Physiography and Structure of the Inner Floor of the FAMOUS Rift Valley: Observations with a Deep-Towed Instrument Package

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Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package

BRUCE P. LUYENDYK Department of Geological Sciences, University of California, Santa Barbara, California 93106 KEN C. MACDONALD* Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

This article is one of a series appearing in the April and May issues of the Geological Society of America Bulletin on the scientific results of Project FAMOUS. These studies were undertaken in the axial area of the Mid-Atlantic Ridge between approximately 36°30' and 37°N latitudes.

ABSTRACT INTRODUCTION

A deep-towed instrument survey was made of the floor of the rift Numerous publications have described the regional geophysics valley in the Mid-Atlantic Ridge near lat 37 °N (FAMOUS project). of the FAMOUS area on the crest of the Mid-Atlantic Ridge at lat Near-bottom bathymetry, side-looking sonar (SLS), and wide- 37 °N (Fig. 1). Our program used the deep-towed instrument pack- angle photography are among the data brought to bear on the age of the Marine Physical Laboratory of Scripps Institution of definition of the American-African plate boundary and the Oceanography (Spiess and Mudie, 1971; Spiess and Tyce, 1973) intrusion-extrusion zone between these plates. The valley floor is 1 and investigated the entire crestal region. In previous papers we de- to 4 km wide and contains eight elongate shield volcanoes that oc- scribed major structural and geophysical features of the crest cupy 40% of the axis. Where the volcanoes — called central highs — are absent, there are sometimes shallow depressions — called central lows. Photographic data show three components to the near-bottom environment: massive pillow lavas, well-sorted rock fragments, and sediment cover. Sediment is ubiquitous in all the photos but is scarcest along the axis of the valley floor. Pillows appear the freshest on the central highs except for one locality found at the extreme east side of the valley floor. The rock fragments are evidently pillow joint blocks and are associated with spalling off steep flow fronts and fracturing from faulting. Less well sorted fragments are associated with the large-throw step faults at the edges of the valley floor. Pillow elongation approaches 10:1 and possibly indicates flow directions both across and along contour. Numerous fissures and small-throw step faults were also seen. The SLS records show that the smoothest areas are the well-sedimented regions off the central highs that are yet to be broken by faulting at the floor edges. More than 700 faults were mapped in the valley- floor region. They trend parallel to valley strike (N17 °E) and are generally absent within 500 m of the axis. Fault density is highest at the east inner floor edge, reaching 35/km2. Fissures are present in photos but are not readily recognized with SLS data. Flow edges or ridges also align near N17 °E, but some trend across strike. SLS point targets 25 m high and about 50 by 50 m represent volcanic conelets (haystacks). These align in groups and often are associated with faults. They are not detected away from the axis. The floor is intensely fractured with fissures and small-throw vertical faults as are parts of the inner walls; this is evidence that the region of tensional faulting is at least 5 km wide. The absence of faults near the axis due to volcanic burial suggests that the extrusion zone is as much as 1 km wide. The construction process in the floor is virtually entirely volcanic, but much of this relief is obscured by the intensive shear (normal) faulting found beyond the inner walls. Many geomorphic features of the floor have direct analogy with those in the Icelandic rift valley and the East Pacific Rise, although there are differences in scale. Figure 1. General bathymetry of the FAMOUS area (uncorrected fathoms). Box marks the primary study area. This paper deals only with observations within the inner floor (approximately outlined by the 1,300- * Present address: Institute of Geophysics and Planetary Physics, University of fm contour). Depths greater than 1,300 fm shaded. Map courtesy of J. D. California, San Diego, La Jolla, California 92093. Phillips and H. Fleming.

Geological Society of America Bulletin, v. 88, p. 648 -663, 14 figs., May 1977, Doc. no. 70504.

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Figure 2. Bathymétrie chart of northern valley-floor region. Contours in uncorrected fathoms, mainly from deep-tow soundings (dots are fish tracks).

(Macdonald and others, 1975; Macdonald and Luyendyk, 1977). Other work deals with the near-bottom magnetic field at the crest (Macdonald, 1977). In the following paper we describe and present Figure 3. Map of rift valley showing locations of camera runs (roman our observations of the valley floor itself. Our purpose in this dis- numerals) and of faults (Ft) and fissures (F) seen in photos. Arrows show cussion lies in answering the same or similar questions dealt with direction of fish travel. Rose diagrams of flow directions determined in sev- by many other valley-floor surveys, including submersible dive eral camera runs are also shown. Central highs indicated are Mercury (M), programs (Bellaiche and others, 1974; Heirtzler, 1975; Ballard, Venus (V), Pluto (P), and Uranus (U) as well as unnamed highs (H) and 1975; ARCYANA, 1975; Ballard and others, 1975): Where is the lows (L). Bathymetry is after unpublished U.S. Navy soundings by H. Flem- actual crustal accretion zone and how is it expressed? What rela- ing and J. D. Phillips, modified in the north by our deep-tow measurements. tionship exists between constructive processes in the valley floor Bottom texture indicated by SLS measurements is smooth (white), inter- and the structure of the greater rift valley itself? Our near-bottom mediate (finely dotted), and rough (coarsely dotted). Dotted lines labeled observations include narrow-beam bathymetry (40 kHz); low- H-H' and J-J' are locations of echo soundings (see Fig. 14). power seismic penetration (4.0 kHz), side-looking sonar (SLS, 110 structive processes in the floor region do not bear on the origin of kHz), simultaneous wide-angle (10-m field) and normal (5-m field) the gross relief of the entire rift-valley structure (Macdonald and photography, magnetic field, and water temperatures. All mea- Luyendyk, 1977). surements are located to a precision of about ±20 m by an acoustic Many descriptions of the rift valley exist, and we will not belabor transponder network that is situated in the latitude-longitude grid these (see, for example, Needham and Francheteau, 1974; Moore via satellite fixes so that our navigation is accurate to 200 m. and others, 1974; besides the above citations). Briefly, the valley The bathymetry, photographs, and SLS records form the primary floor is a 1- to 4-km-wide relatively flat region that trends N17 °E data base for the discussion below. These and other data demon- and butts against the steep inner walls of the rift valley on the east strate that the crustal accretion zone is largely confined to the valley and west. The axial depths are near 1,400 fm, but in the center of floor but that crustal extension occurs over a larger region. The the valley half way between the two offsetting fracture zones to the main extrusion zone is restricted to a central region about 1 km north and south (fracture zones A and B, respectively), the axial wide where faults are either uncommon or are covered by rather depth is some 150 fm shallower than at the north and south valley copious volcanic extrusions (Macdonald, 1977; Ballard and van ends. Aligned along the center line of the floor are eight elongate Andel, 1977). The width of the intrusion zone is not determined. low hills, which are called central highs (Figs. 2, 3, 10). The largest Relief-forming processes in the valley floor are primarily volcanic, central high in the study area occurs at lat 36°50'N and is named but this relief is modified considerably by small- to moderate-scale Mount Venus. Separating the various highs along strike are several faulting at the edges of the floor and inner walls. Thus, the con- depressions. These features, called central lows, occur between

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/648/3418432/i0016-7606-88-5-648.pdf by guest on 25 September 2021 Figure 4. (A) Large pillows photographed on the north slope of Mount Venus in the northernmost section of pillow lavas observed in camera run I-3B (Fig. 6C). (B) Large pillows (some hollow) covered with thin sediments seen on the north slope of Mount Venus atop the small conelet photographed during run I-3B (Fig. 6C). (C) Highly elongate, small-diameter pillows, photographed at the base of the east inner wall (1,400 fm deep) traversed in run I-3F (Fig. 6A). Called "tripe" by Bellaiche and others (1974). (D) Rock fragments at the base of the east inner wall (1,525 fm deep) in run I-3D (Fig. 3). Note how the fragments are lying atop the sediments.

TABLE 1. PILLOW MEASUREMENTS

Camera N L S.D., Sk.t W S.D.«, K-w E S.D.£ run (m) (m) (m) (m) (m) (m) (m) (m)

I-3A, south approach, Mount Venus 155 1.02 0.30 0.5 -0.3 0.65 0.15 -0.4 0.07 1.57 0.27 I-3B, north slope, Mount Venus 73 1.15 0.38 1.3 4.3 0.76 0.22 -0.4 0.2 1.52 0.27 I-3F, east inner wall 571 0.90 0.23 0.6 0.4 0.54 0.48 9.2 89.4 1.85 0.53 IMA, east inner wall 243 1.18 0.38 -1.7 9.5 0.80 0.25 0.4 3.7 1.52 0.42 II-4B, east slope, Mount Venus 665 0.96 0.34 1.8 7.8 0.50 0.14 0.5 0.6 2.01 0.86 II-4C, west slope, Mount Venus 351 1.04 0.27 0.7 1.6 0.65 0.16 0.7 1.1 1.61 0.34 II-4D, Mount Jupiter to west inner wall 153 0.87 0.22 -3.2 17.3 0.55 0.12 -0.8 3.4 1.65 0.55

Note: N is the number of measurements. Average length (L), width (W), and elongation (£) of pillow basalts measured from camera run data. S.D.,, S.D.„,, and S.D.£ are the standard deviations of the length, width, and elongation, respectively. Sk. and K. are skewness and kurtosis. Negative skewness means that the median is greater than the mean and vice versa. A negative kurtosis means that the distribution has fewer members near the tails and mean than a normal distribution, and a positive kurtosis means the opposite. Analysis by W. Kempner.

3 Mount Pluto and Mount Uranus and south of Mount Uranus. umes near 0.1 m . It can also be seen from Table 2 that the larger- Other relative highs are located at the margins of the floor (Mount volume pillows are more elongated. Divers in submersibles ob- Mercury, Mount Jupiter, and the east marginal high). These are served that elongation is directly related to steep bottom slope (Bel- either laterally transported central highs or more recent in situ laiche and others, 1974; ARCYANA, 1975; Ballard and others, volcano-tectonic constructions. 1975). We observed maximum elongations of greater than 10 on the east slope of Mount Venus and on Mount Jupiter (Fig. 7). PHOTOGRAPHY These extreme elongations are not obviously related to the steeper slopes shown by the near-bottom bathymetric data; this suggests The deep-tow fish has two cameras mounted front and rear. The that the divers observed pillows on shorter and narrower slopes front camera has about a 5-m-wide field at a towing height of 10 m, than our data woi ld indicate to exist. and the rear camera field is 10 m wide. Only rear-camera photos Pillow-elongation azimuth was determined for 348 pillows to de- are shown here. The cameras are triggered remotely and simul- termine sense (not direction) of flow (Fig. 3). Direction of flow can taneously from the shipboard laboratory. The minimum repetition be determined by pillow tapering, branching, and by bottom slope. time is 10 sec, which represents a separation of 7 or 8 m at typical Because only sense is measured, the rose diagrams show 180° am- tow speeds of 1.5 knots. Detailed bathymetry and SLS data are biguity. Figure 3 shows a grouping of elongation orientations. taken during the camera runs; this allows unique and highly de- Runs II-4A, II-4B (also II-4C?), and I-3F show a preferential flow tailed textural correlations. The azimuth of the photos is estimated sense across the regional contour. However, runs II-4B and II-4C from the azimuth of the fish track. also show alignment along the regional contour. This possibly indicates that flow directions are also influenced by topography on the Pillow Dimensions smaller scales, perhaps those less than 0.5 km. Our photos were also inspected by James Moore of the U.S. About 1,600 frames were taken with each camera during eight Geological Survey. He believes that pillows with large elongation camera runs in the floor and inner-wall regions (Fig. 3). The bot- give the most correct indication of the flow direction. Where the tom environment has essentially three components: pillow lavas, elongation is small (that is, 3 or less) the direction of elongation is rock fragments, and sediment cover (Figs. 4A through 4D). Also generally random; Moore believes that this indicates such a gentle seen were various forms of bottom life (Fig. 7), open fissures (Fig. slope of the overall surface that each pillow lobe has merely flowed 5A), and small-throw step faults (Fig. 5B). Using the narrow-field into a neighboring depression. Hence, in such cases, the local mi- (5-m) frames, we measured the size and elongation (length/width crorelief rather than the average slope is determining the direction ratio) of the pillows, as well as the elongation orientations (Fig. 3). Some 2,211 measurements were made of pillow lengths and TABLE 2. EXTREME DIMENSIONS OF PILLOWS, widths. The average and extreme lengths and widths are listed on WITH THEIR RESPECTIVE ELONGATIONS Tables 1 and 2, respectively. The more elongate pillows are found on steeper slopes; the length of a pillow on a slope will be foreshor- Camera .Lmax, E Wmax, E L.min, E Wmin, E tened in the horizontal plane of the photographs — for instance, on run (m) (m) (m) (m) a 45° slope, the apparent length would be about 75% of the actual 1-3 A 2.70, 4.82 1.20, 1.51 0.38, 1.08 0.25, 2.16 length. 1-3 B 2.65, 2.91 1.29, 1.05 0.43, 1.23 0.33, 2.21 The average pillow dimensions are remarkably uniform in a wide variety of locations (Table 1). Average length is about 1.0 m; I-3F 2.99, 8.54 1.12, 1.31 0.30, 1.00 0.20, 2.70 width, 0.6 m; elongation, 1.6. The standard deviation of the II-4A 5.68, 6.76 1.85, 1.18 0.36, 1.16 0.22, 1.8 lengths is very uniform, but that of the widths is more variable. II-4B 4.22, 5.28 1.40, 1.28 0.21, 1.47 0.18, 1.61 The greatest elongations were seen in runs I-3F (Fig. 4C) and II-4B. II-4C 2.89, 3.48 1.56, 1.41 0.40, 1.00 0.21, 2.76 Table 2 shows the extreme value for each pillow dimension ob- II-4D 2.04, 8.50 1.11, 1.00 0.30, 1.00 0.25, 2.08 served during a camera run, along with the respective elongation of each extreme pillow. Table 2 shows that there are two extreme Note: Description of camera run locations and number of measurements shapes of elongate pillows that result from maximum and mini- as in Table 1. Extreme lengths (L) and widths (W) of pillows measured from camera run data. The particular pillow elongation (£) for each in- mum pillow volumes: even though the longest pillows have greatly stance of an extreme dimension is given. Some pillow elongations were different elongations than the widest pillows, both shapes of large greater than 10, but these pillows did not have extreme lengths or widths. 3 pillows have volumes on the order of 10 m (calculated assuming Analysis by W. Kempner. cylindrical shape). The shortest and narrowest pillows have vol-

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Figure 5. (A) A 1- to 2-m-wide fissure photographed in run 1-2 (Figs. 3 and 6B) on the east inner wall. No vertical offset is indicated. (B) A step fault with a throw of a few metres seen in run 1-2 (Figs. 3 and 6B) on the east inner wall. The center of the valley floor is to the right, which shows that the fault is down to the east. (C) A gja on the Reykjanes peninsula in Iceland. Note the similarity with Figure 5A.

of flow of pillows on the top surface of the flow. The average slope, however, will control the direction of movement of the flow as a whole. Where the elongation is large (that is, generally greater than 3), the pillow lobes are more nearly parallel and define a general direction of flow, probably because either (1) the top surface of the flow is steep (most likely to occur on a flow front) or (2) slumping or faulting has exposed steep-dipping, foreset-bedded elongate pillows within the flow.

Bottom Environments

All available wide-angle (10-m field) frames were analyzed ac- cording to the areal percentage of pillows, rock fragments, and sed-

iments and the occurrence of several types of bottom life.1 Some valley floor exist in the extreme north and northeastern regions of demonstrative examples are shown in Figures 6 and 7. The rock the survey (Fig. 3). Data are recorded on two facsimile recorders so fragments are remarkably ubiquitous. Most commonly they are that a continuous shadowgraph "picture" of the sea floor emerges, angular blocks of 10 to 30 cm in size which evidently are pillow with reflected areas showing dark and shadows showing white (see, joint blocks (see, for example, Fig. 4D). In the vast majority of for example, Figs. 9A, 9B). From the length of the shadow, the photos, these fields of blocks are very well sorted. Less frequently, height of the reflecting target can be estimated. Basically, two cate- poorly sorted fragments are seen (Fig. 8). The fragments or rubble gories of information are obtained: bottom texture and target loca- are commonly found at the base of scarps or slopes and often lie tion and identification. In our analysis, three categories of bottom atop sediment. These talus piles probably result from both faulting texture (Fig. 3) and four target types were mapped (Figs. 10, 12). of the flows and from spalling off of flow fronts. Therefore, they Textures were classified as rough, intermediate, or smooth, depend- are not believed to be unique indicators of faulting. The poorly ing on returns observed in the near-field (approximately the first sorted talus is more commonly associated with obvious fault 200 m). A rough area characteristically shows crisp discrete or scarps, for example at the base of the west wall (Fig. 8). Several linear returns with white shadows indicative of significant relief hundred photos were taken immediately atop the crest of Mount (several to tens of metres) over the entire near-field. An inter- Venus (run II-4X, Fig. 3). These showed vast fields of very well mediate area shows both crisp and muted returns (no or limited sorted rubble or rock fragments with virtually no sediment cover. grazing shadows) in the near-field indicating less average relief and The camera run down the east inner wall (I-3F, Figs. 6A and 3) (or) lower areal density of targets. Smooth areas showed only showed significant sediment cover on pillow surfaces no more than muted returns or no returns, indicating low relief or rolling, 300,000 yr old (an estimate from the spreading rate). Extensive sediment-covered sea floor. Textural classifications depend fields of well-sorted rubble are apparently associated with the strongly on fish height. The closer the fish to the bottom, the highly faulted topography here. Near the southwest end of this run greater the apparent roughness. Nevertheless, classifications do at the edge of the valley floor is probably the youngest lava flow agree between well over half the fish track intersections; the most that we photographed (see Fig. 4C), including photo coverage on consistent results are in the east-central and southeast parts of the Mount Venus. Run 1-2 (Fig. 6B) is on faulted terrain just out of the valley floor. valley floor and on the east inner wall (Fig. 3). Faults and fissures The bottom-texture variations form three or four strike-parallel shown in Figure 5 are from here. Runs I-3A and I-3B cross Mount bands in the valley floor (Fig. 3). The axis or center of the floor Venus from south to north (Fig. 3). Run 1-3 A is actually at the base shows only intermediate roughness. The roughest regions are at the of Mount Venus, and run I-3B lies on its north slope. Sediment has valley-floor margins where step faulting of the inner walls becomes accumulated in significant amounts, evidently in topographic pock- predominant (discussed below). The smoothest areas are off to the ets. The lava flows seen in run I-3B exhibit some of the largest pil- sides of the central highs and lows at the base of the inner walls. lows photographed (Figs. 4A, 4B). The occurrence of rubble seems Generally, camera runs showing greater than 50% areal coverage nonuniformly distributed; it is found on slopes, in pockets, and in of sediment over most of their traverse are indicative of smooth the flatter, low areas. areas. This is particularly true of runs 1-1, I-3C, I-3E, and II-4D A photo and topographic traverse was made across the valley (Fig. 7). Where large areas of rubble were photographed, the SLS floor in runs II-4A through II-4D (Fig. 7, location in Fig. 3). The bottom textures are rough (I-3F, II-4D; see Figs. 6A and 7). How- areal sediment cover increases away from Mount Venus at the cen- ever, the roughness indicated by the SLS is due to closely spaced ter of the floor. Estimates of sediment thickness also show increases faults and (or) flow fronts, not the rubble seen in the photos. These toward the edges (explained in Fig. 7 caption). These thickness es- common occurrences do support a faulting origin for some of the timates indicate sedimentation rates of only 0.2 to 0.3 cm/103 yr, well-sorted pillow fragments. which are unexpectedly low. Possibly the estimation method is wrong, or sediments are being redistributed to the north and south Lineations and Discrete Targets into the deeper fracture-zone valleys (Detrick and others, 1973). Rubble in the area of run II-4 is associated with the base of the west Four types of returns were noted on the SLS records: three linea- inner wall, the east foot of Mount Jupiter and Mount Venus, and tion types — corresponding to step faults, fissures, and flow edges the base of the east inner wall. Photos immediately at the west wall and (or) lava ridges — and discrete point targets (called "targets" show poorly sorted talus (Fig. 8) and large blocks several metres in below) — corresponding to volcanic conelets. diameter. A wide variety of bottom life was noticed (Fig. 7 cap- Small-throw (2 to 30 m) step faults appear as dark and crisp tion). Gorgonian corals and sponges are attached to pillows and linear bands accompanied by a white grazing shadow downrange (or) rubble. Sponges are distributed across the entire floor, but sed- (Figs. 9A and 9B [left side]). Fissures show as thin gray strips iment dwellers (holothurians and life forms indicated by crater against a darker background indicating no vertical separation rows and tracks) are more common at the floor margins. The (some visible in near-field of Fig. 9B [left]). These two types of re- sponges may be more numerous here also. The distribution of bot- turns were separately digitized, corrected for position, slant range, tom life bears little correlation with tectonic regime or crustal age. and fish height, and then plotted automatically via an incremental Evidently only a very thin sediment cover is needed by sediment plotter. Both types are shown together in Figure 10 but are discrim- dwellers as they are also seen on Mount Venus. inated in Figure 13. In contrast to the hundreds of step faults mapped, only a few tens of fissures were noted. This does not mean SIDE-LOOKING SONAR that they are less common, but rather that they are harder to detect (for example, Fig. 9B). Textures The fault and fissure chart (Fig. 10) shows that the vast majority of the faulting is on the east side of the inner floor. The west inner Two 1-m-long side-looking sonar (SLS) transducers are mounted wall is mainly one major fault scarp with associated minor slivers on opposite sides of the deep-tow fish. Every 1 s they insonify a re- (Macdonald and others, 1975; Macdonald and Luyendyk, 1977). gion about 500 m lateral to the fish at 110 kHz. Considering a On the east, the inner wall is a staircase of smaller-throw faults that 1,000-m-wide effective search path, data gaps in coverage of the intensely break up the terrain. Little faulting is seen associated with the central high and low system, but a band of faulting seems to 1 This analysis on bottom environments is by James Moore of the U.S. Geological Sur- occur just to either side. In fact, four bands of faulting are noted: vey; the interpretation, however, is our own. one either side of the central highs and lows, one at the base of the

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-1367m- -1314 m- • 353° "i—•007c ° 36'48.7 N 36* 4896'N 36° 49.66'N 36° 50.36'N - G 33* 15.9' W 33" 16.37'W 33 16.37'W 33 16.57'W Figure 6. (A) Summary diagram for run I-3F (Fig. 3) down the east inner wall. Bar graph indicates relative areal percentage of pillows (solid), rubble (dotted), and sediment (open). "F" indicates a fissure trending 037°. This and the following analyses by J. Moore. Figure 4A was taken at the base of the slope at the southwest end of the run. (B) Run 1-2 on the east inner wall on the south (Fig. 3). Locations and azimuths of fissures (F) and faults (Ft) are indicated. Figures 5A and 5B are from the south end of this run. (C) Runs I-3A and I-3B going north to south over Mount Venus (Fig. 3). Figures 4A and 4B were taken on run I-3B.

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36- 49.76 N 36" 49.66'N 36° 49.66 N 36" 49.76' N 36" 49.76'N 36" 49,96'N 36° 49 .56' N 36" 50.06 N 33° 16.77'W 33° 17.07'W 33" 16.87' W 33" 16.47' W 33° 16.37'W 33° 15.77'W 33" 15.47' W 33° 14.57'W Figure 7. Photo and topographic traverse across the valley floor. The photo runs indicated by the bar branching sea fan, (7) single sea whip, (8) benthic fish, (9) stalked sponge, (10) sea urchin, and (11) sea graphs are, from east to west, IMA, 1MB, IMC, and H-4D (two bars on the west). See Figure 3 for fan. Maximum sediment thicknesses are estimated from the bottom photos. It is assumed that the pil- location. The spreading rates and estimated ages are from Macdonald (1977). The presence (not abun- lows are cylinders 0.5 m wide. From this, the maximum thickness of sediment in pockets between pil- dances) of eleven different types of bottom life at various locations along the traverse are indicated: (1) lows can be estimated from the areal percentage of sediment. This analysis primarily by J. Moore. spiral sea whip, (2) sponges, (3) crater rows and tracks, (4) holothurian, (5) multiple sea whip, (6)

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N17 °E with a standard deviation of only 6° (Fig. 11). The faults were mapped along subparallel tracks which were transponder navigated, so that the trends are accurate to within about 2°. Some workers in the FAMOUS area have observed north-trending lineations and have interpreted them as evidence that the ridge is rotat- ing to a north-south direction to form an orthogonal system with east-trending transform faults (J.D. Phillips and H. Fleming, in prep.). We also have mapped more than 40 north-trending faults and fissures. However, they appear to be only part of the expected scatter of a natural process and are quite consistent with a normal distribution of lineations about a median of N17 °E (Fig. 11). The east marginal high is shown bracketed by two bands of faults in Figure 10 (see also Fig. 13). The faults on its flanks are spaced at 20 to 120 m and have throws of 4 to 30 m (Fig. 13). Submersible observations indicate that some small-throw faults dip away from the valley axis so as to decrease the net relief of the east marginal high (Ballard and van Andel, 1977). Thus this high is probably an elongate volcano, similar to Mount Venus, modified by postvolcanic tectonism. The east marginal high further defines a graben on its east at the base of the east inner wall. A similar marginal graben is found at the base of the west inner wall (Fig. 7).

Lava Ridges, Flow Fronts, and Haystacks

Two additional types of features revealed by side-looking sonar are mapped in Figure 12. One, a lineation type, is a feature interpreted as either a flow edge or a lava ridge emanating from a fissure or step fault (see right side of Fig. 9B). The second is a discrete target a few tens of metres in diameter and tens of metres high (Fig. 13). The U.S. diving team has visually inspected these features and dubbed them "haystacks" (Heirtzler, 1975). They are volcanic piles associated with small vents. The flows and ridges strongly trend N17 °E. Although some are seen to trend in other directions, many more with differing orientations would be anticipated if these returns actually represent the edges of lava flows. More probably they represent volcanic ridges. Some faults and fissures change along strike into this type of feature (Fig. 13). Admittedly, a lava flowing down over a small-throw step fault could be imagined to generate the same type of return. We have not yet been able to correlate visual sightings by the divers with this lineation type. However, they do report both the lava- ridge structure and steep and high flow edges. The haystack features are probably the same structures noted by Luyendyk (1970) and Larson (1971) associated with abyssal hills in the eastern Pacific. They are most frequently found in the central regions of the valley floor (Fig. 12). These haystacks or "knobs" (Luyendyk, 1970) show a preferred orientation in groups along N17 °E, which again suggests very strong control of the volcanic morphology by the faulting and fracturing associated with the plate boundary. Haystacks are seen aligned along both lava ridges and faults (Fig. 13). In the Icelandic rift valley, individual pyroclastic cones occur in rows along faults and fissures called "gjar" (Thorarinsson and others, 1959). These pyroclastic cones are simi- Figure 8. Photo taken in the extreme east end of II-4D (Fig. 3) at the lar in scale to the haystacks noted here. The frequent associations base of the west inner wall (Fig. 7). Poorly sorted talus and large blocks are of linearly distributed cones with gjar in Iceland and haystacks with seen. Compare with Figure 4D taken on east inner wall. faults and fissures in the FAMOUS area suggest a common genesis of these plate-boundary structures. east marginal high, and one farther up the east inner wall. The areal density of faulting increases systematically away from the floor axis CENTRAL HIGHS AND CENTRAL LOWS (Fig. 10). In the center, fault density is less than 5/km2. From here, the density increases to a maximum of 35/km2 on the east and Mount Venus is 3.7 km long and 1.1 km wide; it rises as much as 10/km2 on the west. Most faults average 600 m in length, but some 250 m above the surrounding floor (Fig. 13). Studies of crustal as long as 2 km were mapped. Throws are about 8 m in the inner magnetization show that the most highly magnetized rocks occur floor. The throws evidently increase from 4 m within 1 km of the along the axis of the inner floor of the rift valley and that the mag- axis to 15 m at a distance of 2 km from the axis. netization is particularly high over Mount Venus and other central Virtually all faults and fissures trend near N17 °E. The fault and highs nearby (Macdonald and others, 1975; Macdonald, 1975, fissure trends form an almost perfect Gaussian distribution about 1977). The freshest rock samples have been collected from Mount

Figure 9. (A) Side-looking sonar records taken running south along the east inner wall east-northeast of Mount Venus (lat 36°50'N to 36°52'N, long 33°14'W). Depth ranges from 1,500 fm in the north to 1,400 fm in the south. This record section shows returns from steep faults on the east slope. Note the apparent change in fault strike associated with a course change. Bottom texture — rough. (B) SLS records from just east of Mount Pluto and Mount Uranus following the 1,400 fm contour (lat 36°47'N to 36°48'N, long 33°16'W). This record shows step faults of the east inner wall on the left; flow fronts (or ridges) and discrete targets associated with the two central highs are shown on the right. The contact between the valley floor and inner wall is seen at the top left center where the smooth returns are in contact with the step faults. Bottom texture — smooth (top left center), intermediate (right), and rough (left).

Venus and Mount Pluto (Bellaiche and others, 1974; Ballard and are probably more common, but side-looking sonar is rather insen- others, 1975). Ages inferred from the thickness of manganese and sitive to this type of target. Some extend for more than 300 m along palagonite coatings calibrated against C14 dates on coral indicate strike. that the youngest basalts at the center of the valley floor may be Immediately north of Mount Venus a major volcanic feature, only a few hundred years old (Moore and Bryan, 1975). The active Mount Mercury, is offset to the west from the inner floor axis (Figs. spreading-center plate boundary lies along the inner floor axis and 2, 3, 10, 12). SLS coverage of Mount Mercury is rather sparse, but passes through or near central highs such as Mount Venus. Average it appears to be volcanic with flows directed away from its axis. slopes on Mount Venus are generally between 20° and 40°, but The flow fronts or lava ridges are precisely parallel to its axis, strik- within the regional slopes there are 5- to 50-m-high drop-offs with ing N15 °E. This suggests that most of Mount Mercury was con- dips of 50° to 70°. Such steep slopes suggest small-scale normal structed by linear fissure eruptions emanating from a system of faulting at first glance; however, side-looking sonar mapping vents near the Mount Mercury axis. On its east side, Mount Mer- shows that most of the apparent drop-offs are actually lobate and cury is truncated by a major fault scarp 4.8 km long with as much sinuous along strike (Fig. 13), which suggests that even the steepest as 400 m of throw. Mount Mercury may have erupted in place at slopes of Mount Venus are constructed by volcanic flow fronts or the edge of the fault block after faulting occurred. Alternatively, it lava ridges. It may be argued that Mount Venus is constructed by may have been a central high at one time, like Mount Venus, which small step faults that have been masked by posttectonic volcanism. has been uplifted and transported away from the inner floor axis. It However, linearity of the scarps on a scale of tens to hundreds of is almost impossible to distinguish posttectonic volcanism from metres would persist (except when a flow is thicker than the scarp it postvolcanic tectonism; however, the latter explanation is favored masks), and this is not observed on the side-looking sonar records. for several reasons: (1) Mount Mercury has almost exactly the A small number of faults occur on the northern flanks of Mount same height, length, width, strike, and constructional characteris- Venus, mostly on the east side, but make up very little of the total tics as Mount Venus. (2) "Lips," or volcanic features perched at the relief (Fig. 13). very edge of fault scraps, are common in the FAMOUS area (Mac- More than 50 discrete targets were mapped on and near Mount donald and Luyendyk, 1977). If there is any systematic relationship Venus. Here they range in height from 2 to 25 m and in width from between block faulting and volcanism, eruptions would occur at 15 to 70 m. Many of these small discrete eruptions occur in linear the base of scarps along fractured fault planes, not systematically at groups trending north-northeast (Fig. 13). Only 12 tectonic fissures the top of fault blocks on their outer edges. (3) From a least-energy were observed in the Mount Venus region (Fig. 5B); in reality they consideration, far less work would be done in an eruption on the

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Figure 10. Faults and fissures in the FAMOUS area as recorded from side-looking sonar records. EMH is the east marginal high, and J is Mount Jupiter. Other symbols as in Figure 3.

54' + 54'

Figure 11. Rose diagram of the orientation of more than 700 faults 52' 52' and fissures mapped in the inner floor (Fig. 10). This indicates a normal distribution about a N17 °E strike (the median value).

50' 50' +

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36° 50' N -\ 36°50'N

36° 49' N H 36° 49 N

36° 48'N 36° 48' N

33° 16 W 33° 15 W 33° 14' W Figure 13. Bathymetry and side-looking sonar features in the Mount Venus area. Soundings are in fathoms from charts published by Moore and others (1974). Camera runs (Fig. 3) are shown as very finely dotted lines. SLS features shown are step faults (medium lines) with throws indicated in fathoms, fissures (dashed lines), flow edges and (or) lava ridges (heavy lines), and point targets (haystacks; heavy dots), again with heights indicated in fathoms.

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DISCUSSION AND CONCLUSIONS

CENTRAL H We can learn much about the geologic processes in the FAMOUS

LOW 2300 area by drawing analogies with the east Icelandic rift valley (Thorarinsson and others, 1959; van Bemmelen and Rutten, 1955; 2400 Walker, 1964). Here, the plate boundary and crustal accretion 2500 zone are characterized by sets of vertical tension cracks called gjar 2600 and small-throw step faults. These zones are a few hundred metres wide and a few to tens of kilometres long (compare with Fig. 10) J' and are sites of active volcanism. The volcanism takes two forms: pyroclastic cone rows aligned along the fissures and faults (com- 2300 pare haystacks, Fig. 12) and flood basalts. The cone rows some- 2400 times coalesce to form elongate shield volcanoes (compare central 2500 highs). The zones of gjar occasionally contain small grabens (com- 2600 pare central lows). Displacements on the zones of gjar are less than 20 m on the step faults, but most offset is buried by flood basalts that tend to fill the grabens and bury the faults. Almost none of the relief within the Icelandic rift valley is due to faulting Figure 14. Line drawings of near-bottom echo soundings over central (Thorarinsson, 1966), and we can say almost the same of the valley lows (arrows). Location shown in Figure 3. floor in the FAMOUS area. valley floor than in routing a conduit to the top of the block fault, The definition of the plate-boundary zone in the FAMOUS area in this case involving an extra 400 m of head to overcome. can be considered as either the seismically active region or the re- Three other topographic highs occur on the inner floor in the gion where the crust-lithosphere is being extended under tension. Mount Mercury area; they are 0.8 to 1.3 km long and 80 to 150 m The zone of crustal accretion is that region within the plate- high (Fig. 2). The flanks of all three are mantled by flows, and little boundary zone where intrusion and extrusion of magma occurs or no faulting is evident; therefore, they apparently are volcanoes. most frequently, so that most of the crust within the zone would be The flow pattern and directions are not as linear and systematic as of "zero" age (Luyendyk and Macdonald, 1976). The plate- on Mount Venus and Mount Mercury. It is unlikely that these boundary zone could obviously be defined to extend out to the smaller volcanoes are products of linear fissure eruptions (like outer walls of the rift valley (total width, 30 km) and shortly be- Mount Venus and Mount Mercury), but probably represent ¿entral yond, assuming that the valley is a steady-state feature requiring eruptions with one or several point sources. In fact, the density of continual uplift of crust along fault planes up to the level of the haystacks in this region is very high (Fig. 12). About 14 of these are crest mountains (Macdonald and Luyendyk, 1977). lineated along the axis of the floor. The definition of the extrusion zone is demonstrated by the re- Just south of Mount Pluto and Mount Uranus are two well- gions in the valley-floor axis where faults are not detected. developed central lows, each about 2 km long and 100 m deep Presumably, tensional faulting occupies the entire floor including (Figs. 3, 10, 12). They are 600 to 800 m wide with floors that range the axis. However, active extrusion of flows masks the faults much from being flat to U- or V-shaped (Fig. 14). These central lows as in Iceland. This "fault-free" band clearly shows a width of 200 could be grabens, created by inward-facing normal faults, or topo- to 1,000 m in Figure 10. This is somewhat narrower than the width graphic troughs, representing a gap between flows erupted near the of the central highs, which reach a maximum width of 1.2 km. inner floor edges, or regions of caldera collapse. Side-looking sonar Complicating this description of the extrusion zone is the observa- records indicate that the lows are bounded by linear scarps (Fig. tion that limited volcanism (extrusion) evidently occurs at the edges 10); therefore, it is unlikely that the depression is simply a gap be- of the valley floors (discussed above) and over the inner walls (cam- tween flows emanating from the floor edges. The difference be- era run I-3-F, Fig. 6A). The width of the intrusion zone is not de- tween formation of a central high and central low is probably the terminable from our observations. presence or absence of a local magma source. Where a magma Volcanic construction dominates the topography of the inner source is not present, continual normal faulting and fracturing of floor of the rift valley. At the inner walls and beyond, tectonic the crust, concommitant with spreading, results in a graben and forces are in control, and large-throw normal faults (at least 600-m downdropping of the floor. Alternatively, the low may be caused throw) dominate the relief. The central highs are transported out of by caldera collapse due to a pressure drop in an underlying magma the inner floor atop the faulted blocks to the inner walls and ter- chamber. Magma may migrate along the strike of the floor with races and become a second-order component in the relief (Mac- sources concentrating at the central highs. For example, the recent donald and Luyendyk, 1977; Ballard and van Andel, 1977). This eruption at Heimaey in Iceland began as a linear fissure eruption does not appear to be true for faster-spreading ridges such as the several kilometres long. Within a few days, magma sources mi- East Pacific Rise, where Luyendyk (1970) and Larson (1971) ob- grated along strike; this resulted in a linear fissure eruption less served that volcanic knobs (compare haystacks) coalesce and join than half its original length (H. Samuelsson, 1973, oral commun.). with lava flows to form abyssal hills (compare central highs). These Crust overlying the depleted zone may collapse; this would result in features are only moderately modified by faulting within the an alternating central high—central low morphology. plate-boundary zone and are a dominant component in the relief of The central low south of Mount Uranus has more than 20 vol- most of the Pacific floor. It is interesting that although the construc- canic cones or haystacks forming north- and north-northeast— tional analogies between the central highs and abyssal hills proba- trending lineations on its floor. The floor of the central low is not bly hold, the scales do not generally compare. Larson's "hay- devoid of volcanic activity, but simply has extrusive activity on a stacks" (knobs) were on the same scale as those observed in the much smaller scale than where a central high is present. This FAMOUS area, but knobs seen by Luyendyk were about five times small-scale volcanism takes place through a series of small point as large. Also, abyssal hills in the Pacific commonly are about five sources, rather than along a fully developed linear fissure system to ten times as large as the central highs mapped here. Possibly our such as is the source of the larger central highs. analogies are incorrectly drawn, but a certainly exciting hypothesis

is that the crustal generation process for fast-spreading ridges (East Heirtzler, J. R., 1975, Where the earth turns inside out: Natl. Geog. Mag., Pacific Rise) has a larger extrusive component than we are dealing v. 147, p. 586-603. with in the FAMOUS area. Larson, R. L., 1971, Near-bottom geologic studies of the East Pacific Rise crest: Geol. Soc. America Bull., v. 82, p. 823-842. Our study has therefore contributed to the geologic mapping of Luyendyk, B. P., 1970, Origin and history of abyssal hills in the northeast the plate-boundary region that is an aim of the FAMOUS project. Pacific Ocean: Geol. Soc. America Bull., v. 81, p. 2237-2260. Obviously, future studies should be aimed at extending the geology Luyendyk, B. P., and Macdonald, K. C., 1976, Spreading center terms and mapped here to other rifted ridges in the manner initiated by concepts: Geology, v. 4, p. 369-370. Needham and Francheteau (1974). In spite of our efforts and those Macdonald, K. C., 1975, Detailed studies of the structure, tectonics, near of others to construct this geologic map (marine version), we have bottom magnetic anomalies and microearthquake seismicity of the shed little light on the forces responsible for moving the plates or Mid-Atlantic Ridge near 37 °N [Ph.D. thesis]: Cambridge, Mass. Inst. even those that cause the rift valley itself. We can only hope that the Technology-Woods Hole Oceanographic Inst., 248 p. geologic data presented here will provide necessary constraints to 1977, Near-bottom magnetic anomalies, asymmetric spreading, the many hypotheses that bear on these subjects. oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N: Geol. Soc. America Bull., v. 88, p. 541-555. Macdonald, K. C., and Luyendyk, B. P., 1977, Deep-tow studies of the ACKNOWLEDGMENTS structure of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS): Geol. Soc. America Bull. We are grateful for the cooperation of the Scripps Institution of Macdonald, K. C., Luyendyk, B. P., Mudie, J. D., and Spiess, F. N., 1975, Oceanography and for the aid of F. N. Spiess and J. D. Mudie and Near-bottom geophysical study of the Mid-Atlantic Ridge median val- the members of the deep-tow technical staff, who assisted in the ley near lat 37 °N: Preliminary observations: Geology, v. 3, p. 211- expedition of Knorr 31 in 1973 and aided greatly in its prepara- 215. tion. We also thank Captain E. Hiller and his crew for exemplary Moore, J. G., and Bryan, W. B., 1975, Compositional zoning in rift valley support under oftentimes trying circumstances. The photographic basalts at 36°50'N, FAMOUS area: EOS (Am. Geophys. Union Trans.), v. 56, p. 375. interpretations were initiated by J. Moore. W. Kempner and Pa- Moore, J. G., Fleming, H. S., and Phillips, J. D., 1974, Preliminary model tricia Allen assisted in the analysis of the photo and side-looking for extrusion and rifting at the axis of the Mid-Atlantic Ridge, sonar interpretations. The Woods Hole Oceanographic Institution 36°48' North: Geology, v. 2, p. 437-440. provided generous financial assistance. Deva Richards, S. Gregg, R. Needham, H. D., and Francheteau, J., 1974, Some characteristics of the rift Groman, and Susan Fletcher, of WHOI, aided in the field and valley in the Atlantic Ocean near 36°48'N: Earth and Planetary Sci. analysis program. Funding for this research was supplied primarily Letters, v. 22, p. 29-43. by the National Science Foundation through grants 36818 and Spiess, F. N., and Mudie, J. D., 1971, Small-scale topographic and magne- 31377X and by the U.S. Office of Naval Research. tic features, in Maxwell, A. E., ed., The sea, vol. IV, Pt. I: New York, J. Wiley & Sons, p. 205-250. Spiess, F. N., and Tyce, R. C., 1973, Marine physical laboratory deep tow REFERENCES CITED instrumentation system: Scripps Inst. Oceanog., Reference 73-74, 37 p. ARCYANA, 1975, Transform fault and rift valley geology by bathyscaphe Thorarinsson, S., 1966, The median zone of Iceland, in Irvine, T. N., ed., and diving saucer: Science, v. 190, p. 108-116. The world rift system: Canada Geol. Survey Pub. 66-14, p. 187-211. Ballard, R. D., 1975, Dive into the great rift: Natl. Geog. Mag., v. 147, Thorarinsson, S., Einarrson, T., and Kjartansson, G., 1959, On the geology p. 604-615. and geomorphology of Iceland: Geog. Annaler, v. 11, p. 135-169. Ballard, R. D., and van Andel, T. H., 1977, Morphology and tectonics of van Bemmelen, P. U., and Rutten, M. G., 1955, Tablemountains of north- the inner rift valley at 36°50'N on the Mid-Atlantic Ridge: Geol. Soc. ern Iceland: Leiden, E. J. Brill Co., 217 p. America Bull., v. 88, p. 507-530. Walker, G.P.L., 1964, Some aspects of Quaternary volcanism in Iceland: Ballard, R. D., Bryan, W. B., Heirtzler, J. R., Keller, G., Moore, j. G., and Leicester Lit. and Philos. Soc. Trans., v. 51, p. 25-40. van Andel, Tj., 1975, Manned submersible observations in the FA- MOUS area, Mid-Atlantic Ridge: Science, v. 190, p. 103-108. Bellaiche, G., Cheminee, J. L., Francheteau, J., Hekinian, R., Le Pichon, X., MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 26, 1975 Needham, H. D., and Ballard, R. D., 1974, Rift valley's inner floor: REVISED MANUSCRIPT RECEIVED AUGUST 25, 1976 First submersible study: Nature, v. 250, p. 558—560. MANUSCRIPT ACCEPTED SEPTEMBER 13, 1976 Detrick, R., Mudie, J. D., Luyendyk, B. P., and Macdonald, K. C., 1973, WOODS HOLE OCEANOGRAPHIC INSTITUTION CONTRIBUTION NO. 3656 Near bottom observations of an active transform fault: Mid-Atlantic CONTRIBUTION OF THE SCRIPPS INSTITUTION OF OCEANOGRAPHY, NEW Ridge at 37 °N: Nature, v. 246, p. 59-61. SERIES

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