CATASTROPHIC GRAVITY SLIDING OF VOLCANIC FIELDS DURING RAPID GROWTH OF BATHOLITHS: INSIGHTS FROM THE CENOZOIC MARYSVALE GRAVITY SLIDE COMPLEX, SOUTHWEST UTAH (Invited Presentation)

Continued work in the Cenozoic Marysvale volcanic field has documented at least three gigantic gravity slides resulting from the southward collapse of the field. Slide masses form an overlapping contiguous complex covering an area >8000 km², with the largest slide (Markagunt gravity slide) being >3500 km². New age constraints show sliding events progressed from oldest to youngest (Sevier gravity slide ~25 Ma, Markagunt gravity slide ~23 Ma, and Black Mountains gravity slide between ~19.5-18 Ma) and followed a westward progression of volcanism in the field. Slide masses moved along three distinct zones: (1) a slide zone (along with a breakaway area) containing a bedding-plane-parallel segment ~55-65 km long, (2) a ramp zone ~1-2 km long where the slide masses cut up section from the slide zone, and (3) a former land surface zone with >35 km of movement over the latest Oligocene and Miocene erosional landscape. Catastrophic emplacement of each slide is indicated by features such as basal layers of sandstone-conglomerate-like material, clastic dikes (injectites) of the same material, ultracataclastically deformed rocks, jigsaw puzzle fracturing, and pseudotachylytes (frictionites).

The Marysvale volcanic field was produced by a series of arc-derived sub-volcanic batholiths formed during the roll-back of the Farallon Plate following the classic shallow-plate induced Laramide Orogeny. Plate roll-back produced transient volcanic flare-up events (also referred to as ignimbrite flare-ups) of high volcanic flux that are the surface manifestations of batholith formation at depth (de Silva and Gosnold, 2007). Work by Petford et al. (2000) demonstrates that granitic plutons can be emplaced rather rapidly (a large 5 km thick pluton can be emplaced into the upper crust in ~10⁵ years). It is our premise that continuous growth of the Marysvale volcanic field was punctuated intermittently by rapid inflation of batholiths that elevated the Marysvale volcanic field to produce the necessary unstable slopes for volcanic field collapses. The resulting mega-slides (some of the largest on Earth) represent a new class of low frequency, but high impact events associated with volcanic field development that could have important implications on hazard assessment of modern fields undergoing rapid magmatic inflation.