TECTONIC SIGNIFICANCE OF IGNEOUS FOLIATION AND LINEATION IN THE MOUNT WALDO PLUTON, WALDO COUNTY, MAINE

Can plutons record stress in the earth’s crust? This study shows that pluton fabrics can record crucial information about the role of crustal kinematics in melt migration and pluton emplacement during orogenesis. We focus on the Mount Waldo pluton, which outcrops over 150 square kilometers to the west of Penobscot Bay in Waldo County, Maine. It is a late Devonian, medium-grained, magnetite-rich granite that shows a weak fabric. We characterize its fabric by documenting (1) the foliation defined by porphyritic K feldspar crystals and (2) the magnetic foliation and lineation defined by anisotropy of magnetic susceptibility (AMS). This fabric is magmatic in origin and records the late Devonian strike-slip stress regime that drove slip along the Norumbega Fault during the last stages of the Acadian Orogeny.

We report data from twenty-six sites across the pluton. On glacial pavement exposures, long axes of feldspar phenocrysts trend north-south. In rare three dimensional exposures, feldspar foliation strikes north-south and dips steeply. In thin section, quartz and feldspar lack textures characteristic of crystal plasticity or brittle deformation. This observation suggests that fabric in the pluton is entirely magmatic in origin. AMS data reflects observations on oriented cores measured using a SI2B Sapphire Magnetic Susceptibility and Anisotropy Meter. Of the twenty-six sites, all but four show susceptibility greater than 10E-3 SI units. We interpret such high susceptibilities as evidence that magnetite controls the AMS signal. This interpretation is supported by petrographic observations of multi-domain magnetite (individual crystals of which are commonly as large as 500 microns) and Jelinek plots of the data (which show no clear relationship between susceptibility and total anisotropy). Magnetic anisotropy in these samples ranges from 1.02 to 1.16 (a dimensionless ratio). Petrographic observations of magnetite grain shape and distribution suggest that anisotropy results from shape-preferred orientation of inequant magnetite grains and/or clustering of equant magnetite grains (a distribution anisotropy).

Our AMS data agree with previous 2D studies and our new 3D data on feldspar foliation in the pluton. In the Mount Waldo pluton, foliation is steep and strikes north-south. Lineation is shallow and trends north-south. We interpret the magmatic fabric of the Mount Waldo pluton as having formed during a small increment of strain that the pluton absorbed late during crystallization (the late Devonian) for two reasons. First, the round shape of the pluton in map view indicates that the pluton has not enjoyed a large strain. Second, the magnitudes of the AMS anisotropy are small. These data then serve as a record of a late Devonian stress field in the crust. We infer a late Devonian maximum principal stress (s1) oriented perpendicular to the foliation in the pluton. We also infer a late Devonian minimum principal horizontal stress (s3) oriented parallel the lineation in the pluton. This interpretation of the data suggests that the intermediate principal stress (s2) was vertical, a crustal stress regime that corresponds to strike slip faulting. In fact, the orientation of late Devonian principal stresses that we report here are consistent with dextral strike-slip movement on the Norumbega Fault, synchronous with crystallization of the Mount Waldo pluton.