Spreading rate, opening time, and kinematic history of the Ayu Trough

Abstract

The Ayu Trough, located in the equatorial western Pacific, is the only divergent boundary surrounding the Philippine Sea Plate. A mid-ocean ridge spreading in the N-S direction near low-latitude regions presents low geomagnetic anomalies, thereby complicating the constraining of the trough's spreading rate based on the geomagnetic signal from the magnetized seafloor. According to previous studies, the Ayu Trough has been open since & SIM;25 Ma, with a half-spreading rate of 4.1-8.1 mm/yr. This implies that the trough belongs to the ultraslow-spreading mid-ocean ridge category with a full-spreading rate below 20 mm/yr. However, the geochemical signatures of the rock samples from the Ayu Trough do not exhibit the unique characteristics of ultraslow-spreading ridges, including the enriched mid-ocean ridge basalts (E-MORB) with high rare earth element (REE) concentrations and abundant light rare earth elements (LREE). Rather, the sampled rocks from the entire trough-axis exhibit the typical features of the normal mid-ocean ridge basalts (N-MORB) with low REE concentrations. The contradictory inferences made from geochemical results suggest that the Ayu Trough did not undergo an ultraslow-spreading but rather a considerably rapid spreading over a short period compared to the estimates made by previous studies. The modified interpretation for the spreading rate of the Ayu Trough can be associated with the global plate reconstruction model, considering the major plate reorganization in East Asia. Around 15 Ma, a series of significant tectonic events occurred in East Asia and the western Pacific. The global models for East Asia imply that the spreading of the Ayu Trough could have been initiated afterwards. Based on our geochemical analysis of the MORB on the Ayu Trough and the global plate model, we argue that the spreading of the Ayu Trough was initiated after & SIM;15 Ma during the major plate reorganization in East Asia, and not in & SIM;25 Ma as reported by previous studies. Therefore, the spreading occurred only for a period of about 10 Myr at a relatively rapid full-spreading rate of >20 mm/yr. The massive sediments with a thickness of approximately 50-70 m on the spreading-axis of the Ayu Trough reinforce the estimated cessation time of the trough. In addition, the V-shaped seafloor of the Ayu Trough and the unclear transform fault may be interpreted as topography formed by a propagation of rift or mantle melting. Therefore, the trough does not have to be simultaneously opened throughout the axis, and its spreading rate could be higher than previously estimated.