REMAPPING THE KEWEENAW FAULT AND DISCOVERY OF RELATED STRUCTURES IN MICHIGAN’S HISTORIC COPPER DISTRICT

The Keweenaw Fault (KF) in Michigan’s historic copper district is a significant structural feature of the Midcontinent Rift System, with postulated 9 km of reverse slip juxtaposing Cu-bearing, Portage Lake Volcanics (PLV, 1.1 Ga) above younger Jacobsville Sandstone (JS). The age, geometry, and number of slip events of the KF has been debated for nearly 150 years due to a lack of datable materials, absence of fossils, and challenging landscape. The 1950s USGS mapping campaign captured and documented the area’s geology using the best methods and concepts of the time. While the resulting maps are beautifully detailed and highly informative, locally they display anomalous fault geometries, imply implausible slip dynamics, and oversimplify a complex fault system. Recent mapping along Bête Grise Bay has revealed that the KF in this area, previously mapped as a single sinuous trace, is a system of left-stepping, ESE-trending, fault segments with a 2:1 ratio of dextral strike slip to reverse slip. The PLV-JS unconformity, identified at several locations, and its nature suggest fault slip before and after JS deposition, with an intervening hiatus in fault motion.

To better constrain the nature of the KF, we are remapping (1:10,000) the fault and related structures between Bête Grise Bay and Mohawk (MI). Equipped with a dGPS unit and state-of-the-art smartphone data collector apps to enhance our conventional mapping toolset, we focused on four areas with anomalous fault geometry previously mapped as: (1) a deep reentrant of JS; (2, 3) a splay branching from and rejoining the KF; and (4) a cross fault and adjacent stratal geometry. Work in 2019 has modified the reentrant geometry such that it resembles a left step in the fault system, and the splay fault also has a geometry consistent with a left step, similar to the KF system at Bête Grise Bay. While difficult to directly observe the fault zones, a few new exposures provide opportunities to define the nature of fault-zone deformation in the area. While previous mapping portrayed stratal geometry in the hanging wall as being a simple homocline dipping NNW, we have identified folds in PLV that are discussed in the companion talk. The 2020 field season will focus on mapping specific exposures with kinematic indicators to be used in paleostress and fault slip inversion analyses.