The concept of plate tectonic subduction has helped explain the compressive deformation of the U. S. Cordillera, but continued convergent plate movements have not explained as satisfactorily the end of compression from Alaska to southern Mexico about 40 m.y.B.P., nor the extensional faulting from Canada through Mexico since about 20 m.y.B.P. Accordingly, it may be appropriate to reconsider Hess' original concept of a two-sided mantle convection sink.

The axis of a deep Bouguer gravity anomaly (below 125 km or 78 mi) extending from Mexico through western Colorado into Canada may represent the axis of a mantle convection sink. The low-density rocks along the axis of the anomaly may originate from sinking mantle convection currents and/or magmatic differentiation of upper mantle rocks which were carried into the deeper, hotter mantle.

If mantle convection currents that transported the Pacific subduction plate eastward and those that transported the North American plate westward both met and sank in the Cordilleran mantle sink, then the two limbs of the sink would be defined. However, a "shadow zone" between the top of the subduction plate and base of the lithosphere offers the possibility that westward-directed mantle convection currents may have exerted enough stress on the subduction plate to end compression and start extension of the lithosphere, despite continued convergent plate movements and subduction.

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