Eos, Vol. 72, No. 8, February 19, 1991 Eos, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION VOLUME 72, NUMBER 8 FEBRUARY 19, 1991 PAGES 81-96 Lake Superior basin are part of the same sys­ tem. (See introduction to Wold and Hinze High Resolution [1982] for discussion.) Development of the MCR in the Lake Su­ perior region is attributed to extensive volca- Aeromagnetic Survey of nism and subsidence at approximately 1.1 b.y. [Palmer and Davis, 1987; Davis and Sutcliffe, 1985; Davis and Paces, 1990]. The Lake Superior MCR is a series of segmented grabens or—as suggested by Behrendt et al. [1988]— PAGES 81, 85-86 a series of juxtaposed half-grabens contain­ ing weakly deformed and metamorphosed D. J. Teskey, M. D. Thomas, R. A. Gibb, S. D. Dods, R. P. Kucks, sequences of sedimentary and volcanic V. W. Chandler, K. Fadaie, and J. D. Phillips rocks more than 30 km thick [Cannon et al., 1989]. In the west and central part of the lake, the Keweenaw fault (Figure 3) appears A 57,000 line kilometer, high-resolution Lake Superior area, a major objective of the to form a southern bounding-fault. The aeromagnetic survey was flown in 1987 as a program is to develop thermal, tectonic, and northern bounding-fault in the central por­ contribution to the Great Lakes International petrogenetic models for the evolution of the tion, the Isle Royale fault, continues to the Multidisciplinary Program on Crustal Evolu­ MCR and to evaluate these in the broader west, as indicated on this new aeromagnetic tion (GLIMPCE). Existing aeromagnetic data context of the tectonic evolution of the North map. from the United States and Canada were American continent. combined with the new data to produce a Pre-1982 geological and geophysical GLIMPCE Reflection Seismic Studies composite map and gridded data base of the knowledge of the MCR in the Lake Superior Lake Superior region (Figure 1). region has been summarized by Wold and Five deep-penetration multichannel re­ Analysis of the new data permits more Hinze [1982]. The Lake Superior region pro­ flection seismic profiles were obtained in accurate definition of faults and contacts vides a unique window on this Proterozoic 1986; their locations are shown in Figure 2. within the Midcontinent Rift system (MCR). rift system, exposing igneous rock of the Cannon et al. [1989] present a discussion of The aeromagnetic map provides important Keweenawan Supergroup that disappears the seismic program and resultant interpreta­ information supplemental to the seismic pro­ under Paleozoic cover to the southwest. tion. The processed seismic sections indi­ files acquired under the GLIMPCE program cate that thick sequences of volcanic and in 1986, allowing lateral extension of the sedimentary strata were deposited in grabens seismic interpretation. In particular, model­ Geology and broadening basins, respectively. The ing of the data provides an independent as­ strata were subsequently subjected to uplift, The geology of the MCR in the Lake Su­ sessment of a reflection seismic model de­ accompanied by reverse faulting along origi­ perior region has been extensively reviewed rived along line A (Figure 2). The profile and nal graben-bounding normal faults, possibly and discussed by Halls [1978], Davidson gridded digital data are available to geosci- caused by a regional compressive event [1982], Morey and Green [1982], Klasner et entists through the Geophysical Data Centre [Cannon et al., 1989]. Basins change in al [1982], and Van Schmus and Hinze of the Geological Survey of Canada (GSC), asymmetry from north lower to south lower [1985]. A generalized geological map of the while the gridded data are available from the between lines C and A, whereas line F ap­ region adapted from Cannon and Davidson USGS-EROS Data Center. pears to be approximately symmetrical. It is [1982] is shown in Figure 2. In this region, assumed that the three corresponding blocks GLIMPCE was established in 1985 to Middle Proterozoic (—1.1 b.y.) Keweenawan are separated along two relatively narrow study the nature and genesis of the crust in sedimentary and volcanic rocks of the MCR accommodation zones: one west of Isle Roy- the Great Lakes region. Program participants are flanked by Archean rocks of the Superior ale [Cannon et al., 1989] is inferred by a include the GSC, the U.S. Geological Survey Province and by overlying Early Proterozoic break in the gravity pattern; and the second, (USGS), provincial and state surveys, and sedimentary rocks which formed a continen­ Canadian and American universities. In the tal margin prism. The Superior Province is east of the Keweenaw Peninsula in the posi­ characterized by east-northeast trending, al­ tion of the Thiel fault [Hinze et al., 1966, ternating belts of metasedimentary gneiss 1982], is evident on the aeromagnetic map and granite-greenstone. South of Lake Supe­ (Figure 1). An interpreted crustal cross-sec­ rior the Early Proterozoic prism is in contact tion for seismic line A [Cannon et al., 1989] D. J. Teskey, M. D. Thomas, R. A. Gibb, S. D. with magmatic terranes of the 1.9-1.8 b.y. across the central portion of the lake is Dods, and K. Fadaie, Geological Survey of Canada, Penokean orogen along the east-west trend­ shown in Figure 3. 1 Observatory Crescent, Ottawa, Ontario, K1A 0Y3; R. P. Kucks, U. S. Geological Survey, P.O. Box ing Niagara suture zone. Southwest of Lake 25046, DFC MS 964, Denver, CO 80225-0046; V. W. Superior the MCR cuts across the western Chandler, Minnesota Geological Survey, 2642 Uni­ extension of the suture, where relationships Gravity Data and Pre-1987 versity Ave., St. Paul, MN 55114-1057; and J. D. are obscured by Phanerozoic cover. Gravity Aeromagnetic Data Phillips, U. S. Geological Survey, MS 927, Reston, and magnetic signatures continue south­ Magnetic and gravity surveys have a sig­ VA 22097. Geological Survey of Canada Contribu­ ward, however, and the gravity and magnetic tion 18390; U.S. Geological Survey Contribution nificant role in the geologic interpretation of R90-0548. pattern shows, in fact, that the MCR and the MCR in the Lake Superior basin, particu- This page may be freely copied. Eos, Vol. 72, No. 8, February 19, 1991 50° t~— 1 # 49° f 48£ N 47° V 46° c c 93 c 92 )0C 88°W 86 ttg. 7. Magnetic-anomaly map of Lake Superior region, compiled H500 -500 -300 -200 -100 0 100 200 300 500 1500 Nanoteslas from 1987 aeromagnetic survey of Lake Superior and from existing • i • • i i _L_^t _L_ data for Canada and the United States, for the Great Lakes Interna­ tional Multidisciplinary Program on Crustal Evolution (GLIMPCE). ^ 80 m [Original color image appears in the back of this volume.} wmmmmmn IHMM larly when constrained by the new high-reso­ tary boundaries, based on existing seismic, measurer1 at a ground base station, was less lution, deep-penetration seismic data and gravity, magnetic, and geologic information. than five nanoteslas from a linear mean, the mapped Keweenawan surface geology on They also calculated detailed models for two over a time period corresponding to the the shores and islands of the lake. Gravity gravity profiles across the western and east­ travel time between the control lines (ap­ and magnetic modeling can refine and ex­ ern portions of Lake Superior. Both models proximately 4 minutes). The GSC compiled tend interpretations based on isolated seis­ indicated a rift-type structure. Below the rift, the total field anomaly map (with the Defini­ mic lines. Gravity data are particularly useful in each case, an anomalous high-density tive Geomagnetic Reference Field for 1987.5 for modeling deep structure and for differen­ (3.00-3.05 g/cm3) "hybrid" crust was mod­ subtracted). Data from surveys surrounding tiating between sedimentary and volcanic eled as extending upward from the mantle the lake were datum-adjusted and combined rocks, whereas magnetic data are very effec­ from approximately 50-20 km deep. This with the new data by R. P. Kucks for the tive for delineating near-surface boundaries. crust is considered to be more dense, possi­ American area, and by S. D. Dods for the The gravity and pre-1987 aeromagnetic data bly as a result of multiple intrusions from Canadian areas, resulting in the map in Fig­ bases, and their previous interpretations, the mantle emplaced into a weakened lower ure 1. The Minnesota Geological Survey gath have been reviewed by Hinze et al. [1982]. ered the Minnesota data with support from In summary, the gravity data base consists of crust during rifting. the Legislative Commission on Minnesota approximately 1300 lake bottom gravity sta­ Resources, while the University of Wiscon- tions, most of which are on an 8-km grid. An New High-Resolution Aeromagnetic sin-Oshkosh gathered the Wisconsin data aeromagnetic survey of Lake Superior was Survey with support from the Wisconsin Geological carried out in 1964 using a proton-preces­ and Natural History Survey. Parts of northern sion magnetometer along flight lines approx­ The GSC carried out the 57,000 line kilo­ imated 10 km apart, oriented at right angles meter high-resolution aeromagnetic survey of Michigan were flown under a cooperative to the lake [Wold and Ostenso, 1966; Hinze the entire lake with its Queenair aircraft. program between the USGS and the Geologi­ et al., 1966]. Hinze et al. [1982] derived North-south survey lines were spaced at 1.5 cal Survey Division, Michigan Department of composite maps showing the interpreted minutes of longitude (1900 m) and flown at Natural Resources. Other areas of the map position of major faults (Douglas, Isle Roy- an altitude of 305 m. East-west control lines were compiled from data available from the ale, Michipicoten, Thiel, and Keweenaw), were flown 20 km apart.