ABSTRACT
A survey of the literature that deals with the gabbroic rocks
believed
to comprise the foundation of the oceanic crust indicates that the
overwhelming
majority of these rocks are recovered from escarpments associated
with
transform faults. The wide range of mineral and chemical
compositions characterizing
oceanic gabbroic rocks suggests that the lower oceanic crust is
much more
heterogeneous in nature than was previously suggested by the
results of
geophysical investigations. The examination of gabbroic rocks
recovered
in situ from the walls of the Mid-Cayman Rise rift valley by the
submersible
ALVIN not only supports the notion that oceanic gabbroic rocks are
heterogeneous
in nature but also that widely varying gabbroic rock types are
found distributed
heterogeneously on the walls at a scale of tens of meters.
Observations
that the largest escarpments on the walls of the Rise have only
several
hundreds of meters of vertical offset, and that gabbroic rocks
were recovered
to within roughly 100 meters of the tops of the rift valley walls,
indicate
that the shallow intrusive and extrusive carapace of the oceanic
crust
here must be anomalously thin. It has been suggested that thin
oceanic
crust characterizes slowly-slipping. ridge-transform intersections
elsewhere;
the thin crust of the Mid-Cayman Rise may be attributable to the
presence
of the two long transform faults that bound the 110 km long Rise
segment.
The two transforms may also have an instantaneous effect on the
structural
evolution of the Rise creating the well-defined tectonic grain
that strikes
at a high angle to the axis of the rift valley.
Stroup, J.B., 1981. Geologic investigations in the Cayman Trough
and
the nature of the plutonic foundation of the oceanic crust.
Unpublished
MSc. thesis, State University of New York at Albany. 189pp., +xi.
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