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Chronology of geomagnetic field reversals magnetic anomaly “number” Ocean floor age, millions of years (Ma), determined largely from deep sea drilling Seafloor spreading is a tape recorder of the geomagnetic field! Age, Ma The recorded reversal chronology crust moho upper mantle The tape drive The recording head of the tape recorder Ocean Ridge system Modern 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 bathymetry Map 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 1960’s by the Columbia University research vessel Eltanin. 21 20 19 The famous Eltanin 19 profile 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. Eltanin profiles of magnetic anomalies 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. The incredible symmetry of the Eltanin 19 profile +500 ESE WNW 0 -500 +500 WNW 0 ESE -500 total intensity anomaly calculated from model mirror image of measured profile to show symmetry measured profile of total intensity anomalies magnetic field intensity,Fobs intensiy anomaly, DF Smoothly varying global field plus small, short wavelength effects of crustal magnitizations 0 distance along ship track subtract global field to yield total intensity anomaly 0 distance along ship track Strike of spreading ridge direction of cross section (perpendicular to axis of spreading) thickness of magnetized prisms Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) perpendicular to strike Vertical, downwards Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) thickness, t width, w Vertical direction of cross section, z axis horizontal direction of cross section, x axis strips extend to “infinity” vertical downwards magnetization equivalent to two parallel strips of magnetic poles as shown: top strip of negative poles and bottom strip of positive poles perpendicular to strike Vertical, downwards strips extend to “infinity” Magnetic field lines for vertically downwards magnetization in cross-sectional view ---------------J + + + + + + + + + + + + +++ Magnetic field lines for vertically upwards magnetization + ++ + + + + + + + + + + + + - - - - - - - - - -J- - - - - - Magnetic field due to magnetized prism taken along the surface above the prism (directions only) ocean surface ---------------J + + + + + + + + + + + + +++ Earth’s field, He Vertically downwards magnetization parallel to vertical earth’s field Magnetized prism field adds to Earth’s field, DF positive ---------------J + + + + + + + + + + + + +++ Earth’s field, He Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Magnetized prism field adds to Earth’s field, DF positive ---------------J + + + + + + + + + + + + +++ Earth’s field, He Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Magnetized prism field perpendicular to He, DF = 0 ---------------J + + + + + + + + + + + + +++ Earth’s field, He Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Magnetized prism field subtracts from He, DF negative ---------------J + + + + + + + + + + + + +++ Earth’s field, He Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Magnetic total intensity anomaly from single vertically magnetized prism DF/2J width = 10 km distance, x, along cross section perpendicular to the trend of the rectangular prism Magnetic total intensity anomaly from single vertically magnetized prism DF/2J width = 80 km distance, x, along cross section perpendicular to the trend of the rectangular prism Combination of rectangular prisms DF/2J Earth’s field, He d t axis of spreading Five 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 separately Wo W DF/2J Multiple prisms vertically magnetized in alternate directions: combined effect Map of magnetic anomaly numbers Deep Sea Drilling sites magnetic anomaly number paleontological age, Ma Deep sea drilling in the South Atlantic Ocean Seafloor ages from deep sea drilling versus geomagnetic reversal chronology Age (Ma) from geomagnetic reversal chronology extrapolate in South Atlantic assuming constant rate of spreading Chronology of geomagnetic field reversals magnetic anomaly “number” Ocean floor age, millions of years (Ma), determined largely from deep sea drilling 4600 My Geologic time scale 600 My Age range of modern ocean floor http://www.geo.ucalgary.ca/~macrae/timescale/timesc Transform faults 7000 3500 1500 -500 0 500 -1500 -2500 -3500 -4500 -5500 -6500 South Atlantic Ocean South Atlantic Ocean inactive fracture zone 7000 3500 active transform fault 1500 -500 0 500 -1500 -2500 -3500 -4500 -5500 -6500 active ridge crest active ridge crest inactive fracture zone inactive fracture zone active transform fault Transform faults and fracture zones Map pattern of magnetic anomaly number Deep sea drilling calibration of chronology of geomagnetic field reversals back to 180 Ma. Map pattern of ocean floor age Age of ocean floor anomaly no. 5 10.9 6 13 18 21 25 31 34 M10N M0 M4 M16 M21 M25 120.4 154.3 From Muller, et al., 1997 180 Pacific 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