Structural Geology of Parautochthonous and Allochthonous Terranes of the Penokean Orogeny in Upper Michigan Comparisons with Northern Appalachian Tectonics U.S. GEOLOGICAL SURVEY BULLETIN 1904-Q AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey 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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UNITED STATES GOVERNMENT PRINTING OFFICE: 1993 For sale by USGS Map Distribution Box 25286, Building 810 Denver Federal Center Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Cregg, William J. Structural geology of parautochthonous and allochthonous terranes of the Penokean orogeny in upper Michigan comparisons with northern Appalachian tectonics / by William j. Cregg. p. cm. (U.S. Geological Survey bulletin ; 1904-Q) (Contributions to Precambrian geology of Lake Superior region; ch. Q) Includes bibliographical references. Supt. of Docs, no.: I 19.3:B1904-Q 1. Geology, Stratigraphic Precambrian. 2. Geology, Structural Michigan. I. Title. II. Series. III. Series: Contributions to Precambrian geology of Lake Superior region; ch. Q. QE75.B9 no. 1904-Q [QE653] 557.3 s dc20 92-27843 [551.7'1'09774] CIP CONTENTS Abstract Ql Introduction Ql Acknowledgments Q3 Structural setting in northern Baraga County Q4 Structures in the Huron River parautochthon Q5 Folds Q5 Foliations Q5 Strain associated with BI deformation Q5 Faults Q6 Structures in the Falls River slice Q6 Folds Q6 Foliations Q12 Strain associated with BI deformation Q14 Faults Q15 Implications for Penokean tectonics in Michigan and Minnesota Q15 Nappe structures Q15 Correlation of thrust boundaries Q17 Correlation of mesoscopic structures Q17 Comparisons with northern Appalachian tectonics Q20 Similarity in terrane assemblages Q20 Origin of gneiss domes Q21 Basement detachments in the Penokean orogen Q21 Origin of metamorphic nodes Q25 Rotation of gneiss domes Q25 References cited Q26 FIGURES 1. Map showing tectonic setting of the Early Proterozoic of Upper Michigan Q2 2. Map showing macroscopic structural setting of the Baraga belt Q4 3. Sketches of typical BI fold profiles in Huron River parautochthon and Falls River allochthon Q7 4. Equal-area hemispherical plots showing mesoscopic structures from Keweenaw Bay to the Falls River and Little Mountain Q8 5. Photomicrographs of chlorite-white mica aggregates in slate of the Michigamme Formation of the Huron River parautochthon Q9 6. Photomicrograph showing microstructure of large opaque minerals in slate of the Michigamme Formation of the Huron River parautochthon Q10 7. Modified Flinn diagram of reduction spot measurements from the slate of the Michi­ gamme Formation of the Baraga belt Qll 8. Sketch of telescoped iron-rich cherty bed in slate of the Michigamme Formation Q12 9. Equal-area hemispherical plots of faults and related features in the Baraga belt Q13 10. Photomicrographs of microstructures in slate of the Michigamme Formation Q14 Contents III 11. Graph of strain distribution shown by reduction spots in slate of the Michigamme Formation Q15 12. Photographs showing mylonites from Penokean thrust faults in Upper Michigan Q16 13. Equal-area hemispherical plots showing rotation of BI structural elements by the Covington section fault Q18 14. Photomicrographs showing microscopic structures in slates, Carl ton County, Minnesota Q19 15. Sketch section comparing tectonic terranes in the Northern Appalachians and Upper Michigan Q22 16. Map and structural model for the development of late Penokean faults in northern Michigan Q24 TABLES 1. Correlation chart of Early Proterozoic rocks in northern Michigan Q3 2. Reduction spot data for slate of the Michigamme Formation Q12 IV Contents CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION Structural Geology of Parautochthonous and Allochthonous Terranes of the Penokean Orogeny in Upper Michigan Comparisons with Northern Appalachian Tectonics By William J. Gregg1 Abstract In northern (Upper) Michigan this structural setting is part of a regional terrane assemblage that is strikingly similar to that The Precambrian terrane of Michigan's Upper Peninsula of the northern Appalachians. Both regions contain a parau­ consists largely of Archean massifs and mantled gneiss domes tochthonous and allochthonous foreland, followed laterally by surrounded by weakly to highly strained belts of Proterozoic a detached basement massif, a tightly appressed metasedimen­ metasedimentary rocks deformed during the Penokean orog­ tary synform, and a compressional mantled gneiss dome- eny, about 1 .85 Ca. The rocks in these Early Proterozoic belts synform complex. The two regions differ in that the deforma­ consist of a thin lower sequence of shelf and shallow-water tion of both basement and cover rocks is of higher intensity in sediments, overlain by a thick upper sequence of slate, silt- the Appalachians. stone, and discontinuous graywacke units, known as the upper In northern Michigan, curved metamorphic isograds may slate member of the Michigamme Formation. The Baraga belt, be related to relative structural position in the gneiss dome- in northern Baraga County, typifies the structural setting of the synform terrane, with the highest metamorphic grade being in upper slate member. The belt can be divided into a weakly the deep synformal hinges. The gneiss domes may have been deformed terrane, the Huron River parautochthon, to the rotated from an initial east-west trend by late Penokean faults north, and a polyphase-deformed terrane, the allochthonous produced by north-northeast-directed tectonic indentation by Falls River slice, to the south. The change in structural style and an irregular projection on the north edge of the Wisconsin finite strain between the two terranes occurs abruptly along the magmatic arc terranes. east-west-trending Falls River thrust, which appears to corre­ The tectonic interpretations presented in this report sug­ late with a similar, previously recognized boundary in Carlton gest that at the beginning of the Proterozoic, the southern com­ County, Minnesota. plex and possibly part of the