Geologic Strip Map Along the Hines Creek Fault Showing Evidence for Cenozoic Displacement in the Western Mount Hayes and Northeastern Healy Quadrangles, Eastern Alaska Range, Alaska By Warren J. Nokleberg, John N. Aleinikoff, Thomas K. Bundtzen, and Maiana N. Hanshaw Pamphlet to accompany Scientific Investigations Map 3238 2013 U.S. Department of the Interior U.S. Geological Survey ii Contents Introduction.......................................................................................................................................................................................1 General Information......................................................................................................................................................................1 Major Geologic Units ....................................................................................................................................................................1 Summary of Hines Creek and Adjacent Faults.............................................................................................................................2 Topographic Setting of Eastern Alaska Range and Sources of Geologic Mapping .....................................................................2 Acknowledgments ........................................................................................................................................................................3 Evidence for Cenozoic Displacement on Hines Creek and Associated Faults.................................................................................3 Cenozoic Displacement Along the Hines Creek Fault..................................................................................................................3 Evidence for Cenozoic Displacement Along the Trident Glacier Fault .........................................................................................4 Evidence for Cenozoic Displacement Along the McGinnis Glacier Fault .....................................................................................5 SHRIMP U-Pb Geochronology of Zircon ..........................................................................................................................................5 Methods........................................................................................................................................................................................5 Samples........................................................................................................................................................................................6 Results..........................................................................................................................................................................................6 Description of Map Units ..................................................................................................................................................................6 Surficial Units................................................................................................................................................................................6 Sedimentary Units ........................................................................................................................................................................8 Mélange....................................................................................................................................................................................9 Post-Accretionary Plutonic Rocks ................................................................................................................................................9 Terranes (North to South)...........................................................................................................................................................11 Yukon-Tanana Terrane ..........................................................................................................................................................11 Jarvis Creek Glacier Subterrane ........................................................................................................................................11 Pingston Terrane—South of Hines Creek Fault and North of Nenana Glacier and Denali Faults .........................................15 Aurora Peak Terrane—South of Nenana Glacier Fault and North of Denali Fault.................................................................18 Windy Terrane—Within Splays of Denali Fault ......................................................................................................................21 Maclaren Terrane—South of Denali Fault and North of Broxson Gulch Thrust .....................................................................23 Wrangellia Terrane.................................................................................................................................................................25 References Cited............................................................................................................................................................................26 Figure Figure 1. Representative images and SHRIMP U-Pb data from zircon from plutonic rocks. A, CL images of zircon from 10ANK001 (leucodiorite). B, Tera-Wasserburg plot of SHRIMP data from 10ANK001. C, CL images of zircon from 10ANK004 CL images of zircon from 10ANK004 (porphyritic granodiorite). D, Tera-Wasserburg plot of SHRIMP data from 10ANK004 (porphyritic granodiorite). ............................................30 Table Table 1. SHRIMP U-Th-Pb data for zircon from two granitic plutons near the Hines Creek Fault, Alaska. ...............................31 iii iv Geologic Strip Map Along the Hines Creek Fault Showing Evidence for Cenozoic Displacement in the Western Mount Hayes and Northeastern Healy Quadrangles, Eastern Alaska Range, Alaska By Warren J. Nokleberg John N. Aleinikoff, Thomas K. Bundtzen, and Maiana N. Hanshaw Introduction General Information This pamphlet augments the companion geologic map by providing an interpretative text describing Cenozoic faulting in the region and by providing a detailed description of map units. The companion geologic map is a strip geologic map along the Hines Creek Fault from the central and northwestern Mount Hayes quadrangle into the northeastern Healy quadrangle. The strip geologic map is provided in ArcMap GIS and Adobe PDF formats. For a listing of files and directories for the map, read the file titled 1_Hines_Creek_Fault_Map_readme.txt. Major Geologic Units Nine major groups of surficial units and bedrock units are present in the map area: (1) upper Pliocene, Pleistocene, and Holocene surficial units—mainly various types of alluvium, colluvium, and glacial deposits; (2) Oligocene through Pliocene sedimentary units—mainly the Nenana Gravel and an older unit of sandstone, siltstone, and lesser conglomerate; (3) an extensive suite of plutonic units— mainly Cretaceous granodiorite and quartz monzonite; (4) Devonian and possibly older Jarvis Creek Glacier subterrane that comprises the southern part of the Yukon-Tanana terrane; (5) Silurian through Triassic metasedimentary and metavolcanic rock structural mélange (that is, a structural breccia formed of highly sheared rock fragments) of the Pingston terrane; (6) Aurora Peak terrane with Paleozoic metasedimentary rock and Cretaceous and lower Tertiary metagranitic rock; (7) sedimentary and volcanic rock structural mélange of the mainly Cretaceous Windy terrane; (8) Maclaren terrane with metamorphosed Upper Jurassic and Lower Cretaceous flysch and Cretaceous and lower Tertiary metagranitic rock; and (9) upper Paleozoic sedimentary and volcanoclastic rock, ultramafic plutonic rock, gabbro, and metagabbro of the Wrangellia terrane. North of the Denali Fault, the bedrock geology is dominated by the Devonian and older Yukon- Tanana terrane, a complex of multiply deformed and metamorphosed sedimentary, volcanic, and plutonic rocks (Jones and others, 1987; Aleinikoff, 1984a,b; Aleinikoff and Nokleberg, 1985a,b; Nokleberg and Aleinikoff, 1985). To the south of the fault, the bedrock geology is dominated by the Mesozoic Maclaren and Paleozoic and Mesozoic Wrangellia terranes (Jones and others, 1987; 1 Nokleberg and others, 1982, 1985, 1989). A number of granitic to gabbroic plutons, chiefly of Mesozoic age, intrude the terranes both north and south of the Denali Fault. Major faults or sutures separate terranes, and many younger Cenozoic faults occur along the margins of, and within, terranes. Summary of Hines Creek and Adjacent Faults Geologic mapping of the Hines Creek Fault and the adjacent Trident Glacier and McGinnis Glacier Faults to the north in the eastern Alaska Range, Alaska, reveals that these faults were active during the Cenozoic. Previously, the Hines Creek Fault, which is considered to be part of the strike-slip Denali Fault system (Ridgway and others, 2002; Nokleberg and Richter, 2007), was interpreted to have been welded shut during the intrusion of the Upper Cretaceous Buchanan Creek pluton (Wahrhaftig and others, 1975; Gilbert, 1977; Sherwood and Craddock, 1979; Csejtey and others, 1992). Our geologic mapping along the west- to west-northwest-striking Hines Creek Fault in the northeastern Healy quadrangle and central to northwestern Mount Hayes quadrangle reveals that (1) the Buchanan Creek pluton is truncated by the Hines