Main Article Content
The average median valley of the Mid-Atlantic Ridge between 12° and 18° N is described as a smooth depression flanked on both sides by a high. This applies both to the bathymetry and to the gravity anomalies. This picture of the median valley and its walls was obtained by stacking profiles across the valley in 50- to 70-km-wide bands. The reduced median valley can then be interpreted as the result of the parting of the lithosphere and the response of the asthenosphere as a viscous layer to repeated unloading. Fluid dynamic equations show that the response is in general broader than the original load disturbance. We describe this as a viscous lag of the shorter wavelength components. A steady-state solution was reached by numerical methods, showing a depression accompanied by a high on both sides. For the asthenosphere under the Mid-Atlantic Ridge at these latitudes a value followed for the kinematic viscosity of 1.5 x 1019 stokes. The model can be extended to other parts of the mid-ocean ridge system by adapting the time-dependent constants (viscosity and spreading rate). If the viscosity is a factor 5 lower, no median valley results. Rising to isostatic equilibrium of a light body under the floor of the median valley then accounts for the existence of a median ridge like found at Reykjanes Ridge and at the East Pacific Rise. The coefficient of viscosity under the East Pacific Rise would be about 0.4 x 1018 stokes. The concept of a viscous lag of the short-wavelength components replaces Sleep's (1969) original notion of a 'loss of head'. The secondary valleys and ridges found in the median valley and on the flanks of the Mid-Atlantic Ridge crest cannot be explained by the model. They represent essentially a non-continuum process, in which presumably an episodic jumping of the inner valley plays an important role. Additional faulting occurs at the hinge line between the floor and the walls of the median valley.
Authors who publish with this journal as of Vol. 63 agree to the following terms:
a. Authors share the copyright with this journal in equal parts (50% to the journal, 50% to the lead author), and grant the journal right of first publication, with the work after publication simultaneously licensed under Creative Commons Attribution License CC BY-NC-ND 4.0 that allows others to share the work with an acknowledgment of the work's authorship and initial publication in this journal.
b. Authors may enter into separate, additional contractual arrangements for the non-exclusive distribution of the journal's published version of the work (e.g., post it to an institutional repository or publish it in a book), with an acknowledgment of its initial publication in this journal, and a reference to this copyright notice.
c. Authors are permitted and encouraged to post their work online (e.g., in institutional repositories or on their website) during the submission process, as this can lead to productive exchanges and earlier and greater citation of published work and better sales of the copyright.
Authors retain copyright and grant the Journal of Geophysics right of first publication, with the work two years after publication simultaneously licensed under the Creative Commons BY-NC-ND 4.0 License that allows others to share the work (with an acknowledgment of the work's authorship and initial publication in this journal), except for commercial purposes and for creating derivatives.
Authors can enter into separate, additional, but non-commercial contractual arrangements for the non-exclusive distribution of the journal's published version of the work (e.g., post it to an institutional repository, but not publish it in a book), with an acknowledgment of its initial publication in this journal.
Authors are permitted and encouraged to post their work online (e.g., in institutional repositories or on their website) before and during the submission process, as that can lead to productive exchanges, as well as earlier and greater citation of published work (See The Effect of Open Access).
This journal is one of a handful of scholarly journals that publish original scientific works under CC BY-NC-ND 4.0 - the only Creative Commons license affording the authors' intellectual property absolute worldwide protection.
Journal of Geophysics is published under the scholar-publishers model, meaning authors do not surrender their copyright to us. Instead, and unlike corporate publishers like Elsevier or Springer Nature that resell copyright to third-parties for up to $80,000 (per paper, per transaction!), the Journal of Geophysics authors share copyright equally with this journal.
Therefore, all the proceeds from reselling copyright to third parties get shared to equal parts (50% to the journal, 50% to the lead author). Under the Berne Convention, this protection is an inheritable right that lasts for as long as the rightsholder lives + 50 years.
By submitting to this journal, the lead author, on behalf of all co-authors, grants permission to this journal to represent all co-authors in negotiating copyright sales and collecting proceeds. The lead author should negotiate with his/her co-authors the modalities of distributing the lead author's portion of the proceeds. Usually, this is per pre-agreed percentage of each co-author's contribution to creating the copyrighted work. (more...)
Cochran, J.R. (1979) An analysis of isostasy in the world's oceans: Part 2. Mid-Ocean Ridge crests. J. Geophys. Res. 84:4713-4729
Cochran, J.R., Talwani, M. (1977) Free-air gravity anomalies in the world's oceans and their relationship to residual elevation. Geophys. J. R. Astron. Soc. 50:495-552
Cochran, J.R., Talwani, M. (1978) Gravity anomalies, regional elevation, and the deep structure of the North Atlantic. J. Geophys. Res. 83:4907-4924
Collette, B.J., Rutten, K., Schouten, H., Slootweg, A.P. (1974a) Continuous seismic and magnetic profiles over the Mid-Atlantic Ridge between 12° and 18° N. Mar. Geophys. Res. 2:133-141
Collette, B.J., Schouten, H., Rutten, K., Slootweg, A.P. (1974b) Structure of the Mid-Atlantic Ridge province between 12° and l8°N. Mar. Geophys. Res. 2:143-179
Collette, B.J., Slootweg, A.P., Twigt, W. (1979) Mid-Atlantic Ridge crest topography between 12° and 15° N. Earth Planet. Sci. Lett. 42:103-108
Courtillot, V., Tapponnier, P., Varel, J. (1974) Surface features associated with transform faults: a comparison between observed examples and an experimental model. Tectonophysics 24:317-329
Fowler, C.M.R. (1976) Crustal structure of the Mid-Atlantic ridge crest at 37° N. Geophys. J. R. Astron. Soc. 47:459-491
Heiskanen, W.A., Vening Meinesz, F.A. (1958) The Earth and its gravity field. McGraw-Hill, New York
Lachenbruch, A.H. (1973) A simple mechanical model for oceanic spreading centers. J. Geophys. Res. 78:3395-3417
Lachenbruch, A.H. (1976) Dynamics of a passive spreading center. J. Geophys. Res. 81:1883-1902
Laughton, A.S., Searle, R.C. (1979) Tectonic Processes on Slow Spreading Ridges. 2nd Maurice Ewing Symposium, Am. Geophys. Union (In press)
Laughton, A.S., Searle, R.C., Roberts, D.G. (1979) The Reykjanes Ridge crest and the transition between its rifted and nonrifted regions. Tectonophysics 55:173-179
Luyendyk, B.P., Macdonald, K.C. (1977) Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package. Bull. Geol. Soc. Am. 88:648-663
Menard, H.W., Atwater, T. (1968) Changes in direction of sea-floor spreading. Nature 219:463-467
Needham, H.D., Francheteau, J. (1974) Some characteristics of the rift valley in the Atlantic Ocean near 36°48' North. Earth Planet. Sci. Lett. 22:29-43
Peter, G., Merrill, G., Bush, S. (1973) Caribbean Atlantic Geotraverse. NOAA-IDOE 1971, Report No. 1, Project Introduction. Bathymetry
Ramberg, LB., Gray, D.F., Raynolds, R.G. (1977) Tectonic evolution of the FAMOUS area of the Mid-Atlantic Ridge, lat 35° 50' to 37° 20' N. Bull. Geol. Soc. Am. 88:609-620
Rea, D.K. (1978) Spreading-center offsets that are not transform faults. Eos. 59:370
Searle, R.C., Laughton, A.S. (1977) Sonar studies of the Mid-Atlantic Ridge and Kurchatov Fracture Zone. J. Geophys. Res. 82:5313-5328
Sleep, N.H. (1969) Sensitivity of heat flow and gravity to the mechanism of sea-floor spreading. J. Geophys. Res. 74:542-549
Sleep, N.H. (1975) Formation of oceanic crust: some thermal constraints. J. Geophys. Res. 80:4037-4042
Sleep, N.H., Biehler, S. (1970) Topography and tectonics at the intersections of fracture zones with central rifts. J. Geophys. Res. 75:2748-2752
Sleep, N.H., Rosendahl, B.R. (1979) Topography and tectonics of midoceanic ridge axes. J. Geophys. Res. 84 (In press)
Slootweg, A.P. (1978) Computer contouring with a digital filter. Mar. Geophys. Res. 3:401-405
Talwani, M. (1970) Gravity. In: Maxwell, A.E. (Ed.) The sea, vol. 4 Part 1, pp. 251-297. Wiley-Interscience, New York
Talwani, M., Le Pichon, X., Ewing, M. (1965) Crustal structure of mid-ocean ridges. 2. Computed model from gravity and seismic refraction data. J. Geophys. Res. 70:341-352
Talwani, M., Windish, C.C., Langseth, M.G., Jr. (1971) Reykjanes ridge crest: A detailed geophysical study. J. Geophys. Res. 76:473-517
Tapponnier, P., Francheteau, J. (1978) Necking of the lithosphere and the mechanics of slowly accreting plate boundaries. J. Geophys. Res. 83:3955-3970, 4497