REVERSE FAULT OR COMPLEX FEEDER ZONE?: EVALUATING GEOPHYSICAL OBSERVATIONS OF THE 1.1 GA MIDCONTINENT RIFT FROM ISLE ROYALE TO SUPERIOR SHOAL AND BROADER GEOLOGIC IMPLICATIONS FOR NORTHERN LAKE SUPERIOR

As much as 3 km of subaerial basalts exposed on the NE-elongate island of Isle Royale (IR) in northwestern Lake Superior record the last major phase of Midcontinent Rift volcanism. The basalts dip SE and are commonly regarded as part of the upthrown block of a post-rift reverse fault just off the northern IR shore. Numerous parallel, NE-trending, linear aeromagnetic anomalies follow the strike of exposed basalt layers on IR and basaltic dikes near and onshore Ontario to the NW. A prominent, narrow, aeromagnetic high-low pair (IR-SS anomaly) emanates from the linear anomalies off the NE tip of IR, curving toward the SE to a strong negative anomaly at Superior Shoal (SS), a bathymetric high near the center of the lake. Two broad, curvilinear gravity anomalies parallel the IR-SS anomaly but are not continuous with a linear gravity anomaly that follows the southern IR shore. The IR-SS anomaly is commonly interpreted as an extension of the IR reverse fault, involving both younger (normal magnetic polarity) and older (reversed magnetic polarity) basalts. A ~N-S seismic-reflection section that crosses the IR-SS anomaly just west of SS shows a 12-km-wide disrupted zone extending vertically below the anomaly’s extent. The zone divides packages of subhorizontal reflections, with package tops at depths of ~2.5 km on the south and ~4.5 km on the north. A vertical reverse fault with ~4 km of throw has been interpreted at the disrupted zone, with the separated packages representing basalt sections of dissimilar ages all overlain by late-rift sediments. Gravity models of this interpretation are permissive, but are difficult to reconcile with magnetic models of the IR-SS anomaly, which call for igneous rocks of both magnetic polarities extending above 2.5 km depth within the disrupted zone. Moreover, analysis of the IR-SS anomaly at the NE tip of IR indicates its sources must lie below the upturned basalts, and the gravity data suggest the causative source underlying IR may be separate from that under the IR-SS anomaly. An alternate hypothesis invokes a long-lived, eroded, and likely faulted, feeder zone, with the subhorizontal reflections on the north side of the seismic section representing sills intruded into an older sedimentary basin underlying northern Lake Superior instead of subaerial basalts overlain by younger sediments.