A two-ship, wide-aperture, multichannel seismic reflection profile obtained across a small western North Atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones. Reflections from the crust-mantle boundary, typically observed at about 2 s below the basement reflection, rise to only 0.8 s beneath the fracture zone trough. This corresponds to crustal thicknesses of perhaps as little as 2 to 2.4 km in the fracture zone, less than half the thickness of normal oceanic crust. This is the first time that Moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset. The existence of thin crust at both large- and small-offset fracture zones suggests that the magnitude of the thermal edge effect at ridge-transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones.

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