Chronology of geomagnetic field reversals magnetic anomaly “number” Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

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  • Chronology of geomagnetic field reversalsmagnetic anomaly numberOcean floor age, millions of years (Ma), determined largely from deep sea drilling

  • Age, MaSeafloor spreading is a tape recorder of the geomagnetic field!The recording head of the tape recorderThe tape driveThe recorded reversal chronology

  • Ocean Ridge systemEast Pacific RiseMid-Atlantic RidgeModern view of ocean bathymetry derived from satellite altimetry. see EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics

  • Global bathymetryMap shown in next slide

  • Ship tracks across the East Pacific Rise which obtained the magnetic anomalies shown in the next slide. The measurements were made in the 1960s by the Columbia University research vessel Eltanin.212019

  • The Eltanin 19 profile is among the most influential geophysical profiles ever published. It provided the smoking gun evidence for seafloor spreading, evidence that turned a majority of skeptics into a majority of believers. The profile was published together with three others in 1966 by Pitman and Heirtzler in Magnetic Anomalies over the Pacific-Antarctic Ridge (Science, 154, 1164-1171). The figures above comes from that paper. The track lines (ELT 19-21) of the research vessel Eltanin are shown together with the correlated magnetic anomalies (numbered dashed lines) and the 2000 fathom bathymetric contour. The crosses are earthquake epicenters. The inferred active spreading center would be between anomalies 1 and 1. The voyage occurred in 1965.The famous Eltanin 19 profile

  • The four profiles show total intensity anomalies and bathymetry (ocean depth in km) along the four tracks shown on the previous map. Note that track 20 crosses the ridge system twice.

    The vertical scale for total intensity anomaly, DF, is shown in gammas. This is the same as nanoTeslas or nT. The horizontal lines are at zero anomaly; the scale is thus minus 500 to plus 500 nT.Eltanin profiles of magnetic anomalies

  • The incredible symmetry of the Eltanin 19 profileESEWNWtotal intensity anomaly calculated from modelWNWESEmeasured profile of total intensity anomaliesmirror image of measured profile to show symmetry+500-500+500-500 0 0

  • magnetic field intensity,Fobsdistance along ship trackintensiy anomaly, DF00Smoothly varying global field plus small, short wavelength effects of crustal magnitizationsdistance along ship tracksubtract global field to yield total intensity anomaly

  • Magnetized rectangular prisms (stripes) parallel to spreading axisdirection of cross section (perpendicular to axis of spreading)thickness of magnetized prismsStrike of spreading ridge

  • Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)longitudinal or strike direction, assumed infiniteVertical, downwardsperpendicular to strike

  • Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)longitudinal or strike direction, assumed infinitethickness, twidth, wVertical direction of cross section, z axishorizontal direction of cross section, x axis

  • vertical downwards magnetization equivalent to two parallel strips of magnetic poles as shown: top strip of negative poles and bottom strip of positive polesVertical, downwardsperpendicular to strikestrips extend to infinitystrips extend to infinity

  • +++++++++++++++----------------JMagnetic field lines for vertically downwards magnetization in cross-sectional view

  • +++++++++++++++----------------JMagnetic field lines for vertically upwards magnetization

  • +++++++++++++++----------------JMagnetic field due to magnetized prism taken along the surface above the prism (directions only)Earths field, HeVertically downwards magnetization parallel to vertical earths fieldocean surface

  • +++++++++++++++----------------JMagnetic field due to magnetized prism taken along the surface above the prism (directions only)Earths field, HeMagnetized prism field adds to Earths field, DF positive

  • +++++++++++++++----------------JMagnetic field due to magnetized prism taken along the surface above the prism (directions only)Earths field, HeMagnetized prism field adds to Earths field, DF positive

  • +++++++++++++++----------------JMagnetic field due to magnetized prism taken along the surface above the prism (directions only)Earths field, HeMagnetized prism field perpendicular to He, DF = 0

  • +++++++++++++++----------------JMagnetic field due to magnetized prism taken along the surface above the prism (directions only)Earths field, HeMagnetized prism field subtracts from He, DF negative

  • DF/2JMagnetic total intensity anomaly from single vertically magnetized prismdistance, x, along cross section perpendicular to the trend of the rectangular prismwidth = 10 km

  • DF/2JMagnetic total intensity anomaly from single vertically magnetized prismwidth = 80 kmdistance, x, along cross section perpendicular to the trend of the rectangular prism

  • DF/2JFive rectangular prisms: one central one plus two pairs symmetrically on either side (seafloor spreading model in the region near the spreading axis); magnetic anomaly of each prism is plotted separatelyEarths field, HeCombination of rectangular prismsWoaxis of spreading

  • DF/2JMultiple prisms vertically magnetized in alternate directions: combined effect

  • Map of magnetic anomaly numbersDeep Sea Drilling sites

  • magnetic anomaly numberAge (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreadingpaleontological age, MaSeafloor ages from deep sea drilling versus geomagnetic reversal chronologyDeep sea drilling in the South Atlantic Ocean

  • Chronology of geomagnetic field reversalsmagnetic anomaly numberOcean floor age, millions of years (Ma), determined largely from deep sea drilling

  • Geologic time scalehttp://www.geo.ucalgary.ca/~macrae/timescale/timescale.html4600 My600 MyAge range of modern ocean floor

  • Transform faults

  • South Atlantic Ocean

  • South Atlantic Oceaninactive fracture zoneactive transform faultactive ridge crest

  • inactive fracture zoneactive transform faultactive ridge crestinactive fracture zone

  • Transform faults and fracture zones

  • Map pattern of magnetic anomaly numberDeep sea drilling calibration of chronology of geomagnetic field reversals back to 180 Ma.Map pattern of ocean floor age

  • 5610.9120.4anomaly no.181325313421M16M10NM4M0M21M25154.3180Age of ocean floorFrom Muller, et al., 1997

  • PangeaTethys OceanPacific Ocean

  • Creation of Atlantic & Indian oceans and associated passive margins

  • closing of Tethys Ocean: collisional tectonics of Mediterranean-central and southeast Asia

  • Interaction of East Pacific Rise with western North America

  • Continued subduction beneath western South America as East Pacific Rise comes closer

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