RELATIONS BETWEEN SILL INTRUSIONS AND BEDDING-PARALLEL EXTENSIONAL SHEAR ZONES IN THE MIDCONTINENT RIFT SYSTEM OF THE LAKE-SUPERIOR REGION

The Mid-continent Rift System is a continuous U-shaped feature extending from Kansas north through the Lake Superior region and then south through Michigan. The Logan Sills on the northwest side of the lake were intruded during the rifting stage (1109 Ma). Deformation features in the country rock adjacent to the Logan sills indicate two episodes of shear strain. The first consists of minor thrusts and mesoscopic shear bands, both suggesting top-to-the-south (southwest) movement parallel to the bedding. The second rotates the previously formed shear bands in the opposite sense, top-to-the-north (northeast) and is localized at the basal contact between the sills and the country rock. We attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding-parallel shear zones that are the surface expression of a south-dipping detachment plane. The detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting. In this scenario the first episode of shear strain might be related to bedding-parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south. The polarity of both events is supported by small scale kinematic indicators such as veins, fiber growths and foliation,