CONSTRAINTS ON MINIMUM RATES OF IGNEOUS EMPLACEMENT FROM PALEOMAGNETIC DATA, TRACHYTE MESA LACCOLITH, 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 from numerous distinct meter- to decameter-scale igneous sheets on the east side of the much larger Mount Hillers intrusive center, which has been dated to 24.75 ± 0.50 Ma. Uncertainties associated with radiometric dating, however, too large to investigate sheet-by-sheet emplacement rates in small, rapidly emplaced intrusions like Trachyte Mesa. Instead, we propose to use the secular variation of Earth’s magnetic field to investigate rates of emplacement at the sheet scale. Holocene secular variation is highly variable from year to year with a maximum rate of secular variation in the Holocene of 0.065°/yr. The geologic record of secular variation does not allow rates of emplacement to be tightly constrained, although the minimum time of emplacement can be calculated. As a proof of concept, we analyze precisely dated lava flows from Hawaii that also record changes in the virtual geographic position (VGP) of the magnetic north pole position. If the maximum rate of secular variation recorded in the Holocene is applied to the Hawaiian flows, the minimum time elapsed between the flows can be accurately obtained. The greater the amount of time between the Hawaiian flows, the higher the amounts of secular variation, and time elapsed is more accurate. This approach is sound in the Holocene and if it applies to the Oligocene, than the minimum amount of time between the emplacement of sheets in Trachyte Mesa can be calculated. Fifty cores were collected for paleomagnetic analysis from ten different sheets within the stack that comprises Trachyte 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˚. All ten sites taken together define a mean orientation consistent with emplacement ca. 25 Ma. The sites record a minimum of 95° of secular variation. If the maximum amount of wander seen in the Holocene applies to the Oligocene, then emplacement of Trachyte Mesa took at least 2500 years.