Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater

High temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 $\pm$ 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 $\pm$ 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth

Origin and implications of two Verwey transitions in the basement rocks of the Vredefort meteorite crater, South Africa

International audienceTwo populations of magnetite exist in the shocked basement rocks of the Vredefort meteorite impact crater: one associated with original crustal genesis and metamorphism around 3.0 Ga, and the other related to the impact itself at 2.02 Ga. Pre-impact magnetite is mostly micron to millimeter in size, lying within the multidomain to pseudo-single domain range. The second population of magnetite is less than 10 μm in size and formed within the interstices of planar deformation features or within the reaction rims of biotite, both of which were created during impact. Our study shows that each of these populations possesses specific Verwey transition temperatures: one around 124 K associated with pre-impact magnetite and the other around 102 K associated with impact-related magnetite. The high temperature Verwey transition is attributed to stoichiometric magnetite while the low temperature Verwey transition to non-stoichiometric magnetite. Pre-impact rocks containing both Verwey transitions are ubiquitous throughout the crater. Pseudotachylites formed during impact have a single Verwey transition spanning temperatures from 94 to 111 K. Heating the basement rocks above not, vert, similar 550–600 °C for 3 min or above not, vert, similar 500 °C for 1 h irreversibly modifies the 124 K Verwey transition by shifting it to lower temperatures. Based on these findings, it is possible that no wholesale heating of the crater occurred above 550–600 °C for 3 min or above 500 °C for 1 h during or since the time of impact, although some places of more localized heating are identified. An unresolved problem remains to reconcile these data with temperatures thought to exist in the crust during and after impact

Magnetic imaging of the Vredefort impact crater, South Africa

International audienceWhile most impact craters are characterised by negative magnetic anomalies over their central regions, aeromagnetic surveys over the Vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre. We performed ground magnetic surveys across a portion of a prominent negative magnetic anomaly that extends in a broad semicircular belt about half way into the basement floor of the crater. Magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths. The longest wavelength negative anomaly coincides well with aeromagnetic data. We find that this feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor. The transition zone is analogous to the Conrad discontinuity, observed at depths of about 20 km elsewhere in the Kaapvaal craton. Petrographic studies show a marked increase in the intensity of the impact-related thermal and shock metamorphism at this transition, which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact. We therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event. A numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact. On the other hand, negative anomalies occurring over smaller (100 to 20 m) wavelengths often do not coincide with the surface geology. These features cannot be modeled using the same criteria as that for the long wavelength anomaly

New Tertiary paleomagnetic poles from Mongolia and Siberia at 40, 30, 20, and 13 Ma: Clues on the inclination shallowing problem in central Asia

International audienceWe report results of a paleomagnetic study of 490 cores from 59 sites, corresponding to 52 distinct basaltic flows from Mongolia and Siberia: Khaton Sudal (39.4 Ma, 44.5°N/101.4°E), Taatsyn Gol (1, 31.5 Ma, 45.4°N/101.3°E; 2, 28.0 Ma, 45.5°N/101.1°E), Ust Bokson (19.9 Ma, 52.1°N/100.3°E), and Taatsyn Gol (3, 12.7 Ma, 45.5°N/101.0°E). Stepwise thermal and alternating field demagnetizations isolated a stable high-temperature component (HTC) of magnetization in most specimens, which we interpret as the primary magnetization of these basaltic lava flows. The four corresponding paleopoles appear consistent with coeval paleopoles from other Asian effusive formations. However, except for the 12.7 Ma paleopole, the paleopoles are systematically far-sided from the European apparent polar wander path (APWP) with respect to site locations, corresponding to anomalously shallow inclinations in Tertiary Asian effusive formations. In the hypothesis of a dipolar magnetic field in the Tertiary, this indicates a ∼1000–1500 km position of the Siberia craton and Amuria block farther south than expected at 40 and 30 Ma. Tectonically, this interpretation implies decoupling and relative rotations between the western and eastern parts of Eurasia between the Cretaceous and Present. We show that if Siberia were located more to the south, the ∼15°–20° paleolatitude anomaly generally observed in sedimentary formations from central Asia reduces to a more reasonable average value of ∼7°, which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of Asian blocks under the effect of ongoing penetration of India into Eurasia in the Tertiary

Lightning remagnetization of the Vredefort impact crater: No evidence for impact-generated magnetic fields

The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact-generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact-generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single-domain magnetite found along shock-generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact-generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact-generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos.National Science Foundation (U.S.) (Grant EAR-0810244