RECONSTRUCTING LATE MIOCENE BASIN AND RANGE EXTENSION ALONG THE OLDEST PART OF THE YELLOWSTONE HOT SPOT TRACK IN NORTHWESTERN NEVADA

The earliest volcanic rocks attributed to the Yellowstone hot spot erupted from the McDermitt Caldera and related volcanic centers in northwestern Nevada at 17-15 Ma. At that time, a major episode of extensional faulting was underway to the south in central Nevada, leading some to suggest that the nascent hot spot caused or facilitated Miocene Basin and Range extension. Regional geologic relations indicate that the total magnitude of extension in northwestern Nevada is low compared to the amount documented in central Nevada, and that extension was accommodated almost entirely by the widely spaced, high-angle normal fault systems that formed the modern ranges. The Pine Forest Range is located on the westernmost margin of the Basin and Range in northwestern Nevada and exhibits a well-exposed record of Tertiary volcanism and extensional faulting. On the west side of the range, 38-15 Ma volcanic rocks form a conformable ~1200m thick sequence that is tilted ~35 degrees to the west and was uplifted along a major east-dipping normal that bounds the range to the east. The absence of angular unconformities within this section demonstrates that no faulting and tilting took place over the interval 38-15 Ma. Tilting of the volcanic rocks thus postdates 15 Ma, and apatite fission-track data from the northern Pine Forest Range indicate that faulting was underway at 8-10 Ma, and began only slightly earlier. To the east, fission-track data from the Santa Rosa Range indicate a similar 7-8 Ma age for faulting, demonstrating that Basin and Range extension in northwestern Nevada is significantly younger than the 17-15 Ma volcanism attributed to the hot spot. This argues against a direct link between the Yellowstone hot spot and the initiation of extension in this area and casts additional doubt on the role of the hot spot in extension across the broader Basin and Range Province.