Starting around 16 Ma in Miocene time and continuing into the present, a large part of the North American Plate in the region has been under extension by literally being pulled apart. Debate still surrounds the cause of this crustal stretching, but an increasingly popular idea among geologists called the slab gap hypothesis states that the spreading zone of the subducted Farallon Plate is pushing the continent apart. Whatever the cause, the result has been the creation of a large and still-growing region of relatively thin crust; the region grew an average of per year initially and then slowed to per year in the last 5 million years. Geologists call this region the Basin and Range Province.