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Regional Tectonics

Regional Tectonics. Geos 425/ 525. Oceanic lithosphere I. LECTURE 7. 24 slides , 70 mins. The oceanic lithosphere. 1. The architecture of the oceanic crust 2. Fast vs slow spreading ridges 3. Ultra slow spreading ridges 4. Continental breakup.

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Regional Tectonics

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  1. Regional Tectonics Geos 425/525 Oceanic lithosphere I LECTURE 7 24 slides, 70 mins

  2. The oceanic lithosphere 1. The architecture of the oceanic crust 2. Fast vs slow spreading ridges 3. Ultra slow spreading ridges 4. Continental breakup

  3. ocean ridges: where new oceanic lithosphere is created lithosphere moves away from crest of ridge seafloor spreading …at ridge axis, extensional tectonics dominates schematic section through oceanic crust and upper mantle at spreading ridge ophiolites (more in a minute) from: van der Pluijm and Marshak, 1997 from: http://www.earth.nwu.edu/personal/seth

  4. deeper parts of ocean basins correspond to older crust… (…this ignores trenches…) relationship noted of age vs. depth d = 2500 + 350(t)1/2 only true for lithosphere < 80 Ma age relationship for seafloor > 60-80 Ma is different d = 6400 - 3200exp(-t/62.8) from: Kearey and Vine, 1996

  5. thermal boundary layer model for seafloor topography (Turcotte and Oxburgh, 1967) as seafloor moves away from ridge, it cools and thermally contracts …increasing its density

  6. bathymetry of North Atlantic: consistent with age relationships? from: http://topex.ucsd.edu/marine_topo

  7. bathymetry/morphology of oceanic ridges depends on spreading rate: slow slow: < 5 cm/yr; axial rift valley (depression) fast from: http://www.soc.soton.ac.uk/CHD/classroom@sea/carlsberg from: van der Pluijm and Marshak, 1997

  8. fast-spreading: magma enters a large magma chamber in crust; …broad bulge exists at ridge slow-spreading: magma chamber freezes between pulses of …spreading; axial rift valley occurs from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

  9. bathymetry, spreading rate, ridge axis characteristics slow intermediate fast from: Kearey and Vine, 1996

  10. slow-spreading ridge from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270 cross-section (Mid-Atlantic Ridge) diagram on left shows alternating phases of construction and extension from: Kearey and Vine, 1996

  11. “classic ridge” (slow-spreading) map from: http://www.soc.soton.ac.uk/CHD/classroom@sea/carlsberg

  12. seismic refraction data fast spreading ridge model from seismic data fissure eruption on Mauna Loa from: http://gore.ocean.washington.edu/classpages/ocean410/

  13. what about along axis variations? mid-ocean ridges composed of segments of various lengths… …bound by transform faults/fracture zones (1st order discontinuity) …50 km on average…more in Atlantic than in East Pacific Rise… mid-Atlantic Ridge East Pacific Rise segments thought to behave independently from each other …each segment related to source deep in mantle below… from: http://gore.ocean.washington.edu/classpages/ocean410/

  14. fast-spreading ridges…2nd order discontinuity divided into segments 10-2000 km long by overlapping ridge tips …overlapping spreading centers (OSC) can you find OSC(s) on image to the right? from: http://gore.ocean.washington.edu/classpages/ocean410/

  15. slow-spreading ridges… 2nd order discontinuity divided into segments by non-transform offsets …typically average 50 km… axial valley has hourglass shape (narrower in center than ends) from: http://gore.ocean.washington.edu/classpages/ocean410/

  16. discontinuities classified by shape, size, longevity first: transform fault second: fast: large OSC offsets ridge by > 2 km slow: NTO jog in valley third: fast: small OSC offsets ridge by 0.5 to 2.0 km slow: gap between volcanoes from: McDonald and Fox, 1990

  17. OSC can generate off-axis features: …new tips develop and old ones move away through spreading East Pacific Rise at 21°S from: McDonald and Fox, 1990

  18. what about an extremely slow (< 1.5 cm/yr) spreading ridge? predictions? Gakkel Ridge, Arctic SW Indian Ridge

  19. SW Indian Ridge drill holes contain • peridotite (green dots) • basalt (red dots) peridotite: mantle rock

  20. Gakkel Ridge: mantle exposed over large regions of seafloor-- --peridotite altered to serpentinite • crust very thin • abundant hot springs • widely-spaced volcanoes • lavas more Na, K rich • no transform faults mantle emplaced as blocks where there is no seafloor processes similar to early stages of continental break-up and drift …structure of ultraslow ridges similar to continental margins at depth…

  21. Gakkel Ridge

  22. peridotite: green; basalt: red; gabbro: yellow

  23. Summary of ocean ridge tectonics • Fast spreading - typical crustal section with the 5-7 km crustal thickness as seen in many ophiolites; no axial valley; • Slow spreading ridges - more extension, mantle sections exposed; axial rift valley;

  24. Next Lecture • Preserving oceanic lithopsherein the geologic record.

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