Three‐dimensional seismic inhomogeneities in the lithosphere and asthenosphere: Evidence for decoupling in the lithosphere and flow in the asthenosphere

Newly discovered three‐dimensional seismic velocity anomalies in the earth's lithosphere and asthenosphere are interpreted in terms of decoupling in the lithosphere and flow in the asthenosphere. A rough estimate of flow in the asthenosphere under California was made using Stokes' law on the basis of the size of anomalies and density contrast estimated from the velocity contrast. The result depends on the cause of velocity anomalies. If they are due to any effect which obeys Birch's law, the flow is as rapid as that of Morgan's (1972) plume driving the plate motion. On the other hand, if they are due to partial melts in thin cracks or at grain boundaries, the flow is negligible in accordance with Elsasser's (1971) model of plate tectonics driven quietly by the ridge‐push and slab‐pull in which the asthenosphere under the base of the plate is only passively dragged. The result of Cockerham and Ellsworth (1979, 1980) that the low‐velocity zone in the mantle under central California appeared to have preserved the shape of the downgoing slab for up to 10 m. y. supports Elsasser's rather than Morgan's model. The shapes of low‐velocity bodies found by Iyer and his colleagues under several geothermal areas in the western United States may be interpreted in terms of decoupling between the upper brittle lithosphere and the ductile substratum. The misalignment of the axis of the low‐velocity column from the center of the granite outcrop in the Roosevelt Hot Springs area, Utah, and the inclined low‐velocity column under the Geysers‐Clear Lake region, California, indicate a decoupled lithosphere in these areas, whereas the upright column found under Yellowstone National Park suggests an intact lithosphere there.