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.

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*Different Pangea reconstructions

The underlying problem is illustrated here in twoways: *

Left: a 3500 km megashear to arrive at Pangea A from which the Atlantic opens.

Below: the Gondwana and Laurussia APWPs in a Pangea-A type fit deviate most for the Late Permian Early Triassic (~25010 Ma)*

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 geophysicistsPaleomagnetic directions from sedimentary rocks may have undergone inclination shallowingThe 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. *

New paleomagnetic results from Europe for 280 Ma (from Ukraine) to be presented at AGU, San Francisco, December 2009. *

Bart Hendriks, Norw. Geol. SurveyTrond Torsvik, U-Oslo, NorwayAda Dominguez, UofM undergraduate/graduateEric Tohver, PhD UofM, now lecturer in Perth,AustraliaRenata Tomezzoli Research Scientist,Buenos AiresHaroldo Vizan, Research Scientist, Buenos AiresMatt Domeier, UofM graduate studentThe Pangea teamKen Yuan UofM graduate student*

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 ? * ? ? ??Producing a tighter fit reduces overlap in Pangea A?

*Extension from gravity inversion (km) for the 12 sections shown in the maps. Extension from seismic refraction (km)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.).

*Oslo dikes

*From: The Oslo Rift: new palaeomagnetic and 40Ar/39Ar age Constraints T. H. Torsvik, E. A. EideJ. G. Meert, M. A. Smethurst and H. J.Walderhaug

Geophys. J. Int. (1998) v. 135, 10451059

Mean age is about 240.5 Ma

*Mean age is about 270 Ma

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250Age (Ma)0 20 40 60 80 100% Ar cumulativeSite 36, plag (Age spectrum 1s)Site 36 plag (inverse isochron 270.73 2.55 Ma0.004

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0.00036Ar/40Ar0.00 0.01 0.02 0.03 0.04 39Ar/40ArNew age date from Bart Hendriks, NGU Trondheim Laboratory

~ 270 Ma

Magnetite only Age ~ 270 Ma Age ~ 240 maFe-sulfide onlyMagnetite and Fe-sulfideFor the paleomagnetists in the audience Zijderveld plots of the Oslo dikes*

The sulfide magnetization has a slightly steeper upward inclination, and is therefore slightly younger. We interpret it as an Early Triassic remagnetization of regional extent, which likely was also responsible for the 39Ar/40Ar plateau ages of ~ 240 Ma.

The magnetiteremanence, on the other hand, may be linkedwith ages of about 275 Ma.More dating is still in progress.* Paleolatitude 27 N Paleolatitude 21.2 N Age ~ 240 Ma Age ~ 270 Ma

Mendoza Province, Argentina*

Puesto Viejo site mean directions(and bootstrap fold test results)Positive fold test*

Plateau age: 239.8 2.2 MaPV02 (K-feldspar)40Ar/39Ar geochronologyThe 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.*

Puesto Viejo 30 site-mean poles give mean paleopole: 77.8S, 328.7E, K=26.8, A95 = 5.2Paleolatitude 44.1 S, age ~ 240 Ma Paleogeographic position of South America with PVs paleolatitude. *240 Ma

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

The large discrepancies between the Laurussia and Gondwana APWPs (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). beforeafterPrognosis*