PALEOMAGNETIC CONSTRAINTS ON THE TIMING OF INTRUSIONS IN GOLD CREEK CANYON, MT. HILLERS, HENRY MOUNTAINS, UTAH

Intrusive rock bodies exposed within Gold Creek canyon on Mt Hillers in the Henry Mountains of southern Utah preserve multiple stages of shallow crustal magma emplacement and host rock deformation. Previous studies provide evidence that plutons progressively grow as magma pulses of different sizes and geometries coalesce. The Henry Mountains in southeastern Utah are mid-Tertiary intrusions of diorite porphyry that intruded into subhorizontal late Paleozoic and Mesozoic strata of the Colorado Plateau. These intrusions are younger than regional Laramide deformation, suggesting that rotation of host strata in the Henry Mountains is due solely to magma emplacement.

Paleomagnetic data from tilted rocks in the Henry Mountains provide constraints on the chronology of emplacement of the igneous bodies. Sites with a paleomagnetic orientation consistent with either depositional age or mid-Tertiary emplacement were likely emplaced in situ, whereas those that show deviation from the expected direction have experienced some post-emplacement deformation. In Gold Creek, located the eastern flank of Mount Hillers, we collected paleomagnetic data at 13 stations. Four of the igneous sites yield thermoremanent paleomagnetic signatures. These samples demonstrate good internal consistence (i.e. α₉₅ ≤ 5˚). Sites HL13-2 and HL13-7 are sills whose paleomagnetic signatures match the expected direction for a body cooling in the mid-Tertiary, suggesting they were emplaced into already tilted strata. Sites HL13-5 and HL13-8 record post-emplacement rotation and suggest, respectively, approximately 90˚ and 30˚ of vertical-axis, counter-clockwise rotation.

These preliminary data suggest a three-phase emplacement history for this flank of the Mount Hillers laccolith. During the first phase of igneous activity, the strata were already tilted, likely due to intrusion of the main body of Mt. Hillers. HL13-5 and -8 were then emplaced, followed by more main-body emplacement that resulted in vertical axis rotation of these sills. HL13-2 and -7 were emplaced after tilting ceased. Additional paleomagnetic data from Gold Creek should allow for further testing of this hypothesis.