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Marine Geophysical Researches

, Volume 18, Issue 1, pp 13–52 | Cite as

Abundant seamounts of the Rano Rahi seamount field near the Southern East Pacific Rise, 15° S to 19° S

  • Daniel S. Scheirer
  • Ken C. Macdonald
  • Donald W. Forsyth
  • Yang Shen
Article

Abstract

A widespread seamount province, the Rano Rahi Field, is located near the superfast spreading Southern East Pacific Rise (SEPR) between 15°–19° S. Particularly abundant volcanic edifices are found on Pacific Plate aged 0 to ∼ 6.5 Ma between 17°–19° S, an area greater than 100,000 km2. The numbers of seamounts and their volume are several times greater than those of a comparablysurveyed area near the Northern East Pacific Rise (NEPR), 8°–17° N. Most of the Rano Rahi seamounts belong to chains, which vary in length from ∼ 25 km to >240 km and which are very nearly collinear with the Pacific absolute and relative plate motion directions. Bends of 10°–15° occur along a few of the chains, and some adjacent chains converge or diverge slightly. Many seamount chains have fluctuations in volume along their length, and statistical tests suggest that some adjacent chains trade-off in volume. Several seamount chains split into two lines of volcanoes approaching the axis. In general, seamount chains composed of individual circular volcanoes are found near the axis; the chains consist of variably-overlapping edifices in the central part of the survey; to the west, volcanic ridges predominate. Near the SEPR, the volume of nearaxis seamount edifices is generally reduced near areas of deflated cross-sectional area of the axial ridge. Fresh lava flows, as imaged by sidescan sonar and sampled by dredging, exist around some seamounts throughout the entire survey area, in sharp contrast to the absence of fresh flows beyond ∼ 30 km from the NEPR. Also, the increases in seamount abundance and volume extend to much greater crustal ages than near the NEPR. Seamount magnetization analysis is also consistent with this wider zone of seamount growth, and it demonstrates the asynchronous formation of most of the seamount chains and volcanic ridges. The variety of observations of the SEPR seamounts suggests that a number of factors and mechanisms might bring about their formation, including the mantle upwelling associated with superfast spreading, off-axis mantle heterogeneities, miniplumes and local upwelling, and the vulnerability of the lithosphere to penetration by volumes of magma. In particular, we note the association of extensive, recent volcanism with intermediate wavelength gravity lineaments lows on crust aged ∼ 6 Ma. This suggests that the lineaments and some of the seamounts share a common cause which may be related to ridge-perpendicular asthenospheric convection and/or some manner of extension in the lithosphere.

Key words

East Pacific rise seamounts seafloor volcanism MELT 

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Copyright information

© Kluwer Academic Publishers 1996

Authors and Affiliations

  • Daniel S. Scheirer
    • 1
  • Ken C. Macdonald
    • 1
  • Donald W. Forsyth
    • 2
  • Yang Shen
    • 2
  1. 1.Department of Geological SciencesUCSBSanta BarbaraUSA
  2. 2.Department of Geological SciencesBrown UniversityProvidenceUSA

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