Late Oligocene to Present Contractional Structure in and Around the Susitna Basin, Alaska—Geophysical Evidence and Geological GEOSPHERE; V

Research Paper THEMED ISSUE: Geologic Evolution of the Alaska Range and Environs

GEOSPHERE Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological GEOSPHERE; v. 12, no. 5

doi:10.1130/GES01279.1 implications R.W. Saltus1,*, R.G. Stanley2, P.J. Haeussler3, J.V. Jones III3, C.J. Potter4, and K.A. Lewis1 7 figures; 1 supplemental file 1U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA CORRESPONDENCE: rick.saltus@noaa.gov 3U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508-4626, USA 4U.S. Geological Survey, Piscataway, New Jersey 08854, USA CITATION: Saltus, R.W., Stanley, R.G., Haeussler, P.J., Jones, J.V., III, Potter, C.J., and Lewis, K.A., 2016, Late Oligocene to present contractional structure in ABSTRACT morphic rocks of the Talkeetna Mountains, and on the southeast by the Castle and around the Susitna basin, Alaska—Geophysical Mountain strike-slip fault. evidence and geological implications: Geosphere, The Cenozoic Susitna basin lies within an enigmatic lowland surrounded Existing structural information for the basin is sparse; aside from the Castle v. 12, no. 5, p. 1378–1390, doi:10.1130/GES01279.1. by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountain fault, none of the bounding structures are exposed. Subsurface data Mountains), and Talkeetna Mountains in south-central Alaska. Some previ- consist of a modest number of vintage seismic lines (e.g., Lewis et al., 2015) Received 1 October 2015 Revision received 8 June 2016 ous interpretations show normal faults as the defining structures of thebasin and a few wells, none of which drilled deep enough to reach crystalline base- Accepted 6 July 2016 (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data ment. Other data include 1970s and newer gravity data, mostly collected by the Published online 11 August 2016 shows predominantly (Late Oligocene to present) thrust and reverse fault State of Alaska and the U.S. Geological Survey (USGS; see Supplemental File1) geometries in the region, as previously proposed by Hackett (1978). A key ex- and aeromagnetic data from surveys collected in 2000 and 2012 by the USGS ample is the Beluga Mountain fault where a 50-mGal gravity gradient, caused (http://mrdata.usgs.gov/magnetic/show-survey.php?id = 4247, http://mrdata / ------/ XYZ EXPORT [10/01/2015] / DATABASE [.\SusitnaGravAll.gdb] / ------by the density transition from the igneous bedrock of Beluga Mountain to .usgs.gov /magnetic /show -survey .php?id = 10001). / / NAME LON LAT Xutm5 Yutm5 ElevM FAA CBA CBAfix /======/ the >4-km-thick Cenozoic sedimentary section of Susitna basin, spans a hori Based on access to early exploration data and models, Kirschner (1988, T628 -147.18000 62.98917 794620.7 6997531.1 786.09 35.61 -51.79 -51.79 01TK -147.68883 62.99867 768820.8 6996354.6 1158.25 78.92 -44.71 -44.71 01TK -147.68233 62.97767 769342.7 6994048.0 1005.85 75.65 -33.98 -33.98 T629 -147.45383 62.96550 781012.5 6993674.4 765.67 58.81 -25.54 -25.54 T596 -148.02450 62.98367 751986.5 6993326.6 809.56 17.66 -70.88 -70.88 zontal distance of ~40 km and straddles the topographic front. The location 1994) depicted the Susitna basin as bounded entirely by normal faults. How- T626 -147.01367 62.94200 803512.1 6993066.3 800.72 6.45 -82.96 -82.96 HS37 -148.55167 62.99700 725218.4 6992851.1 762.92 9.82 -67.39 -67.39 T601 -148.32083 62.98783 736965.2 6992661.5 979.33 50.33 -57.49 -57.49 99TM -148.27083 62.98533 739514.5 6992568.9 1170.45 61.24 -66.59 -66.59 and shape of the gravity gradient preclude a normal fault geometry; instead, ever, this depiction was inconsistent with an earlier gravity interpretation of T536 -148.56100 62.99467 724764.5 6992559.3 778.47 10.91 -69.29 -69.29 99TM -148.25300 62.98417 740425.8 6992506.5 1056.75 48.42 -68.92 -68.92 99TM -148.22050 62.97533 742142.9 6991645.6 978.42 34.33 -74.45 -74.45 01TK -148.99200 62.99333 702972.6 6990975.8 1575.84 83.71 -73.43 -73.43 it is best explained by a southwest-dipping thrust fault, with its leading edge Hackett (1977a, 1977b) that showed a reverse geometry for the Beluga Moun- FP25 -147.40500 62.93667 783761.5 6990685.7 838.21 53.33 -39.91 -39.91 T493 -148.88200 62.98733 708580.5 6990660.5 743.11 10.11 -65.68 -65.68 T489 -149.17633 62.98600 693694.7 6989591.4 708.36 -0.07 -67.20 -67.20 located several kilometers to the northeast of the mountain front, concealed tain fault that bounds Susitna basin to the southwest. 1Supplemental File. Gravity data in and around beneath the shallow glacial and fluvial cover deposits. Similar contractional The dip of the Beluga Mountain fault is an ideal target for gravity anomaly Susitna basin, Alaska. Please visit http://dx .doi .org fault relationships are observed for other basin-bounding and regional faults investigation. The significant lateral density contrast between the igneous bed- /10.1130/GES01279 .S1 or the full-text article on www as well. Contractional structures are consistent with a regional shortening rock of Beluga Mountain and the Cenozoic sedimentary deposits of the Susitna .gsapubs.org to view the Supplemental File. strain field inferred from differential offsets on the Denali and Castle Mountain basin creates an easily measurable gravity gradient. If the surface projection right-lateral strike-slip fault systems. of the presumed fault structure can be identified, then the overall dip of the OLD G structure can be uniquely determined from gravity modeling (e.g., Saltus and Blakely, 2011, and references therein). Hackett (1977b) showed a reverse fault INTRODUCTION geometry model for the Beluga Mountain fault based on an assumption of the surface fault projection at the topographic mountain front. In this report we OPEN ACCESS The Susitna basin of south-central Alaska (Figs. 1 and 2) consists of ~4–5 km present an update of his model, using best available gravity and airborne mag- of Cenozoic strata (Stanley et al., 2014). The basin is bounded on the southwest netic data for the Beluga Mountain fault. Using this well-constrained model, by the intrusive, metavolcanic, and sedimentary rocks of Mount Susitna and we point out other likely thrust or reverse structures based on similar gravity Beluga Mountain, on the northwest by the Kahiltna flysch sequence (Wilson anomaly-topographic front associations in the Susitna lowland region and dis- et al., 2012) that outcrops in low hills, on the northeast by igneous and meta- cuss the relevance of these features in the context of southern Alaska margin geodynamics. An improved structural model for Susitna basin is important for

This paper is published under the terms of the *Now at Cooperative Institute for Research in Environmental Sciences, University of Colorado, better understanding of hydrocarbon resources and modern seismic hazards CC‑BY license. Boulder, Colorado 80305, USA close to the population center of Anchorage, Alaska (e.g., Gillis et al., 2013).

GEOSPHERE | Volume 12 | Number 5 Saltus et al. | Susitna basin contractional structure Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/5/1378/3334604/1378.pdf 1378 by guest on 29 September 2021 Research Paper

153°W 150°W 147°W

BP 63°N

Figure 1. South-central Alaska physiog Alaska Range raphy. The Susitna lowland is surrounded by the Western Alaska Range, Central Central Alaska Range, and Talkeetna Mountains and is separated from the upper Cook Inlet basin by the Castle Mountain fault (CMF). The Susitna basin (SB) and Yentna basin (YB) fall within the lowland. A triangular Talkeetna Mtns inter-lowland region of higher relative YB topography includes Beluga Mountain Susitna (BM), Mount Susitna (MS), and Little Mount Susitna (LMS). The small Beluga depo lowland center (BD) lies to the southwest of the Western Alaska Range SB elevated Beluga-Susitna triangle. The back- (including Tordrillo Mtns) BM ground topography is a shaded rendition of a 300-m digital elevation model. Gray lines indicate mapped faults by Haeussler (2008). MS CMF LMS ANC—Anchorage; BP—Broad Pass. BD

t ANC 61°N

Cook Inle

100 km

GEOLOGIC AND GEOPHYSICAL SETTING Previous authors have stated that the Susitna basin is a northern continuation of the Cook Inlet forearc basin (e.g., Rouse and Houseknecht, 2012, p. 2; Geology Craddock et al., 2014, p. 48), but we disagree with this interpretation because the two basins differ from each other in significant ways. The Cook Inlet basin The Susitna basin is separated from the adjacent Cook Inlet basin by the is a 200-million-year-old forearc basin with Late Paleocene to Quaternary non- seismically active Castle Mountain fault (CMF, Fig. 1) with estimated right- marine strata up to 8 km thick (LePain et al., 2013); these strata rest unconform- lateral displacements of ~26 km since 35 Ma (Haeussler and Saltus, 2005) and ably on a pile of Late Triassic to Late Cretaceous marine strata more than 10 km ~110–130 km since the Late Jurassic (Trop et al., 2003). We suggest that the thick (included in the Peninsular terrane of Nokleberg et al., 1994). In contrast difference between these two estimates may indicate that tens of kilometers to the Cook Inlet basin, the Susitna basin is less than 60 million years old and of right-lateral movement occurred along the Castle Mountain fault during the consists of nonmarine strata of Late Paleocene to Quaternary age that have a Cretaceous and/or Paleogene. If this interpretation is correct, then the Cook maximum thickness of ~4–5 km (Stanley et al., 2014). The nonmarine strata of Inlet and Susitna basins did not form adjacent to each other and may have the Susitna basin rest unconformably on Triassic and older igneous, metamor- formed in different tectonic settings. phic, and sedimentary rocks of the Wrangellia terrane (Nokleberg et al., 1994;

and aeromagnetic data (Shah et al., 2014; Stanley et al., 2014; Lewis et al., 2015) 150°W indicate that a deeply buried Paleogene sequence, known primarily from two deep exploratory wells (the Pure Kahiltna Unit 1 and Trail Ridge Unit 1; see

ange N Fig. 2), is unconformably overlain by a Neogene and Quaternary sequence that BPF ka R is present in several exploratory wells and scattered surface outcrops (Gillis l Alas Talkeetna Mtns et al., 2013). The Paleogene basin fill includes a lower interval, more than PCF Centra r 800 m thick, of interstratified nonmarine sedimentary and volcanic rocks, including basaltic and andesitic rocks that have yielded whole-rock 40Ar/39Ar ages SUSITNA BASIN of ca. 57.3 to ca. 54.3 Ma (Stanley et al., 2014). This volcanic-bearing interval is YH conformably overlain by a nonmarine sequence ~1300 m thick that includes Yentna depocente 62°N PK sandstone, conglomerate, siltstone, mudstone, and coal with Paleocene to Middle Eocene palynomorphs. The Paleogene volcanic and sedimentary se- SS quence was interpreted by Stanley et al. (2013, 2014) to record volcanism, W TR estern Alas subsidence, and sedimentation that accompanied eastward passage of a slab

B M Susitna depocenter CMF window related to subduction of a spreading ridge, consistent with tectonic BM F ka BC models proposed by Ridgway et al. (2012) and Benowitz et al. (2012a). R ang WLR The Paleogene sequence in the Susitna depocenter is unconformably e TZ MS overlain by a package of Neogene and Quaternary nonmarine conglomerate, LMS Beluga sandstone, siltstone, mudstone, and coal with Early Miocene to Quaternary depocenter palynomorphs (Stanley et al., 2013). The Miocene-on-Paleogene unconformity is not precisely dated but may record uplift and erosion that accompanied the early stages of Yakutat microplate subduction beneath south-central Alaska; 61°N speculatively, this uplift and erosion may have coincided with an episode of 100 km exhumation ca. 23 Ma inferred from thermochronologic data by Benowitz et al. (2012a, p. 13). The thickest Neogene deposits in the Susitna depocenter, ~2500 m thick in the Trail Ridge Unit 1 well, occur in a broad synform that is Figure 2. Susitna basin location map. Background image is geology from Wilson et al. (2012). Gray lines are faults from Haeussler (2008). The Susitna, Yentna, and Beluga depocenters are bounded on its western and eastern margins by north-striking reverse faults indicated. The black dashed lines are additional faults noted by Haeussler (2008) including the (Stanley et al., 2014; Lewis et al., 2015). We hypothesize that Neogene sub Broad Pass fault (BPF) and the Skwentna structure (SS), as well as the Talachulitna zone (TZ) sidence and deposition in the synform, as well as movement along the reverse of Flores and Doser (2005). Beluga Mountain fault (BMF) is located along the Beluga to Susitna mountain front. Dots show the approximate location of the Pure Kahiltna Unit 1 (PK) and Trail faults, resulted from contractional deformation associated with Yakutat micro- Ridge Unit 1 (TR) hydrocarbon exploration wells. Other labeled features include Beluga Moun- plate subduction. It is possible that the north-striking Neogene reverse faults tain (BM), Mount Susitna (MS), Little Mount Susitna (LMS), Bear Creek (BC), Wolf Lake region seen on seismic-reflection profiles (Lewis et al., 2015) are reactivated Paleo- (WLR), Yenlo Hills (YH), Pass Creek fault (PCF), and the Castle Mountain fault (CMF). gene normal faults, but this hypothesis remains to be evaluated. Seismic-reflection and aeromagnetic data show that Paleogene strata in Schmidt and Rogers, 2007) and Cretaceous to Paleogene plutonic intrusions the eastern part of the Susitna depocenter are folded and cut by reverse and that represent the roots of a subduction-related magmatic arc. We believe that thrust faults (Shah et al., 2014; Lewis et al., 2015); the timing of the folding these differences in age, thickness, and tectonic setting (forearc basin versus is unclear, but at least some of the northeast-striking faults appear to have magmatic arc) indicate that the Cook Inlet and Susitna basins are distinct from surface expression and therefore may be geologically young. Geophysical one another, and that the Susitna basin is not a northward continuation of evidence, discussed in detail in this report, indicates that the southwestern the Cook Inlet basin. Furthermore, we note that comparative stratigraphic margin of the Susitna depocenter is the Beluga Mountain fault, a northwest- studies (Trop et al., 2003; LePain et al., 2013) indicate that the likely northern striking, southwest-dipping thrust fault. continuation of the Cook Inlet forearc basin is located in the Matanuska Valley In the Yentna depocenter (Peters Hills basin of Haeussler, 2008), surface and southeastern Talkeetna Mountains on the opposite (northern) side of the geologic investigations show that Neogene nonmarine strata consist of con- Castle Mountain fault. glomerate, sandstone, siltstone, mudstone, and coal, which have yielded The Susitna basin can be divided into two depocenters, the Susitna depo palynomorphs of Middle to Late Miocene and Pliocene age (Wolfe et al., 1966; center and the Yentna depocenter (Figs. 1–3). In the Susitna depocenter, recent Haeussler, 2008; Gillis et al., 2013; LePain et al., 2015). The thickness of the investigations using data from exploratory wells, seismic-reflection, gravity, Neogene strata is ~3 km (Stanley et al., 2014; based on gravity modeling by

151°30′W 151°00′W 150°30′W

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1 1 depocenter0

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basement –130 mGal – 7 0

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– region 61°45′ Figure 3. Southwest margin of Susitna Beluga Mtn basin—gravity and topography. Shaded

0 topography (300-m digital elevation 7

– model) with complete Bouguer anomaly

contour lines (2 mGal interval). The heavy

2 blue line marks the geomorphic mountain

1 – Wolf Lake Profile front (also shown on Fig. 4). The dashed

0 black line outlines a triangular region of

– elevated topography. The heavy red line

9 – shows the location of the Wolf Lake profile – 8 (models shown in Fig. 5). 0 0 0 – 9 0 7 7 – – –70 mGal Mt Susitna 61°30 ′N

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R.W. Saltus). The Neogene strata rest in angular unconformity on strongly the Broad Pass fault is strongly linked to right-lateral displacement along the deformed and locally metamorphosed marine strata of the Kahiltna flysch se- Denali fault system (Haeussler, 2008). quence of Cretaceous and Jurassic age (Wilson et al., 2012; Hults et al., 2013), Possible sources of sediment for Tertiary strata in the Susitna basin include which is interpreted to record collision of the Wrangellia terrane with North the Talkeetna Mountains and the Alaska Range. Thermochronologic studies America (Trop and Ridgway, 2007). No seismic-reflection profiling or explora document exhumation episodes in the western Alaska Range ca. 56–35 Ma, tory drilling has been done in the Yentna depocenter. The Neogene basin in 23 Ma, and 6 Ma (Benowitz et al., 2012a; Gillis et al., 2014), prolonged exhu- this area is interpreted as a piggyback or wedgetop basin (Ingersoll, 2012, mation of the central Alaska Range near the Denali fault starting ca. 28 Ma p. 22) that developed above a hypothetical, southeast-directed thrust fault, the (Benowitz et al., 2012b), and the eastern Alaska Range since ca. 24 Ma (Beno- Broad Pass fault of Haeussler (2008). In this interpretation, movement along witz et al., 2011, 2014).

The Susitna lowland was heavily glaciated during Pleistocene time (e.g., age between Mount Susitna and Little Mount Susitna. Wilson et al. (2012) do Karlstrom, 1964; Schmoll and Yehle, 1986) and is largely mantled by glacial not draw a fault along the Beluga-Susitna mountain front. Gillis et al. (2013) and fluvial deposits in the shallow subsurface. Wahrhaftig (1951) notes that the report that they did not find any surface evidence of a fault contact during a northern part of the lowland has a “great system of parallel north-south ridges, reconnaissance traverse through Quaternary cover and into volcaniclastic and 10 to 50 feet high, and looks as if it had been furrowed by a great plow.” He metavolcanic rocks within a deeply incised drainage along the Beluga Moun- also notes a broad medial moraine that extends southwest from the Talkeetna tain topographic front. However, a normal fault was drawn along or parallel to Mountains and forms a hilly country with numerous lakes. The significant gla- this mountain front by Kirschner (1994) and Trop and Ridgway (2007). cial morphology in the lowland region obscures direct observation of many structural features including the fault trace of the Beluga Mountain thrust fault. Geophysics The northeastern slope of Beluga Mountain follows a generally linear trend striking about N50W, roughly paralleling the trend of the Yentna River, which A low gravity anomaly has long been identified in the Susitna basin (Hackett, lies ~15–20 km to the northeast. The Beluga-Susitna mountain front is the north- 1977a, 1977b, 1978). Barnes et al. (1994) showed a roughly oval gravity low eastern edge of a triangle-shaped region of elevated topography (Figs. 1–3). The of ~50 mGal. Hackett (1977b, 1978), Meyer and Boggess (2003), and Meyer southwestern edge of the elevated triangle trends about N20°W. The shorter (2005) collected and published gravity data for the Susitna basin. For this southeastern edge of the triangle parallels the Castle Mountain fault. Beluga study, we acquired additional gravity data along several transects across the Mountain, with a maximum elevation of 3670 ft, occupies the northwestern Beluga Mountain to Susitna basin gravity gradient to support more detailed point of the elevated triangle and consists of a broad complex of northeast- and profile interpretation (Fig. 3). We compiled historical USGS and State of Alaska northwest-trending ridges. Northeast-trending Bear Creek follows the south- data and data from our new transect onto the IGSN71 datum and calculated eastern flank of the Beluga Mountain complex and drains to the northeast into complete Bouguer anomalies using the standard Bouguer reduction density of Alexander Lake. Mount Susitna occupies the southeastern corner of the ele- 2670 kg/m3 (gravity data are available in the Supplemental File [see footnote 1]). vated triangle and attains a maximum elevation of 4396 ft. The southern flank of New gravity data better constrain the southwestern portion of the gravity Little Mount Susitna (maximum elevation 3035 ft) occupies the southern corner gradient from the elevated Beluga-Susitna region to the gravity low centered of the elevated triangle. Between Beluga Mountain and Little Mount Susitna, on the Susitna depocenter (Fig. 3). This allows for more robust modeling of the intervening Wolf Lakes elevated region has more subdued topography, cul- the gradient sources. The deepest portion of the Susitna gravity low is located minating in a broad north-trending ridge with a peak elevation of 2209 ft. ~15 km northeast of Beluga Mountain. A steep gravity gradient (4.3 mGal/km) The elevated triangular region encompassing Beluga Mountain, Mount parallels the Beluga-Susitna mountain front. The highest gravity anomaly val- Susitna, and Little Mount Susitna consists of Cretaceous to Paleocene mag- ues occur in the elevated region, generally ~10 km southwest of the mountain matic rocks underlain by rocks of the late Mesozoic and Cenozoic magmatic arc front, so that the total width of the gradient zone is ~15 km. (Wilson et al., 2012). Beluga Mountain, the central Wolf Lakes region, and the Two USGS public-domain, moderately low level (flown with a nominal valley between Mount Susitna and Little Mount Susitna consist of metamor- draped flight height of 1000 ft above ground) aeromagnetic surveys encom- phosed intermediate volcanic and sedimentary rocks of Cretaceous age (map pass the Beluga-Susitna mountain front and the transition from the elevated unit “Kivs” of Wilson et al., 2012). Mount Susitna is mapped as Late Cretaceous region into the Susitna basin (http://mrdata .usgs .gov /magnetic /show -survey granodiorite, tonalite, and quartz monzodiorite. Little Mount Susitna is made .php?id = 4247). These surveys were collected and released as part of the up of granitic plutons of Paleocene age. A small region at the top of Beluga Anchorage Urban Region Aeromagnetics project of the USGS, which focused Mountain is mapped as intermediate to mafic volcanic rocks of Paleocene age. primarily on interpretation within the Cook Inlet basin (Saltus et al., 2001). Haeussler (2008) cites three seismically active structures at the margins of Complex high-amplitude magnetic anomalies are observed within the the Susitna basin: (1) an actively uplifting structure, with an inferred N-S trend study area (Fig. 4), reflecting highly magnetic igneous rocks within and be- and east-directed reverse motion, crossing a broad bend in the Skwentna neath the elevated Beluga-Susitna region and within the deep basement of the River (Willis and Bruhn, 2006; “SS” in Fig. 2); (2) the Pass Creek fault (PCF in Susitna basin. In particular, the basin-facing edge of the Wolf Lakes (Fig. 2) cen- Fig. 2), which is roughly 17 km long, strikes northeasterly, and is identified as a tral section of the Beluga-Susitna highland, as well as Little Mount Susitna and north-side-down normal fault; and (3) N-S–striking normal faults to the west of the western part of Mount Susitna, correlate with very high magnetic anomaly the Skwentna structure. Flores and Doser (2005) describe a north-south zone values. The Beluga Mountain region correlates with moderately high magnetic (the Talachulitna zone) of active seismicity striking north-northwest at the east- anomaly values. The central section of the Beluga-Susitna triangle, southwest ern edge of the Beluga basin and dipping to the northeast beneath the Beluga of the basin-facing high, is a region of low magnetic anomaly values with triangular highland (TZ in Fig. 2). Flores and Doser (2005) cite one reliable abrupt linear boundaries in an orthogonal pattern parallel and perpendicular to focal mechanism within this zone to indicate reverse-oblique motion. A fault is the trend of the Beluga-Susitna mountain front. The moderate and high mag- mapped (Wilson et al., 2012) in the north-south Wolverine–Lewis Creek drain- netic anomalies along the Beluga-Susitna mountain front extend basinward

151°30′W 151°00′W 150°30′W

500 62°00′ N

Wolf Lake pro le

61°45′ Figure 4. Southwest margin of Susitna basin—magnetic anomalies. Shaded color N relief display of magnetic anomaly data. Magnetic front 0 The topographic front is shown as a heavy black line. (See Wolf Lake profile model in Fig. 5; location shown by heavy red line.) The magnetic front follows (magenta line) Beluga the buried nose of the Beluga Mountain fault. The triangular region of elevated Mtn basement topography is also shown for reference.

Mtn front 61°30

rdrillo MtnsTo ′N

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~5 km into the region of mapped cover deposits of the Susitna basin (magenta that contains the most interpretable reflections at the Beluga Mountain margin line on Fig. 4). (Lewis et al., 2015, their fig. 30) shows no evidence for a normal fault offsetting Proprietary industry seismic-reflection data were acquired in Susitna basin a basement reflector. Although this line is heavily disrupted in the region of from the 1960s to the 1980s (Lewis et al., 2015). The data have relatively low the magnetic edge (magenta line on Fig. 4), it shows evidence for both the signal-to-noise ratios but clearly show layered reflectors throughout the basin northeast-vergent Skwentna reverse fault and Beluga Mountain thrust fault. consistent with the significant sedimentary section known from exploratory This seismic line and several other lines that approach the basin margin show wells including the Pure Kahiltna Unit 1 and Trail Ridge Unit 1. Six seismic strong northeasterly apparent dips on Tertiary reflectors on the upthrown lines are close enough to the Beluga-Susitna mountain front to potentially (southwest) sides of the Beluga Mountain and Skwentna faults, a structural yield useful structural information about the basin margin. Of these, the one pattern that is indicative of contractional fault-propagation folding and reverse

or thrust faulting along the southwest margin of the Susitna basin, rather than inferred position for the toe of the thrust fault. Hackett (1977a, 1977b) placed normal faulting. While sparse and somewhat indistinct, this seismic-reflection the toe close to the mountain front. Also we have more complete gravity data evidence for the Beluga Mountain and Skwentna faults can be interpreted as coverage that shows the gravity gradient to be somewhat broader than shown steeper-dipping, near-surface splays that have propagated from the deeper, in Hackett’s original data (1978). regional thrust fault modeled by the potential field data. Model (B) shows that a normal fault geometry does not fit the data. The fundamental problems with this or any related model are the broad and GEOPHYSICAL INTERPRETATION smooth nature of the gradients and the fact that the gravity gradient extends well to the southwest of even the most mountain-ward possible location (i.e., Profile Interpretation—Beluga Mountain Fault the edge of the surficial basin sediments) for the surface fault trace. Any normal (or vertical) fault geometry for a two-body solution will not match the ob- Three key observations relate to the structure of the Beluga Mountain fault served gravity gradient. along the southwest margin of Susitna basin: (1) a mapped geologic and/or Model (C) fits the position of the gravity gradient but is not preferred be- physiographic boundary that juxtaposes high-density and variably magnetic cause it relies on insertion of an ad hoc structural element at depth. We con- igneous rocks on the southwest against surficial glacial and/or fluvial deposits struct this model as an illustration of uncertainties in potential field modeling. on the northeast; (2) a broad gravity gradient transition from low-gravity It is generally possible to concoct a model that incorporates additional unseen anomaly values over the Susitna depocenter to high-gravity values over the (and therefore unconstrained) geologic elements to force a match to a given igneous rocks of the Beluga highland; the gradient is roughly centered on the potential field profile. For this model, we add an ad hoc deeper body with den- physiographic boundary; (3) moderate to high magnetic anomaly values asso- sity intermediate to the two surface blocks. The ad hoc body is centered in the ciated with the exposed igneous rocks along the Beluga-Susitna physiographic middle of the gradient with flanks that dip (in a “normal” geometry) in both boundary; these moderate to high magnetic values extend ~5 km basinward directions. On average, this body allows for a smoothing of the gravity gradi- from the physiographic front (magenta line on Fig. 4). ent by smearing the overall lateral density contrast in both directions from the To explore the implications of these key observations, we model a south- center. We can think of no reasonable geologic rationale for such a body, and west to northeast cross-section profile spanning the Beluga mountain front furthermore, there is no seismic indication of a normal fault interface on the (location shown on Figs. 3 and 4). The gravity gradient shows a similar profile if basin side (as discussed above). drawn across the mountain front anywhere along the front from north of Beluga Mountain southeast to Mount Susitna; so gravity models crossing the front will Other Features in the Gravity and Magnetic Anomaly Data yield similar results along this part of the margin. The magnetic signal, on the other hand, is more complex and shows considerable lateral variation along As previously noted by Hackett (1977b, 1978), other major faults in the re- the mountain front. On the chosen profile, parallel to Bear Creek, the magnetic gion around Susitna basin have a gravity gradient signature consistent with anomaly pattern is relatively simple and appears to reflect primarily the geom- contractional fault geometries (Fig. 6). The gravity gradient associated with etry of the basin-bounding fault. The magnetic signature along this profile is a the Castle Mountain fault undercuts the southeast flanks of Mount Susitna and single magnetic high that terminates at the toe of the inferred thrust front. Little Mount Susitna. Along with the magnetic expression, in which an intense For discussion and demonstration of modeling uncertainties, we show magnetic high is truncated along a line parallel to the Castle Mountain fault but three models for this profile (Fig. 5): (A) a thrust fault geometry using observed located ~5–7 km to the northwest (coincident with the high edge of the gravity geologic constraints; (B) a normal fault geometry using observed geologic con- gradient), this supports a contractional geometry with a dip to the northwest. straints; and (C) a more complicated model constructed specifically to force a Similarly, the gravity gradient on the northern margin of the Matanuska low- normal fault geometry. Model (A) is our preferred model. It fits the data and lands undercuts the southern flank of the Talkeetna Mountains as expected requires the fewest ad hoc assumptions. The fundamental assumptions under for a reverse geometry. On the northwest flank of the greater Susitna basin, pinning model (A) are: (1) the simplification of the geology into two bodies, a the gravity gradient bounding the Yentna (Peters Hills) basin undercuts the dense (igneous and metamorphic) upper plate and a lower density “Susitna southeast flanks of the adjacent ridges of the Alaska Range, again suggesting a basin” lower plate allows for reasonable estimation of fault geometry; (2) the reverse geometry. Gravity gradients undercut the flanks of the Yenlo Hills, but shallow subsurface magnetic rocks modeled are part of the hanging wall (i.e., not as steeply as along the Beluga-Susitna front, suggesting a steeper angle to their location can be used to map the hanging wall where it is concealed by the reverse structures facing both the Susitna basin to the southeast and the the shallow basin sediments); (3) there is no hidden density contrast lurking Yentna (Peters Hills) basin to the northwest. In contrast, the gravity gradient beneath the basin-bounding structure (as in model C below). The main reason on the eastern side of the Susitna basin, adjacent to the Talkeetna Mountains that our preferred model has a shallower dip angle for the Beluga Mountain front, tapers into the basin suggesting an onlapping or normal fault geometry fault compared to the steeper dips of Hackett (1977a, 1977b) results from our (as shown in Stanley et al., 2014).

A 1200 B 1200

800 Magnec relief indicates 800 magnec property Magnec low maps nT) B Calculation variability buried thrust front s( A Calculation 400 400 Magnetic

Magnetics (nT) 0 0 C Calculation =Observed, =Calculated =Observed, =Calculated –60 –60 Broad gravity gradient extends well into region of mapped high density rocks B Calculation –80 –80 Smooth shape of C Calculation

gravity gradient consistent mGal)

–100 y( with planar density interface (fault) –100 A Calculation Gravit Gravity (mGal) –120 =Observed, =Calculated –120 =Observed, =Calculated

–1 S=0.02 B – Normal Fault Model 0

S=0.08 0 Dense, variably magnec Dense, magnetic 1 D=2670, S=–0.02 1 Low density,

2 Low density, non-magnec km) non-magnetic

h( 2

Depth (km) D=2300, S=0 Dense, non-magnetic 3

Dept 3 4 20° 4 VE =1 5 0510 15 5 0 6 12 18

–1 C – Extra Body Model 0

Terary sedimentary 0 Meta-Igneous complex Low density, 1 ault rocks ain f 1 Dense, non-magnetic Dense, magnetic non-magnetic

2 km) ga Mount schemac h(

Depth (km) Belu 2 3 folding Intermediate density, of sedimentary Dept 3 20° non-magnetic 4 layers VE =1 4 5 ??? 0510 15 5 Distance (km) 061218

Figure 5. Two-dimensional forward models (gravity and magnetic) across the Beluga Mountain front. (A) Preferred model. The gravity gradient extends well to the southwest from the mountain front, which is best modeled by a reverse and/or thrust geometry for the basin-bounding structure. (B) Alternate models. Model B is a test of the normal fault hypothesis, but it fails to match the gravity gradient. Model C shows that the introduction of an ad hoc body with intermediate density is required to improve the gravity fit and preserve an apparent normal fault geometry.

152°W 151°W 150°W149°W 62°30′ N

Figure 6. Regional topographic and gravity gradients. Gravity contour lines with key

62°00′ fault-related gravity gradients labeled. Red lines map middle of gravity gradient, and black lines show the width of the grav- N6 ity gradient. The gradients are labeled as follows: Beluga (Beluga Mountain fault), Broad Pass fault (inferred; Haeussler, 2008), Susitna (Castle Mountain fault along the southeast margin of Mount Susitna and Little Mount Susitna), and Matanuska (Castle Mountain fault along the southeast margin of the Talkeetna Mountains). 1°30 ′N

Middle of gravity gradient 25 km Width of gravity gradient

The USGS Anchorage Urban Region Aeromagnetics (AURA) magnetic sur- pose high-density basement of the Wind River uplift (2670 kg/m3) in the hang- vey data (Fig. 4; Saltus et al., 2001) reveal complex magnetic anomalies in ing wall against the low-density (2370–2600 kg/m3) sedimentary section of the Beluga-Susitna region. Shah et al. (2014) report on preliminary interpreta- the Green River basin in the footwall block. The amount of lateral overthrust tion of magnetic anomalies and possible relationships to basement lithology (heave) is ~15 km (Smithson et al., 1978), again similar to our model. In the and structure beneath the Susitna basin. A detailed analysis of the magnetic Wind River case, the total gravity gradient spans nearly 80 mGal, almost twice anomalies of the Beluga-Susitna triangular highland and surrounding region our gravity range, but the thickness of the overthrust Green River basin section is beyond the scope of this paper. is nearly 10 km compared with our modeled 4 km. The excellent COCORP seismic data have much better signal penetration and lateral coverage across the DISCUSSION thrust compared to the shallow exploration seismic data available for Susitna basin. The COCORP data were collected continuously from the basin, through The thrust-fault geometry model for the Beluga-Susitna front is similar the Wind River uplift, and into the Wind River basin on the other side of the in some ways to the COCORP deep seismic-reflection and/or gravity model range, imaging the entire crust down to the Moho; whereas the Susitna basin for the Wind River thrust fault in Wyoming (Smithson et al., 1978). Like our industry seismic data, at best, only image to pre-Tertiary basement on one side preferred model, the Smithson et al. (1978) model uses a thrust fault to juxta of the fault.

Figure 7. Regional tectonic context. Figure from Haeussler (2008) with the addition of two thrust faults: BF—Beluga fault and BPF—Broad Pass fault. Background figure is an interpretation of active tectonics of southern Alaska (Haeussler, 2008). Active structures shown as thick black lines. Fault names: WDF—western Denali fault; CDF— central Denali fault; CMF—Castle Moun- tain fault; TF—Totschunda fault. Fault or block slip rates shown in white ovals are in mm/yr. Pacific–North America velocity BPF from NUVEL-1A model. Yakutat–North America velocities from Fletcher and Frey- mueller (2003) and Leonard et al. (2007). Eastern and western Denali fault slip rate estimates are from Matmon et al. (2006) BF and Haeussler (2008) but are influenced by Lahr and Plafker (1980) and GPS data from Fletcher (2002) and Leonard et al. (2007). Black arrow indicates the interpreted northwestward migration of the southern Alaska block (the region south of the Denali fault). The location of the subducted Yakutat terrane slab to a depth of less than 50 km and a region of thick- ened Alaskan continental crust (Eberhart- Phillips et al., 2006) is shown. Gray arrows pointing toward each other show regions of significant shortening. Thick gray circle illustrates small circle along which the southern Alaska block is rotating counterclockwise. Dashed part of circle shown to illustrate where known faults do not line up with the small circle (Haeussler, 2008).

We reiterate that we are not the first to interpret the gravity gradient along CONCLUSIONS the Beluga-Susitna mountain front as a contractional fault. Hackett (1977b) states: “This gravity feature is therefore interpreted to be the expression of We concur with the earlier gravity interpretations of Hackett (1977a, 1977b, a high-angle reverse fault dipping 60-75 degrees, up-thrown on the south- 1978) that the boundary between the high-density igneous rocks of the Beluga west along the Susitna lowland-Beluga Mountain boundary.” His reverse fault Mountain–Mount Susitna region has a contractional fault geometry against interpretation (Hackett, 1977b, 1978) has not been widely appreciated in our the low-density Tertiary sedimentary rocks of the adjacent Susitna basin. We opinion. For example, both Kirschner (1994) and Trop and Ridgway (2007) cite jointly modeled gravity and magnetic anomalies across this margin with con- Hackett (1977b) but draw the Beluga-Susitna edge of Susitna basin as a normal straints from legacy industry seismic data to show an overthrust structure that fault. Flores and Doser (2005) cite both Hackett’s 1977 and 1978 reports and resembles the Wind River thrust fault in Wyoming (Smithson et al., 1978). A mention his reverse fault interpretation. thrust fault structure on the southwest margin of the Susitna basin fits well A thrust-fault geometry for the southwest side of the Susitna basin im- with current geodynamic models (e.g., Haeussler, 2008) and is consistent with plies that a significant thickness of Tertiary nonmarine strata, including coal the variation in observed slip between the Denali fault to the north and the and other possible hydrocarbon source rocks, may have been overthrust and Castle Mountain fault system to the south (Benowitz et al., 2012c; Riccio et al., therefore experienced greater pressure and temperature histories relative to 2014). A significantly buried and heated nonmarine, coal-bearing sedimentary the rest of the basin. We speculate that thrusting may have promoted thermal section has implications for a possible overthrust hydrocarbon play and fluid maturation of petroleum source rocks, including coal and organic-rich shale, migration in the Susitna basin. Although this particular thrust fault structure and migration of petroleum-bearing fluids from beneath the hanging wall to does not appear to be seismically active, the presence, history, and nature of the foreland. These aspects of a thrust interpretation have implications for thrust (and reverse-oblique) faults in this region have implications for seismic oil and gas potential, including a speculative overthrust play adjacent to the hazard evaluation. southwestern margin of Susitna basin. The Beluga Mountain fault does not appear to be seismically active today. ACKNOWLEDGMENTS Instead, Flores and Doser (2005) note a linear band of seismicity (“TZ” on We are grateful for careful journal reviews by Leland O’Driscoll and Bob Gillis and for excellent Fig. 2) that forms an apparent northeast-dipping band, with epicenter depths editorial suggestions from Jeff Benowitz. Our work has benefitted from discussions withMarwan Wartes and Bob Gillis. Thanks also to the pilots of Pollux Aviation and the proprietors of the from the surface to 16 km, that they call the Talachulitna zone. This band of Skwentna Roadhouse. We acknowledge our appreciation for David Barnes (USGS, deceased), Bob seismicity has a surface intercept in the Beluga basin on the western flank Morin (USGS, retired), and John Meyer, Jr. (Alaska Division of Oil and Gas, retired) for collecting of the Beluga-Susitna triangular highland region. Flores and Doser (2005) and publishing gravity data in Susitna basin and the surrounding region. We wish to recognize Steve W. Hackett (deceased) for the original observation of the associa- note Hackett’s (1978) interpretation of the southwest-dipping structure asso tion between gravity gradients and topographic features in and around the Susitna basin. Steve ciated with the Beluga-Susitna mountain front to the northeast but point recognized the significance of the gravity features and made the initial models to illustrate reverse out that, in general, earthquake solutions for the Talachulitna seismicity and thrust structures related to the Susitna and Beluga basins. His “Yentna-Beluga” lineament is essentially the same as our Beluga Mountain fault. have very large inversion uncertainties, making it difficult to determine first- motion directions. 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