Abstract
A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25′S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/106 years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts.
Comparison of the EPR at 3°25′S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.
Similar content being viewed by others
References
Anderson, R. N. and Noltimier, H. C.: 1973, ‘A Model for the Horst and Graben Structure of Midocean Ridge Crests Based upon Spreading Velocity and Basalt Delivery to the Oceanic Crust’, Roy, Astron. Soc. Geophys. J. 34, 137–147.
Atwater, T. M. and Mudie, J. D.: 1973, ‘Detailed Near-Bottom Geophysical Survey of the Gorda Rise’, J. Geophys. Res. 78, 8665–8686.
Ballard, R. D. and van Andel, Tj. H.: 1977, ‘Morphology and Tectonics of the Inner Rift Valley at Lat. 37°50′N on the Mid-Atlantic Ridge’, Geol. Soc. Am. Bull. 88, 507–530.
Batiza, R., Rosendahl, B. R., and Fisher, R. L.: 1977, ‘Evolution of Oceanic Crust. 3. Petrology and Chemistry of Basalts from the East Pacific Rise and the Siqueiros Transform Fault’, J. Geophys. Res. 82, 265–276.
Bellaiche, G. and Cheminee, J.: 1976, ‘Les Formes Volcaniques Observées Pendant l'Operation FAMOUS dans le Plancher Interné du Rift Medio-Atlantique par 37°50′N’, Compt. Rend. Acad. Sci. Paris 282D 519–522.
Blakely, R. S. and Lynn, W. S.: 1977, ‘Reversal Transition Widths and Fast-Spreading Centers’, Earth and Planetary Sci. Letters 33, 321–330.
Bonatti, E.: 1967, ‘Mechanisms of Deep-Sea Volcanism in the South Pacific’, in P.Abelson (ed.), Researches in Geochemistry, 2, John Wiley & Sons, Inc., New York, 453–491.
Bonatti, E.: 1968, ‘Fissure Basalts and Ocean-Floor Spreading on the East Pacific Rise’, Science 161, 886–888.
Cann, J. R.: 1974, ‘A Model for Oceanic Crustal Structure Developed’, Geophys. J. R. Astron. Soc. 39, 169–187.
Francheteau, J., Hekinian, R., Le Pichon, X., Needham, D., Choukroune, P., Tapponnier, P., Bellaiche, G., and Cheminee, J.: 1975, ‘Transform Fault and Rift Valley Geology by Bathyscaph and Diving Saucer’, Science 190, 108–116.
Francis, T. J. G.: 1974, ‘A New Interpretation of the 1968 Fernandina Caldera Collapse and Its Implications for the Mid-Oceanic Ridges’, Roy. Astron. Soc. Geophys. J. 39, 301–318.
Handschumacher, D. W.: 1976, ‘Post-Eocene Plate Tectonics of the Eastern Pacific’, Ch. 16 of Sutton, G. H., Manghnani, M. N., and Moberly, R. (ed.), The Geophysics of the Pacific Ocean Basin and its Origin, Geophysical Mono.19, American Geophysical Union, Washington, D.C. 177–202.
Harrison, C. G. A.: 1976, ‘Magnetization of the Oceanic Crust’, Roy. Astron. Soc. Geophys. J. 47, 257–283.
Heiskanen, W. A. and Vening-Meinesz, F. A.: 1958, The Earth and its Gravity Field, McGraw-Hill, New York, 470 pp.
Hey, R., Johnson, G. L., and Lowrie, A.: 1977, ‘Recent Tectonic Evolution of the Galapagos Area and Plate Motions in the East Pacific’, Geol. Soc. Am. Bull. (in press).
Jackson, D. B., Swanson, D. A., Koyanagi, R. Y., and Wright, T. L.: 1975, ‘The August and October 1968 Flank Eruption of Kilauea Volcano, Hawaii’, U.S. Geol. Surv. Prof. Paper 890, 33 pp.
Johnson, D. A.: 1972, ‘Ocean-Floor Erosion in the Equatorial Pacific’, Geol. Soc. Am. Bull. 83, 3121–3144.
Klitgord, K. D. and Mudie, J. D.: 1974, ‘The Galapagos Spreading Centre: A Near-Bottom Geophysical Survey’, Roy. Astron. Soc. Geophys. J. 38, 563–586.
Klitgord, K. D., Heustis, S. P., Parker, R. L., and Mudie, J. D.: 1975, ‘An Analysis of Near-Bottom Magnetic Anomalies: Sea-Floor Spreading, the Magnetized Layer, and the Geomagnetic Time Scale’, Roy. Astron. Soc. Geophys. J. 43, 387–424.
Kusznir, N. J. and Bott, M. H. P.: 1976, ‘A Thermal Study of the Formation of Oceanic Crust’, Roy. Astron. Soc. Geophys. J. 47, 83–95.
Lachenbruch, A. H.: 1973, ‘A Simple Mechanical Model for Oceanic Spreading Centers’, J. Geophys. Res. 78, 3395–3417.
Larson, R. L.: 1971, ‘Near-Bottom Geophysical Studies of the East Pacific Rise Crest’, Geol. Soc. Am. Bull. 82, 823–842.
Lonsdale, P. F.: 1977a, ‘Regional Shape and Tectonics of the Equatorial East Pacific Rise’, this issue, p. 295.
Lonsdale, P. F.: 1977b, ‘Abyssal Pahoehoe with Lava Coils at the Galapagos Rift’, Geology 5, 147–152.
Lonsdale, P. F.: 1977c ‘Clustering of Suspension-Feeding Macrobenthos near Abyssal Hydrothermal Vents at Oceanic Spreading Centers’, Deep-Sea Res. (in press).
Luyendyk, B. P. and Macdonald, K. C.: 1976, ‘Spreading Center Terms and Concepts’, Geology 4, Valley Inner Floor Observed with a Deeply Towed Instrument Package’, Geol. Soc. Am. Bull. 88, 648–663.
McBirney, A. R.: 1971, ‘Oceanic Volcanism: A Review’, Rev. Geophys. and Space Physics 9, 523–556.
Macdonald, G. A.: 1956, ‘The Structure of Hawaiian Volcanoes’, Koninkl. Ned. Geol. Mijnbouwk. Genoot. Verhandel. 16, 274–295.
Macdonald, K. C.: 1977, ‘Near-Bottom Magnetic Anomalies, Asymmetric Spreading, Oblique Spreading, and Tectonics of the Accreting Plate Boundary on the Mid-Atlantic Ridge near 37°N’, Geol. Soc. Am. Bull. 88, 541–555.
Macdonald, K. C. and Luyendyk, B. P.: 1977, ‘Deep-Tow Studies of the Structure of the Mid-Atlantic Ridge Crest near Lat. 37°N’, Geol. Soc. Am. Bull. 88, 621–636.
Malahoff, A. and McCoy, F.: 1967, ‘The Geologic Structure of the Puna Submarine Ridge, Hawaii’, J. Geophys. Res. 72, 541–548.
Mammerickx, J., Anderson, R. N., Menard, H. W., and Smith, S. M.: 1975, ‘Morphology and Tectonic Evolution of the East Central Pacific’, Geol. Soc. Am. Bull. 86, 111–118.
Menard, H. W.: 1960, ‘The East Pacific Rise’, Science 132, 1737–1746.
Menard, H. W. and Mammerickx, J.: 1967, ‘Abyssal Hills, Magnetic Anomalies, and the East Pacific Rise’, Earth and Planetary Sci. Letters 2, 465–472.
Minster, J. B., Jordan, T. H., Molnar, P., and Haines, E.: 1974, ‘Numerical Modelling of Instantaneous Plate Tectonics’, Roy. Astron. Soc. Geophys. J. 36, 541–576.
Mohr, P. A.: 1962, ‘The Ethiopian Rift System’, Geophys. Obs. Addis Ababa Bull. 3, 33–62.
Moore, J. G. and Krivoy, H. L.: 1964, ‘The 1962 Flank Eruption of Kilauea Volcano and Structure of the East Rift Zone’, J. Geophys. Res. 69, 2033–2045.
Moore, J. G. and Koyanagi, R. Y.: 1969, ‘The October 1963 Eruption of Kilauea Volcano, Hawaii’, U.S. Geol. Surv. Prof. Paper 614C, 13 pp.
Moores, E. M. and Jackson, D. B.: 1974, ‘Ophiolites and Ocean Crust’, Nature 250, 136–138.
Mudie, J. D., Grow, J. A., and Bessey, J. S.: 1972, ‘A Near-Bottom Survey of Lineated Abyssal Hills in the Equatorial Pacific’, Marine Geophys. Res., 1, 397–411.
Needham, H. D., Choukroune, P., Cheminee, J. L., Le Pichon, X., Francheteau, J., and Tapponnier, P.: 1976, ‘The Accreting Plate Boundary: Ardoukoba Rift (Northeast Africa) and the Oceanic Rift Valley’, Earth and Planetary Sci. Letters 28, 439–453.
Normark, W. R.: 1976, ‘Delineation of the Main Extrusion Zone of the East Pacific Rise at 21°N’, Geology 4, 681–685.
Rea, D. K.: 1975, ‘Model for the Formation of Topographic Features of the East Pacific Rise Crest’, Geology 3, 77–80.
Rea, D. K.: 1976a, ‘Changes in the Axial Configuration of the East Pacific Rise near 6°S during the Past 2 m.y.’, J. Geophys. Res. 81, 1495–1504.
Rea, D. K.: 1976b, ‘Analysis of a Fast-Spreading Rise Crest: The East Pacific Rise, 9° to 12° South’, Marine Geophys. Res. 2, 291–313.
Richter, D. H., Ault, W. U., Eaton, J. P., and Moore, J. G.: 1964, ‘The 1961 Eruption of Kilauea Volcano, Hawaii’, U.S. Geol. Surv. Prof. Paper 474D, 34 pp.
Rosendahl, B. R.: 1976, ‘Evolution of Oceanic Crust. 2. Constraints, Implications, and Inferences’, J. Geophys. Res. 81, 5305–5314.
Rosendahl, B. R., Raitt, R. W., Dorman, L. M., Bibee, L. D., Hussong, D. M., and Sutton, G. H.: 1976, ‘Evolution of Oceanic Crust. 1. A Physical Model of the East Pacific Rise Crest Derived from Seismic Refraction Data’, J. Geophys. Res. 81, 5294–5304.
Salisbury, M. H. and Christensen, N. I.: 1976, ‘Sonic Velocities and Densities of Basalts from the Nazca Plate, DSDP Leg 34’, Chap. 45, of Yeats, R. S., Hart, S. R. et al., Initial Reports of the Deep Sea Drilling Project 34, Washington, 543–546.
Sleep, N. H.: 1975, ‘Formation of Oceanic Crust: Some Thermal Constraints’, J. Geophys. Res. 80, 4037–4042.
Spiess, F. N. and Mudie, J. D.: 1970, ‘Small-Scale Topographic and Magnetic Features’, in A. E.Maxwell (ed.), The Sea 4, Wiley, New York, N.Y., 205–250.
Spiess, F. N. and Tyce, R. C.: 1973, ‘Deep-Tow Instrumentation System’, Scripps Inst. of Oceanog. Reference 73-4, 37 pp.
Stearns, H. T.: 1966, Geology of the State of Hawaii, Pacific Books, Palo Alto, Calif. 266 pp.
Swanson, D. A., Jackson, D. B., Koyanagi, R. Y., and Wright, T. L.: 1976, ‘The February 1969 East Rift Eruption of Kilauea Volcano, Hawaii’, U.S. Geol. Surv. Prof. Paper 89, 30 pp.
Thorarinsson, S.: 1969, ‘The Lakigigar Eruption of 1783, 1969’, Bull. Volcanologique, Ser. 2, 33, 910–927.
van Andel, Tj. H. and Bowin, C. O.: 1968, ‘Mid-Atlantic Ridge Between 22° and 23° North Latitude and the Tectonics of Mid-Oceanic Rises’, J. Geophys. Res. 73, 1279–1298.
van Andel, Tj. H. and Komar, P. D.: 1969, ‘Ponded Sediments of the Mid-Atlantic Ridge between 22° and 23° North Latitude’, Geol. Soc. Am. Bull. 80, 1163–1190.
van Andel, Tj. H., Heath, G. R., and Moore, T. C.: 1975, ‘Cenozoic History and Paleooceanography of the Central Equatorial Pacific Ocean’, Geol. Soc. Am. Mem. 143 134 pp.
Vine, F. J.: 1968, ‘Magnetic Anomalies Associated with Mid-Ocean Ridges’, R. A.Phinney (ed.), The History of the Earth's Crust: A Symposium, Princeton Univ. Press, Princeton, New Jersey, 73–89.
Walker, G. P.: 1973, ‘Lengths of Lava Flows’, Phil. Trans. Roy. Soc. London 274A, 107–118.
Walker, G. P. L.: 1974, ‘Eruptive Mechanisms in Iceland’, Kristjansson, L. (ed.), Geodynamics of Iceland and the North Atlantic Area, D. Reidel Publ. Co., Dordrecht, Holland, 189–202.
Weiss, R. F., Lonsdale, P. F., Lupton, J. E., Bainbridge, A. E., and Craig, H.: 1977, ‘Hydrothermal Plumes in the Galapagos Rift’, Nature (in press).
Whitmarsh, R. B. and Laughton, A. S.: 1976, ‘A Long-Range Sonar Study of the Mid-Atlantic Ridge Crest near 38°N (FAMOUS Area) and its Tectonic Implications’, Deep-Sea Res. 23, 1005–1023.
Wright, T. L. and Fiske, R. S.: 1971, ‘Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii’, J. Petrology 12, 1–65.
Author information
Authors and Affiliations
Additional information
Contribution of the Scripps Institution of Oceanography, new series.
Rights and permissions
About this article
Cite this article
Lonsdale, P. Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S. Marine Geophysical Researches 3, 251–293 (1977). https://doi.org/10.1007/BF00285656
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00285656