PALEOMAGNETIC INSIGHT INTO THE EMPLACEMENT HISTORY OF THE NORTHEASTERN FLANK OF MOUNT PENNELL, HENRY MOUNTAINS, UTAH

Paleomagnetic data provide constraints on the igneous emplacement history of the northeastern flank of Mount Pennell of the Henry Mountains, southeastern Utah. Mount Pennell is a laccolith consisting of a series of igneous sills and dikes of mid-Tertiary age intruded into shale and sandstone of Jurassic to Cretaceous age. The interior of the Colorado Plateau, where the Henry Mountains are located, has sub-horizontal strata that experienced little tectonic deformation. The emplacement of a shallow crustal pluton, consisting of multiple intrusive bodies, caused deformation in this area, making this area an ideal setting to study emplacement processes. Emplacement of each new magmatic pulse resulted in the deformation of earlier igneous intrusions and sedimentary host rock. Paleomagnetic vector orientations from both igneous and sedimentary units help to constrain the emplacement history.

Seven paleomagnetic sites, four igneous and three sedimentary, were drilled on the northeastern flank of Mount Pennell, generating 75 specimens. One- and two-component system demagnetization styles were observed. For systems comprised of two components, the high coercivity component was selected as the thermoremanent magnetization. Relative ages of intrusions are interpreted based on increased deviation, in older igneous rocks, between expected and observed magnetization orientations. The magnetic vectors obtained from the seven sites indicate that the rocks obtained their vector in situ, suggesting the rocks were emplaced in their inclined state, after tilting of the host strata. The sedimentary rocks contain the same magnetic unit vector as the igneous rocks, suggesting that the emplacement of the pluton reset the magnetic vector of the sedimentary rocks.