The Slate Islands meteorite impact site: a study of Shock Remanent Magnetization

Summary. A detailed palaeomagnetic study of the Slate Islands meteorite impact site, northern Lake Superior, has successfully isolated a secondary Component of remanence which on the basis of field relationships, models of impact crater formation and laboratory investigation appears to be a Shock Remanent Magnetization (SRM).
Evidence for this hitherto rare type of naturally formed remanence is that:
(a) the component is confined to rocks having experienced intense shock (50 to more than 100 kb) on the basis of shatter-cone development and the presence of planar features in quartz and felspar
(b) the component appears to have been acquired virtually instantaneously in terms of normal geological processes as a combined palaeomagnetic—shatter cone analysis suggests that it was formed within a time interval (perhaps several minutes in duration) between the moment of impact and the formation of the central uplift
(c) the extent of magnetic resetting decreases with increasing distance from the centre of the impact site, and
(d) the degree of resetting increases with the abundance of low coercivity magnetic grains as given by H_cr , a relationship found by other investigators for experimentally-produced SRM.
Intrusive breccias found on the islands yield a secondary component of TRM or TRCM origin which was frozen in after central uplift formation. Increased convergence of shatter cone axes to a central focus after rotation of host rock blocks according to palaeomagnetic data, suggests that the SRM had the same direction as that found in the breccias and hence was aligned with the ambient field at the time of impact.