Abstract

The explosive rhyolitic eruptions that define the track of the Snake River Plain–Yellowstone volcanism have produced a large volume of tephra found in late Miocene and younger basin-fill sediments throughout the western United States. Here we use 40Ar/39Ar isotopic dating, paleomagnetic analysis, major- and trace-element geochemistry, and standard optical techniques to establish regional tephra correlations. We focus on tephra deposits in three Neogene basins in spatially separated areas—Grand Valley, in eastern Idaho; Jackson Hole, in northwestern Wyoming; and the Granite Mountains area, in central Wyoming. These basins have experienced relatively continuous deposition from the late Miocene to the Holocene. We found tephra layers that directly tie the stratigraphy between all three basins. Using these correlations we found that basins experienced discrete pulses of extension separated by long periods of relative quiescence, the dates of which are staggered between basins. An early pulse of extension occurred in Grand Valley and Jackson Hole between 10.3 Ma and 16.33 Ma with a second pulse initiating between 4.54 Ma and 2.09 Ma. The Granite Mountains basin experienced a single pulse of extension sometime between 11.14 Ma and 6.75 Ma. These pulses of accelerated extension, along with evidence of similar pulses in other basins, present a pattern of west-to-east migration that we suggest is related to the Yellowstone hotspot. The later pulse of activity in Grand Valley and Jackson Hole corresponds to the migration of the North American Plate over the tail of the Yellowstone hotspot. We speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location, where the Columbia River Plateau Flood Basalt Province initiated in southeastern Oregon, to its outermost edge under central Wyoming. Our results are consistent with this model of a plume head, though not unique to it.

The results of our study also indicate that previous suggestions that the rate of Snake River Plain explosive volcanism has decreased by a factor of 2 or 3 since emplacement of the middle Miocene Trapper Creek tuffs likely underestimate post–Trapper Creek eruption rates. We have discovered a large number of previously unidentified post–middle Miocene major eruptive events, both as ash-flow tuffs and as vitric air-fall tuffs. Recalculation to include these newly discovered events results in a rate of major eruptions that is fairly uniform until ca. 4.54 Ma. However, there is a substantial gap in major silicic eruptions in the interval between 4.54 Ma and 2.09 Ma, which we call the “post–Heise eruptive gap.” With the exception of this gap, the rate of major eruptions on the Snake River Plain has been roughly constant since inception of the eastern Snake River Plain–Yellowstone volcanic track between 16 Ma and 17 Ma.

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