Asymmetric rifting of the northern Mariana Trough

The evolution of rifting in the northern Mariana Trough was studied, based on single‐channel seismic reflection profiles and heat flow. The rift showed structural asymmetry. The northernmost part of the Mariana Trough at 24°N, just south of Minami‐Iwojima Island, is now in an incipient rifting stage and shows a half‐graben structure. The arc crust just behind the volcanic front is cut by a few major east‐dipping normal faults. The major faults extend southward behind the Hiyoshi seamounts around 23°30′N. The rift develops to a full‐graben stage at ∼ 23°N, where the width of the trough increases to 80 km. The trough is comprised of several faulted and tilted blocks of island‐arc crust. Maximum subsidence occurs along a row of small grabens on the eastern margin of the trough. These grabens are separated by arc volcanoes, and their depths increase southward from 2500 m at 23°20′N to 4500 m at 22°N. The strike of each graben is north‐northwest–south‐southeast, which is close to the trend of the remnant West Mariana Ridge, but oblique to the active Mariana arc. Crustal extension becomes concentrated along the eastern margin of the trough as rifting progresses. The transition from rifting to sea floor spreading may occur at ∼ 22°N, where the width of the trough is ∼ 120 km. The possible spreading center lies along the southern extension of the grabens on the eastern margin. The period of back‐arc rifting before spreading begins is estimated to be less than 3 million years. Heat flow is asymmetric in the rift. High heat flow was observed only in or close to the row of grabens along the eastern margin of the trough. The asymmetric pure shear extension model fits the observed heat flow distribution better than the simple shear extension model.