PALEOMAGNETIC CONSTRAINTS ON THE RATE OF EMPLACEMENT OF THE PINE VALLEY MOUNTAINS LACCOLITH, SOUTHWESTERN UTAH

The goal of this research is to place constraints on the rate of emplacement of the Pine Valley Mountain laccolith in southwest Utah. On a much larger scale, the structure of this ancient magma chamber is analogous to Mt. St. Helens. Prior to the eruption in 1980, magma filled the chamber, inflating the sides of the volcano and when over steepened, caused a landslide that decreased pressure on the magma chamber, leading to the eruption. The northern flank of the Pine Valley laccolith is covered by large landslide deposits that are buried by lava flows from the intrusion. Previous workers have hypothesized that rapid inflation of magma into the shallow crust destabilized the surface and resulted in landslides. If the Pine Valley Mountain laccolith was emplaced rapidly, then there should have been very little (or no) time for the Earth’s magnetic pole to wander resulting in a paleomagnetic data set that records very little secular variation. To test this hypothesis, we collected 20 oriented hand-samples across a vertical profile from the base of the laccolith to close to its highest preserved point. Samples collected in the field were then cored, cut, and subjected to alternating field demagnetization up to 200 mT in the lab. Preliminary data suggests the Pine Valley Mountain laccolith has a strong, reproducible paleomagnetic signal composed of two components. The high coercivity component is interpreted to be the thermoremanent magnetization acquired during cooling, while the low coercivity component is interpreted to arise from chemical weathering post emplacement. The orientation of the high coercivity component is consistent with the expect orientation based on the North American polar wander curve for the time of emplacement (approximately 20 Ma). Once a larger volume of samples has been analyzed the rapid emplacement hypothesis will be tested by calculating the total amount of secular variation recorded by the laccolith. Rapid emplacement predicts that there should be no statistically significant variation.