Around 30 million years ago the western u. S. Experience a large amount of crustal extension.
Stretching of the crust causes brittle deformation of the upper crust, breaking into debris oriented perpendicular to the direction of stretching. This can be manifested by a group of faults that slope in the opposite direction (symmetrical) or faults that all slope in the same direction (asymmetric).
Causes of this crustal expansion first discussed in the report include mantle convection, mantle plume, the opposite force of slab pull and ridge push, and gravitational collapse.
Where does the crustal extension occur?
Divergent Plate Boundaries are actively expanding zones due to the inclusion of the newly formed crust in the mid-ocean ridge system in the opening process.
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