Abstract:

According to the seismological observations, there is usually a layer called as a transition layer which generally has an intermediate speed between P-wave speeds in typical crust and mantle rocks. This layer is often called as the crust   mantle mixing layer in the Earth material science fields. This is not a term with a definite mechanism. In this paper, an other alternative choice is provided. In fact, the geophysical transition layer may be a product of mantlederived magma underplating and crustal differentiation. The transitional P-wave velocity in this layer is caused by gradual variation of its density with pressure. Therefore, the P-wave transition layer is the mark of lithosphere instability. The layer may also be a product of asthenosphere transformation to lithosphere. In this case, the existence of the layer implies regional lithosphere cooling. Our model could better explain lithosphere delamination, crustmantle material exchange and magmatism in orogens, and hence suggests important implications for the continental geodynamics.

Key words: continental geodynamics, crust-mantle transition layer, delamination, underplating, crustal differentiation