NON-MAGNETIC ANOMALOUS LOWER CRUST OF THE ADIRONDACK MOUNTAINS, NORTHEASTERN U.S. FROM DEFRACTAL SPECTRAL MAGNETIC BOTTOM DETERMINATIONS

A blind test of spectral magnetic bottom determinations from the Defractal method in the Adirondack mountains in New York from the long-wavelength corrected NURE_NAMAG2008 magnetic anomaly data showed that the crust in the region below the depth of 15-20 km has either very little magnetization or is effectively non-magnetic. On the other hand, the Grenville and the Taconic/Appalachian domains to the west and the east of the mountains, respectively, have magnetization extending to deeper levels of the crust and, in places, up to the Moho, within the vertical resolution of the magnetic and the seismic methods (about 5 km for magnetic bottoms in this region and about 3-4 km for the seismic refraction Moho). Based on previous investigations of geologic, gravity, magnetic property, magnetic anomaly, seismic refractions, seismic reflection, tele-seismic receiver functions in the region, the top of the Adirondack non-magnetic middle-lower crust corresponds to the anomalous seismic reflectivity zone previously named as the Tahawus complex. The complex also corresponds to the zone of high P- and S-wave velocities (about 7.1 and 4 km/s, respectively); below the complex the velocities abruptly decrease prior to increasing gradationally to the upper mantle velocities near the Moho. Below the high velocity Tahawus complex, there is a zone of high electrical conductivity throughout the Adirondacks. The isostatic gravity anomaly high over the Central Granulite Terrane of the Adirondacks could be partly interpreted by the higher density of this middle crustal mafic layer. Because the present-day geotherms in the Adirondacks are low, the lack of magnetization of the middle crustal mafic layer and of the high conductivity lower crust may be attributed to non-magnetic mineralogy rather than the Curie temperature phenomenon. The magnetic and other geophysical results are consistent with the Adirondacks being anomalous with respect to the surrounding regions. The anomalous topography and geophysical properties of the Adirondacks crust are consistent with the magmatism during the passage of the region over the Great Meteor hotspot which may have led to faulting near its edges and these faults may have reactivated during the glacial rebound resulting in the post-glacial anomalous uplift of the Adirondacks.