NATURE OF THE BEAVER CREEK FAULT, ADIRONDACK LOWLANDS, NEW YORK
The Beaver Creek Fault is a prominent lineament with pronounced topographic and aeromagnetic expression. Lithologies within the fault zone are predominantly marble, calc-silicates and meta-pelitic gneisses. There are also minor occurrences of the Hermon Gneiss. Throughout the length of the fault the dominant foliation, defined by transposed lithologic layering, has an attitude of approximately 230/70NW but locally displays fold interference patterns similar to interference patterns observed throughout the Lowlands. Subhorizontal mineral lineation trending subparallel to the fault was observed in the meta-pelitic gneiss unit; however mineral lineation in the marble is sub-vertical. Marbles within the fault zone display spectacular mylonitized texture in large bands (~15 m) parallel to foliation. The rocks grade from relatively characteristic white-gray marble to unusual black marble in the center of the bands. The latter appears to have undergone the greatest grain-size reduction although it contains rounded clasts up to .5 m. As seen in thin section, calcite grains have an opaque coating, likely graphite. Asymmetric porphyroclasts, S-C fabrics and steep lineations are suggestive of sinistral transpression consistent with some models of the CCMZ and recent work on other faults within the Lowlands. However, it appears likely from several different attitudes of slickenlines and lineations within the fault zone that the Beaver Creek Fault, like the CCMZ, has a complex history of reactivation.