Spreading process of the northern Mariana Trough: Rifting-spreading transition at 22°N

We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting-to-spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with side-scan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free-air gravity anomaly from satellite altimetry. The rifting-to-spreading transition occurs at about 22°N, which is proved by seafloor-spreading fabric in the bathymetry, clear magnetic lineations, and the bull's-eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma-starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma-rich, magma-starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor [2002]. The magnetic anomaly revealed the detailed history of the spreading. The seafloor spreading between 19°N and 20°N began prior to 5 Ma, and that between 20°N and 21°30'N began at about 4 Ma. Spreading half-rates in the western side of the spreading center were 2 to 3 cm/year before 2.58 Ma south of 21°30'N and during the Matuyama Chron north of 21°30'N, but an average during the Brunhes Chron is 1 cm/year or less. Orientations of the ridge axes, which range from -20° to 0° at present, have rotated about 20° clockwise since the start of the spreading. These changes in rate and direction might be associated with changes in the motion of the Philippine Sea plate. Spreading has been asymmetric in the northern Mariana Trough. The spreading rates of the western side of the spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.