Embed Size (px)
Citation preview
mag
net
ic f
ield
inte
nsit
y,F
obs
distance along ship track
inte
nsi
y an
omal
y,
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
Mag
netiz
ed re
ctang
ular p
risms
(“stri
pes”
) para
llel to
sprea
ding a
xis
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)
long
itudi
nal o
r stri
ke di
recti
on, a
ssum
ed in
finite
Vertical, downwards
perpendicular to strike
Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)
long
itudi
nal o
r stri
ke di
recti
on, a
ssum
ed in
finite
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”
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, HeVertically 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
Wo Wd
t
axis of spreading
F/2J
Multiple prisms vertically magnetized in alternate directions: combined effect
5 6
10.9 120.4
anomaly no. 1813 25 31 3421 M16M10N
M4M0 M21 M25
154.3 180
Age of ocean floor
From Muller, et al., 1997
Pangea
Tethys 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
5 6
10.9 120.4
anomaly no. 1813 25 31 3421 M16M10N
M4M0 M21 M25
154.3 180
Age of ocean floor
From Muller, et al., 1997
