Rochechouart impact crater melt breccias record no geomagnetic field reversal

A new high‐precision40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic–Jurassic boundary

“…The 2σ full external uncertainty age obtained for the impact (206.91 ± 0.32 Ma) overlaps with a period of reverse magnetic polarity (white bars), and is thus consistent with the reversed polarity of melt‐bearing rocks from the structure (Eitel et al. ). This figure also demonstrates that the Rochechouart impact is considerably older (5.6 ± 0.4 Ma) than the Triassic–Jurassic boundary.…”

“…These results demonstrate that the impact occurred during a period of reversed magnetic polarity (Eitel et al. ). According to the Geologic Time Scale 2012 (Gradstein et al.…”

“…Thirty samples possessing mostly titanium‐free hematite as the magnetic remanence carrier have solely reverse‐polarity directions, and also pass data selection criteria from Thellier–Thellier experiments (Eitel et al. ). These results demonstrate that the impact occurred during a period of reversed magnetic polarity (Eitel et al.…”

A paleomagnetic and rock magnetic study of the Manicouagan impact structure: Implications for crater formation and geodynamo effects

“…The black circle with black 1σ uncertainty envelope denotes the virtual dipole moment from Manicouagan, and gray circle with gray uncertainty envelope denotes the virtual dipole moment from the Rochechouart crater for 214 ± 8 Ma [Kelley and Spray, 1997] and 201 ± 2 Ma [Schmieder et al, 2010]. the Manicouagan paleointensity result falls close to the average trend-again suggesting that secular variation at Manicouagan has been averaged out in our data set. In contrast, the virtual dipole moment obtained from the Upper Triassic Rochechouart (France) crater likely did not include enough time averaging [Eitel et al, 2014] and lies away from the average global trend (Figure 11b). Nonaveraging of secular variation of the Rochechouart data makes it difficult to use paleomagnetism (direction and intensity) to test the hypothesis for a fragmented, or Shoemaker-Levy 9-type, impact to explain the origin of both craters [Spray et al, 1998], although different magnetic polarities recorded in their impact melts suggest different formation times [Kent, 1998;Eitel et al, 2014].…”

“…The energy released during impact events can exceed that of the largest terrestrial earthquakes by several orders of magnitude. Some of this kinetic energy penetrates deep within the planet, potentially influencing the magnetohydrodynamic regime in the core, which could in turn perturb the dynamo generation process (see Eitel et al [2014] for a more detailed discussion). Evidence linking meteorite impacts to anomalous magnetic field behavior arises from correlating tektite layers with the paleomagnetic records in marine sediments [Glass and Heezen, 1967;Durrani and Khan, 1971;Glass and Zwart, 1979;Hartl and Tauxe, 1996]; however, the correlations are highly debated [Burns, 1990;deMenocal et al, 1990;Schneider and Kent, 1990;Glass et al, 1991;Schneider et al, 1992].…”