SLIP KINEMATICS OF THE KEWEENAW AND HANCOCK FAULTS WITHIN THE MIDCONTINENT RIFT SYSTEM, UPPER PENINSULA OF MICHIGAN: A POTENTIAL KEY TO THEIR ORIGIN AND EVOLUTION

The Keweenaw fault is a major compressional structure along the southern edge of the Midcontinent Rift System (MRS). The smaller Hancock fault connects with the hanging wall of the Keweenaw fault and, together, the two faults define a thrust slice. The MRS formed ~1.1 billion years ago when a major extensional event split a significant portion of the ancient North American continent across the Upper Midwest. The rifting was accompanied by eruption of large volumes of basaltic magma, roughly ending with the Portage Lake Volcanics that have an exposed thickness of 3-5 km along the Keweenaw Peninsula. A common interpretation of the Keweenaw fault is that it began as a normal fault during MRS extension, and then inverted to become a reverse fault during a post-rift compressional event, hypothesized to be the Grenville Orogeny. Another interpretation is that the Keweenaw and Hancock faults are parts of a detached fault system initiated during the Grenville Orogeny. Measurement of kinematic slip indicators on these and related faults provides new data with which to help resolve these contrasting ideas.

Until a few years ago, ideas about these and similar faults in the region considered only dip slip with either normal or reverse sense of motion. Recent bedrock mapping and measurements of fault slip lineations, however, have revealed a significant component of right-lateral strike slip on the Keweenaw fault system near its northeastern end. In contrast, the Hancock fault is shown on published maps with apparent left-lateral offset of units across it. Our work utilizes additional bedrock mapping and fault slip measurements southwest of earlier studies and in the historic Quincy Mine adit to clarify slip kinematics of the Keweenaw and Hancock faults and to relate both faults to their causative tectonic regime. Ongoing work and analyses suggest that the Keweenaw fault has a lower ratio of strike to dip slip near the Hancock fault (1:1) than farther northeast (2.5:1), and that strike and dip slip components may be partitioned to a greater degree near the Hancock fault. We hypothesize that apparent left-lateral strike slip on the Hancock fault may represent conjugate motion with respect to right-lateral strike slip on the Keweenaw fault elsewhere in the system.