Mode of seafloor spreading in the northern Mariana Trough

Abstract

A geophysical study was conducted to clarify the mode of seafloor spreading in the northern Mariana Trough north of 20°N. Magnetic anomaly data and single-channel seismic reflection profiles were taken along ship's tracks typically with a 2–4 mile spacing. The transition from back-arc rifting to true seafloor spreading occurs at a structural boundary, which has NE-SW strike, at about 22°N. South of this boundary, clear magnetic lineations occur, and axial rift valleys on the spreading ridges are visible on the seismic profiles. Our identification of the magnetic lineations shows that the accretion of oceanic crust started at about 3.5 Ma with a spreading half-rate of ~ 2 cm/yr. The average spreading rate during the Brunhes chron is of the order of 1 cm/yr. The rate increases northward and is accompanied by a decrease in the relative depths of the axial rift valleys. A ridge-axis discontinuity is found at about 21°N. Two spreading axes overlap each other about 50 km along-strike, and are offset about 30 km. We interpret this discontinuity as an on-going ridge jump from the western axis to the eastern one. The southern end of the western axis is cut by a transform fault but the eastern axis is now propagating northwards. Arc volcanoes comprising the present volcanic front occur on the eastern flank of the spreading ridges, suggesting basinward migration of the volcanic arc. The spreading center of the Mariana Trough is located not at the center but on the eastern, or the volcanic arc, side of the trough in general. Offset is caused by the arcward ridge jump and the basinward migration of the volcanic arc. Asymmetric rifting before the beginning of the accretion of oceanic crust would be another reason for the offset. A row of rift grabens along the eastern margin of the trough north of 22°N, where the extension of island-arc crust is concentrated, continues to the spreading ridge south of 22°N. The arcward offset of spreading ridges is also found in the Lau Basin, suggesting that it is a common feature of the evolution of back-arc basins.