Overlapping spreading centers

Overlapping spreading centers (OSC) are a feature of spreading centers at mid-ocean ridges.

Schematic of oceanic overlapping spreading centers. Plates A and B are spreading apart. Black regions denote spreading centers.

Appearance

In the normal case adjacent spreading centers not on the same trend, end at a transform fault that appears to offset each center. At an overlapping spreading center the adjacent centers overlap and are not connected by a transform fault.[1][2][3] In plan view, overlapping spreading centers look like two people about to shake hands but over-reaching by a distance that is three times the distance between the hands.[2] This 3:1 overlap to offset ratio (observed at a wide range of scales, from hundreds of meters to tens of km) holds the key to their mechanism of formation. In fracture mechanics, when two cracks propagate toward each other, they tend to deflect away from each other, propagate past each other, and then stall out when the ratio is 3:1 because at this point the crack propagation force drops to zero.[4]

What results are two spreading centers overlapping, apparently pushing against each other, which would violate one of the rules of plate tectonics: plates are rigid. This paradox is solved by the fact that the overlapping spreading centers are non-steady-state and propagate along the ridge; as one segment lengthens, the neighboring segment shortens. Which segment lengthens, is arbitrary; evidence shows that at times the spreading center tips have alternated migrating directions.[2] The discovery of overlapping spreading centers revealed the time-varying behavior of mid-ocean ridge segments, with implications for magmatism, volcanism and patterns of hydrothermal activity.[5]

Occurrence

Overlapping spreading centers are found mostly on mid-ocean ridge spreading centers in the Pacific Ocean basin where the rates of separation between the plates are higher than in other ocean basins.[6][7] They occur on all fast-spreading rate centers (>90 mm/yr) including the East Pacific Rise, and on most intermediate-rate spreading centers (50–90 mm/yr), such as the Galapagos Spreading Center, the Juan de Fuca Ridge and the Pacific-Antarctic Ridge.[8][9]

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References
  1. Macdonald, Ken C.; Fox, P. J. (1983). "Overlapping spreading centres: new accretion geometry on the East Pacific Rise". Nature. 302 (5903): 55–58. Bibcode:1983Natur.302...55M. doi:10.1038/302055a0. ISSN 1476-4687.
  2. Macdonald, Kenneth C.; Fox, Paul J. (1990). "The Mid-Ocean Ridge". Scientific American. 262 (6): 72–81. Bibcode:1990SciAm.262f..72M. doi:10.1038/scientificamerican0690-72. JSTOR 24996826.
  3. Lonsdale, P. (1983). "Overlapping rift zones at the 5.5°S offset of the East Pacific Rise". Journal of Geophysical Research: Solid Earth. 88 (B11): 2156–2202. Bibcode:1983JGR....88.9393L. doi:10.1029/JB088iB11p09393.
  4. Sempere, J.-C. and K.C. Macdonald (1986). "Overlapping spreading centers: Implications from crack growth simulation by the displacement discontinuity method". Tectonics. 5 (1): 151–163. Bibcode:1986Tecto...5..151S. doi:10.1029/TC005i001p00151.
  5. Macdonald, K. C.; Sempere, J.-C.; Fox, P. J.; Tyce, R. (1987-11-01). "Tectonic evolution of ridge-axis discontinuities by the meeting, linking, or self-decapitation of neighboring ridge segments". Geology. 15 (11): 993. Bibcode:1987Geo....15..993M. doi:10.1130/0091-7613(1987)15<993:teordb>2.0.co;2. ISSN 0091-7613.
  6. Macdonald, K. C. (1982). "Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone". Annual Review of Earth and Planetary Sciences. 10 (1): 155–190. Bibcode:1982AREPS..10..155M. doi:10.1146/annurev.ea.10.050182.001103.
  7. Macdonald, Ken C. (2019), "Mid-Ocean Ridge Tectonics, Volcanism, and Geomorphology", Encyclopedia of Ocean Sciences, Elsevier, pp. 405–419, doi:10.1016/b978-0-12-409548-9.11065-6, ISBN 9780128130827
  8. Macdonald, K., Sempere, J.C. and Fox, P.J. (1984). "East Pacific Rise from Siqueiros to Orozco fracture zones: Along‐strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers". Journal of Geophysical Research: Solid Earth. 89(B7) (B7): 6049–6069. Bibcode:1984JGR....89.6049M. doi:10.1029/JB089iB07p06049.CS1 maint: multiple names: authors list (link)
  9. Perram, Laura Jean; Macdonald, Ken C. (1994). "An overlapping propagating spreading center at 87°30′W on the Galapagos Spreading Center". Earth and Planetary Science Letters. 121 (1–2): 195–211. Bibcode:1994E&PSL.121..195P. doi:10.1016/0012-821x(94)90040-x.
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