NEW CONSTRAINTS ON THE EXTENT, AGE, AND EMPLACEMENT HISTORY OF THE EARLY MIOCENE MARKAGUNT MEGABRECCIA, SOUTHWEST UTAH—ONE OF THE WORLD'S LARGEST SUBAERIAL GRAVITY SLIDES

New geologic mapping of the Panguitch and Beaver 30´x60´ quadrangles―which straddle the Basin and Range and High Plateaus in southwest Utah and which include the southern Marysvale volcanic field―redefines the extent, age, and emplacement history of the early Miocene Markagunt Megabreccia. The Markagunt Megabreccia, the remains of the catastrophically emplaced Markagunt gravity slide (MGS), consists of Miocene and Oligocene, locally derived andesitic volcanic mudflow breccias and subordinate lava flows and volcaniclastic rocks, and intertonguing regional ash-flow tuffs. With an apparent aerial extent of at least 3400 km², the MGS is among the largest known subaerial gravity slides on earth. From its inferred breakaway zone in the Tushar Mountains to the southern limit of its debris avalanche deposits, the MGS is nearly 95 km long and locally at least 50 km wide; it slid at least 30 km over the former early Miocene land surface.

Striations and Riedel shears on basal slip surfaces, thin basal cataclastic and sheared breccia, clastic dikes, and crushed and rehealed clasts provide strong evidence of catastrophic emplacement from the north by gravity sliding. Pseudotachylyte occurs as thin layers along subhorizontal secondary internal shear planes and as veins injected into fractures oblique to these planes. The uniformity of directional indicators, the stratigraphic sequence of volcanic rocks in the upper plate, and the overall geometry of the MGS show that it represents a single emplacement event and not multiple, smaller gravity slides derived from various sources. The basal slip surface was in the landslide-prone Eocene-Oligocene Brian Head Formation, a non-resistant tuffaceous sedimentary unit at the base of the volcanic section. The MGS was emplaced on rocks as young as the 22 Ma Harmony Hills Tuff and is overlain by the undeformed 21.6 Ma Haycock Mountain Tuff. The MGS represents catastrophic gravitationally induced collapse, about 22 to 21 million years ago, of the southwestern sector of the Oligocene to early Miocene Marysvale volcanic field, possibly triggered by pre-caldera inflation of the 20-18 Ma Mount Belknap caldera. Subsequent basin-range extension complicates estimates of the initial dip of the slip surface; it may have been just a few degrees at the breakaway zone and subhorizontal south of the ramp.