CATASTROPHIC GRAVITY SLIDING OF THE MARYSVALE VOLCANIC FIELD DURING RAPID GROWTH OF LACCOLITHIC BATHOLITHS: INSIGHTS FROM THE CENOZOIC MARYSVALE GRAVITY SLIDE COMPLEX, SOUTHWEST UTAH

The Cenozoic Marysvale volcanic field contains at least three documented mega-scale 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 east to west (Sevier gravity slide ~25 Ma, Markagunt gravity slide ~23 Ma, and Black Mountains gravity slide ~21.5 Ma) following a westward progression of volcanism in the field. 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 magma chambers 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). Detailed structural cross sections through the Marysvale volcanic field, utilizing deep petroleum exploration wells and geophysical data, indicate magma intruded into a thick package (~4-5 km) of Sevier age thrust faulted Paleozoic and Mesozoic strata, forming numerous shallow sills and laccoliths. Continuous volcanic growth of the Marysvale volcanic field was punctuated intermittently by rapid inflation of laccolithic intrusions that elevated the Marysvale volcanic field (as much as ~2-3 km) 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, high impact events associated with volcanic field development that could have important implications on hazard assessment of modern fields undergoing rapid magmatic inflation.