The Midcontinent rift system is a 1.1‐b.y.‐old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on‐shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low‐angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben‐bounding normal faults were reactivated as high‐angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.
In 1988 a stratigraphic test core drilled in southwestern Ohio penetrated a previously unknown structure and sedimentologic unit. Core analyses disclosed a sequence of lithic arenite and siltstone of probable pre-Phanerozoic age, deposited within an alluvial-fluvial environment. A seismic reflection profile across the core site shows a sequence of strong, horizontal Paleozoic reflectors unconformably overlying eastward dipping, layered units of poor reflectivity. Beyond the core hole total depth, a regime of excellent reflectors is seen. The pre-Mount Simon (Upper Cambrian) sedimentary sequence encountered has been defined as the type section of the Middle Run Formation. Point counter analyses of typical Middle Run Formation samples indicate that this sequence is similar in composition to middle Proterozoic units associated with the Midcontinent Rift Systemin the Lake Superior region. The newly discovered structure in Ohio is also similar to half-graben structures composing the Lake Superior Basin portion of the Midcontinent rift. On the basis of similarities in lithology, stratigraphy, structure, and proximity to regional gravity anomalies, the Middle Run Formation and its apparent half-graben basin is proposed as evidence for the probable extension of the Midcontinent Rift System southward, from its normally accepted termination in southeastern Michigan, into southwestern Ohio. Hinze, 1985; Dickas, 1986]. The western arm of this system, termed the Midcontinent Gravity High by Thiel [1956], is identified on the gravity anomaly map of the United States [Lyons and O'Hara, 1982] as a sharply defined trend of positive anomalies and associated flanking gravity lows extending from western Lake Superior to central Kansas (Figure 1). Subsequently, King and Zietz [1971] equated this gravity trend, and its related magnetic signature, with a rift system and named it the Midcontinent Rift System. Additional gravity and aeromagnetic surveys [Lyons, 1959; Yarger, 1983] suggest an extension can be traced further southwest to the Kansas-Oklahoma border. The eastern arm of the Midcontinent Rift System is defined by the Mid-Michigan Gravity High [Rudman et al., 1965], traced across the eastern axis of Lake Superior and then south into the Lower Peninsula of Michigan. This anomaly is more subdued in amplitude than its western counterpart due to burial beneath 4000 m of Phanerozoic strata that fill the Michigan Basin. The southern terminus of the Mid-Michigan Gravity High is commonly placed in the vicinity of Detroit, Michigan [Halls, 1978; Klasner et al., 1982]. Various potential field anomalies have been used by Lyons [1970], Halls [1978], Keller et al. [1982, 1983], Wheeler [1983], and Van Schmus and Hinze [1985] as evidence that this rift system extends to the south of Michigan. These extensions, named the East-Continent Gravity High [Bryan, 1975; Keller et al., 1982], are based on limited crustal refxaction seismology, gravity and magnetic data, and basement petrographic data [Fox, 1988]. The basement rocks associated with thi...
Integrated geophysical investigations of the North American Midcontinent RiftSystem have resulted in a new understanding of the structure, stratigraphy, and evolution of this 1,100-Ma aborted continental rift. Interpretation of seismic reflection, gravity and magnetic anomaly, seismic refraction, rock physical property, and geologic data has identified a great degree of structural heterogeneity of the rift system in eastern Minnesota, northwestern Wisconsin, and western Lake Superior. In the western Lake Superior region, two ridges of pre-rift basement rocks are identified by pinch out of the rift's volcanic strata and lower portion of the overlying sedimentary sequence. In addition, regional rift reverse faults terminate above these ridges. Three-dimensional gravity modeling, constrained by seismic reflection profiles, suggests that both ridges are underlain by and composed of a belt of granitic rocks within the buried Archean greenstone-granite province beneath the rift basin, suggesting a significant influence of ancestral structures throughout the evolution of the rift system. Magnetic modeling indicates that magmatism did not occur uniformly along the length of the rift. In Minnesota and Wisconsin, the great majority of the rift's volcanic rocks are normally polarized and probably younger than the recorded ca. 1,098-Ma magnetic reversal, in contrast to western Lake Superior, where the lower half of the volcanic sequence is reversely polarized and was probably erupted before ca. 1,098-Ma. Gravity modeling suggests that the mass deficiency associated with crustal thickening along the rift is probably compensated by the positive effect of dense rift intrusions in the lower crust. The volume of magma trapped in the lower crust may be similar to that erupted into the rift basin.
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