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International Research
19 N
(a)
West Mariana Ridge
Mariana Trough
Mariana Arc
(b)
17 10'N
Segment End
18 N
17 N Segment
Segment Center
17 N
17 00'N
Spreading Axis
16 N
142 E 143 E 144 E 145 E 146 E 147 E
-7000 -6000 -5000 -4000 -3000 -2000 -1000 0 Depth (m)
Figure 1: (a) Bathymetry of the central Mariana Trough showing the location
of the study area. (b) Bathymetry of the Mariana Trough 17°N segment.
Black box indicates sidescan sonar survey area. Red, blue, and yellow
lines show Shinkai 6500 dives #1088, #1089, and #1090, respectively.
Median Valley
16 50'N
Spreading Axis
144 40'E 144 50'E 145 00'E
2. YK08-08 Leg-1 cruise
Three Shinkai 6500 dives were devoted to the Mariana Trough
at 17°N (Dives #1088, #1089, and #1090). We dove to the median
valley in the segment center to observe lava morphology
both of smooth and bumpy surfaces indicated in the sidescan
sonar imagery. The submersible carried a magnetometer and a
sub-bottom profiler, and geophysical measurements were operated
continuously during all dives. We also conducted a surface-ship
geophysical survey to collect swath bathymetry, magnetic
and gravity data in the central part of the trough between
the Mariana Arc and the West Mariana Ridge (remnant-arc).
3. Submersible dive observations
3.1 Dive #1088
The landing point was the axial portion of the median valley
and the eastern foot of a small ridge of ~150-m relative height
elongating in NNW-SSE (~N20°W), sub-parallel to the median
valley direction (Fig. 2). The dive traversed the ridge and
continued on the western flank of the median valley at ~3420-
m depth towards the western wall. The small ridge corresponds
to bumpy sidescan sonar imagery, while the portion around
the landing point and the western flank are smooth surfaces
with high backscatter intensity. Morphology at the landing
point was jumbled-wrinkled lavas (Fig. 3). The lava morphology
suggests sheet and turbulent flow at high eruption rates.
The lava flows could be very young, because they show no or
little accumulation of sediment, potentially the freshest lava
flows observed among the three dives.
The small ridge mainly consists of pillow, ~2 m in length and
~1m in diameter, and pahoehoe lavas. The small ridge is covered
with thin sediment and is probably an old construction
compared to the portion around the landing point. The western
half of the dive route had a mixture of pahoehoe, folded sheet,
lobate sheet, and jumbled flows produced by sheet flows with
high effusion rate (Fig. 3). Topography consists of a series of
ridges and troughs with relative height ~5-10 m. The topography
may be originated to be tectonic structures such as faults or
volcanic structures such as fissures or lava channels. Strikes of
the ridges and troughs are in the NNW-SSE or N-S direction.
According to tectonic interpretation of the sidescan sonar images
(Asada et al., 2007), the submersible passed across an area
of N-S lineated structures, and then across an area of NNW-
SSE lineated structures. Sediment coating in the area of N-S
structures tends to be thinner than that in the area of NNW-
SSE structures, perhaps suggesting that the N-S structures are
newer. Sediment cover more or less gradually increases toward
the margin of the median valley. The sub-bottom profiler indicated
very thin sediment cover through the dive route, however
it was difficult to obtain clear reflection images from the sediment/lava
boundary due to rough seafloor relief.
3.2 Dive #1089
The eastern margin of the median valley where we landed has
lobate sheets covered with thin sediments. The submersible
landed on a tumulus feature that is several meters in height
and ~10 m in diameter with wrinkled surface and wide and
deep inflation cracks. In adjoining places, spherical to bulbous
pillows are exposed on the sedimentary cover. Swollen and
fractured pillows and irregular pillow lobes were present on the
sedimentary plain.
The sidescan sonar imagery shows a small volcano complex that
consists of bumpy aggregates of dome-like structures superposed
on fan-shaped lava terraces along the foot of the eastern
valley wall. The terraces appear to be overlain by large domes
100-300 m across that form a broad ridge 1 km east-west and
700 m north-south and 30-40 m in height (Fig. 2). The terraces
are floored by sedimentary plains, with sporadically exposed
InterRidge News 25 Vol. 17, 2008