The Quaternary thrust system of the northern Alaska Range

Sean P. Bemis1, Gary A. Carver2, and Richard D. Koehler3 1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, USA 2Carver Geologic, Inc., P.O. Box 52, Kodiak, Alaska 99615, USA 3Alaska Division of Geological and Geophysical Surveys, 3354 College Road, Fairbanks, Alaska 99709, USA

ABSTRACT INTRODUCTION et al., 2009). However, concurrent studies were just beginning to document the widespread The framework of Quaternary faults in South-central Alaska is a large geographic nature of Quaternary deformation in the north- Alaska remains poorly constrained. Recent region characterized by rugged, remote terrain ern Alaska Range (Bemis, 2004; Carver et al., studies in the Alaska Range north of the and low population density. Available geologic 2006, 2008; Bemis and Wallace, 2007) and Denali fault add signifi cantly to the recog- and elevation data are generally of low resolu- these preliminary data were insuffi cient to con- nition of Quaternary deformation in this tion, and detailed geologic studies related to strain the inferences of Matmon et al. (2006) active orogen. Faults and folds active dur- Quaternary faults are lacking in many areas. and Mériaux et al. (2009). Therefore, a more ing the Quaternary occur over a length of Studies of strain accumulation measured with systematic understanding of the style and activ- ~500 km along the northern fl ank of the global positioning system (GPS) (Freymueller ity of Quaternary faulting in the northern Alaska Alaska Range, extending from Mount et al., 2008) and strain release observed through Range is required to approach the problem of McKinley (Denali) eastward to the Tok seismicity (Ruppert, 2008) demonstrate that interaction between the dominantly right-lateral River valley. These faults exist as a con- all of south-central Alaska is actively deform- strike-slip Denali fault and modern growth of tinuous system of active structures, but we ing. However, for the reasons given, many of the Alaska Range. divide the system into four regions based the structures accommodating this deformation This paper presents the current state of knowl- on east-west changes in structural style. At remain elusive. Comparing the three published edge on Quaternary faulting in the northern the western end, the Kantishna Hills have summaries of Alaskan neotectonics (Brogan Alaska Range, introduces previously unde- only two known faults but the highest rate et al., 1975; Plafker et al., 1994; Haeussler, scribed faults in this region, and synthesizes the of shallow crustal seismicity. The western 2008) demonstrates that the number of known regional character of this system of Quaternary northern foothills fold-thrust belt consists of active faults has increased signifi cantly over faults. In addition to our maps, table, and discus- a 50-km-wide zone of subparallel thrust and the past 35 years, but the parameters essential sion of these Quaternary faults presented here, reverse faults. This broad zone of deforma- for seismic hazard assessments (e.g., slip rate, we have contributed our Quaternary fault data- tion narrows to the east in a transition zone paleoearthquake timing, and time since the most base to the development of the Quaternary fault where the range-bounding fault of the west- recent event) are published for only a limited and fold database for Alaska by the Alaska Divi- ern northern foothills fold-thrust belt ter- number of these faults. Thus, the current level of sion of Geological and Geophysical Surveys. minates and displacement occurs on thrust fault characterization is insuffi cient to properly Once completed, this database will serve as the and/or reverse faults closer to the Denali model seismic hazards for the region. Further- repository for access to up-to-date Quaternary fault. The eastern northern foothills fold- ing our understanding of these active faults is fault data for Alaska, including the northern thrust belt is characterized by ~40-km-long important for mitigating seismic hazards for Alaska Range. This paper summarizes major thrust fault segments separated across left- several major proposed and planned infrastruc- advances in the understanding of central Alaska steps by NNE-trending left-lateral faults. ture projects crossing interior Alaska. neotectonics since the last statewide summary Altogether, these faults accommodate much The 2002 M7.9 Denali fault earthquake by Plafker et al. (1994). The summary also of the topographic growth of the northern sequence (Eberhart-Phillips et al., 2003) ini- includes many important contributions beyond fl ank of the Alaska Range. tiated a renewed interest in the Quaternary Haeussler (2008) and suffi cient detail, where Recognition of this thrust fault system tectonics of central Alaska. While this event available for individual faults, to guide regional represents a signifi cant concern in addition provided an opportunity to characterize this interpretations and future studies. to the Denali fault for infrastructure adja- major continental strike-slip fault in detail, it cent to and transecting the Alaska Range. also highlighted how little was known about REGIONAL TECTONIC SETTING Although additional work is required to the structures accommodating growth of the characterize these faults suffi ciently for Alaska Range. Several studies suggested that Southern Alaska acts as a diffuse plate bound- seismic hazard analysis, the regional extent deformation of the Alaska Range north of the ary, where convergence between the Pacifi c and structural character should require Denali fault (referred to here as the northern and North American plates is accommodated the consideration of the northern Alaska Alaska Range) could help to accommodate the both along the Aleutian megathrust and by Range thrust system in regional tectonic observed westward decrease in slip rate across translating strain more than 600 km into central models. the Denali fault (Matmon et al., 2006; Mériaux Alaska (Fig. 1). Helping to drive this distributed

Geosphere; February 2012; v. 8; no. 1; p. 196–205; doi:10.1130/GES00695.1; 6 fi gures; 1 table.

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zone between the Denali and Tintina faults and 500 km between 152° W and 146° W (Ruppert et al., 70°N Arcc Ocean 2008). In this area, prominent NNE-trending lineaments of seismicity with predominantly left-lateral focal mechanisms (Page et al., 1995; Brooks Range Ruppert et al., 2008) are associated with the Salcha, Fairbanks, Minto Flats, Rampart, and Dall City seismic zones. However, these linea- 65°N CANADA ments have no known surface traces, despite the ALASKA Tintina f. occurrence of three M>7 earthquakes associated with these seismic zones since 1900 (Page et al., Alaska Range 1995; Ratchkovski and Hansen, 2002).

QUATERNARY FAULTS OF THE SCA Denali f. NORTHERN ALASKA RANGE 60°N Most of our current understanding of Quater- Castle Mtn. f. YB nary tectonics in the northern Alaska Range is derived from the following studies: (1) Wahr- haftig’s geologic mapping and related studies Bering Sea Fairweather fault Pacific in the western northern foothills (Fig. 2) (Wahr- Plate haftig, 1958, 1968, 1970a–1970h; Wahrhaftig 55°N et al., 1969); (2) active faulting investigations Aleutian Megathrust for the Trans-Alaska pipeline system (TAPS) Pacific Ocean prior to development (Woodward-Lundgren and 160°W 150°W 140°W Associates, 1974; Brogan et al., 1975); (3) post– 2002 Denali fault earthquake investigations Figure 1. Alaska and the location of the Alaska Range relative to major active crustal struc- (Carver et al., 2006); (4) synthesis of published tures. The dashed black line encloses the area of Figure 2 and illustrates the portion of the and unpublished neotectonic data for Alaska Alaska Range in which Quaternary faults and uplift occur north of the Denali fault. The (Plafker et al., 1994); (5) structural and geomor- motions of the Pacifi c plate, Yakutat Block (YB), and the portion of south-central Alaska phic studies in the western northern foothills adjacent to the Denali fault system (SCA) relative to North America are shown with black (Bemis, 2004, 2010; Bemis and Wallace, 2007); arrows and are not scaled for relative velocity. F.—fault. and (6) the Alaska Division of Geological and Geophysical Surveys geologic framework stud- deformation is the subduction and accretion of the Nenana Gravel, a thick sequence of coarse ies for the Alaska Highway corridor between the eastern and central Yakutat terrane (Fig. 1), alluvial fan and coalescing braidplain deposits. the Delta River and Canadian border (Carver which began in the late Miocene to Pliocene This unit signals a major drainage reversal after et al., 2008, 2010). Taken together, these stud- (e.g., Bruns, 1983; Plafker and Berg, 1994; 6.7 Ma (Wahrhaftig, 1958, 1987; Triplehorn ies defi ne a system of Quaternary faults in the Chapman et al., 2008). The strain from this col- et al., 2000). The Nenana Gravel fi lled the for- northern Alaska Range extending from Denali lision is transferred into central Alaska through mer foreland basin of the Alaska Range until in the west, to near the town of Tok in the east the translation and counterclockwise rotation of widespread deformation propagated northward, (Fig. 2). For most of this distance, the topo- south-central Alaska (Freymueller et al., 2008; and motion across thrust and/or reverse faults graphic range front is defi ned by active faults Haeussler, 2008). The Denali fault system uplifted these deposits above local base level or folds. These structures form the boundary defi nes the northern margin of the south-central (Thoms, 2000; Bemis and Wallace, 2007). As between the actively uplifting Alaska Range Alaska rotation and accommodates a large the former basin surface, the upper surface of and the subsiding Tanana Basin. To describe proportion of the strain translated into central the Nenana Gravel forms a distinct geomor- the Quaternary faulting of the northern Alaska Alaska (Fig. 1). phic surface of possibly early Quaternary age Range, we divide it into regions based on along- Deformation related to the modern Alaska (Wahrhaftig, 1987; Bemis, 2010) and is an strike structural changes, which essentially cor- Range was probably under way by the early important marker for determining cumulative respond with the manner in which each region Miocene as indicated by the formations of the uplift and deformation since that time. Where has been previously described. For each region, Oligocene–Miocene Usibelli Group (Wahrhaftig the Neogene sedimentary sequence has been we describe the major faults and style of defor- et al., 1969; Ridgway et al., 1999, 2007) and stripped in the northern Alaska Range, the mation and introduce new observations from thermochronologic exhumation ages (Haeussler , topography of the more resistant crystalline previously unpublished studies. Additional data 2008; Benowitz et al., 2009). Stratigraphy, basement rocks of the Yukon-Tanana terrane for each fault are contained in Table 1. paleofl ow directions, and unconformities within often exhibits a subplanar surface representing the Usibelli Group indicate minor syndeposi- the exhumed unconformity between the bed- Kantishna Hills tional foreland deformation (Ridgway et al., rock and Usibelli Group and/or Nenana Gravel. 2007). As the locus of Alaska Range deforma- Prior to the 2002 Denali fault earthquake The Kantishna Hills region (Fig. 3) is the least tion migrated northward, this foreland defor- sequence, most of the shallow crustal seis- studied in terms of Quaternary deformation mation was overwhelmed by the deposition of micity of interior Alaska occurred in a broad despite the high rate of instrumental seismicity

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0408020 km Fairbanks Tanana R. YUKON - TANANA UPLAND O TANANA BASIN

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Denali fault ( ( Mt. McKinley 63°N (Denali)

152°W 150°W 148°W 146°W 144°W TF 142°W Transion Kanshna Hills Western NFFTB Zone Eastern NFFTB

Figure 2. Quaternary faults of the northern Alaska Range. Map shows the extent of known Quaternary faults north of the Denali fault dis- cussed in this paper and the direct relationship of these faults with the topographic extent of the northern Alaska Range. Regions noted in text are shown in all-caps, towns and other locations shown in title case, rivers labeled with white italicized letters, and major crustal faults labeled with black italicized letters. Faults shown here with simplifi ed traces. NFFTB—northern foothills fold-thrust belt; TF—Totschunda fault. Base imagery is a slope map (black—steep, white—fl at) overlain with the U.S. Geological Survey 30-m digital elevation model (eleva- tion gradient from green [low] to white [high]). The white portion of the elevation scale roughly corresponds with the glaciated high peaks along the axis of the Alaska Range. Figures 3 through 6 use the same base image and will show each major region of the northern Alaska Range deformation from west to east.

associated with the Kantishna cluster (Ruppert Also within these mountains is the East Fork ment of earthquakes associated with the southern et al., 2008). Seismicity here does not clearly fault, which is only documented by Plafker et al. end of the Minto Flats seismic zone. The western correspond with known faults, although (1994) and displays an unvegetated, 4-m-tall, northern foothills fold-thrust belt is also bound cumulative deformation results in two primary late Holocene fault scarp with open fi ssures that by the Tanana basin to the north, the Hines Creek topographic elements, the Kantishna Hills are visible on recent satellite images. Based on fault to the south, and the Wood River to the east proper, and a band of mountains immediately the style of bedrock deformation (Reed, 1961) (Fig. 4). The general bedrock geology and Qua- north of the Denali fault. Occurrences of the and topographic trends, we suspect that addi- ternary stratigraphy are documented in a series of Nenana Gravel on the fl anks of the Kantishna tional Quaternary faults occur adjacent to the eight geologic maps (Wahr haftig, 1970a–1970h) Hills (Reed, 1961) demonstrate the anticlinal East Fork fault, as well as along the Minto Flats and a number of related papers (Wahrhaftig, origin of this landform. Geomorphic evidence seismic zone, but the resolution and focus of our 1958, 1968; Wahrhaftig et al., 1969). Subsequent of late Quaternary folding at the southwest mapping has not been suffi cient to recognize workers have revised and refi ned the understand- end of the Kantishna Hills and convex profi les late Quaternary deformation. The zone of defor- ing of the Neogene stratigraphic record (e.g., of an antecedent stream (Lesh and Ridgway, mation north of the Denali fault becomes nar- Ridgway et al., 1999, 2007; Thoms, 2000) and 2007) indicate that the anticline is active and rower west of the Kantishna Hills as indicated recently developed the framework of faults that propagating to the southwest. Based on the by the relatively narrow band of hills between accommodates the Quaternary uplift and defor- presence of uplifted Nenana Gravel–like sedi- the Denali fault and the basin to the north. mation of this region (Bemis and Wallace, 2007; ments near the Denali fault (Reed, 1961) and Bemis, 2010). our reconnaissance mapping on air photos and Western Northern Foothills Faults and folds of the western northern satellite images north of Mount McKinley, foothills fold-thrust belt trend approximately we infer the existence of a thrust fault that An abrupt change in topographic grain east-west, with both north- and south-vergent accommodates uplift along the range front between the broad, NE-SW–trending anticlinal structures. Additional faults occur oblique to this (herein named the Peters Dome fault; Fig. 3). ridge of the Kantishna Hills (Fig. 3) and the trend, and these appear to be subvertical faults Although fi eld investigations have not exam- E-W–trending ridges and broad, plateau-like that correspond with lateral changes in the struc- ined this fault, late Pleistocene activity of the uplift to the east (Fig. 4) constitutes the bound- tural style of the fold-thrust belt (Fig. 4; Bemis Peters Dome fault is inferred from the pres- ary (Fig. 2) between the Kantishna Hills and the and Wallace, 2007; Bemis, 2010). The North- ence of apparent scarps in moraine deposits western northern foothills fold-thrust belt. This ern Foothills thrust extends the entire length of along the fault trace. boundary also corresponds with the NNE align- the western northern foothills fold-thrust belt,

198 Geosphere, February 2012

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/196/3341856/196.pdf by guest on 25 September 2021 Quaternary thrust system of the northern Alaska Range 007) 2 ( e c 6, 2008) a l l a eld studies W (1992) d 006) n a s i m e B , ) e 0 Source 7 9 1 ) ) ) , 8 0 6 a 0 1 0 0 0 0 0 7 2 2 2 9 ( ( ( ) 1 . . . ( 0 l l l 1 a a a g i 0 t t t t f 2 e e e ( a r r r ed from Carver et al. (2006) h s e e e i r v v v h r r r m Nokleberg et al. (1992) Carver et al. (2006) Bemis (2010) a a a a e C W B C C p p p # u u - - u p - p u E W - u S - N , N E , S L , S L motion Relative L R R L ? 0 § 0 0 0 0 6 6 6 3 3 – (°) Dip > > > > 5 1 E ? ? S S 15–45? S-up This study S 15–45 S-up Carver et al. (2006) S S >30? RL, S-up (2007) (1970e), Bemis and Wallace Wahrhaftig N 10–30 N-up Bemis (2010) S Dip SW? >60? SW-up Modifi cates that geomorphic evidence or paleoseismic studies establish surface offset within the cates that geomorphic evidence or paleoseismic studies establish surface offset direction cant displacement has occurred, but recent offset geologic markers or detailed fi cant displacement has occurred, but recent offset s t ned by monocline ssures in tundra S? >60 N-up? Reed (1961), Plafker et al. (1994) n s e t ? n m p e g r e a m s c a s s oodplain deposits S 15–45? S-up Carver et al. (2008) e p p r n r i m l a a r c , c ssures, offset morainesssures, offset ? >60 LL, NE-up Holmes and Péwé (1965), Brogan et al. (1975), e s s s t s - t p p t r r r l g o a a u n surface in Nenana Gravel beds lineaments scarp Gravel c c h a o S S Scarp on terrace surface N 60–90 N-up (2007), (1970g), Bemis and Wallace Wahrhaftig F S L Scarps, offset morainesScarps, offset ? >60 LL, NW-up Carver et al. (2008) ? s e n i uvial a r o uvial terraces Scarps across multiple terraces S 15–30 S-up Bemis (2010) uvial terraces Fold scarps in multiple terraces N 15–30 N-up (2007), Bemis (2010) Bemis and Wallace uvial terrace Scarps, range front monocline S 15–45? S-up (2007), Bemis (2010) Bemis and Wallace m TABLE 1. SUMMARY OF QUATERNARY FAULTS IN THE NORTHERN ALASKA RANGE ALASKA THE NORTHERN IN FAULTS OF QUATERNARY 1. SUMMARY TABLE m m u i u , s i v l l e v u e e l c l u v v l a l o a a f r r a c r u G G e e s n n a a l e e n n a i c c a a v o o l n n l u terrace sediments l e e o o l H N A N H e e n n e e c c o o y t t r s s e e i i a n n e e n l l r e e P P e c c t o o e e a l l t t u o o a a Quaternary Nenana Gravel breaks-in-slope Topographic SW 30–60? S-up Nokleberg et al. (1992) Late Pleistocene Late Pleistocene fl L H QuaternaryHolocene Nenana Gravel Alluvial fans, modern surface Open fi Long-term scarp defi Q L Holocene Alluvial/colluvial deposits Steep scarp facet at base of older H t l t l u u a f a f s d n t i i l p a u t a a n f t t l l R u k u l u o e a a a f f r e M r t d r c n e e i C t t e h Queried dip values indicate that these were estimated based on geomorphic expression and the mapped trace of fault. LL—left-lateral motion, RL—right-lateral n/a—not applicable. s y t n d l Section B Section A Section B Section A *Time period during which the most recent displacement of fault is constrained. Use Quaternary indicates that signifi *Time fault § # i l y a u i r Donnelly Dome fault Holocene Holocene colluvium Scarp in outwash and moraines S 45–90? S-up Péwé and Holmes (1964), Carver et al. (200 Dot “T” Johnson faultEast Fork faultEva Creek fault HoloceneGlacier Creek fault Holocene Quaternary Holocene loess Quaternary Nenana Gravel Nenana Gravel Modern surfaceGranite Mountain fault – Scarps in fl breaks-in-slope Topographic scarp, open fi Fresh lineaments Topographic S ? 30–60? S-up >60 (2007), (1970e), Bemis and Wallace Wahrhaftig N-up Athey et al. (2006) Gold King fault – do not exist. Late Pleistocene indicates offset or deformation of landforms/deposits within the past ~130,000 yr. Holocene indi or deformation of landforms/deposits within the past ~130,000 yr. do not exist. Late Pleistocene indicates offset past ~12,000 yr. M Healy Creek fault Late Pleistocene Mid/late Pleistocene fl Gold King fault – C Ditch Creek fault Quaternary Nenana Gravel Long-term scarp on Nenana Gravel Kantishna Hills anticline Quaternary Pleistocene(?) alluvial surface Folded alluvial surfaces n/a n/a n/a and Ridgway (2007) Lesh Granite Mountain fault – Molybdenum Ridge Northern Foothills thrust Late Pleistocene Late Pleistocene fl Potts fault Quaternary Alluvial surface Lineaments, disrupted drainages ? >60? NE-up Bemis (2010) T Glacier faultTrident Quaternary Nenana Gravel breaks-in-slope Topographic S 30–60? S-up Nokleberg et al. (1992) Healy fault Holocene Holocene buried soil Late Scarp across multiple terraces N 45 N-up (2007), Bemis (2010) Bemis and Wallace H Kansas Creek fault Quaternary Usibelli Group Breaks-in-slope, topographic Macomb Plateau faultMcGinnis Glacier fault Quaternary Holocene Erosion surface Modern surface Long-term scarp on erosion surface gashes in modern surface Tension S SW? 15–60? >45? S-up SW-up This study Brogan et al. (1975), Nokleberg Canteen fault Holocene Holocene loess, pond Park Road faultPeters Dome fault Pleistocene Late Quaternary Alluvial fans Nenana Gravel, moraines? Long-term scarp uplifting Nenana Lineaments and intermittent scarps N 30–90 N-up (2007) Bemis and Wallace Stampede fault Pleistocene Late Pleistocene fl Late Fault name Activity* unit offset Youngest Geomorphic expression B Panoramic fault Late Pleistocene Alluvial surface across Panoramic Creek Surface offset ? >60 NE-up Carver et al. (2008) Red Mountain fault Late Pleistocene Pleistocene alluvium Breaks-in-slope, scarps S 30–60? S-up Csejtey et al. (1992), Carver (2 Rex fault Late Pleistocene Pleistocene outwash Late across large drainage Surface offset S >30 S-up Carver et al. (2006)

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152°W 150°W and deformation of the Japan Hills anticline 64°N north of the Northern Foothills thrust (Fig. 4). The western part of the Gold King fault is a north-vergent thrust fault that progressively off- ( sets a sequence of late Pleistocene terraces. The O ( eastern portion of this fault has a south-vergent 01020km SF surface trace and is clearly defi ned at the surface McKinley River by mono clinal folding of the uppermost beds of the Nenana Gravel (Bemis, 2010). Because the individual faults and folds within Minto Flats Seismic Zone this zone are superimposed on the uplift of the entire region, Bemis and Wallace (2007) inter- incised Kanshna Hills ancline preted that the Northern Foothills thrust is the meanders HCF surface trace of a basal detachment that extends underneath this region at least to the Hines Creek fault. Therefore, the smaller-wavelength folds within this zone (such as the folds associ- ated with the Stampede and Park Road faults; East Fork Fig. 4) are the expression of shortening within fault ? the hanging wall of the Northern Foothills

( thrust. Based on slip rates inferred from the

( Denali fault deformation of Pleistocene fl uvial terraces by

( Bemis (2010), we estimate a maximum value

? ( of ~3 mm/yr of horizontal shortening across

( the northern foothills fold-thrust belt west of the ( Peters( Dome fault Mt. McKinley Nenana River. 63°N ? (Denali) The Hines Creek fault juxtaposes rocks of the Yukon-Tanana terrane (a Cretaceous meta- morphic assemblage of Precambrian–Paleozoic Figure 3. Quaternary faults and deformation of the Kantishna Hills area. The large dotted metamorphic rocks and Mesozoic plutons [e.g., line is along the Minto Flats seismic zone, which separates the Kantishna Hills from the Pavlis et al., 1993; Hansen and Dusel-Bacon, western northern foothills fold-thrust belt. Incised meanders of the McKinley River where 1998; Dusel-Bacon et al., 2004]) to the north, it crosses the axis of the Kantishna Hills anticline demonstrate recent activity of this struc- with Paleozoic to Paleogene rocks representa- ture (Lesh and Ridgway, 2007). HCF—Hines Creek fault; SF—Stampede fault. This and tive of marine environments to the south (Wahr- subsequent fi gures use the following typical fault symbology: solid—certain, long dashes— haftig et al., 1975). This fault also approximates approximately located, short dashes—inferred, dotted—concealed, and queries indicate a boundary between the rugged, recently gla- existence is uncertain. ciated terrain of the main axis of the Alaska Range and the northern foothills of the Alaska typically occurring near the base of a large 1600 yr ago (Bemis, 2010). The Healy Creek Range, which contain widespread Neogene monocline that marks the northern margin of fault has a distinct scarp where it offsets older deposits and landforms that span Quaternary the Alaska Range. Studies of this fault near the fl uvial terraces, but latest Pleistocene terraces time. Locally the Hines Creek fault is described Nenana and Wood rivers (Fig. 4; Bemis, 2010) are not offset (Wahrhaftig, 1970g; Brogan et al., as deforming Neogene deposits, although differ- suggest that these parts of the Northern Foothills 1975; Bemis, 2010). Cumulative displacement ent workers have mapped this fault in different thrust have not produced a surface rupture during across the Healy Creek fault appears to decrease locations (e.g., Wahrhaftig, 1958; Sherwood the Holocene, although late Pleistocene activity signifi cantly west of the Nenana River, presum- and Craddock, 1979; Csejtey et al., 1992). is apparent due to offsets of fl uvial terraces. The ably transferring slip onto the adjacent Stam- Bemis and Wallace (2007) reinterpreted some Stampede fault is clearly defi ned by the steep pede fault (Bemis and Wallace, 2007). The Eva of this previous mapping to suggest that this forelimb of a bedrock-cored anticline and a fold Creek fault is mapped by Athey et al. (2006) to late Cenozoic deformation results from motion scarp that progressively offsets several middle offset the Nenana Gravel and corresponds with on adjacent contractional faults. At one loca- and late Pleistocene fl uvial terraces (Bemis and a topographic lineament, but the recent activity tion, a short segment of the fault mapped near Wallace, 2007; Bemis, 2010). The Park Road of this fault is unknown. The Mystic Mountain, the Nenana River and Denali Park headquar- fault also occurs on the south fl ank of a bed- Kansas Creek, and Ditch Creek faults appear to ters offsets late Pleistocene glacial outwash rock-cored anticline, with the trace of the fault be a complex system of NW-striking, SW-side- deposits ~6 m down to the south (Wahrhaftig, defi ned by intermittent scarps in alluvial fans, up oblique-slip faults connected by east-west– 1958; Wahrhaftig et al., 1975). deformed Nenana Gravel, and mapped bedrock striking thrust and/or reverse faults (Bemis and Plafker et al. (1994) document several faults offsets (Sherwood and Craddock, 1979; Bemis Wallace, 2007). All three faults are associated between the Denali and Hines Creek faults as and Wallace, 2007). The Healy fault has a well- with deformation of the Nenana Gravel, and “suspicious” and two as being active during defi ned scarp where it offsets several fl uvial ter- Carver et al. (2006) indicate the presence of the Quaternary. Subsequent reconnaissance races. Paleoseismic trenching demonstrates that late Pleistocene scarps. The Gold King fault work in the region has not identifi ed additional the most recent rupture was between ~500 and and related structures accommodate the uplift evidence for Quaternary activity of these suspi-

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1500°W 1488°W 0 5 10 20 km

(

( O

Northern Foothills thrust Wood R.

(

(

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(

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64°N64 N ( Mysc Mtn. f. (

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( ( ? Healy Creek fault Kansas Creek fault

( ( ( ( ( ( ( ( ? ? ( Red Mtn. f.( Minto Flats Seismic Zone ? ( (

Stampede fault (

HF ( @ ( ( ? ( ( ? ( ( ( Park Road fault Hines Creek fault

Figure 4. Quaternary faults of the western northern foothills fold-thrust belt. The framework of faults is relatively well defi ned in the west near the Nenana River, where the Northern Foothills thrust, Stampede fault, Healy Creek fault, and Park Road fault accommodate the majority of deformation. To the east, the structural framework is complicated by the occurrence of several NW-striking faults near the Wood River. Furthermore, topographic and geologic trends suggest the presence of additional faults in the region between the Healy Creek and Kansas Creek faults, east of the Eva Creek fault, and west of the Park Road fault. The Hines Creek fault is shown as a dashed line because Quaternary activity is unknown and the mapped trace varies between authors. Trace shown from Csejtey et al. (1992). Abbreviations: f.—fault; BF—Bear Creek fault; HF—Healy fault; HuF—Hunter fault; JHA—Japan Hills anticline; NFT—Northern Foothills thrust; PoF—Potts fault. (A) and (B) indicate sections A and B of the Gold King fault, with their individual characteristics described in Table 1.

cious faults, because these faults do not intersect of the western northern foothills fold-thrust belt Post–2002 Denali fault earthquake investiga- Pliocene or younger rocks and do not obvi- (Fig. 4), as defi ned by the deformed Nenana tions for the Alyeska Pipeline Service Company ously deform surfi cial deposits. The Healy Gravel, transition to structures of the eastern identifi ed evidence for additional Quaternary Creek fault, as defi ned by Brogan et al. (1975), northern foothills fold-thrust belt coincident faults. The results of these studies are only pre- is incorrectly labeled by Plafker et al. (1994). with a southerly step in the topographic range sented in an abstract by Carver et al. (2006), This fault is located north of the Hines Creek front and a narrowing of the Alaska Range north and thus Table 1 and Figure 5 summarize the fault (Fig. 4). Therefore, although we agree of the Denali fault (Fig. 5). evidence for these faults from their mapping. with the suspicious nature of faults indicated Active faulting investigations performed prior Although Carver et al. (2006) did not include by Plafker et al. (1994) between the Denali and to the development of the Trans-Alaska pipe- detailed fi eldwork west of the Delta River, their Hines Creek faults, the East Fork fault (Fig. 3) line system (Woodward-Lundgren and Asso- regional reconnaissance identifi ed late Pleisto- is currently the only fault with demonstrable ciates, 1974; Brogan et al., 1975) identifi ed the cene and Holocene fault scarps associated with Quaternary offset in this zone. McGinnis Glacier fault as an active fault. These several previously mapped faults, including the studies describe evidence suggestive of a Holo- Red Mountain fault (Csejtey et al., 1992), Gla- Transition Zone between the Western and cene surface rupture including north-south– cier Creek fault (Wahrhaftig 1970e; Bemis and Eastern Northern Foothills oriented , en echelon tension cracks across Wallace, 2007), and the eastern end of the North- glacio fl uvial deposits in several valleys. How- ern Foothills thrust (Bemis and Wallace, 2007). Most geologic mapping of the northern foot- ever, the tectonic history and structural relation In addition to late Quaternary fault scarps, the hills between the Wood and Delta rivers of the to the northern foothills fold-thrust belt and/or Red Mountain and Glacier Creek faults bound Alaska Range has been published at 1:250,000 the Denali fault system remain unclear. Nokle- the north side of bedrock uplifts and are closely scale (Csejtey et al., 1992; Nokleberg et al., berg et al. (1992) provide additional map docu- associated with folded Neogene deposits. The 1992), and as a result, only captured some of the mentation of the McGinnis Glacier fault, and eastern end of the Northern Foothills thrust is basic details of possible Quaternary deforma- also map the Trident Glacier fault as displacing associated with a large anticline, under which tion. What is clear from this previous mapping Nenana Gravel–like deposits, indicating Quater- the fault becomes a blind thrust as the anticline is that the major east-west–trending anticlines nary activity of this fault. begins to plunge to the east. Carver et al. (2006)

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Nor 010205 km front for the next ~40 km between the Canteen

ther thrust n thills ( ( ( foo fault and the town of Dot Lake (Fig. 6). Geo-

Lile Delta R. morphic mapping and trench studies indicate

Glacier Crk. f. O

Wood R. Rex ancline that the Dot “T” Johnson fault is a Holocene-

(

( 64°N active thrust fault (Carver et al., 2008, 2010). (

( Rex f. ( East of Dot Lake, the range front steps to the

Delta R.

( Red Mtn. f. south, and evidence for active faulting is absent

( in the late Pleistocene glacial deposits of the ( (

( ? Molybdenum Ridge f.

( UF Robertson River. ( ( (

KCF ( MRA

( ? ( East of the Robertson River, the Cathedral

Rapids fault extends for ~40 km near the Tok

DDF

( UF ( ( ? River valley (Fig. 6). The western half of this

TF ( ? Tride ( fault is characterized by well-developed trian-

nt Gl. f. gular facets on the range front and three sub- (

( McGinnis Gl. f. parallel, sinuous, south-dipping thrust splays.

( ( These fault splays offset late Pleistocene gla-

( ( cial deposits and Holocene alluvium (Koehler

et al., 2010). The eastern half of this fault is ( ( ( characterized by an anticline that progressively

Denali fault ( deforms alluvial fan deposits along the north- ern range front. Offset and folded terraces as well as paleoseismic excavations indicate late 146°W Pleistocene and Holocene displacement of the Cathedral Rapids fault (Carver et al., 2010). Figure 5. Quaternary faults and deformation of the transitional zone between the west- Although Plafker et al. (1994) noted numerous ern and eastern northern foothills fold-thrust belt (NFFTB). The Northern Foothills thrust, suspect lineaments in the Tok River valley and which is continuous along the entire range front of the western NFFTB, terminates into to the northeast as potential Quaternary faults, large, east-plunging Rex anticline. This anticline deforms late Pleistocene terraces along investigations by Carver et al. (2010) did not the Little Delta River. Beyond this to the east, the range-bounding faults step southward, fi nd evidence for Quaternary activity on any of closer to the Denali fault. The Molybdenum Ridge fault becomes blind to the east and is these features. interpreted to lie beneath the Molybdenum Ridge anticline (MRA), which impounds a sub- Outside of the range-bounding faults, there stantial lake in an antecedent drainage immediately upstream of the anticlinal axis. Abbre- are two additional Quaternary faults in this por- viations: f.—fault; DDF—Donnelly Dome fault; KCF—Kansas Creek fault; TF—Trident tion of the northern Alaska Range. Carver et al. fault; UF—unnamed faults (from Carver et al., 2006, 2008). (2008, 2010) investigated geophysical and geo- morphic lineaments north of the Tanana River, and only documented evidence for Quaternary also document several previously unknown Donnelly Dome glacial advance are offset displacement on the Billy Creek fault (Fig. 6). Quaternary faults, including the Molybdenum 3–9 m (Brogan et al., 1975), and trenching dem- Within the Alaska Range, we infer the Macomb Ridge fault, the Rex fault, and the Trident fault onstrates that this fault has ruptured during the Plateau fault based on the large (100–200 m) (Fig. 5). The Molybdenum Ridge fault is clearly Holocene (Carver et al., 2006; Table 1). Directly sinuous scarp that appears to offset the broad defi ned by fault scarps for ~30 km. Farther east, east of the Donnelly Dome fault, the northeast- Macomb Plateau surface (Fig. 6). surface deformation from this fault appears as striking portion of the Granite Mountain fault Slip-rate information for the eastern northern subtle folding of the landscape, which has dis- displays SE-side-up, left-lateral slip where it foothills fold-thrust belt is limited to measure- turbed surface drainage across the anticlinal axis offsets late Pleistocene glacial deposits. Here, ments on offset late Pleistocene moraines cut by (Fig. 5). The Rex and Trident faults have low the northeast-trending Panoramic fault offsets the Canteen fault, part of the Dot “T” Johnson cumulative displacements, and appear to offset Holocene alluvium on the south side of the fault system that suggests a left-lateral slip rate Nenana Gravel–equivalent and late Pleistocene Tanana River valley and, together with the Gran- of 1.6 mm/yr (Carver et al., 2008). We can also glacial deposits. ite Mountain fault, may be kinematically linked estimate a slip rate for the adjacent, and likely to the Donnelly Dome fault (Fig. 6). The Granite kinematically connected Granite Mountain Eastern Northern Foothills Fold-Thrust Belt Mountain fault follows the range front around fault. Carter (1980) measured a 1000 m section an ~90° bend to the southeast, and becomes a of tilted Nenana Gravel on the footwall of the Along the northern margin of the Alaska predominantly reverse-slip fault (Carver et al., Granite Mountain fault, which has uplifted a Range east of the Delta River, the range front 2008). The predominantly left-lateral Canteen bedrock unconformity containing relict patches defi nes a series of left steps, including from west fault forms a link between the Granite Moun- of Nenana Gravel (Holmes and Péwé, 1965) on to east, the Donnelly Dome, Granite Mountain, tain fault and the Dot “T” Johnson fault across its hanging wall ~1 km above the exposure. This and Dot “T” Johnson faults. The Donnelly another left step in the range front. The Canteen suggests up to 2 km of vertical separation on the Dome fault was originally defi ned as a normal fault is delineated by left-laterally offset latest upper surface of the Nenana Gravel. Therefore, fault (Péwé and Holmes, 1964), but was subse- Pleistocene glacial moraines (Carver et al., assuming a 30° dip for the Granite Mountain quently redefi ned as a reverse fault (Nokleberg 2008). The previously unknown Dot “T” John- fault and allowing for generous uncertainties in et al., 1992). Moraines of the late Pleistocene son fault is nearly continuous along the range the early Quaternary age of the upper Nenana

202 Geosphere, February 2012

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144°W 010205kmO Granite Mtn. f.

(

( PF Billy Creek f. Donnelly (A) (

Dome f. (B)

( ( ( (

Delta R.

(

( (

Canteen f. (( (

(

( Dot “T” Johnson f. Dot Lake

? (

( 63°30′N

Macomb ?

( Plateau f. (

Robertson R.

Tanana R.

( Cathedral Rapids f.

( ( ( Tok

( Denali fault (

Tok R. ( (

Figure 6. Quaternary faults of the Dot “T” Johnson fault system. Note the left-stepping pattern of faulting between Dot Lake and the Delta River. PF—Panoramic fault. (A) and (B) refer to sections A and B of the Granite Mountain fault, with their individual characteristics described in Table 1.

Gravel and the fault dip, we suggest a horizontal Péwé, 1965; Holmes and Foster, 1968) defi ne the intersection between the Totschunda and shortening rate of 1–4 mm/yr. This range is con- a similar style of regional uplift. Within the Denali faults (Fig. 2), suggesting that the angu- sistent with the slip rate for the Canteen fault, hanging wall of the basal thrust are additional lar difference between these two faults may play and suggests that either the Granite Mountain thrust faults that superimpose localized zones a role in the occurrence of shortening north of fault has a similar slip rate or, if faster, that the of deformation upon the regional uplift. Long- the Denali fault. Canteen fault accommodates only a portion of term subsidence of the foreland basin to the The NNE-trending faults of the eastern the shortening across this portion of the northern north is indicated in the Tanana basin (Fig. 2) by northern foothills fold-thrust belt, the Gran- Alaska Range. the thick sequence of sedimentary fi ll (Hanson ite Mountain fault, Canteen fault, and perhaps et al., 1968). a similarly oriented portion of the Dot “T” EXTENT AND CHARACTER OF The mapped western and eastern extents Johnson fault (Fig. 6), are aligned with faults THE NORTHERN ALASKA RANGE of Quaternary faulting in the northern Alaska and geophysical lineaments mapped in the THRUST SYSTEM Range correspond with signifi cant geometric Yukon-Tanana Uplands well to the north of the complexities of the Denali fault system. The Alaska Range (e.g., Foster, 1970; Carver et al., Active faults exist in an essentially continu- Peters Dome fault occurs immediately north of 2008). A widely accepted regional model pro- ous system as part of the northern Alaska Range an abrupt ~17° bend in the Denali fault, inside posed by Page et al. (1995) suggests that the for ~500 km, from Denali to the town of Tok of which is the high topography of Denali (Figs. NNE-trending faults could be throughgoing (Fig. 2). Faults or folds that exhibit late Pleis- 2 and 3). West of this fault bend, the Denali fault active structures accommodating clockwise tocene to Holocene activity defi ne the range displays Quaternary activity (Reed and Lan- block rotations and offsetting the range front front for essentially this entire distance. The phere, 1974; Plafker et al., 1994; Bundtzen et al., of the Alaska Range. However, because the relatively persistent style of plateau-like uplift 1997), but signifi cant topography north of the Quaternary displacement across the NNE- suggests that much of the uplift occurs over a fault is absent. At the eastern end of the northern trending faults appears to be mostly restricted south-dipping basal thrust fault that underlies Alaska Range, Carver et al. (2010) did not iden- to between thrust fault segments (Carver et al., much of the northern Alaska Range. In the tify any evidence for Quaternary faulting east of 2008, 2010), we argue that these NNE-trending west ern northern foothills fold-thrust belt, the Tok, and the topography immediately north of faults are older bedrock structures that have widespread preservation of the upper surface the Denali fault is subdued relative to the rug- been reactivated locally as lateral tears in the of the Nenana Gravel defi nes this uplift (Fig. 4). ged mountains of the eastern northern foothills Dot “T” Johnson fault thrust sheet. This would Along the eastern northern foothills fold-thrust fold-thrust belt. This eastern termination of the suggest that, although active, the NNE-trending belt, bedrock surfaces with local remnants of northern foothills fold-thrust belt near the Tok faults may not rupture independent of the adja- Nenana Gravel–like deposits (Holmes and River valley (Fig. 6) lies immediately north of cent thrust fault segments.

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Wahrhaftig, C., 1958, Quaternary geology of the Nenana basis for pipeline design for active-fault crossings for Ridgway, K.D., Trop, J.M., and Jones, D.E., 1999, Petrology River valley and adjacent part of the Alaska Range, in the Trans-Alaska pipeline system: Houston, Texas, and provenance of the Neogene Usibelli Group and Wahrhaftig, C., and Black, R.F., eds., Quaternary and unpublished report prepared for Alyeska Pipeline Service Nenana Gravel: Implications for the denudation history Engineering geology in the Central Part of the Alaska Company, 115 p. of the central Alaska Range: Journal of Sedimentary Range: Washington, D.C., U.S. Geological Survey Pro- Research, v. 69, no. 6, p. 1262–1275. fessional Paper 293, p. 1–73. Ridgway, K.D., Thoms, E.E., Layer, Paul W., Lesh, Wahrhaftig, C., 1968, Schists of the central Alaska Range: M.E., White, J.M., and Smith, S.V., 2007, Neogene U.S. Geological Survey Bulletin 1254-E, 22 p. transpressional foreland basin development on the Wahrhaftig, C., 1970a, Geologic map of the Fairbanks A-2 MANUSCRIPT RECEIVED 28 MARCH 2011 north side of the central Alaska Range, Usibelli quadrangle, Alaska: U.S. Geological Survey Geologic REVISED MANUSCRIPT RECEIVED 19 AUGUST 2011 Group and Nenana Gravel, Tanana basin, in Ridgway , Quadrangle Map GQ-808, 1 sheet, scale 1:63,360. MANUSCRIPT ACCEPTED 5 OCTOBER 2011

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