A record of crustal-scale stress; igneous foliation and lineation in the Mount Waldo Pluton, Waldo County, Maine

Abstract

The Late Devonian Mount Waldo pluton is a porphyritic granite that has a magmatic foliation and lineation. Foliation is defined by: (1) preferred orientation of potassium feldspar phenocrysts and biotite crystals, and (2) the anisotropy of magnetic susceptibility (AMS). AMS alone defines the lineation. The carrier of the AMS signal is multidomain magnetite, confirmed by petrographic observation, high bulk susceptibility (generally >3 E3 SI units) of samples, and results of isothermal remanent magnetization experiments. AMS data from 26 sites in the pluton consistently show a subhorizontal north–south-trending lineation (defined by the orientation of the maximum axis of the AMS ellipsoid) and a subvertical north–south-striking foliation (whose pole is the minimum axis of the AMS ellipsoid). At individual sites, the feldspar and AMS foliations are parallel. The lack of evidence for significant solid state deformation in the pluton suggests that both the lineation and foliation are entirely magmatic in origin. The pluton's relatively small size (15 km diameter) and shallow emplacement depth (≤3 kbar) suggest that it was in its magmatic state for a short time, probably less than one million years. The orientation of the lineation and foliation is consistent with their formation due to a small increment of crustal-scale strain absorbed by the pluton in its geologically brief magmatic state. If so, these fabrics record a Late Devonian crustal-scale strike-slip stress regime that may also have driven dextral movement on the neighboring Norumbega Fault Zone.