PALEOMAGNETIC INSIGHT INTO THE CONSTRUCTION OF THE BUCKHORN RIDGE SILL, HENRY MOUNTAINS, UT

The Henry Mountains in south central Utah provide an ideal place to study the emplacement history of igneous intrusions. The country rock originally consisted of nearly horizontal Triassic and Jurassic shales and sandstones that have been tectonically inactive prior to and since emplacement. We analyzed paleomagnetic data to study the construction of the Buckhorn Ridge sill, a 40-m-thick intrusion that is part of the larger Mount Holmes intrusive center. Presently, this sill dips approximately 19 degrees N. Alternating field demagnetization of samples from the Buckhorn Ridge sill yielded two or three component natural remanent magnetization signal in almost all samples. The high coercivity component of individual cores at each site was consistently oriented (alpha95 < 4˚), while the low coercivity components were scattered and inconsistent. Three of four sites analyzed yield natural remanent magnetization orientations consistent with the stable North American reference pole for the age of the intrusion (33-23 Ma). This suggests the sill was emplaced in its present orientation – i.e. the sill was intruded while the surrounding sedimentary strata were already inclined due to earlier intrusion of an underlying laccolith. Our result contrast with the conclusions of earlier work by other authors on a distinct but nearby intrusive center. On the Mount Hillers intrusive center, paleomagnetic analysis of marginal sills suggested the sills were intruded and cooled while host rock strata were still horizontal.