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 Unpublished MSc.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
K Idleman MS thesis scanned text with colour photos pdf (8.6MB)
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