40AR/39AR AND PALEOMAGNETIC CONSTRAINTS ON THE AGE AND AREAL EXTENT OF THE PICABO VOLCANIC FIELD: IMPLICATIONS FOR THE YELLOWSTONE HOTSPOT

The Picabo Volcanic Field (PVF) is one of the key silicic volcanic fields in the time-transgressive track of the Yellowstone hotspot. The location of this volcanic field along the Yellowstone-Snake River Plain track has been used to mark a major change in the apparent rate of the North American Plate over a presumed fixed hotspot mantle source. The first eruption of the PVF is the 10.41 ± 0.01 Ma Arbon Valley Tuff A and the last is the 7.58 ± 0.02 Ma tuff of American Falls. We sampled several ignimbrites based on their age and location that we suspected may be part of the PVF but we found they all originate from the Twin Falls Volcanic Field (TFVF). For example, base on age, paleomagnetics, and petrography the Picabo Tuff (PVT, 9.02 ± 0.11 Ma) and the tuff of Cotterel Mountains (C10A, 9.05 ± 0.13) correlated with the TFVF tuff of McMullen Creek (9.05 ± 0.07 Ma). All three ignimbrites of the Lake Hills were thought to be from the PVF but we found them to be part of the TFVF. This includes the oldest (Tivo, 9.38 ± 0.64 Ma) and the two younger ignimbrites we tentatively correlate with the TFVF Lincoln Reservoir member (7.98 ± 0.30; 8.70 ± 0.90 Ma). Based on the age and location of the first PVF eruption, and that of the first of the Yellowstone Volcanic Field, we calculated a migration rate of the Yellowstone volcanic track over the last ~ 10 m.y. to be 2.27 cm/yr. This is somewhat less than the migration rate of the deformation field (2.38 ± 0.2 cm/yr) over the same interval.

We also discovered a new ignimbrite marking the last eruption of the Heise Volcanic Field that we call the tuff of Birch Creek Sinks and dated at 4.37 ± 0.08 Ma. This ignimbrite was found directly overlying the Kilgore Tuff but yields a normal polarity. We believe others have misidentified this unit as the Kilgore Tuff.