Inception of ridge-ridge-ridge triple junctions: Morphostructural analysis and dynamics in the early back-arc extension of the northern Okinawa Trough

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.