Simplified models of continental lithospheric extension demonstrate that the strength of the lower crust is an important factor in the evolution of rifting. When the lower crust is strong, both the crust and the mantle lithosphere should extend in the same place. When the lower crust is weak, however, the upper crust can mechanically decouple from the lithospheric mantle during extension. Two distinct styles of extensional decoupling can be recognized. Diffuse decoupling occurs when the lower crust flows laterally in response to topographically induced pressure gradients. Offset decoupling occurs when stretching of the upper crust is horizontally displaced from mantle lithospheric stretching. We show that diffuse decoupling is expected for a range of possible crustal mineralogies, but that offset decoupling only occurs for extremely weak mineralogies.

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