Abstract
Recently Brune and Singh (1986) reported evidence for anomalous continent-like crustal structure beneath the Bay of Bengal, possibly caused by perturbation in the temperature-pressure regime and consequent phase chage, partial melting, or mass transport (e.g. convection or underplating). Recent refraction results indicate the existence of an anomalous lower crustal or subcrustal layer ofP-wave velocity about 7·3 km per second along the eastern North America passive margin, possibly a result of underplating of the oceanic crust just after initial rifting. We have searched for other evidence of anomalous crustal structure. The data suggest some mechanism may cause a general increase in the anomalous thickness of the crust with increasing thickness of the accumulated sediments, up to a thickness of about 6–7 km. On the other hand, anamolous crustal structure may in fact be transitional between oceanic and continental, or may have been modified by aseismic ridges, thus requiring no sediment related structure modification mechanism. The explanation for all the data may require more than one mechanism, all probably involving severe temperature perturbations. The general tendency is for perturbations of normal oceanic crust to make it more continent-like, suggesting that normal oceanic crust is an unstable end-member in crustal states.
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