PALEOMAGNETIC INVESTIGATION OF PULSED ASSEMBLY OF THE MAIDEN CREEK SILL, HENRY MOUNTAINS, UT

The Maiden Creek sill is a small igneous intrusion located on the east side of the Mount Hillers intrusive center in the Henry Mountains, UT. The sill has a tabular geometry with several protrusions that extend from the main body to the northeast and to the south. Previous investigation of the sill revealed meter-scale variations in the magnetic susceptibility measured along vertical profiles in the field. This finding was corroborated by an analogous pattern observed in the thermoremanent magnetization. The paleomagnetic data are consistent with the 25 Ma pole position and vary on a meter-scale over the 20 m thick sill, suggesting the intrusion is constructed of at least 20 1-m-thick discrete pulses of magma.

In the present study, paleomagnetic data were collected from 22 sites spaced ~5 cm apart over ~3.7 m vertical distance on the same face where the original magnetic susceptibility study was conducted. Preliminary results from a 1-d thermal model suggest heat flow from fresh magma pulses into the surrounding porphyry will not cause the surrounding rocks to exceed the blocking temperature. Therefore we expect the hypothesized 1 –m-thick pulses of magma to retain their original thermoremanent magnetization. The samples were found to be resistant to alternating field demagnetization, with only 80% of the magnetic signal being removed in most cases. A comparison of samples within each site revealed a high degree of internal variation, taken as evidence of a chemical overprint. Under alternating field demagnetization the measurable portion of the magnetic signal was inconsistent with 25 Ma pole positions. However, the remaining 20% of the signal must be analyzed as it may contain a significant part of the high coercivity component of the magnetic vector. For this reason, the next step will be to analyze the samples under thermal demagnetization, which will allow for analysis of the magnetic vector in its entirety.