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1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
2 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
During middle Miocene time, western North America was subject to flood-basalt volcanism, dike-swarm injection, and broad-scale fracturing and folding of the crust. We propose a simple model to account for these events and for a regional pattern of geologic and geophysical features. Aeromagnetic maps reveal some of the most important elements of this pattern, which are several narrow, arcuate anomalies, here referred to as the Northern Nevada rifts. These rifts extend hundreds of kilometers across Nevada and are likely caused by highly magnetic, middle Miocene mafic dikes. With the aid of filtering techniques, the anomalies can be traced into Oregon. Together with other geologic features, such as fold axes, dike swarms, and faults, they produce a spoke-like pattern fanning over 220° of arc that converges toward a point near the Oregon-Idaho border (lat 
44°N). A possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the Yellowstone hotspot. The spoke-like pattern, however, does not persist at large distances from the emerging hotspot; several hundred kilometers to the south, the Northern Nevada rifts deviate significantly (>30°) from a radial trend. We show that a simple model—imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle Miocene stress direction—fits the observed fracture pattern. It thus accounts for both the radial pattern present near the nascent hotspot and the far-field pattern due to regional stresses.
Key Words: Yellowstone hotspot • Northern Nevada rifts • Snake River Plain • dike swarm • flood basalts • Columbia River Plateau
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