Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N KEN C. MACDONALD* Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093 BRUCE P. LUYENDYK Department of Geological Sciences, University of California, Santa Barbara, California 93106 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 large faulted blocks toward nearby fracture Le Pichon, 1974). We discuss here the de- zones suggests that spreading-center tec- tailed structure and evolution of the rift val- A detailed study of the structure of the tonics is affected by fracture-zone tectonics leys between fracture zones A and B (rift Mid-Atlantic Ridge median valley and rift throughout the length of the rift in the valley 2) and B and C (rift valley 3) and the mountains near lat 37°N (FAMOUS) was FAMOUS area. Both the crustal accretion rift mountains in these areas. Fine-scale conducted using a deep-tow instrument zone and transform fault zone are narrow, deep-tow studies of the inner floor of rift package. The median valley may have either only 1 to 2 km wide, over short periods of valley 2 are discussed by Luyendyk and a very narrow inner floor (1 to 4 km) and time. In the course of millions of years, how- Macdonald (1977). Studies of near-bottom well-developed terraces or a wide inner floor ever, they apparently migrate over a zone 10 magnetic anomalies and a tectonic synthesis (10 to 14 km) and narrow or no terraces. to 20 km wide. of rift valleys 2 and 3 are presented by The terraces appear to be non-steady-state Macdonald (1977). features of the rift valley. The entire depth OBJECTIVES and gross morphology of the median valley SETTING may be accounted for by normal faulting, Recently, several detailed surveys have while volcanic relief contributes to the been made on the Mid-Atlantic Ridge (at lat The FAMOUS area lies south of the short-wavelength topography (<2 km). 45°N, Aumento and others, 1971; at lat Azores triple junction on the North Most faults dip toward the valley axis an 37°N, Needham and Francheteau, 1974; American-African plate boundary (Fig. average of 50°, and the blocks are tilted back and at lat 26°N, McGregor and Rona, 1A). Rotation of magnetic anomalies about 2° to 3°. Fault dip is asymmetric about the 1975). However, the limitations of wide- the North America-Africa pole of opening valley axis. Active crustal extension in the beam echo sounders and other conventional and absence of a distinct median valley in- inner floor and inner walls has the same geophysical tools have made it difficult to dicate that the ridge crest is influenced by sense of asymmetry as the local spreading investigate the fine-scale structure and tec- triple-junction tectonics as far south as lat rates, reaching a maximum of 18 percent. tonics of the median valley and rift moun- 38°N (J. D. Phillips and H. S. Fleming, in Thus, asymmetric spreading appears to be tains. Several fundamental problems have prep.). South of lat 38°N, however, the me- accomplished by asymmetric crustal exten- remained unsolved: (1) the width and mor- dian valley is well defined, and magnetic sion on a fine scale as well as by asymmetric phologic expression of the crustal accretion anomalies 2' prime and 5 overlap properly crustal accretion. Spreading is 17° oblique to zone within the median valley, (2) the roles when rotated back about the pole of open- the transform faults and shows no indication of volcanism and faulting in creating relief ing. The part of the ridge studied consists of of readjusting to an orthogonal system, even in the median valley and in the rift moun- two segments, each 40 km long, trending on a fine scale. Eighty percent of the decay or tains, (3) the morphologic and structural N17°E (Figs. 1A, IB). Fracture zones A, B, transformation of median-valley relief into expression of asymmetric and oblique and C each right-laterally offset the valley rift-mountain topography is accomplished spreading, (4) the degree of horizontal ex- about 20 km and trend east. Spreading is by normal faults that dip away from the tension of crust within the plate-boundary currently 17° oblique, and detailed studies valley axis. Most of the outward-facing zone, (5) the tectonic interaction between of the tectonic and magnetic trends in the faulting occurs near the median-valley- transform faults and spreading centers, (6) inner floor of the rift suggest that the rift-mountain boundary. Tilting of crustal the relationship between microearthquakes spreading is stably oblique—that is, the blocks accounts for only 20 percent of the and the fine-scale structure of ridge crests, ridge is not readjusting to an orthogonal decay of median-valley relief. Most long- and (7) the evolution of the median valley configuration (Macdonald, 1977; Luyen- wavelength topography in the rift moun- into the relief of the rift mountains. dyk and Macdonald, 1977). Oblique tains has a faulted origin. As in the median To address these problems, a near- spreading may have been stable here for at valley, volcanic relief is short wavelength bottom geophysical study of the Mid- least 6 m.y., even through a change in (<2 km) and appears to be fossil, originating Atlantic Ridge near lat 37°N was conducted spreading direction (Macdonald, 1977). In in the median-valley inner floor. Bending of from the R/V Knorr (cruise 31) of the addition, spreading is highly asymmetric in Woods Hole Oceanographic Institution, rift valley 2, with rates of 7.0 mm/yr to the using the deep-tow instrument package of west and 13.4 mm/yr to the east. At 1.7 the Marine Physical Laboratory (Scripps m.y. B.P., the sense of asymmetry reversed, * Present address: Marine Physical Laboratory and with rates of 13.3 mm/yr to the west and Geological Research Division, Scripps Institution of Institution of Oceanography). This effort is Oceanography, La Jolla, California 92093. part of the FAMOUS project (Heirtzler and 10.8 mm/yr to the east (Macdonald, 1977). Geological Society of America Bulletin, v. 88, p. 621-636, 16 figs., May 1977, Doc. no. 70502. 621 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/621/3418363/i0016-7606-88-5-621.pdf by guest on 23 September 2021 622 MACDONALD AND LUYENDYK 38°W Figure 1A. Regional setting and plate boundaries in FAMOUS area (unpub. U. S. Navy bathymetric chart, courtesy of J. Phillips and W. Perry). E-E'" is long deep-tow traverse into eastern rift mountains. FZ=Fracture zone. Spreading is only slightly asymmetric in rift tains is discussed here first. Rift valley 3, looking sonar has a range of 400 to 700 m valley 3, with rates of 9.8 mm/yr to the west which has a much wider inner floor than rift to either side, resulting in an effective search and 10.6 mm/yr to the east (Macdonald, valley 2, is discussed for comparison. path approximately 1 km wide. The side- 1977). looking sonar is particularly effective in Most of the FAMOUS investigations, in- DATA INTERPRETATION mapping linear scarps on a small scale and cluding use of manned submersibles, have is useful in determining the linearity of fea- focused on rift valley 2 (Fig. IB) (Reid and The structural studies are based on in- tures. We (Luyendyk and Macdonald, Macdonald, 1973; Needham and Fran- terpretation of three types of deep-tow 1977) discuss photographic data, and cheteau, 1974; Macdonald and others, sonar records: high-frequency (40 kHz) (Macdonald, 1977) discusses near-bottom 1975; Laughton and Rusby, 1975; Bel- narrow-beam sonar for depth, low- magnetic data. laiche and others, 1974; Moore and others, frequency (4.0 kHz) sonar for sediment Within the outer walls of the median val- 1974; ARCYANA, 1975; Ballard and penetration, and left and right side-looking ley the fish was navigated using bottom- others, 1975; Ballard and van Andel, 1977; sonar (110 kHz) (Spiess and Tyce, 1973). moored acoustic transponders (Spiess and Bryan and Moore, 1977; Luyendyk and When the fish is towed at normal heights Tyce, 1973). Relative accuracy is 10 to 50 Macdonald, 1977; Macdonald, 1977). Rift above bottom (50 to 150 m), the 40-kHz m. Outside the transponder net the fish was valley 3 has also been studied but in some- sonar can track slopes greater than 70°, as tracked using satellite navigation and a what less detail (Laughton and Rusby, verified by submersible observations of the cable trajectory program by Ivers and 1975; Macdonald, 1977; Ramberg and van same scarps (FAMOUS dive team, 1975, Mudie (1973), for which accuracy is 500 to Andel, 1977; J. D. Phillips and H. S. Flem- personal commun.). The 4.0-kHz sonar re- 1,000 m. Location relative to latitude and ing, in prep.). The deep-tow studies concen- liably penetrates and detects sediment longitude was determined by comparing trating on rift valley 2 and the rift moun- thicknesses from 5 to 100 m. The side- some 40 satellite fixes with transponder- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/621/3418363/i0016-7606-88-5-621.pdf by guest on 23 September 2021 STRUCTURE OF MID-ATLANTIC RIDGE CREST 623 Inner Walls The inner floor is bounded by the inner walls (Figs.