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The Megasplay Fault Zone (6K#579, 6K#889~891):
Field Observations
Four dives focused on the structures near the megasplay fault and its bifurcations
exposed on the 5th ridge (Fig. 5 ). Faults on the 5th ridge are associated with active
cold seepages. Five fault zones (dashed lines in Fig. 5a ) were deduced based on the
bathymetric features and distribution of chemosynthetic bio-communities such as
vesicomyid bivalves (mainly Calyptogena nautilei : Fig. 6e, f ) and vestimentiferan
tubeworms. Bathymetric features used here are EW-trending depressions along the
crest, knicks along the foot of the ridge, and a transverse ridge composed of silt-
stone (Fig. 5b ) and developed across the mid-axis of the canyon. The megasplay
fault was inferred to be located along the southern foot of the 5th ridge (fault in
Fig. 5a ). Three bifurcations of the megasplay fault (faults to ) were also rec-
ognized, which are continuous with the EW-trending depressions developed in the
crest of the ridge. A hump developed between faults and extends westward
from the crest of the 5th ridge. The bifurcating faults displaced the surface of the
canyon wall. A boundary fault (fault ) was deduced in the north that separates the
5th ridge from its hinterland, and was determined to continue to the northern mar-
gin of the transverse ridge developed across the mid-axis of the canyon (Fig. 5a ).
Mudstone-dominant turbidites comprise the northern part of the 5th ridge. The
sequence becomes rich in sandstones with increasing thickness southward (Fig. 6a ).
Orientations of the selected bedding planes are shown in Fig. 5a . A plot of S-poles
to the bedding planes (Fig. 5c ) indicates that the mudstone-dominant strata in the
northern part (area of Dive 6K#891) are folded about an axis plunging 17° to the
west. The folds have wavelength of few hundred meters. S-pole concentration near
the center implies that the majority of the strata are sub-horizontal and/or dip gently
to the southwest. The sub-horizontal bedding is steeper near the northern boundary
fault and fault .
Sandstone-dominant turbidite strata dipping steeply to the southwest (Fig. 6a, b )
are predominant in the middle part of the 5th ridge (6K#889: Fig. 5 ). The distribu-
tion of S-poles to bedding implies folds with axes plunging 40° to the west
(Fig. 5d ). Oblique fracture cleavage was observed to have developed in siltstones
interbedded with sandstones (Fig. 6b ). The fracture cleavages dip steeply to the
north. Sub-vertical strata are commonly developed near faults to (Fig. 6c ).
Deformation bands (Ujiie et al. 2004 ) and web structures (Hirono 2005 ) are abun-
dant in this area.
The southern part of the 5th ridge (6K#890) consists of sandstone-dominant
turbiditic strata dipping gently to the south (Fig. 5e ). Sub-horizontal strata in the
frontal part are locally deflected and steepened along the megasplay fault (near
890R4 outcrop). The distribution of S-poles implies that bedding is folded about a
sub-horizontal axis plunging gently to southeast. The central part of the 5th ridge
dips steeply to the south. The sandstone-dominant turbidites are covered by con-
glomerate with imbricate pebbles (Fig. 6d ) that imply transportation from the
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