Geological Survey Professional Paper 754—C The Portage Lake Volcanics (Middle Keweenawan) on Isle Royale, Michigan

The layered bedrock sequence on Isle Royale is tilted toward the southeast. Dips of the strata range from less than 10° to 55°; they are generally steeper on the north side of the island than on the south side and average less than 20°. The increase in dip toward the north side is probably related in part to increasing proximity to the Isle Royale fault in that direction (fig. 1). The existence of a high-angle thrust fault along the north side of Isle Royale, similar to the well-documented Keweenaw fault on the Keweenaw Peninsula, was postulated on physiographic grounds by Irving and Chamberlain (1855). Considerable geophysical evidence has since been marshalled in support of its existence (Wold and Ostenso, 1966; Halls and West, 1971), and Halls and West estimated that the southeast side has been upthrown at least 1—2 kilometers. The structural setting of Isle Royale is thus analogous to that on the Keweenaw Peninsula, where dips increase markedly toward the Keweenaw fault; dips exceeding 30° are found only within 4 or 5 miles of the Keweenaw fault (White, 1968). A slight flexure in the strata between McCargoe Cove and Pickerel Cove on the northeast end of Isle Royale also may be related to movement on the Isle Royale fault.

The only major distortion superimposed upon the homoclinal structure of Isle Royale is in the area between Siskiwit Lake and Rock Harbor, where the upper part of the Portage Lake Volcanics is warped around a node of apparent uplift and fracturing. The style of deformation there is different on opposite sides of a fault that extends through Chippewa Harbor and Lake Richie. On the southwest side of that fault, the stratigraphic units from the Edwards Island Flow upward, though cut by numerous minor faults, all swing with relative continuity around a point centered near the south end of Lake Richie. On the northeast side, only the stratigraphically highest sedimentary units in the Portage Lake Volcanics maintain continuity. The volcanic sequence between those units and the Scoville Point Flow is broken by a series of closely spaced normal faults trending parallel to the strike direction of the flows and consistently downthrown on the north, resulting in extreme duplication of that part of the stratigraphic section.

Lane (1898, p. 210), in discussing this area, noted that "the strata appear as though wrapped over some large mass below" and that "this suggests either that there is an intrusion below or that we are on the flanks of an old volcanic focus." In support of this suggestion, he cited the presence of thin "apparently intrusive sills," which in the present study are interpreted as pegmatitic differentiates of extrusive volcanic rocks; no intrusive rocks are now thought to be exposed in the area. Nevertheless, the area seems to have been domed upward, and it is possible that a buried intrusive was the cause.

Lane's map of this deformed area is somewhat misleading and can be interpreted as indicating an appreciable nontectonic thickening of the stratigraphic section in the Lake Richie-Chippewa Harbor region, an interpretation made by Halls and West (1971, p. 622), among others. Lane was aware of the limitations of his map, however, when he stated that "I can hardly hope to have connected all the isolated outcrops exactly" (Lane, 1898, p. 209). Geologic mapping during the present study indicates that what appears to be stratigraphic thickening on Lane's map is due partly to decrease in dip of the strata in the central part of the upwarped zone and partly to extensive duplication of section by a complex pattern of normal faults. Consequently, the outcrop pattern is strongly influenced by topography, and the map pattern of the thin sedimentary units gives the impression of a more complex structure than actually exists. It is only fair to state that most of the refinements of the present mapping were made possible solely through the use of resources unavailable to Lane, in particular modern aerial photographs and modern mosquito repellent that permitted the field use of those photographs.

Topographic lineaments and smaller scale fractures related to a regional fracture system are superimposed upon the homoclinal structure. One prominent joint set of this system trends northward at the west end of Isle Royale and changes progressively eastward to trend about 30° E. of N. at McCargoe Cove. Minor faulting has occurred along some of the lineaments related to this set, but displacements are generally small and probably rarely exceed 100 feet. All displacement appears to be near vertical, with the east side upthrown relative to the west side. Some fractures of a second set, trending east-west, commonly are faults showing somewhat greater displacements but still no more than a few hundred feet. Displacements along this set also are vertical, with the south side upthrown relative to the north side. The systematic orientation and nature of displacement of these fracture sets attest to the regional nature of their pattern. The fractures were probably formed in response to warping of the bedrock strata on the flank of the Lake Superior syncline, probably in part caused by thrust movement on the Isle Royale fault.