Resolving Pangea reconstructions with new paleomagnetic results from Late Permian – Early Triassic igneous rocks in No

1. Resolving Pangea reconstructions • with new paleomagnetic results from • Late Permian – Early Triassic igneous rocks in Norway and Argentina. • Rob Van der Voo, Matthew Domeier, Ada Dominguez Dept. Geological Sciences, University of Michigan • and • Trond Torsvik • Centre for Physics of Geological Processes, • Univ. of Oslo, Norway.

2. Different Pangea reconstructions

3. The underlying problemis illustrated here in two ways:

4. Left: a 3500 km megashear to arriveat Pangea A from which the Atlantic opens. Below: the Gondwana and Laurussia APWP’sin a Pangea-A type fit deviate most for the Late Permian – Early Triassic (~250±10 Ma)

5. Pangea B (or C) must morph into Pangea A, but the necessary Permotriassic megashear (>3000 km!!) is not acceptable to geologists, and so, Pangea B tends to be ignored by them. • Other possible solutions could be that: • The magnetic field is not purely dipolar, but this has been treated with disdain by geophysicists • Paleomagnetic directions from sedimentary rocks may have undergone inclination shallowing • The paleomagnetic results are not very reliable, and ages must be carefully and more reliably determined. • We have therefore started a program to improve these results, with collections of well-dated, structurally unambiguous, igneous rock units.

6. So, what has changed? • A tightened and improved North America – Greenland – Europe fit • New paleomagnetic results for the Early Triassic from Europe • New paleomagnetic results for theEarly Triassic from South America • (this talk) New paleomagnetic results from Europe for 280 Ma (from Ukraine) to be presented at AGU, San Francisco, December 2009.

7. The “Pangea team” Ada Dominguez, UofM undergraduate/graduate Matt Domeier, UofM graduate student Trond Torsvik, U-Oslo, Norway Bart Hendriks, Norw. Geol. Survey Eric Tohver, PhD UofM, now lecturer in Perth, Australia Renata TomezzoliResearch Scientist, Buenos Aires Ken YuanUofM graduate student Haroldo Vizan, Research Scientist, Buenos Aires

8. Producing a tighter fit reduces overlap in Pangea A Reconstructions based on fracture zones and marine magnetic anomalies are relatively (!) clear-cut. But how to account for the crustal extension that occurred before new ocean crust formed? A thinned continental crust between the cratonic interior and the true oceanic crust is often inferred, but how much do we correct for ? ? ? ? ? ?

9. The answer is: through estimating lithospheric thermal expansion, doing a gravity inversion, estimating of original crustal thickness, and comparing all of this with seismic profiles (plot at bottom). The new fit (below left) is much tighter than the Bullard et al. fit (left). From Alvey et al. (GJI, 2009 in review) and Torsvik (pers.com.). Extension from seismic refraction (km) Extension from gravity inversion (km)for the 12 sections shown in the maps.

10. Oslo dikes

11. From: “The Oslo Rift: new palaeomagnetic and 40Ar/39Ar age Constraints” T. H. Torsvik, E. A. Eide J. G. Meert, M. A. Smethurst and H. J. Walderhaug Geophys. J. Int. (1998) v. 135, 1045–1059 Mean age is about 240.5 Ma

12. Mean age is about 270 Ma

13. Site 36, plag (Age spectrum 1s) 320 310 300 290 280 270 260 250 New age date fromBart Hendriks, NGUTrondheim Laboratory Age(Ma) 0 20 40 60 80 100 % Ar cumulative Site 36 plag (inverse isochron 270.73 ± 2.55 Ma 0.004 0.003 0.002 0.001 0.000 36Ar/40Ar ~ 270 Ma 0.00 0.01 0.02 0.03 0.04 39Ar/40Ar

14. For the paleomagnetists in the audience – Zijderveld plots of the Oslo dikes Fe-sulfide only Magnetite and Fe-sulfide Magnetite only Age ~ 270 Ma Age ~ 240 ma

15. The sulfide magnetization has a slightly steeper upward inclination, and is thereforeslightly younger. We interpret it as an Early Triassic remagnetization of regional extent,which likely was also responsible forthe 39Ar/40Ar plateau ages of ~ 240 Ma. The magnetite remanence, on the other hand,may be linked with agesof about 275 Ma. More dating is still in progress. Paleolatitude 27º N Paleolatitude 21.2º N Age ~ 240 Ma Age ~ 270 Ma

16. Mendoza Province, Argentina

17. Untilted In situ Puesto Viejo site mean directions (and bootstrap fold test results) Positive fold test

18. 40Ar/39Ar geochronology PV02 (K-feldspar) Plateau age: 239.8± 2.2 Ma The PV thus has an age of ~240 Ma, as the above plot indicates. Stratigraphic (and some paleontologic) evidence also supports this Early Triassic age, as does older (but preliminary) geochronologic work.

19. Puesto Viejo 30 site-mean poles give mean paleopole: 77.8ºS, 328.7ºE, K=26.8, A95 = 5.2º Paleolatitude 44.1º S, age ~ 240 Ma Paleogeographic position of South America with PV’s paleolatitude. 240 Ma

20. Conclusions: Pangea A fit now allowed, Pangea B no longer necessary This reconstruction is similar to that for 240 Ma, albeit showing some minor northward drift of Pangea. Argentina and Norwaysites with new paleolatitudes 240 Ma

21. Prognosis after The large discrepancies between the Laurussia and Gondwana APWP’s (left) are disappearing fast with a new EUR-NAM fit, well-dated results from igneous rocks, and inclination corrections of results from sedimentary rocks (Torsvik et al). before