ANISOTROPY OF MAGNETIC SUSCEPTIBILITY ANALYSIS SAMPLES FROM THE PINE VALLEY MOUNTAIN LACCOLITH, SOUTHWESTERN UTAH

Volcanic systems that fill quickly can be accompanied by devastating landslides. This occurred repeatedly from the flanks of laccoliths in the Miocene of southwestern Utah, including from the emplacement of the gigantic Pine Valley Mountain laccolith. Previous geologic mapping suggests that rapid emplacement of this monzogranitic laccolith in the shallow crust (<<1 km) resulted in surface uplift and initiation of large-scale landslides of the overlying sedimentary country rock. We seek to better understand the construction of the Pine Valley Mountain laccolith using paired magnetic susceptibility and paleomagnetic analyses of samples from the Pine Valley laccolith collected at regular vertical intervals throughout the intrusion. Anisotropy of magnetic susceptibility (AMS) fabrics should show the flow direction of magma. If the hypothesis is correct, then the AMS fabrics from the entire laccolith should originate and spread from the same location. Additionally, if the Pine Valley Mountains laccolith was assembled from a series of magmatic sheets to grow into a laccolith, we expect to see K3 (the minimum axis of the AMS ellipsoid and the pole to the magnetic foliation) to be nearly vertical. All AMS analyses will be paired with estimates of the orientation of the thermoremanent magnetization acquired during emplacement. These data should point in the expected direction for North America in the Miocene. Alternatively, if the laccolith has been disturbed by post-emplacement deformation then restoration of the AMS fabric to horizontal should result in paleomagnetic orientations parallel to the expected direction. Preliminary analysis shows that the AMS fabrics are consistent at the hand sample scale. Fabrics vary from oblate to slightly prolate and are characterized by a well-defined maximum axis (K1). The minimum axis (K3), however, is less well defined and varies from steeply to moderately plunging.