MICROSTRUTURAL ANALYSIS OF SHEARED QUARTZITE IN THE CANADA LAKE SYNFORM, SOUTHERN ADIRONDACKS, NEW YORK

The southern Adirondacks are underlain by numerous map-scale folds. Many of these folds were shown to have formation histories consistent with high-temperature sheath folding. The Canada Lake synform is one of these folds. The core of the synform contains interlayered quartzite (Irving Pond), pelitic and granitic gneiss. All rock types contain penetrative L>S, S=L, or S>L deformation fabrics. This study involved detailed petrofabric analyses of the Irving Pond quartzite and interlayered granitic gneiss to estimate the strain and shear sense to better understand the mechanics of sheath fold development at the map-scale.

The Irving Pond quartzite consists of dynamically recrystallized quartz, with minor biotite, sillimanite, microcline and garnet. Interlayered pelitic and semipelitic gneiss contains garnet, biotite, sillimanite, K-feldspar and quartz. Granitic gneiss layers (upward of a few meters thick) occur within the Irving Pond quartzite. The contacts of these layers are parallel to the penetrative foliation in the quartzite. Additionally, they too contain the penetrative foliation and lineation. Foliation and lineation in the quartzite is defined by aggregates of dynamically recrystallized quartz and quartz ribbons. Garnets in the quartzite are mostly broken into small lineated aggregates. Deformation fabrics in the granitic gneiss are defined by aggregates of dynamically recrystallized microcline, plagioclase and quartz, and minor pyroxenes that show signs of brittle deformation. Quartz c-axis fabrics obtained from the quartzite on both limbs and hinge region of the fold show strongly symmetric patterns indicative of a component of pure shear. The grain-shape fabrics obtained from the granitic gneiss from both limbs of the fold reveal grain-shape aspect ratios ranging from 30:1 to 60:1 in the X-Z plane. Although shallowly plunging lineations consistently trend NW-SE, kinematic indicators show opposite shear sense on either limb of the fold. When plotted relative to structural position on the fold, the kinematic data is consistent with development of a large-scale sheath fold with the dominant transport direction northwestward. Our observations are consistent with earlier reports on sheath folds from the southern Adirondacks.