MAGNETIC CONSTRAINTS ON THE SOUTHWESTERN CONTACT OF THE BOTTLE LAKE IGNEOUS COMPLEX NEAR PASSADUMKEAG MOUNTAIN, ME

Sparse bedrock exposures hinder field mapping in the forested areas of rural Maine. We used field magnetic methods to better constrain the location of the contacts between the Passadumkeag River granite, western component of the Bottle Lake igneous complex, and its host rocks near Passadumkeag Mountain in east-central Maine. The Bottle Lake complex is one of the largest Devonian plutons intruded ~380 Ma into chlorite grade Cambro-Ordovician and Silurian metasediments northwest of the Norumbega fault system. Our motivation for this study is estimating the width of the contact metamorphic aureole for thermal modeling of what appears to have been a multiphase emplacement process. Previous geophysical work has included seismic lines and gravity modeling to delineate the pluton, which is estimated to be a lens up to 5 km thick.

Total magnetic field data were collected using a proton precession magnetometer at 160 m (0.1 mi) intervals along a highway, a network of logging roads, and a new access road for the installation of wind turbines along the crest of Passadumkeag Mountain. Using two sensors, we sampled at different elevations to be able to compute vertical magnetic gradients, and monitored time variation in the local magnetic field at base stations midway along two of our three transects. Following the field campaign, the data were plotted according to latitude (for the S-N sections) and longitude (for the W-E section) and compared to the regional magnetic gradient reduced by 53000 nT extracted from the ME state magnetic map.

Little temporal magnetic field variation was noted and only the longer W-E transect needed correction for time, with deviations up to 30 nT. A positive magnetic anomaly nearly 100 nT from the regional trend, accentuated on the vertical gradient plot, suggests that the contact lies near the western end of the W-E transect. Positive anomalies near the southern ends of the two S-N sections, approximately 6 km apart, suggest that the southern contact of the pluton lies high on the slope of Passadumkeag Mountain. On the easternmost transect, the anomaly is 300 nT greater than the regional trend, and on the westernmost transect, the anomaly is nearer 50 nT, but the vertical gradient is reversed. Further modeling will explore the geometry and magnetic properties of subsurface bodies that can account for such anomalies.