BRITTLE TO DUCTILE TRANSITIONAL STRUCTURES IN THE LOWER PALEOZOIC OCTARARO FORMATION BELOW HOLTWOOD DAM, SUSQUEHANNA RIVER, PENNSYLVANIA

Structures spanning the brittle-ductile transition occur in lower garnet-grade metamorphic rocks of the early Paleozoic Octararo Formation in a 3-km2 exposure on the floor of the Susquehanna River below Holtwood Dam, PA. An early schistosity (S1) is associated with mica and albite porphyroblast development and release of excess silica as pervasive quartz stringers that largely define S1. A second schistosity (S2), sub-parallel to S1 forms the dominant N40-60W, 10-30 S foliation. Both these probable Taconian age structures involved almost purely ductile deformation. A spaced cleavage (S3) involving flexing of the micas and minor recrystallization is pervasively developed and locally passes into a well-developed fold set (F3) with several decimeter wavelengths and 1:1wavelength/ amplitude ratios. The N40E, 80S orientation of S3, asymmetry of the folds, and association with the Tucquan antiform suggest NW transport origin during the Alleghanian orogeny. Intensity of development of these structures seems to follow 100-200 m thick packages of S2 schistosity suggesting localization of younger ductile strain by older anisotropy. The best brittle-ductile transition appears in an outcrop of quartz veins on a bluff beneath power pylons on the west side of the river, 1 km below the dam. At higher elevations, N30E, 80 SE en echelon veins are sub-parallel with local S3 foliation and indicate injection under brittle conditions but pass downward into blunted ductile structures and F3 folds superimposed on S2 planes. The veins appear to have acted as a buttress for compression in the last stages of F3 folding. They indicate massive pumping of fluids through spaces developing during S3 time. Additional, completely brittle features overprint all the riverbed exposures. An enigmatic but widespread set of N80W macrojoints exposes surfaces up to 10x150 m below Norman Wood bridge. Average dips are 50S but locally as low as 25o. A poorly developed conjugate set at N80W, 50S suggests compressive origin but with no indication of shear movement. Other prominent joint sets and domains evident in aerial photographs are difficult to separate age-wise in outcrop but seem to carry over into areas of low exposure in the adjacent hills. Current work is focused on defining and constraining F3 events within the ductile -brittle transition.