Alvin Dive 3348 - Summary

7 February, 1999
John Sinton (transcript)
PIT Bruce Strickrott
Blee Williams

Objectives: This dive will explore the graben north of the South Hump lava flow and study the young flow. The dive will initially traverse a re-entrant into the western wall of the asymmetric graben that is a feature of the Hump segment. Upon entering the graben we will zig-zag to the south until the young flow is found. There are two prime suspects for the northern boundary based on 120 KHz side-scan sonar imagery. Once found, the western contact will be followed for a distance (to WP 5) in order to attempt to determine its thickness, the extent to which it mantles pre-existing topography, plummets into bottomless cracks and ramps up onto various walls. Post-eruptive deflation should be able to be determined.
To the south, cracks (fissures?) and possible inflationary structures will be visited and appreciated. The goal is to reach at least to 18°33.41'S, sample the flow in various places, and also rocks making up the graben walls. Approximately 4.5 km of track is planned.

Way Points

1  18° 31.54'S 113° 25.03'W (Launch)
2  18° 31.78   113° 24.68
3  18° 31.82   113° 24.78
4  18° 31.99   113° 24.72
5  18° 32.23   113° 24.90
6  18° 32.50   113° 24.82
7  18° 32.59   113° 24.95
8  18° 32.78   113° 24.88
9  18° 32.83   113° 25.00
10 18° 33.01   113° 24.92
11 18° 33.14   113° 25.00
12 18° 33.22   113° 24.92
13 18° 33.30   113° 25.05
14 18° 33.30   113° 24.95
15 18° 33.41   113° 25.03

Summary: This dive began outside the western bounding flank of the ridge axis, which in this area is a narrow, deep, asymmetric graben. Pillow lavas and lobate lavas make up the western flank of the axis. The western side of the graben along the track descends over several faults, of which the two largest have scarps of 21 and 12 m. The innermost east-facing fault has a scarp of about 3-4 m. This fault appears to be buried near 18° 31.5'S by a younger graben-filling fault. Lobate lavas making up the graben floor to the north of the younger flow is intensively faulted and fissured. The younger flow also shows some sign of tectonism, with open fissures, and some minor faults cutting it. However it is much less tectonized that the seafloor to the north or the walls on either side of it. The northern contact of the flow consists of a 4 m-high flow front of rubbly debris that passes up onto the flow surface, which consists of lineated sheet flows, sloping gently to the north. The sheet flow is locally broken and dropped down up to 10 cm along cross-flow cracks. To the south toward the source of the flow, it progressively steps up through several different flow lobes, from flat sheet, to folded and jumbled sheets and eventually to lobate and pillow lavas. The graben-filling flow has significant sediment cover, up to 1-2 cm on flat surfaces.

Samples collected:

1. 18° 31.41S -113° 25.95'W: pillow lava outside of western high
2. 18° 31.48'S 113° 24.83'W: pillow lava on top of western high
3. 18° 31.67'S 113° 24.80'W: lobate lava in graben floor
4. 18° 31.78'S 113° 24.72'W: sheet flow, just above flow front
5. 18° 31.84'S 113° 24.76'W: folded sheet flow
6. 18° 32.42'S 113° 24.87'W: pillow lava, young flow
7. 18° 32.71'S 113° 24.93'W: pillow lava, young flow