Structural analysis of shatter cones from the Slate Islands, northern Lake Superior

Abstract

Shatter cones, an unusual type of fracturing produced by intense shock, are found widespread on the Slate Islands in northern Lake Superior. The islands have been interpreted as the central uplift of an eroded meteorite impact crater about 30 km in diameter. The cones are best developed in certain rock types: Keweenawan basalt flows and chilled margins of associated feeder dikes, and Archean diorite and foliated feldspar porphyry. At certain sites cones show an elongate cross section and anomalous orientation caused by foliation-induced elastic anisotropy in the host rock.In general the cones point upward and inward toward the centre of the Slate Islands group. After structural correction of some sites according to paleomagnetic data, there is increased convergence of cone axes to a central focus (the inferred impact point) that occurs at a height of about 1 km above the present land surface.