Observers: William Ryan(transcript) and Mario Aigner-Torres (transcript)
Pilot: Dudley Foster
Launch: 18° 34.90' S, 113° 24.90 W
Dive plan:
Objectives:
Notes:
Summary: Dive Alivin 3347 completed thirteen zig-zag
transects across the so-called "Auzende Flow" within
the axial depression (graben or valley) working north from -18°
34.7' S to -18° 33.5' S (for an along-axis distance of 4 km).
The first three transects at the south end of the explored corridor
encountered numerous crustal slivers bounded by faults. The slivers
deepen from the east rim into the valley floor, whereas northward
the valley floor is more or less one intact surface that is broad,
flat and cut by a single or sometimes by two N-S running deep
fissures (clefts) repeatedly visited on sequential transects.
Age-relationships: The terrain outside the valley (either
directly on its rim or in the case of the first transect, extending
several hundred meters down the east flank of the axial high)
is everywhere pillow lava characterized by elongated pillows and
tubes with a pronounced orientation indicating a flow direction
downslope and away from the valley. This "valley exterior"
terrain is old (age 3.0 to 2.5). An identical pillowed terrain
is observed on the surfaces of the individual crustal slivers
with in the southern part of the valley itself. This mostly pillowed
and subordinately lobated terrain in the center of the southern
valley becomes somewhat younger and reaches a minimum of age 2.0.
Where the valley floor is broad, locally smooth and dips ever-so
slightly northward (starting around -18° 34.3' S) it is extensively
surfaced with sheet flows (wavy, jumbled, striated and "billiard
table" smooth) of an even still younger age 1.5. The valley
floor also deepens gradually the south from this mid point. However
to the south the surficial valley floor flooding flows are of
the lobate rather than sheet-type. These southern lobates are,
as best as can be ascertained from visual observation, the same
apparent age (1.5) as the northern sheets. The very youngest flows
(age 1.0) are very limited in areal extent and are found only
in the floor of the clefts (age 1.0). Scattered talus aprons are
shed onto the edges of the valley floor from the bounding walls.
Nevertheless, for the most part, the talus that makes up steep
ramps at the base of the east and west bounding walls is older
than the last valley floor flooding events and is overlapped by
their flows.
Tectonics: The bounding walls of the axial valley are clearly
of displacement origin, though they also reveal the subsequent
effects of mass wasting. As near vertical cliffs they expose the
now buried interior substrate of the former axial high that we
presume to have predated the formation of the broad, deep valley
(speculation). This substrate is almost exclusively comprised
of pillow lavas exposed in the cliffs as round and radially cracked
cross-sections of either pillow heads, trunks or tubes. The truncated
pillows appear in the wall in horizontal layering (looks like
bedded rock) even though they consists of individual appendages
of the flows. Very little of the wall is massive rock. The bounding
wall is on several occasions a narrow sliver (2-4 m deep perpendicular
to the cliff face and ten or more meters in height) detached along
a vertical joint that forms an opening which widens upwards. The
cliff edge on one transect consists of at least three of these
slivers in succession pealed off and tilting ever so slightly
into the valley. Every time it could be inspected the top of a
sliver is the same heavily sedimented terrain as that found on
the topmost rim and on the flank of the axial high exterior to
the valley floor (age 2.5 to 3.0). Glass has spalled from the
rocks exposed in the opening joints and it has accumulated on
steep scree surfaces made of sand to pea-sized glittering fragments
that further down near the base of the cliffs pass into duller
pebble, cobble, boulder and even car-sized blocks of talus. Round
pillow trucks and heads are common as allochtonous blocks in the
valley edge talus. On one occasion a sliver of the wall was observed
to have fallen inward onto the valley floor so that its pillowed
cap dipped 45° towards the valley center and its smooth joint
surface along which it had separated from the wall now dipped
45° back towards escarpment. Fissures on valley floor slice
into terrain that is either not vertically offset on either side
of the cut, or at the fissure steps downward or upward on the
scale of 1 to 2 meters. These fissures and the deeper and wider
ones called clefts provide excellent exposures of the valley floor
substrate. Such fissures and clefts are commonly but not everywhere
associated with facets stained white by presumable hydrothermal
exhalations. Staining was more prevalent in cracked pillow lavas
near the base of the valley-bounding walls, less in the valley
floor lobates and very rare in the sheet flows except in the immediate
vicinity of the valley floor clefts. It is all together absent
outside the valley (rim or flank of the axial high). Around these
clefts the submersible induced turbulence scattered into the water
the "spaghetti" worms that colonize the rims and uppermost
walls of the clefts.
Stratigraphic relationships of the lavas: Several excellent
exposures on cliffs and in the walls of fissures revealed the
cross-sections of "giant" tubes one or more meters in
diameter with a clearly visible exterior skin (<10 cm. in thickness).
This skin displays radial patterns like those of a pillow but
radiate from a far away center so that the fabric on the truncated
skin consisted of a parallel indentations and ridges perpendicular
to the circumference of the tube (looks like megascopic sparry
calcite). The tubes were not ideally rounded, but squashed horizontally
and with the skin folded to accommodated the plastic deformation
(the crenulated skin was uncracked). More than once we saw the
"giant" tubes filled or partly filled with uniformly
thin (2 cm thickness) horizontally-bedded flows that in vertical
exposure look like a cut through a stack of Neco Wafers".
These are not bathtub rings on the walls or pillows of a lava
pond. Where one could look down on the top flat surface of these
thin flows within the "giant" tubes, it was very flat
and dull with no signs of reflective glass. The tubes that were
partly occupied by these thin flows either had voids at their
tops, or were filled the rest of the way by what looked like welded
talus. The talus here was tabular and did not contain fragments
of pillows but pieces that resembled roof rock or the skin of
the tubes itself. The tops of the tubes lay just below the seafloor
(the rim of the exposure) and were covered either by pillows or
lobates that were only one or two flows in thickness. The exposure
between the "giant" tubes was exclusively pillow terrain
seen in round cross-section and with their typical radial fracture
patterns showing that they had been severed by the fissuring or
faulting process. Lobates were conspicuously absent in the lavas
between and below the tubes in the exposures we observed and when
present were only the surficial capping flow or two at the top
of the escarpment. More than once we clearly observed the lobate
to have dripped down the face of the exposure as if it were a
flow contemporaneous with the fissuring/faulting event. The descending
tongues were never more than 0.5m in length and did not reach
the base of the fissure or fault escarpment. In the context that
we saw them as the stratigraphically highest flows, the lobate
lava in cross-section was lens shaped, more or less flat on the
bottom, convex upward at the top with 3-5 cm. thick thin walls
(skin) and typically hollow (empty) with drips hanging from the
bottom of their roofs. Thus we may infer that they were both cut
by the faulting/fissuring and emptied by pouring into the crack
formed. The "giant" tubes were only seen in vertical
cross-sections exposed in the floor of the valley by small relief
faulting and fissuring. We did not report any such tubes from
the more vast exposure we looked at from the valley bounding walls.
However, it is important to remark that we were almost always
rapidly ascending these walls, so the opportunity to stop and
hunt for such tubes in the valley wall context did not present
itself. When exploring the valley floor where the sheet flows
lay adjacent to lobate flows, we typically saw the tongues of
the lobates flow out onto the sheets. The lavas in this context
seemed to be of the same relative age. The sheet flows rapidly
changed their morphology in a spatial sense of a few meters from
wavy, to jumbled, to striated, to "billiard table" smooth.
However, almost invariably the "billiard table" smooth
sheets were cracked into large polygon slabs and slightly sagged
in very shallow (<0.5m) depressions five or more meters in
breadth. The white staining in cracks of presumed hydrothermal
origin was never associated with an aureole on the sediment adjacent
to the crack and no color change in the sediment cover was observed
towards a stained crack.
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