Abstract

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault.

The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse (∼95% volumetrically) off the Columbia River flood basalts in northern Oregon ∼15.5-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The site of the initial breaching of the North America plate by this plume is probably the McDermitt volcanic center at the north end off the rift near the Oregon-Nevada border. The present north-northwest trend of the rift and its internal elements, such as dikes and lava-filled grabens, record the orientation of the arc-normal extensional stress in this back-arc region at the time of emplacement. Paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range and Rio Grande rift provinces.

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