Testing the Shock Remanent Magnetization Hypothesis at the Slate Islands Impact Structure, Canada

Abstract:

The ubiquity of cratering events in the solar system motivates understanding how the magnetizations of planetary surfaces are affected by hypervelocity impacts. Passing shock waves may impart target rocks with shock remanent magnetization (SRM) which can, in principle, be preserved if a region does not experience substantial impact-related heating. While SRM has been proposed as an explanation for enigmatic magnetizations observed at impact craters and in extraterrestrial samples, SRM has not yet been conclusively identified in natural samples. At present, the most suggestive evidence for naturally occurring SRM exists at the ~450 Ma Slate Islands Impact Structure, Canada. The islands represent the ~10 km diameter central uplift of a ~30 km diameter complex crater. Target rocks, which consist of Archean and Paleoproterozoic dikes and metamorphic rocks as well as ~1.1 Ga dikes and lava flows, contain a pervasive impact-related secondary magnetization component which is not present in coeval rocks of corresponding lithologies outside the impact crater (Halls 1975, 1979).

Our paleomagnetic study aims to determine whether the Slate Islands overprint is an SRM. Samples were collected from 18 igneous dikes and 17 lava flows across the impact structure from regions which experienced variable (2-25 GPa) shock pressures (Dressler et al., 1998). Alternating field (AF) and thermal demagnetization results indicate that the secondary component persists to AF levels reaching several tens of mT and unblocking temperatures approaching ~250-525°C. At all studied sites, grains at higher AF levels and unblocking temperatures retain pre-impact magnetizations. Secondary magnetization blocked within the lower coercivity and blocking temperature fractions of samples is consistent with an origin as either SRM or thermoviscous remanence (TVRM) resulting from impact-related heating. Viscous remanence acquired at Earth surface temperatures is precluded by the component’s high unblocking temperature and distinct direction from the present local field. Comparison of natural remanent magnetization with laboratory-induced pressure and thermal remanences paired with thermochronometric and geologic constraints is being used to differentiate between the overprints originating as SRM or TVRM.