Paleomagnetic Pole Determination

Process:

(1) Find Rocks of right age.

(2) Sample extensively to a)eliminate secular variation effects

b)determine or average out local tilt

c)fold test where possible

d)determine alpha95 on pole

3) Determine inclination and declination. Infer latitude and orientation.

Paleomagnetic Pole Determination

•! Problems:

1.!No longitudinal control

2.! deformation of rocks can change the orientation of the RM (best to measure untilted, undeformed rocks)

3.!RM can be partly or wholly reset (weathering, metamorphism)

4.! need a control on the age of the rock

5.! errors of uncertainty are always quite large because the magnetic pole is moving around the geographic pole – requires several analyses

tan I =2 tan (latitude)

Inclination

Paleomagnetism

Plot Two ways: 1.! the way it actually happens –

the continent is plotted in a series of positions

2.! Fix continent: plot the apparent pole position with time •! apparent pole wander path •! avoids longitude issue + can display data for several

continents

APW for NA and EU (b) is after closing N altlantic.

Triple Junction Stability

•! See discussion in text

•! We constructed several examples in class

•! The North America - Pacific – Fallon Plate

evolution was examined. – see animation

on web site

Understanding “Lithosphere”

•! A mechanical description of layering – not

compositional

•! Review of brittle – ductile behavior

•! Role of P and T discussed

•! Questions you should be able to answer:

–!Thickness? (typical –thinnest – thickest?)

–!What controls thickness?

–!Sharp or diffuse boundary?