RAPID EMPLACEMENT OF THE PINE VALLEY LACCOLITH DETERMINED BY PALEOSECULAR VARIATION ANALYSIS, SOUTHWESTERN UTAH

The Pine Valley laccolith was emplaced into the shallow crust along the western edge of the Colorado Plateau in the Early Miocene. The emplacement of this approximately kilometer-thick pluton is associated with both volcanic activity and massive gravity slides. Previously identified field relationships show emplacement of the pluton occurred first, then gravity sliding, and lastly volcanic venting. This suggests a potential causal relationship between events: subsurface magma flux leading to oversteepening along the flanks of the laccolith, landslide generation, followed by a subaerial eruption. This sequence of events is similar to the 1980 Mt. St. Helens landslide and eruption and suggests a rapid emplacement model for the Pine Valley laccolith. This hypothesis is evaluated with paleomagnetic data collected in a vertical section throughout the Pine Valley Mountain pluton. Instantaneous emplacement on a geologic time scale should record very little movement of the magnetic pole about true north (i.e. secular variation). The paleomagnetic data suggests the pluton records a minimum of 160±60˚ of secular variation, which would correspond to a minimum of 3200±1200 years elapsed during construction. Emplacement on this timescale indicates a magma flux rate on the order of 0.01-0.10 km³/yr. In comparison to other igneous systems, this would either be a slow volcanic system or a fast plutonic system.