![]() ![]() ![]() rifting event is considered to be related to a major change in the tectonic setting of the western United States that occurred after the deposition of the Belt Supergroup and related rocks. The Belt Supergroup, which is locally as much as 20 km thick, was deposited, according to this theory, in one of these abandoned arms. It is considered to have developed along a system of three-armed rifts in which the abandoned arm is preserved as deep epicratonic troughs (aulacogens) extending into the craton at a high angle to the trend of the main rift. rifting event is seen as cutting across trends of Precambrian crystalline basement rocks. Speculation concerning Precambrian rifting has focused on the possibility of major continental rifts about 1,450 m.y002E and 900? to 650 m.y. Rifting has occurred episodically in the western United States from the Precambrian to the Cenozoic.
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