Project FAMOUS: Its origin, programs, and setting
J. R. HEIRTZLER Woods Hole Oceanographic Institute, Woods Hole, Massachusetts 02543 TJEERD H. VAN AND EL"' Department of Geology, Oregon State University, Corvallis, Oregon 97331
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 ingly few details on which adequate ley lies at the ridge crest. It is now known theories can be based. that such a rift valley occurs in many, but Project FAMOUS was organized to pro- Some of the first basalts from mid-ocean not all crestal areas. Yet its occurrence is so vide data on the details of the spreading regions were recovered on the Challenger common that it must be accounted for in process of the Mid-Atlantic Ridge. From Expedition (Murray, 1895), but it was only any theory of the evolution of the tectonic 1971 until 1974, when a series of manned as recently as 1961 (E. Hayes, 1974, per- plates. submersible dives to the inner rift valley sonal communs.) that extremely fresh Vine and Matthews (1963), in account- floor took place, numerous cruises were un- basalt was dredged from the Atlantic Ocean ing for the magnetic anomaly band over the dertaken to define the chief characteristics of floor. Holmes (1929, 1944) proposed that center of the Carlsberg Ridge and approxi- the American and African plates and the line basaltic magma rises to create islands and mately symmetric bands to either side, at- of their common origin. New technology swells in the oceans, although he probably tributed it to volcanism on the axis, and was required and used on several cruises. At thought of those volcanic processes as being subsequently Vine (1966) provided an age the same time, major outfitting of submersi- very similar to volcanic processes on land scale for near-axial processes. However, the bles and training of scientists and pilots for and illustrated them only in a general Vine-Matthews theory does not resolve submersible diving were undertaken. schematic way. In 1952 on Capricorn events within the axial anomaly (about 15 A detailed picture of the inner rift valley Cruise of Scripps Institution of Oceanog- km in the Atlantic and 100 km or more in has emerged between two fracture zones at raphy, Maxwell, using the new heat-flow parts of the Pacific) in view of the fact that latitudes of about 36°30' and 37°N. The probe that he and Bullard developed, first the present normal interval of the Earth's width of the inner valley floor here is approx- found high heat flow on the Albatross magnetic field has lasted —700,000 yr. The imately 1 to 5 km, with the narrow part Plateau (East Pacific Rise) and hypothesized Lamont group (Heirtzler and others, 1968) nearly midway along this 40-km-long inner (Maxwell, 1958) that this could be due to showed, with their world-wide collection of valley. There is a series of low, apparently the rising magma of a convective cell. magnetic profiles, that the volcanic plate- young hills along the center line of the valley Shortly thereafter, Bullard obtained high generating process is operative throughout floor, which are intensely fissured. Most of heat-flow values on the floor of the Mid- the oceans, rather than in a few places, and the American submersible dives occurred on Atlantic Ridge (Bullard and others, 1956). that two-dimensional magnetized blocks the floor of this rift valley between the While heat-flow values are frequently high offer a surprisingly good model for the top latitudes of 36°47' and 36°50'N. near the axes of mid-ocean ridges, this is of the oceanic basement. On the north, this rift valley segment is not always so, and the scatter has not been Seismic refraction proved to be difficult offset approximately 20 km to the east (right satisfactorily explained. in the axial region, partly because of the laterally) by fracture zone A. The dives by At a key meeting of the Royal Society in rough and complex relief, and possibly also French submersibles were primarily in this 1954 and in the subsequent proceedings, because of the lack of simple horizontal fracture zone and in the rift valley floor Rothe (1954) showed that, at least for the velocity stratification. The relief is so com- north of the American dive area. Atlantic, earthquake epicenters are concen- plex that the precision echo sounders car- To the south, the rift valley segment is trated near the crest of the ridge. The inac- ried by oceanographic vessels with a beam offset about 20 km to the west by fracture curacy of epicenter locations for such tele- width of 30° cannot resolve features at a zone B. Two of the American dives took seismic events prohibits their use, even to- scale of 1-km dimension in mid-ocean place there. Although in these fracture zones day, in detailed theories of axial processes. regions. the zone of sheared rocks has a width of It is, however, sufficient to show that they In spite of the many bits of general in- nearly 20 km, the currently active part of occur mainly on transform fractures offset- formation on crustal generation on the each fracture zone is less than 1 km wide. ting ridge axes (Sykes, 1967). Numerous mid-ocean ridge, there was, in the late events were known to occur in other than 1960s, no comprehensive body of data that INTRODUCTION transform faults. scientists could turn to in order to under- At the same symposium, Hess (1954) stand the details of the axial process. In an The mode of generation of lithosphere suggested that the rising basalt at mid- attempt to provide such information in the plates at mid-ocean spreading centers is of ocean ridges carries blocks of peridotite Atlantic, Woods Hole Oceanographic In- fundamental importance to Earth science. with it. Ewing and Heezen (1956) illus- stitution initiated a study of the rift at 22°N However, a review of marine geoscience re- trated the continuity of the mid-ocean ridge (van Andel and Bowin, 1968); the British in search over the last century reveals surpris- system and, following an observation by 1965, followed by the Canadians in 1966, * Present address: Department of Geology, Stanford Marie Tharp (Heezen and Tharp, 1957), made an intensive study at 45°N (Lon- University, Stanford, California 94305. they noted that a 30- to 50-km-wide rift val- carevic and others, 1966; Aumento, 1967).
Geological Society of America Bulletin, v. 88, p. 481-487, 3 figs., April 1977, Doc. no. 70401.
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Largely as a result of studies there, sideration of projects for a bilateral creasingly finer density of coverage. The Matthews and Bath (1967) proposed an in- oceanographic program devised by the finest-scale surveys would produce maps of jection mechanism with a probability dis- United States and France. The initial discus- features that would be useful to scientists in tribution about the ridge center line. Inde- sions in late 1971 concerned the possible the submersibles during their time on the pendently, Harrison (1968) proposed a use of manned submersible dives to the bottom. Naturally, the voluminous data similar model. Both papers showed that the floor of the Mid-Atlantic rift valley in a would have to be worked up in the short linearity of the magnetic anomaly could be unique marine project. time available before their dives. Through accounted for by most of the dikes being in- This project was named the French the support of many major participants, jected in a zone 3 to 5 km wide, with the American Mid-Ocean Undersea Study this was effectively accomplished. Attention probability of injection falling off rapidly at (FAMOUS). Unfortunately, the project was given to techniques traditionally used greater distances. could not build upon the body of pre- for geological exploration on land, espe- Additional information about the injec- existing data and work at 45°N, because the cially in volcanic areas, and to any lessons tion zone has been obtained from studies of chances of unfavorable weather prevented that could be gleaned from the manned the magnetic polarity transition zones on submersible operations there. The work at lunar exploration program. Initially, an the ridge flanks. Schouten and McCamy 45°N was nevertheless of considerable help executive board of prominent scientists to (1972) and Blakely and Cox (1972) used in planning this project. However, an review the American program was consi- analytical techniques; Atwater and Mudie aeromagnetic survey (Phillips and others, dered. However, a number of such people (1973) used a deep-towed magnetometer to 1975) had just been completed across the were already involved in the project, and study these transitions and to derive the Mid-Atlantic Ridge south of the Azores the project moved so fast that the formation width of the injection zone. (35° to 37°N). It showed a central magnetic of such a board never came to pass. After the recovery of the first oceanic anomaly and bands of anomalies to either As illustrated in Figure 1, there were four basalts by the Challenger, knowledge of the side. To a first approximation, then, here components to the FAMOUS Project. The petrology of the oceanic basement ad- was a typical section of ridge within sub- first was the marine geological-geophysical vanced little until, in the late 1940s, cruises mersible operating range of a good work which included several unique re- of the R/V Atlantis produced rich collec- port (Ponta Delgada on the island of Sao search instruments and techniques. The tions (Quon and Ehlers, 1963) which, with Miguel). In December of 1971, the R/V second was the scientific diver training many others obtained subsequently, be- Knorr on Cruise 24 made the first crossing program which included field trips with came the subject of intensive study. Engel of the ridge area between 36°30' and 37°N French pilots of submersibles, and scientists and others (1965) pointed out the distinct with a precision echosounder and towed and "test dives" where familiarization with characteristics of oceanic tholeiites and magnetometer. That cruise showed the exis- instruments ranked equally in importance their remarkable global uniformity. Shortly tence of a rift valley with an inner floor that with scientific mission. These test dives afterward, Melson and van Andel (1966) lies within the depth capabilities of French were made largely in areas of convenience. found the first metamorphosed rocks of the and American research submersibles. This The third component of the work was the ocean floor, and noted that, in the Atlantic, section of the rift valley was bounded by diving program on the ridge itself. Early in assemblages of basalts and ultrabasic rocks, fracture zones at about 37°N (fracture zone the program, it was hoped that the sub- including serpentinites, are restricted A) and 36°30'N (fracture zone B). The mersible dives on the ridge could be under- mainly to the fracture zones, but basalts cruise also showed that the central anomaly and their metamorphic equivalents domi- was near the inner rift valley. ze> nate on the rift zones. Since then, extensive In 1972, the U.S. National Academy of z petrological and geochemical studies have Sciences sponsored a meeting at Princeton z greatly improved the understanding of < CO University to put in perspective the major Z LCUO petrogenesis and magmatic processes at scientific programs required for the study of (J) -j i- < t LLI mid-ocean ridges. Notwithstanding this, > < ÜJ the Atlantic sea floor (Heirtzler, 1972). Of CO £E LLI LU o however, the conclusions remained severely the several major efforts identified, a de- CO nCE O LjJ
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taken in the summer of 1973. A delay in the magnetic measurements (Needham and crotopographic relief (Macdonald and delivery of the new pressure hull for the Francheteau, 1974; Greenewalt and Taylor, others, 1975; Reid and Macdonald, 1973; American submersible Alvin and continued 1974; Hekinian and others, 1974; Bougault Macdonald and Luyendyk, 1977). work required on the new French submer- and Hekinian, 1974; Lecaille and others, The British R.R.S. Shackleton made a sible Cyana precluded their diving on the 1974). comprehensive seismic refraction program ridge until 1974. The French submersible The R/V Atlantis II was in the area in in June. Ocean-bottom seismographs were Archimede, however, was able to make November of 1972 on Cruise 73. She un- used (Fowler and Matthews, 1974). seven preliminary dives in the summer of dertook dredging and bottom photography Because the bathymetric charts were in- 1973. with an acoustic transponder positioning complete for the area of fracture zone A, a The fourth component of the overall system that had been first tested the previ- prime dive site for the French, the French program was the French-American Plan- ous spring on one of the diver training exer- hydrographic survey ship B. O. D'Entrecas- ning and Review meetings held alternately cises in the Gulf of Maine. In addition, teau made a narrow-beam bathymetric sur- in the two countries. These meetings were water-temperature measurements were vey in that area (Renard and others, 1974). very important for discussions of recent made, and sonobuoys were used in a The line spacing was 100 to 200 m, and data that had just come from the ships, for seismic-refraction experiment (Poehls, acoustic navigation was employed. This informing scientists with closely allied in- 1974). survey extended from June to July of 1973. terest of the status of knowledge of the area, The year 1973 was an intense one for In July, the British R.R.S. Discovery and for coordinating studies and logistics. cruises in the FAMOUS area. In March, the towed the 7-ton GLORIA long-range side- In addition to the exchange of scientific in- U.S. Naval Oceanographic Office made scan sonar instrument systematically over formation, such items as allocation of un- another cruise with one of its multi-beam the area between fracture zones A and B derwater sound frequencies, schedules for echo sounder ships, spending some five days (Laughton and Rusby, 1975). The initial scientific reporting by ships in the area, re- in the area. Combining these data with analysis of the records showed major struc- sponsibility for analysis of samples, and so those of the previous narrow-beam survey tural features paralleling the strike of the on were subjects of discussion. yielded seven preliminary bathymetric rift valley but bending at the fracture zone In 1973, the JOIDES group was making charts with a 5-fm contour interval. These to approach the fracture zone strike. A line plans for a test of the deep hard-rock drill- charts were completed by Phillips and Flem- of small hills down the axis of the inner rift ing capabilities of the Glomar Challenger. ing and will be published in the near future. valley floor was prominent in the echo re- As the ship was scheduled to go from the On Cruise 31 of the R/V Knorr in May of turns. Several photographic runs were made equatorial latitudes to the far North Atlan- 1973, the Scripps deep-tow geophysical on this cruise in the inner valley, and seis- tic, plans were formulated to have her drill package was deployed over the planned mic refraction lines were shot in the median as close to the FAMOUS rift area as possi- primary dive sites. The deep-tow instru- valley (Whitmarsh, 1973). ble. Drilling was undertaken at several sites ment produced good records of magnetic The preliminary dives of Archimede on starting at about 38 km west of the rift axis anomalies, sediment thickness as deter- the Mid-Atlantic Ridge were also made dur- on Leg 37, simultaneous with the main mined by 4-kHz depth sounder, side-scan ing July and August (Bellaiche and others, FAMOUS operation in the summer of sonar results to about 500 m to either side 1974). The dives proved the feasibility of 1974. of the fish, and good records of the mi- submarine use of transponders in the rift
Cruises Prior To the Main Diving Program TABLE 1. CRUISES AND DIVES IN FAMOUS AREA
A list of all cruises to the FAMOUS area Date Cruise Special activities is given in Table 1. Many of these cruises Oct. 1971 NRL/WHOI aeromag. Magnetics were on ships-of-opportunity in the sense survey that they had not been planned long before Dec. 1971 R/V Knorr-24 Bathym., magnetics, gravity they were undertaken. Feb. 1972 USNS Hayes Seismic profiling, medium-beam bathym., magnetics After the brief reconnaissance of the area Mar. 1972 NAVOCEANO ship Narrow-beam bathym. by KTVKnorr in December of 1971, the first Sept. 1972 N/O Charcot (Midlant 72) Seismic refraction, camera, dredge, deep-tow magnetics research cruise was by the R/V Hayes. She Nov. 1972 R/V Atlantis-73 Dredge, camera, bathym., magnetics, gravity, water was able to add greatly to magnetic and temp., refraction Mar. 1973 NAVOCEANO ship especially to the regional bathymetric Complete narrow-beam bathym. survey May 1973 R/V Knorr-31 Deep-tow, seismic reflection and refraction, sonobuoy coverage, being equipped with a directive, observation of earthquakes medium-width (6°) beam echo sounder; June 1973 R.R.S. Shackleton Refraction with bottom seismographs also, the first estimate of sediment thickness June-July 1973 D'Entrecasteaux Medium-beam survey with a seismic profiler was obtained on that July 1973 R.R.S. Discovery GLORIA side-scan system, camera cruise. July-Aug. 1973 LebihanlArchimede Preliminary dives on Mid-Atlantic Ridge (7 dives) Shortly thereafter, in March 1972, one of Aug. 1973 R/V Atlantis-77 Camera, dredge, earthquakes located with bottom the special survey ships of the U.S. Naval transponders Oceanographic Office, with a multi-beam Sept. 1973 N/O Charcot (Rift-73) Seismic refraction Oct. 1973 USNS Hayes Harris array (see Maley and others, 1974, Refraction by bottom seismographs Oct. 1973 USNS Mizar LIBEC for brief description), was able to make sev- Oct. 1973 USNS Mizar Deployed bottom-current meters eral passes over the area. This ship pro- Dec. 1973 USNS Lynch Retrieved bottom-current meters duced, in real time, strip contours to a May 1974 D'Entrecasteaux Medium-beam survey 10-fm contour interval. June 1974 N/O Charcot (Rift-74) Cameras in fracture zone A In September 1972, the R/V Jean Charcot June-July 1974 R/V Glomar Challenger Deep drilling completed its "Midlant 72" cruise, covering June-Aug. 1974 AlvinlLulu Diving (17 dives) June-Aug. 1974 R/V Knorr^2 especially the area near fracture zone A. She Camera, water temp., dredging, sonobuoy attempted the first seismic-refraction mea- observation of earthquakes, drilling July-Aug. 1974 CyanalNoroit surements, obtained bottom photographs Diving (15 dives), dredges June-Sept. 1974 ArchimedelLebihan Diving (12 dives) with the troika sled, collected dredge sam- Nov .-Dec. 1974 R/V Verna 32/2 Refraction with bottom seismographs ples, and undertook the first deep-tow
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valley terrain, but some evidence of bottom light scattering back to the camera and was vironment of the actual dive. These trips currents of about 1 knot were noticed. As thus able to take very large area bottom also allowed them to become familiar with this is about the same as the speed of the photographs. These photographs were usu- volcanic materials and forms and with their submersibles in still water, there appeared ally taken in an overlapping fashion so that nomenclature. to be a possible hazard to submarine con- extensive mosaics were possible. This The French group visited Iceland in Sep- trol. Accordingly, a sea-floor current-meter coverage gave the first clear evidence of tember of 1972 and the Afar region of East project was developed for immediate de- multiple fissures of the inner valley floor, Africa in January of 1974. The American ployment. paralleling the valley axis (Brundage and group visited Hawaii in January 1974, In August, the R/V Atlantis II (Cruise 77) Cherkis, 1975). examining the flows of Kilauea Volcano again came to the area, concentrating its After a brief port stop, the Mizar re- above and below sea level. work on areas where American dives were turned to the area and deployed three Pilots and diving scientists participated in planned, especially in the area of fracture ocean-bottom current meters (Koelsch and several submersible dives before diving on zone B. Aside from underway geophysics Anderson, 1974). These meters were acous- the ridge. The Americans dived in the Gulf programs (magnetics, gravity, bathymetry, tically recalled by the USNS Lynch in De- of Maine in the spring of 1972 and in the seismic reflection), she took bottom heat- cember of the same year after having been Bahamas in February of 1974. The French flow measurements, dredge samples, and on the bottom for approximately six weeks. used the Archimede in the summer of 1972 photographs with bottom acoustic trans- One meter malfunctioned, but the other off Madeira and in the fall of that year off ponder tracking. Using the same bottom two showed currents, predominantly flow- Corsica. This submersible made other dives transponders to track a group of drifting ing along the direction of the valley and off Toulon in the spring of 1973, and the sonobuoys, microseisms were very accu- with a tidal component which was essen- Cyana had dives off southern France in rately located (Spindel and others, 1974). tially in phase with tides in Ponta Delgada. May of 1974. Largely because of the work on this cruise, The magnitude did not exceed 0.5 knots, a the true width of the sheared rock zone of value which we felt offered no hazard to The Greater FAMOUS Area fracture zone B was discovered. The peak- submarine operation (Keller and others, ing of heat flow along the active axis of 1975). The FAMOUS area along the ridge be- fracture zone B was also discovered, and In May of 1974, the B. O. D'Entrecas- tween 3 6°30' and 37°N was chosen because clustering of microseisms near the base of teaux returned to complete its detailed preliminary surveys showed it had charac- the eastern inner rift valley wall was ob- bathymetric survey of the northern part of teristics thought to be typical of ridge areas. served. the rift valley and fracture zone A. The R/V Other studies, including many new ones The French R/V Jean Charcot (Cruise Jean Charcot (Cruise Rift 74) returned to undertaken during the course of this proj- Rift 73) carried out a seismic-refraction fracture zone A in June of 1974 and con- ect, put the area (Fig. 2) in better regional program in September; the USNS Hayes centrated on bottom photography. perspective as a typical, active spreading completed a similar program in October, center. using ocean-bottom seismographs. These Field Programs of Diving Scientists and One of the most comprehensive regional results are still being analyzed. Submersible Plots analyses of topographic elevations in the The project profited greatly from the U.S. North Atlantic has been made by Sclater Naval Research Laboratory's special cam- Several field excursions were arranged for and others (1975). Their chart of residual era system, deployed by the USNS Mizar in diving scientists and pilots prior to work on elevations, after the systematic subsidence October of 1973. This system uses the the Mid-Atlantic Ridge in order to give the with age has been subtracted, shows that high-intensity method known as "Lights various participants a chance to evolve with values ranging from 1,200 to 400 m, BEhind the Camera" (LIBEC) to reduce team working habits before the intense en- the deviations near the Azores are similar to those of the remainder of the North Atlan- tic. The same paper examines regional grav- ity anomalies. A free-air gravity anomaly map, averaged over 1 degree square, shows a general decrease of the anomaly south- ward but no especially pronounced effect over the Azores. Regan and others (1975) have looked for broad-scale magnetic anomalies as recorded on the Pogo satellite. Their global magnetic anomaly map shows irregular variations from +6 to less than —4 gammas over the North Atlantic Ocean but does not show the Azores to be espe- cially anomalous. Thus, the geochemical anomalies interpreted as evidence for the presence of an Azores plume or hot spot (for example, see Schilling, 1975) do not have a topographic or geophysical counter- part. With the earliest identification of mag- netic anomalies around the Azores (Pitman and others, 1971), it was recognized that east of the Mid-Atlantic Ridge different spreading rates existed north and south of the Azores. Although analysis of seismic first motions (Banghar and Sykes, 1969) Figure 2. Greater FAMOUS and FAMOUS areas. The 2,000-m isobath is shown in the lower figure. was inconclusive, it was believed that dif-
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ferential spreading rates caused the seismic rently oblique spreading — one over the about 7° on the west and only about 4.5° on activity along an irregular transform fault past 9 m.y. and one in the period 14 to 25 the east. Thus, the western peaks are only that extends from Gibraltar through most m.y. B.P. about 12 km from the center line, but the of the Azores islands. Studies of regional While the region around the FAMOUS eastern peaks appear at about 19 km. This bathymetry and magnetics by Krause and area shows great diversity, and some parts gross asymmetry is also reflected in the Watkins (1970), Laughton and Whitmarsh are in need of further study, it does not ap- spacing of magnetic anomalies and there- (1974), and Phillips and Fleming (unpub. pear to be especially atypical in any regional fore the spreading rates in the two direc- data) and bathymetry by Uchupi (1971) way. In fact, the regional geological and tions. have greatly improved our knowledge of geophysical studies of the Azores area have The axial portion of a typical bathymet- the region around the Azores platform and failed to define any demarcation of the ric profile in the rift between fracture zones eastward. Azores platform as a geophysical entity. A and B shows a relatively flat inner floor Although the region between Gibraltar with a depth of about 2,400 m and bounded and the Azores has been called the Azores- The FAMOUS Rift Valley and by relatively steep inner walls. The northern Gibraltar Ridge, the more detailed surveys Inner Rift Valley Floor half of the inner rift valley floor and walls, revealed little evidence of a continuous ridge between fracture zones A and B, has been but rather a region of considerable topo- The most comprehensive bathymetric described by Needham and Francheteau graphic complexity. A seismically active fea- survey of the entire rift valley area was car- (1974), Moore and others (1974), and ture, the East Azores Fracture Zone, can be ried out by the USNS Hayes (Phillips and Macdonald and others (1975). The first of followed from about 15°W to the eastern- Fleming, unpub. data), although numerous these papers was based upon a closely most of the Azores islands, Santa Maria, at individual ship's tracks have added data to spaced survey by the R/V ]ean Charcot, about 25°W. At that point, the epicenter this compendium (Fig. 3). A typical using a standard 12-kHz wide-beam preci- zone turns northwest, following the line of bathymetric profile shows the peak of the sion echo sounder and taut moored buoys the islands along the "Terceira rift" while rift mountains on the east at a depth of with radar reflectors for navigation. The the topographic expression of the fracture about 1,300 m and a similar depth in the rift second paper was based upon an interpreta- zone continues westward to about 29° or mountains on the west of the Mid-Atlantic tion of the preliminary U.S. Navy 30°W but stopping short of the ridge at Ridge center. The eastern peaks are sepa- bathymetric chart for the area. The ships about 32°30'W. rated from the Western peaks by 30 to 32 collecting the data for these charts probably According to magnetic anomaly patterns, km. The regional slope of the valley walls is had the best overall navigation of any ship the section of African plate immediately south of the Azores islands has spread at a rate of 1.2 cm/yr for the entire period of 0 to 60 m.y. before present. Older crust east of the islands spread in a slightly different di- rection and at a considerably faster rate of 3.5 cm/yr for a period extending as far back as 80 m.y. B.P. To the west of the Mid-Atlantic Ridge, both south and north of the latitude of the Azores, the sea floor is less well known. The Pico Fracture Zone has been identified at a latitude of approximately 38°N, extending from perhaps 33° to 39°W. Present-day bathymetric charts do not, however, show this feature reaching the ridge, which is at about 31°W at this latitude. Details of ridge topography are still poorly defined between the FAMOUS area at 37°N (fracture zone A) and the area where the ridge passes through the Azores at 39°N between the islands of Flores on the west and Faial on the east. Krause and Wat- kins (1970) showed an axial anomaly on their residual magnetic anomaly map and, in fact, suggested the geometrical frame- work for a triple junction at this point. However, they were unable to work out the details of the age relationship of the mag- netic anomalies in this area. South of fracture zone B at 36°30'N to the latitude of 34°N, the 1971 aeromagnet- ic survey (Phillips and others, 1975) reveals nine distinct fracture zones, the largest and longest being the Oceanographer Fracture Zone at 35°N. Rates of spreading near the ridge are about 1.3 cm/yr. Bird and Phillips (1975) have analyzed the strikes of the anomalies in this latitude zone and iden- tified two periods when there was appa-
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that participated in the project. The third range. As all of the inner valley floor lies ently active scarp just discussed for fracture paper is based largely upon the Scripps within the Brunhes normal period, the es- zone A. For some 10 km on either side of deep-tow navigation system for relative po- tablishment of a chronology has been a the active zone in fracture zone B, sheared sitions. The first two papers, especially, major problem for analysis of samples and rocks were recovered with dredges. This characterize the bathymetric profile as con- observations. suggests that the line of activity in this frac- sisting of an inner floor, east and west inner Initial interpretation of submersible ob- ture zone has oscillated during geologic wall, east and west terrace, and east anc servations in the rift valley and fracture time. Dredging in fracture zone A has not west outer wall. The inner floor is 1 to 3 km zone A have been reported by Ballard and been so thorough, but a similar situation is in width; the inner walls extend 3 to 4 km others (1975) and ARCYANA (1975). likely to exist there as well. beyond the inner floor but locally may be Relatively fresh basalt seems to extend quite steep, especially on the western side, The Bounding Fracture Zone down the rift valley axis and into fracture which is frequently found to be nearly ver- zone B as far as the presently active line. In tical. The terraces, at a depth of about Fracture zone A was first mapped in de- addition to basalt, consolidated sediments 1,850 m, are 14 km wide on the east and only tail by RIV Knorr Cruise 31 during a deep- were found in the fracture between the rift 8 km on the west. The broad-scale asym- tow operation (Detrick and others, 1973) valleys. metry of the rift valley is thus reflected in and was followed shortly thereafter by a the asymmetry of its components, namely GLORIA side-scan sonar survey of the ACKNOWLEDGMENTS the inner valley walls and the terraces. Rift juncture of the fracture and the rift valley valley segments elsewhere in the FAMOUS (Laughton and Rusby, 1975). Morphologi- For the most part, the activities of Project area show essentially the same topographic cally, this fracture zone is a U-shaped val- FAMOUS evolved as the needs arose, and units with somewhat different dimensions ley. It is deepest (about 3 km) and widest there was no rigid preconceived plan. There and symmetries. (about 10 km) where it intersects the rift were invaluable contributions from numer- The center of the inner rift valley floor valley. In other places, it is of the order of 4 ous colleagues and laboratories in France, consists of a line of elongated hills, which km wide. Its length between 33°00' and England, and the United States. It is impos- photography and analysis of bottom sam- 33°16'W, the distance between the offset sible to list the names of all of these persons ples suggested were the youngest part of the rift valleys, is about 20 km. The main trend and groups here. Most of the financial sup- rift. These hills are about 300 to 1,000 m of the fracture valley is east-west and the port for the American work was from the wide, several times as long, and about 100 main bounding fault scarps trend in the National Science Foundation (both the to 200 m in height. The crest of these hills is same direction. However, there are major Office of the International Decade of Ocean remarkably uniform in absolute depth for north-south—trending structural elements, Exploration and the Division of Environ- each unit at about 2,550 ± 50 m over most especially at 33°04'W. The French sub- mental Sciences). The National Oceanic of the length of the valley, except near the sequently mapped this feature in some de- and Atmospheric Administration supported intersection with the fracture zones. Be- tail and showed that it extended back some the submersible and support ship opera- tween the hills, the terrain is either flat or 10 km into the African and American plates tions. The U.S. Navy supplied ships and forms a depression of about 100 m. On on either side of the fracture. special facilities on numerous occasions. either side of the axial zone, there are other An east-west—trending scarp, about 120 One of us especially (Heirtzler) would like depressions (marginal basins) separated by m high and down on the south, was ob- to express his appreciation for the overall shallow saddles (Luyendyk and Mac- served at 33°06' and 33°08'W; it was later support he received as Chief Scientist of the donald, 1977). mapped farther to the east. Microseismicity United States effort since 1972. The inner valley of the rift is about 40 km (Detrick and others, 1973) seems to be as- long and varies in width from about 1 km in sociated with this feature, suggesting that it the central part to well over 3 km at the marks the active part of the transform. Ob- REFERENCES CITED northern and southern ends. Its depth servations by the French (ARCYANA, ranges from an average of 2,500 m (exclud- 1975) near this scarp suggested hydrother- ARCYANA, 1975, Transform fault and rift val- ing local hills) in the middle to more than mal activity. ley geology by bathyscaph and diving saucer: Science, v. 190, p. 108—116. 2,800 m near but not in the deep depres- The American research cruises devoted Atwater, T., and Mudie, J. D., 1973, Detailed sions that mark the intersection of the rift more effort to fracture zone B than to frac- near-bottom geophysical study of the valley with the fracture zones. ture zone A, because the American submers- Gorda Rise: Jour. Geophys. Research, v. The location of the Brunhes-Matuyama ible was to dive there. Fracture zone B was 78, p. 8665-8686. magnetic anomaly boundary has been a explored intensively on R/V Atlantis II Aumento, F., 1967, Magmatic evolution on the convenient way for several authors to de- Cruise 77, immediately after R/V Knorr Mid-Atlantic Ridge: Earth and Planetary termine an average spreading rate for the Cruise 31 completed a microseismicity sur- Sci. Letters, v. 2, p. 225-230. last 700,000 yr to the eastern and western vey of the area. Other data from this area Ballard, R. D., Bryan, W. B., Heirtzler, J. R., Kel- sides of the topographic center line of the were obtained on Knorr Cruise 42 during ler, G., Moore, J. G., and van Andel, Tj., 1975, Manned submersible observations in ridge (Needham and Francheteau, 1974; the 1974 dive period. the FAMOUS area: Mid-Atlantic Ridge: Greenewalt and Taylor, 1974; Macdonald, At the intersections of the northern and Science, v. 190, p. 103-108. 1977; Phillips and Fleming, unpub. data). southern rift valley segments with fracture Banghar, A. R., and Sykes, L. R., 1969, Focal Assuming that the central line of hills is the zone B, large and deep basins are formed, mechanisms of earthquakes in the Indian zero isochron, they obtain values for just as at fracture zone A. In fracture zone Ocean and adjacent regions: Jour. spreading of 0.7 to 1.0 cm/yr to the west B, however, these two basins are separated Geophys. Research, v. 74, p. 632-649. and 1.2 to 1.4 cm/yr to the east. These vari- by a shallow sill; a distinct narrow trough Bellaiche, G., Cheminee, J.-L., Francheteau, J., ations on each side probably reflect a real runs across this sill and connects the two Hekinian, R., Le Pichon, X., Needham, D., difference in values along strike rather than basins. Heat flow (D. Williams, 1973, per- and Ballard, R. D., 1974, Inner floor of the rift valley: First submersible study: Nature, an inaccuracy in the observed values. sonal commun.) and seismic activity (Reid v. 250, p. 558-560. Magnetic anomalies cannot help in the and Macdonald, 1973; Spindel and others, Bird, P., and Phillips, J. D., 1975, Oblique dating of crust that is less than 700,000 yr 1974) both peak along this trough, suggest- spreading near the Oceanographer Fracture in age, and other techniques have been dif- ing that it is now the active part of the Zone: Jour. Geophys. Research, v. 80, p. ficult to apply to the chronology of this time transform, and the counterpart to the pres- 4021—4027.
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