Magnetic field intensity,F obs distance along ship track intensiy anomaly,  F 0 0 Smoothly varying global field plus small, short wavelength effects of.

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Presentation transcript:

magnetic field intensity,F obs distance along ship track intensiy anomaly,  F 0 0 Smoothly varying global field plus small, short wavelength effects of crustal magnitizations distance along ship track subtract global field to yield total intensity anomaly

Magnetized rectangular prisms (“stripes”) parallel to spreading axis direction of cross section (perpendicular to axis of spreading) thickness of magnetized prisms Strike of spreading ridge

Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) longitudinal or strike direction, assumed infinite Vertical, downwards perpendicular to strike

Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) longitudinal or strike direction, assumed infinite thickness, t width, w Vertical direction of cross section, z axis horizontal 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 poles Vertical, downwards perpendicular to strike strips extend to “infinity”

J 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) Earth’s field, He Vertically downwards magnetization parallel to vertical earth’s field ocean surface

J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth’s field, He Magnetized prism field adds to Earth’s field,  F positive

J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth’s field, He Magnetized prism field adds to Earth’s field,  F positive

J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth’s field, He Magnetized prism field perpendicular to He,  F = 0

J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth’s field, He Magnetized prism field subtracts from He,  F negative

 F/2J Magnetic total intensity anomaly from single vertically magnetized prism distance, x, along cross section perpendicular to the trend of the rectangular prism width = 10 km

 F/2J Magnetic total intensity anomaly from single vertically magnetized prism width = 80 km distance, x, along cross section perpendicular to the trend of the rectangular prism

 F/2J 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 Earth’s field, He Combination of rectangular prisms WoWd t axis of spreading

 F/2J Multiple prisms vertically magnetized in alternate directions: combined effect

anomaly no M16 M10N M4M0M21M Age of ocean floor From Muller, et al., 1997

Pangea Tethys Ocean 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

anomaly no M16 M10N M4M0M21M Age of ocean floor From Muller, et al., 1997