SIT07-P01

Room:Convention Hall

Time:May 25 18:15-19:30

Tectonic evolution of the Philippine Sea: Magnetic data collected during the Japanesecontinental shelf survey
OKINO, Kyoko1∗
1AORI, The University of Tokyo

Marine magnetic anomalies have been used to date the seafloor, characterize the oceanic crust and reconstruct the evolution
process of ocean basins. Japanese continental shelf survey project has collected high-quality, dense magnetic data in the Philippine Sea and the adjacent areas for two decades. The compiled and processed magnetic anomaly data improve our understandingof the tectonic history of the area. Clear magnetic lineation patterns in the Shikoku, Parece Vela basins allow us to elaborate thespreading history of these basins, including the initiation and cessation process of the backarc opening. The anomalies associatedwith the Kyushu-Palau Ridge also record the transitional phase from arc volcanism to backarc rifting-opening. The West Philippine Basin, Daito Ridges and its intervening small basins, that was formed before the formation of the paleo-IBM arc, show their
specific magnetic characteristics. These areas are considered to have moved northward and rotated after their formation. Theskewness analysis of magnetic data can provide us some insights about paleo-latitude and/or rotation.
Keywords: tectonics, Philippine Sea, magnetic anomaly, backarc basin, arc

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SIT07-P02

Room:Convention Hall

Time:May 25 18:15-19:30

Petrography of mantle xenoliths from Lyudao, in the Luzon arc, Taiwan
KOMARU, Chihiro1∗ ; MORISHITA, Tomoaki1 ; TAMURA, Akihiro1 ; ARAI, Shoji11

1Graduate School of Natural Science, Kanazawa University

Lyudao, island of southeastern Taiwan, is located at the north end of the Luzon arc. Lyudao is volcanic island, which is madeup of andesitic rocks and pyroclastic rocks. Peridotite xenoliths have been found in andesite of Lyudao and Lanyu (Chen, 1988
Acta Geol. Taiwanica). Lanyu is the island, which is located on the south of Lyudao. Peridotite xenoliths have been found inthe Luzon arc, such as Batan Island (Arai and Kida, 2000 Island Arc; Arai et al., 2004 J. Petrol.), Pinatubo volcano (Kawamotoet al., 2013 PNAS) and Siayan Island (Y. Iizuka, unpublished date). Peridotite xenoliths from Luzon arc can provide spatial
information of mantle wedge peridotites at the subduction zone. Because Lyudao is located at the north end of the Luzon arc,
it is important for addressing the spatial variations. Peridotite xenoliths in Lyudao Island have never been, however, studied indetails since Chen (1988). Here we report petrological characteristics of peridotite xenoliths from Lyudao.
Peridotite xenoliths are mainly harzburgite with one olivine websterite. Harzburgite shows two types in terms of grain size;
fine-grained (less than 0.1mm across) and coarse-grained (several millimeters across). These textures coexist in hand-specimen.
There is no difference in mineral mode between them. These petrological textures are the same as those of Batan Island (Arai etal., 1996). Amphibole occurs at the border part of the host andesite and peridotite xenoliths. The Fo content [=100Mg/(Mg+Fe2+ )atomic ratio] of olivine is from 91 to 92. The Al2 O3 content and Mg# [=Mg/(Mg+Fe2+ ) atomic ratio] of orthopyroxene are from
2.4 to 3.5 wt%, and from 0.90 to 0.92, respectively. The Al2 O3 and Cr2 O3 content, and Mg# of clinopyroxene are from 1.7 to4.3 wt%, about 1 wt%, and from 0.90 to 0.94, respectively. The TiO2 and Na2 O contents of clinopyroxene are very low, beingfrom 0 to 0.2 wt%. Fe3+ /(Cr+Al+Fe3+ ) atomic ratio of spinel is very low, mostly being less than 0.2, There Cr# [=Cr/(Cr+Al)
atomic ratio] and TiO2 content is from 0.4 to 0.6, and from 0 to 0.1wt%, respectively.
Keywords: xenolith, mantle, peridotite, Luzon arc, Taiwan, Lyudao

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SIT07-P03

Room:Convention Hall

Time:May 25 18:15-19:30

Deformation history of the Chimei Fault, eastern Taiwan: Insights from paleostress andfold analysis
KUO, Szu-ting1∗
1

Institute of Geosciences, National Taiwan UniversityThe Chimei Fault, the only fault cutting across the Coastal Range in eastern Taiwan, is the boundary fault that thrusts theforearc basement (Tuluanshan Formation) over the forearc sedimentary rocks (Paliwan Formation). Although previously deemed
to be a reverse fault with strike-slip component, paleostress pattern along the Chimei Fault zone has not yet been established. Inorder to reconstruct the deformation history of the fault, this study carries out paleostress and fold analysis along a well-exposed
outcrop in the central part of the Coastal Range.
The Chimei Fault zone is composed of 100 and 500-m wide damage zones in the hanging wall and the footwall, with several
sets of subsidiary faults developed intensely. Based on crosscutting relationship, the fault-slip data could be divided into threestages. The earliest stage is characterized by the left-lateral fault slickensides that crosscut mineral veins related to heat flowactivity in the Tuluanshan Fm. In the footwall, a 100 m-wide fold zone, including boudins and mud-filled veins, indicates
deformation of buried unconsolidated sedimentary rocks during the second stage. The third stage is characterized by brittle
subsidiary faults. The predated folds and postdated brittle faults indicate that the deformation depth of fault rocks decreased
during faulting. In addition, both folds and faults show N-S compression, suggesting that two structural processes record thesame paleostress status.
The three-stage evolution could be comparable to previous reconstruction from paleomagnetic analysis: (1) The initial leftlateral component is consistent with island-arc movement since late Miocene; (2) Folding and faulting of the footwall thenillustrate the N-S forearc closure since late Pliocene.

Keywords: fault damage zone, paleostress analysis, fold analysis, reverse fault

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