Active and Potentially Active Faults in Or Near the Alaska Highway Corridor, Dot Lake to Tetlin Junction, Alaska

Division of Geological & Geophysical Surveys

PRELIMINARY INTERPRETIVE REPORT 2010-1

ACTIVE AND POTENTIALLY ACTIVE FAULTS IN OR NEAR THE ALASKA HIGHWAY CORRIDOR, DOT LAKE TO TETLIN JUNCTION, ALASKA

by Gary A. Carver, Sean P. Bemis, Diana N. Solie, Sammy R. Castonguay, and Kyle E. Obermiller

September 2010

THIS REPORT HAS NOT BEEN REVIEWED FOR TECHNICAL CONTENT (EXCEPT AS NOTED IN TEXT) OR FOR CONFORMITY TO THE EDITORIAL STANDARDS OF DGGS.

Released by

STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES Division of Geological & Geophysical Surveys 3354 College Rd. Fairbanks, Alaska 99709-3707

$4.00

CONTENTS

Abstract ...... 1 Introduction ...... 1 Seismotectonic setting of the Tanana River valley region of Alaska ...... 3 2008 fi eld studies ...... 5 Field and analytical methods ...... 5 Dot “T” Johnson fault ...... 7 Robertson River step-over ...... 16 Cathedral Rapids fault ...... 17 Giant Moletrack ...... 18 Terrace profi les ...... 19 Cliffhanger trench...... 22 Giant Moletrack trench ...... 26 Mansfi eld Creek lineaments ...... 31 Dennison Fork lineament - Tok River valley ...... 32 Conclusions ...... 33 Acknowledgments ...... 37 References ...... 38

FIGURES

Figure 1. Alaska Highway corridor map 2008 ...... 2 2. Regional neotectonic map ...... 4 3. Field area map ...... 6 4. Sears Creek Trench log ...... 8 5. D“T”J trench III photo view to the north ...... 9 6. Close-up view, meter 7–10, east wall D“T”J III trench ...... 10 7. Trench D“T”J III east wall photo mosaic ...... 11 8. Trench D“T”J III west wall photo mosaic ...... 12 9. Trench D“T”J III log, east wall ...... 13 10. Trench D“T”J III log, west wall ...... 14 11. Stereogram of faults and shears, trench D“T”J III ...... 15 12. Bear Creek lineament oblique air photo ...... 16 13. Bear Creek lineament trench photo ...... 17 14. Cathedral Rapids fault map ...... 18 15. Giant Moletrack oblique air photo view to the southeast ...... 19 16. AirSAR image and profi les across the Giant Moletrack anticline and Cathedral Rapids fault scarp ...... 20 17. Catastrophe Creek terrace profi les ...... 21 18. Catastrophe Creek study site map ...... 22 19. Oblique air photo of the Cliffhanger trench site ...... 23 20. Cliffhanger trench photo ...... 23 21. Cliffhanger trench photo mosaic ...... 24 22. Cliffhanger trench log ...... 25 23. GMT I trench photo ...... 26 24. GMT I trench photo mosaic ...... 27 25. GMT I trench close-up photo ...... 28 26. GMT I trench log ...... 29 27. Cathedral Rapids fault seismicity map ...... 30 28. Mansfi eld lineament oblique air photo ...... 31 29. Mansfi eld trench log ...... 32 30. Dennison Fork lineament, Tok River valley map ...... 33 31. Dennison Fork lineament trenches photo mosaics ...... 34 32. Trench I log - Dennison Fork lineament, Tok River valley ...... 35 33. Trench II log - Dennison Fork lineament, Tok River valley ...... 36

TABLES

Table 1. Location of trench sites excavated as part of active fault investigations...... 5

APPENDIX

14C ages for samples collected from trenches in 2008 ...... 41 ACTIVE AND POTENTIALLY ACTIVE FAULTS IN OR NEAR THE ALASKA HIGHWAY CORRIDOR, DOT LAKE TO TETLIN JUNCTION, ALASKA

by Gary A. Carver1,2, Sean P. Bemis3, Diana N. Solie4, Sammy R. Castonguay3, and Kyle E. Obermiller5

ABSTRACT In 2005 the Alaska Division of Geological & Geophysical Surveys initiated a multi-year geologic fi eld study focused on a corridor centered along the Alaska Highway between Delta Junction and the Canada border. The purpose of this project is to provide geologic information relevant to a proposed Alaska–Canada natural gas pipeline and other future development in the corridor. Identifi cation of active faults and characterization of seismic hazards were included in the project. During the 2006 and 2007 field seasons, lineaments and geologic features indicative of possible youthful surface faulting in or near the western half of the corridor between Delta Junction and Dot Lake were identified and evaluated. Four of the lineaments were found to be active (Holocene) faults. These are: (1) the Dot “T” Johnson fault, a thrust fault on the south side of the Tanana River valley along the base of the Alaska Range, (2) the Canteen fault, a strike-slip tear fault joining segments of the Dot “T” Johnson fault at a left step in the range front in the Little Gerstle River valley, (3) the Panoramic fault, also a left-lateral tear fault connecting the west end of the Dot “T” Johnson fault with the east end of the Donnelly Dome fault at a left step of the Alaska Range on the west fl ank of Granite Mountain, and (4) the Billy Creek fault, a northeast-trending left normal oblique fault north of the Tanana River valley in the Billy Creek drainage. The Dot “T” Johnson fault is interpreted to be an eastern extension of the Northern Foothills Fold and Thrust Belt (NFFTB), a system of active thrusts and fault-generated folds along the northern fl ank of the Alaska Range west of the Delta River.

During the 2008 fi eld season the active fault studies were focused on the central part of the corridor between Dot Lake and Tetlin Junction. Field studies included helicopter and fixed-wing air reconnaissance augmented by interpretation of stereo air photos, remotely sensed images, ground reconnaissance, and fi eld mapping. Detailed investigations were conducted where the reconnaissance identifi ed lineaments indicative of Holocene surface faulting. In addition, detailed fi eld studies were conducted of several lineaments identifi ed in the published literature as possible active faults. The detailed studies included fi eld mapping, topographic profiling, trenching, and 14C dating of surficial sediments associated with lineaments exhibiting characteristics of active faults. Two faults and a large fault-related anticline were found to have generated late Pleistocene and Holocene surface deformation in the Dot Lake–Tetlin Junction section of the corridor. These structures, the eastern part of the Dot “T” Johnson fault, the Cathedral Rapids fault, and the Giant Moletrack anticline, are active structures in the eastern part of the NFFTB. Detailed investigation of the Mansfi eld and Dennison Fork lineaments, identifi ed as candidate active faults in the literature, produced conclusive evidence that they are not active faults. Detailed study of a third suspect lineament, the Bear Creek lineament, was inconclusive.

INTRODUCTION The Alaska Highway is the principal land transportation route connecting interior Alaska to Canada and the lower 48 United States and is the locus of signiﬁ cant planned and proposed development. In the Tanana River valley between Delta Junction and the Canada border, the Alaska Highway corridor is a 25-km-wide, 320-km- long swath centered on the highway (ﬁ g. 1). The corridor includes proposed routes for an Alaska–Canada natural gas pipeline and an extension of the Alaska Railroad through Canada (Solie and Burns, 2006, 2007). In order to allow informed evaluations of future development plans and guide engineering and design decisions regarding the proposed natural gas pipeline, the Alaska railroad extension, and other development in the corridor, the State

1Department of Geology, Humboldt State University, Emeritus, Arcata, CA 95521; [email protected] 2Carver Geologic Inc., Kodiak, AK 99615 3Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 4Baseline Geoconsulting, LLC. P.O. Box 82293, Fairbanks, AK 99708-2293 5Department of Geological Engineering, University of Alaska, Fairbanks, AK 99775 2 Preliminary Interpretive Report 2010-1

Map Alaska

Highway miles 0 10 Corridor 0 10 Junction kilometers

2008 Study Area N

Figure 1. The Alaska Highway corridor is a 25-km-wide, 320-km-long area centered on the Alaska Highway between Delta Junction and the Canada border where the Alaska Division of Geological & Geophysical Surveys (DGGS) is conducting ﬁ eld studies to develop information about the geology in support of proposed development including the Alaska–Canada natural gas pipeline and an extension of the Alaska Railroad through Canada. During the 2008 ﬁ eld season the studies were focused on the central part of the corridor between Dot Lake and Tetlin Junction.

Legislature in 2005 authorized a multi-year geologic framework assessment project to be conducted by the Alaska Division of Geological & Geophysical Surveys (DGGS). One of the components of the project includes mapping and characterization of active and potentially active faults in and near the Alaska Highway corridor. This report summarizes the active and potentially active fault studies of the framework project conducted in the central section of the corridor between Dot Lake and Tetlin Junction during the 2008 fi eld season, and presents the preliminary results and conclusions from these investigations. Active faults are defi ned as those that show evidence of surface displacement during the Holocene (last 11,000 years), and have the potential for future movement. This report summarizes results of studies of active and potentially active faults conducted during the 2008 fi eld season in and adjacent to the central section of the corridor that could impact existing and future development. The studies in 2008 build on mapping and fi eld investigations of the western section of the Alaska Highway corridor completed in 2006 and 2007 (Carver and others, 2008). These studies are intended to guide users to information about specifi c active and potentially active faults in the region. Applications of this information to specifi c development projects may require additional investigation to more thoroughly determine fault locations and displacement parameters. Due to the reconnaissance nature of this work, additional active faults may be found in the region with more detailed mapping and study. In this report, in order to distinguish active and potentially active faults from inactive faults other lineaments, we have reserved the term “active fault” for those on which we observe evidence of recent (Holocene) movement. The term “lineament” as used in this report includes (a) active faults not yet recognized as such, (b) older faults that ceased activity more than 11,000 years ago, and (c) linear features on air photos and remotely sensed images that are not related to faulting. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 3

SEISMOTECTONIC SETTING OF THE TANANA RIVER VALLEY REGION OF ALASKA The active tectonics of interior Alaska is driven by the ~65 mm/yr north–northwest convergence of the Pacifi c and North American plates. The process that transfers tectonic forces inland from the Pacifi c plate to the interior of Alaska is complex and not well understood. It involves effects of an ongoing collision between the internally deforming Yakutat microplate and counter-clockwise rotation of the Wrangell microplate, two recently identifi ed crustal blocks that comprise part of the complex North American–Pacifi c plate boundary (Chapman and others, 2008; Freymueller and others, 2008). The Denali–Totschunda fault system defi nes the broadly arcuate northern boundary of the Wrangell microplate (Haeussler, 2008). Right-lateral slip along the Denali–Totschunda fault system accommodates the rotation of the Wrangell microplate in south-central and southwestern Alaska (fi g. 2). The slip rates on the central Denali fault, source of the 2002 M7.9 Denali fault earthquake, are about 9 to 14 mm/yr (Matmon and others, 2006). Quaternary uplift, youthful faulting and folding in the Alaska Range, subsidence of the Tanana lowlands, and signifi cant levels of seismicity as far north as the southern margin of the Brooks Range are evidence of active tectonics in central Alaska. Right-lateral slip on the Denali fault accommodates much of the westward component of motion of the Yakataga and Wrangell microplates. A component of north-directed contraction is transferred across the Denali fault and accommodated by east–west-oriented youthful thrust faults and folds in the foothills region on the north fl ank of the Alaska Range. These youthful folds and faults compose the Northern Foothills Fold and Thrust Belt (NFFTB) (Ridgway and others, 2002; Bemis, 2004; Carver and others, 2008). West of the corridor study area between the Delta River and Denali National Park and Preserve, the ~50-km-wide NFFTB imparts a pronounced east–west grain to the topography (fi g. 2). North-fl owing rivers heading in the Alaska Range are antecedent to the folds and faults and their long profi les display perturbations related to active growth of these structures (Bemis, 2004; Carver and others, 2006, Bemis and Wallace, 2007; Lesh and Ridgway, 2007; Carver and others, 2008). The folds and faults deform late Neogene and Quaternary alluvial surfaces and fl uvial terraces. Shortening across the NFFTB near the Nenana River, based on a balanced cross-section analysis of deformed late Tertiary Usibelli Group and Nenana Gravel, is about 8.8 km (Bemis, 2004). Assuming the onset of thrust faulting and folding in the region coincided with the end of deposition of the Nenana Gravel about 3 my ago, the average shortening rate across the NFFTB in the vicinity of the Nenana River is at least 3 mm/yr (Bemis, 2004; Bemis and Wallace, 2007). Seismicity in the northern foothills region supports this interpretation (Doser, 2004). Reconnais- sance of the thrust faults between the Delta and Nenana rivers shows many are marked by steep, uneroded scarps in late Pleistocene and Holocene sediments (Bemis, 2004; Carver and others, 2006, 2008). West of the corridor study area, the northern foothills of the Alaska Range are bordered on the north by the actively subsiding Tanana foreland basin that underlies an extensive alluvial and swampy lowland comprising the Tanana River valley (Ridgway and others, 2002). The Tanana basin narrows and shallows to the east in the vicinity of the study area and the north side of the basin bounds the Yukon–Tanana Upland, a region of low hills and mountains dissected by mature drainages. Seismicity north of the northern foothills includes several northeast- trending zones of shallow seismicity that include fi ve historic earthquakes between M6 and M7.3 (Ruppert and others, 2008). In the Fairbanks region these include the Minto Flats, Fairbanks, and Salcha seismic zones (fi g. 2). Focal mechanisms for earthquakes in these seismic zones are dominantly left lateral (Ratchkovski and Hansen, 2002). The largest was a M7.3 in 1937 in the Salcha seismic zone. East of Fairbanks, northeast-oriented streams parallel to the seismic belts suggest structural control of the drainage along faults. The northeast-trending seismic zones and faults are interpreted to refl ect deformation from clockwise horizontal rotation of fault-bounded blocks between the Denali and Tintina fault systems (Page and others, 1995; Lesh and Ridgway, 2007). Upper crustal seismicity in the Tanana River valley region includes many earthquakes in the Alaska Range and NFFTB west of the study area and modest seismicity along the Denali fault south of the corridor, but few earthquakes have been located within the corridor (fi g. 2). The seismicity shown on fi gure 2 is from the National Earthquake Information Service (NEIS) database for the period 1979 to 2002. To provide a view of the regional background seismicity, we have not included the main shocks (M6.7 and 7.9) or aftershocks from the 2002 Denali fault earthquake sequence. It is noteworthy that the seismicity in the northern foothills region west of the study area is similar to the northern fl ank of the Alaska Range adjacent to the corridor. The 2002 Denali fault earthquake sequence generated surface displacement on the previously unknown Susitna Glacier fault, the central section of the Denali fault, and the northern part of the Totschunda fault (fi g. 2) (Eber- hart-Phillips and others, 2003). Surface displacement also occurred on part of the central section of the Denali fault near the Delta River in 1912 (Carver and others, 2004). No other historic surface faulting events are known in central Alaska. 4 Preliminary Interpretive Report 2010-1 M>5 Historic Holocene Late Pleistocene Late Faults Depth < 33 km ank of the Alaska Range. ank of the Seismicity fl M<5 Center (NEIC) 1979 - 2002 Center BF - Billy Creek Fault (LL) Fault BF - Billy Creek (T) Fault Creek BCF - Buzzard (LL) Fault CF - Canteen (T) Rapids Fault CRF - Cathedral (RL) (west) DFw - Denali Fault (RL) (central) DFc - Denali Fault (east) (RL) - Denali Fault DFe (T)DDF - Donnelly Dome Fault (T) Johnson Fault “T” DTJ - Dot (T) Fault GCF - Glacier Creek (LL) Mountain Fault GMF - Granite (RL) Fault HF-Hines Creek (T) Fault HCF - Healy Creek (T)JHS - Japan Hills Splay (T) - Kansas Fault Creek KCF (RL) MGF - McGinnis Glacier Fault (LL?) MF - Minto Fault (T)MRF - Molybdenum Ridge Fault (T)MMF - Mystic Mountain Fault (T)Thrust NFT - Northern Foothills (LL?) Fault PF - Panoramic (T) Road Fault PRF - Parks (T)RF - Rex Fault (T)RMF - Red Mountain Fault SF - Stampede Fault (T)SGF - Susitna Glacier Fault (LL?) Fault Trident TF - (RL) Fault Totschunda - TsF National Earthquake Information ion west of the study area, but in ion west of the study area, (RL) - Right Lateral (LL ) - Left Lateral (T) Lateral (LL ) - Left (RL) - Right Lateral Thrust) - north black. 64° 63° 65° 62° Range. The 2002 rupture segments of Range. The 2002 rupture Range and the thrust faults of North- N Canada Canada FMP YMP km km Gulf of Alaska PP

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PRF Rex Rex HCF 150° 149° 148° 147° 146° 145° 144° 143° 142° 141° 150° 149° 148° 147° 146° 145° 144° 143° 142° 141° Principal faults in the region include the Denali fault, a major right lateral fault that extends along the axis of the Alaska include the Denali fault, a major right lateral fault that extends along axis of Principal faults in the region Alaska folds in the foothills north of ern Foothills Fold and Thrust Belt, a system of thrusts associated fault-related Holocene faults in orange, and late Pleistocene shown in red, faults are Denali, and Totschunda the Susitna Glacier, the vicinity of the Alaska Highway corridor shallow seismicity was sparse with most earthquakes located in the foothills on the vicinity of 65° 64° 63° 62° Figure 2. Regional seismicity and fault map. During the period 1979–2002 many upper crustal earthquakes were located in the reg 2. Regional seismicity and fault map. During the period 1979–2002 many upper crustal earthquakes were Figure Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 5

Recent mapping in the northern foothills of the Alaska Range has identifi ed a number of faults with late Pleis- tocene and Holocene displacement (fi g. 2) (Bemis, 2004, 2006; Thoms, 2005; Carver and others, 2006, 2008). The majority of these are relatively low-slip-rate thrust faults interpreted to accommodate regional north–south-directed contraction resulting from convergence of the Pacifi c plate and collision of the Yakataga block in southeastern and south-central Alaska with the North American plate north of the Denali fault. During the 2007 corridor studies, we documented four faults with Holocene displacement: the Panoramic, Canteen, Dot “T” Johnson, and Billy Creek faults (Carver and others, 2008). The Dot “T” Johnson fault is a generally east–west-trending, south-dipping, low-angle thrust fault that borders the northern fl ank of the Alaska Range along the southern margin of the Tanana River valley. It is interpreted to be part of the eastern extension of the NFFTB. The Panoramic and Canteen faults are north-trending left-oblique-slip faults. The Panoramic and Granite Mountain faults connect the east end of the Donnelly Dome fault and the west end of the Dot “T” Johnson fault where the range front steps left about 15 km along the northwestern fl ank of Granite Mountain (fi g. 2). The left-lateral Canteen fault joins left-stepping segments of the Dot “T” Johnson thrust fault. The Billy Creek fault is a left-lateral strike-slip fault on the north side of the Tanana River valley. This fault has a northeast trend parallel to the Minto Flats, Fairbanks, and Salcha seismic zones.

2008 FIELD STUDIES During the summer of 2008, fi eld studies and mapping focused on the section of the Alaska Highway corridor between the village of Dot Lake and Tetlin Junction (fi g. 1). Field efforts included trenching investigation of the eastern part of the Dot “T” Johnson fault and evaluation of lineaments identifi ed as potentially active faults in the published and unpublished literature and maps. We also analyzed stereo air photos and conducted helicopter reconnaissance covering the Dot Lake to Tetlin Junction section of the corridor and adjacent parts of the Alaska Range and Yukon–Tanana Uplands to identify additional linear features indicative of geologically youthful faults and surface folds not reported in the literature. Although many lineaments were observed during the helicopter reconnaissance, few exhibited morphology or other geologic evidence of Holocene surface faulting or folding. The reconnaissance resulted in selection of six sites for detailed study. These are the eastern end of the Dot “T” Johnson fault near Dot Lake, the previously unrecognized Cathedral Rapids fault, its associated Giant Moletrack anticline, and the Mansfi eld, Bear Creek, and Dennison Fork lineaments (fi g. 3). AirSAR digital images were used to help map and measure deformation associated with the eastern end of the Cathedral Rapids fault and the Giant Moletrack anticline.

FIELD AND ANALYTICAL METHODS All fi eld location coordinates were collected using a handheld GPS unit (no differential correction was applied). Where lineaments with fi eld characteristics indicative of active or potentially active faults were identifi ed, the features were mapped by low-altitude helicopter overfl ights with GPS coordinates Table 1. Location of trench sites excavated as part of active fault taken at closely spaced intervals along investigation. the lineaments. GPS locations were also taken where surface fi eldwork, includ- Trench site locations UTM; NAD 27 ing topographic profi ling and trenching, Sears Creek trench 6 V 625229 7064849 was performed. Table 1 presents UTM DTJ III trench 6 V 638102 7064659 coordinates (based on the Clark 1866 GMT I trench 7 V 379831 7023325 spheroid, NAD 27 datum, UTM zone 6 Cliffhanger trench 7 V 379831 7023325 or 7 projections) for the trench sites. Dennison Fork Tok trench I 7 V 380556 6999341 Trench sites were chosen to transect Dennison Fork Tok trench II 7 V 380531 6999311 features suspected to be active faults, to expose a cross-sectional view of Mansfi eld Scarp trench 7 V 390221 7064121 faults and the most recent stratigraphy. Bear Creek trench 6 V 647503 7053411 Trenches across the Dot “T” Johnson fault (Sears Creek and D“T”J III) and the Dennison Fork lineament (Dennison Fork lineament–Tok River valley trenches I and II) were dug with a large, track-mounted excavator. Trenches at the Mansfi eld scarp and Bear Creek lineament sites as well as the Dot “T” Johnson trenches I and II and the Giant Moletrack and Cliffhanger trenches at the Catastrophe Creek site were inaccessible to motorized excavation equipment. These trenches were hand dug. To facilitate fi eld interpretation and logging we used nails to pin colored fl agging along exposed contacts and

6 Preliminary Interpretive Report 2010-1

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nelly Highway that were the focus of 2008 that were identi conducted in 2008. The Cathedral Rapids fault and Giant Moletrack anticline, both found to be active, were effort. The Mans inconclusive.

Richardson Richardson Delta River Delta Don Figure 3. Map of the 2008 study area, showing active and potentially faults lineaments in near the central part 3. Map of the 2008 study area, Figure Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 7 faults on the cleaned trench walls except at the Sears Creek site where the instability of the trench walls required contacts to be delineated with spray paint. Logging of the trench walls was done by placing a grid of horizontal and vertical string lines over the exposed trench wall. The grid was used to accurately measure and transfer each feature to cross section Mylar. Each stratigraphic layer and feature of interest was described and locations of radiocarbon samples were recorded. Soil and sediment colors for each stratigraphic unit were defi ned using a Munsell Soil Color chart. Where possible, we collected radiocarbon samples from layers bounding faulting events to bracket the age of paleo-earthquakes interpreted from the stratigraphy. Upon completion, we backfi lled the trenches, re- contoured the trench sites and replaced the natural vegetation. The radiocarbon samples were carefully extracted to minimize contamination. Samples were air dried and stored in the fi eld in aluminum foil bindles placed in labeled plastic bags. The dry samples were transferred to labeled glass vials after leaving the fi eld. Before being sent to the laboratory for analyses, each sample was inspected through a binocular microscope, picked clean of mineral debris, and for some samples individual plant macro- fossils or charcoal grains were separated for analysis. Beta Analytic Radiocarbon Dating Laboratory in Miami, Florida, performed the radiocarbon age analyses on the 14C samples. All analysis was done using AMS methods and included 13C corrections. Laboratory ages were calibrated using the IntCal 04 calibration curve (Reimer and others, 2004). All ages are reported as calibrated two sigma ranges before present (cal BP). The lab age, dating method, material dated, paleoseismic signifi cance, sample and lab numbers, 13C/14C ratio, and two-sigma calendar calibration age ranges for each of the radiocarbon samples are shown in Appendix I. Scarp profi les and slope profi les across lineaments and scarps were measured using a hand level stabilized on an instrument rod of known height and vertical distances between survey points (foresites and backsites) read from horizontal sightings on a stadia-surveying rod. At the Catastrophe Creek site the stream thalweg and terrace profi le data were collected across the Giant Moletrack anticline with a handheld Trimble GeoXH using the Terrasync collection software, and post-processed against three continuously operating GPS stations within ~120 km of the site. We also conducted a hand leveling survey along the part of terrace T4 near the fault tip where it was strongly deformed in the forelimb of the anticline. Slope distances were measured with a 50 m tape. Horizontal distances were calculated and slope and scarp profi les were plotted from the hand level and GPS survey measurements.

DOT “T” JOHNSON FAULT The Dot “T” Johnson fault is an active, south-dipping thrust fault that daylights along the southern margin of the Tanana River valley between Granite Mountain and Dot Lake (fi gs. 2 and 3) (Carver and others, 2008). The fault is the eastern extension of the Donnelly Dome fault and the principal fault in the previously unmapped eastern continuation of the NFFTB. The west end of the Dot “T” Johnson fault is connected with the east end of the Donnelly Dome fault by the left-normal-oblique Granite Mountain and Panoramic faults (fi g. 2). The Dot “T” Johnson fault is approximately 80 km long with two distinct segments, the Granite Mountain and Dot Lake segments (30 and 50 km long, respectively). The segments are separated by a 10 km left step along the Little Gerstle River, where they are connected by the left-lateral Canteen fault (Carver and others, 2008). At the east end of the Dot Lake segment, between Berry Creek and Dot Lake, the fault forms a ~1-km–wide, 10-km-long pop-up thrust wedge. In 2007 two hand-dug trenches (Dot “T” Johnson I and II, fi g. 3) were excavated across a down-to-the-south 3–5-m-high moletrack scarp on the south side of the pop-up wedge (Carver and others, 2008). Detrital charcoal from faulted loess between two colluvial wedges exposed in one of the trenches (D“T”J I, fi g. 3) yielded 14C ages that document two episodes of early Holocene faulting (Carver and others, 2008). Two additional trenches were dug with a large track-mounted excavator across the Dot “T” Johnson fault during the 2008 fi eld season, the Sears Creek and D“T”J III trenches (fi g. 3). The Sears Creek trench was located across a 2-m-high north-facing scarp about 20 m north of the Alaska Highway and 100 m west of Sears Creek. The trench exposed a south-dipping low-angle thrust fault that offsets interbeds and lenses of fl uvial sand, gravel, and overlying silt and loess (fi g. 4). The fault truncates beds and lenses in both the hanging wall and footwall and is marked in the trench by a single 2–4-cm-wide planar zone of mixed silt and sandy silt containing tabular pebbles and cobbles aligned along the fault (fi g. 4). The fault dips 12° to 14° to the south and daylights at the base of the scarp. No secondary or branch faults were observed in the trench. Beds could not be matched across the fault in the 1.5-m-high trench wall exposures, and no evidence of multiple slip episodes were found in the trench, suggesting the dip-slip displacement on the most recent event was at least 3 m and resulted from a single earthquake. No organic material was found in the fl uvial gravels, but detrital charcoal recovered from the base of the overlying silt and loess yielded 14C ages that limit the fault displacement to more recent than 4,430–3,230 cal BP. The base of an unfaulted organic mat capping the silt and loess dated to 650–560 cal BP. 8 Preliminary Interpretive Report 2010-1 North S1 R R R ocations and 2-sigma R R PBS root PSS Granitic boulder Granitic Lenses and layers of small to of small to and layers Lenses medium (1-3 cm) clast supported sandy matrix. pebbles with sparce - silt matrix supported Silty gravel small (1-2 cm) pebbles. rounded sand - 1-3 cm rounded Pebbly pebbles in poorly sorted medium matrix supported. sand, coarse to - 1-4 cm rounded Sandy silty gravel pebbles in silty coarse medium to matrix supported. sand, - 2-4 cm rounded gravel Sandy coarse pebbles in poorly sorted mediun to matrix supported. sand, coarse - 1-2 cm rounded Sandy fine gravel pebbles in poorly sorted mediun to matix supported. sand, coarse sub-angular granitic Bouldery sand, sorted and boulders in well cobbles matrix supported. medium sand, fine to R R R SG PBS SG4 SG1 SG2 SG3 O GO1 SG3 Logged June 28-29, 2008 June 28-29, Logged Garyby Obermiller Carver and Kyle R R Legend Legend SEARS CREEK TRENCH LOG TRENCH LOG SEARS CREEK L2 R R GO SG3 GO1 Organic mat - moss, plant parts, plant parts, mat - moss, Organic loose peat brown - light yellow Loess silt massive dark grey - dark brown, Loess silt. black massive to medium silty sand, Pebbly brown. 1-3 cm colluvium, Gravely pebblesin siltyrounded sandy matrix. sorted medium sand, Well small rounded scattered few pebbles. sorted of moderately Lenses sand with coarse medium to pebbles. small rounded small of rounded Lenses clast (1-2 cm) pebbles, no matrix. supported, GO1 O GO L1 S1 S2 L2 GC PSS S1 R WEST WALL WEST WALL North S2 meters meters PBS SG3 S1 Cal BP 680 - 560 Cal S2 GO1 45678910 Cal BP 4430 - 4196 Cal SG3 GO1 S2 L2 SG S1 Cal BP 3390 - 3230 Cal SG4 O EAST WALL EAST WALL (Reflected view) S1 FAULT FAULT L1 meters meters SG1 SG3 roots roots GO1 23 SG4 GO SG2 SG3 O L1 SG1 FAULT FAULT 1 S2 GO1 SG2 GO1 GO FGO SG3 234567 0 South GC South calibrated ages of radiocarbon samples that limit the timing of faulting are shown on the log. calibrated ages of radiocarbon samples that limit the timing faulting are meters meters meters 3 4 3 2 2 0 1 0 1 Figure 4. Log of the Sears Creek trench across the Dot “T” Johnson fault adjacent to the Alaska Highway near Sears Creek. The l Alaska Highway near Sears Creek. the Dot “T” Johnson fault adjacent to across trench 4. Log of the Sears Creek Figure Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 9

The second trench excavated across North the Dot “T” Johnson fault (trench D“T”J III) during the 2008 fi eld season was about 300 m west of trench D“T”J I, where the moletrack scarp transitions into a 4-m-high, south-facing planar scarp. The trench was excavated in three sections, a 10-m-long, 2-m-deep north section in the hanging wall above the crest of the scarp, a 9-m-long, 3.5-m-deep benched middle section across the scarp, and an 8-m-long, 2-m-deep south section in the footwall (fi gs. 5–8). It was not possible to accurately map discrete time–stratigraphic units through- out the trench because the stratigraphy exposed in the trench is very complex, especially in the hanging wall, where it is strongly deformed. Instead we delineated rock–stratigraphic units based on sediment texture and composition and defi ned these units in a generalized time–stratigraphic context. The oldest stratigraphy exposed in the trench walls consists primarily of interbedded, thin, well-sorted medium and coarse sand deposits with pebble and cobble basal lags. These deposits represent three unconformable packages (oldest to youngest: Ccc and S2; Cms and S1; Cc and Cs/Vcs) that are interpreted to be glaciofl uvial deposits laid down during South large outburst fl oods (Reger and others, 2009). Although no datable material was Figure 5. Dot “T” Johnson trench D“T”J III viewed from the south. found in the fl ood sands, they were likely The trench consisted of a benched central part with shallower un- deposited during the maximum extent of benched north and south sections. Two people are standing at the the Donnelly glaciation last glacial maxi- north end of the trench for scale. mum (LGM) about 20 ka. Two folded and locally overturned sequences of thinly bedded silt, silty sand, poorly sorted to well sorted fi ne and medium sand and well sorted coarse to very coarse sand (folded sequences 1 and 2, fi gs. 9 and 10) are stratigraphically above the fl ood deposits. These folded sequences appear to be preserved in fault-bounded detached fold noses above the main thrust, a nearly horizontal shear zone that extends through the coarse sand of unit Cs. This shear zone is shown on the trench logs as unit Sz (fi gs. 9 and 10). We speculate that this unit of loose coarse sand has low shear strength, acted as a stress guide, and controlled the location of the principal fault displacement. An upper sequence of interbedded fl uvial silt and well sorted fi ne and medium sand (units Ms, Fs, Vfs, and Si) also overlies the fl ood deposits. In the hanging wall, sediments of the upper sequence are highly deformed, probably by cryogenic processes and liquefaction. Radiocarbon ages of 12,840–12,650 and 11,350–11,210 cal BP were obtained from small fragments of detrital charcoal in the deformed fi ne-grained fl uvial silt (unit Si, fi gs. 9 and 10). A sandy deposit (unit C), interpreted to be colluvium, unconformably overlies the upper sequence of fl uvial sands and the folded sequences (fi gs. 9 and 10). A 14C age of 7,980–7,850 cal BP was obtained from a small fragment of detrital charcoal found at the base of unit C. The sand is capped by a layer of loess (unit S) with weak A–C soil development and a 20–30-cm-thick organic mat. A nearly horizontal zone of closely spaced micro-faults and shears in the coarse fl ood sand (unit Sz) defi nes the principal thrust exposed in the trench. This shear zone dips very gently (15°–25°) to the north in the northern end of the trench beneath the uplifted hanging wall and becomes nearly horizontal or slightly south-dipping under 10 Preliminary Interpretive Report 2010-1

the scarp. The shear zone is more than a meter thick in the north part of the trench and thins beneath the scarp to a tapered fault tip beneath the toe of the scarp. The fault does not daylight, but ends at the fault tip in the top of the fl ood deposits about a meter beneath the ground surface. Overlying fl uvial and colluvial sediments and soil are warped up over the tapered shear zone and fault tip. The geometry of the shear zone indicates much of the growth of the scarp has resulted from internal thickening of the shear zone by nearly horizontal thrust displacement on the closely spaced micro-faults and inter-granular shears in the coarse fl ood sands. The dip of micro-faults and shears in the primary thrust zone were between about 10° and 25° and their average strike about N28°W, nearly perpendicular to the N70°E trend of the scarp (fi g. 11). Faults exposed in the trench include both south- and north-dipping, small displacement, secondary conjugate thrusts in the hanging wall above the low-angle zone of closely spaced micro-faults and shears that composed the primary thrust (fi g. 6). Dip-slip displacement of the folded sequences and upper fl uvial sands on these secondary faults range from a few centimeters to more than a meter, showing that part of the total displacement on the fault is accommodated by hanging-wall deformation. Some of the conjugate faults offset the upper sequence of fl uvial silts and sands (units Ms, Fs, Vfs, and Si; fi gs. 8 and 10) and the contact between the upper sequence of fl uvial silt and sand and the underlying fl ood deposits (units Vcs) on discrete faults, but they could not be traced as well defi ned fault planes down through the fl ood sand, probably because the very-coarse-grained fl ood sand deformed internally by distributed inter-granular shear. Dips on most of the secondary conjugate faults range from about 20° to 50° (fi g. 11). The secondary conjugate fault pattern defi nes a principal compression axis (sigma 1) of about N21°W, also nearly perpendicular to the N70°E trend of the scarp (fi g. 11). Both the secondary hanging-wall conjugate faults and the micro-faults and shears in the principal thrust zone indicate a maximum compression direction (sigma 1) of north 21° to 28° west.

East Wall

North South 8 9 10

-1 Shear Zone -1 7 8 9 10 Trench Floor

Figure 6. Close-up view of a section of the east wall of trench D“T”J III. The red lines mark small conjugate thrust faults in the hanging wall near the top of the scarp. The steeply dipping beds marked by the light green, blue, and orange ﬂ agging are in the detached noses of overturned fault propagation folds. At the base of the trench, the red-ﬂ agged contact marks the top of a shear zone that constitutes the principal thrust fault. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 11 7980 - 7850 cal BP 1420 - 1300 cal BP 12840 - 12650 cal BP Figure 7. Photo mosaic of the east wall of trench D“T”J III. 7. Photo mosaic of the east wall trench Figure 12 Preliminary Interpretive Report 2010-1 11350 - 11210 cal BP Figure 8. Photo mosaic of the west wall of trench D“T”J III. 8. Photo mosaic of the west wall trench Figure Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 13

Dot “T” Johnson Fault Trench III, East wall

0 1 2 3 4 5 m 7 8 9 10 11 1 12840 - 12650 cal BP 1420 - 1300 cal BP North section O O S S O North S S O S Ms South C C C m S F1 18 F6 S Ms Vcs 18 F4 E5 Cal BP 12840 - 12650 C Ms Ms Css F2F F7 E2 E3E4 0 F3 F8 C Ms Si Ms F5 Vfs Si MsMs Vfs Vfs Ms E1 Vcs Css10 29 32 Cs Css 14 26 Vcs 23 27 31 39 41 Css Ms E2 33 39 Si 36 CsCs 10 E3 25 Vcs Css Css Si E4 16 Cs Vcs E5 Si 8 22 Si Vcs Si 30 Ms Vfs Vcs 42 -1 Si Vcs 1 Sz

O West East 11 12 13 14 m 15 16 17 18 19 North S 0 C C Bench section Si O Cs S Cs S Si 7980 - 7850 cal BP Cs m -1 C O South Sz S S East O West SiCs Ms C S S Bottom of benc Sz North end of bench h Shear zone Cs Ms Ms Cs 9 Cs measurements -2 7 Sz Sz Sz Cs Bottom Bottomof upper bench CsS2 of benc CsCs 5 no exposure h CmsS1 S2 BottomBottom of upper of bench bencS2Cc Cs h South end of bench -3 S2Vcs Cms Ccc Ccc -1 19 20 22 23 m 24 25 26 27 North South section O South East O West S S S Ms Ms East wall Ms C m -2 Cs Bench Sz Cs North section Bench section South section N Bench S2 West wall S2 Trench layout plan view -3 Logged: July 23-27, 2008 By: G.Carver, S.Bemis, D.Solie, K.Obermiller, & S.Castonguay South end of trench

Unit Key

O Organic mat

S Soil A/B

C Upper colluvium Folded sequence 1 E1 Poorly sorted fine and medium sand Si Silt E2 Well sorted medium sand Vfs Very fine sand, sandy silt E3 Silt

Fs Fine sand, silty sand E4 Silty coarse sand

Ms Medium sand E5 Well sorted coarse sand

Cs Coarse sand Sz Shear zone in unit Cs Folded sequence 2 F1 Well sorted very coarse sand Vcs Very coarse sand F2 Well sorted coarse sand Css Coarse silty sand F3 Poorly sorted silty sand

Vcss Very coarse silty sand F4 Well sorted medium sand

F5 Poorly sorted fine and medium sand CcS1 Cobbles in poory sorted fine to coarse sand F6 Well sorted medium sand S1S1 Fine and medium sand and silty sand F7 Silt Cobbles in medium to coarse sand Cms F8 Poorly sorted medium and coarse sand

S2 Poorly sorted medium and coarse sand

Ccc Cobbles in poorly sorted fine to coarse sand

Figure 9. Trench log of the east wall of the D“T”J III trench. 14 Preliminary Interpretive Report 2010-1

Dot “T” Johnson Fault Trench III, West Wall (Reflected View)

1 0 1 2 3 4 5 m 6 7 8 9 10 11 12 North North section O South Cal BP 11350 - 11210 S S O S C Ms C Si O C C Si Ms Cs Vfs S Si Ms F3 Vcss CssCss F4 20 0 m Vfs F5 Cs E5 C Si Si C Si Si 19 6 Fs Si Cs Si 24 2 F4 Cs 21 Cs Cs Cs Vcss 35 37 4 Si F3 28 Vsc Cs F8 Css Ms Si F5 Cs Ms -1 Vsc Vsc Sz Sz West East North 0 1 2 12 13 14 15m 16 17 18 19 20 O South S O Bench section 0 S C C Sides of 30 cm deep O cut into trench wall East West Cs Cs S C 210 -1 Ms O Ms Ms S S Sz 15 13 11 Sz Cs C 17 S Bottom of bench Shear zone Ms Ms m measurements Cs Ms C Sz Shear zone Cs North end of bench Sz Sz Cs Cs 3 Cs measurements Sz Sz Sz 1 Cs -2 Cs Cs Cs Cs Cs Cs Cs Bottom of bench South end of bench -3 Cc Cc S1 S1 Cms m -1 20 21 22 23 24 25 26 27 28 South section North O O C S S S East wall C Sz C S Bench Cs North section Bench section South section -2 m Cs Cs Bench Cs N West wall Cs Trench layout plan view Cc S2 S1 Logged: July 23-27, 2008 -3 By: G.Carver, S.Bemis, D.Solie, K.Obermiller, & S.Castonguay Cms

Unit Key

O Organic mat

S Soil A/B

C Upper colluvium Folded sequence 1 E1 Poorly sorted fine and medium sand Si Silt E2 Well sorted medium sand Vfs Very fine sand, sandy silt E3 Silt

Fs Fine sand, silty sand E4 Silty coarse sand

Ms Medium sand E5 Well sorted coarse sand

Cs Coarse sand Sz Shear zone in unit Cs Folded sequence 2 F1 Well sorted very coarse sand Vcs Very coarse sand F2 Well sorted coarse sand Css Coarse silty sand F3 Poorly sorted silty sand

Vcss Very coarse silty sand F4 Well sorted medium sand

S2 Poorly sorted medium and coarse sand

Ccc Cobbles in poorly sorted fine to coarse sand

Figure 10. Trench log of the west wall of the D“T”J III trench. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 15

Equal Area

Compression

Fault* Azmuth Dip no. 1 85 25N 2 60 21S 3 305 16N N70°E 4 85 25S 5 165 25W 6 37 32S 7 200 31W 8 230 7N Scarp 9 208 26W 10 100 35S

11 302 32N N21W 12 80 3S 13 325 21N 14 50 26S 15 305 26N 16 85 21S N = 38 C.I. = 2.0%/1% area 17 114 27S 18 60 38S (a) Secondary conjugate faults in the hanging 19 82 54S wall of the Dot “T” Johnson fault 20 135 40S 21 81 52S Equal Area 22 140 19S 23 104 45S Compression 24 35 10S 25 310 25N 26 40 29S 27 95 35S 28 50 48S 29 20 44E N70°E 30 45 31S 31 57 38S 32 40 20S 33 42 36S 35 104 37S Scarp 37 118 44S 39 44 30S 41 30 32S N28W

* Fault number shown on trench

logs (Figures 10 and 11) N = 35 C.I. = 2.0%/1% area (b) Shear surfaces and micro-faults in the shear zone of the principal thrust of the Dot “T” Johnson fault

Figure 11. Stereogram plots of poles to planes of secondary conjugate faults in the hanging wall (a) and shears and micro-faults in the principal thrust (b) of the Dot “T” Johnson fault measured in trench D“T”J III. The stereograms also show the trend of the scarp at the trench site and best ﬁ t orientation of the axis of principal compression (sigma 1) from the secondary conjugate faults in the hanging wall and the micro-faults and shears in the principal thrust fault shear zone. 16 Preliminary Interpretive Report 2010-1

Multiple episodes of faulting are evident by stratigraphic bracketed upward terminations of secondary thrust faults and faults offsetting faults in the trench. The most recent faulting event is represented by several small faults that cut unit C and terminate at the base of unit S at the south end of the benched section in the west wall of the trench (fi g. 10). Evidence for this event is limited, possibly because the surface displacement was small at this location and the fault slip was largely accommodated by shear in the underlying shear zone. Alternatively, displacement was predominantly blind and accommodated by folding with little surface fault offset during this event. The event occurred more recently than 7,980–7,850 cal BP. Several faulting events are represented by the hanging-wall thrusts that offset the base of the upper sequence of fl uvial silt and sand (units Ms, Fs, Vfs, and Si) but do not cut unit C. Most of these faults die out down-section or are truncated at internal contacts within the fl uvial sequence. The severe internal deformation within these sediments precludes differentiation of individual faulting events. The overprint of cryogenic deformation and liquefaction further complicates the interpretation of paleoseismic events in this part of the trench. These events predate 7,980–7,850 cal BP and may be older than 11,350–11,210 cal BP. Additional earlier paleo-earthquakes are indicated by crosscutting faults within the folded sequences in the fault-bounded fold noses (fi gs. 9 and 10).

ROBERTSON RIVER STEP-OVER The Cathedral Rapids fault and Giant Moletrack anticline represent the principal active range-front structures at the eastern end of the NFFTB (fi g. 3). The Robertson River step-over is an approximately 32 km right step in the Alaska Range foothills, Tanana River valley, and NFFTB between the eastern end of the Dot “T” Johnson fault and western end of the Cathedral Rapids fault (fi g. 3). The nature and location of faults or other active structures that connect the Dot “T” Johnson and Cathedral Rapids faults across the step-over have not been identifi ed. One candidate for a connecting fault is the Bear Creek lineament (fi g. 12). This north–south-trending lineament is east

Donnelly Moraine

Bear Creek Trench

Bear Creek Lineament

Donnelly Outwash

Figure 12. Oblique air photo of the Bear Creek lineament and trench site. View is to the east. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 17 of and parallel to Bear Creek. A linear west-facing scarp that extends from large Donnelly moraines along the west side of the Robertson River north to the Tanana River ﬂ oodplain marks the Bear Creek lineament. A dense spruce forest covers the Bear Creek lineament, precluding effective observation from aircraft. Where it crosses the Alaska Highway the scarp has been eroded by Bear Creek. We attempted to trench the scarp at its southern end where it intersects the Robertson River moraines, but were unable to excavate an adequate trench in ice-rich silty sediments to allow determination of the origin of the scarp (ﬁ g. 13). Whether the Bear Creek lineament is a fault, and is active, remains indeterminate.

West

Scarp Ice Rich Loess

Donnelly OutwashLoess

East

Figure 13. Oblique photo of the Bear Creek trench. No evidence of faulting was observed in the trench, and the contact between the Donnelly outwash and loess was not offset in the upper part of the trench. However, the ice-rich frozen loess prevented excavation of the outwash–loess contact in the lower part of the trench at the base of the scarp where a fault would be expected to daylight. Thus we consider the trench to be inconclusive in demonstrating a lack of faulting.

CATHEDRAL RAPIDS FAULT The Cathedral Rapids fault was identifi ed and mapped during the 2008 Alaska Highway corridor studies. The western section of the fault includes two principal imbricate splays, the north and south traces, both sub-parallel south-dipping thrust faults that extend east along the south side of the Tanana River valley for about 21 km from Sheep Creek to the vicinity of Moon Lake (fi g. 14). The fault traverses a 3- to 5-km-wide north-sloping alluvial and colluvial apron at the base of the steep range front. The apron is made up of coalesced alluvial fans, debris fl ows, landslide deposits, outwash, and Delta and Donnelly age till (Reger and others, 2009). The age of the surface deposits along the range front vary from place to place. The faults form north-facing scarps in these sediments that range from a few to more than 30 m high, with the higher scarps developed in the older deposits. A third young 18 Preliminary Interpretive Report 2010-1

normal fault, the Range Front fault, was mapped along the steep range front south of the Cathedral Rapids fault between Sheep Creek and Moon Lake (ﬁ g. 14). This fault offsets late Pleistocene moraines, outwash, alluvium and colluvium (Reger and others, 2009). We interpret this fault as a large bending-moment fault in a fault-bend anticline that forms the range front above the Cathedral Rapids thrust. East of Moon Lake, Cathedral Rapids fault continues for an additional 16 km as a single sinuous surface trace with an associated large growth anticline. We informally named the anticline the “Giant Moletrack” because its geomorphic form is similar to moletrack-type scarps characteristic of some thrust faults. East of Moon Lake, Cathedral Rapids fault is sinuous and locally blind. Here its trend varies by about 90°, from nearly north–south to nearly east–west. Fault scarps are small, even in Delta age deposits, but the fold scarp associated with the anticline is up to 50 m high. Along this section of the fault most of the near surface displacement on the fault is accommodated by growth of the Giant Moletrack anticline.

reek 0 1 2 Sheep C miles 0 1 2 kilometers N

Rapids Cathedral North Trace Fault Moon Lake Cathedral Rapids South Trace Fault D U D Range D D U D Front Fault U U U Giant

Moletrack

Figure 14. Map of the western part of the Cathedral Rapids fault between Sheep Creek and the west end of the Giant Mole track. This section of the Cathedral Rapids fault includes two imbricate south-dipping thrusts that daylight on the large alluvial apron at the base of the range.

GIANT MOLETRACK The 16-km-long section of the Cathedral Rapids fault east of Moon Lake (fi g. 15) is marked by variation in its structure and near-surface expression. We term this section of the Cathedral Rapids fault the Giant Moletrack section because its northwestern 8 km and eastern 4 km are characterized by a sinuous growth anticlinal ridge up to 70 m tall (see inset topographic profi les on fi g. 16). The anticlinal ridge disappears in the central 4-km-long section of the Giant Moletrack segment where the Cathedral Rapids fault has a linear map trace and small north- facing scarp (fi g. 16). The Giant Moletrack section of the Cathedral Rapids fault and associated anticline deform a broad piedmont slope at the toe of the range front composed of coalescing alluvial fans derived from the small drainages in the mountains immediately to the southwest. At their distal end, these coalescing fans grade into the broad, fl at Tok Fan (Cararra, 2004; Reger and others, 2009) (fi g. 16). Previous workers interpreted the Giant Moletrack to be a moraine (Foster, 1970), a series of bedrock knobs (Cararra, 2004), or landslide deposits (Carter and Galloway, 1978). However, there is no bedrock exposed in the ridges on the piedmont slope, no evidence of the former presence of a large piedmont glacier along this part of the range front that would be required to emplace a moraine in this position, and no source for landslides large enough to generate the ridges. The compositions of clasts in different parts of the Giant Moletrack deposits cor- respond directly with the bedrock lithologies immediately upstream and the texture of the deposits matches that of the adjacent piedmont slope deposits. The combination of the shape of the ridge, and the fault scarp adjacent to the ridge’s downslope margin, are characteristic of a fault-generated fold. That the fault is parallel to the sinuous ridge indicates they are genetically related. The location of the ridge and fault scarp on the piedmont slope well above the valley fl oor rules out an erosional origin from the glacial outburst fl oods that fl owed down the Tanana River during the LGM (Reger and others, 2009). Finally, the small streams that head in the range to the south fl ow across the piedmont slope and through the ridge in deeply incised canyons, evidence that the ridge postdates the development of the local drainage pattern. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 19

East Stream Catastrophe Alaska Range Front West Terrace Creek Profiles

Giant Moletrack Cathedral GMT I and Rapids Anticline Cliffhanger

Trench Sites Fault

Figure 15. Oblique aerial view looking to the southeast along the central section of the Cathedral Rapids fault and western segment of the Giant Moletrack anticline ridge. The location of the measured stream terrace proﬁ les along Catastrophe Creek and sites of the Cliffhanger and GMT I trenches are shown in the upper left part of the photo.

To test whether the Giant Moletrack anticline and scarp are active fault-related features, we undertook studies along a drainage incised across the ridge we have informally termed “Catastrophe Creek” (fi g. 16). We selected Catastrophe Creek for these detailed investigations following helicopter reconnaissance that revealed the presence and excellent preservation of several fl uvial terraces along the creek, the visually obvious folding of one of these terraces, and actively raveling exposures of the alluvial deposits in the canyon walls where the creek cut through the anticline. Two investigative approaches were pursued: (1) paleoseismic trenches were excavated across the main fault scarp and secondary grabens, interpreted to be the surface expression of a bending-moment fault at the crest of the anticline, and (2) differential GPS surveys of longitudinal topographic profi les were measured across the anticline and fault in the modern stream channel and on several nested terraces in the stream valley (fi g. 17). We also mapped the terraces, faults, associated landforms, and late Quaternary geology in the vicinity of the terrace surveys and trenches (fi g. 18).

TERRACE PROFILES Antecedent streams have incised through the Giant Moletrack anticline, which through progressive uplift, has locally created abandoned fl uvial terraces above the modern stream level. Our investigation identifi ed six raised terraces along Catastrophe Creek. Terraces T1, T2a and b, and T4 (T1 is youngest, with the rest sequentially older) have distinct risers and generally well preserved terrace treads. Two poorly preserved terraces, T3 and T5, occur as distinct landforms, but have experienced signifi cant erosion of the terrace treads and were not assessed in detail. Terrace T1 is characterized by an irregular, bouldery surface with the incipient development of a thin organic mat. This terrace is generally 1–2 m above the modern streambed and appears to continue to receive overbank deposits during fl ood fl ows in Catastrophe Creek. A boreal forest fl oor dominated by sphagnum mosses and prickly rose 20 Preliminary Interpretive Report 2010-1 beneath a dense stand of large, fi rst-generation white spruce trees mantles terraces T2a and T2b. We saw only one relatively fresh stump of a large white spruce on T2b. Cores taken with an increment bore from four of the oldest living white spruce trees were examined using a binocular microscope and had 180, 185, 195, and 197 annual rings. Assuming a short ecesis period for the spruce, the tree rings indicate an age for the terrace of a few hundred years. T4 is the highest well-defi ned terrace and is preserved continuously (except where it is crossed by one small, incised drainage) across the Giant Moletrack anticlinal ridge. The surface of this terrace is covered with a thick sphagnum mat with abundant low shrubs and black spruce trees. We collected differential GPS data to construct topographic profi les of the modern Catastrophe Creek thalweg and terrace treads on T2a, T2b, and T4 across the anticlinal ridge. These profi les, shown in fi gure 17, illustrate the progressive uplift and deformation of the terrace surfaces relative to the Catastrophe Creek channel profi le. Terraces T2a and T2b are composed of terrace surfaces vertically separated by a few meters and could only be traced across part of the anticline. The profi le of T2a is parallel to the present stream channel, but the northern end of terrace T2b is slightly higher relative to T2a and the stream channel and appears to show relatively recent growth of the fold (fi g. 17). Terrace T4 is signifi cantly tilted to the south (upstream) and elevated where it crosses the anticline. Two segments of T4, one upslope of the anticline and the other across the anticline, were fi t by linear regressions to distinguish planar panels that are separated by narrow hinges (fi g. 17). The upstream segment of T4 is nearly parallel to Catastrophe Creek and thus has likely uplifted monotonically over a planar thrust fault segment

Cathedral 0 1 2 3 4 km 850 Rapids Profile A Fault 800 GMT Profile B Profile C GMT 750 700 Tok Fan 750 700 650 700 600 Elevation (m) Elevation CRF 650 CRF CRF

650 (m) Elevation Elevation (m) Elevation 600 550 0 500 1000 0 500 1000 1500 0 500 1000 1500 2000 Profile distance (m) Profile distance (m) Profile distance (m)

Giant A Trenches and Tok Fan B Terrace Profiles Profile D Profile E (Figures 16, 17) 700 700 GMT 650 650 Moletrack 600 CRF CRF 600 Elevation (m) Elevation C (m) Elevation 550 D 0 500 1000 0 500 1000 1500 Catastrophe Profile distance (m) Profile distance (m) Creek Giant Tok Fan Symbol Key Thrust Fault Moletrack E GMT Anticline

Cathedral N Rapids Fault

Figure 16. Shaded-relief digital elevation model (DEM) view of the Giant Moletrack and adjacent Cathedral Rapids fault scarp. A 5-m-resolution AirSAR DEM covers the area of the Giant Moletrack anticline and adjacent Cathedral Rapids fault scarp. This DEM is superimposed on the USGS National Elevation Dataset DEM with 30-m resolution to show the relief of the adjacent areas south of the coverage of the AirSAR. The sinuous Cathedral Rapids fault scarp (dashed thrust fault symbol) and Giant Moletrack anticline axis (dotted line) occur outboard of the range front on a wide piedmont apron of coalescing alluvial fans where small streams emanating from the mountains cross the piedmont apron. Profi les A–E (labeled solid lines on the image and solid lines on the inset profi le diagrams) were measured using the AirSAR digital data across the anticline and fault scarp. The anticline ridge is labeled GMT and the Cathedral Rapids fault is labeled CRF on the profi les. The gray dashed lines on the profi le diagrams illustrate the minimum vertical offset of Quaternary alluvial deposits across the fault. The ages of the alluvial surfaces that these profi les transect are unknown, but the surfaces downhill (on the footwall) of the scarp are younger due to alluvial deposits from drainages that have incised through the scarp. Note the anticline does not appear on profi les C and D. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 21 at depth. The downstream segment of T4 is distinctly back-tilted relative to the stream gradient, indicating a change in fault geometry (a listric ramp) as the fault plane approaches the surface. Amos and others (2007) observed similar terrace geometry on active thrust faults in New Zealand, and demonstrated that the back tilting of terraces could most easily be explained by a listric fault in the subsurface. To the north, beyond the back-tilted segment of T4, the terrace surface gradually steepens to where it abruptly drops off at the north-facing forelimb and scarp (fi g. 17). This terrace does not appear at the surface downslope of the fault scarp due to presence of younger alluvial fan deposits covering the footwall. Slope processes and lateral incision during alluvial fan formation has locally modifi ed the steep face of the anticline forelimb and fault scarp, but it is clear that the T4 terrace surface is dramatically folded to form this scarp face.

740 South North

720 y = -0.1018x + 654.59 Leveling survey - Terrace T4 GPS survey - Terrace T4 a b GPS survey -Terrace T2a and T2b a GPS survey - Stream thalweg 700 a a Hinge b y = -0.081Giant Moletrack Anticline a a 9x + 664.4 b Pan 680 el y = -0.1069x + 646.73 Bending a Hinge moment Panel grabens Hinge 660 Panel Elevation (m) Elevation 640

620

Cathedral Rapids Fault 600

VE - 5x Fault shown diagrammatically

-700 -600 -500 -400 -300 -200 -100 0 100 200 Distance from fault (meters, positive to the north)

Figure 17. Differential GPS topographic profi les of stream terraces across the Giant Moletrack anticline and Cathedral Rapids fault scarp. Stream survey points (green diamonds) were collected along the thalweg of the streambed of the ephemeral, informally named Catastrophe Creek. The stream thalweg profi le is shown with the long-dashed line. Terraces T2a and T2b are discontinuous, low terraces with <2 m elevation differences (yellow circles and dotted lines). Terrace T4 (green squares and short-dashed line) is a distinct, continuous terrace that is easily followed for its entire length. Equations are linear regressions for the two long, near- planar segments of T4 and the thalweg profi le of Catastrophe Creek. The red triangles were obtained from a leveling survey across the forelimb panels of folded Terrace T4. The back tilting of the central panel of Terrace T4 relative to the stream profi le likely results from a listric fault in the near surface (Amos and others, 2007). Location and dip of the fault in the subsurface is shown diagrammatically. 22 Preliminary Interpretive Report 2010-1

Active floodplain T1 Terrace 1 T2a Younger terrace 2

Holocene Holocene T2b? T2b Older terrace 2

T3 Terrace 3

cene cene T2b? Terrace 4

Terrace 5 Pleisto

T1 T2b

T2b

T2a T1

Figure 18. Geomorphic map of the informally named Catastrophe Creek ﬁ eld site on the Giant Moletrack anticline and Cathedral Rapids fault scarp. Catastrophe Creek has incised a small canyon through the anticline formed by progressive uplift of the hanging-wall of the Cathedral Rapids fault and growth of the Giant Moletrack anticline. Several ﬂ uvial terraces are preserved in this canyon and record progressive uplift of the anticline. The most prominent of these terraces are mapped here, with additional uncorrelated, discontinuous terrace remnants present but not shown. Contour interval = 5 m.

CLIFFHANGER TRENCH Above the fault scarp face on T4, near the crest of the fault-generated fold (location shown on fi gs. 17 and 18), are two east–west-trending shallow (<1 m) troughs that parallel the fold crest. These troughs are exposed in cross-section in the actively raveling Catastrophe Creek canyon wall cut into the T4 terrace riser (fi g. 19). The exposure shows that these troughs are small grabens that are fi lled with faulted fi ne-grained sediments, buried soils, and colluvium (fi g. 20). We interpret these structures as bending-moment grabens that accommodate extension at hinges in the crest of the anticline during folding events associated with displacement on the underlying thrust fault (Carver and McCalpin, 2009; Carver and McCalpin, 1996). Thus, these grabens contain evidence of coseismic deformation and therefore a record of paleo-earthquakes on the underlying fault. To interpret this paleo-earthquake record, we cleaned off a 7-m-long by 1.5-m-deep exposure on the face of the cliff across the southern, more topographically distinct graben (fi g. 20). An interpreted photo mosaic of this exposure is shown in fi gure 21 and a log of the graben cross-section in fi gure 22. The graben structure is clearly defi ned by steeply dipping normal faults that separate the coarse-grained alluvial and colluvial deposits of terrace T4 from the fi ne-grained colluvium and loess that fi lls the graben. We did not expose the bottom of the graben. Within the graben we tentatively identifi ed at least fi ve discrete faulting events. The oldest offsets units Cpsc, a graben-fi lling colluvium, T, a very fi ne silty volcanic ash, and Pss, a poorly sorted, pebbly, silty, sandy colluvium on faults 4 and 7 (fi gs. 21 and 22). Units Ps, Cps, and L1 cap faults 4 and 7. The age and origin of the ash (unit T) is unknown. The timing of this faulting event is older than ~12,330–11,630 cal BP from a charcoal sample collected in unit Cfss, about 0.5 m above unit T. Since units Cpsc, T, and Pss are preserved in a pre-existing graben, this faulting event is not the fi rst associated with the graben’s formation; the graben was initially generated during one or more earlier episodes of coseismic folding of the Giant Moletrack anticline and associated bending-moment faulting. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 23

Giant Moletrack Anticline Tanana River

Cathedral Rapids Fault N

Cathedral Rapids Fault

Terrace T4 Cliffhanger Trench

Giant Moletrack Anticline

Catastrophe Creek

GMT I Trench Terrace T3

Terrace T4

Figure 19. Oblique photo (view to the northwest) of the Catastrophe Creek study site showing the Giant Moletrack anticline, Cathedral Rapids fault, Catastrophe Creek terraces and Cliffhanger and GMT I trench sites.

Terrace T4 Terrace T4 Graben

Graben margin Graben margin

Figure 20. Photo of the Cliffhanger trench exposure. The trench exposure provides a cross section through one of two grabens on bending moment faults in the crest of the Giant Moletrack anticline. 24 Preliminary Interpretive Report 2010-1

South

North 0.5

-0.5

-1

0 1 2 3 4 5 6 8,550–8,390 cal BP 8,600–8,430 cal BP O 10,290–10,220 cal BP 0.5 L 11,120–10,610 cal BP C1

C2 C2 0 L C1 L C3 C2 Cps C2c C2b C3 Cps O1 -0.5 O2 Cps Csfs Css Cps Ps1 Csp 3 L2 2 Ps1 Csp C3 1 Cpsc L1 Cfss -1 Ps1 Css Cpsc 12,230–11,630 cal BP L1 8 Pss Ps 7 10,720–10,520 cal BP Cpsc 4 Cpsc T 5 6 Cpsc Unit Key O Organic mat; moss, plant parts, loose peat Ps1 Colluvium,Colluvium; pebblypebbly, poorly sorted sandy silt Loess,Loess; well sortedsorted, veryvery finefine sandsand andand silt,silt; Cfss Colluvium;Colluvium, wellwell-sorted sorted fine fine sand sand and and silt silt L massive.massive FaultingFaulting (fault (fault 5) 5)

Colluvium,Colluvium; cobblescobbles, andand bouldersboulders withwith poorlypoorly y stratigraphy stratigraphy

Post-faulting Post-faulting C1 sorted coarse sand matrix L1 LoessLoess, (buried); (buried) well well-sorted sorted silt. silt. Faulting (faults 1, 2, and 8) Css Colluvium,Colluvium; poorly sorted granular sand, grus grus C2c Colluvium,Colluvium; sandysandy siltsilt Ps Colluvium,Colluvium; pebblypebbly, poorly sorted sandy silt y y O1 Buried soil;soil, organic silt Colluvium,Colluvium; angularangular pebblespebbles andand granulesgranules inin Csp Colluvium;Colluvium, pebbly,pebbly poorly sorted sandy silt C2b Faulting siltysilty matrixmatrix stratigraph fill Graben Faulting (faults(faults 4, 4, 7) 7) Cps Colluvium,Colluvium; pebblypebbly, poorlypoorly sortedsorted sandsand T White to tan very fine tephra Faulting (fault 6) Pss Colluvium;Colluvium, pebbly,pebbly poorly sorted sandy silt O2 Buried soil;soil, organic silt Colluvium,Colluvium; poorlypoorly sortedsorted, pebbly pebbly, sandy sandy silt silt with with Graben fill stratigraph fill Graben Cpsc scatteredscattered cobblescobbles L2 LoessLoess, (buried); (buried) well well-sorted sorted silt silt. FaultingFaulting (multiple ( multiple graben graben forming forming events?) events?) Colluvium,Colluvium; cobbles in siltysilty, poorly sorted silty Csfs Colluvium,Colluvium; siltsilt, andand well-sortedwell-sorted finefine sandsand C2 sand Faulting (fault 3) Colluvium,Colluvium; poorlypoorly sortedsorted, sandy sandy silt silt with with C3

abundantabundant cobblescobbles andand bouldersboulders

stratigraphy stratigraphy Pre-graben Pre-graben

Figure 21. Photo mosaics of the Cliffhanger trench exposure without interpretation (above) and with interpretation (below). Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 25

Fault 5 (fi gs. 21 and 22) offsets units Li and Ps and is truncated by unit Cfss, documenting a second episode of faulting in the graben. The age of the charcoal sample from unit Cfss also provides a maximum age for this event. A third faulting event is interpreted from units L1 and Cpsc that are offset by fault 3. Unit Csfs caps fault 3. The age of this faulting event is bracketed by 14C ages on charcoal from unit Cfss of 10,720–10,520 cal BP and from unit Cps of 10,290–10,220 cal BP (fi gs. 21 and 22). A fourth faulting event offsets units Csp and L2 on fault 6 (fi gs. 21 and 22). Unit Cps caps fault 6. Its age is also constrained by a maximum age from unit Cfss of 10,720–10,520 cal BP and a minimum age from the base of unit Cps of 10,290–10,220 cal BP. The most recent faults in the graben (1, 2, and 8) offset colluvial units 2C, C2b and C2c. Unit C2c contains detrital charcoal pieces near its base that date to 8,550–8,390 cal BP and 8,600–8,430 cal BP. These provide maximum limiting ages for the most recent faulting in the graben. The minimum age for the most recent faulting in the

North South meters 0 1 2 3 4 5 6 7 8,550–8,390 cal BP 8,600–8,430 cal BP O 10,290–10,220 cal BP 0.5 11,120–10,610 cal BP C1 C2 L L 0 C1 O L L C A13 m C23a C2c C2b Cps O1 A1 Cps -0.5 Cps Cps Css Csfs O2 Csp Cpsc Ps1 A1 2 L2 Csp Cfss10a Ps1 3 L1 12,230–11,630 cal BP 1 -1 L1 Cpsc Ps1 Css Cpsc L1 8 Ps 7 Pss 10,720–10,520cal BP T 6 4 Cpsc 5 Cpsc Cliffhanger Trench I Logged 8/13/08 By: G.Carver, S.Bemis, D.Solie, S.Castonguay Unit Key

O Organic Organic mat; mat moss, - moss, plant plant parts, parts, loose loose peat peat Ps1 Colluvium;Colluvium, pebbly, pebbly poorlypoorly sortedsorted sandysandy siltsilt y y Loess, well sorted very fine sand and silt, L Loess; well sorted, very fine sand and silt; Cfss Colluvium; Colluvium, well well-sorted sorted fine fine sand sand and and silt silt massivemassive. FaultingFaulting (fault (fault 5) 5)

Colluvium;Colluvium, cobbles cobbles and and boulders boulders with with poorly poorly stratigraph

Post-faulting Post-faulting C1 sortedsorted coarse coarse sand sand matrix matrix L1 LoessLoess, (buried); (buried) well well-sorted sorted silt silt. FaultingFaulting (faults (faults 1, 2, and 1, 2, 8) and 8)

tigraphy tigraphy Css Colluvium, poorly sorted granular sand, grus C2c Colluvium;Colluvium, sandysandy siltsilt Colluvium; poorly sorted granular sand, grus Ps Colluvium, pebbly poorly sorted sandy silt

y y Colluvium; pebbly, poorly sorted sandy silt O1 Buried Buried soil; soil, organic organic silt silt Csp Colluvium;Colluvium, pebbly, pebbly poorly poorly sorted sorted sandy sandy silt silt C2b Colluvium;Colluvium, angularangular pebblespebbles andand granulesgranules inin tigraph Faulting siltysilty matrixmatrix stra fill Graben Faulting(faults (faults 4, 7) 4, 7) Cps Colluvium;Colluvium, pebbly,pebbly poorly sorted sand T WhiteWhite toto tantan veryvery finefine tephratephra FaultingFaulting (fault (fault6) 6) Pss Colluvium;Colluvium, pebbly, pebbly poorly poorly sorted sorted sandy sandy silt silt O2 Buried Buried soil; soil, organic organic silt silt Colluvium;Colluvium, poorly sorted,sorted pebblypebbly, sandy silt with Graben fill stra fill Graben Cpsc scatteredscattered cobblescobbles L2 LoessLoess, (buried); (buried) well well-sorted sorted silt silt. FaultingFaulting ( multiple (multiple graben graben forming forming events?) events?) Colluvium, cobbles in silty poorly sorted silty Csfs Colluvium; Colluvium, silt silt and and well well-sorted sorted fine fine sand sand C2 Colluvium; cobbles in poorly sorted silty sand sand FaultingFaulting (fault (fault 3) 3) Alluvium;Alluvium, poorly sorted sandy silt with A1

abundantabundant cobblescobbles andand bouldersboulders

stratigraphy stratigraphy Pre-graben Pre-graben

Figure 22. Log of the Cliffhanger trench exposure. At least ﬁ ve paleo-earthquakes are interpreted from upward terminations of faults in the graben ﬁ ll sediments. 26 Preliminary Interpretive Report 2010-1

graben is unconstrained. Unit C2c is partially overlain by unit C1, which caps fault 1, and the entire exposure is capped by an unfaulted loess layer (unit L). Since bending-moment grabens are secondary fault features formed at hinges between panels of fault-generated folds in the hanging walls of thrusts and migrate away from the hinge as displacement accrues on the underlying thrust, they become inactive when they are no longer at or near the hinge. The graben we studied is no longer at the hinge that likely generated it. Thus it is probable that additional slip events have occurred on the underlying fault since the graben was active.

GIANT MOLETRACK TRENCH On the east side of Catastrophe Creek, opposite the Cliffhanger trench, we identifi ed an ~2-m-high gently sloping scarp offsetting a young terrace, possibly correlative with terrace T2b. This scarp aligns with an abrupt slope break immediately to the east that we interpret as the fold scarp of the Cathedral Rapids fault (fi g. 19). A steeper, ~1-m-high facet is present along sections of the fold scarp including the section where it offsets the Catastrophe Creek terrace. The steeper facet is not present on the west side terrace. We excavated a 7-m-long and approximately 1-m-deep trench (trench GMT I) across the scarp where the steeper facet ends (fi gs. 23 and 24). In general, the stratigraphy exposed in the trench is characterized, from top to bottom, North by: (1) a 20–30 cm section of interbedded organic mats and well sorted fl uvial sands Base of (units O, S1, O1, S2, and O2); fold scarp (2) 40–70-cm-thick stratifi ed colluvial deposits, with local Base of fluvial sand horizons (units facet C1, C1a, and C1b) containing Top of discontinuous thin streaks of facet very-fi ne-grained black organics and charcoal (white fl agging and white dashed lines on the trench wall photo shown in fig. 25 and black dashed lines on the trench log shown in fig. 26); and (3) frozen, boulder-rich colluvial units at the bottom of the trench (units C2 and C3). These frozen sediments were the lower limit of our excavation. This stratigraphy is for the most part parallel bedded and gently inclined beneath the fold scarp at the same angle as the ground surface. A small pit excavated into the undeformed terrace surface above the scarp encountered the same stratigraphy as in the trench. Bedding in the pit is horizontal (fi g. 26). We interpret the gently sloping scarp to Top of be a fold scarp at the tip of the Cathedral Rapids fault. fold Scarp There are distinct differences in the scarp morphology Figure 23. Photo of trench GMT I viewed to the north from the top of the scarp between the two trench walls, on terrace T2b. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 27

Top of facet

Base of facet

Top of fold scarp

East wall (a) Top of facet

Base of facet

Base of fold scarp (b)

2 Fault

O S1 S2 O1 O2 S2 O2 S1 East wall O1 S2 O North S1 C1 1 South O2

S2 C1 O2 310-0 cal BP C1 m 940-780 cal BP S1 C1 O C1 300-0 cal BP O1 0 310-0 cal BP S1 14 S1 O1 O C age cal BP S2 C1 (c) O1 Charcoal layer O2 S2 C2 O2 C1 C2 C3 Fault C1 C3 -1 C2

01234567m

Figure 24. (a) Oblique view of the east wall of trench GMT I showing the steep facet on the fold scarp. (b) Photo mosaic of the east wall of trench GMT I. (c) Interpreted photo mosaic of the east wall of trench GMT I. Unit designations from the trench log (fi g. 26). 28 Preliminary Interpretive Report 2010-1 as well as subtle differences between the colluvial sections. The east wall exposed a section across the ~1-m-high steep scarp facet, the most pronounced morphological step in the terrace surface. Underlying this step is a convex- up, south-dipping thrust fault trace that offsets the unit C1/C2 contact, buried organic mat O2, and several thin organic/charcoal horizons, all by 10–15 cm (fi gs. 25 and 26). Looking simply at the fault trace, it would appear that buried organic mat O1 truncates the fault, suggesting the age of this horizon would provide a minimum limiting age for this faulting event. However, we fi nd it diffi cult to envision a scenario in which the thin, well-sorted fl uvial sands S1 and S2 could be deposited and preserved continuously across the scarp and steep facet, as seen in the east wall of the trench. Our favored interpretation is that the sands were deposited during overbank fl oods of the creek and covered with an organic mat before the terrace was deformed. The 2-m-high gentle scarp represents a fold scarp generated by a single episode of recent displacement on a largely blind south-dipping thrust with most of the displacement accommodated by folding at the surface. Based on the 2-m-high fold scarp and an assumed dip of 30° for the fault at depth (the dip of the small fault exposed in the trench), geometric relations suggest about 4 m of dip-slip displacement on the fault at depth. As the fault approached the surface, slip on the fault decreased and warping of the sediments increased, producing the fold scarp. With the small (10–15 cm) displacement of the fault at the surface, and a strong mechanical contrast between the essentially cohesionless fl uvial sand unit S2 and the fi brous, densely rooted overlying organic mats (O and O1), we speculate that the surface trace of the fault-exploited unit S2 is a

North South

Top of trench 3m 2m 0 m O S1 O1 S2 O2 S2 O2 0 m C1

-1 m C1

C2 -1 m

2m C3 3m Trench floor

Trench floor

Figure 25. Close-up view of the fault in the east wall of GMT I trench. The light green dashed line marks the base of the lower layer of a well sorted fl uvial sand (unit S2). The blue dashed line is the base of a charcoal-rich organic silt (unit O2). The white dashed lines mark thin streaks of fi ne-grained charcoal and organic material in the colluvium (unit C1). The pink dashed line shows the base of this colluvium. The fault (red line) offsets the charcoal streaks and contacts between the colluviums C2 and C1 and the overlying organic layer O2 and base of the fl uvial sand S2. Where the fault crosses the trench fl oor it is defl ected around large boulders on the west side of the trench and is oblique to the orientation of the trench. Unit designations are from the trench log (fi g. 26). Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 29 detachment horizon and translated the overlying units downslope where they formed a secondary small fold (the steep facet) in the larger fold scarp, creating the apparent offset in unit O1 at the odd-shaped protrusion of modern organic mat O into the underlying deposits. In the west wall the fault is less distinct, and where it extends through the lower colluvial units it is about 1.5 m downslope from its position in the east wall, indicating the strike of the fault is highly oblique to the trench. In the lower colluvial units the fault intersects large boulders that appear to control its location and shape in the trench. The fault tip dies out in the uppermost part of the upper colluvium and does not appear to offset the upper sands and organic layers (fi g. 26). We infer that the most recent faulting occurred after deposition of unit S1 and sometime during the formation of unit O. Two radiocarbon ages from the youngest units involved in this deformation result in ages that, at a two-sigma confi dence level, are indistinguishable from Modern with age ranges of 310–0 cal BP and 300–0 cal BP (samples GMT-T1-06 and GMT-T1-05, respectively). This indicates the most recent event on the Giant Moletrack segment of the Cathedral Rapids fault occurred sometime after ~1,650 AD. A review of recent seismicity in the region shows at least seven earthquakes with mechanisms compatible with slip on the down-dip projection of the Cathedral Rapids fault have been recorded (AEIC earthquake catalog; Natasha Ruppert, written commun., 2010) (fi g. 27). These earthquakes further support the interpretation that the Cathedral Rapids fault is active.

Cathedral Rapids Fault Trench GMT I

North m South 01234567Terrace 2b O S1 2 O1 S2 O2 Cal BP 14C age C1 Charcoal layer East wall O1 S2 Fold Scarp C1 O

1 O2 S2 lt Tip S1 Fau C1 R m facet 310 - 0 cal BP 300 - 0 cal BP O R Terrace 2b 310 - 0 cal BP O1 2 0 O Terrace 2b S1 940 - 780 cal BP C3 S2 O2 C1 O1 C2 West wall O S2 C3 (Reflected view) S1 Fold Scarp 1 270 - 0 cal BP O1 -1 S2 C1 O2 S1 530 - 340 cal BP O m C1 S2 O2 Terrace 2b O1 C1b 0 O C1a C2 S1 S2 C1 O2 ? C2 C1a C2 C3 C2 C3

Organic silty sand with abundant C1a detrital charcoal

Lower sand, laminated C1b Thinly bedded silt and silty sand O Modern organic mat S2 well sorted medium sand Colluvium, pebbles, and cobbles in silt Upper sand, laminated C2 S1 O2 Lower buried organic mat and sand with charcoal laminations well sorted medium sand Colluvium, pebbles, and cobbles in C3 Boulders in pebble- and cobble- O1 Upper buried organic mat C1 silty sand with charcoal laminations rich sand and silt

Figure 26. Log of trench GMT I. The fault offsets all strata in the trench except uppermost thin ﬂ uvial sand (S1) and surface organic mat (O). Radiocarbon ages of 310–0 and 300–0 cal BP from the faulted organic layer O1 are minimum limiting ages for slip on the fault and show most recent activity of the Cathedral Rapids fault at this site postdates AD 1650. 30 Preliminary Interpretive Report 2010-1

The Cathedral Rapids fault appears to be the easternmost active thrust in the NFFTB. Our mapping indicates the active range-front folds and thrust faults do not continue east of the Tok River. Reconnaissance of the range front between the Tok River and the Canada border found no evidence of late Quaternary thrust faulting. It is in- teresting to note that the eastern end of the NFFTB is directly north of the junction of the Denali and Totschunda faults (ﬁ g. 2). We speculate that slip on the Totschunda fault, which intersects the Denali fault at an angle of about 20°, results in a north-directed component of compression across the Denali fault west of the intersection. This compression is partitioned across the Denali fault onto the NFFTB and drives the thrusting and folding along the north ﬂ ank of the Alaska Range (Haeussler, 2008; Bemis and Wallace, 2007). East of the Totschunda–Denali fault intersection, the eastern segment of the Denali fault crosses to the northern side of the range and is purely dextral strike-slip. GPS velocity vectors suggest little compression is occurring across the range east of the Tok River (Freymueller and others, 2008).

63°30'N

Cathedral Alaska Highway Rapids

y Fault

Long. Lat. Depth Date Magnitude (km) -143.4895 63.2030 5.06 11/23/2009 M3.4 63°15'N -143.6914 63.3026 5.45 1/20/2008 M2.8 -143.2771 63.2424 15.93 9/15/2003 M2.9 Glenn Highwa -143.5203 63.1282 4.54 2/04/2006 M3.2 -143.7750 63.1270 1.80 11/06/2002 M4.1 -143.7841 63.0827 1.00 11/05/2002 M4.4 -143.4990 63.0823 10.60 9/14/2005 M3.0

9/14/2005_M3.0

63°00'N 144°00'W 143°30'W 143°00'W

Figure 27. Focal mechanisms (red) for recent earthquakes that have locations, depths, and mechanisms compatible with slip on the down-dip extension of the Cathedral Rapids fault. (AEIC earthquake catalogue; Natasha Ruppert, written commun., 2009). Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 31

MANSFIELD CREEK LINEAMENTS The Mansfi eld Creek lineaments include several northeast-trending linear breaks in slope and scarps in the Mansfi eld Creek drainage north of the Tanana River (fi g. 3). These lineaments are shown on the Neotectonic Map of Alaska (Plafker and others, 1994) and classifi ed as “Suspicious—Age of fault or lineament displacement is unknown. Nearby shallow seismicity, and/or known faults with similar orientation indicates the possibility of Neogene displacement.” The trend of the Mansfi eld Creek lineaments is similar to that of the Billy Creek fault, a documented active fault located north of the Tanana River about 32 km to the northwest (Carver and others, 2008). We conducted helicopter reconnaissance and limited surface studies of these features and excavated one trench across a southeast-facing 1.5-m-high scarp on the most prominent lineament. The reconnaissance showed the lineaments include several semi-continuous linear alignments of distinct vegetation contrasts between black spruce trees and grassy tundra, down-slope facing side-hill scarps locally up to 10 m high, and springs. These linear features were most prominent on gentle slopes along the northwestern side of the Mansfi eld Creek drainage where the bedrock was mantled by thin colluvium (fi g. 28). The lineaments were discontinuous and did not extend across thicker alluvium and colluvium in small southeast-trending intermittent tributary drainages to Mansfi eld Creek. The trench was sited across a well-defi ned 1.5-m-high southeast-facing scarp on the most continuous and prominent lineament about 4.8 km north–northeast of Mansfi eld Lake (fi gs. 3 and 28). The trench exposed pervasively weathered and intact horizontally interlayered muscovite schist and quartzite (fi g. 29). A mantle of thin layers of colluviums, a weak A–C soil horizon, and organic silt (loess) unconformably overlies the bedrock. The interlayered bedrock crosses the scarp with no evidence of faulting, and the unconformable contact between the bedrock and overlying colluviums was not offset. No evidence of a fault was found in the trench. We interpret the scarp at the trench site to be the result of differential erosion of the resistant layers of quartz and soft-weathered schist. We conclude the Mansfi eld Creek lineaments refl ect differential erosion of the bedrock underlying the slopes in the Mansfi eld Creek drainage. While some of the lineaments may be the surface expression of bedrock faults, they are not expressed in the valley fi ll alluvium where they trend across drainages containing young alluvial sediments. We do not believe these lineaments are active faults.

Mansfield Lineaments

Mansfield Scarp Trench

Figure 28. Oblique air photo of the Mansﬁ eld lineaments and the Mansﬁ eld trench site. View is to the northwest. 32 Preliminary Interpretive Report 2010-1

Mansfield Scarp Trench Northwest Unit Key Logged 8/08/2008 1.5 By: G. Carver, S. Bemis, S. Castonguay O Organic silt with abundant charcoal and modern forest fire debris O Sandy, silty colluvium with weak A/C S soil and scattered quartzite pebbles Sandy, silty colluvium with 1.0 Scarp C abundant angular quartzite clasts BS S Pervasively weathered foliated C BS muscovite schist meters

0.5 BQ BQ Massive quartzite O Southeast BS O S 0 C BQ

5 4 3 meters 2 1 0

Figure 29. Mansfi eld scarp trench log. Contacts between the shist (BS) and quartzite layers (BQ) in the bedrock and the overlying colluvial (C), loess (O), and soil (S) units are not offset where they cross the base of the scarp that defi nes the Mansfi eld lineament at this location, showing the scarp is not the result of faulting.

DENNISON FORK LINEAMENT—TOK RIVER VALLEY Plafker and others (1994) show the Dennison Fork lineaments on their Neotectonic Map of Alaska as several “suspicious” northeast-trending lineaments along the west margin of the Tok River valley south of Tok and along the upper reaches of the Dennison Fork of the Fortymile River northeast of Tok (fi g. 3). During the 2008 fi eld season we conducted helicopter reconnaissance of these lineaments in the upper Dennison Fork of the Fortymile River area and a detailed investigation of the most prominent of the lineaments in the upper Tok River valley. The Dennison Fork lineaments northeast of Tok are similar to the Mansfi eld Creek lineaments, characterized by linear vegetation contrasts, side-hill scarps, and spring lines. The lineaments terminate where they intersect valley fi ll deposits in small drainages. We found no evidence that these lineaments are youthful faults that offset Holocene landforms or deposits. Along the west margin of the upper Tok River valley, the Dennison Creek lineament crosses several alluvial fans where small, unnamed tributaries to the Tok River enter the valley. A single prominent northeast-trending, east-facing scarp is present on each of these alluvial fans (fi g. 30). These scarps are up to 10 m high. A similar discontinuous west-facing scarp is present on the east margin of the Tok River valley. This scarp defi nes one of several lineaments that make up the Caribou Creek lineaments shown as “suspicious” (possible active faults) on the Neotectonic Map of Alaska (Plafker and others, 1994). The scarps on both sides of the valley closely follow the valley margin and are at similar elevations above the valley fl oor. We excavated two trenches across the Dennison Fork lineament on the west side of the Tok River valley about 3.7 km west–southwest of the Glenn Highway bridge across the Tok River (fi g. 30). The trenches were excavated across the base of a 4-m-high scarp on a large alluvial fan where a small, unnamed south-fl owing creek enters the Tok River valley. Both trenches exposed stacked sequences of debris-fl ow deposits interbedded with thin layers of loess deposited against alluvial fan sediments at the base of the scarp (fi gs. 31–33). Contacts between the debris fl ows and intervening layers of loess were not faulted where they crossed the scarp. No evidence of faulting was found in the trenches. Reger and others (2009) present evidence of large glacial-outburst fl oods, probably originating from glacial lake Atna, which repeatedly fl owed down the Tok River valley during the Donnelly glaciation. We propose that these fl oods removed the distal parts of the alluvial fans on the margins of the valley and eroded scarps into the heads of the fans. The toes of these scarps have subsequently been buried during severe storm events by debris fl ows, alluvium, and colluvium derived from the fan heads above the scarps. Between episodes of debris-fl ow deposition, loess accumulated on the deposits. A 14C age of 1,510–1,310 cal BP was obtained from the stratigraphically lowest layer of buried loess in the debris-fl ow sequence in trench II, demonstrating a Holocene age for the debris fl ows Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 33

Scarp on the Dennison Fork Lineament - Upper Tok River Valley

Scarp

Trenches T-I T-II

Tok River

Figure 30. Scarp and trench locations on the Dennison Fork lineament in the upper Tok River valley.. that postdate the scarp. We conclude that the Dennison Fork lineament in the Tok River valley is a product of erosion by late Pleistocene glacial outburst ﬂ oods and is not associated with faulting. Based on its similar elevation and geomorphic expression we also interpret the Caribou Hills lineament on the east side of the Tok River valley to be a glacial outburst ﬂ ood scarp.

CONCLUSIONS Field investigations of active and potentially active faults in and near the Alaska Highway corridor in 2008 included excavation of two trenches on the eastern segment of the Dot “T” Johnson fault. This fault was fi rst mapped in 2007 (Carver and others, 2008). Additionally a previously unknown active fault, which we named the Cathedral Rapids fault, and an associated active fault-generated anticline, named the Giant Moletrack anticline, were identifi ed during this study. The Cathedral Rapids fault is a low-angle thrust that trends along the south margin of the Tanana River valley at the base of the Alaska Range foothills east of the Dot “T” Johnson fault. The Cathedral Rapids fault and its associated Giant Moletrack anticline are the easternmost active structures in the eastern extension of the NFFTB. Field reconnaissance, mapping, and trenching were also conducted on three suspected active faults, the Bear Creek, Mansfi eld Creek, and Dennison Fork lineaments. Paleoseismic investigation of the Dot “T” Johnson fault in 2007 showed the fault generated two episodes of latest Pleistocene and early Holocene displacement, each with more than 3 m of dip-slip displacement (Carver and others, 2008). The earlier of the two slip events dated to between 12,100–11,700 cal BP and 12,380–11,980 cal BP. The maximum age of the second slip event is constrained by a 14C date of 9,740–9,540 cal BP. The minimum age for this event is unconstrained (Carver and others, 2008). The two trenches excavated across the Dot “T” Johnson fault in 2008 added to the paleoseismic record for the fault and improved understanding of its slip parameters. The Sears Creek trench revealed an additional slip event on the Dot “T” Johnson fault that dated to between 4,430–3,230 cal BP and 680–560 cal BP. This event produced about 3 m of dip-slip displacement. Trench D“T”J III showed several displacement events with the most recent occurring after 7,980–7,850 cal BP. Earlier events predate the 7,980–7,850 cal BP age but their timing is not well constrained. The presently available paleoseismic data for the Dot “T” Johnson fault suggests it has a recurrence interval of several thousand years and that it generates about 3–4 m of dip-slip displacement per event. Its Holocene dip-slip displacement rate is approximately 1 mm/yr. Structural analysis of the complex faulting pattern in the hanging wall of the fault exposed in D“T”J III trench shows the displacement on the thrust is nearly pure dip-slip and the orientation of the principal compression axis 34 Preliminary Interpretive Report 2010-1

Trenches on the Dennison Fork Lineament - Tok River Valley Northwest Southeast 10 9876543210 meters Trench I 1 North Wall

0 meters

-1

Trench I 1 South Wall (reflected)

0 meters

-1

Trench II 1 North Wall

0 meters

-1

Trench II 1 South Wall (reflected)

0 meters

-1

Figure 31. Photo mosaics of trenches across the Dennison Fork lineament scarp in the upper Tok River valley. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 35

10 West 9 876543210 meters Dennison Fork Lineament - Tok River Valley Scarp Trench I O 1 1 L1 South Wall C5 L1

S1 L1 O East C1 S1 0 0 meters L1 meters C5 C2 S1 L2 L3 West L3 C5 C4 CH C3 -1 -1 O Scarp S1

1 North Wall (reflected) Logged 8/3/2008-8/4/2008 1 By: G.Carver, D.Solie, S.Bemis, S.Castonguay C2 O C5 S1 East C1 O 0 C2 S1 L1 0 C2 meters L2 S1 meters L2 C5 C4 CH L3 L3 L3 CH C5 OR2 OR1 C3 -1 OR3 -1 CH Unit Key

O Organic mat

L1 Loess with ash?

S1 Fine to medium sand and silt; scattered organics

C1 Alluvium, large cobbles in poorly sorted sandy matrix

C2 Alluvium, small cobbles and pebbles in poorly sorted sandy matrix

L2 Medium and fine sand and silt with abundant wood and charcoal

L3 Medium and fine sand and silt with scattered charcoal

CH Charcoal, charred wood, twigs, leaves, plant parts

OR1 Medium to coarse sand with abundant pebbles

OR2 Fine to medium poorly sorted sand with scattered detrital charcoal

OR3 Fine to medium poorly sorted sand with abundant detrital wood and charcoal

C3 Alluvium, pebbles and cobbles in poorly sorted sandy matrix

C4 Alluvium, large cobbles in poorly sorted sandy matrix

C5 Alluvium, large cobbles and boulders in poorly sorted sandy matrix

Large cobbles and boulders visible on the photomosaics

Figure 32. Log of trench I across the base of the Dennison Fork lineament scarp in the upper Tok River valley. No evidence of faulting was found in the trench. 36 Preliminary Interpretive Report 2010-1

10 9876543210 West meters Dennison Fork Lineament - Tok River Valley Trench II 1 Scarp 1 O South Wall C4 S1 C5 L1 0 S1 East 0 meters L2 O meters L1 C1 O C2 C5 L2 C1 -1 -1 West C4

O Scarp

North Wall (reflected) 1 C4 O 1 L1 Logged 8/2/2008 L2 By: G.Carver, S.Bemis, S.Castonguay S1 C5 C1 L1 Root L1 O East S1 Root Root 0 C2 O 0 meters meters C5 C1 x C4 -1 x -1 CH C5 1,510–1,310 cal BP L2

C6 CH Unit Key

O Organic mat

L1 Loess with ash?

S1 Fine to medium sand and silt; scattered organics

C1 Alluvium, large cobbles in poorly sorted sandy matrix

L2 Medium and fine sand and silt with abundant wood and charcoal

C2 Alluvium, small cobbles and pebbles in poorly sorted sandy matrix

CH Charcoal, charred wood, twigs, leaves, plant parts

C4 Alluvium, large cobbles in poorly sorted sandy matrix

C5 Alluvium, large cobbles and boulders in poorly sorted sandy matrix

C6 Alluvium, large cobbles in poorly sorted sandy matrix Large cobbles and boulders visible on the photomosaics

Figure 33. Log of trench II across the base of the Dennison Fork lineament scarp in the upper Tok River valley. No evidence of faulting was found in the trench. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 37 nearly north–south. Although this general interpretation is evident from the other trenches we have studied on this fault, the measurements of secondary conjugate faults in the hanging wall and the micro-faults and shears in the principal thrust zone exposed in trench D“T”J III allowed quantifi cation of this parameter. The azimuth of the principal compression axis across the fault (sigma 1) is north 332° to 339°. The Cathedral Rapids fault is a 37-km-long, low-angle, south-dipping thrust fault in the eastern part of the range-front thrust system on the south margin of the Tanana River valley. It is separated from the Dot “T” Johnson fault by the Robertson River step-over; a 20 km right step in the range front, the Tanana River valley, and the thrust system. Faults crossing the step-over and connecting the Dot “T” Johnson and Cathedral Rapids faults have not been identifi ed and the mechanics of the right step in the range-front thrust system are not well understood. The Cathedral Rapids fault is mapped from near Sheep Creek southwest of Cathedral Rapids on the Tanana River east to near the Tok River. The eastern 16 km of the fault has a prominent fault-generated anticline in the hanging wall, the Giant Moletrack anticline. Scarps along the fault and folded terraces bordering streams crossing the anticline show late Pleistocene and Holocene activity. The paleoseismic record of the Cathedral Rapids fault is deduced from trench studies of a faulted stream terrace and a bending-moment graben in the crest of the anticline. Four Holocene slip events are recognized. Three of these are interpreted from upward terminations of fault traces in deformed sediments that fi ll the bending-moment graben. Radiocarbon ages that bracket these paleoearthquakes include: event 1 (oldest), greater than 12,230–11,630 cal BP; event 2, 11,120–10,610 cal BP to 10,290–10,220 cal BP; event 3, 10,290–10,220 cal BP to 8,600–8,430 cal BP. The most recent surface faulting is interpreted from the trench across a faulted late Holocene stream terrace and dates to more recent than AD 1650. This event generated about 4 m of dip-slip displacement on the fault in the shallow subsurface and offset the terrace vertically about 2 m. While this paleoseismic record is likely incomplete, it shows the recurrence interval; the single-event displacement and slip rate of the Cathedral Rapids fault are in general similar those of the Dot “T” Johnson fault. Two of the three lineaments investigated in 2008, the Mansfi eld Creek and Dennison Fork lineaments, displayed no evidence of faulting in trenches excavated across well defi ned scarps. The third, the Bear Creek lineament in the Robertson River step-over, was also trenched but the exposure failed to provide defi nitive evidence regarding the presence or absence of a fault. In summary, our investigations of active and potentially active faults in and near the Alaska Highway corridor have shown the principal active structures are low-angle, south-dipping thrust faults that make up the eastern part of the NFFTB. This system of thrust faults is divided into three segments separated by large lateral steps. From west to east the segments are the Granite Mountain and Dot Lake segments of the Dot “T” Johnson fault, and the Cathedral Rapids fault segment. The segments are about 30, 50, and 37 km long, respectively. The two segments of the Dot “T” Johnson fault are joined by the Canteen fault, a left-lateral tear fault in the upper thrust sheet of the NFFTB (Carver and others, 2008). The structural nature of the step between the Dot “T” Johnson and Cathedral Rapids faults has not been identifi ed. No other active faults were found in the central part of the corridor. Several lineaments, identifi ed as possible active faults in earlier studies, including the Mansfi eld and the Dennison Fork lineaments, were investigated and found not to be active faults.

ACKNOWLEDGMENTS This project is funded by the State of Alaska and managed by the Alaska Division of Geological & Geophysi- cal Surveys. We thank Peter Haeussler (USGS) and Gordon Seitz (San Diego State University) for their insightful ﬁ eld reviews of the Dot “T” Johnson trench III and Sears Creek trench. Valuable discussions with Richard D. Reger (Reger Geologic Consulting), Trent D. Hubbard and De Anne Stevens, (DGGS), Robert F. Swenson (DGGS Director and State Geologist), and Rodney A. Combellick (DGGS Deputy Director) are gratefully acknowledged. We also appreciate suggestions and insight provided by the DGGS bedrock-mapping group, Joe Andrews, Larry Freeman, and Melanie Werdon. The skill, experience, and ability of our helicopter pilot, Tom “Rat” Ratledge con- tributed immeasurably to our ﬁ eld effort. The hospitable owners and operators of the Cathedral Creeks B&B, Chris Bentele and Dave Thorp, provided excellent meals and comfortable camp facilities. Richard Koehler reviewed the manuscript, and we appreciate his constructive and insightful comments. 38 Preliminary Interpretive Report 2010-1

REFERENCES AEIC, Alaska Earthquake Information Center, AEIS Earthquake Database Search, http://www.aeic.alaska.edu/ html_docs/db2catalog.html. Amos, C.B., Burbank, D.W., Nobes, D.C., and Read, S.A.L., 2007, Geomorphic constraints on listric thrust faulting; Implications for active deformation in the Mackenzie Basin, South Island, New Zealand: Journal of Geophysical Research, v. 112, issue B3. Bemis, S.P., 2004, Neotectonic framework of the north-central Alaska Range foothills: Fairbanks, Alaska, Uni- versity of Alaska Fairbanks, unpublished M.S. thesis, 142 p. ———2006, Overview of Quaternary structures and deformation of the Northern Foothills Fold and Thrust Belt, central Alaska Range, Alaska [abs]: American Geophysical Union Chapman Conference on Active Tectonics and Seismic Potential of Alaska, Abstracts with program, 11–14 May, Alyeska Resort, Girdwood, Alaska, p. 41. Bemis, S.P., and Wallace, W.K., 2007, Neotectonic framework of the north-central Alaska Range foothills, in Ridg- way, K.D., Trop, J.M., Glen, M.G., and O’Neill, J.M., eds., Tectonic growth of a collisional continental margin; Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 549–572. Carrara, P.E., 2004, Surfi cial geologic map of the Tanacross B-5 Quadrangle, east-central Alaska: U.S. Geological Survey Scientifi c Investigations Map 2856, version 1.0, 9 p., 1 plate, scale 1:63,360. Carter, L.D., and Galloway, J.P., 1978, Preliminary engineering geologic maps of the proposed natural gas pipeline route in the Tanana River valley, Alaska: U.S. Geological Survey Open-fi le Report 78–794, 26 p., 3 plates, scale 1:125,000. Carver, G.A., and McCalpin, J.P., 1996, Paleoseismology of compressional tectonic environments – Chapter 5, in McCalpin, J.P., ed., Paleoseismology: New York, Academic Press, International Geophysics Series, v. 62, p. 183–270. ———2009, Paleoseismology of Compressional Tectonic Environments - Chapter 5; in McCalpin, J.P., ed., Pa- leoseismology, 2nd Edition, International Geophysical Series, v.95, Elsevier, Burlington, MA., p. 315–419. Carver, G.A., Plafker, George, Metz, M.C., Cluff, Lloyd, Bemis, S.P., Roddick, Jim, Redington, J., and Sorensen, Steve, 2006, Late Quaternary growth of thrust faults and associated folds in the eastern part of the Northern Foothills Fold and Thrust Belt, central Alaska Range, Alaska: [abs], American Geophysical Union Chapman Conference on Active Tectonics and Seismic Potential of Alaska, Abstracts with program, 11–14 May, Alyeska Resort, Girdwood, Alaska, p. 42–43. Carver, G.A., Plafker, George, Metz, M.C., Cluff, Lloyd, Slemmons, Burt, Johnson, Elden, Roddick, Jim, Sorensen, Steve, 2004, Surface rupture on the Denali fault interpreted from tree damage during the 1912 Delta River

Mw 7.2–7.4 Earthquake; Implications for the 2002 Denali fault earthquake slip distribution, in Rowe, C.A., Christensen, Douglas, and Carver, G.A., eds., The 2002 Denali fault earthquake sequence, dedicated issue: Bulletin of the Seismological Society of America, v. 94B, p. S58–S71. Carver, G.A., Bemis S.P., Solie, D.N., and Obermiller, K.E., 2008, Active and potentially active faults in or near the Alaska Highway corridor, Delta Junction to Dot Lake, Alaska: Alaska Division of Geological & Geophysical Surveys Preliminary Investigative Report 2008-3d, 32 p. Chapman, J.B., Pavlis, T.L., Gulick, S.P.S., Berger, A.L., Lowe, Lindsay, Spotila, J.A., Bruhn, Ronald, Vorkink, Michael, Koons, Peter, Barker, Adam, Picornell, Carlos, Ridgway, K.D., Hallet, Bernard, Jaeger, J.M., and McCalpin, J.P., 2008, Neotectonics of the Yakutat collision; Changes in deformation driven by mass redistribu- tion, in Freymueller, J.T., Haeussler, P.J., Wesson, R.L., and Ekstrom, Goran, eds., Active tectonics and seismic potential of Alaska: American Geophysical Union Geophysical Monograph Series, v. 179, p. 65–82. Doser, D.I., 2004, Seismicity of the Denali–Totschunda fault zone in central Alaska (1912–1988) and its relation to the 2002 Denali fault earthquake sequence, in Rowe, C.A., Christensen, Douglas, and Carver, G.A., eds., The 2002 Denali fault earthquake sequence, dedicated issue: Bulletin of the Seismological Society of America, v. 94, no. 6B, p. S132–S144. Eberhart-Phillips, Donna, Haeussler, P.J., Freymueller, J.T., Frankel, A.D., Rubin, C.M., Craw, P.A.C., Ratchkovski, N.A., Anderson, Greg, Carver, G.A., Crone, A.J., Dawson, T.E., Fletcher, Hilary, Hansen, R.A., Harp, E.L., Harris, R.A., Hill, D.P., Hreinsdottir, Sigrun, Jibson, R.W., Jones, L.M., Kayen, R.E., Keefer, D.K., Larsen, C.F., Moran, S.C., Personius, S.F., Plafker, George, Sherrod, B.L.,Sieh, Kerry, Sitar, Nicholas, and Wallace, W.K., 2003, The 2002 Denali fault earthquake, Alaska; A large magnitude, slip partitioned event: Science, v. 300, issue 5622, p. 1,113–1,118. Foster, H.L., 1970, Reconnaissance geologic map of the Tanacross quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-593, 1 plate, scale 1:250,000. Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 39

Freymueller, J.T., Woodard, Hilary, Cohen, S.C., Cross, R.S., Elliot, Julie, Larsen, C.F., Hreinsdottir, Sigrun, and Zweck, Chris, 2008, Active deformation processes in Alaska, based on 15 years of GPS measurements; in Frey- mueller, J.T., Haeussler, P.J., Wesson, R.L., and Ekstrom, Goran, eds., Active tectonics and seismic potential of Alaska: American Geophysical Union, Geophysical Monograph Series, v. 179, p. 1–42. Haeussler, P.J., 2008, An overview of the neotectonics of interior Alaska: Far-fi eld deformation from the Yakutat microplate collision; in Freymueller, J.T., Haeussler, P.J., Wesson, R.L., and Ekstrom, Goran, eds., Active tectonics and seismic potential of Alaska: American Geophysical Union, Geophysical Monograph Series, v. 179, p.83–108. Lesh, M.E., and Ridgway, K.D., 2007, Geomorphic evidence of active transpressional deformation in the Tanana foreland basin, south-central Alaska; in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O’Neill, J.M., eds., Tectonic growth of a collisional continental margin; Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 573–592. Matmon, Ari, Schwartz, D.P., Haeussler, P.J., Finkel, R., Lienkaemper, J.J., Stenner, H.D., and Dawson, T.E., 2006, Denali fault slip rates and Holocene–late Pleistocene kinematics of central Alaska: Geology, v. 34, no. 8, p. 645–648. Page, R.A., Plafker, George, and Pulpan, Hans, 1995, Block rotation in east-central Alaska; A framework for evaluating earthquake potential?: Geology, v. 23, no. 7, p. 629–632. Plafker, George, Gilpin, L.M., and Lahr, J.C., 1994, Neotectonic map of Alaska: in Plafker, George, and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-l, 12 plates, scale 1:2,500,000. Ratchkovski, N.A., and Hansen, R.A., 2002, New constraints on tectonics of interior Alaska; Earthquake locations, source mechanism, and stress regime: Bulletin of the Seismological Society of America, v. 92, p. 998–1,014. Reger, R.D., Hubbard, T.D., and Carver, G.A., 2009, Surfi cial geology of the Alaska Highway corridor, Robertson River to Tetlin Junction, Alaska: Alaska Division of Geological & Geophysical Surveys Preliminary Investiga- tive Report 2009-6a, in press. Reimer, P.J., Baillie, M. G. L., Bard, Edouard, Bayliss, Alex, Beck, J.W., Bertrand, J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, Michael, Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, Bernd, McCormac, Gerry, Manning, Sturt, Ramsey, C.B., Reimer, R.W., Remmele, Sabine, Southon, J.R., Stuiver, Minze, Talamo, Sahra, Taylor, F.W., van der Plicht, Johannes, and Weyhenmeyer, C.E., 2004, IntCa104 terrestrial radiocarbon age calibration, 0–26 cal kyr BP: Radiocarbon, v. 46, no. 3, p. 1,029–1,058. Ridgway, K.D., Trop, J.M, Nokleberg, W.J., Davidson, C.M., and Eastham, K.R., 2002, Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range; Progressive basin development and deformation in a suture zone: Geological Society of America Bulletin, v. 114, no. 12, p. 1,480–1,504. Ruppert, N.A., Ridgway, K.D., Freymueller, J.T., Cross, R.C., and Hansen, R.A., 2008, Active tectonics of interior Alaska; Seismology, GPS geodesy, and local geomorphology, in Freymueller, J.T., Haeussler, P.J., Wesson, R.L., and Ekstrom, Goran, eds., Active tectonics and seismic potential of Alaska: American Geophysical Union Geophysical Monograph Series, v. 179, p. 109–133. Solie, D.N., and Burns, L.E., 2006, Geology, geophysics, and geohazards along the Alaska Highway corridor; A project of the Alaska Division of Geological & Geophysical Surveys (poster): Yukon Geosciences Forum, November 2006, Whitehorse, Yukon Territory, Canada. ———2007, Alaska Highway corridor geology and geophysics, in Alaska GeoSurvey News: Alaska Division of Geological & Geophysical Surveys newsletter, NL 2007-1, p. 1–4. Thoms, E.E., 2005, Neotectonics of the central Alaska Range; A guidebook for the 2005 Alaska Cell, Friends of the Pleistocene fi eld trip, 42 p.

Active and potentially active faults in or near the Alaska Highway corridor, Dot Lake to Tetlin Junction 41 Age Age Cal BP 600–560 4,190–4,190 4,190–4,190 C 12 ‰ C/ 13 BP Lab Age Lab B252294 7,090 ± 40 40 -23.6 7,980–7,850 ± B252294 7,090 50 -22.8 11,350–11,210 ± B252292 9,870 B252293 10,690 ± 50 -23.5 12,840–12,650 B252291 1480 ± 40 -25.9 1,420–1,300 B252303 70 ± 40 40 -25.2 270–0 ± 40 -23.3 B252303 70 ± 310–0 40 -23.3 B252302 200 ± 300–0 B252301 170 B252300 430 ± 40 B252299 -23.6 210 ± 40 B252312 -24.3 530–340 960 ± 40 -27.2 310–0 940–780 B252298 680 ± 40 -24.6 680–630 B252297 3,900 ± 40 40 -25.1 4,430–4,230 ± B252297 3,900 B252295 3,100 ± 40 40 -26.2 3,390–3,230 ± B252295 3,100 Number Number Paleoseismic Significance APPENDIX 1 Maximum limiting age for last event last age for limiting Maximum trench in recognized Maximum limiting age for penultimate trench in event recognized Maximum limiting age for penultimate trench in event recognized Stratigraphy may be reworked be reworked mayStratigraphy on this segment of fault on this segment of fault on this segment of fault on this segment Minimum limiting age for last event on event on last age for limiting Minimum fault of this segment Maximum limiting age for last event last age for limiting Maximum of fault on this segment Maximum limiting age for last event last age for limiting Maximum of fault on this segment Maximum limiting age for last event last age for limiting Maximum of fault on this segment event last age for limiting Maximum of fault on this segment Maximum limiting age for penultimate event on fault Dot “T” Johnson fault Cathedral Rapids Cathedral fault Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal Sample Sample Material Material Detrital twig twig Detrital event last age for limiting Maximum Detrital wood wood Detrital uncertain; or Indeterminate Detrital wood wood Detrital event last age for limiting Maximum wood Detrital event last age for limiting Maximum Location H = 17.15 m H = 17.15 m V = -1.6 m H = 3.15 m V = -0.10 m H = 1.45 m V = -0.05 H = 2.5 m m -0.05 V = H = 1.5 m m V = 0.05 H = 1.9 m V = 0.1 m m H = 2.15 m V = 0.05 m H = 2.95 m V = 0.50 m H = 2.75 V = 0 m H = 5.90 m H = 5.90 m V = -1.6 H = 3.5 m m V = 0.55 H = 5.7 m m H = 5.7 m V = -1.65 H = 2.95 m H = 2.95 m V = -1.05 C AGES FOR SAMPLES COLLECTED FROM TRENCHES IN 2008 14 Sample Sample Number DTJIII-10 West wall DTJIII-10 West wall DTJIII-11 East GMT-T1-06 East wall GMT-T1-06 East wall GMT-T1-05 East GMT- T1-03 West wall Wall GMT-T1-01 East wall GMT-T2-04 East Sears1-05 East wall Sears1-05 East Sears1-01 West wall Sears1-01 West SITE DTJIII-12 East wall East DTJIII-12 D“T”J III trench D“T”J III trench DTJIII-09 East wall Sears Creek trenchSears Creek Sears1-07 East wall GMT I trench GMT-T1-12 West wall 42 Preliminary Interpretive Report 2010-1 Age Age Cal BP C 12 ‰ C/ 13 BP Lab Age Lab B252310 10,210 ± 70 -25.2 12,230–11,630 B252308 9,400 ± 40 -26.8 10,720–10,520 B252307 9,530 ± 60 -23.6 11,120–10,610 B252309 9,110 ± 40 -25.3 10,290–10,220 B252305 7,750 ± 40 -24.7 8,600–8,430 B252306 7,670 ± 50 -24.0 8,550–8,390 Number Number e for penultimate continued Paleoseismic Significance Minimum limiting age for earliest earliest age for limiting Minimum trench in event Maximum limiting age for limiting Maximum trench in event antepenultimate Minimum limiting age for limiting Minimum trench in event antepenultimate Maximum limiting age for penultimate trench in event Minimum limiting ag event in trench trench in event Maximum limiting age for last event in event last age for limiting Maximum anticline on trench trench this in No paleoseismic record B252313 1,500 ± 40 -24.1 1,510–1,310 Dennison Fork lineament Fork Dennison Giant Moletrack Giant anticline APPENDIX 1— Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital Detrital charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal charcoal Sample Sample Material Material charred twig twig charred charred twig twig charred Location V = -0.75 m V = -0.75 V = -1 m V = -0.6 m m V = -0.6 V = -0.65 m V = -0.65 V = -0.4 m m V = -0.4 V = -0.4 m m V = -0.4 H = 5.4 m m V = -1.1 Sample Sample Number CHT-I-07 m H = 4.2 CHT-I-04 m H = 4.4 CHT-I-03 m H =4.05 CHT-I-06 m H = 4.3 CHT-I-01 H =4.3 m =4.3 CHT-I-01 H SITE Cliffhanger trenchCliffhanger CHT-I-02 m H = 4.5 IITrench TEII-04 North wall