120-21 Seismic-Velocity Models and Converted Time-To-Depth Seismic Reflection Sections Provide Insights into the Eastern Arm of the North American Midcontinent Rift, Eastern Lake Superior

Abstract:

The ca. 1.1 Ga North American Midcontinent Rift System (MRS) is recognized as two arms of a series of elongated volcanic and sedimentary troughs with related intrusions. The western arm stretches northeastward from Kansas to central Lake Superior where it joins the eastern arm, which extends southeastward through eastern Lake Superior into southern Michigan. The MRS initiated with voluminous magmatism followed by an extensive period of clastic sedimentation and was eventually arrested by ongoing compressional events. Based on interpretations of 1980s seismic-reflection data acquired from Lake Superior and outcrops around the western lake, both arms have been interpreted as a series of superposed volcanic/sedimentary basins bounded by thrust faults that placed basalts over the youngest of the Mesoproterozoic sedimentary rocks. To better interpret seismic data for the poorly exposed eastern arm, we used legacy seismic-refraction studies to develop velocity models for converting reflection sections in two-way travel time to depth. The velocity models and resulting reflection depth sections improve the definitions of the lithology, thickness, and depth of MRS geologic units. Along the western margin of the eastern arm, upturned wedges of NE-dipping, subparallel seismic reflections flatten and collectively thicken from about 5 to 15 km to the NE. They were inferred to be the hinges of half-grabens or edges of subsiding basins composed of layered basalts that were later faulted to place older basalts over younger sedimentary rocks. Five-km-wide zones of vertically stacked, concave-down reflections near the upturned edges were interpreted as fault rollovers. Contrary to this view, however, our models indicate that the rock units under the wedges must have a different lithology and/or age than expected and that the zones of concave patterns penetrate the layered reflections rather than connect to them as rollovers would. Several alternate hypotheses are proposed: (1) intrusions or buried volcanic structures are located at the edges of basalt-filled basins with only minor thrust faulting; (2) reverse faulting here occurred much earlier than what is recorded in outcrops in the western lake; or (3) the subparallel reflections represent multiple conformable sills that intruded a pre-existing sedimentary basin. There are arguments against all these hypotheses, but the problems suggest that the tectonic history of the eastern arm is dissimilar from the western arm and warrants further study.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-17874

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