Tectonic Imbrication and Foredeep Development in the Penokean Orogen, East-Central Minnesota An Interpretation Based on Regional Geophysics and the Results of Test-Drilling The Penokean Orogeny in Minnesota and Upper Michigan A Comparison of Structural Geology U.S. GEOLOGICAL SURVEY BULLETIN 1904-C, D AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the cur­ rent-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Sur­ vey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. 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HO 1ST Chapters C and D are issued as a single volume and are not available separately U.S. CEO LOGICAL SURVEY BULLETIN 1904-C, D CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION P.K. SIMS and LM.H. CARTER, Editors U.S. DEPARTMENT OF THE INTERIOR MANUEL LUJAN, JR., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Southwick, D.L. (David Leroy), 1936- Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota an interpretation based on regional geophysics and the results of test-drilling / by D.L. Southwick and G.B. Morey. The Penokean orogeny in Minnesota and upper Michigan a comparison of structural geology / by Timothy B. Hoist. p. cm. (Contributions to Precambrian geology of Lake Superior region ; ch. C-D) (U.S. Geological Survey bulletin ; 1904-C-D) "Chapters C and D ... are not available separately." Includes bibliographical references (p. ) Supt. of Docs, no.: I 19.3:1904-C 1. Geology, Stratigraphic Proterozoic. 2. Geology Minnesota. 3. Geology, Structural Minnesota. 4. Plate tectonics. 5. Geology Michigan. 6. Geology, Structural Michigan. I. Morey, G.B. II. Hoist, Timothy B. Penokean orogeny in Minnesota and upper Michigan a comparison of structural geology. 1991. III. Title. IV. Title: Penokean orogeny in Minnesota and upper Michigan a comparison of structural geology. V. Series. VI. Series: U.S. Geological Survey bulletin ; 1904-C-D. QE75.B9 no. 1904-C-D [QE653.5] 557.3 s dc20 [551.7'15'09776] 90-14023 CIP Chapter C Tectonic Imbrication and Foredeep Development in the Penokean Orogen, East-Central Minnesota An Interpretation Based on Regional Geophysics and the Results of Test-Drilling By D.L SOUTHWICK and G.B. MOREY U.S. GEOLOGICAL SURVEY BULLETIN 1904 CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION P.K. SIMS and LM.H. CARTER, Editors CONTENTS Abstract Cl Introduction Cl Acknowledgments Cl Previous investigations Cl New work C4 Tectonic conclusions and stratigraphic implications C7 Regional considerations Cll Possible equivalence of Niagara fault zone and Malmo discontinuity CH Correlation of stratigraphic sequences north of Niagara-Malmo zone- Possibilities and problems C13 Summary C13 References cited C14 FIGURES 1. Index map of Lake Superior region, showing Penokean orogen and Midconti- nent rift system C3 2. Generalized tectonic map of Penokean orogen showing major subdivisions and place names mentioned in text C4 3. Map showing medial zone of Penokean orogen in Minnesota, including part of deformed fold-and-thrust belt and part of Animikie basin C5 4. Schematic diagram illustrating evolution of an oceanic trench into a foredeep C8 5. Jensen plot of volcanic rock compositions from various parts of fold-and-thrust belt C9 6. FeO-MgO-Al2O3 plot showing distribution of basaltic rock compositions from fold-and-thrust belt C9 7. Diagram of Zr:Y plotted against Zr, showing distribution of basaltic rock compositions from fold-and-thrust belt CIO 8. Chondrite-normalized rare-earth element plots for basaltic rocks of fold-and- thrust belt CIO 9. Section of inferred stratigraphic relationships among rock units in Minnesota segment of Penokean orogen CH IV Contents CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION Tectonic Imbrication and Foredeep Development in the Penokean Orogen, East-Central Minnesota An Interpretation Based on Regional Geophysics and the Results of Test-Drilling ByD.L. Southwick1 andG.B. Morey1 Abstract Group (Early Proterozoic) accumulated therein. Iron- formations were deposited in three distinct tectono- Regional aeromagnetic and gravity data, substantiated stratigraphic settings as the Penokean orogen evolved by data from follow-up shallow drilling, archived exploration through time. drilling, and sparse bedrock outcrop, are the basis for a plate-tectonic reinterpretation of the segment of the Early Proterozoic Penokean orogen that lies in Minnesota, immediately west of the crosscutting Midcontinent rift system INTRODUCTION (Middle Proterozoic). This segment occupies a major northwest-facing salient in which structural trends curve from The Penokean orogen is an Early Proterozoic east-west near Lake Superior to north-south in central deformed belt that trends west-southwest along the south Minnesota. Within that broad arc the orogen consists of a margin of the Archean Superior craton (Card and southerly internal fold-and-thrust belt that contains tectoni- Ciesielski, 1986; Hoffman, 1988). It is truncated on the cally shuffled sequences of sedimentary and volcanic rock in east by the Grenville orogen, and on the southwest by the several terranes of contrasting stratigraphy, structural style, Central Plains orogen (Sims and Peterman, 1986) at a and metamorphic grade, together with an external turbidrte poorly defined location in the subsurface of Nebraska. basin (Animikie basin) that has the attributes of a migrating Sedimentation, volcanism, deformation, and plutonism foredeep. The deformed supracrustal rocks in the medial part of the fold-and-thrust belt are characterized by mainly occurred variably