TPW unplugged: Absolute plate motions and true polar wander in the absence of hotspot tracks, 320 — 130 Ma Bernhard Steinberger In collaboration with Trond

TPW unplugged:

Absolute plate motions and true polar wander

in the absence of hotspot tracks, 320 — 130 Ma

Bernhard Steinberger

In collaboration with Trond Torsvik (NGU)

Center for Geodynamics, NGU, Trondheim, Norway

TPW unplugged:

Absolute plate motions and true polar wander

in the absence of hotspot tracks, 320 — 130 Ma

What is true polar wander? (kinematics)

Why distinguish between plate motions

and true polar wander?

The physics of true polar wander (dynamics)

How to distinguish between plate motions

and true polar wander

--> with hotspot tracks (130 — 0 Ma)

--> without hotspot tracks (320 — 130 Ma)









What is true polar wander?

Motion of “Earth as a whole”

relative to its spin axis




in practice: mantle reference frame

with no net rotation of whole mantle

regarded as suitable for “Earth as a whole”










TPW vs. plate motions -- A bit of history ...

Both are old ideas Detailed TPW discussion by Darwin (1876)“Continental drift” Wegener (1915)Supported by paleomagnetism in the 1950'sQualitative theory of true polar wander by Gold (1955)Plate tectonics in the 1960'sTPW “Renaissance” since 1990s (Kirschvink, 1997, ...)


Why distinguish between plate

motions and true polar wander?






Why distinguish between plate

motions and true polar wander?

Reconstructed LIP eruption sites (circles) and hotspots (crosses) plotted on smean tomography model in lowermost mantle.Blobs with other color scale for steep gradients

Plume heads from the edges (-1% contour)of Large Low Shear Velocity Provinces (LLSVPs)Steep gradients along the same contour









30-60 60-30 30-60 600geoid relative to equilibrium shape [m]

seismic velocity anomaly average below 1900 km0.4% 0.4%0.8% 0

2 cm/yr flow

smean tomography model

chemically distinct D''regions?

Flow, density and the geoid:●degree two pattern of seismic anomalies in lower mantle and inferred flow●inferred low-density upwellings correspond to geoid highs, high- density downwellings in lower mantle to geoid lows.

Rotation axis = axis of maximum non-hydrostatic moment of inertiaexpect little tpw, if there is dominant, stable degree-2 geoid, with three different moments of inertialarger tpw possible if there isn't

axis of intermediate moment of inertia

axis of minimum moment of inertia

Maximum speed of true polar wander

With realistic Earth parameters,maximum speed in case (2)about 1 degree / million years

geoid [m]

Reconstruction100 Ma

Withoutgeoid highs related to subduction (presently at ~ equatorial position) axis of maximum non-hydrostatic moment of inertia could have possibly moved along blue circle~ corresponds to rotation of continents relative to spin axis but not overlower mantle

TPW vs. continent rotation over lower mantle

smean lowermost mantle

Center of massAfricanPacificLLSVP









●Use “hotspot tracks” (geometry,

age progression)

● Consider modelled hotspot motion

● Analyse plate motion properties

for the times when hotspot tracks

are available in order to proceed

towards “absolute” plate motions

for times before the oldest hotspot

tracks

How to obtain “absolute” plate motions?

76 Ma

61 Ma

56 Ma

49 Ma47 Ma

0 Ma

Modellinghotspotmotion -an example

Paleomagnetic (declination, inclination) --> “Virtual Geomagnetic Pole” (VGP) Relative plate motions --> common reference frame (here: South Africa)Group data in age bins (“running mean”) to obtain global apparent polar wander path (APWP)

(from Torsvik et al., 2007,Rev. Geophys., in review)

Apparent polar wander

converted from African apparent polar wander path (Torsvik et al., 2007, Rev. Geophys., in review)

Event at 100-110 Ma follows indeedapproximately the “blue circle”

colors for lowermostmantle seismicanomalies(smean)

“True Polar Wander” in mantle reference frame








--> without hotspot tracks (“unplugged”; 320 — 130 Ma)

TPW unplugged: the challengeWithout hotspot tracks, can only use paleomagnetic reference frame:by definition, no polar wanderno longitude constraint

Plate motions in hotspot paleomagneticreference frame

(from Torsvik et al., 2007,Rev. Geophys., in review)

100 Ma

Paleomagnetic reference frame

110 Ma



110 — 100 Ma

TPW unplugged: the toolFor each time step, compute, for all continents combinedCenter of massInertia tensor IAngular momentum L mean rotation L=I

Three components alongEarth's spin axis(coherent E-W motion)Equatorial axis; longitude as center of mass(coherent rotation)Equatorial axis; longitude90 from center of mass(coherent N-S motion)

TPW unplugged — results

mean rotationmean N-S motion

paleomagnetic global mantle Africa mantle ref. frame

135 Ma

145 Ma

145 — 135 Ma

Center of masscontinentsAfrican LLSVPPacific LLSVP

110 — 100 Ma110 — 100 Ma



mean rotationmean N-S motion

paleomagnetic global mantle Africa mantle ref. frame

145 Ma

195 Ma

195 — 145 Ma

CAMP (200 Ma)


220 Ma

250 Ma

250 — 220 Ma

CAMP (200 Ma)



195 — 145 Ma

CAMP (200 Ma)



mean rotation

mean N-S motion

paleomagnetic global mantle Africa mantle modified pm ref. frame

Summary

Rotations in paleomagnetic frame, interpreted as TPW events

Amount time period axis longitude~18 250-220 Ma~-18 195-145 Ma~-10 145-135 Ma near center of mass~10 110-100 Ma of continentssmall 320 Ma - present

10-15 W near CAMP

20-40 E

Northward motionAmount time period~30 320-190 Ma~15 190Ma-present

Documents

TPW unplugged: Absolute plate motions and true polar wander in the absence of hotspot tracks, 320 — 130 Ma Bernhard Steinberger In collaboration with Trond