Shatter cones developed in an amygdaloidal Keweenawan basalt flow within the Slate Islands Impact Structure

Shatter cones developed in an amygdaloidal Keweenawan basalt flow within the Slate Islands Impact Structure


One powerful way we can learn about the ancient history of Earth and other bodies in the solar system is through study of the magnetization of planetary materials. Earth’s internally-generated magnetic field is a defining characteristic of our planet that shields us from dangerous solar particles. When rocks form, they can record both the direction and strength of magnetic fields as a magnetization that can be preserved for potentially billions of years. Ongoing research in the Earth and planetary sciences seeks to understand whether the magnetization of extraterrestrial materials (including lunar rocks and meteorites) are the result of long-lived fields internally-generated by planetary bodies, or transient magnetic fields generated by impacts. One of the processes advanced as an explanation of sometimes enigmatic magnetizations observed on planetary surfaces, as well as in lunar and meteorite samples, is known as shock remanent magnetization. Meteorite impacts lead to high pressures and the short duration shock can lead to the acquisition of new magnetization in the presence of a magnetic field. This context provides strong motivation to understand shock magnetization within impact basins on Earth both in relation to its acquisition from Earth’s internally-generated field as well as potential impact-generated fields.

The most suggestive evidence for the presence of naturally occurring shock remanent magnetization (SRM) on Earth is from the ca. 30 km diameter Slate Islands impact structure in northern Lake Superior where there is a pervasive magnetic overprint. Advances in the understanding of SRM acquisition, in the analytical and theoretical framework of thermochronometry and of the geology of the Slate Islands impact structure and shock pressures recorded therein, make this an excellent time to test the hypothesis that the secondary magnetization within the Slate Islands is an SRM. We also seek to evaluate whether the putative shock-induced remanence is consistent with resulting from the Earth’s geodynamo field or if it may record a transient impact-induced field. Stepped heating thermochronometry combined with detailed characterization of natural remanent magnetization and rock magnetic experiments on samples from high to low shock levels across the structure will help us determine if the magnetic overprint is the result of shock, or whether it could have originated as a thermal overprint from impact-related heating.
WCO_Slate-2287

Luke Fairchild, Sonia Tikoo and Nick Swanson-Hysell standing in front of a large composite shatter cone developed in pseudo-autochonous target rocks of the Slate Islands Impact Structure.


This material is based upon work supported by the National Science Foundation Division of Earth Sciences under Award No. 1316395. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.