsidence of the arc at the remnant stage. These stages of the evolution of oceanic island arc are quite similar to those of the volcanic islands such as the Hawaiian volcanoes.
This study presents the images of the evolution of oceanic island arcs by the compilation of the sediment accumulation rates, lithologies and frequency of the volcanic ash layers of the cores recovered during the Deep Sea Drilling Programs.
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