We explored the early stages of back-arc extension in the vicinity of the Tokara Islands in the northern Okinawa Trough to gain insight into the inception of ridge-ridge-ridge (RRR) triple junctions. Detailed bathymetric surveys within the trough identified numerous lineaments indicative of active faulting. Using k-means clustering based on the orientation of these lineaments, we identified three distinct fault systems corresponding to the Okinawa Trough, the Tokara Gap, and the Yokoate Echelon Knoll Chains. Further analysis of centroid moment tensor (CMT) solutions near each fault system suggested that normal faulting predominates in all three systems. Stress inversions of the CMT solutions showed that these systems were formed under a unified north-south extensional stress regime, corresponding to the rifting of the Okinawa Trough. We inferred the following mechanism: The curved arc causes arc-parallel rifting to occur during back-arc rifting; thus, a single stress field drives extension in two directions, thereby achieving the conditions necessary for RRR formation. The persistence of this tectonic regime, contingent on the continued expansion of the Okinawa Trough, may foster conditions conducive to the future development of an apparent RRR triple junction.
High‐resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A.1n subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.
A geophysical swath survey and a Deep-Towed Proton Magnetometer survey were carried out using the R/V Yokosuka of JAMSTEC at the Parece Vela Rift and the Central Basin Fault and its adjacent area, during a 20 day cruise from Guam (departure January 26, 2000) to Guam, (arrival February 14, 2000). A huge megamullion called Giant Core Complex was fully mapped and corrugated surface structure, termination and break away were recognized. A deeptowed proton magnetometer survey was performed across one segment of the Parece Vela Rift to obtain fairly good magnetic signals for a better understand the spreading history. The easternmost segment of the Central Basin Fault was also mapped near its junction with the Kyushu-Palau Ridge. Two N-S lines were acquired across the extinct spreading center, normal to the fabrics for age determination from magnetic anomalies. These new data sets contribute to our knowledge on the last spreading phases of both the Parece Vela Basin and the West Philippine Basin.
Rock and sediment samples were collected from petit-spots in the northwestern Pacific. The sampling was conducted using deep-submergence vehicle (DSV) Shinkai 6500 and its mother ship, research vessel (RV) Yokosuka during YK20-14S and YK21-07S cruises. The collected rock samples are basalt and peperite. Some of the basalts include small mantle xenoliths (∼3 cm in diameter). The dataset of rock and sediment samples from the petit-spots located on >130 Ma northwestern Pacific plate are presented herein. The peperites are a reaction product between petit-spot magma and wet sediment, and the mantle xenoliths are fragmented mantle materials transported by the petit-spot magmas. Therefore, the petit-spot samples are of significant importance to elucidate modification process of the surface condition by petit-spot magma and to characterize the deep lithospheric mantle. The dataset presented herein provides in a sense a unique insight into the whole Pacific plate just before its subduction beneath the Japan arc.
