Basin and Range rifting in northern Nevada: Clues from a mid-Miocene rift and its subsequent offsets

Abstract

A linear rift composed of dike swarms and graben-filling volcanic rocks marks the inception of Basin and Range rifting in northern Nevada 17 to 14 m.y. ago. This mid-Miocene rift, with its associated aeromagnetic anomaly, indicates S68/sup 0/W-N68/sup 0/E (+-5/sup 0/) extension. A small strike-slip (transform) fault at Sawtooth dike, one short element of the rift, confirms the mid-Miocene extension direction. The present extension, based on data throughout the Basin and Range province, is N65/sup 0/W-S65/sup 0/E (+-20/sup 0/), which is consistent with younger fault offsets of the rift. In the Sawtooth dike region this 45/sup 0/ change in direction of extension took place between 15 and 6 m.y. B.P. The Nevada rift is thought to be a part of a linear rift zone roughly 700 km long, which includes feeder dikes of Columbia River basalts and the mid-Miocene location of the Yellowstone hot spot. Extension in this zone was probably perpendicular to a then-active trench along the western plate margin. The clockwise change in extension direction is consistent with a superposition of right-lateral shear along the plate margin when the San Andreas fault was activated.