ABSTRACT
A thin (<200 m.) mafic suite and well developed
mafic/ultramafic transition zone are exposed above a flat lying
peridotite contact on northwestern Table Mountain. The igneous
layering and sedimentary features indicate mineral deposition
under conditions which promoted adcumulate growth, were capable of
minor transport, and were subjected to at least minor tectonic
activity during consolidation. Feldspathic,. mafic, and ultramafic
dikes and veins cross-cut the layering. Microscopic futures
indicate deformation at elevated temperature and/or low strain
rates. Deformation is best developed within the transition zone,
but cataclastic zones are most common in the hornblende gabbros.
Orientations of layering, foliation, and lineation indicate a
variable mafic/ultramafic transition and macroscopic folding.
Geometric analysis indicates three distinct fold axis
orientations: an east-west horizontal fold axis, a northeast
trending modestly plunging axis, and a vertical though poorly
defined axis. Such features demonstrate that an apparently simple
contact relationship may be extremely complex. This has important
implications for ocean floor accretion. The relatively simple
ocean floor seismic stratigraphy masks very complex petrological
and structural processes. Such processes may involve deposition in
an actively convecting magma chamber with a differentially
subsiding wedge (Dewey and Kidd, 1977), in which folding occurs in
response to the steepening angle between the cumulate banding and
the base of the magma chamber. The instability is enhanced by the
different accumulation rates and densities of the minerals
involved. The lineation may originally be a sedimentary feature
indicative of transport direction from the convection cell, and
perpendicular to the compressive stress which produced. the
folding. The different orientations of lineations and fold axes
could be produced by rotation of the ocean crustal blocks during
lateral transport along the ocean floor and/or obduction. Further
detailed study of ophiolite complexes will continue to shed light
upon the nature and development of oceanic crust.
O'Connell, S., 1979. Geology of the Mafic/Ultramafic Transition,
Table Mountain, Western Newfoundland. Unpublished MSc. thesis,
State University of New York at Albany. 145pp., +xiv.
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