KEWEENAW FAULT GEOMETRY AND KINEMATICS NORTH OF PORTAGE LAKE, MICHIGAN

The Keweenaw Fault (KF) is perhaps the most significant fault of the Midcontinent Rift System based on its length of 325 km, postulated slip of 9 km, and thrusting of Cu-bearing Portage Lake Volcanics (PLV, 1.1 Ga) over younger Jacobsville Sandstone (JS). A long-held view is that it began as a steep, rift-bounding, normal fault during crustal extension and later was inverted during crustal shortening. This scenario would produce a fault dipping > 45° NW with high cut-off angles across PLV layers. However, published maps and cross-sections show the KF dipping < 45⁰ NW (often < 30°) for much of its length, with PLV layers sub-parallel to its surface. Prior work also documents layer-parallel slip within the PLV. These observations suggest a thrust system detached along PLV layers. To better define the nature of the KF, we have begun a structural analysis project along the Keweenaw Peninsula, combining published maps and core data with new mapping and fault slip measurements to build balanced cross-sections.

Two regional cross-sections were constructed ~25 km apart across the KF and lesser faults in its hanging wall. Over this stretch the KF changes strike from N35°E to N55°E going SW to NE, often dips less than 25°NW and, together with HW faults, bounds PLV layers with similar to counter-regional dips. Cross-section work suggests that observed geometry was produced by a SE-propagating thrust system with a basal detachment 15-20 km beneath Lake Superior. Farther northeast, previous maps show the KF as a single sinuous trace that curves clockwise even more to about S70°E. However, our new mapping of the fault’s eastern extent reveals instead a series of left-stepping, ESE-trending, fault segments. Slip indicators show mostly dextral strike slip with lineations plunging ≤ 30°. Segment connections are not yet well defined, but tilt directions and dip reversals in PLV strata indicate that areas between segments have shortened. Outcrops of the JS-PLV unconformity here, unlike to the SW where these units are in fault contact, and regional constraints suggest that vertical separation across the KF decreases from ~3000 m near Portage Lake to ~300 m at the fault’s eastern extent. We therefore interpret the KF thrust system to lose displacement northeastward, ultimately terminating in a series of left-stepping splays with dextral strike slip.