Abstract
The Wilkes fracture zone offsets the East Pacific Rise about 200 km right-laterally near 9°S. The bathymetric expression of the fracture zone ranges from a simple slope or step along its inactive extension to a 100 km wide zone of oblique structural features in the active portion. A low ridge 200 to 300 m high, 5 to 15 km wide and 185 km long is the dominant oblique structure; it trends 23° north of the main transform trend. A high-amplitude magnetic anomaly trends 097° along the southern part of the active portion and apparently marks the main transform direction. The structurally simple, inactive portions of the Wilkes fracture zone trend 105°. Plots of epicenter locations reveal two groupings of earthquakes, one along an 082° trend in the central part of the fracture zone, and a cluster near the southwestern fracture zone — spreading center intersection.
Taken together the data suggest that some event, other than a shift in the Nazca-Pacific pole of rotation, occurred 0.9 m.y. ago to change the Wilkes fracture zone from a simple fault to a complex zone of shearing. Since that time the long oblique ridge, probably the surface expression of a Riedel shear, was formed. At present the entire 200 km long, 100 km wide region between the offset axes is seismically active, but transform motion may be largely confined to the southern margin of the active zone, coincident with the high-amplitude magnetic anomaly there.
Similar content being viewed by others
References
Choukroune, P., Francheteau, J., and LePichon, X.: 1978, In Situ ‘Structural Observations Along Transform Fault A in the FAMOUS Area, Mid-Atlantic Ridge’, Geol. Soc. Am. Bull. 89, 1013–1029
Cochran, J. R.: 1973, ‘Gravity and Magnetic Investigations in the Guiana Basin, Western Equatorial Atlantic’, Geol. Soc. Am. Bull. 84, 3249–3268.
Crane, K.: 1976, ‘The Intersection of the Siqueros Transform Fault and the East Pacific Rise’, Mar. Geol. 21, 25–46.
Cyamex: 1978. ‘First Submersible Study of the East Pacific Rise: RITA (Rivera-Tamayo) Project 21°N, Trans. Amer. Geophys. Union 59, 1198.
Detrick, R. S., Mudie, J. D., Luyendyk, B. P., and Macdonald, K. C.: 1973, ‘Near-bottom Observations of an Active Transform Fault (Mid-Atlantic Ridge at 37°N)’, Nature Phys. Sci. 246, 59–60.
Eittreim, S. and Ewing, J.: 1975, ‘Vema Fracture Zone Transform Fault’, Geology 5, 555–558.
Fox, P. J., Schreiber, E., Rowlett, H., and McCamy, K.: 1976, ‘The Geology of the Oceanographer Fracture Zone: A Model for Fracture Zones’, J. Geophys. Res. 81, 4117–4128.
Gumma, W. J.: 1974, An Interpretation of the Gravity and Magnetic Anomalies of the Rivera Fracture Zone, East Pacific Ocean, Unpub. M. S. Thesis, Oregon State Univ., Corvallis, 50 p.
Herron, E. M.: 1972. ‘Sea-floor Spreading and the Cenozoic History of the East-Central Pacific’, Geol. Soc. Am. Bull. 83, 1671–1692.
I.A.G.A. Commission 2 Working Group 4, Analysis of the Geomagnetic Field: 1969, ‘International Geomagnetic Reference Field 1965.0’, J. Geophys. Res. 74, 4407–4408.
LePichon, X. and Hayes, D. E.: 1971, ‘Marginal Offsets, Fracture Zones, and the Early Opening of the South Atlantic’, J. Geophys. Res. 76, 6283–6293.
Lonsdale, P.: 1977, ‘Structural Geomorphology of a Fast-Spreading Rise Crest: The East Pacific Rise Near 3°25′S’, Mar. Geophys. Res. 3, 251–293.
Lonsdale, P.: 1978, ‘Near Bottom Reconnaissance of a Fast-Slipping Transform Fault at the Pacific-Nazca Plate Boundary’, J. Geology 86, 451–472.
Lynn, W. S.: 1976, A Geophysical Analysis of the Orozco Fracture Zone and the Tectonic Evolution of the Northern Cocos Plate, Unpub. M. S. Thesis, Oregon State Univ., Corvallis, 80 p.
Lynn, W. S. and Lewis, B. T. R.: 1976, ‘Tectonic Evolution of the Northern Cocos Plate’, Geology. 4, 718–722.
Macdonald, K. C., Kastens, K., Spiess, F. N., and Miller, S. P.: 1979, ‘Deep Tow Studies of the Tamayo Transform Fault’, Mar. Geophys. Res. 4, 37–70.
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.
Mammerickx, J. and Smith, S. M.: 1978, ‘Bathymetry of the Southeast Pacific’, Geol. Soc. Am. Map and Chart Series MC-26.
Mammerickx, J., Herron, E., and Dorman, L.: 1980, ‘Evidence for Two Fossil Spreading Ridges in the Southeast Pacific’ Geol. Soc. Am. Bull. 91, 263–271.
Matthews, D. J.: 1939, Tables of the Velocity of Sound in Pure Water and Sea Water for Use in Echo-Sounding and Sound Ranging (second ed.), Hydrographic Dept., Admiralty, London, 52 p.
Menard, H. W.: 1964, Marine Geology of the Pacific, McGraw-Hill, Inc., New York, 271 p.
Phillips, J. D. and Luyendyk, B. P.: 1970, ‘Central North Atlantic Plate Motions Over The Last 40 Millions Years’, Science 170, 727–729.
Prothero, W. A., Reid, I., Reichle, M. S., and Brune, J. N.: 1976, ‘Ocean Bottom Seismic Measurements on the East Pacific Rise and Rivera Fracture Zone’, Nature 262, 121–124.
Rea, D. K.: 1970, ‘Changes in Structure and Trend of Fracture Zones North of the Hawaiian Ridge and Relation to Sea-Floor Spreading’, J. Geophys. Res. 75, 1421–1430.
Rea, D. K.: 1972, ‘Magnetic Anomalies Along Fracture Zones’, Nature Phys. Sci. 236, 58–59.
Rea, D. K.: 1975, Tectonics of the East Pacific Rise 5° to 12°S, Unpub. Ph. D. Thesis, Oregon State Univ. Corvallis, 139 p.
Rea, D. K.: 1976a, ‘Changes in the Axial Configuration of the East Pacific Rise Near 6°S During the Past Two Million Years’, 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’, Mar. Geophys. Res. 2, 291–313.
Rea, D. K.: 1977, ‘Local Axial Migration and Spreading Rate Variations, East Pacific Rise, 31°S’, Earth and Planet. Sci. Lett. 34, 78–84.
Rea, D. K.: 1978a, ‘Evolution of the East Pacific Rise Between 3°S and 13°S Since the Middle Miocene’, Geophys. Res. Lett. 5, 561–564.
Rea, D. K.: 1978b, ‘Asymmetric Sea-Floor Spreading and a Nontransform Axis Offset: The East Pacific Rise 20°S Survey Area’, Geol. Soc. Am. Bull. 89, 836–844.
Schouten, J. A.: 1971, ‘A Fundamental Analysis of Magnetic Anomalies Over Oceanic Ridges’, Mar. Geophys. Res. 1, 111–144.
Searle, R. C. and Laughton, A. S.: 1977, ‘Sonar Studies of the Mid-Atlantic Ridge and Kurchatov Fracture Zone’, J. Geophys. Res. 82, 5313–5328.
Sleep, N. H. and Biehler, S.: 1970, ‘Topography and Tectonics at the Intersections of Fracture Zones with Central Rifts’, J. Geophys. Res. 75, 2748–2752.
Twight, W., Slootweg, A. P., and Collette, B. J.: 1979, ‘Topography and a Magnetic Analysis of an Area South-East of the Azores (36°N, 23°W), Mar. Geophys. Res. 4, 91–104.
vanAndel, T. H., vonHerzen, R. P., and Phillips, J. D.: 1971, ‘The Vema Fracture Zone and the Tectonics of Tranverse Shear Zones in Oceanic Crustal Plates’, Mar. Geophys. Res. 1, 261–283.
vanAndel, T. H., Rea, D. K., vonHerzen, R. P., and Hoskins, H.: 1973, ‘Ascension Fracture Zone, Ascension Island, and the Mid-Atlantic Ridge’, Geol. Soc. Am. Bull. 84, 1527–1546.
Wilcox, R. E., Harding, T. P., and Seely, D. R.: 1973, ‘Basic Wrench Tectonics’, Am. Assoc. Pet. Geol. Bull. 57, 74–96.
Wilson, J. T.: 1965, ‘A New Class of Faults and Their Bearing On Continental Drift’, Nature 297, 343–347.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Kureth, C.L., Rea, D.K. Large-scale oblique features in an active transform fault, the Wilkes fracture zone near 9°S on the East Pacific Rise. Marine Geophysical Researches 5, 119–137 (1981). https://doi.org/10.1007/BF00163474
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00163474