Melting features along the western Ryukyu slab edge (northeast Taiwan): Tomographic evidence

Abstract

Behind the sedimentary Ryukyu arc lies the Okinawa Trough, whose termination is located at the tip of the Ilan plain (northern Taiwan), just above the Ryukyu slab edge. The present‐day active volcanic front is located 80–100 km above the Ryukyu slab and extends from Japan to Kueishantao Island, an islet situated 10 km offshore the Ilan plain. Between December 1990 and May 1999, 3370 earthquakes recorded in northern Taiwan by 65 seismic land stations were used to determine the three‐dimensional Vp and Vs velocity structures and Vp/Vs ratios. A low Vs but high Vp/Vs sausage‐like body, ∼30 km in diameter, lies within the Eurasian mantle wedge, on top of the western Ryukyu slab extremity, at depths ranging between 20 and 100 km. We suggest that the H2O‐rich component formed by dehydration processes from subducting sediments, oceanic crust, and serpentinized mantle above the Ryukyu slab and along the vertical portion of the slab edge might explain the presence of the sausage‐like body. A low Vs but high Vp/Vs channel rises obliquely from the sausage‐like body at a depth of 40 km in direction of the andesitic Kueishantao Island. We propose that the H2O‐rich component and/or melt rise up from the sausage‐like body and interfere with the Okinawa Trough back arc basin magmas formed in the upper mantle/lower crust. Then magmas propagate upward within the upper brittle crust through veins and/or narrow conduits.

Insight into the Geothermal Structure in Chingshui, Ilan, Taiwan

“…Based on the V P and V S velocity structures and V P /V S ratios, a sausage-like body with a low V S and high V P /V S ratio, approximately 30 km in diameter, was found at depths ranging between 20 and 100 km (Lin et al 2004;their Fig. 10).…”

“…Some magma chambers remain in the shallow crust. They have produced northern Taiwan's most recent volcanism (Lin and Yeh 2001;Chen et al 2002;Lin et al 2004) and provide the heat source of the Chingshui geothermal field (Yu and Tsai 1979;Lin and Yeh 2001).…”

“…10). This might be associated with a partially molten magma body (Lin et al 2004). The magma propagates upward within the upper brittle crust through veins and/or narrow conduits (Lin et al 2004).…”

“…This might be associated with a partially molten magma body (Lin et al 2004). The magma propagates upward within the upper brittle crust through veins and/or narrow conduits (Lin et al 2004). Therefore, it is possible that the upper part of the upward magma body has cooled to below its Curie temperature to appear as the high magnetic anomaly shown in Fig.…”

An Investigation of the 3D Electrical Resistivity Structure in the Chingshui Geothermal Area, NE Taiwan

“…The Chingshui geothermal area southwest of the Ilan plain is identified as the western extension of the Okinawa Trough in the northern Taiwan subduction system (Suppe 1984;Yeh 1989;Liu 1995;Lin et al 2004). The Chingshui Fault traverses this region from the northwest to southeast along the Chingshui stream (Tseng 1978;Hsiao and Chiang 1979), and a northeast-southwest trend anticlinorium axis parallel to the stratum is present in this region, as shown in Fig.…”