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

Abstract

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.

A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA

“…2). Elsewhere, the existence of the rift is inferred from magnetic and gravity anomalies and limited deep‐well data (Morey 1974; Witzke 1990; Allen 1994; Allen et al . 1997).…”

“…Its structure and stratigraphy are constrained by interpretations of basin‐scale seismic reflection data and by detailed potential‐field modeling (Cannon et al . 1989; Allen 1994; Allen et al . 1997).…”

“…1997). In cross‐section, the rift‐filling sequence forms a broad basin with little evidence of large‐scale growth faulting (Cannon 1992; Allen 1994; Allen et al . 1997).…”

“…The thickest portion of the basin fill is delineated by the Lake Superior syncline, which coincides with the axis of Lake Superior east of Houghton, Michigan. North of the White Pine region, the syncline axis is deflected to the north‐west between the Grand Marais and White's ridges and apparently terminates near Tofte, Minnesota (Allen 1994; Allen et al . 1997).…”

“…These structural and stratigraphic characteristics, combined with high‐precision chronostratigraphy (Davis & Sutcliffe 1985; Palmer & Davis 1987; Davis & Paces 1990), suggest a four‐phase model of rift evolution in the Lake Superior basin: (i) a 16‐Myr period (1109–1093 Ma) of active rifting characterized by rapid extension (approximately 80 km) and the emplacement of approximately 20 km of volcanics in depocenters along the Lake Superior syncline axis; (ii) an abrupt cessation of extension, waning of volcanism, local isostasy, and immature clastic sedimentation (lower Oronto Group); (iii) a transition to regional isostasy and deposition of the Freda Formation (upper Oronto Group); and (iv) mature clastic sedimentation (Bayfield Group) in a broad flexural basin, later accompanied by regional compression (1070–1050 Ma) and partial inversion (thrusting) of the rift‐filling sequence along major reverse faults (Bornhorst et al . 1988; Cannon 1992; Ohr 1993; Allen 1994; Allen et al . 1997; Bornhorst 1997).…”