Ultramafic-mafic plutonic rock suites exposed along the mid-atlantic ridge (10°N-30°N). Symmetrical-asymmetrical distribution and implications for seafloor spreading processes

Geophysical Monograph Series

Volumn 106, Issue , 1998, Pages 153-176

  Lagabrielle, Yves ^a   Bideau, Daniel ^b   Cannat, Mathilde ^c   Karson, Jeffirey A ^d   Mével, Catherine ^c  

^a INSTITUT UNIVERSITAIRE EUROPÉEN DE LA MER   (France)

^b IFREMER   (France)

^c CNRS   (France)

^d DUKE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 11644258896     PISSN: 00658448     EISSN: 23288779     Source Type: Book Series    
DOI: 10.1029/GM106p0153     Document Type: Chapter

References (91)

1. Aubry, M.P., W.A. Berggren, A. Schaaf, J.M. Auzende, Y. Lagabrielle, and V. Mamaloukas-Frangoulis, Paleontological evidence for early exposure of deep oceanic crust along the Vema Fracture Zone Southern wall (Atlantic Ocean, 10°45'N), Marine Geol., 107, 1-7, 1992.
2. Aumento, F„ and H. Loubat, The Mid-Atlantic Ridge near 45°N, XVI, Serpentinized ultramafic intrusions, Can. J. Earth Sci, 8, 631-663, 1971.
3. Auzende, J.M., D. Bideau, E Bonatti, M. Cannat, J. Honnorez, Y. Lagabrielle, J. Malavieille, V. Mamaloukas-Frangoulis, and C. Mével, Direct observation of a section through slow-spreading oceanic crust, Nature, 337, 726-729, 1989.
4. Auzende, J.M., M. Cannat, P. Gente, J.P. Henriet, T. Juteau, J.A. Karson, Y. Lagabrielle, C. Mével, and M. Tivey, Affleurements de roches profondes de la croflte océanique et du manteau sur le mur sud de la zone de fracture Kane (Atlantique central): observations par submersible, C. R Acad. Sc. Paris, 317, Serie II, 1641-1648, 1993.
5. Auzende, J.M., M. Cannat, P. Gente, J.P. Henriet, T. Juteau, J.A. Karson, Y. Lagabrielle, C. Mével, and M. Tivey, Observation of sections of oceanic crust and mantie cropping out on the Southern wall of Kane F.Z. (North Atlantic), Terra Nova, 6, 143-148, 1994.
6. Bideau, D., R. Hékinian, C. Bollinger, M. Constantin, E. Gracia, G. Guivel, B. Sichler, R. Apprioual, and R. Le Gali, Submersible investigation of highly contrasted magmatic activities recorded on two segments of the Mid-Atlantic Ridge near 34°52'N and 33°55'N, InterRidge News, 5, 9-14, 1996.
7. Blackman, D.K., and D.W. Forsyth, Isostatic compensation of tectonic features of the Mid-Atlantic Ridge: 25-27°30'S, J. Geophys. Res., 96, N.B7, 11741-11758, 1991.
8. Bougault, H., J.L. Charlou, Y. Fouquet, H.D. Needham, N. Vaslet, P. Appriou, P. Jean, Baptiste, P.A., Rona, L. Dmitriev, and Silantiev, S., Fast and slow spreading ridges: structure and hydrothermal activity, ultramafic topographic highs and CH4 output, J. Geophys. Res., 98, 9643-9651,1993.
9. Bonatti, E., J. Honnorez, and G. Ferrara, Peridotite-gabbro-basalt complex from the equatorial Mid-Atlantic Ridge, Phil. Trans. R. Soc. London, 268, 385-402, 1971.
10. Bonatti, E., Serpentinite intrusions in the oceanic crust, Earth Planet. Sci. Lett., 32, 107-113, 1976.
11. Bonatti, E., and J. Honnorez, Sections of the Earth’s crust in the Equatorial Atlantic, J. Geophys. Res., 81, B23, 4104-4116, 1976.
12. Bonatti, E., and P. Hamlyn, Oceanic ultramafic rocks, in The Sea (Vol. 7), The Oceanic Lithosphere, edited by C. Emiliani, pp. 241-283, New York (Wiley)., 1981.
13. Bonatti, E., A. Peyve, P. Kepezhinskas, N. Kurentsova, M. Seyler, S. Skolotnev, and G. Udintsev, Upper mantie heterogeneity below the Mid-Atlantic Ridge, 0°-15°N, J. Geophys. Res., Vol. 97, N.B4, 4461-4476, 1992.
14. Brown, J.R., and J.A. Karson, Variations in axial processes on the MAR: the median valley of the MARK area, Mar. Geophys. Res., 10, 109-138, 1988.
15. Cannat, M., Emplacement of mantie rocks in the seafloor at Mid-Ocean ridge, J. Geophys. Res., 98, B3, 4163-4172, 1993.
16. Cannat, M., How thick is the magmatic crust at slow-spreading ocean ridges?, J. Geophys. Res., 101, 2847-2857, 1996.
17. Cannat, M., T. Juteau, and E. Berger, Petrostructural analysis of the Leg 109 serpentinized peridotites, in Proceedings of the O.D.P., Sci. Results, 106/109, pp. 47-56., 1990.
18. Cannat, M., D. Bideau, and H. Bougault, Serpentinized peridotites and gabbros in the Mid Atlantic Ridge axial valley at 15°37'N and 16°52'N, Earth Planet. Sci. Lett., 109, 87-106,1992.
19. Cannat, M. and J. Casey, An ultramafic lift at the Mid-Atlantic Ridge: successive stages of magmatism in serpentinized peridotites from the 15°N region, in Mantie and Lower crust exposed in oceanic ridges and in ophiolites, edited by R.L.M. Vissers and A. Nicolas, pp. 5-34, Kluwer Academic Publishers, The Netherlands., 1995.
20. Cannat, M., C. Mével, M. Maia, C. Deplus, C. Durând, P. Gente, P. Agrinier, A. Belarouchi, G. Dubuisson, E. Humler, and J. Reynolds, Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22°-24°N), Geology, 23, 1, 49-52, 1995.
21. Cannat, M., F. Chatin, H. Whitechurch, and G. Ceuleneer, Gabbroic rocks trapped in the upper mantie at the Mid-Atlantic Ridge, in Proceedings of the Ocean Drill. Program, Iniţial Rep, edited by J.A. Karson, M. Cannat, D.J. Miller, and D. Elthon, Vol.153, pp. 243-264, College Station, TX., 1997a.
22. Cannat, M., Y. Lagabrielle, N. de Coutures, H. Bougault, J. Casey, L. Dmitriev, and Y. Fouquet, Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region, Tectonophysics, 279, 193-213, 1997b.
23. Casey, J. F., M. Cannat, and the FARANAUT Scientific Party, Crustal architecture and mechanisms of ultramafic and gabbroic exposure along rift valley walls of the Mid-Atlantic Ridge north and south of the Fifteen Twenty transform, in Am. Geophys. Union, Fall Meeting (abstract), 72, p. 537, EOS 7>ansactions, 1992.
24. Charlou, J.L., H. Bougault, P. Appriou, T. Nelsen, and P. Rona, Different TDM/CH4 hydrothermal plume signature: TAG site at 26°N, and serpentinized ultrabasic diapir at 15°05'N on the Mid-Atlantic Ridge, Geoch. Cosm. Acta, 55, 3209-3222, 1991.
25. Detrick, R.S., P.J. Fox, N. Schulz, R. Pockalny, L. Kong, L. Mayer, and W.B.F. Ryan, Geologic and tectonic setting of the MARK area, in Proceedings. Ocean Drilling Program, Init. Repts. (PartA), 106/109, edited by R. Detrick., J. Honnorez., W.B. Bryan., T. Juteau. et al., pp. 15-22, Ocean Drilling Program, College Station, TX., 1988.
26. Detrick, R.S., H.D. Needham, and V. Renard, Gravity anomalies and crustal thickness variations along the Mid-Atlantic ridge between 33°N and 40°N, J. Geophys. Res., 99, B3, 3767-3787, 1995.
27. Dick, H.J.B., Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism, in Magmatism in the Ocean Basins, edited by A.D. Saunders and M.J. Norris, 42, pp. 71-105, Geol. Soc. London Spec. Publ, 1989.
28. Dick, H.J.B., G. Thompson, and W.B. Bryan, Low-angle faulting and steady-state emplacement of plutonic rocks at ridge-transform intersections, Amer. Geophys. Union Trans., EOS, 62, p. 406, 1981.
29. Dick, H.J.B., R.L. Fisher, and W.B. Bryan, Mineralogic variability of the uppermost mantie along mid-ocean ridges, Earth Plan. Sci. Lett., 69, 88-106, 1984.
30. Dilek, Y., A. Coulton, and S.D. Hurst, Serpentinization and hydrothermal veining in peridotites at Site 920 in the MARK area, in Proceedings of the Ocean Drilling Program, Scientific Results, v. 153, edited by Karson, J.A., Cannat, M., Miller, D.J., and Elthon, D., Ocean Drilling Program, Texas A and M University, College Station, TX., 1997.
31. Dubois, J., and Scientific Party, The Gravinaut cruise: Seafloor gravity and electromagnetism at the MARK-Snake Pit area, InterRidge News, 3, 1, 3-6, 1994a.
32. Dubois, J., V. Ballu, P. Beuzart, S. Bonvalot, C. Deplus, M. Diament, A. Dubreule, C. Durand, M. F. Esnoult, A. Schultz, and P. Tarits, Sea-bottom gravity and electro-magnetic measurements on MARK area, in EGS 19, Grenoble 25-29 april 1994, Vol.P025, p. 49, 1994b.
33. Durand, C., V. Ballu, P. Gente, and J. Dubois, Horst and graben structures on the flanks of the Mid-Atlantic Ridge in the MARK area (23°22'N): submersible observation, Tectonophysics, 265, 275-297, 1996.
34. Escartin, J., and J. Lin, Ridge offsets, normal faulting, and gravity anomalies of slow spreading ridges, J. Geophys. Res., 100, B4, 6163-6177, 1995.
35. Fox, P.J., and B.C. Heezen, Sands of the Mid-Atlantic Ridge, Science, 149, 1367-1370, 1965.
36. Fox, P.J., and D.G. Gallo, A tectonic model for ridge-transform-ridge plate boundaries: implications for the structure of the oceanic lithosphere, Tectonophysics, 104, 205-242, 1984.
37. Fox, P.J., N.R. Grindlay, and K.C. Macdonald, The Mid-Atlantic ridge (31°S-34°30’S): temporal and spaţial variations of accretionary processes, Mar. Geophys. Res., 13, 1-20, 1991.
38. Francis, T.J.G., Serpentinization faults and their role in the tectonics of slow-spreading ridges, J. Geophys. Res., 86, 11616-11622,1981.
39. Fryer, P., A synthesis of Leg 125 drilling of serpentinite seamounts on the Mariana and Izu-Bonin forearcs, in Proceedings. Ocean Drill. Program, Iniţial Rep, edited by P. Fryer, J.A. Pearce, L.B. Stokking, et al., pp. 593-614, College Station, TX., 1992.
40. Gente, P., C. Mével, J.M. Auzende, J.A. Karson, and Y. Fouquet, An example of a recent accretion on the Mid-Atlantic Ridge: The Snake Pit neovolcanic ridge (MARK Area, 23°22’N), Tectonophysics, 190, 1-29, 1991.
41. Gente, P., C. Durand, R. Pockalny, M. Maia, C. Deplus, C. Mével, G. Ceuleneer, M. Cannat, C. Laverne, D. Aslanian, A. Campan, H. Leau, E. Marion, and M. Seyler, Structures obliques sur les flancs de la Dorsale Médio-Atlantique: traces fossiles de la propagation le long de ľaxe de segments ďaccrétion, C. R. Acad. Sc. Paris, 318, 2, 1239-1246, 1994.
42. Gente, P., R.A. Pockalny, C. Durand, M. Maia, C. Deplus, G. Ceuleneer, C. Mével, M. Cannat, and C. Laverne, Characteristics and evolution of the segmentation of the Mid-Atlantic ridge between 20°N and 24°N during the last 10 million years, Earth Planet. Sci. Lett., 129, 55-71, 1995.
43. Gente, P., G. Ceulenner, O. Dauteuil, J. Dyment, C. Honsho, A. Laverne, C. Le Turdu, M.C. Mitchell, M. Ravilly, and R. Thibaud, On-and off-axis submersible investigations on an highly magmatic segment of the Mid-Atlantic Ridge (21°40'N): the TAMMAR Cruise, InterRidge News, 5, 2, 27-31,1996.
44. Gracia, E., D. Bideau, R. Hekinian, Y. Lagabrielle, and L.M. Parson, Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33°43'N to 34°07'N). Geology, 25, 12, 1059-1062.
45. Grindlay, N.R., P.J. Fox, and K.C. Macdonald, Second order ridge axis discontinuities in the South Atlantic: morphology, structure and evolution, Mar. Geophys. Res., 13,21-49, 1991.
46. Hess, H.H., History of ocean basins, in Petrologie Studies, Burlington Volume, A.E.J., 1962.
47. Juteau, T., E. Berger, and M. Cannat, Serpentinized, residual mantie peridotites from the MAR median valley, ODP hole 670a (21°10’N, 45°02'W, leg 109): primarily mineralogy and geothermometry, in Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 106/109, pp. 27-45, 1990a.
48. Juteau, T., M. Cannat, and Y. Lagabrielle, Serpentinized peridotites in the upper oceanic crust away from transform zones: a comparison of the results of previous DSDP and ODP legs, in Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 106/109, pp. 303-308, 1990b.
49. Karson, J.A., Seafloor spreading on the Mid-Atlantic Ridge: implications for the structure of ophiolites and oceanic lithosphere produced in slow-spreading environments, in Proceedings of the Symposium "Troodos 1987", edited by J. Malpas, E.M. Moores, A. Panayiotou. et C. Xenophontos, pp. 547-555, Geological Survey Department, Nicosia, Cyprus., 1990.
50. Karson, J.A., and H.J.B. Diek, Tectonics of ridge-transform intersections at the Kane fracture zone, Mar. Geophys. Res., 6, 51-98, 1983.
51. Karson, J.A., Faulting and Magmatism at Mid-Ocean Ridges: A Perspective from Tectonic Windows in the Oceanic Lithosphere, in Proceedings of the 4th RIDGE Theoretical Institute, edited by W.R. Buck, P.A. Delaney and J.A. Karson, American Geophysical Union, this volume.
52. Karson, J.A., G. Thompson, S.E. Humpris, J.M. Edmond, W.B. Bryan, J.R. Brown, A.T. Winters, R.A. Pockalny, J.F. Casey, C. Campbell, G. Klinkhammer, M.R. Palmer, R.J. Kinzler, and M. Sulanowska, Along-axis variations in seafloor spreading in the MARK area, Nature, 328, 681-685,1987.
53. Karson, J.A., and J.B. Brown, Geologic setting of the Snake pit hydrothermal site: an active vent field on the Mid-Atlantic ridge, Mar. Geophys. Res., 10, 91-107, 1988.
54. Karson, J.A., and P.A. Rona, Block-tilting, transfer faults, and structural control of magmatic and hydrothermal processes in the TAG area, Mid-Atlantic ridge 26°N, Geol. Soc. Am. Bull., 102, 1635-1645, 1990.
55. Karson, J.A., and A.T. Winters, Along-axis variations in tectonic extension and accomodation zones in the MARK area, Mid-Atlantic ridge 23°N latitude, in Ophiolites and their modem oceanic analogues, edited by L.M. Parson, B. J. Murton, and P. Browning, pp. 107-116, Geol. Soc. Special Publ, 60, 1992.
56. Karson, J.A., and R.M. Lawrence, Tectonic setting of serpentinite exposures on the western median valley wall of the MARK area in the vicinity of ODP Site 920, in Proceedings of the Ocean Drill. Program, Iniţial Rep, edited by J.A. Karson, M. Cannat, D.J. Miller, and D. Elthon, Vol.153, pp. 5-21, College Station, TX., 1997a.
57. Karson, J.A., and R.M. Lawrence, Tectonic window into gabbroic rocks of the middle oceanic crust in the MARK area near ODP Sites 921-924, in Proceedings of the Ocean Drill. Program, Iniţial Rep, edited by J.A. Karson, M. Cannat, D.J. Miller, and D. Elthon, Vol.153, pp. 61-76, College Station, TX., 1997b.
58. Karson, J.A., M. Cannat, D.J. Miller, and D. Elthon, Proceedings of the Ocean Drill. Program, Iniţial Rep, Voi. 153, College Station, TX., 1997.
59. Kong, L.S., R.S. Detrick, P.J. Fox, L.A. Mayer, and W.B.F. Ryan, The morphology and tectonics of the MARK area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23°N), Mar. Geophys. Res., 10, 59-90, 1988.
60. Kuo, B.Y., and D.W. Forsyth, Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5°S: upwelling centers and variations in the crustal thickness, Mar. Geophys. Res, 10, 205-232, 1988.
61. Lagabrielle, Y., and M. Cannat, Alpine jurassic ophiolites resemble the modem central Atlantic basement, Geology, 18, 319-322, 1990.
62. Lagabrielle, Y., and M. Lemoine, Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model, C. R. Acad. Sci, 325, 909-920, 1997.
63. Lawrence, R.M., J.A. Karson, and S.D. Hurst, Dike orientations and fault-block rotations in slow-spread oceanic crust at the SMARK Area, Mid-Atlantic Ridge at 22°40'N, J. Geophys. Res, in press.
64. Lin, J., G.M. Purdy, H. Shouten, J.-C. Semperé, and C. Zervas, Evidence from gravity data for focused magmatic accretion along the Mid-Atlantic Ridge, Nature, 344, 627-632, 1990.
65. Lin, J., and J. Phipps Morgan, The spreading rate dependence of three-dimensional Mid-Ocean ridge gravity structure, Geophys. Res. Lett., 19, N. l, 13-16, 1992.
66. Lombardo, B., and U. Pognante, Tectonic implications in the evolution of the western Alps ophiolite metagabbro. Ofioliti, 7, 2/3, 371-394, 1982.
67. Mével, C., J.M. Auzende, M. Cannat, J.P. Donval, J. Dubois, Y. Fouquet, P. Gente, D. Grimaud, J.A. Karson, M. Segonzac, and M. Stievenard, La ride du Snake Pit (dorsale médio-Atlantique, 23°22'N): résultats préliminaires de la champagne Hydrosnake, C. R. Acad. Sc. Paris, 308, 2, 545-552, 1989.
68. Mével, C., M. Cannat, P. Gente, E. Marion, J.M. Auzende, and J.A. Karson, Emplacement of deep crustal and mantie rocks on the west median valley wall of the MARK area (MAR, 23°N), Tectonophysics, 190, 31-53, 1991.
69. Michael, P.J., and E. Bonatti, Peridotite composition from the North Atlantic: regional and tectonic variations and implications for parţial melting, Earth Plan. Sci. Lett., 73, 91-104, 1985.
70. Morris, E., and R.S. Detrick, Three-dimensional analysis of gravity anomalies in the MARK area, Mid Atlantic Ridge, 23°N, J. Geophys. Res., 96, 4355-4366, 1991.
71. Mutter, J.C., and J.A. Karson, Structural processes at slow-spreading ridges, Science, 257, 627-634, 1992.
72. Neuman, G.A., and D.W. Forsyth, High resolution statistical estimation of seafloor morphology: Oblique and orthogonal fabric on the flanks of the Mid-Atlantic Ridge, 34°-35.5°S, Marine Geophys. Res., 17, 221-250, 1995.
73. Niu, Y., and R. Batiza, Magmatic processes at a slow spreading ridge segment: 26°S Mid-Atlantic Ridge, J. Geophys. Res., Vol.99, B10, 19719-19740, 1994.
74. OTTER Group: Karson, J.A., P.J. Fox, H. Sloan, K.T. Crane, W.S.F. Kidd, E. Bonatti, J.B. Stroup, D.J. Fornari, D. Elthon, P. Hamlyn, J.F. Casey, D.G. Gallo, D. Needham, and R. Sartori, The geology of the Oceanographer transform: the ridge-transform intersection, Mar. Geophys. Res., 6, 109-141, 1984.
75. Pockalny, R.A., R.S. Detrick, and P.J. Fox, Morphology and tectonics of the Kane transform from Sea Beam bathymetry data, J. Geophys. Res., 93, B4, 3179-3193, 1988.
76. Ranero, C. R., E. Banda, and P. Buhl, The crustal structure of the Canary Basin: accretion processes at slow spreading centers, J. Geophys. Res., 97, B5, 10185-10201, 1997.
77. Rommevaux, C., C. Deplus, P. Patriat, and J.C. Sempéré, Three-dimensional gravity study of the Mid-Atlantic ridge: evolution of the segmentation between 28° and 29°N during the last 10 My, J. Geophys. Res., 99, B2, 3015-3029, 1994.
78. Rona, P.A., L. Widenfalk, and K. Bostrom, Serpentinized ultramafics and hydrothermal activity at the Mid-Atlantic ridge crest near 15°N, J. Geophys. Res., 92, B2, 1417-1427, 1987.
79. Sempéré, J.C., P. Blondei, A. Briais, T. Fujiwara, L. Géli, N. Isezaki, J.E. Pariso, L. Parson, P. Patriat, and C. Rommevaux, The Mid-Atlantic ridge between 29°N and 31°30'N in the last 10 Ma, Earth Planet. Sci. Lett., 130, 45-55, 1995.
80. Severinghaus, J. P., and K. C. Macdonald, High inside comers at ridge-transform intersections, Mar. Geophys. Res., 9, 353-367,1988.
81. Shaw, P.R., and J. Lin, Causes and consequences of variations in faulting style at the Mid-Atlantic ridge, J. Geophys. Res., 98, B12, 21839-21851, 1993.
82. Shaw, P. R., and J. Lin, Models of ocean ridge lithospheric deformation: Dependence on crustal thickness, spreading rate, and segmentation, J. Geophys. Res., 101, B18, 17977-17993,1996.
83. Shemenda, A.I., and A. L. Grocholsky, Physical modelling of slow seafloor spreading, J. Geophys. Res., 99, B5, 9137-9153, 1994.
84. Shipboard Scientific Party Site 670, in Proceedings of the Ocean Drilling Program, Iniţial Rep., edited by W.B. Bryan, T. Juteau, et al., Vol. 109, pp. 203-240, College Station, Texas., 1988.
85. Shipboard scientific Party Sites 920-924, in Proceedings of the Ocean Drilling Program, Iniţial Rep, edited by J.A. Karson, M. Cannat, D.J. Miller, and D. Elthon, Vol. 153, College Station, TX., 1995.
86. Skolotnev, S. G., N. V. Tsukanov, A.A. Peyve Raznitisin and A. Yu, 15°20'N fracture zone bedrocks: General characteristics, in Structure of the Cape Verde fracture zone, Central Atlantic, edited by Y. M. Pushcharovsky, 439, pp. 40-60, Academy of Sciences of the USSR, Order of the Red Banner of Labour, Geological Institute Transactions., 1989.
87. Sloan, H., and P. Patriat, Kinematics of the north American-African plate boundary between 28° and 29°N during the last 10 Ma: evolution of the axial geometry and spreading rate and direction, Earth Planet. Sci. Lett., 113, 323-341, 1992.
88. Tiezzi, L.J., and R.B. Scott, Crystal fractionation in a cumulate gabbro, Mid-Atlantic Ridge, 26°N, J. Geophys. Res., 85, 5438-5454, 1980.
89. Tolstoy M., A.J. Harding, and J.A. Orcutt, Crustal thickness on the Mid-Atlantic Ridge: bull's eye gravity anomalies and focused accretion, Science, Vol.262, 726-729, 1993.
90. Tucholke, B.E., and J. Lin, A geological model for the structure of ridge segments in slow-spreading ocean crust, J. Geophys. Res., 99, 11937-11958, 1994.
91. Wolfe, C.J., G.M. Purdy, D.R. Toomey and S.C. Solomon, Microearthquake characteristics and crustal velocity structure at 29°N on the Mid-Atlantic Ridge: the architecture of a slow-spreading segment, J. Geophys. Res., 99, 24449-24472, 1995.