CONJUGATE STRUCTURES AT THE NORTHERN MARGIN OF THE MARCY MASSIF, ADIRONDACK MOUNTAINS, NEW YORK, USA

The Marcy Massif of the Adirondack Mts. of northern New York is a Proterozoic anorthosite batholith of the Grenville Province. History of this area involves convergent deformation and magmatism during the Shawinigan orogeny, emplacement of the AMCG plutonic suite (including the massif anorthosites), and continued orogenesis during the Ottawan phase that ended with late syn-to-post kinematic emplacement of the Lyon Mountain Granite. This history is more comprehensively documented at the southern margin of the massif. Prior work at the Copper Kiln Landslide outcrops at the northern margin identified melt-filled conjugate fractures that may have played a role in the disaggregation and attenuation of layers and xenoliths in the meta-anorthosite. This study focuses on those features and includes observations from additional nearby outcrops.

Conjugate structures are present at Copper Kiln Landslide, Cobble Lookout, and Catamount Mountain. The conjugate structures at Copper Kiln and Catamount are melt-filled fractures. At Cobble Lookout they are shear planes that cut and transpose the host gneissosity. At all outcrops, the conjugate structures are moderately-to-steeply northward dipping with west trending and shallowly plunging (~13 deg.) estimated sigma one orientations. Overall, the sense of shear is poorly developed but is consistent with dominantly strike-slip motion. Field observations suggest a genetic relationship between upright folds and the melt-filled fractures at Catamount outcrops. At Copper Kiln and Catamount, the fractures are filled with an assemblage similar to the host rock, making them difficult to identify in outcrop, esp. areas where the host rock gneissosity is poorly defined. At Cobble Lookout, the mineral assemblage of the conjugate shear planes is distinct from the host rock and defined by a garnet corona texture. The comparable orientations and character of the conjugate structures suggests that they are related. The connection to upright folds and the corona texture is consistent with late Ottawan deformation. The Marcy Massif is interpreted to have behaved as a rigid body during the Ottawan localizing deformation at its margins, and study of these areas, including spatial variation in orientations and kinematics around the margin, is key for developing a model for orogenesis.