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? (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

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”

What is true polar wander? Motion of “Earth as a whole” relative to its spin axis

What is true polar wander? Motion of “Earth as a whole” relative to its spin axis

What is true polar wander? Motion of “Earth as a whole” relative to its spin axis

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's Qualitative theory of true polar wander by Gold (1955) Plate tectonics in the 1960's TPW “Renaissance” since 1990s (Kirschvink, 1997,...)

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)

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

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)

geoid relative to equilibrium shape [m] seismic velocity anomaly average below 1900 km 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 inertia expect little tpw, if there is dominant, stable degree-2 geoid, with three different moments of inertia larger 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] Reconstruction 100 Ma Without geoid 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 over lower mantle TPW vs. continent rotation over lower mantle smean lowermost mantle Center of mass African Pacific LLSVP

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)

● 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 Ma 47 Ma 0 Ma

Modelling hotspot motion - 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 Ma follows indeed approximately the “blue circle” colors for lowermost mantle seismic anomalies (smean) “True Polar Wander” in mantle reference frame

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 (“unplugged”; 320 — 130 Ma)

TPW unplugged: the challenge Without hotspot tracks, can only use paleomagnetic reference frame: by definition, no polar wander no longitude constraint Plate motions in hotspot paleomagnetic reference frame (from Torsvik et al., 2007, Rev. Geophys., in review)

100 Ma Paleomagnetic reference frame

110 Ma Paleomagnetic reference frame

110 — 100 Ma

TPW unplugged: the tool For each time step, compute, for all continents combined Center of mass Inertia tensor I Angular momentum L mean rotation  L=I   Three components along Earth's spin axis (coherent E-W motion) Equatorial axis; longitude as center of mass(coherent rotation) Equatorial axis; longitude 90  from center of mass(coherent N-S motion)

TPW unplugged — results mean rotation mean N-S motion paleomagnetic global mantle Africa mantle ref. frame

135 Ma

145 Ma

145 — 135 Ma Center of mass continents African LLSVP Pacific LLSVP

110 — 100 Ma Center of mass continents African LLSVP Pacific LLSVP

TPW unplugged — results mean rotation mean N-S motion paleomagnetic global mantle Africa mantle ref. frame

145 Ma

195 Ma

195 — 145 Ma CAMP (200 Ma) Center of mass continents African LLSVP Pacific LLSVP

220 Ma

250 Ma

250 — 220 Ma CAMP (200 Ma) Center of mass continents African LLSVP Pacific LLSVP Center of mass continents African LLSVP Pacific LLSVP

195 — 145 Ma CAMP (200 Ma) Center of mass continents African LLSVP Pacific LLSVP

TPW unplugged — results 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  Ma ~-18  Ma ~-10  Ma near center of mass ~10  Ma of continents small 320 Ma - present 10  -15  W near CAMP 20  -40  E Northward motion Amount time period ~30  Ma ~15  190Ma- present