Interpretation of seismic reflection and gravity profile data in western Lake Superior

Abstract

The interpretation of 1047 km of seismic reflection data collected in western Lake Superior is presented along with reflection traveltime contour maps and gravity models to understand the overall geometry of the Midcontinent Rift System beneath the lake. The Douglas, Isle Royale, and Keweenaw fault zones, clearly imaged on the seismic profiles, are interpreted to be large offset detachment faults associated with initial rifting. These faults have been reactivated as reverse faults with 3–5 km of throw. The Douglas Fault Zone is not directly connected with the Isle Royale Fault Zone. The seismic data has imaged two large basins filled with more than 22 km of middle Keweenawan pre-Portage Lake and Portage Lake volcanic rocks and up to 8 km of upper Keweenawan Oronto and Bayfield sedimentary rocks. These basins persisted throughout Keweenawan time and are separated by a ridge of Archean rocks and a narrow trough bounded by the Keweenaw Fault Zone to the south. Another fault zone, herein named the Ojibwa fault zone, previously interpreted as the northeastern extension of the Douglas Fault Zone, has been reinterpreted as a reverse fault that closely follows the ridge of Archean rocks. Previous researchers have stated that neighboring segments of the rift display alternating polarity of basins associated with large detachment faults. Accommodation zones have been previously interpreted to exist between rift segments; however, the seismic data do not image a clearly identifiable accommodation zone separating the two basins in western Lake Superior. Thus, the seismic profile may lie directly above the pivot of a scissors-type accommodation fault zone, there is no vertical offset associated with the zone, or the zone does not exist. Seismic data interpretations indicate that application of a simple alternating polarity basin – accommodation zone model is an oversimplification of the complex geological structures associated with the Midcontinent Rift System.

North America’s Midcontinent Rift: When rift met LIP

“…This polarity reversal along a series of adjacent half-graben segments (Fig. 5) (Sexton and Henson, 1994;Dickas and Mudrey, 1997) is analagous to that observed in the East African rift.…”

Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota

“…13). Moreover, the seismic data in Lake Superior show no evidence for an eastern extension of the Douglas fault, and are incompatible with interpretations (e.g., Sexton and Henson, 1994) that suggest the fault extends into the lake.…”

“…6). This small fault is imaged only along GLIMPCE profile C and is therefore a local feature, in contrast to the interpretation by Sexton and Henson (1994) who name this feature the Ojibwa fault.…”

“…As shown in Figure 3, only three of the onshore profiles in Wisconsin and Minnesota have been previously interpreted. In addition, just one prior investigation (Sexton and Henson, 1994) has focused on the application of the Grant-Norpac/Argonne profiles to the investigation of the rift's structure in western Lake Superior.…”

Bedrock geologic map of the Ashland and the northern part of the Ironwood 30' x 60' quadrangles, Wisconsin and Michigan

“…A region of low density rocks, probably Archean basement, surrounded by rift basalts was inferred beneath the Bayfield Peninsula by White (1966) on the basis of analysis of gravity, magnetic, and geologic data. This structure has been confirmed and clarified by more recent analyses including seismic reflection data (Sexton and Henson, 1994;Allen, 1994) and named White's Ridge. The Keweenawan Supergroup volcanic strata progressively thin toward the ridge (see cross sections A-A' and B-B') and eventually pinch to a feather edge, which is approximately located on the map.…”