evidence for coeval late triassic terrestrial impacts from the rochechouart france meteorite crater

8/14/2019 Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart France meteorite crater

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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: [email protected]

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

Alexandrescu, M., S. Gilder, V. Courtillot, J. L. Le Moul, and D. Gilbert (

Looking back on a half century of repeat magnetic measurements in Franc

AGU, 80 , 25-26.

Besse, J., and V. Courtillot (2002), Apparent and true polar wander and the geom

the geomagnetic field over the last 200 Myr, J. Geophys

doi:10.1029/2000JB000050.

Bischoff, L., and W. Oskierski (1987), Fractures, pseudotachylite, veins and breccia


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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

216.

Henry, B., D. Jordanova, N. Jordanova, M. Le Goff (2005), Transformations of ma

mineralogy in rocks revealed by difference of hysteresis loops measure

stepwise heating: theory and case studies, Geophys. J. Int., 162 , 64-78.

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.

McFadden, P. L., and M. W. McElhinny (1990), Classification of the reversal

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

114, Vieweg, Braunschweig/Weisbaden, Germany.

Reimold, W. U., W. Oskierski, and J. Huth (1987b), The pseudotachylite

Champagnac in the Rochechouart meteorite crater, France, J. Geoph

E737-E748.

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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evidence for coeval late triassic terrestrial impacts from the rochechouart france meteorite crater

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