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| 題名: | Seismicity and volcanism in the southwestern Okinawa trough (northeast Taiwan) |
| 作者: | Lin,JY;Sibuet,JC;Hsu,SK;Lee,CS;Klingelhoefer,F |
| 貢獻者: | 地球物理研究所 |
| 關鍵詞: | FREQUENCY-MAGNITUDE DISTRIBUTION;PHILIPPINE SEA PLATE;RYUKYU SUBDUCTION ZONE;EAST SCOTIA RIDGE;BACK-ARC BASIN;MANTLE FLOW;GAGUA RIDGE;B-VALUES;REGIONAL SEISMICITY;OBLIQUE SUBDUCTION |
| 日期: | 2009 |
| 上傳時間: | 2010-06-29 18:40:51 (UTC+8) |
| 出版者: | 中央大學 |
| 摘要: | Most of the volcanic activities observed in the Okinawa backarc basin are linked to the subduction of the Philippine Sea plate (dehydration, upwelling of asthenosphere). We have analyzed data acquired from a passive OBS experiment and available geophysical data (magnetic anomalies, seismic profiles and onland recorded earthquakes) to determine the depth of magnetic sources, earthquake locations, crustal and lithospheric 3D velocity models. spectral analyses of P-wave and seismic tomography. The calculations of the top and bottom of magnetic sources were performed in the East China Sea and in the southern Okinawa trough. This analysis shows the presence of a thin crust along 123.3 degrees E longitude and a thick crust beneath the Cross Backarc Volcanic Trail (CBVT) volcanic area. A N-S trending high-magnetization feature appears along the 123.3 degrees E meridian. in the northern prolongation of the Gagua ridge. On each side of this boundary. the lithospheres are characterized by different crustal thicknesses and rheologies and the two portions of slab display different strain patterns and dipping angles oil each side of this boundary suggesting the presence of a slab tear along the 123.3 degrees E meridian. In November 2003, 15 ocean bottom seismometers (OBS) Were deployed during a period of 12 clays in the Southwestern Okinawa trough. More than 3300 earthquakes were localized and 20 of then were simultaneously recorded by land stations. According, to the distribution of the 2823 relocated hypocenters, four Clusters of earthquakes were recognized. The seismicity is essentially restricted to the central part of the Southwestern Okinawa trough and terminates abruptly against the N-E trending prolongation of the Lishan fault. Most of the microearthquakes are aligned along E-W trending normal faults, showing where the present-day active normal faulting occurs and how it accounts for the N-S extension in the Okinawa trough. According to the P-wave velocity spectra estimated from some deep earthquakes located beneath the CBVT, the existence of a lower crustal / upper Mantle magma chamber is suggested by the presence of low frequency earthquakes in the 3 - 10 Hz bandwidth. Based on numerous earthquakes recorded by land stations and the microseismicity acquired during this Cruise, two tomographic inversions were performed. (1) A low Vp, Vs but high Vp/Vs sausage-like body, about 30 km in diameter, lies within the Eurasian mantle wedge, oil top of the western Ryukyu slab edge, at depths ranging between 20 and 125 km. We suggest that partial melting and/or a H2O-rich component formed by dehydration processes from subducting sediments, oceanic crust, and serpentinized mantle belonging to the Ryukyu slab and along the vertical portion of the slab edge might explain the presence of this sausage-like body. A low Vp, Vs but high Vp/Vs channel rises obliquely from the sausage-like body from a depth 40 km in direction of the andesitic Kueishantao Island. We propose that the melt and/or a H2O-rich component rises up from the sausage-like body and interferes with the Okinawa trough backarc basin magmas formed in the Upper mantle/lower Crust of the wedge. Then, magmas propagate Upward within the Upper brittle Crust through veins and/or narrow conduits. (2) A mantle inflow characterized by low Vp, Vs and high Vp/Vs passing through the Slab tear was imaged. This melt and/or fluid component is rising obliquely from the Slab tear in direction of the CBVT, of the northern slope of the Southern Okinawa trough and north of Iriomote Island. In addition, the asthenospheric intake is also imaged by all inclined chip-like high Vp/Vs and low Vp, Vs body dipping northerly. which might be linked to the slab retreat. b-values estimations give more credit to the existence of magmatic activities illustrated by tomographic results. High b-values were observed around the low Vp, Vs but high Vp/Vs anomalies above and along the slab tear, in the CBVT area, Under the sausage-like anomaly located in the western extremity of the Ryukyu slab. and along the magma supply conduit to Kueishantao Island. |
| 關聯: | BULLETIN DE LA SOCIETE GEOLOGIQUE DE FRANCE |
| 顯示於類別: | [地球物理研究所] 期刊論文
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