Back Arc Extension in the Okinawa Trough

Abstract

The Okinawa Trough, lying to the east of China, is a back arc basin formed by extension within continental lithosphere behind the Ryukyu trench‐arc system. Middle to late Miocene uplift, associated with normal faulting of the initially adjacent Ryukyu nonvolcanic arc and the Taiwan‐Sinzi folded belt, corresponds to the first rifting phase. The timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern Okinawa Trough. The rifting occurred after a major early Miocene change in the motion of the Philippine plate with respect to Eurasia and ceased during the Pliocene. A second rifting phase started about 2 m.y. ago, at the Plio‐Pleistocene boundary and has continued until the present time. It has proceeded to a more advanced stage in the middle and southern Okinawa Trough than it has farther north. Detailed bathymetric (Sea Beam), seismic reflection, and magnetics data collected during the POP 1 cruise of the R/V Jean Charcot reveal the principal features of the extensional processes. The back arc spreading phase started very recently in the southern and middle Okinawa Trough, as exemplified by several en échelon and, in some cases, overlapping active, central graben oriented N70°E–N80°E. Some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies. Transform faults between these en échelon active rifts are not obvious. We suggest that the major part of the southern Okinawa Trough is underlain by a thinned continental crust and that except for the system of en échelon rifts of the southern Okinawa Trough, the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough. The system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of Okinawa Island whereas active volcanoes in the Ryukyu arc occur only north of Okinawa Island. We refer to this transition between active arc and back arc volcanism as the volcanic arc‐rift migration phenomenon (VAMP). Globally, back arc volcanism propagated from the southern Okinawa Trough to the Okinawa VAMP area. Rifting continues to occur in the northern Okinawa Trough but is not yet accompanied by associated volcanism. The Okinawa VAMP area is characterized by a series of parallel basaltic ridges oriented N75°E with associated linear magnetic anomalies characteristic of dyke intrusions. We suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending N75°E and that this zone presently ends at the southern extremity of the active volcanic chain. The initial phase of formation of back arc basin oceanic crust is non‐steady state and is characterized by the lack of a developed fracture zone pattern. The termination of the VAMP area in the direction of the volcanic zone of the arc is consistent with the suggestion of Molnar and Atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc...

Actively forming Kuroko-type volcanic-hosted massive sulfide (VHMS) mineralization at Iheya North, Okinawa Trough, Japan

“…Arc volcanism predominantly occurs in the northern part of the Ryukyu Arc and in the middle and southern parts of the Okinawa Trough . The transition from arc to back-arc volcanism in the middle Okinawa Trough has been termed the VAMP area (Sibuet et al, 1987), with present day activity in the middle and southern Okinawa Trough associated with en echelon intra-trough grabens (Glasby and Notsu, 2003;Shinjo and Kato, 2000). The VAMP area is coincident with the subduction of the WNW-ENE trending Daito Ridge (Sibuet et al, 1998; Fig.…”

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

“…27.2). The northern to middle part of the trough, which lies along the Kyushu to Amami Islands, consists of numerous half-grabens within and/or next to the massive Tokara Sub-Basin (Sibuet et al 1987). The stratigraphy of this area was documented by the borehole "TO-KA1" (Nash 1979).…”

“…While the volcanic front in the Ryukyu Arc disappears from the sea-surface in west area of the Iwo-Torishima Is., an area of extensive intra-trough volcanism along the Iheya Graben occurs in the middle of the Okinawa Trough. Sibuet et al (1987) inferred this was a product of the volcanic arc migrating into the back-arc basin, and named it the Volcanic Arc Migration Phenomenon, or VAMP area. Although the volcanism takes place within the Okinawa Trough, its compositions consistently vary from rhyolite to basalt (Ishikawa et al 1991;Shinjo and Kato 2000).…”

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

“…Figure A7). Active back-arc rifting has occurred along the Okinawa Trough since the late Miocene [e.g., Sibuet et al, 1987;Lee et al, 1980]. Large, anticlockwise paleomagnetic rotations (up to 30°) have been observed for the last 2-6 Ma in southern Kyushu [e.g., Kodama and Nakayama, 1993;Kodama et al, 1995], and have occurred simultaneously with both rifting in the Shimabara/ Beppu Graben (SBG) [Kamata, 1989], and collision of the Kyushu-Palau ridge at the Nankai Trough/Northern Ryukyu Trench.…”