A portion of the upper mantle that is directly below the lithosphere which is plastic enough for rock flowage to occur; extends from a depth of 30-60 miles (50-100 kilometers) to about 240 miles (400 kilometers) and is seismically equivalent to the low velocity zone. A zone of low strength in the upper mantle defines the top of the asthenosphere. This weak zone allows the plates of the lithosphere to slide across the top of the asthenosphere.
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