Presentation is loading. Please wait.

Presentation is loading. Please wait.

Resolving Pangea reconstructions with new paleomagnetic results from Late Permian – Early Triassic igneous rocks in Norway and Argentina. Rob Van der Voo,

Similar presentations


Presentation on theme: "Resolving Pangea reconstructions with new paleomagnetic results from Late Permian – Early Triassic igneous rocks in Norway and Argentina. Rob Van der Voo,"— Presentation transcript:

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. 1

2 2 Different Pangea reconstructions

3 The underlying problem is illustrated here in two ways: 3

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

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: 1.The magnetic field is not purely dipolar, but this has been treated with disdain by geophysicists 2.Paleomagnetic directions from sedimentary rocks may have undergone inclination shallowing 3.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. 5

6 So, what has changed? 1.A tightened and improved North America – Greenland – Europe fit 2.New paleomagnetic results for the Early Triassic from Europe 3.New paleomagnetic results for the Early Triassic from South America (this talk) New paleomagnetic results from Europe for 280 Ma (from Ukraine) to be presented at AGU, San Francisco, December

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

8 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 ? 8 ? ? ? Producing a tighter fit reduces overlap in Pangea A ?

9 9 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.).

10 10 Oslo dikes

11 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 Ma

12 12 Mean age is about 270 Ma

13 Age (Ma) % Ar cumulative Site 36, plag (Age spectrum 1s) Site 36 plag (inverse isochron ± 2.55 Ma Ar/40Ar Ar/40Ar New age date from Bart Hendriks, NGU Trondheim Laboratory ~ 270 Ma

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

15 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 39 Ar/ 40 Ar plateau ages of ~ 240 Ma. The magnetite remanence, on the other hand, may be linked with ages of about 275 Ma. More dating is still in progress. 15 Paleolatitude 27º N Paleolatitude 21.2º N Age ~ 240 Ma Age ~ 270 Ma

16 Mendoza Province, Argentina 16

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

18 Plateau age: ± 2.2 MaPV02 (K-feldspar) 40 Ar/ 39 Ar geochronology 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. 18

19 Puesto Viejo 30 site-mean poles give mean paleopole: 77.8ºS, 328.7ºE, K=26.8, A 95 = 5.2º Paleolatitude 44.1º S, age ~ 240 Ma Paleogeographic position of South America with PV’s paleolatitude 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. 240 Ma 20 Argentina and Norway sites with new paleolatitudes

21 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 after Prognosis 21


Download ppt "Resolving Pangea reconstructions with new paleomagnetic results from Late Permian – Early Triassic igneous rocks in Norway and Argentina. Rob Van der Voo,"

Similar presentations


Ads by Google