Paleomagnetic poles
Brief review of plate tectonics
Apparent polar wander paths
Paleomagnetism and plate reconstructions
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Tectonic applications
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Continental Drift?
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e.g., Antonio Snider-Pellegrini (1858): “before” and “after” maps
http://pubs.usgs.gov/gip/dynamic/historical.html & UC Berkeley
An old idea (from 1596 even)
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http://pubs.usgs.gov/gip/dynamic/historical.html
Wegener (1880-1930)
Gondwana land
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But most Northern hemisphere geologists/geophysicists didn’t buy
it
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Paleomagnetic polesAssume the magnetic field is that of a GAD on average
If we sample over sufficient time to average out PSV, the average of all the VGPs is a PALEOMAGNETIC POLE (Exactly what is “sufficient time” is not known but must be > 10,000 years
Hospers (1955): paleomagnetic poles could be used to test the idea of continental drift (of e.g., Wegener, 1915) or polar wandering (Milankovitch, 1933)
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Wandering continents or wandering poles?
a) b)
AA
BB
CC
Drifting continent
Fixed pole
B
C
Wandering poles
Fixed continent
A
present present
Chapter 16Tuesday, May 17, 2011
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Australia
Europe
India
North America
You need data from more than one continent AND
older poles
Irving 1958
Data from 4 different
continents support
continental drift
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World encircling rift - Heezen and Tharp
(1957)
meanwhile - mapping of the
sea-floor
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At the time Heezen and Tharp thought the rift was from an expanding earth
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Paleomagnetists became “drifters” in the 50’s
No viable mechanism was proposed until the idea of sea-floor spreading (Hess, 1962)
Sea floor spreading (versus expanding earth) gained credibility with Morely-Vine-Matthews hypothesis (Vine and Matthews, 1963) which added in polarity reversals
Plate tectonics is “how it works”.
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http://pubs.usgs.gov/gip/dynamic/dynamic.html
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The major lithospheric plates
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http://www.pbs.org/wgbh/nova/teachers/activities/2515_vesuvius.html
Get locations and present relative motions from earthquakes
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16240 270 300 330 0
-60
-30
0
30
60
13
13
13
13
25
25
M25
13
13
13
25
1325
34
25
13
13
34
13
M0
M0
5
13
1325
34
34
34M0
M0
M0
13
1325
34
34 M0
13 2525 34
3434
M0
13 25
-60
-30
0
30
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last week we talked about relative
motions over the last 200 Ma from
magnetic anomalies
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Moving plates around on a sphere
Move continuously using an angular velocity vector: Euler pole specified by latitude, longitude and rate of rotation
Can describe any rotation by a finite pole of rotation: latitude, longitude and total angle
or by a sequence of “stage poles” which sum up to some total finite rotation pole
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!
" ,#e e
NAM
EUR
Euler poles: pairs of plates separate by rotation around a pole at a rate of (λe, φe) ω◦/Myr
v
v = aω sin θ
a = Earthradius
Chapter 16
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How to find Euler polesridges point to them
fracture zones make small circles around them
magnetic anomalies give rates of spreading which via equation for velocity versus colatitude equation can give you rate of rotation
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How to find finite rotation poles
find the rotation that “puts things back together best” by trial and error
can use a paleomagnetic pole to rotate a plate or plate fragment back to past orientation wrt north and latitude
or find finite rotations that cluster poles the best
see Table A.4 for set of finite rotations that put Gondwana continents back together..
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a)
b) c)
but be ware of ambiguity!
which polarity?(a versus b)
longitude unconstrained(b versus c)
Chapter 16Tuesday, May 17, 2011
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180-200 Ma Pangea A
Making the paleomagnetic poles fit
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Back to apparent polar wander
There have been over 10,000 paleomagnetic poles published since 1925.
Range in age from Archean to recent
Range in quality from abysmal to excellent
most are available in the pole database (GPMDB) available online: http://www.ngu.no/geodynamics/gpmdb/
More and more available with all the supporting site level data through earthref.org/MagIC
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All the poles from Australia in the GPMDB
Chapter 16
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But how to select?(see book for details)e.g., criteria of Van der Voo (1990) or Besse and Courtillot (2002)
Gotta know the age!
Must be geomagnetic field and average out PSV
Tectonic tilt (and rotation!) must be accounted for
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Australian pole set (<200 Ma) after selection (using BC02)
200 Ma
Chapter 16Tuesday, May 17, 2011
Creating “master” APWP paths
Say we have a bunch of poles from different continents whose ages are well known.
finite poles of rotation connecting the different continents are known (from sea floor spreading data, e.g.) - can rotate all poles to common reference frame
average everything together to create a master APWP
then export the poles back to each continent.
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200 AU
140
80
50
200IN120
90
AF
200
SAANT
present
50 Ma 200 Ma
100 Maa)
b) d)
c)
e)
Exported master APWP for Gondwana continents
Chapter 16Tuesday, May 17, 2011
Watch out for “discordant” poles in the database!
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circles: “stable” North America
triangles: “displaced terrains” from
western US (e.g., Mojave desert)
Chapter 16Tuesday, May 17, 2011
Take home message
Plate Tectonics was one of the major discoveries of the 20th century
Motions of the plates can be reconstructed using earthquake locations, deep sea mapping and paleomagnetic data (including anomalies)
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