The Great Pangea Controversy

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Regina Quaranta
6 anni fa
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1 The Great Pangea Controversy

2 Premises we saw earlier that the time-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.

3 Example: Pleistocene (ie, recent) Lavas from Japan: The GAD north magnetic pole coincides with the geographic north pole (rotation axis).

4 but plates move with respect to the geographic north pole (rotation axis) due to plate tectonics.

5 Due to plates motion, GAD north paleomagnetic poles calculated from the magnetization (dec/inc pairs) of ancient rocks do not correspond to the geographic north pole (rotation axis)

6 Exampe from the mid-cretaceous of North 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 geographic north pole (rotation axis) because of continental drift of North America since the mid-cretaceous

7 In this example, we also notice that the different localities (1-4) yielded statistically indistinguishable GAD north magnetic poles, which implies tectonic coherence of the investigated rock units (same rigid plate; no internal rotation/motion).

8 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.

9 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)

10 and therefore, we can reconstruct the geologic history of motion of that plate (with respect to the rotation axis).

11 Application of these notions to the configuration of Pangea

12 Incastro geografico dei continenti una delle prove della Deriva dei Continenti di Wegener Wegener propose che i continenti erano riuniti nel supercontinente Pangea A

ma Pangea ebbe sempre una configurazione stabile di tipo wegeneriano (Pangea A) durante il suo periodo di esistenza dal Carbonifero al Giurassico

13 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) Ted Irving oppure Pangea ebbe configurazioni diverse, e non fu un supercontinente stabile, come ha proposto Irving (1977)?

14 Studio paleomagnetico di ignimbriti permiane sudalpine

15 Studio paleomagnetico di ignimbriti permiane sudalpine

16 Ignimbriti di età radiometrica nota, ca. 280 Ma.

17 Poli paleomagnetici da ignimbriti sudalpine coincidenti con poli paleomagnetici coevi da nord Africa. -> coerenza tettonica tra Adria (proto-italia) e Africa

18 Possiamo utilizzare i poli paleomagnetici medi di nord Africa e Adria per ricostruire la posizione paleogeografica di Africa (+Sud America) nel Permiano Inferiore. Possiamo fare la stessa cosa per Europa usando dati paleomagnetici dalla letteratura.

19 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

20 In summary, from our paleomagnetic analysis we infer the existence of Pangea B in the Early Permian (~280 Ma).

21 evolving into Wegenerian Pangea A in the Late Permian (~265 Ma)..

22 still Pangea A in the Early Jurassic ( Ma)

23 until mid-jurassic break-up and continents dispersal

24 that eventually led to the present-day geographic configuration.

25 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. NO because in our analysis we used paleomagnetic data from igneous rocks, which are typically not affected by (sedimentary) Inclination shallowing.

26 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, we assumed 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. BUT NO, WAIT! ( very complicated business just believe in what I say---)

29 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 )

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