USING ZIRCON GEOCHRONOLOGY TO CONSTRAIN THE AGE OF THE BLACK MOUNTAINS GRAVITY SLIDE, MARYSVALE VOLCANIC FIELD, UTAH, USA

The newly discovered Black Mountains gravity slide (BMGS) is the youngest of three catastrophic collapse events of the Oligocene-Miocene Marysvale gravity slide complex. These mega-scale gravity slides represent southward collapse of a large portion of the Marysvale volcanic field that developed during roll-back of the Farallon Plate that also produced transient Cenozoic volcanic flare-up events (also referred to as ignimbrite flare-ups) following the classic shallow-plate induced Laramide Orogeny. The three gigantic gravity slides form an overlapping contiguous complex covering an area >8,000 km², with the largest slide (Markagunt gravity slide) being >3,500 km². Field mapping relationships and new age constraints show sliding events progressed from oldest in the east to youngest in the west (Sevier gravity slide at 25 Ma, Markagunt gravity slide at 23 Ma, and Black Mountains gravity slide younger than 22 Ma), following a westward progression of volcanism in the field. Both the Sevier and Markagunt gravity slides have tight age constraints by U-Pb and/or ⁴⁰Ar/³⁹Ar dates on rock units deposited above and below the slide masses. However, current mapping of the BMGS (covering an area of ~3,000 km²) constrains the age only as being younger than the underlying 22 Ma Harmony Hills Tuff that it was emplaced upon. Here, we present age constraints determined through analysis of zircons collected from a sandstone and a tuffaceous sandstone deposited on top of the BMGS to provide a lower bracket on its emplacement age. Zircons were separated from these rock units using traditional gravitational and magnetic techniques and later analyzed at the University of Arizona Laserchron Center. The sandstone unit (n=142) has zircons that range in age from 19-2700 Ma and has a maximum depositional age of 21.09 +/- 0.37 Ma. The tuffaceous unit (n=186) has zircons that range in age from 19.2-2884 Ma and has a maximum depositional age of 21.04 +/- 0.40 Ma. The maximum depositional ages of these two localities overlap in error and indicate that the BMGS was emplaced before 21 Ma. Our data now tightly constrains the emplacement of the BMGS between 22-21 Ma.