In geology, the slab gap hypothesis is one of the explanations put forward to explain several instances of crustal extension that occur inland near former subduction zones.
In this view, there is no mantle upwelling, so once the crustal rift is overridden, the only residual effects are from the remnant descending plate slab.
However, actual observations of the crust in western North America where the Farallon plate's trench and rift was snuffed out millions of years ago by the westward movement of the North American plate, and replaced by the San Andreas Fault, show not compression inland, but extension.
The fast melt is because the portion of the subducted plate nearest the spreading zone is thin and still warm from its recent creation.
This idea has been used to explain the extension and very large flood basalts that occurred in what is now southern Washington, Oregon and northern California about 17 million years ago (see Columbia River Plateau).