Geoscience Reference
In-Depth Information
Serpentinite Body
Dive and dredge results show that the northern and southern parts of the serpentinite
exposure consist of differing lithotypes. The southern part is dominated by massive
serpentinite, accompanied by local and minor schistose serpentinite. In the northern
part, however, schistose serpentinite is dominant, with relatively small bodies of
massive serpentinite. Foliation (schistosity) of the schistose serpentinite shows
scattered orientations generally dipping to the east, southeast, or northeast (Fig. 8 ),
with subhorizontal stretching lineations trending to the north to northeast. Massive
serpentinite in the southern part thus structurally overlies dominantly schistose ones
in the northern part. The two parts seem transitional without distinct boundary.
Localities of amphibole schist floats lie at the structurally lowermost horizons of
the serpentinite exposure.
Two kinds of folds are assumed for the varying orientation of foliation planes.
One has a southeast-plunging fold axis with variable foliation strikes as typically
seen in the variation of the 6K#1065 route (Fig. 8 ). This type of folds is occasion-
ally found in schistose serpentinite samples (Fig. 6g-h ). The fold axes cross the
stretching lineations at high angles, and bend both the foliation and the lineation
(Fig. 6g ). These folds are commonly associated with slip surfaces and fractures
filled by low-T serpentinites, carbonates, or magnetite (Fig. 6h ). The other type of
fold has presumably near-horizontal and north-northeast trending axes, and is
mainly responsible for the variation of foliation dips from near-horizontal to near-
vertical. The assumed axis is subparallel to the stretching lineation. Fractureless
microfolds with lineation-parallel axes are seen in several samples of schistose
serpentinite (Fig. 6c ) and in an amphibole schist sample (Fig. 6a ), and they are
regarded as specimen-scale equivalents of the map-scale folds. This type of fold
may predate the southeast-trending folds, which bend the lineation.
Comparing the varying orientation of foliation surfaces with specimen-sized
fold structures, it is considered that the schistose parts of the serpentinite body are
entirely folded (Fig. 9 ). Foliation dips are dominantly gentle in the northernmost
sections (dives 6K#609 and 6K#1064), whereas steeper in the southern section
(dive 6K#1066). Therefore, an anticline is assumed in the northernmost part of the
serpentinite exposure.
Overlying Formations
Based on the distribution of rocks and nearly horizontal layering of tuff breccia
observed during the dive 6K#609, a nearly flat-lying boundary is assumed between
the serpentinite body and the Paleogene volcanic rocks at 3100-3,200 mbsl.
Although no outcrop or float stones of the Paleogene volcanic sequence was
observed during submersible dives to the south of 29°04.5'N, volcanic rocks and
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