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Evidence for coeval Late Triassic terrestrial impacts from theRochechouart (France) meteorite crater
Laurent Carporzen * and Stuart A. Gilder
Institut de Physique du Globe de Paris, Equipe de Palomagntisme, 4 place Jussieu
75252 Paris Cedex 05 France *Corresponding author: lcarpo@ipgp.jussieu.fr
Abstract
High temperature impact melt breccias from the Rochechouart (France) meteorite
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meteorite, like that observed on Jupiter in 1994 [ Orton et al. , 1995]. Evidenc
the Spray et al. [1998] hypothesis for a fragmented, or Shoemaker-Levy 9-type
relied mostly on radiometric dating of the Manicouagan (Canada) and Roche
(France) craters at 2141 Ma (U-Pb [ Hodych and Dunning , 1992]) and
(40Ar/ 39Ar [ Kelley and Spray , 1997]), respectively. Late Triassic ages for the ot
craters were loosely constrained by biostratigraphy. Controversy arose when
pointed out that the Manicouagan melt rocks recorded only normal magnetic po
[ Larochelle and Currie , 1967; Robertson , 1967], while those of Rochechouar
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poles (VGPs) from Rochechouart are indistinguishable with those from Manicouag
Late Triassic reference frame.
2. Paleomagnetic results
We sampled 52 cores at 12 sites using a gas-powered drill. The core
oriented with magnetic and sun compasses, the latter to correct for local decl
anomalies. Four sites were drilled in impact melt breccias, while eight sites com
basement rocks of variable lithologies lying within a radius of 16 km from the
estimated center at 45.825N, 0.785E, which lies near the town of Roche
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(antipodal magnetization components in a single sample, etc.) to support a self-re
mechanism to explain the polarity differences. Thermal demagnetization of two
five samples from the Babaudus-type suevites at Site 7, located ca. 50 m from Site 6
a component with easterly declinations and shallow inclinations that decays l
toward the origin. Three samples subjected to AF demagnetization have two ma
components: one resembling the two others from Site 7 at fields below 22 m
another isolated above 22 mT, with the same normal polarity direction observed in
(Fig. 1d). One difference between sites 6 and 7 is that the natural remanent magnet
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being statistically antipodal within 95% confidence limits (Class B [ M
McElhinny , 1990]). We interpret these directions as recording a magnetic field
during cooling (e.g., a primary, thermal remanent magnetization). However,
[1974 and 1977b] mapped the Rochechouart impact melt sheet as covering an area
km with a medium thickness of 12 m. He estimated that the original thickness was
over 280 km, which corresponds to a characteristic cooling time of ~400 years [
et al. , 1978]. Although this seems too rapid to have recorded a field reversal (rever
thought to last at least a few thousand years, e.g., Quidelleur et al. [20
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yet no evidence for a phase with a Curie temperature around 350C, such as pyrr
Moreover, hysteresis loops measured at 200C before and after heating to 400C [
et al. , 2005] are identical, confirming the absence of a magnetic phase with a Curi
between 200C and 400C. Our preferred explanation is that the Champagnac bas
was heated to 350C during impact, which remagnetized the magnetite grains cap
acquiring remanences up to 350C. This would explain why nearly all of the rem
unblocks in a very narrow temperature range around 350C, despite the absenc
magnetic mineral with this characteristic Curie point. If true, this would constr
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from the Chabanais quarry (Site 11) has very stable magnetizations carried mai
magnetite whose directions are reasonably well clustered (Figs. 1i and 2a, Ta
Finally, granite sampled along Highway N141 (Site 12), northeast of the crater, pos
unstable magnetizations with weak intensities (
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directions and not on principal component analyses derived from s
demagnetization. However, Pohl and Soffel [1971] performed magnetic vis
and they compared the mean direction based on blanket AF demagnetization w
NRM directions, finding no compelling reason to reject the NRM directions. We c
their analysis in that the mean direction based on the NRMs from our study (D= 35
43.9, N= 22, k= 46.7, 95= 4.6, excluding Site 7) is virtually identical to the o
on principal component analysis.
So either a systematic bias exists between the two laboratories or
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Tauxe , 2005], the corrected pole lies within 95% confidence limits of our Roche
pole. Thus, we think the differences between the virtual paleomagnetic poles of
Soffel [1971] and our own is that the former recorded instantaneous fields wher
latter are representative of a time-averaged field.
Magnetic data from the basement rocks may shed light on the thermal ef
the impact at Rochechouart. That all sites yielding stable magnetic remanence
reversed polarities could suggest that they were thermally remagnetized duri
impact. Alternatively, because the basement rocks have radiometric ages ranging fr
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km from the craters center and has suevites directly covering the basement
Finally, the serpentinite pole from the Merlis quarry (Site 4) lies near the 240 Ma p
the Eurasian APWP [ Van der Voo , 1993; Torsvik et al. , 2001], far from the p
suevites. Magnetic remanences from this site (>10 km from the craters center) ar
likely pre-impact.
A final point to address is whether tilting during post-impact rebound
Rochechouart crater reoriented the magnetic directions of the suevites and ba
rocks. Observations of most complex craters show that pre-impact rocks dip awa
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mean pole of = 58.9N, = 90.3E (A 95= 5.8) for the Manicouagan crater.
shows a paleogeographic reconstruction of Eurasia and North America at 210 Ma
the parameters from Van der Voo [1993] and Besse and Courtillot [2
reference frame, the Rochechouart and Manicouagan poles are indistinguishable
confidence limits. Figure 3b shows an Euler rotation at 210 Ma of Eurasia relativ
fixed North America following Torsvik et al. [2001]. In this reference
Rochechouart and Manicouagan poles are again indistinguishable at 95% conf
limits. This finding, together with the dual magnetic polarities now recogn
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References
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Grieve, R. A. F., and A. M. Therriault (2004), Observations at terrestrial
structures: Their utility in constraining crater formation, Meteor. Planet. S
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Hodych, J. P., and G. R. Dunning (1992), Did the Manicouagan impact trigger e
Triassic mass extinction?, Geology, 20 , 51-54.
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Larochelle, A., and K. L. Currie (1967), Paleomagnetic study of igneous rocks fr
Manicouagan structure, Quebec, J. Geophys. Res., 72 , 4163-4169.
Mazur, M. J., R. R. Stewart, and A. R. Hildebrand (2000), The seismic signa
meteorite impact craters, Canadian Society of Exploration Geophysicists
Recorder , 10-16.
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palaeomagnetism, Geophys. J. Int., 103 , 725-729.
McFadden, P. L., and F. J. Lowes (1981), The discrimination of mean directions
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Reimold, W. U., and W. Oskierski (1987a), The Rb-Sr age of the Rochechouart I
Structure, France, and geochemical constraints on impact melt-target rock-me
compositions, in Research in Terrestrial Impact Structure , edited by J. P
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Robertson, W. A. (1967), Manicouagan, Quebec, paleomagnetic results,
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Site Name Lithology n/N/C Dec Inc k 95 P lat P long dp dm1&2 Champagnac quarry Gran., leuco. & pseudo. 7/9/8 230.3 -20.8 63.6 7.6 34.8 113.7 3.3 6.2 45.85
3 Champonger quarry Metamorphic & granite 0/7/7 454 Merlis quarry Serpentinite 5/5/4 203.5 -18.5 243.0 4.9 48.5 144.4 2.1 4.0 45.5 Montoume quarry Suevite 12/12/10 219.2 -43.5 113.7 4.1 52.7 110.4 2.5 4.0 45.76 Fonceverane forest Suevite 6/6/4 33.1 40.8 174.4 5.1 457 Fonceverane forest Suevite 3/5/3 29.0 46.4 44.5 18.7 45
6&7 Fonceverane forest Suevite 9/11/7 31.8 42.7 93.3 6.9 57.0 119.5 4.1 6.6 45.8
8 Fonceverane Suevite 4/4/3 215.9 -41.8 177.8 6.9 53.9 115.4 5.2 8.5 45.9 Fonceverane Metamorphic 0/3/3 10 D160 road Granite 3/3/3 345.8 62.1 18.4 29.6 411 Chabanais quarry Granite 4/4/3 228.0 -6.8 145.8 7.6 30.6 121.3 2.9 5.8 45.812 N141 highway Granite 0/3/3
Table 1. Paleomagnetic results for the Rochechouart impact melt breccias and ba
rocks.
Abbreviations/notations are: Gran., granodiorite; leuco., leucogranite; p
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and Currie [1967] and Robertson [1967]). All VGPs are projected in thhemisphere. In light grey is the Eurasian apparent polar wander path (APWP) fro
der Voo [1993] and Besse and Courtillot [2002], in dark grey is the Eurasian A
Torsvik et al. [2001]numbers next to the poles are time-window ages in mil
years. The four VGPs in light grey are from Pohl and Soffel [1971]. The m
of the suevites increases from the Chassenon-type (square), to the Montoum(triangle) and finally to the Babaudus-type (circle) [ Lambert , 1974; 1977b; C
al. , 1996]. The trajectory of these VGPs possibly record geomagnetic secular v
~214 million years ago during progressive cooling of the impact melt breccias.
Figure 3. a) Paleogeographic reconstruction at 210 Ma using the parameters from
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