ABSTRACT
The most commonly observed structures in studies of the ocean floor in the vicinity of mid-ocean ridges are inward- and outward-facing, axis-parallel and axis-oblique normal faults. The characteristics of these faults at depth is not well known due to the limitations inherent in marine geological work. Shear zones within the plutonic portion of the North Arm Mountain massif, Bay of Islands Ophiolite Complex, western Newfoundland, are likely to have formed as deep level expressions of mid-ocean ridge bounding normal faults.
The mineral assemblage of the shear zones is hornblende + calcic plagioclase, with minor clinopyroxene, sphene, chlorite, opaques, and quartz. Plagioclase and hornblende compositions suggest that shear zone formation took place under lower amphibolite conditions. A minimum temperature of formation of 420 oC is estimated.
The shear zones are S-C mylonites in which the c-surfaces form the main mylonitic foliation. The sense of shear across each of the shear zones was determined, mainly using microstructural criteria. This sense of offset information, combined with reconstruction of the massif to its pre-obduction configuration, suggests that the shear zones formed as mid-ocean ridge related axis-parallel and axis-oblique normal faults. Some of these faults formed dipping towards the spreading axis, while others dipped away from the axis.

 Idleman, K.A.J., 1986. The Significance of Shear Zones Within the Plutonic Section of North Arm Mountain, Bay of Islands Ophiolite Complex, Newfoundland. Unpublished MSc. thesis, State University of New York at Albany.
 155pp., +viii, +1p. abstract.
University at Albany Science Library call number:  SCIENCE Oversize (*)  QE 40 Z899 1985 I34

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