Three dimensional model of an ultramafic feeder system to the

http://dx.doi.org/10.1029/2011GC003508

1. Introduction

[2] One of the most complete sections of alarge igneous province (LIP) worldwide is in theAmphitheater Mountains, Alaska (Figure 1) where anear‐continuous section from basal gabbroic sillsand submarine‐to‐subaerial basalts of the NikolaiGreenstone Formation is exposed. Abundant mafic‐to‐ultramafic (MUM) intrusions in the Amphithe-ater Mountains and southern Alaska Range to thenorth and east suggest that this area was a primaryvent site for Nikolai LIP volcanism. Two of theMUM intrusions, the cyclically layered and com-positionally graded Fish Lake (also referred to as theAlpha) complex and the Tangle (also referred to asthe Beta) complex, as well as their sedimentary hostunits, form the asymmetrical Amphitheater synformbeneath the thickest exposures of the Nikolai floodbasalts.

[3] This study models the structure of the Amphi-theater synform and adjacent areas in 3‐D to betterconstrain its geometry, lend insight into how itformed, and provide more accurate estimates of thevolumes of Nikolai basalts and related ultramaficsills, which are relevant to determining the area’smineral potential. Distinct density and magneticproperties of the Nikolai basalts, and ultramafic andsedimentary rocks make the synform particularlyamenable to potential field (magnetic and gravity)modeling.

[4] Goals of the modeling were (1) to distinguishbetween postextrusive (syncline) and synvolcanic(sag basin) models for formation of the Amphithe-ater Mountains structure, (2) to determine the vol-umes of Nikolai lavas and ultramafic rocks withinthe synform, (3) to help resolve spatial relationshipsbetween the Fish Lake and Tangle intrusions andnearby ultramafic bodies (Canwell, Eureka, Rainy)which have been interpreted as other segments of theNikolai LIP, and address the “magmatic connectivity”of these complexes, (4) to assess the role of synvol-canic structural controls on the emplacement of theNikolai magmatic system and the size, shape, andlocation of magma chambers (mafic‐ultramaficcomplexes); and (5) to possibly identify shallow,drill‐accessible portions of ultramafic intrusions thatcould host prospective Ni‐Cu ± PGE magmatic sul-fide deposits.

1.1. Wrangellia Terrane Overview

[5] South central Alaska consists of Paleozoic toMesozoic tectonostratigraphic terranes (Figure 1b)

accreted to North America during Jurassic toCretaceous subduction. The lithostratigraphic ter-rane underlying the Amphitheater Mountains isWrangellia, a sequence of Mississippian to MiddleTriassic metasedimentary and metavolcanic rockscharacterized by extensive flood basalts of theMiddle to Late Triassic Nikolai Greenstone, over-lain by Triassic and Jurassic shallow marine sedi-mentary rocks, and cut by Nikolai‐related gabbroic,ultramafic, and dolerite sills [Nokleberg et al., 1994].

[6] Wrangellia and other intraoceanic terranes insouth central Alaska [Berg et al., 1972; Jones et al.,1972; Berg et al., 1978; Plafker and Berg, 1994;Nokleberg et al., 1994] were accreted to the conti-nental margin as subduction‐related convergenceclosed a series of intervening marginal seas.Wrangellia was likely well south (∼30° latitude) ofits present position during extrusion of the Nikolai inMiddle to Late Triassic time [Hillhouse and Coe,1994]. Following accretion, it was splintered, andsome segments moved northward by oblique trans-lation along coast‐parallel strike‐slip systems. Theassembled southern Alaska terranes were locallystitched by Mid‐Cretaceous and Tertiary plutonsand volcanic fields and cut by Cenozoic to Recentstrike slip and thrust fault systems such as the HinesCreek and McKinley strands of the Denali FaultZone.

[7] Wrangellia in the study area, contains a differentpre‐Nikolai stratigraphy than at its type section inthe Wrangell Mountains (Figure 3a) [e.g., Werdonet al., 2001; Schmidt et al., 2003; Greene et al.,2010]. Amphitheater Mountain Nikolai basalts areunderlain by fine‐grained argillites and shales ofPermian to Triassic age, which are variably intrudedby gabbroic sills interpreted as feeders to the NikolaiGreenstone. Pennsylvanian to Permian volcaniclasticand volcanic rocks (e.g., Tetelna Formation) morecharacteristic of the type section of Wrangellia arelimited to the northeastern part of the study area,outside of the Amphitheater synform. These twostratigraphies were interpreted as the Tangle andSlana River subterranes, respectively, by Nokleberget al. [1992].

[8] Minor post‐Nikolai calcareous and argillaceousrocks of Late Triassic age (assigned to the Clear-water terrane ofNokleberg et al. [1992]) are exposedin the study area between Wrangellia and theMaclaren metasediments. Some sills and basalts ofthe Amphitheater Nikolai sequence yield 40Ar/39Arages of plagioclase (160–169 Ma) suggestive ofthermal resetting by Middle Jurassic plutons which

GeochemistryGeophysicsGeosystems G3G3 GLEN ET AL.: A 3‐D MODEL OF A FEEDER TO THE NIKOLAI LIP 10.1029/2011GC003508

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Figure 1. (a) Digital shaded relief map of the Amphitheater Mountains and surrounding region, south central Alaskashowing physiographic features, roads and trails (black lines), andmapped faults (orange lines [Nokleberg et al., 1992]).The red box outlines the 3‐D model area (area of Figures 2a and 2b). (b) Major tectonostratigraphic terranes in theAmphitheater Mountains and surrounding region, south central Alaska [after Glen et al., 2007a]; terrane‐boundingfaults are a subset of those indicated in Figure 1a. The red box outlines the 3‐D model area (area of Figures 2a and 2b).Green lines show the locations of profile lines 1, 2, and 3 that were extracted from the 3‐D model.


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intrude Wrangellia southwest of the study area[Greene et al., 2010].

[9] A series of Jurassic‐Cretaceous flysch basinsdeveloped as overlap assemblages inboard ofthe leading margin of Wrangellia during suturing[Eastham and Ridgway, 2002; Ridgway et al., 2002;Trop et al., 2002]. Remnants of one flysch sequence,the Maclaren unit in the northwest part of the studyarea (Figures 1 and 2), include metamorphosedfine‐grained clastic rocks of unknown provenance,intruded by Late Cretaceous to Tertiary plutons (theMaclaren terrane and East Susitna Batholith ofNokleberg et al. [1992]). Maclaren metasedimentsare thrust southward over Wrangellia along theBroxson Gulch and related thrust faults and aretruncated to the north by the McKinley strand of theDenali fault.

1.2. Nikolai Large Igneous Province

[10] The term “large igneous province” (LIP) refersto continental or oceanic flood basalt eruptions[Coffin and Eldholm, 1994] typically with arealextents exceeding 100,000 km2, in which large vo-lumes (>100,000 km3) of mafic magma erupted overa relatively short period of time (commonly >75%erupted in 1–5 Myr [Bryan and Ernst, 2008]). Theyare often ascribed to the ascent and impact of amantle plume on the lithosphere which, accompa-nied by rifting, leads to ensuing eruptions. Widevariations in magma type, tectonic setting, durationof magmatism, etc., of different LIP provinces,however, suggests that a variety of mechanisms,rather than a single (plume) model, may be requiredto explain the origin and evolution of different LIPs[Bryan and Ernst, 2008].

[11] Regardless of their ultimate origin, mechanismsof magma emplacement at middle to shallow crustaldepths and magma venting are likely similar inmany LIPs. The eruptive histories, the number andgeometry of vents, and the role of structure, stra-tigraphy, and tectonics in controlling magmaticpathways for LIPs, are poorly understood becausefew root systems or midcrustal levels of LIPs areexposed worldwide. Studies of vent geometries andtheir structural and tectonic settings may shed lighton LIP origins (e.g., plume versus rift or back‐arcmodels), help predict the locations of possible ventareas, and aid in assessing the relative importance ofcrustal heterogeneity, stress fields, and preexistingstructural grain on magma emplacement. This is thescope of the present study which focuses on theMiddle to Late Triassic Nikolai Greenstone–one of

the oldest (∼230 Ma) and best exposed flood basaltevents preserved in the world.

[12] The Nikolai flood basalts (sometimes referredto as the Wrangellia flood basalts) formed as anoceanic plateau basalt province [Panuska, 1990;Ernst and Buchan, 2001] near a continental margin,perhaps in a back‐arc–island arc setting [Nokleberget al., 1994]. Although controversy remains overwhether Nikolai petrochemistry suggests a mantleplume [Richards et al., 1991; Lassiter et al., 1995]or back‐arc rifting [Barker et al., 1989] origin, mostoceanic plateaus of Nikolai scale are explained bymantle plume sources [Kerr and Mahoney, 2007].Recent geochemical data from the Nikolai lavassuggest a plume‐type Pacific mantle source similarto that of basalts from the Ontong Java and Carib-bean plateaus [Greene et al., 2009].

[13] The Nikolai volcanic province is one of the20 largest LIPs worldwide (estimated at ∼1 ×106 km3). Its remnants are preserved along morethan 2,500 km of the western North Americanmargin from south central Alaska to BritishColumbia and Vancouver (Figure 3). The originalshape of the Nikolai flood basalt province is diffi-cult to determine; its present ribbon‐shaped distri-bution of fragments results from a long history ofoblique accretion and margin‐parallel strike‐slipfaulting [Jones et al., 1977].

[14] Paleozoic rocks of the Wrangellia terranewere deposited in settings ranging from continentalmargins to island arcs (Figure 4). Nikolai basaltsin south central Alaska were extruded primarily overMississippian to Triassic siliceous argillite, siltstone,chert and limestone, rather than Pennsylvanian‐Permian volcanic rocks as seen at the Nikolai typesection in the Wrangell Mountains. Nikolai basaltshave undergone metamorphism ranging from verylow grade (zeolite facies, in the AmphitheaterMountains) to greenschist facies (most exposures).Gabbro sills intrude the base of the volcanic pilein the Amphitheater Mountains and the sedimentarysection immediately below it. Basal Nikolai lavas(∼500 m of section) in the Amphitheater Mountains[Greene et al., 2008] are submarine, while themajority of the 3.5–4 km thick Nikolai section therewas erupted subaerially [Greene et al., 2008], sug-gesting relatively shallow water depths for initialemplacement and emergence through a combinationof uplift (?) and infill of the submarine basin.

[15] Nikolai flood basalts, like many other LIPs,probably erupted over a relatively short period oftime. Ladinian (late Middle Triassic) bivalves arepreserved in argillaceous sedimentary rocks imme-


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Figure 2. (a) Digital shaded relief map of the 3‐D model area (orange box) showing physiographic features, majorroads (gray lines), rivers and lakes (light blue). Dark blue lines (profiles G, H, J, K, L,M, N, R, and S) show the locationsof 2‐D profile models that were used to construct the initial 3‐Dmodel. Green lines (lines 1–3) show 2‐Dmodel profilesextracted from the 3‐D model. (b) Geologic map of the 3‐D model area (orange box in Figure 2a). Map compiled fromNokleberg et al. [1992];Greene et al. [2010]; unpublished mapping of NevadaStar, Inco, and USGS; and interpretationsof magnetic and gravity data. Green and dark blue lines are model profiles shown and labeled in Figure 2a.


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diately below the Nikolai at its type section inthe Wrangell Mountains and on Vancouver Island[Greene et al., 2010]. SparseMiddle or Late Triassicbivalves in argillite are interbedded with basal pil-lowed Nikolai in the study area [Nokleberg et al.,1992]. Late Carnian to early Norian (early LateTriassic) fossils are found in limestones immediatelyoverlying the Nikolai at the type section and in the

Talkeetna Mountains [Jones et al., 1977; Schmidtet al., 2003]. Radiometric ages (40Ar/39Ar andU–Pb) of peridotite and gabbro intrusions inter-preted as feeders to the Nikolai in Yukon and cen-tral Alaska, and 40Ar/39Ar ages of basalts from theWrangell Mountains, range from 227 to 232 Ma[Lassiter et al., 1995; Bittenbender et al., 2007;Schmidt and Rogers, 2007; Greene et al., 2010],

(a)

(b)

Figure 3. (a) Map showing distribution of Nikolai Greenstone and correlative basalts along western North Americanmargin (adapted from Jones et al. [1977]). VI, Vancouver Island; WM, Wrangell Mountains; SEAK, southeast Alaska;QC, Queen Charlotte Islands; KB, Kluane Belt; CAR, Central Alaska Range. Dashed box indicates the approximatebounds of the study area. (b) Map of south central Alaska showing extent of Nikolai Greenstone and related sillcomplexes, 3‐D model area (orange box), topographic contours (gray lines), roads (dark blue lines), rivers (light bluelines), and faults (thick black lines).


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indicating a Middle–Late Triassic age for theNikolai, and suggesting that the bulk of the provincemay have been emplaced in only a fewmillion years.

1.3. Amphitheater Mountains: Geology

[16] The Amphitheater Mountains, study area,located north of the Copper River basin, form atopographic outlier to the Alaska Range that liesimmediately to the north and northeast (Figure 1).Several MUM intrusive complexes are exposed inthe study area; all are interpreted as parts of theNikolai LIPmagmatic system. The two largestMUMcomplexes, the Fish Lake and Tangle (Figures 2and 5) occur along the northern and southeasternflanks, respectively, of the Amphitheater synform.The smaller Eureka, Rainy and Canwell MUMcomplexes are exposed north and northeast of thesynform, along the southern flank of the AlaskaRange itself; they have been cut and structurallystacked by south directed thrust faulting along theBroxson Gulch and related fault systems.

[17] The Fish Lake Complex [Stout, 1976] is a thick(1–1.5 km), extensive (>30 km), layered sill presumed

to be comagmatic with the petrologically and geo-chemically similar, overlying Nikolai Greenstonelavas [Ellis, 2000]. It contains 4 ultramafic‐to‐maficcycles of cumulate dunite, peridotite, pyroxeniteand gabbro, and is the only known cyclically zoned,layered complex in Wrangellia. Exposures of theTangle Complex are less extensive, and consistprimarily of unlayered gabbroic and minor ultra-mafic sills of complex structural orientation. TheFish Lake and Tangle Complexes structurally andstratigraphically underlie pillowed and subareal basaltlavas of the Nikolai Greenstone in a west‐northwesttrending, gently west plunging synformal structure[Stout, 1976] that may have been a major feeder tothe Nikolai magmatic system.

[18] Because the Amphitheater Mountains are wellexposed and the basalts are relatively unmetamor-phosed and undeformed compared to other expo-sures of the Nikolai LIP, this area provides a uniqueopportunity to study the plumbing of a large mag-matic system. In addition, the area is of particularinterest for mineral exploration because of its Ni,Cu, and platinum group element PGE) prospects,the largely unexplored ultramafic complexes, and its

Figure 4. Generalized stratigraphic column and age control of Nikolai‐related rocks within the Central Alaska Rangebelt. Time scale shown is that of Gradstein and Ogg [2004]. Fossil and isotopic age data are summarized in Supple-mental Data File 5 of Greene et al. [2010]. Figure is modified after Schmidt and Rogers [2007, Figure 3].


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proximity to both the Denali and Richardson high-ways (Figures 1 and 2).

[19] One goal of this study is to differentiate betweentwo hypotheses for development of the Amphithe-ater synform. A purely structural syncline, withstratigraphic and intrusive units folded long afterdeposition, would have different characteristics thana structure that formed synvolcanically (similar to asag basin) due to subsidence as it filled with lavasand as the underlying magma chamber evacuated.Table 1 identifies characteristics that distinguishthese two scenarios.

[20] 2‐D and 3‐D geophysical modeling should alsoresolve details (size, shape, and orientation) of the

intrusions and their bounding structures. Thesemodels may therefore shed light on the geometryand possible connectivity between various Nikolaimagmatic chambers (e.g., Rainy, Eureka, Fish LakeMUM complexes) at the time of basalt extrusion byclarifying the relative offset between stratigraph-ically different segments of the Nikolai LIP. Becausemany oceanic plateaus remain submerged and rela-tively inaccessible, little is known about their vol-canic stratigraphy, their vent areas, or the intrusionsthat supplied them with magma. The AmphitheaterMountains, which expose both the Nikolai basaltsand the MUM complexes of this large accretedoceanic plateau, offer a rare opportunity to investi-gate the plumbing system of a major oceanic LIP,

Figure 5. Schematic cross section showing a possible model for the structural relationships between mafic‐ultramaficcomplexes within the Central Alaska Range belt. Aside from the Fish Lake and Tangle sills which coalesce at depth, it isunclear whether any of the other intrusive complexes were adjacent or connected during emplacement [after Schmidt andRogers, 2007, Figure 11].


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and may provide information useful to the study ofother LIPS such as the Proterozoic Midcontinent riftin the north central United States.

1.4. Mineral Resources

[21] Mafic LIPs worldwide host resources of plati-num group elements (PGE), Ni, and Cu, the mosteconomically significant of which are magmaticsulfide deposits in MUM intrusive complexes thatsupplied magma to the extrusive portions of the LIP.

[22] Mineral deposit types associated with theNikolai LIP include (1) magmatic sulfide Ni‐Cu,PGEs that are hosted inMUM intrusives, (2) basalticcopper deposits found in the Nikolai volcanics, and(3) Cu‐Ag mineralization that occurs in the overly-ing sedimentary rocks. Disseminated to net‐texturedNi‐Cu sulfide mineralization with elevated values ofPGEs has been identified in several Nikolai MUMintrusions in the study area. Massive sulfide depositshave not yet been identified, but are the primarytarget of current exploration activity. Schmidt andRogers [2007] provide a detailed discussion of themetallogeny and mineral potential of the Wrangelliaterrane in southern Alaska.

[23] The Nikolai LIP is similar geologically to theworld class Noril’sk District in Russia (a layeredintrusive feeder to the Permian Siberian Traps floodbasalts) that has supplied abundant Ni and Cufrom magmatic sulfide deposits. A genetic modelfor Noril’sk mineralization [Naldrett et al., 1995,1996], which ties Ni‐Cu, PGE mineralization tothe magma chemistry, magma flow dynamics, andmagma plumbing of the layered MUM intrusions,has been applied to exploration in and near theAmphitheater structure [Ellis, 2000].

[24] Magmatic sulfide Ni‐Cu‐PGE deposits form byimmiscibility between sulfide and silicate melts,

with metals partitioning preferentially into the sul-fide phase [Naldrett and Lightfoot, 1999]. The sul-fide droplets typically collect in low‐velocity areaswithin magma chambers and the plumbing sys-tems that connect them. The volume and grade ofmineralization is directly tied to the geometry anddynamics of the magmatic system, and the volumeand flow rate of magma that passes through each partof the plumbing system. Besides velocity gradientsand flow volume, the formation of magmatic sulfidemineralization also depends on sulfur availabilityfrom magma and wall rock, and the geochemicalconditions (O, S fugacity) which favor immiscibility[Naldrett and Lightfoot, 1999].

[25] Because the geometry of the magmatic plumb-ing system is a critical factor controlling the distri-bution of sulfide minerals and PGE mineralization,defining the subsurface structure of the Nikolai ventsystem may help identify areas favorable for theaccumulation of sulfide deposits.

2. Potential Field Methods

[26] Geophysical methods allow imaging of sub-surface geologic bodies and structures and are par-ticularly useful for regions that are poorly mappedand difficult to access. Variations in gravity andmagnetic fields occur due to lateral contrasts in rockdensity and magnetic properties (magnetic suscep-tibility and remanent magnetization). Rock propertycontrasts may occur within a rock unit (e.g., lateralfacies changes), across geologic structures (faultsor folds), or at contacts with other rock units. Thegeometry and depth to sources, the character of thegeomagnetic field, and the rock properties of sourcesall determine the character of a source’s potentialfield anomaly. Despite the complexity and non-uniqueness of potential fields, gravity and magneticdata can be effectively used to resolve the geometry

Table 1. Features Expected in Syncline Versus Structural Basin Settings for the Amphitheater Mountains Synform

Feature SynclineSynvolcanic Basin Related to Magmatic

Emplacement

Flow thickness Uniform across synform axis Thicker toward center of synformFlow orientation Parallel from top to base of sequence Steeper flows at base; shallower dip toward topTotal thickness of basalt Uniform in all areas Increases toward center of synformMUM sill distribution Random within sediments across region Increase in number toward center of synformMUM sill geometry Variable across region Increase in volume, thickness and continuity

toward center of synformFish Lake/Tangle relations Continuous sheet at same stratigraphic

level; or no relationshipConnected but intruded at different stratigraphichorizons, or both thicken toward centerof synform

Sub‐Nikolai base Sedimentary rocks ± sills below syncline MUM magmatic root or keel below centerof basin

Timing of formation Late Cretaceous‐Tertiary Middle Triassic


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and origin of sources, particularly when combinedwith other geologic constraints.

[27] Evaluating a region’s potential for magmaticmineral deposits depends on understanding itsintrusive history and how the deposit models ofinterest relate to the distribution of lithologic unitsand structures. Potential field methods are useful inexploration for magmatic mineral resources becausethe ore minerals of interest (e.g., chalcopyrite, pyr-rhotite) are dense and/or magnetic and because theysuccessfully map dense and often magnetic maficand ultramafic igneous rocks that host the deposits.

[28] In the Amphitheater Mountains area, the phys-ical properties of mafic volcanic rocks and ultra-mafic sills of the Nikolai magmatic system contraststrongly with the surrounding metasedimentaryrocks to produce prominent gravity and magneticanomalies [Glen et al., 2007a]. At some Ni‐Cuprospects in the Rainy complex, net‐textured anddisseminated sulfides occur with magnetite‐bearing

dunite which produces coincident gravity and mag-netic anomalies.

[29] Because detailed potential field data are avail-able for the study area, the Amphitheater Mountainsprovide a unique opportunity to integrate geologicand geophysical methods to understand the structureand character of a major vent to the Nikolai LIP andits influence on metallogeny.

[30] Magnetic data were compiled from threeregional aeromagnetic surveys (AK08, Delta River,and survey 193; Figure 7). Gravity and magneticprofiles for modeling were derived from 250m gridsgenerated from these data.

2.1. Gravity

[31] Over 175 new gravity stations were collectedalong several profiles (Figure 6) through the Amphi-theater Mountains study area and, combined withexisting regional data [Morin and Glen, 2002, 2003],provide roughly 2 km station spacing along the

Figure 6. Regional isostatic gravity map centered on the Amphitheater Mountains. Triangles indicate the location ofgravity stations. The 3‐D model area is indicated by a red box. Profiles from the 2‐D models are shown by blue lines.Green lines (lines 1–3) represent 2‐D profiles extracted from the 3‐D model.


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profiles. An isostatic gravity map of the study area(Figure 6), derived from the data described above,reflects anomalies produced by contrasts in crustaldensity, with long‐wavelength anomalies with smoothgradients originating from sources at depths greaterthan those of short‐wavelength anomalies with steepgradients. To obtain data reflecting lateral variationsin crustal density, raw gravity measurements werereduced using standard methods [Dobrin and Savit,1988; Blakely, 1995] to remove the effects of eleva-tion, topography, and the total mass, rotation, andellipsoidal shape of the Earth, yielding the completeBouguer gravity anomaly (CBA). Although the CBAreveals lateral density variations at short‐wavelengthscales, it does an inferior job isolating longer wave-length features, because these are often masked bybroad anomalies due to crustal roots that isostaticallycompensate topographic loads. The isostatic correc-tion attempts to correct for the effects of these com-pensating masses. Although there is some indicationthat the southern Alaska margin is out of isostaticbalance due to effects of the subducting pacific plate[Barnes, 1976; Saltus et al., 2007], use of the isostaticanomaly in our modeling is considered to have little ifany effect on our modeling results and conclusions,given that the extent of the study area is small com-pared to the wavelength of the isostatic imbalance.

2.2. Magnetics

[32] The magnetic map (Figures 7 and 8) for themodel area was compiled from three surveys: AK08

flown at 1000′ above ground level (AGL) alongnorth‐south lines 3/4 mile apart [Connard et al.,1999]; Delta River, flown 200 feet AGL alongnorthwest and northeast oriented lines 1/8 and1/4 mile apart [Burns, 2003], and survey 193, flownat 4000′ barometric elevation along north‐southlines 1 and 2 miles apart. Survey 193, which coversonly the southernmost study area, was derived froma grid of digitized contours of the original surveymaps [Andreasen et al., 1958; Saltus and Simmons,1997].

[33] The magnetic map reveals variations in themagnetic field that arise from contrasting magneticproperties of rocks, such as variations in remanentmagnetization or the amount and type of magneticminerals. The shallower the depth of a body, thehigher the amplitude, the shorter the wavelength, andthe sharper the gradients of its magnetic anomaly.

[34] Although crustal fields depend on both inducedand remanent crustal magnetization, remanence isoften ignored because in many cases its magnitudeis negligible or because its direction lies close to theinduced field direction. Remanence however, mayhave a significant effect, particularly in the case ofstrongly magnetic units such as mafic and ultramaficrocks like the Nikolai Greenstone and associatedintrusions.

[35] Uncertainties in the magnetic response alsoarise from deviations of the instrument‐bearing air-craft from the designated draped or fixed elevation.This uncertainty particularly effects the interpreta-tion of older surveys for which aircraft elevation dataare not available. The uncertainty is most significantin steep or highly variable terrain where the devia-tions in elevation are likely to be the greatest.

2.3. Rock Properties

[36] Because gravity and magnetic field anomaliesreflect variations in the density and magnetic sus-ceptibility of the underlying bedrock, these rockproperties are essential components to potential fieldmodeling. To aid modeling of the potential fielddata, magnetic susceptibility (>700 measurements)and densitymeasurements (>300 samples) weremadeon rock samples from the area, and combined withrock data from previous surveys in south centralAlaska. For details on gravity, magnetic, and rockproperty data, refer to Glen et al. [2007a, 2007b],Morin and Glen [2002, 2003], and Sanger and Glen[2003]. In general, average grain density of rocksin the region increases from sedimentary, and felsic‐to intermediate‐igneous rocks, to mafic igneous and

Figure 7. Index map of the boundaries of aeromagneticsurveys covering the AmphitheaterMountains study area.Red outline indicates the 3‐D model area. Individual sur-veys are denoted by colored polygons and labels. Surveyboundaries outside the margin (black outline) are schema-tic and indicate only the general direction in which thesurveys extend.


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metamorphic rocks, consistent with general trends[Olhoeft and Johnson, 1989]. Magnetic suscept-ibilities (measured from outcrop and hand sample)are low for sedimentary and felsic intrusive rocks;moderate for metamorphic and intermediate igneousrocks; and highest for mafic and ultramafic igneousrocks that generally contain more abundant stronglymagnetic minerals (note that the magnetization of arock depends primarily on its content of magneticminerals such as magnetite [Carmichael, 1982]).Magnetic remanence can also be an important fac-tor in controlling magnetic anomalies, particularlyin strongly magnetic volcanic rocks. For manypotential field studies, magnetic remanence data isnot available. Fortunately though, remanence is ofteneither negligible, due to a much stronger inducedcomponent that is controlled by the magnetic sus-ceptibility, or dominated by a present field overprintmagnetization that can be considered equivalentto an induced component of magnetization. In ourstudy we make use of previously published rema-

nence data for the Nikolai Greenstone. Rock prop-erties assigned to the model presented here (Table 2)are based on a combination of samples and outcropmeasurements taken from the study area, and dataderived for similar lithologies obtained from annational database (D. Ponce, USGS, unpublished data,2010) consisting of over 17,000 measurements.

3. Geophysical Maps

[37] Gravity and magnetic highs in south centralAlaska (Figures 6 and 8) reflect mafic and ultramaficrocks associated with the Nikolai Greenstone [Glenet al., 2007a]. Gravity high G1 (Figure 9a), centeredon the axis of the Amphitheater synform, reflectsdense Nikolai Greenstone and associated ultramaficintrusive rocks in the shallow to middle crust. A lesspronounced high occurs over the Canwell ultramaficcomplex (feature G6, Figure 9a).

Figure 8. Regional residual magnetic anomaly map centered on the Amphitheater Mountains. The 3‐D model area isindicated by a red box. Profiles from the 2‐D models are shown by blue lines. Green lines (lines 1–3) represent 2‐Dprofiles extracted from the 3‐D model.


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[38] Similarly, magnetic highs M1, M2, M6 andM7 (Figure 9b) result from the Fish Lake, Tangle,Rainy, and Canwell MUM complexes, respectively.

[39] Gravity and magnetic contrasts from north(G7, M10) to south (e.g., G3 and G4, M5) acrossthe margins of the Amphitheater structure, reflectweakly magnetic, low‐density metasedimentary rocksin contact with dense, magnetic rocks of the Nikolaisystem. Gravity high G2 and the correspondingmoderate magnetic anomaly (M11) are located overNikolai basalts whose basement is poorly known.

[40] Magnetic high M6 (Figure 9b), which corre-sponds with anomaly 17 of Campbell and Nokleberg[1997], is inferred to result from the Rainy intrusivecomplex. Gravity high G5 is the northern edge of avery large high over the Alphabet hills (Keg high ofGlen et al. [2007a]) and corresponds in part with anelongate magnetic low south of the study area (southof feature M4). Prominent gravity lows (G3 andG4, Figure 9a) coincident with a regional magneticlow (M5) reflect weakly magnetic, low‐density sedi-mentary rocks, probably Tertiary basin fill.

[41] A narrow E‐W trending magnetic high (M4,Figure 9b), corresponds in part with the “Maclaren‐Gulkana anomalies” of Andreasen et al. [1964], andwith the “Media high” of Campbell and Nokleberg[1986], along the edge of gravity high G5. The M4anomaly, although partially covered by Quaternarysediments reflects the presence of Jurassic(?) ultra-mafic rocks.

[42] A moderate magnetic anomaly (M8) at thesoutheast edge of the study area corresponds withpart of “Excelsior Creek anomalies” of Andreasenet al. [1964] and is associated with a moderate

gravity high. The source although largely concealed,may reflect mafic volcanic rocks which outcropnearby.

4. Two‐Dimensional Models

[43] Two‐dimensional potential field models wereconstructed along eight geologically selected pro-files (locations shown in Figure 2a) through thestudy area. These eight 2‐Dmodels were used as theinitial input to build the 3‐D model presented here(see Figure 10 for an example of one of these 2‐Dprofile models). Model magnetic fields were calcu-lated on a datum that drapes topography at a nominalheight of 1000 feet (305 m), which reflects the ele-vation of the merged data compilation used [Saltusand Simmons, 1997].

[44] The 2‐D profiles (1) include the highest den-sity of gravity data, (2) coincide with geologic crosssections, and (3) are roughly perpendicular to thestrike of geologic units (Figures 2 and 6). Theywere constructed using GMSYS, a forward model-ing program that allows for nonorthogonal modelstrikes. Sources were approximated by blocks thatvaried in the ± Y directions (commonly referred toas 2 3/4‐D modeling). Forward modeling of thistype [Talwani et al., 1959; Blakely and Connard,1989] can critically constrain viable structural mod-els when combined with geologic (bedrock, drilling)and other geophysical data (magnetotelluric, seismic,etc.). Nonetheless, potential field forward models arecritically dependent on the modeling assumptionsinherent in the simplification of complex geology bysimple geometric block models.

Table 2. Rock Properties of Density, Magnetic Susceptibility, and Magnetic Remanence Assigned to Layers of the 3‐D Modela

Layer Unit

Density (g/cm3)Magnetic Susceptibility

(cgs 10−6) Magnetic Remanence

Min Max Density Min Max SuscMag

(emu/cm3 × 10−6)Inc(deg)

Dec(deg)

1 Maclaren 2.74 2.79 variable 202 Canwell 2.78 2.84 variable 68003 Rainy 2.77 10004 North mix 2.77 2.80 variable 10005 Nikolai 2.82 3000 1000 −26.7 98.56 Tangle and sills 2.68 2.83 variable 0 9360 variable7 UMs 2.83 94008 Gulkana MUMs 2.82 10009 Gulkana 2.74 15010 South mix 2.77 2.78 variable 350 2520 variable11 basal layer 2.8 9000aMax, maximum; min, minimum; Susc, magnetic susceptibility; Mag, magnetization; Inc, inclination; Dec, declination.


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Figure 9. (a) Isostatic gravity map of the Amphitheater 3‐D model area. (b) Residual magnetic anomaly map of theAmphitheater 3‐D model area. Labeled anomalies are discussed in the text.


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[45] Our 2‐D model bodies consisted of horizontaltabular prisms or blocks with long axes parallel tothe regional strike of bedding. Their surface extentswere constrained to be consistent in size, shapeand orientation with mapped geologic units. Thesubsurface geometries of the model bodies weredetermined through a forward method to matchcalculated anomalies with observed anomalies withinthe limits imposed by surface geology, rock propertydata, and maximum horizontal gradients (MHG).The MHG, best expressed over bodies with nearvertical boundaries [Grauch and Cordell, 1987;Cordell andMcCafferty, 1989], reflect abrupt lateralchanges in density or magnetization, and are use-ful for estimating the horizontal extent of buriedsources. Glen et al. [2007a] discuss the details ofthese methods and provide maps and interpretationsof regional‐to‐local scale geophysical domains thatspan the study area.

[46] Density and magnetic properties of 2‐D bodieswere adjusted iteratively to match observed grav-ity and magnetic profiles while staying within thelimits of values (1) derived from the correspondinggeologic units [Sanger and Glen, 2003] or (2) fromvalues derived from similar lithology (based on anunpublished database of western U.S. rock prop-erties (D. Ponce, USGS, 2010)). Magnetizationswere assumed to parallel the present field direction,(56,500 nT, 75.5° inclination, and 27° declination).An exception was made for Nikolai basalts to whicha remanent magnetization [Hillhouse and Gromme,

1984] was assigned, in addition to the inducedcomponent.

[47] Although potential field models are relativelyeffective at constraining the depth to the top ofan anomaly’s source, or the location and dip of itsedges, they are relatively insensitive to the depthof a source’s base and therefore characterize theshallow and deeper crust with different degrees ofdetail. Because of the inherently 3‐D structure of theplunging Amphitheater synform, 3‐Dmodeling wasrequired to adequately characterize the geometry ofthe structure and surrounding features.

5. Three‐Dimensional Model

[48] To construct the 3‐D potential field model, wefirst exported surfaces defined as the top or bottomsurfaces of layers from our 2‐D profile models (e.g.,profile H, Figure 10). This required simplifying thegeology to include a total of eight layers. As a result,some features in the 2‐Dmodels are not representedin the 3‐D model, including layers that boundthe Eureka Complex, or the cyclic layering withinthe Fish Lake sill. In addition, Quaternary units(including alluvial (Qa), fluvial‐lacustrine (Qfl)and undivided Qu), Tertiary volcanic rocks, Tertiarysediments, and various Cretaceous to Tertiary gra-nites across the map were not distinguished in oursimplified layers.

[49] The exported 2‐D surfaces, together with out-crop constraints (Figure 11), were gridded, within a

Figure 10. Two‐dimensional potential fieldmodel along profile H (see Figure 2a for profile location). (top andmiddle)Observed (black circles) and model (red line) anomalies for magnetic and gravity fields, respectively. (bottom) Thepotential field model with individual model bodies colored by rock unit.


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model area of 50 × 70 km, to produce a set of eightgrid layers from which the 3‐D model was ini-tially constructed. Additional layers (Figure 12 wereadded to account for features not crossed by the 2‐Dprofiles. Once all the layers were in place, the 3‐Dmodel was thenmodified through a series of forwardand inverse steps to minimize the error betweenobserved and calculated anomalies (Figure 13). Thefinal model consists of 11 surfaces, including topog-raphy and a basal surface at 3 km depth (Figure 12)representing regional crustal basement.

[50] A list of the model layers and the geologic unitsthey represent is given in Table 2. In the model, eachsurface (layer) represents the top of a particular unit.These include (from top to bottom in model order):

Maclaren (equivalent to the topographic surface),Canwell, Rainy, North mix, Nikolai, Tangle and sills,UMs, GulkanaMUMs, Gulkana, South mix, and thebasal layer. Herein, we will refer interchangeablyto units and the model layers that represent theirtops.We note that this order was imposed largely formodeling convenience and does not imply the actualstratigraphic order of units.

[51] The Maclaren layer includes the granites (TKg)in the northwest portion of the study area (Figure 2b),plus metasedimentary rocks (Jrarg, Jrph, and Jrsch)and a small piece of Clearwater terrane (unlabelledunit in the westernmost part of map). Rainy andCanwell ultramafic rocks are grouped (along withEureka, Fish Lake and Tangle ultramafics) as the

Figure 11. Map of outcrop constraints used in the initial construction of the 3‐D model.

Figure 12. Schematic of model grid layers.


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same lithologic unit (Trum) on the geologic map(Figure 2b), but are represented by unique 3‐Dmodel layers. The North mix model layer includesvolcaniclastics and sediments (PlPu) older than theTangle Formation, together with Tertiary sediments(Ts), Eureka ultramafic rocks, and small outcrops ofNikolai (TrNu) that occur north of the Amphitheatersynform proper. The Nikolai, Tangle and sills, and

ultramafic layers form the core of the synform. TheGulkana MUMs layer represents geophysically dis-tinct gabbro and ultramafic rocks within the Gulkanametamorphic complex. The Gulkana layer includesthe remaining metaintrusive and metasedimentaryrocks of the Gulkana metamorphic complex (Jrmet).The South Mix layer includes Nikolai Greenstoneoutside the synform, some lower Tangle Formation,

Figure 13. Observed, calculated, and misfit maps of gravity and magnetic fields for the Amphitheater study area.Calculated maps are generated from the 3‐Dmodel. Misfit maps reflect the difference (error) between observed and cal-culated maps. Black lines (lines 1–3) show the locations of 2‐D model profiles extracted from the 3‐D model.


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Figure 14. Oblique 3‐D views of the (a) Nikolai Greenstone (viewed from below, looking toward NW) and (b) ultra-mafics (viewed from the SE, looking toward NW).


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Figure 15. Two‐dimensional profiles of the 3‐D potential field model (see Figures 2a, 6 and 8 for profile locations) forlines 1, 2, and 3.


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minor Tertiary volcanic and granitic rocks, andmetahornblende andesite that is probably low‐grademeta‐Nikolai (mha).

[52] The 3‐D model, like the 2‐D models fromwhich it was derived, is more sensitive to shallowcrustal sources. As a result, most of the complex-ity in the model occurs in the shallow‐level crustwith only the deepest, ultramafic portions of theAmphitheater synform extending to depths below3 km. For the 2‐D models, the transition betweenshallow and mid‐crustal levels was based on aregional common depth to the top of matched‐filtered layers of gravity and magnetic data (at 2.5and 3.3 km, respectively), that suggest a change incrustal character at that depth [Glen et al., 2007a,2007b].

[53] The largest misfits in gravity and magneticsremain over the northern edge of the synformover outcrops of ultramafic rocks of the Fish LakeComplex. This may result from an unaccounted forcomponent of remanent magnetization in the intru-sive units of the complex. To illustrate the geometryand fit of the 3‐D model we show 3‐D views of theNikolai and ultramafic layers (Figure 14) as well asseveral 2‐D cross sections (Figure 15) and cutawayslices (Figure 16) taken from the 3‐D model (seeFigures 2a, 6, and 8 for location of profile lines 1–3).

[54] The regional structure is complicated byCretaceous(?) to Recent thrust, strike‐slip, and tearfaults related in part to the Denali Fault Zone. Fur-thermore, the facies architecture of sedimentary andvolcaniclastic rocks into which themagmatic systemwas emplaced is very poorly understood. Despitethis, the model demonstrates that the Amphitheaterstructure forms a relatively simple, coherent syn-

form, gradually thickening and plunging westward,and extending to depths below 5 km, suggesting thatthe magmatic feeder system to the Nikolai LIP islarger and more extensive than previously known.We note that the model is least sensitive to thedeepest extents of the structure, making it difficult todistinguish whether the ultramafic rocks deepen intoa narrow, multikilometer keel similar to the Muskoxintrusion in the Northwest Territories, Canada [Irvine,1980], or diminish at depth. Nonetheless, our modelrequires a significant volume of ultramafic materialat depth, which forms a funnel‐like structure thatwe interpret as the upper portion of a chamberthat supplied magma to the Nikolai lavas and fedemplacement of the largest Nikolai‐related mag-matic intrusions. Volume calculations on the modellayers indicate the Amphitheater synform containsapproximately 2000 km3 of ultramafic rocks andover 900 km3 of Nikolai basalts.

6. Discussion

6.1. The 3‐D Model

[55] The Amphitheater Mountains, which containcyclically layered and differentiated ultramaficintrusives that fed the overlying Nikolai lavas, is theonly known vent for the Nikolai LIP. Even if othervents fed some portions of the volcanic province,the Amphitheater structure must be a major eruptivecenter as the stratigraphic thickness of Nikolaihere is at least equal to that at the type section inthe Wrangell Mountains. Despite the exceptionalexposures of the Amphitheater structure, the vastmajority of the mafic and ultramafic rocks in thecomplex reside in the subsurface. Three‐dimensionalpotential field modeling offers the opportunityto characterize the extent of subsurface units, andallows for volume calculations of potentially min-eral resource‐bearing units like the Nikolai and asso-ciated ultramafic intrusions. The model reveals thepresence of a significant, westward plunging sub-surface ultramafic material below the AmphitheaterMountains, substantially expanding the mineralpotential of the complex (see animations S1–S3 inthe auxiliary material).1 Irregularities in the gener-ally smooth surfaces of the ultramafic intrusionsmay indicate areas of magmatic/hydraulic complex-ity or quiescence conducive to segregation or accu-mulation of sulfide minerals. Estimated volumesof Nikolai basaltic and related ultramafic rocks,based on the 3‐Dmodel are on the order of 1000 and

Figure 16. Cutaway slices of the 3‐D model layers ofNikolai and ultramafics. Slices (excluding the two ends)are 5 km thick. Nikolai and ultramafic layers have beenoffset vertically from each other.

1Auxiliary materials are available in the HTML. doi:10.1029/2011GC003508.


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2000 km3, respectively. Details of the model mayhelp to distinguish between the two end‐membermodels hypothesized for the origin of the Amphi-theater structure (Table 1).

6.2. Syncline Versus Sag Basin Models

[56] One of the goals of this work was to definethe crustal structure below the Amphitheater Moun-tains, in order to determine whether the units exposedrepresent a syncline (a postdepositional fold due toregional deformation, in which the core preservesthe stratigraphically youngest rocks) as previouslymapped, or a sag basin that represents a struc-tural downwarp or extensional basin formed duringemplacement of the Nikolai basalts. The latter couldform structurally, with basin‐bounding faults alsoacting as a conduit for magma eruption, or bydeflation as a magma chamber at middle to shallowcrustal levels was emptied, and collapsed under theweight of the overlying basalts.

[57] Geologic observations favor a synvolcanicbasin (Table 1) that developed during emplacementof the Nikolai Greenstone. Basalt flows thickentoward the center of the synform and dips shallowupward [Nokleberg et al., 1992;Greene et al., 2010].In addition the lack of small‐scale folding in theTangle Formation sediments (B. Ellis, oral commu-nication, 2005) suggests the synform is not a productof postdepositional folding.

[58] Certain features of the geophysical model alsosupport the hypothesis that the Amphitheater struc-ture developed primarily as a sag basin. Density andmagnetic gradients within the Tangle sedimentssuggest that volume of gabbroic sills varies sub-stantially with a general increase toward the centerof the synform. Although they coalesce at depth, theFish Lake and Tangle sills do not occur at the samestratigraphic level below the Nikolai, making itunlikely that they were once a single planar intrusionfolded by regional deformation. A deep (>3 km) rootof dense material along the axis of the sag basinplunges westward alongwith overlying stratigraphiccontacts, but the model suggests asymmetry withmore of the root along the southern (Tangle) side.Asymmetry of the sag basin is also suggested bya central structure (synvolcanic fault?) parallel to7 Mile lake, which divides and offsets two sepa-rately dipping limbs of Nikolai basalts. This centralaxial structure overlies the dense ultramafic rootand may have been the primary magma conduit forthe mafic‐ultramafic sills below the Nikolai basalts.Nonetheless, the 3‐D model cannot preclude the

possibility of later, minor steepening of the limbs ofthe basin during post‐Triassic collision.

[59] Although a synmagmatic sag basin was primarilyresponsible for development of the Amphitheatersynform, preexisting structures and stratigraphicand facies boundaries below the eruptive Nikolaisequence may have played a significant role incontrolling the emplacement, shape, and location ofmagma chambers, and the connectivity and com-plexity of magmatic channels, some of whicheventually vented to the surface.

[60] The alignment of the axis of the Amphitheatersynform, which is roughly parallel to broad foldingin the Tangle Formation and to the strike of southdirected thrust faults, suggests that the synform mayhave been steepened by Cretaceous(?) to Tertiarydeformation superposed on the sag basin. Theapparent incongruity of this with inferences madefrom recent mapping (Table 1) and from the 3‐Dmodel, may be reconciled if the deep, dense keelof the Amphitheater structure, had either locallyinfluenced deformation of the surrounding sedi-ments during later folding and/or rotated into theregional stress field. In summary, the geologicmapping and geophysical modeling are consistentwith a sag basin model for the initial developmentof the Amphitheater synform with subsequent[Cretaceous(?) to Tertiary] deformation that modi-fied the synform through broad warping, limbsteepening, and possible rotation.

7. Conclusions

[61] In the Amphitheater Mountains, central AlaskaRange, a major eruptive center for the subcontinental‐scale Triassic Nikolai Greenstone mafic LIP formsa broad synformal structure of thick, extensivecumulate mafic and ultramafic sills and overlyingNikolai Greeenstone lavas. Because of the volumeof the flood basalt and the abundance of comag-matic and layered intrusions, the Nikolai LIP in thisregion has the potential to host world class depositsof PGE and Ni‐Cu. We undertook 3‐D potentialfield modeling in order to better characterize thegeometry of the Amphitheater synform and to con-strain the extent of subsurface units that may con-tain deposits.

[62] We built a 3‐D potential field model consist-ing of 11 layers, initially constructed from a setof intersecting 2‐D models, and further developedthrough a series of forward and inverse calculations.The 3‐D model confirms the presence of a deep,


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keel‐like, and asymmetric geometry to the synformthat supports a sag basin model for development ofthe Amphitheater Mountains structure. The align-ment of the axis of the Amphitheater synform withthe trend of broad warping and folding in the TangleFormation suggests that the orientation and form ofthe structure may to some extent have been modifiedby, or perhaps influenced, Cretaceous(?) to Tertiarydeformation.

[63] The 3‐D model also allows for volume calcu-lations of potentially important mineral resource‐bearing ultramafic rock units. Estimated volumes ofNikolai basalt and ultramafic rocks, derived from the3‐D model, are on the order of 1000 and 2000 km3,respectively, substantially expanding the mineralpotential of the complex.

[64] A regional geophysical assessment of Nikolai‐related anomalies [Glen et al., 2007a] suggests thatthe Nikolai LIP is far more extensive than previouslyknown, and that the magmatic feeder system in theAmphitheater Mountains may be one segment ofa much larger dissected structure, or one of sev-eral isolated or interconnected complexes occurringthroughout south central Alaska.

Acknowledgments

[65] We thank Peter Bittenbender for field support and assist-ing in field logistics and BobMorin for help with data reduction.We are grateful to Rick Saltus, Sarah Roeske,Warren Nokleberg,and David Ponce for their reviews and comments that helpedimprove themanuscript.We are also grateful to ChrisMusselmanfor assisting with model animations.

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