STRUCTURAL ANALYSIS OF THE INDIAN HILL SHEAR ZONE, HUDSON HIGHLANDS, NY

Traversing across the strike of the Hudson Highlands, there are many shear zones with widths of 1-2 km that cross cut and transpose older contacts. Recent mapping in the Monroe and Sloatsburg 7.5 minute quads has identified another major shear zone, the Indian Hill zone (IHz), that can be traced for more than 30 km. The IHz is defined by steeply dipping penetrative foliation that strikes 030-035 and dips 65 to 80 SE. Dynamically recrystallized minerals, such as feldspars and quartz define the shear zone foliation where it crosses feldspatic rock. Mineral lineations within the zone occur as rods of quartz & feldspar, they are subhorizontal, & trend 030-035. The width of the IHz varies from 1-1.5 km measured perpendicular to the shear zone boundaries, but the transition domains extend outward an additional km or more. Micro-, meso- & megascopic kinematic analysis along the IHz revealed right-lateral shear sense. S-C fabrics with dextral asymmetry are well developed in megacrystic granitic gneiss. Where the IHz crosses metavolcanic rocks, asymmetric boudins of amphibolite in feldspathic gneiss show consistent dextral rotation. Dextral sigma- and delta-type porphyclasts were observed in the field and in polished rock slabs. At the map scale, the trace of older foliation from outside to inside the IHz defines large drag folds that indicate dextral shear. Small sinistral shear zones cross cut the IHz at many locations, & these sinistral zones may be ductile conjugates. The southern extent of the IHz appears to splay into two smaller high-strain zones. Structural blocks of older rock between the high-strain zones contain upright folds with axial planes oblique to parallel with the bounding shear zones. The obliquity of the fold hinges to the shear zones also supports a model of dextral transpression. Metamorphic rims on zircons from a sheared feldspatic gneiss within the IHz yielded a range of U/Pb ages of ~1057-1019 Ma placing an approximate upper limit on the time of shearing.