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The Black Mountain Tectonic Zone—A Reactivated Northeast-Trending Crustal Shear Zone in the Yukon-Tanana Upland of East-Central Alaska

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The Black Mountain Tectonic Zone—A Reactivated Northeast-Trending Crustal Shear Zone in the Yukon-Tanana Upland of East-Central Alaska The Black Mountain Tectonic Zone—A Reactivated Northeast-Trending Crustal Shear Zone in the Yukon-Tanana Upland of East-Central Alaska By J. Michael O’Neill, Warren C. Day, John N. Aleinikoff, and Richard W. Saltus Chapter D of Recent U.S. Geological Survey Studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada—Results of a 5-Year Project Edited by Larry P. Gough and Warren C. Day Scientific Investigations Report 2007–5289–D U.S. Department of the Interior U.S. Geological Survey iii Contents Abstract ........................................................................................................................................................D1 Introduction..................................................................................................................................................D1 Black Mountain Tectonic Zone .................................................................................................................D5 Last Chance-Goodpaster Fault System ..........................................................................................D5 Tibbs Fault ...........................................................................................................................................D5 Brink Fault ...........................................................................................................................................D6 Serpentine Fault .................................................................................................................................D7 Porphyry Fault Zone...........................................................................................................................D7 Lineaments ..........................................................................................................................................D7 Summary.......................................................................................................................................................D8 References Cited.........................................................................................................................................D8 Figures D1. Simplified geologic map showing tectonic assemblage of the ukon-TananaY tectonostratigraphic terrane of east-central Alaska ...........................................................D2 D2. Simplified geologic map of the Big Delta B–1 and B–2 quadrangles, east-central Alaska, showing location of gold mines and prospects and the surficial expression of the Black Mountain tectonic zone. ................................................................D3 D3. Regional aeromagnetic map of the Yukon-Tanana Upland of east-central Alaska showing location of the Black Mountain tectonic zone. .....................................................D4 D4. Structural geologic map of the Big Delta B–1 quadrangle showing major high-angle strike-slip and normal faults, thrust faults, and lineaments in the Black Mountain tectonic zone. ................................................................................................D6 The Black Mountain Tectonic Zone—A Reactivated Northeast-Trending Crustal Shear Zone in the Yukon- Tanana Upland of East-Central Alaska By J. Michael O’Neill,1 Warren C. Day,1 John N. Aleinikoff,1 and Richard W. Saltus1 Abstract Introduction The Black Mountain tectonic zone in the Yukon- The Black Mountain tectonic zone, recently recognized Tanana terrane of east-central Alaska is a belt of diverse and mapped in the Big Delta B–1 quadrangle, east-central northeast-trending geologic features that can been traced Alaska, lies within the Yukon-Tanana Upland (YTU) (fig. D1; across Black Mountain in the southeast corner of the Big Delta see also fig. 1 of the Editors’ Preface and Overview). The YTU 1°×3° degree quadrangle. Geologic mapping in the larger is restricted to the terrane between the Tintina and Denali scale B1 quadrangle of the Big Delta quadrangle, in which dextral-slip fault zones and extends from central Alaska Black Mountain is the principal physiographic feature, has eastward into Yukon, Canada. The YTU is underlain mainly revealed a continuous zone of normal and left-lateral strike- by complexly deformed greenschist to amphibolite-grade slip high-angle faults and shear zones, some of which have metamorphic rocks at least as old as Middle Devonian. The late Tertiary to Quaternary displacement histories. The tectonic supracrustal metasedimentary and metavolcanic Paleozoic zone includes complexly intruded wall rocks and intermingled units were intruded during the Devonian by plutonic rocks apophyses of the contiguous mid-Cretaceous Goodpaster and of intermediate composition, which are now preserved as Mount Harper granodioritic plutons, mafic to intermediate augen and biotite orthogneiss. After the tectonic assembly composite dike swarms, precious metal mineralization, early of the Paleozoic units during the regional early Mesozoic Tertiary volcanic activity and Quaternary fault scarps. These deformation, Jurassic, Cretaceous, and Tertiary plutonism structures define a zone as much as 6 to 13 kilometers (km) flooded the area with mainly granodioritic batholiths wide and more than 40 km long that can be traced diago- (Dusel-Bacon and others, 2002). nally across the B1 quadrangle into the adjacent Eagle 1°×3° The YTU between Fairbanks and Tok is cut by four quadrangle to the east. Recurrent activity along the tectonic prominent northeast-trending lineaments (Wilson and others, zone, from at least mid-Cretaceous to Quaternary, suggests 1985). One such lineament is the Shaw Creek fault (fig. D1), the presence of a buried, fundamental tectonic feature beneath a major left-lateral strike-slip fault that cuts across the Salcha the zone that has influenced the tectonic development of River gneiss dome (Foster and others, 1979; Werdon and this part of the Yukon-Tanana terrane. The tectonic zone, others, 2004). Two other lineaments to the east, the Sixtymile centered on Black Mountain, lies directly above a profound and Tok, are recognized by the alignment of physiographic northeast-trending aeromagnetic anomaly between the Denali features, mainly stream drainages and topographic highlands. and Tintina fault systems. The anomaly separates moderate The Mount Harper lineament, about 100 kilometers (km) east to strongly magnetic terrane on the northwest from a huge, of the Shaw Creek fault and confined mainly to the Eagle weakly magnetic terrane on the southeast. The tectonic zone is 1°×3° quadrangle, directly east of the map area (see fig. D2), parallel to the similarly oriented left-lateral, strike-slip Shaw consists of mapped segments of faults, stratigraphic breaks, Creek fault zone 85 km to the west. and physiographic alignments (Foster, 1976). The Mount Harper lineament appears to separate domains that have characteristic geochronologic signatures; Jurassic plutons do not occur west of this lineament (Foster, 1976). The YTU between Fairbanks and Tok also is bisected by a major linear magnetic anomaly located between the Shaw Creek and Mount Harper lineaments (Saltus and Day, 2006). 1 U.S. Geological Survey. A steep, pronounced, northeast-trending magnetic gradient, D2 Recent U.S. Geological Survey Studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada 146° 145° 147° Qs 144° |w TKis |cf |cf TKis M|s 143° |sc Tintina fault |cf |cf t ul 142° |cf fa 65° k e |ss re C w |n a Fairbanks h |ss S Kg Eagle TKis Kg |n Qs Salcha River |n Dome |n |ss |o J^g Area of figure 2 Kg |f Big Delta Black Mountain tectonicMount zone Harper lineament |ss |o Delta Kg Chicken Junction |gf 64° J^g Qs Location map Qs |o nt UNITED STATES e Northwest m |o CANADA Territories a Alaska ne li Tok lineament Kg ile tym Six Area of figure Yukon TKis |n British Columbia Kg Qs Pacific Ocean Tok |o 0 50 KILOMETERS Kg 63° EXPLANATION Qs Surficial deposit (Quaternary) Contact M|s Igneous and sedimentary rocks, undifferentiated (Tertiary to Cretaceous) Fault TKis Parautochthonous sedimentary rocks (pre-Tertiary) Thrust fault—Sawteeth on upper plate Kg Granitoid (Cretaceous) Trend of tectonic feature—Lineament or tectonic zone J^g Granitoid (Early Jurassic to Late Triassic) |ss Major road Highest Oceanic and associated sedimentary rocks (Paleozoic)—|ss, Seventymile |sc assemblage; |sc, Chatanika assemblage Tectonites of the Yukon-Tanana Upland (Paleozoic) |f Fortymile River assemblage—Oceanic, arc, and marginal-basin tectonites |n Nisutlin assemblage—Marginal-basin strata Structural level |o Orthogneiss assemblage—Continental-affinity paragneiss and peraluminous augen 340-Ma orthogneiss Lowest |cf Rocks of the Chena River and Fairbanks “subterranes”—Parautochthonous continental strata |w Parautochthonous Wickersham assemblage (Paleozoic) Figure D1. Simplified geologic map showing tectonic assemblage of the ukon-TananaY tectonostratigraphic terrane of east-central Alaska. Shows approximate outline of map area (fig. D2), and location of major northeast-trending lineaments of Wilson and others (1985). Modified from Hansen and Dusel-Bacon (1998). The Black Mountain Tectonic Zone—A Shear Zone in the Yukon-Tanana Upland of East-Central Alaska D3 145°00’ 144°30’ 144°00’ 64°30’ |o Goodpaster Kg Kg River 60 River Kd Pogo Kg Qc 40 Kdt Kg 47 Kg 80 32 |o |o Creek 64 48 Shawnee |m Peak 55 Kdt Kbm 55 ibbs 38 T Kd Ka |m 34 | Goodpaster m Qa 65 Kmg Kmg 32 Central Kbm Km 45 45 32 36 Creek 26 Grizzly Bear 52 Creek Qc 40 |m 34 Gray Lead 36 Ka 10 15 Blue Lead Kg 22 Boulder 43 10 45 Ka Km
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