PALEOMAGNETIC CONSTRAINTS ON THE DURATION OF LACCOLITH GROWTH, TRACHYTE MESA INTRUSION, HENRY MOUNTAINS, UT

The Trachyte Mesa laccolith is located in the Henry Mountains, a group of Oligocene igneous intrusions emplaced into flat-lying strata of the Colorado Plateau in Utah. The Trachyte Mesa intrusion is a small (0.05 km3), elliptical body formed on the east side of the much larger Mount Hillers laccolith, near several other satellite intrusions. Mount Hillers has been dated to 24.75 ± 0.50 Ma. The adjacent Black Mesa satellite intrusion (0.4 km3) is eight times larger than Trachyte Mesa, and according to previously published thermal modeling was emplaced in less than 100 years. We therefore expect Trachyte Mesa to be emplaced in even less time if both intrusions are characterized by similar rates of emplacement. In this study we attempt to constrain the duration of emplacement of Trachyte Mesa using paleomagnetic data to quantify the amount of recorded secular variation.

Fifty cores were collected from ten different sheets within the stack that composes Trachtye Mesa. Individual cores typically display either one component or two component systems, in which case the high coercivity component was used. The sites displayed a high degree of internally consistency with 95% confidence intervals less than 5˚.

If thermal modeling results from Black Mesa are applicable to Trachyte Mesa, then Trachyte Mesa should have been emplaced in much less than one hundred years. Such a short time frame for emplacement leaves little time for secular variation of the Earth’s magnetic field. Therefore the paleomagnetic signal recorded by different sheets within Trachyte Mesa should have very similar orientations. Our results, however, indicate the sheets within Trachyte Mesa record secular variation. All ten sites taken together define a mean orientation consistent with emplacement ~25 Ma. Individual sites, however, appear to record secular variation around that mean. Thousands of years of data are typically needed to average out secular variation. This suggests Trachyte Mesa took much longer to emplace than previously thought. Assuming the Holocene rate of secular variation applies to the Oligocene, our data suggest that emplacement of Trachyte Mesa laccolith occurred over a period of 100 to 1000 years.