Abstract
The axis of the East Pacific Rise is defined by a topographic block about 15 km wide and 300 to 350 m high which is flanked by abyssal hills 100 to 200 m high and 3 to 5 km wide. These hills often are tilted such that their steep slopes face the axis. An empirical model explaining these features combines axial extrusion to form the central block and rotational faulting to lower the shoulders of the axial block to the regional depth and tilt them outward.
The axial block is offset about 10 km left-laterally at 10.0°S and a similar amount right-laterally at 11.5°S. Offsets (or lack of offsets) of young magnetic anomalies indicate that these axial displacements occurred between 1.7 and 0.9 m.y. ago and 0.7 m.y. ago and the present in the north and south. respectively. These small axial offsets are interpreted to be the result of either brief episodes of asymmetric see-floor spreading or discrete jumps in the site of spreading activity. Both axial shifts were to the west; a unidirectional sequence of such shifts occurring at the above rate of one per million years would be difficult to differentiate from true regional asymmetric spreading and might explain that phenomenon on other medium-to fast-spreading rises.
Reconnaissance data from the east flank of the East Pacific Rise indicate that spreading activity began on that part of the rise between the 9°S and 13.5°S fracture zones approximately 8.2 m.y. ago when the site of crustal accretion jumped westward from the now dormant Galapagos Rise. Slope change in crust approximately 2 and 6 m.y. old imply faster spreading rates between about 6 and 2 m.y. ago than either before or after that time. Identification and correlation of anomaly 3′ allows an estimate of about 90 mm/y for this higher east flank spreading rate. Since 1.7 m.y. ago spreading rates have averaged about 80 mm/y to the west and 77 mm/y to the east.
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References
AndersonR. N. and NoltimierH. C.: 1973, ‘A Model for the Horst and Graben Structure of Mid-ocean Ridge Crests Based upon Spreading Velocity and Basalt Delivery to the Oceanic Crust’, Roy. Astron. Soc. Geophys. J. 34, 137–147.
AndersonR. N. and SclaterJ. G.: 1972, ‘Topography and Evolution of the East Pacific Rise Between 5° and 20°S’, Earth and Planetary Sci. Letters 14, 433–441.
AtwaterT. and MudieJ. D.: 1973, ‘Detailed Near-Bottom Geophysical Study of the Gorda Rise’, J. Geophys. Res. 78, 8665–8686.
BlakelyR. J.: 1975, ‘Evidence of Local Migration of a Spreading Center’, Geology 3, 35–38.
CoxA.: 1969, ‘Geomagnetic Reversals’, Science 163, 237–245.
DalrympleG. B., CoxA., DoellR. R., and GrommeC. S.: 1967, ‘Pliocene Geomagnetic Polarity Epochs’, Earths and Planetary Sci. Letters 2, 163–173.
DavisE. E. and ListerC. R. B.: 1974, ‘Fundamentals of Ridge Crest Topography’, Earth and Planetary Sci. Letters 21, 405–413.
DobrinM. B.: 1960, Introduction to Geophysical Prospecting, McGraw-Hill, New York, N.Y., 446 p.
FosterJ. H. and OpdykeN. D.: 1970, ‘Upper Miocene to Recent Magnetic Stratigraphy in Deep-sea Sediments’, J. Geophys. Res. 75, 4465–4473.
HafnerW.: 1951, ‘Stress Distributions and Faulting’, Geol. Soc. Am. Bull. 62, 373–398.
Hayes, D. E.: 1976, ‘The Nature and Implications of Asymmetric Sea-floor Spreading’, Geol. Soc. Am. Bull., in press.
HeirtzlerJ. R., DicksonG. O., HerronE. M., PitmanW. C.III, and LePichonX.: 1968, ‘Marine Magnetic Anomalies, Geomagnetic Field Reversals and Motions of the Ocean Floor and Continents’, J. Geophys. Res. 73, 2119–2136.
HerronE. M.: 1971, ‘Crustal Plates and Sea Floor Spreading in the Southeastern Pacific’, Am. Geophys. Union, Antarctic Oceanology I, Antarctic Research Series 15, 229–237.
HerronE. 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.
KlitgordK. D. and MudieJ. D.: 1974, ‘The Galapagos Spreading Centre: a Near-Bottom Geophysical Survey’, Roy. Astron. Soc. Geophys. J. 38, 563–586.
Krause, D. C., Grim, P. J., and Menard, H. W.: 1973, Quantitative Marine Geomorphology of the East Pacific Rise, Nat'l. Oceanic and Atmospheric Admin. Tech. Rept. ERL 275-AOML 10, 73 pp.
LarsonR. L.: 1971, ‘Near-bottom Geologic Studies of the East Pacific Rise Crest’, Geol. Soc. Am. Bull, 82, 823–841.
LuyendykB. P.: 1970, ‘Origin and History of Abyssal Hills in the Northeast Pacific Ocean’, Geol. Soc. Am. Bull. 81, 2237–2260.
MammerickxJ., AndersonR. N., MenardH. W., and SmithS. M.: 1975, ‘Morphology and Tectonic Evolution of the East-central Pacific’, Geol. Soc. Am. Bull. 86, 111–118.
MankinenE. A.: 1972, ‘Paleomagnetism and Potassium-Argon Ages of the Sonoma Volcanics, California’, Geol. Soc. Am. Bull. 83, 2063–2072.
MatthewsD. J.: 1939, Tables of the Velocity of Sound in Pure Water and Sea Water for Use in Echo-sounding and Sound-ranging (2nd ed.), Hydrographic Dept. Admiralty, London, 52 pp.
OpdykeN. D.: 1972, ‘Paleomagnetism of Deep-Sea Cores’, Rev. Geophys. Space Phys. 10, 213–249.
Opdyke, N. D. and Foster, J. H.: 1970, ‘Paleomagnetism of Cores from the North Pacific’, in Hays, J. D. (ed.), Geological Investigations of the North Pacific, Geol. Soc. America Mem. 126, Boulder, Co., pp. 83–119.
ParkerR. L. and OldenburgD. W.: 1973, ‘Thermal Model of Ocean Ridges’, Nature Phys. Sci. 242, 137–139.
ReaD. K.: 1972, ‘Magnetic Anomalies along Fracture Zones’, Nature Phys. Sci. 236, 58–59.
Rea, D. K.: 1975a, Tectonics of the East Pacific Rise, 5° to 12°S', (Ph.D. Thesis). Corvallis, Oregon State University, 139 pp.
ReaD. K.: 1975b, ‘Model for the Formation of Topographic Features of the East Pacific Rise Crest’, Geology 3, 77–80.
ReaD. K., and NauglerF. P.: 1971, ‘Musicians Seamount Province and Related Crustal Structures North of the Hawaiian Ridge’, Marine Geol. 10, 89–111.
ReaD. K., DymondJ., HealthG. R., HeinrichsD. F., JohnsonS. H., and HussongD. M.: 1973, ‘New Estimates of Rapid Seafloor Spreading Rates and the Identification of Young Magnetic Anomalies on the East Pacific Rise, 6° and 11°S’, Earth and Planetary Sci. Letters 19, 225–229.
ScheideggerK. F.: 1973, ‘Temperatures and Compositions of Magmas Ascending Along Mid-ocean Ridges’, J. Geophys. Res. 78, 3340–3355.
SchoutenJ. A.: 1971, ‘A Fundamental Analysis of Magnetic Anomalies over Oceanic Ridges’, Marine Geophys Res. 1, 111–144.
SclaterJ. G. and FrancheteauJ.: 1970, ‘The Implications of Terrestrial Heat Flow Observations on Current Tectonic and Geochemical Models of the Crust and Upper Mantle of the Earth’, Roy. Astron. Soc. Geophys. J. 20, 509–542.
SclaterJ. G., AndersonR. N., and BellM. L.: 1971, ‘Elevation of Ridges and the Evolution of the Central Eastern Pacific’, J. Geophys. Res. 76, 7888–7915.
TalwaniM., WindischC. C., and LangsethM. G.Jr.: 1971, ‘Reykjanes Ridge Crest: a Detailed Geophysical Study’, J. Geophys. Res. 76, 473–517.
vanAndelTj. H., and HeathG. R.: 1970, ‘Tectonics of the Mid-Atlantic Ridge 6–8° South Latitude’, Marine Geophys. Res. 1, 5–36.
VineF. J.: 1968, ‘Magnetic Anomalies Associated with Mid-ocean Ridges’, in PhinneyR. A. (ed.), The History of the Earth's Crust, Princeton Univ. Press, Princeton, N.J., pp. 73–89.
VogtP. R., AveryO. E., SchneiderF. D., AndersonC. N., and BraceyD. R.: 1969, ‘Discontinuities in Sea-floor Spreading’, Tectonophys 8, 285–317.
WatkinsN. D.: 1973, ‘Paleomagnetism of the Canary Islands and Madeira’, Roy. Astron. Soc. Geophys. J. 32, 249–267.
WatkinsN. D., GunnB. M., BaksiA. K., YorkD., and Ade-HallJ.: 1971, ‘Paleomagnetism, Geochemistry, and Potassium-Argon Ages of the Rio Grande de Santiago Volcanics, Central Mexico’, Geol. Soc. Am. Bull. 82, 1995–1968.
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Rea, D.K. Analysis of a fast-spreading rise crest: The East Pacific Rise, 9° to 12° south. Mar Geophys Res 2, 291–313 (1976). https://doi.org/10.1007/BF00426250
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DOI: https://doi.org/10.1007/BF00426250