AN EMPLACMENT MODEL FOR LOW-VOLUME INTRUSIONS BASED ON THE SHONKIN SAG LACCOLITH, MONTANA

Based on observed deformation in the margins of the Shonkin Sag laccolith, we present a multiple-pulse emplacement model for the laccolith. The Shonkin Sag laccolith is a low-volume, hypabyssal intrusion with associated fringing sills emplaced into undeformed sandstones. This site is unique in that the base, roof, and eastern contact between the laccolith and fringing sills are exposed along a cliff face. This outcrop displays substantial deformation within the margins of the laccolith and fringing sills and in the surrounding sandstones. We observed varying levels of deformation using bulk magnetic susceptibility, anisotropy of magnetic susceptibility, and thin section analyses. These zones of deformation are characterized by normal and thrust faulting and separated by internal contacts. Due to a lack of regional deformation, the observed deformation is attributed to seven pulses of magma emplacement. Thermal models were used to constrain the duration of emplacement and indicate that the pulses emplaced over ~ 3 years. We present a three-stage emplacement model for the Shonkin Sag laccolith. In this model, 1) an initial sill intrudes, then 2) inflates near the magma source to produce the footprint of the laccolith, initiating failure of the overlying sandstone and allowing uplift. The newly formed laccolith then 3) continues to inflate as fringing sills emplace.

The emplacement of low-volume intrusions similar to the Shonkin Sag laccolith was previously observed prior to the 1980 Mt. St. Helens and during the 2011 Cordón Caulle eruptions. These intrusions occurred rapidly and caused extensive ground deformation, which induced the 1980 Mt. St. Helens eruption. The model developed for the Shonkin Sag laccolith may provide insight into how similar low-volume intrusions may emplace in the future, allowing for better prediction of potential hazards associated with rapid ground deformation.