STRUCTURAL ANALYSIS AND INTERPRETATION OF DEFORMATION ALONG THE KEWEENAW FAULT SYSTEM FROM LAKE LINDEN TO MOHAWK, MICHIGAN

The Keweenaw fault is likely the most significant and most studied fault associated with the Midcontinent Rift System. The fault roughly bisects the Keweenaw Peninsula and places Portage Lake Volcanics (~1.1 Ga) over much younger Jacobsville Sandstone (~1.0 Ga). Published bedrock geology maps with cross sections from the 1950s show the fault as a single continuous trace that is locally associated with smaller cross faults and splays. The accompanying cross-sections show hanging-wall volcanic strata having a well-defined, listric geometry with dip decreasing away from the fault to the northwest. This M.S. thesis presents a structural analysis and interpretation of the Keweenaw fault system between Lake Linden and Mohawk, MI, which includes the well-known localities of Houghton-Douglass Falls, the St. Louis ravine, the Natural Wall ravine, and the anomalous rhyolite body near Copper City. Objectives of the study were to better define the geometry, movement, and slip kinematics of the Keweenaw fault while also characterizing the fold patterns associated with the fault system. Field observations and data were used to revise existing bedrock geology maps, construct new cross-sections, and analyze fold geometry and fault slip behavior to infer aspects of the tectonic regime that caused the deformation. New field mapping has refined the trace geometry of the Keweenaw fault and smaller associated faults, revised intersections between several splay faults and the main fault, and suggested the existence of several footwall splays not previously recognized. These map changes better define the Keweenaw fault system in this area as consisting of: 1) major NNE-trending segments that define the fault system’s overall trend and probably have mostly reverse slip; 2) NE-trending segments that branch off the major fault segments and define wedge-shaped fault blocks that widen to the northeast in the footwall; and 3) NNW-trending segments that connect faults of the first two types. At a point southwest of Copper City, the main fault surface abruptly changes strike from N16°E to N58°E traveling in a northeasterly direction and it shifts ~650 m deeper stratigraphically within the Portage Lake Volcanics. This abrupt change in trend and stratigraphic level of the main fault occurs at a complex junction of the Allouez Gap fault with two major segments of the Keweenaw fault at the northeast end of the Mayflower fault block. Orientation analysis of Jacobsville Sandstone strata in the footwall of the fault system defines fold axes subparallel to nearby faults and with plunge directions that change from southwest to northeast moving in the same direction. Fault-slip analysis reveals both strike slip and dip slip along the fault system rather than only reverse movement as in the generally accepted model. Measured slip directions collectively define a 0.84:1 ratio of strike-to-dip slip for the fault system, and a fault-slip inversion analysis computes a nearly north-south maximum shortening direction of 2°-182°. Fold axis trends in the current area indicate shortening along an ESE-trending line, whereas fault-slip inversion analysis indicates a north-south shortening direction