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The Great Pangea Controversy

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Premiseswe saw earlier that thetime-averaged geomagnetic field

is a geocentric axial dipole (GAD)field.

A GAD implies that:

1) Tan(Inc) = 2*Tan(lat), or

Inc = invTan(2*Tan(lat))i.e., GAD field inclination

depends on latitude.

2) GAD declination = 0

everywhere.

AT ANY POINT ON EARTH,

THERE IS A DEC=0/INC PAIR

THAT DEFINES A SAME

MAGNETIC NORTH POLE

THAT COINCIDES WITH THE GEOGRAPHIC NORTH POLE.

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Example:

Pleistocene (ie, recent)

Lavas from Japan:

The GAD north magneticpole coincides with the

geographic north pole

(rotation axis).

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but plates move with respect to the geographic north pole

(rotation axis) due to plate tectonics.

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Due to plates motion, GAD north paleomagnetic poles calculated

from the magnetization (dec/inc pairs) ofancient rocks

do not correspondto the geographic north pole (rotation axis)

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Exampe from the

mid-Cretaceous ofNorth America.

GAD north magnetic

poles (gray circles) calculated

from the magnetization(dec/inc pair) of

mid-Cretaceous rocks at

different localities (1-4) are

systematically displaced

from the geographicnorth pole (rotation axis)

because of continental

drift of North America

since the mid-Cretaceous

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In this example, wealso notice that the different

localities (1-4) yielded

statistically indistinguishable

GAD north magnetic poles,

which implies tectoniccoherence of the investigated

rock units (same rigid plate; no

internal rotation/motion).

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By rotating the GAD north magnetic pole of a given plate for a given

time on the geographic north pole (rotation axis), we can reconstruct

the paleogeographic position of that plate at that time by considering

that the plate is attached to its north magnetic pole.

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By studying the magnetization of rocks of different ages belonging to

the same plate, we can reconstruct the sequence of GAD north magnetic

poles of that plate -> apparent polar wander path (APWP)

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and therefore, we can reconstruct the geologic history of motion

of that plate (with respect to the rotation axis).

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Application of these

notions to the configuration ofPangea

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Incastro geograficoIncastro geografico

dei continentidei continentiuna delleuna delle proveprove

della Deriva deidella Deriva dei

ContinentiContinentidi Wegenerdi Wegener

WegenerWegenerproposeproposecheche ii

continenti erano riuniti nelcontinenti erano riuniti nel

supercontinentesupercontinentePangeaPangeaAA

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ma Pangea ebbe sempre una configurazione stabile di tipo

wegeneriano (Pangea A) durante il suo periodo di esistenza dal

Carbonifero al Giurassico (~100 milioni di anni)

oppure Pangea ebbe configurazioni diverse, e non fu un

supercontinente stabile, come ha proposto Irving (1977)?

Ted Irving

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Studio paleomagnetico di ignimbriti permiane sudalpine

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Studio paleomagnetico di ignimbriti permiane sudalpine

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Ignimbriti di et radiometrica nota, ca. 280 Ma.

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Poli paleomagnetici da ignimbriti sudalpine coincidenti con poli

paleomagnetici coevi da nord Africa.

-> coerenza tettonica tra Adria (proto-Italia) e Africa

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Possiamo utilizzare i poli paleomagnetici medi di nord Africa e Adria

per ricostruire la posizione paleogeograficadi Africa (+Sud

America) nel Permiano Inferiore.

Possiamo fare la stessa cosa per Europa usando dati paleomagneticidalla letteratura.

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ma se posizioniamo i continenti assumendo Pangea A,

otteniamo una sovrapposizione crostale di alcune centinaia di km.

come eliminarla?

assumendo una

configurazione

alternativa di Pangea

che comunque soddisfi

i limiti imposti dai dati

paleomagnetici:

-> Pangea B

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In summary, from our paleomagnetic analysis we infer the

existence of Pangea B in the Early Permian (~280 Ma) .

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evolving into Wegenerian Pangea A in the Late Permian

(~265 Ma)..

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still Pangea A in the Early Jurassic (198176 Ma)

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until mid-Jurassic break-up and continents dispersal

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that eventually led to the present-day geographic configuration.

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but could Pangea B be an artifact of the data? For example,

some sediments are affected by paleomagnetic inclination error, that is,

their magnetization may underestimate paleolatitudes, thus artificially

creating the crustal misfit at the basis of the Pangea B model.

NObecause in our analysis we used paleomagnetic data fromigneous rocks, which are typically not affected by (sedimentary)

Inclination shallowing.

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Torsvik and Van der Voo claimed in a number of papers that Pangea B

is an artifact produced by an undetected octupole contamination

of the GAD field. In other words: in our reconstructions, weassumed a GAD field, and calculated paleolatitudes accordingly

(tanI=2tanLat); but the field may not have been a simple GAD field

during the Permian. It may have had a more complicated geometry

(80% dipolar+20% octupolar). Therefore, our paleomagnetic estimates

of paleolatitudes (in assumption of a GAD field) are wrong.

(very complicated businessjust believe in what I say---)

BUT NO, WAIT!

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This is a tribute to Edward Irving,

the father of Pangea B (Irving, 1977).

He first recognized the crustal misfit,

and proposed Pangea B.We are just on his side (against all odds)