Geology topics

Esther K. Stewart

Publications and source records attributed to Esther K. Stewart.

4 recordsLinked to original sources

USGS2026

Syn-magmatic subsidence during the early stages of continental rifting in the Mesoproterozoic—A reanalysis of legacy data for the Midcontinent Rift, western Lake Superior

The Midcontinent Rift system (ca. 1.1 Ga) is a 2000-km-long series of elongated volcanic and sedimentary troughs and associated intrusive centers exposed chiefly in the Lake Superior region of North America. The rift system represents a long history of intense magmatism and subsequent sedimentation that was arrested by far-field tectonic events before sea-floor spreading was established. The premature cessation preserved a record of processes related to the beginning of continental rifting. The rift system under Lake Superior has been long studied using seismic-reflection data collected as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE). We reexamine GLIMPCE Line C by developing a detailed velocity model for time to depth conversion constrained by other legacy data. We corroborate the model and develop a geologic interpretation using gravity and magnetic modeling and ties to geology mapped onshore. We recognize superposed subsiding sedimentary and volcanic basins for the southern half of the Line C depth section. This interpretation differs from previous paradigms that show major crustal faults that bound half-grabens or full grabens. We conclude that high-velocity (6.9 km/s) intrusive zones rather than major crustal faults border the sides of the basins. We speculate that the volcanic basin represents the initiation of seaward dipping reflectors. The syn-magmatic subsidence can be explained by dike injection and volcanic loading. Discrete lava basins throughout the region likely subsided at different times in a disorganized manner along the rift trend, raising questions about the long-term role of lithospheric thinning and melt generation.

Michigan, Minnesota, Ontario, Wisconsin↗

USGS2024

Revisiting geophysical interpretations of the Midcontinent Rift below Lake Superior— Insights from GLIMPCE seismic-reflection line C

The 1.1 Ga Midcontinent Rift System (MRS) has been investigated in the Lake Superior region for more than a century. The most influential geophysical data for modern paradigms has come from seismic-reflection profiles collected by the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) in the late 1980s. We are revisiting seismic-reflection GLIMPCE line C by constructing a detailed velocity model for conversion of the seismic data measured in two-way travel time to a section plotted versus depth. Preliminary results from the velocity modeling, depth conversion, and ties to onshore geology have led to a significantly different view of Line C as primarily a sag basin rather than a half-graben. Narrow intervals of high-velocity material that emanate upwards along both sides of the sag basin are interpreted as mafic intrusions connected to the mantle. This new interpretation suggests that basin subsidence was the dominant process in the development of rift stage troughs rather than major half-graben structures.

Conference Paper↗

USGS2023

Exploring the geology of the Midcontinent Rift under western Lake Superior using a preliminary velocity model of seismic line GLIMPCE C

Seismic-reflection data were collected in the 1980s as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) to investigate the 1.1 Ga Midcontinent Rift System (MRS). GLIMPCE Line C crosses western Lake Superior from north to south shores (Fig. 1 inset). Many previous workers have interpreted the MRS in Line C as an asymmetric central graben filled with 10–20 km of subaerial basalt flows, overlain by 7-10 km of sedimentary section, and underlain by magmatic underplating. The central graben was interpreted to have formed from extensional normal faults, later reactivated as high-angle reverse faults. The northern part of Line C crosses over a prominent gravity low called the Grand Marais Ridge (GMR; Fig. 1 inset), previously interpreted as an Archean granitic basement high.

Lake Superior↗

USGS2020

Integrated geophysical analysis provides an alternate interpretation of the northern margin of the North American Midcontinent Rift System, Central Lake Superior

The Midcontinent Rift System (MRS) is a 1.1 Ga sequence of voluminous basaltic eruptions and multiple intrusions followed by widespread sedimentation that extends across the Midcontinent and northern Great Lakes region of North America. Previous workers have commonly used seismic-reflection data (Great Lakes International Multidisciplinary Program on Crustal Evolution [GLIMPCE] line A) to demonstrate that the northern rift margin in central Lake Superior developed as a normal growth fault that was structurally inverted to a reverse fault during a compressional event after rifting had ended. A prominent, curvilinear aeromagnetic anomaly that extends from Isle Royale, Michigan, to Superior Shoal in central Lake Superior, Ontario (the IR-SS anomaly), is commonly presented as a manifestation of this reverse fault. We have integrated multidisciplinary geophysical analyses (seismic-reflection, seismic-refraction, aeromagnetic, and gravity), physical-property information (density, magnetic susceptibility and remanence, and compressional-wave velocity), and geologic concepts to develop an alternate interpretation of the rift margin along GLIMPCE line A, where it intersects the IR-SS anomaly. Our new model indicates that a normal fault is the dominant structure at the northern rift margin along line A, contrary to the original rift-margin paradigm, which asserts that compressional structures are the dominant features preserved today. Integral to this alternate model is a newly interpreted, prerift sedimentary basin intruded by sills in northern Lake Superior. Our alternate model of the northern rift margin has implications for interpreting the style, scale, and timing of extension, rift-related intrusion, and compression during development of the MRS.

Lake Superior↗