Preliminary Paleomagnetic Data From Santa Mariá Volcano, Guatemala and Their Bearing on the Mono Lake and Hilina Pali Excursions

Paleomagnetic directions determined from oriented block samples collected by Rose et al. in 1977 ( Journal of Geology) and from eight paleomagnetic sites drilled in lava flows from Santa Maria volcano, Guatemala in 1990 define a pattern of variation similar to the pattern of geomagnetic field changes recorded by the sediments of the Wilson Creek Formation near Mono Lake, California. This led Conway et al. in 1994 ( Journal of Geology) to suggest that these flows had recorded the Mono Lake Excursion (MLE). The correlation was made on pattern recognition alone and relied almost entirely the well- defined inclination dataset than on the declination data; no radioisotopic ages were available. In March of 2005 we returned to the crater of Santa Mariá and drilled 23 lava flows from the original sections of Rose et al; block samples for 40Ar/39Ar were also collected. Unfortunately aggradation in the crater due to mass wasting made it impossible to sample all the flows of Rose et al. At each site or lava flow, four to seven cores were drilled and oriented with a sun compass. Samples cut from the drilled cores were magnetically cleaned using alternation field demagnetization and analyzed using principle component analysis. Thermal demagnetization is currently underway. The resulting inclination waveform (over 70° of change from +60° to -12°) is very similar to those previously reported in the literature for the MLE, but the declination waveform shows little variation (<25°; mean declination is 13.4°) throughout the stratigraphic sequence that we collected. Consequently, VGP data from the lava flows do not show the classic clockwise and counterclockwise loops as seen at the Wilson Creek section and at other MLE locations. Instead the directions (VGPs) tend to cluster in three distinct groups with the lowermost lava flows (5) and uppermost lava flows (3) clustering near the expected axial dipole inclination for the region (~28 °) while lava flows from the middle of the stratigraphic section have inclinations near zero (+8 ° to -12°). The transition between the low-inclination middle section and the upper section is marked by flows with inclinations up to +60°. This is also seen in the Conway data set. Preliminary 40Ar/39Ar dates from lava flows having near zero inclinations suggest an age of 20 ka. Therefore the possibility exists that the Santa Maria lava flows have recorded the Hilina Pali Excursion (HPE). In fact the magnitude of the inclination change recorded in the Santa Maria lava flows is very similar to that recorded by the lava flows from the Hawaiian Scientific Drilling Project. This suggests that the HPE is at least a regional geomagnetic event and may be useful as a tool for stratigraphic correlation. However, paleointensity data is needed before any firm conclusions can be drawn.