Reorganization of the Pacific-Izanagi-Farallon triple junction in the late Jurassic; tectonic events before the formation of the Shatsky Rise

It has been suggested that the Shatsky Rise oceanic plateau formation began simultaneously with a reorganization of spreading at a triple junction bordering the northern Pacific plate, and this coincidence has led to speculation about the connections between the two events. We present new marine geophysical data that constrain the seafloor spreading history of the Pacific-Izanagi-Farallon triple junction just before the birth of the Shatsky Rise. Bathymetric data reveal en echelon, abandoned spreading centers trending northwest-southeast located adjacent to the southwest flank of the Shatsky Rise. Magnetic anomalies and bathymetry are interpreted to indicate that segments of the Pacific-Farallon Ridge near the triple junction propagated northwest from chron M23 (153 Ma) to chron M22 (151 Ma) during a spreading ridge reorganization at the edge of a likely microplate. Our detailed examination of bathymetric and magnetic anomaly lineations also shows that the strike of the Pacific-Izanagi Ridge changed gradually on the west side of the triple junction around chron M22. Our observations indicate that the plate boundary reorganization began several million years before the formation of the Shatsky Rise, implying that the eruption of the plateau did not cause the reorganization.