Dive 4606, 1 April 2010 Dive Summary
Observers: Ken Rubin (p), Tim McClinton (s), Mike (pilot)
This dive visited the terrain along the ridge axis immediately due west of the on-axis seamount at 91°
57’N. The region sits astride the axial graben. The dive targeted regions of both faulted and unfaulted
topography, and areas of acoustic backscatter contrast that was evident in the Haymon-White dsl-120 survey
data, driving generally west to east along a trajectory that zig-zagged across this graben. It also visited an area
on the southern margin of the graben wall, where the ridge forming this wall is especially low, and where
relatively continuous and homogenous acoustic backscatter gives the impression of a vast lava sheet spilling
out of this region. This “spillway” area sits at the head of a lobe-shaped high ground on the southern ridge axis
flank.
11 samples of mildly Mn-coated, generally fresh basaltic lava were taken on the dive.
There are two dominant features of the ridge axis that were observed on this dive:
1. The graben, which is asymmetrically faulted: It is bounded by a large near vertical normal fault on the
southern side, with throws of 20 to 34 m, trending roughly 300° to 320° (and 120° to 140°), and with
talus accumulations at its base; The northern side of the graben was defined by a series of faults, and
smaller horst-graben structures with 2 to 5m throws (see map inset for the geometry in cross-section).
2. Lava flows of essentially constant sediment cover throughout (Van Andel age 1 to perhaps 1.5, with
light sediment dusting on the tops of flows and limited accumulation in pockets between the high
standing portions of the flow surface). Pillows are dominant in the center of the graben, whereas
lobates dominate on the southern flank, especially in the “spillway” region. High effusion rate curtain
folded sheet flows occur in the eastern part of the dive transect, particularly in the central part of the
graben, but also in small channels in the western part of the spillway region. All three of these
lithologies were observed to be cut by the large normal fault on the southern graben margin.
The dive started in pillow lavas, hugging a local constructional slope, and then crossed a series of fissures
and large tilted lava blocks, followed by mostly lobate lavas on the transect from WP1 to WP2. The dive visited
the high standing ridge on the southern graben margin (south of the fault) 4 times, and each time this ridge was
topped by lobate lavas. The north-facing side leading up to the ridge was constructed of pillows. Lobates were
also observed on the northern extremes of the dive transect, where local relief was flatter, although these were
not always local highs. The western part of the spillway region had collapsed and shelly lobate lavas and small
lava channels, giving the impression of quite high effusion rate flows. Lobates interspersed with high standing
pillows occurred throughout the spillway region, including on the high ground at the southeastern extreme of the
dive track. By far the highest effusion rate forms were found below and above the large-offset normal fault in
the center of the basin where the dive ended. The orientation of the sheets suggest flow in the diection roughly
perpendicular to the graben (i.e., towards the “spillway” and then off axis).
Our overall impression was that we traversed one lava unit which was subsequently cut by graben forming
faults. It is difficult to indentify source vents for the flow, although we hypothesize that they were on the
southern margin of the graben, with higher effusion rate activity in the eastern part of the area studied.
SubSea2.20100401_195810.jpg (lower fault block)
SubSea2.20100401_201341.jpg (upper fault block)