RELATIVE AGE DATING OF SILLS AND SATELLITE INTRUSIONS FROM PALEOMAGNETIC DATA, MT HILLERS, HENRY MOUNTAINS, UT

The laccoliths of the Henry Mountains in south central Utah provide an ideal place to study the emplacement history of igneous intrusions constructed from multiple magma pulses. The country rock consists of nearly horizontal Triassic and Jurassic shales and sandstones that have been tectonically inactive prior to and since mid-Tertiary igneous emplacement. We analyzed paleomagnetic data to study the construction of the Mt. Hillers intrusive complex. Alternating field demagnetization of samples yielded two or three component natural remanent magnetization signals in almost all samples. The high coercivity component of individual cores at each site was consistently oriented at most sites (α₉₅ < 5˚). Data collected from both steeply dipping sills and a satelite intrusion suggest these igneous units have experience diverse histories. We recognize five categories of intrusions based on the paleomagnetic results and the orientation of bedding in the nearby sedimentary rocks: (1) emplacement into horizontal country rock followed by tilting; (2) emplacement into tilted country rock followed by further tilting; (3) emplacement into tilted country rock followed by vertical axis rotation; (4) emplacement into horizontal country rock followed by inclined axis rotation (i.e. horizontal and vertical axis rotation); and (5) emplacement into dipping country rock followed by inclined axis rotation. The categories listed above are placed in order of increasing complexity of their structural history. We hypothesize that those units with the most complex histories are the oldest, while younger units have progressively more straight forward structural histories.