Abstract
As the quantity of seismic refraction data in the oceans increases it is becoming apparent1–3 that there is a systematic increase in crustal thickness with age, at least out to about 60 Myr (refs 1,2). To explain this phenomenon more precisely the detailed structure of the lower crust is required. We present seismic refraction data from the Northern Cocos plate which shows that thickening occurs by the formation of a low velocity zone (LVZ) at the base of the crust with a velocity < 6.8 km s−1. One possible explanation of this in terms of these relatively low seismic velocities for the lower crust4, is that upper mantle material becomes serpentinised by a hydration reaction as it cools. If so, this implies that the Moho boundary, except at very young ages, is a phase change (hydration) boundary. This supports the ideas of Hess5.
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