U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Northern Cleft Segment: Morphology and Volcanology
The termination of the morphologically distinct inner graben occurs at 44°55.5'N (Plate 2 and Figures 3a and 3b). This sudden change in structure is probably a fourth-order discontinuity as defined by Macdonald et al. [1988]. The western boundary fault continues north to about 45°00'N where it becomes buried by younger flows. North of 44°54'N (Plate 2) the eastern boundary fault becomes a low relief feature only about 20 m high. The 100-m relief trough and ridge on the east side of the axial valley (ETR on Figure 3b) is not the eastern boundary fault of the present axial valley but a slightly older feature. Between 44°56' and 45°01'N, the neovolcanic zone is centered between the 100-m relief western boundary fault and the poorly defined continuation of the eastern boundary fault (Plate 2 and Figure 3).
Plate 2. Sea Beam bathymetry of northern Cleft segment. Contour interval is 5 m and color interval is 20 m. Raw Sea Beam depth is corrected to Alvin depths for this area (15 m subtracted from raw Sea Beam data). Bold line is outline of young sheet flow (YSF).
Figure 3a. Mosaic of three SeaMARC 1A (30 kHz) sidescan swaths of northern Cleft, the same area as Plate 2 and Figure 3b (opposite). White ribbons are center beam of sidescan. White is high reflectivity, and black is low reflectivity or shadows. Each swath is 1 km on each side with 1048 pixels across total swath. See Figure 3b for full names of abbreviations. Approximate latitudinal limits of sidescan swaths shown in Figure 4 are indicated by arrows.
Figure 3b. Interpretative map of sidescan in Figure 3a for northern Cleft segment. Roughly the same area as Plate 2 and Figure 3a. Features outside of Cleft axial valley based on Sea Beam map (Plate 2). Bold line is outline of young sheet flow (YSF). PO is Pipe Organ Vent, M is Monolith Vent, ETR is Eastern Trough, and LRR is Lava Rise Ridge. S indicates location of extinct sulfide chimney fields located north or south of area covered by Figure 5.
Within the axial valley north of 44°55'N, several types of constructional features are present. Bisecting the axial valley from 44°55.7'N to 44°59.5'N is a string of mounds (marked age 2 on Figure 3b on a relative age scale in which age 1 is youngest and age 4 the oldest) which are composed primarily of pillow lavas with a few fissures cutting through them (Figure 3a). The lava flows erupted during the 1980s (marked age 1 on Figure 3b and henceforth designated the new pillow mounds (NPM) are only barely differentiable on sidescan sonar records from the age 2 mounds by the complete lack of fissures on the NPM (confirmed by towed camera and submersible observations). Surrounding the constructional mounds are areas of flat, smooth seafloor. These are composed primarily of sheet flows. One particularly young sheet flow (YSF) that appears to be almost as young as the NPM (described in detail below) lies primarily west of the pillow mounds and extends south to the northern portion of the inner graben at 44°55'N (Plate 2 and Figure 3b).
Another type of constructional feature is somewhat enigmatic. These are ridges lying along the northwestern portion of the axial valley (north of 44°59'N and labeled LRR on Figure 3b, for Lava Rise Ridge) which are covered primarily by jumbled sheet flows. Narrow, 020°N trending fractures penetrate the 40-m high east flank of one ridge, and several 5- to 10-m wide, noneruptive fractures are found along the top (Figures 5a and 5b). Observations from an Alvin dive that followed one of these cracks for several hundred meters revealed that the jumbled flows cap massive, columnar-jointed units. The large fractures located near the top of the steep flanks of the ridge resemble "lava inflation clefts" [Walker, 1991]. The LRR appears to be a large lava inflation feature caused either by the continued introduction of lava into a flow lobe during the solidification of the outer crust or by later intrusion from below. Similar features have been described in Hawaii [Walker, 1991], on Axial Volcano, Juan de Fuca Ridge [Appelgate and Embley, 1992], and near the Kane Fracture Zone on the Mid-Atlantic Ridge [Humphris et al., 1990].
The NPM and YSF are both superimposed on distinctly older flows. Their young age, which is evident from their 05% sediment cover and vitreous luster (Plates 3a and 4h), allows the contacts between young and old lavas to be easily mapped with bottom photography. The young flows are morphologically distinct from each other. The YSF consists primarily of a mixture of ropy, lineated and lobate flows covering an area of about 3.5 km2 between 44°55.5'N and 44°59.5'N. North of the YSF, from 45°01'N to about 45°10'N, the NPM consist of a string of discontinuous pillow lava mounds covering up to several square kilometers along the crest of the ridge that overlaps with the Vance axial valley [Chadwick and Embley, this issue] (Figure 1). Repeat Sea Beam surveys have shown that these mounds were erupted between 1983 and 1987 [Embley et al., 1990b; Chadwick et al., 1991; Fox et al., 1992]. A comparison of bottom photographs between the YPM and YSF shows no distinguishable difference in sediment cover or reflective properties, and therefore we interpret the YSF to be only years to decades old. The young lava flows of the northern Cleft segment are distinctly younger than flows found elsewhere on the segment, including those forming the lava plain to the south, which Normark et al. [1983] estimated to be less than a few hundred years old.
The boundary of the young sheet flow is, for the most part, not distinguishable on sidescan imagery (Figures 3 and 5), because the edge of the YSF is similar in morphology to the surrounding older flows (e.g., southern boundary) and the difference in sediment cover between the young and older flows is not great enough to cause a significant change in backscatter intensity. However, there are distinct backscatter differences between different flow types within the YSF. The most striking differences occur between areas of very flat, lineated lavas characterized by low backscatter and areas of chaotic, jumbled, or collapsed sheet flows characterized by high backscatter (note area between 44°57' to 58'N and 130°13' to 14'W) (Figure 3a). Lobate flows appear to be characterized by an intermediate level of backscatter.
With this understanding of the origin of the backscatter contrasts at this site, we compared sidescan sonar data collected over the same portion of the YSF in 1982 and 1987 (Figure 4) to see if the YSF is present in both. The 1982 data were of significantly lower quality (the ship was turning at the time and only analogue data were available), but the same backscatter patterns are apparent. Therefore we conclude that the YSF was erupted prior to October 1982, at least 7 months before the NPM eruption.
Figure 4. Comparison of 1982 and 1987 SeaMARC sidescan showing similar patterns of backscatter associated with sheet flow. Similar backscatter patterns are identified by number. The low backscatter (white) zones are areas of flat, lineated sheet flows. Note that the 1982 analogue record (no digital data available) is distorted by a large course change.
Near the northern end of the YSF there is a topographic constriction where lava flowed through a constriction at 44°58.1'N (Figure 6e and Figure 5a). The upper surface of the YSF is nearly flat, both north and south of the constriction, but it is about 12 m lower on the south side. The original flow top (before collapse) is at 2263 m at the north end of the flow, at 2273 m at the constriction, and at 2275 m south of the constriction (Figure 6). Another contrast is that the height of the lava pillars within the collapse areas is typically 1 m north of the constriction but 5 m south of it. However, on any given east-west cross section the tops of the pillars are at the same depth to within less than a meter, representing the original surface of the flow. This is evidence that much of the YSF lava was erupted at the northern end of the flow and quickly flowed south through the constriction where it then spread out and ponded in a broad shallow depression between the age 2 pillow mounds and the western boundary fault (Plate 2 and Figure 3).
Figure 5a. Geology and bathymetry of northern part of sheet flow. Bathymetry is based on Alvin and towed camera pressure depth and altitude data. LRR is Lava Rise Ridge, A2PM is age 2 pillow mounds (age 1 is YSF and NPM). Most of the structural information is based on SeaMARC I sidescan imagery (Figure 5b, opposite). Western boundary fault is 80 to 100 m in relief.
Figure 5b. Sidescan mosaic of same area as Figure 5a (opposite).
Figure 6. Cross sections of north Cleft segment using Alvin and towed camera microbathymetry. Bold thick line on cross sections show extent of YSF. F and arrow shows location of hydrothermal activity and/or most recently active fissure. A2PM is age 2 pillow mound. Locations of cross sections for Figures 8 and 9 in inset. Letters on inset map refer to following cross sections: (a) Camera tow 198914, (b) Alvin dive 2429, (c) Alvin dive 2269, (d) Alvin dive 2269, (e) Alvin dive 2267, (f) Alvin dive 2431, (g) Alvin dive 2444, (h) camera tow 19874, and (i) camera tow 19875.
This sheet flow consists of a wide range of lava morphologies because the interior of the flow has drained out and is collapsed in many areas. The original upper surface of the flow is lobate in morphology, where it is preserved at the flow edges and on the top of the pillars (Plate 3a). The collapse areas are floored by lineated, ropy flows, and jumbled sheet flows (Plates 3a3e). South of 44°58'N, the lava onlaps the base of the talus slope of the western bounding fault.
Plate 3. Photographs of northern Cleft geologic features. ALVBC, Alvin bow camera; ALVHH, Alvin hand-held camera; TC, towed camera (approximate scale across bottom of photo). (a) (TC) Contact of young sheet flow with older lobate flows (4 m). (b) (TC) Lineated sheet flow with low-temperature hydrothermal deposits (4 m). (c) (TC) Transition from young lineated sheet flow to ropy and jumbled flows (5 m). (d) (TC) Young Jumbled flow (4 m). (e) (ALVHH) Pushup structure in young sheet flow (3 m). (f) (TC) Looking down on young fissure with remains of collapsed and drained-back lobate flow at edge (5 m). (g) (ALVHH) Lava spires with multiple "bath-tub ring" shelves at western edge of fissure (3 m). (h) (ALVHH) Spire with shelves in southern portion of sheet flow (3 m). (i) (ALVBC) Looking along primary fissure (within a few hundred meters of Plate 3f) showing lava drainback sheets (5 m).
The eruptive fissure for the YSF is interpreted to be a NE-SW (~020°) trending fissure system along the western shoulder of the age 2 pillow mounds, and it is along this trend that most of the high-temperature and low-temperature diffuse vents are concentrated [Embley et al., 1991] (Figures 3b and 5a). This en echelon fissure system along the eastern edge of the YSF is most continuous north of 44°58.5'N. Classic drainback features are seen along the fissure (particularly north of the Cavern Vent at 44°59'N (Figure 5a)), including draped sheets and drainback cavities at the lip of the fissure (Plates 3f3i). In some places, primarily between Cavern and Marker 4 and at the Tripod vent (Figure 5a), fissures cut through the young sheet flows, indicating some posteruptive extension. There is only minimal vertical offset (~12 m) (Figure 6d). The fissure system is generally within the YSF, except at 44°58.3'N (Figure 5a) where it cuts through a section of the age 2 pillow mound.
Zones of lava pillars and "shelly" lobate flows [Embley et al., 1990a] are found along the edges of the flow, predominantly on the eastern side adjacent to the hydrothermally active fissure system (Figures 6b6f) but also on the western side of the sheet flow near the wall (Figure 6f). NE-SW trending lineated lava flows are found in the interior of the YSF, along with small pushup ridges (Plate 3e), which usually have a long axis perpendicular to the lineations. Numerous examples of transitions from lineated to ropy and whorly pahoehoe-like flow surfaces show that the lineated flow surface is created by flow of lava in a southwesterly direction parallel to the lineations (e.g., Plate 3c). The lower flow surface postdates the formation of the pillars since rubble from broken pillars and collapsed roof sections is commonly found on the lower surface.
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