STRAIN HISTORY OF THE PISECO LAKE SHEAR ZONE EASTERN TERMINATION, ADIRONDACK MOUNTAINS, NEW YORK

The Piseco Lake shear zone (PLsz) occurs mostly in a belt of deformed granitoids that cross the Adirondacks with an E-W strike, and is associated with Shawinigan orogenic activity. The eastern termination of the PLsz has undergone apparent dextral transposition to form a map-scale reclined fold, with N-S strike in the Palmerton Range (region where it was proposed a possible syntaxis formed between Mesoproterozoic shear zones). Along the length of the PLsz, sinistral transpression formed a thick (15 km) domain of subvertical penetrative mylonite with subhorizontal lineations, and it's juxtaposed with L-tectonite cored, mylonitic foliation domes. The rocks in the N-S trending segment of the shear zone were studied to determine if they have a similar strain history as the main PLsz, and/or include subsequent strain associated with the apparent dextral transposition. Is the arcuate trace of the PLsz primary to the formation of the structure or related to an event superimposed on the structure?

Within the Palmerton Range, variably developed tectonites (S>L, S=L, L>S and L) occur in granitoids, pelitic gneiss, amphibolite and quartzite, similar to the PLsz farther west. Moderate to shallowly dipping mylonitic foliation strikes generally N-S, with south trending lineations. The foliation and lineation are defined by recrystallized aggregates of quartz and feldspars in granitoids. Quartz ribbons are abundant in granitic and pelitic lithologies. A variety of kinematic indicators were observed, including sigma- and delta-type porphyroclasts, S-C fabrics and broken grains. Low elevation, on the north slope near the Hudson River, the foliation dips S-SW and the shear sense is top to the north. However, at the top of the range, where the foliation dips gently to the E, the lineations are subhorizontal, and shear sense is low-angle sinistral. The apparent inconsistencies in shear sense may be related to large-scale transposition of PLsz fabrics, or related to subsequent development of ductile fabrics in already complexly deformed rocks. It is interesting to note that reports on the PLsz portray consistent sinistral displacement over a vast region where the strike is consistently E-S, and this is inconsistent with the region where the structure has apparently been transposed in orientation and/or cross cut.