The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers

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A 725 km long transect along a flow line in the NW Pacific provides new images of the internal structure of Cretaceous oceanic crust formed at fast spreading rates. Reflections from the Moho transition zone document changes in crustal travel time along the profile: variations with a wavelength of 100–150 km may representing fluctuations in the magma supply with a 2–3 m.y. periodicity; to the south the crust may be several kilometers thicker than “normal” in places, possibly resulting from increased magma supply due to the distal effects of the Shatsky hotspot. At about the depth expected for the boundary between seismic layers 2 and 3, sub‐horizontal reflections are imaged over much of the profile; the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of ∼0.1, implying a sharp velocity and density discontinuity. Northward (ridgeward) dipping reflections in the lower crust are imaged over much of the profile, particularly toward the north (away from the region possibly affected by the hotspot). These reflections stop at the Moho reflection but die out perhaps 1 s beneath top basement; they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation, although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center. The discrete character and, in places, regular spacing (every ∼8 km) of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0.1–0.2 m.y.