THE SOUTHERN ADIRONDACK SHEAR ZONE SYSTEM: PROTEROZOIC (GRENVILLE) PROGRESSIVE STRAIN PARTITIONING

The S. Adirondacks of NY is predominantly meta-igneous rocks and intensely deformed in a system of mostly sinistral high strain zones (the S. Adirondacks shear zone system, SASZ). At all scales, the SASZ is E-W trending defined by consistently intense E-W- to ESE-trending (080° to 120°) subhorizontal mineral elongation and stretching lineations, but variably intense and variably dipping foliation (shallowly- to moderately-N- or S-dipping), all of which developed during dynamic recrystallization. Boundaries of the SASZ are limited only by exposure in the S. Adirondacks, therefore the width of the zone is >75 km wide, identified at minimum by the Snowy Mountain dome-Moose River plain shear zone system in the north, and the Piseco Lake shear zone (PLSZ) in the south, but likely extends farther south to the nonconformity of the Proterozoic rocks with the Lower Paleozoic cover rocks. The E-W extent of the SASZ is unknown due to cover, but, as exposed, the zone is >100 km long, covering the entire width of the S. Adirondacks. Within the entire system asymmetric structures show mostly sinistral sense of shear recorded by tails around Kfs and Pl porphyroclasts, and augen-shaped aggregates of Kfs+Pl+Qtz. Relatively minor transcurrent shear zones are found between these two delimiting structures including one small dextral shear zone which is opposite to the main kinematics of the system, but is parallel to the main zones.

Structural analysis reveals a complex system of low-T zones developed upon an older, relatively high-T structure. This sequence produced an overprinting geometry of discrete transcurrent zones of L-S to L>S E-W-trending shallowly plunging fabrics whose foliation is relict of the older structure to define an E-W-trending set of regional-scale folds. Fabric variations are consistent with crests, troughs and limbs of the folds; L>S fabrics dominate the cores and the L-S fabrics on the limbs. We suggest the formation of the L>S fabrics in the SASZ, within a relatively narrow zone that was last active at greenschist facies conditions, reflects the accommodation of apparent constrictional strain as deformation localized progressively from a much wider zone that was active initially at higher T. Apparent constriction within the core of the PLSZ may be the result of intersection between oppositely-dipping zones of L-S fabrics.