Seismic survey of GB21- 1 Cruise off southwest Tokara Islands

Abstract

A multi-channel seismic survey was conducted off southwest Tokara Islands by GB21-1 survey cruise for marine geological maps. The seismic stratigraphy and geological structures were compiled with reflection profiles, which were obtained by high-density spacing survey. West of the volcanic front is characterized by four sedimentary units (OT1-OT4) divided by unconformities. The faults found in the west of the volcanic front were divided into two types: normal faults affecting vertical displacement in the OT1 unit and affecting cumulative vertical displacement in the OT3 unit sediment layers. The former faults were recognized east of Nishi-Yokoate Knoll with NNE-SSW trending. The later faults are developed widely west of the volcanic front, and found with NE-SW trending along the east edge of Okinawa Trough and with ENE-SWS trending southwest of Takarajima island. The normal faults affecting cumulative vertical displacement in the OT3 unit are considered to be formed by intermittent NW-SE extension related to the back-arc rifting in the northern Okinawa Trough after formation of the OT2 unit. In the east of volcanic front, the sedimentary units are divided into four (TK1-TK4) based on unconformities and acoustic facies differences. Two types of faults were found in the east of the volcanic front: normal faults that trend E-W and affect cumulative vertical displacement in the TK4 unit sediment layers in the northern margin of the Amami Trough, and normal faults that trend WNW-ESE and cause vertical displacement through the TK3 and TK4 units in the southeast of Takarajima Island. Combining fault characteristics based on seismic stratigraphy with analysis of sedimentary age will provide further discussions on tectonics in the northern Ryukyu Arc.