Two‐phase opening model for the Okinawa Trough inferred from paleomagnetic study of the Ryukyu arc

Abstract

Tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central Ryukyu arc and the southern Ryukyu arc for a palcomagnetic and geochronological study, in an attempt to understand the opening of the Okinawa Trough back arc basin. Stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central Ryukyu arc and from 12 sites of two geological units of the southern Ryukyu are. The palcomagnetic results were compared with the Tertiary apparent pole wander path of Eurasia in order to investigate the tectonic movement of the Ryukyu arc with respect to the Asian continent. The comparison and the radiometric ages indicate that the southern Ryukyu arc rotated clockwise through 25 ø between 10 Ma and 6 Ma. In contrast, the central Ryukyu arc experienced little rotational motion since 17 Ma. We propose the following two-phase opening model for the Okinawa Trough opening. The first stage took place between 10 Ma and 6 Ma. The southern part of the trough opened by means of a "wedge" mode to cause the clockwise rotation of the southern Ryukyu are. In the central part the "parallel" opening occurred, and the Ryukyu arc drifted without any rotation. The second phase started recently in the whole Okinawa Trough. It is observed as the present activities in the trough. Geology of the Ryukyu Are An outline of the geology is described here following Kizaki [1986]. The Ryukyu arc is divided into north, central, and southem domains from both geological and geomorphological view points; the Tokara channel forms the boundary between the northern and the central Ryukyu arc, whereas the Miyako depression forms the boundary between the central and the southem Ryukyu arc (Figure 1). Figure 2 shows the schematic stratigraphy of the central and the southern Ryukyu arc. Weakly metamorphosed Mesozoic-Eocene sedimentary rocks are widely distributed in the northern and central Ryukyu domain. The sedimentary rocks of Okinawa-jima Island are named the Kayo Formation (Figure 3) and can be dated as Eocene by the presence of the foraminifera fossils (Nummulites sp. [Konishi et al., 1973]).

Regional seismicity and on‐land deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough

“…Between 6 and 10 Ma, the northern Ryukyu arc did not rotate [KodanTa et al, 1991; Kodama and Nakayama, 1993] and the central part did not experience any significant rotation [Miki, 1995]. For the same 10-6 Ma period the southern Ryukyu arc rotated 25 ø clockwise [Miki et al, 1990;Miki, 1995]. The rotations of the northern arc since 6 Ma and of the southern arc between 6 Ma and 10 Ma appear to be recorded only at the extremities of the arc.…”

“…Kodama et al [1995] showed that the major part of the rotation (-20 ø) occurred during the last 3 m.y. Between 6 and 10 Ma, the northern Ryukyu arc did not rotate [KodanTa et al, 1991; Kodama and Nakayama, 1993] and the central part did not experience any significant rotation [Miki, 1995]. For the same 10-6 Ma period the southern Ryukyu arc rotated 25 ø clockwise [Miki et al, 1990;Miki, 1995].…”

“…(Table I and Figure 2). Since-6 Ma, the southern and central Ryukyu arc has not undergone any rotation [Miki, 1995], while the northern Ryukyu arc and southern Kyushu Island have rotated -30 ø counterclockwise [Kodama and Nakayama, 1993]. Kodama et al [1995] showed that the major part of the rotation (-20 ø) occurred during the last 3 m.y.…”

Collisional model for rapid fore‐arc block rotations, arc curvature, and episodic back‐arc rifting in subduction settings

“…GPS velocities suggest ongoing anticlockwise rotation of the Ryukyu Islands, and rates of rifting in the Okinawa trough increase from 1 cm/a in the north up to 5 cm/a at the southern end of the trough [Nishimura et al, 2004]. Clockwise rotation of the southern Ryukyu arc (near 25°N) from 10 to 6 Ma ago is documented by paleomagnetic declinations [Miki et al, 1990;Miki, 1995], and are similar to the timing of Taiwan collision onset $6-8 Ma ago [e.g., Byrne and Liu, 2002, and references therein]. Clockwise rotation of the southern Ryukyu arc appears to have ceased [Nishimura et al, 2004].…”

Hydrothermal Activity in the Okinawa Trough Back-Arc Basin: Geological Background and Hydrothermal Mineralization

“…Lee et al 1980;Kimura 1996;Miki 1995). The geographical order and timing of the initiation of rifting still remains quite controversial.…”