Transcript 240 Ma Age - Geological Society of America

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
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The underlying problem
is illustrated here in two
ways:
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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)
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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.
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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 2009.
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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
Haroldo Vizan, Research
Scientist, Buenos Aires
Renata Tomezzoli
Research Scientist,
Buenos Aires
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Ken Yuan
UofM graduate student
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 ?
? ?
?
?
?
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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.).
Extension from gravity inversion (km)
for the 12 sections shown in the maps.
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Oslo dikes
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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
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Mean age is about 270 Ma
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Site 36, plag (Age spectrum 1s)
320
New age date from
Bart Hendriks, NGU
Trondheim Laboratory
310
300
Age 290
(Ma)
280
270
260
250
0
20
40
60
80
% Ar cumulative
100
Site 36 plag (inverse isochron 270.73 ± 2.55 Ma
0.004
0.003
36Ar/40Ar
~ 270 Ma
0.002
0.001
0.000
0.00
0.01
0.02
39Ar/40Ar
0.03
0.04
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For the paleomagnetists in the audience –
Zijderveld plots of the Oslo dikes
Magnetite only
Age ~ 270 Ma
Fe-sulfide only
Age ~ 240 ma
Magnetite and
Fe-sulfide
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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 magnetite
remanence, on
the other hand,
may be linked
with ages
of about
275 Ma.
More dating
is still in progress.
Paleolatitude 27º N
Age ~ 240 Ma
Paleolatitude 21.2º N
Age ~ 270 Ma
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Mendoza Province, Argentina
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Puesto Viejo site
mean directions
(and bootstrap fold test results)
Untilted
In situ
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40Ar/39Ar
PV02
geochronology
(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.
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Puesto Viejo
30 site-mean poles give mean paleopole:
77.8ºS, 328.7ºE, K=26.8, A95 = 5.2º
Paleogeographic
position of South
America with PV’s
paleolatitude.
Paleolatitude 44.1º S, age ~ 240 Ma
240 Ma
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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 Norway
sites with new paleolatitudes
240 Ma
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after
before
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).
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