Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota

Abstract

Two contrasting zones of deformation have been previously recognized in the Proterozoic Thomson Formation of east-central Minnesota. The northern zone consists of open, upright E-W-trending folds with a well-developed steeply south-dipping axial-planar cleavage. The southern zone is marked by a pervasive subhorizontal cleavage (S 1), crenulated by a second cleavage that is vertical to steeply south dipping (S 2). Paleomagnetic results and anisotropy of anhysteretic remanent magnetization (AARM) in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction: neither NRM nor AARM were much affected by tectonic deformation. By contrast, the low-field magnetic susceptibility, which is dominated by paramagnetic minerals (particularly chlorite), reflects the tectonic fabric of the rock. Anisotropy of magnetic susceptibility (AMS) is characterized by minima normal to the cleavage in the northern zone and to the S 1 cleavage in the southern zone. Thus, the S 2 deformation did not significantly affect the AMS fabric. “March strains” were obtained by measuring the (002) lattice preferred orientation of chlorite using an X-ray pole-figure goniometer. The principal orientations of March strain match well those of AMS, and AMS and March strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a Flinn diagram. Based on AMS signature, it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone. This could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north. In this scenario, the second deformation, the result of regional late-stage shortening, was only marked by a cleavage (S 2) in the southern zone due to the favorable orientation there of S 1.