ANALYSIS AND INTERPRETATION OF FOLDS AND FAULT SEGMENTS ALONG THE KEWEENAW FAULT SYSTEM, MICHIGAN

The Keweenaw Fault (KF) on the south edge of the Midcontinent Rift System is a major structure that has thrust copper-bearing Portage Lake Volcanics (PLV, ~ 1.1 Ga) over younger Jacobsville Sandstone (JS). Geologic maps and cross sections published in the 1950s by the U.S. Geological Survey combined their field data with mining company data to represent the state of knowledge for that time. Since then, major advances have occurred in understanding fault system kinematics and in geospatial technology for precision mapping. Published cross sections generally show stratal geometry in the KF hanging wall (HW) as a NW-dipping homocline, steeper near the fault and less steep away, implying a simple listric geometry as the fault is projected to depth. Rarely, cross sections show more complex HW geometries, which imply a more variable fault geometry at depth. In an ongoing USGS EdMap project, we are remapping an anomalous portion of the KF between Bête Grise Bay and Mohawk (MI) to better define its geometry and deformation in adjacent rocks using modern structural concepts.

Mapping completed to date has modified the KF trace significantly in some areas, has revealed folded PLV strata in the HW, and has refined footwall fold geometry in JS strata. Folded HW strata near Snake Creek have steep north dip at the fault and define an anticline-syncline pair with NE plunging axes. Footwall JS strata are deformed into a tight syncline with beds overturned to the south near the fault that become horizontal 200 m away. The fold pattern here, and manifested elsewhere in the area, is oblique to the main KF trace and consistent with a component of dextral shear along the fault zone. An updated cross section along the Snake Creek transect is now similar to an older published one to the southwest that is well constrained by drilling. Both sections have HW geometry with an anticline-syncline pair and splay faults off the main slip surface, which we think is the general rule for this area. Field work this year will focus on better definition of deformation elsewhere along the KF and obtaining fault-slip measurements to invert for paleostress-strain.