OTTAWAN TRANSECTION OF SHAWINIGAN STRUCTURES IN THE ADIRONDACKS OF VERMONT: BEARING on LARGE SCALE EXTENSIONAL OR THRUST FAULTING IN THE WHITEHALL SHEAR ZONE, NY AND VT

The 5- 10 km-wide, east dipping Whitehall shear zone(WHSZ) is described as mylonitic and strongly lineated Mesoproterozoic gneiss at the east margin of the Adirondack Highlands formed by west directed Ottawan thrust faulting or eastward extension. Detailed mapping constrains both models. Non- faulted east-west subvertical to inclined YF1 folds and YS1 gneissosity control the distribution of the rock units. A U-Pb SHRIMP zircon age of 1106± 7 Ma (unpub. data J.Aleinikoff) from a metagabbro dike near Dresden Station cross cuts these earlier Shawinigan YF1 folds. Map scale YF1 folds are transected buy a granulite-grade gently east dipping YS2 Ottawan gneissosity throughout the WHSZ that is axial planar to isoclinal folds of YS1 that plunge gently east to northeast parallel to an intersection lineation marked by spears of oriented hornblende, biotite, rods of polygonal quartz. About 100 oriented thinsections show that sections cut parallel to the lineation lack a sense of shear. Sections perpendicular to the lineation and foliation show moderately elliptical quartz, and little sense of shear. Sections parallel to the foliation show minimal and irregular elongation parallel to the lineation. The rods of quartz and granulite-grade minerals form bundles concentrated at the intersection of YS1 and YS2. Microgranular-leucosomes 1-2 mm thick of perthite, quartz and minor biotite form along the YS2 surfaces and record no strain in any orientation. Ottawan recrystallization destroyed many structures of the YS1 fabric leaving a pervasive intersection lineation that is not a mylonitic L>>S fabric.YF1 folds extend km’s across the trace of the YS2 foliation without deflection, and the WSZ does not require extensional or thrust faulting in the Ottawan. Very minor, 100 m long, <10 m- thick post- YS2, ductile extensional shear zones inYS2 that dip to the northwest and southeast contain nonfoliated pegmatites and may record minor late extensional tectonics in the WSZ.