PALEOMAGNETIC CONSTRAINTS ON THE RELATIVE TIMING OF EMPLACEMENT OF IGNEOUS INTRUSIONS FROM THE SOUTH SIDE OF MT. HILLERS, HENRY MOUNTAINS, UTAH

Recent research suggests that many plutons are assembled from a series of magma pulses. We use paleomagnetic data to investigate the relative emplacement timing of mid-Tertiary intrusive pulses at Mt. Hillers, in the Henry Mountains of southern Utah. The igneous centers of the Henry Mountains experienced no syn-tectonic deformation, making it an ideal location to study plutons in their near original state.

We used an alternating field demagnetizer to acquire the orientation of the natural remanent magnetization from approximately thirty specimens from igneous bodies and adjacent sedimentary strata on the south side of Mt. Hillers . The sites primarily show a two-component magnetization, in which the low coercivity component is interpreted as a chemical weathering signal. We interpret the remaining high coercivity component as the thermal remanent magnetization (TRM). Both the igneous and sedimentary data are fairly consistent with a 95% confidence interval <15 degrees. Orientations of regional bedding away from strata tilted by the intrusions constrain the pre-emplacement bedding orientation. We use the magnitude of angular deviation from the expected TRM to infer the relative age of component intrusions – i.e. an intrusion that requires a greater correction than another from the same area has experienced more deformation, implying that it is older.

The magnetization of the igneous rock at sites HL13-18 and HL13-20 falls within the expected range for Oligocene emplacement, suggesting magma at these locations was emplaced while strata were in their presently tilted state. Analysis of a much older package of Entrada Sandstone (HL13-19) yields a similar orientation, suggesting the paleomagnetic signal in this sedimentary rock was reset during igneous emplacement.