AN ABSTRACT of the THESIS of Involves Obduction of An

AN ABSTRACT OF THE THESIS OF

Jonathan D. Williams for the degree of Master of Science in Geolo presented on May 23, 2000. Title: Reconstructing Northern Alaska: Crustal-Scale Evolution of the Central Brooks Range.

Abstract approved: Robert J. Lillie

KinematictectonicmodelsconstrainedbyAiryisostaticequilibrium demonstrate the crustal-scale evolution of the Brooks Range during ocean basin closure, arc-continent collision, and exhumation of the orogen. The Bouguer gravity anomaly low that develops across the orogen is related by wavelength to the amount of shortening during collision, and by amplitude to the combined effects of erosional unroofing and isostatic rebound. Three collision models test a range of pre-collision crustal geometries and investigate a variety of evolution histories.

The preferred solution comprises the best aspect of all three models and involves obduction of an oceanic arc onto a passive continental margin with sedimentary cover 250 km wide. Two distinct periods of convergence and unroofing are identified, separated by strike-slip faulting that influences the hinterland. This model involves -200 km of shortening by crustal overlap and up to 17.5 km of erosional unroofing and isostatic rebound; it results in a symmetric, 40 mGal Bouguer gravity low that is consistent with the observed anomaly across the Brooks Range. The Brooks Range can thus be described as a relatively hard collision that is deeply exhumed compared to other orogens. East of the modeled profile a reversal in asymmetry of the Bouguer gravity low across the Northeastern Brooks Range can be attributed to continuing Tertiary contraction. In the central Brooks Range, thick- skinned thrusting that formed the Doonerak antiform characterized this period of convergence. ©Copyrightby Jonathan D. Williams

Jonathan D. Williams

A THESIS

submitted to

Oregon State University

in partial fulfillment of the requirements for the degree of

Master of Science

PresentedMay 23, 2000 CommencementJune 2001 Master ofScience thesis of Jonathan D. Williams presentedon May 23, 2000

APPROVED:

Major Professor, representingGeology

Chair of Department of Geosciences

Dean of Graduate School

I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. ACKNOWLEDGMENTS

Many thanks to the following for support during this project: 0 Professor Robert Lilliefor providing me with a challenging project that investigated a phenomenal part of Alaska Dr. Andrew Meigs for his open door to discussions on Alaska, cycling and life in general. Professor Wesley Wallace and Dr. Catherine Hanks for developing the structural models that formed the basis of this project, supporting my trips to Alaska, and giving excellent advice regarding the Alaskan aspects of the project. 0 Industry sponsors of the University of Alaska Fairbanks (UAF) Tectonics and Sedimentation Research Group- BP Exploration, ARCO Alaska Inc, Chevron, Phillips, Petrofina, and Union Texas.

0 My wife, Janelle, for living through the last few months of thesis writing. TABLE OF CONTENTS

Page

INTRODUCTION ...... 1

Motivation for KinematicModeling ...... 1

Quantifying Collision: The Value of Bouguer GravityAnomalies ...... 4

GEOLOGY OF THE BROOKS RANGE ...... 9

Tectonic History of the BrooksRange ...... 9

Constraints on the Evolution of the CentralBrooks Range ...... 12

Geothermometry and Cooling History ...... 12 GeophysicalImaging ofCrustal Structure ...... 13

METHODOLOGY ...... 14

Description of Model Units ...... 14

InitialModel Starting Geometry ...... 16

Calculation Procedure ...... 18

Assessmentof the Initial Model ...... 22

ALTERNATIVE TECTONIC MODELS ...... 25

Model A -Introductionof CrustalDuplexing...... 25

Concepts Under Test ...... 25 Kinematic Evolution ...... 26 Evaluation of Model A ...... 26

Model B -WidePassive Margin andThin Overriding Crust ...... 30

Concepts Under Test ...... 30 Kinematic Evolution ...... 31 Evaluation of Model B ...... 33 TABLE OF CONTENTS(Continued)

Model C - Oceanic Arc and Strike-Slip Involvement ...... 35

Concepts Under Test...... 35 KinematicEvolution...... 37 Evaluationof Model C ...... 38

DISCUSSION ...... 40

Assessment of Alternative Tectonic Models ...... 40

Composite Model ...... 42

Flexure, Isostasy and the Colville Basin ...... 46

Along-Strike Variations in Evolution of the Brooks Range ...... 48

GlobalContext ofthe Brooks Range Orogeny ...... 51

CONCLUSIONS ...... 54

Results Summary ...... 54

Implications of Results ...... 55

BIBLIOGRAPHY ...... 57

APPENDICES ...... 62

INTRODUCTION TO APPENDICES ...... 63

Appendix 1 -Initial Model Spreadsheet ...... 65 Appendix 2 -Model A Spreadsheet ...... 71 Appendix 3 - Model B Spreadsheet ...... 75 Appendix 4 - Model C Spreadsheet ...... 81 Appendix 5 -Composite Model Spreadsheet ...... 87 LIST OF FIGURES

Figure

1. Selected tectonic units of northern Alaska ......

2. Previous results from kinematic modeling of continental collisions ...

3. Soft versus hard collisions......

4. Erosion and isostatic rebound ......

5. TACT interpretation of crustal structure......

6. Starting geometry for kinematic modeling and arrangement of 1-D mass columns ......

7. Initial kinematic model ......

8. Initial model exhumation step ......

9. Model A ......

10. Detailed comparison of present-day from kinematic models A, B and C ......

11. Model B......

12. Model C ......

13. Composite model starting geometry ......

14. Composite kinematic model ......

15. Composite model interpretation of present-day crustal structure......

16. Schematic representation of Airy isostasy and regional isostasy ......

17. Along-strike variations in crustal structure of the Brooks Range ......

18. Comparison of the Brooks Range to theAlps ......

19. Example 1D mass column...... RECONSTRUCTINGNORTHERNALASKA: CRUSTAL-SCALE EVOLUTION OF THE CENTRALBROOKS RANGE

INTRODUCTION

Motivation for Kinematic Modeling

The Brooks Range is thought to have developed initially through arc- continent collision, but has since been modified by post-collisional extension, convergence, and erosion. The central Brooks Range has undergone much investigation because it is near the trans-Alaska pipeline and the Dalton Highway (Fig. 1) (Mull and Adams, 1989; Plafker and Mooney, 1997; Oldow and Ave Lallement, 1998; and references therein). However, many questions regarding the crustal geometry of the orogen and its tectonic evolution remain unresolved. For example: 1. How much crustal overlap occurred as a result of collisional shortening? 2. How much exhumation by erosional unroofing and accompanying isostatic rebound took place during and since the collision? 3. What was the geometry and width of the south-facing passive continental margin of the northern Alaska (NAK) crust prior to collision? 4. What is the tectonic affinity of crustal units within the core of the mountain

range? 5. How extensive were processes such as extensional unroofing and strike-slip emplacement of crustal bodies? An attempt to answer these questions is made by modeling the evolution of crustal geometry and topographic relief through periods of collision and exhumation. Kinematic tectonic models that test different configurations of the passive margin prior to collision, and a variety of collision schemes, are constrained in space and time by Airy isostatic equilibrium and conservation of area. Each model concludes 7-1°Nr '165°W 1.60°W 1,55°W 150°W 1,45°W 140°W 70°N,j-. atgravityData 150°W coverage and topography area for BEAUFORT SEA 68°N69°N 66°N67°N.,,. EXPLANATION B rooks Range (undiffere ntiated) Cretaceous and Tertiary I C sedimentary100 depositsI I 200 km I ws0 CHE bIdfootsubterraneammond.ndicott. Mountains subterrarre. alloc hthon (EMA) 0 AngayuchamRuby terrane terrane raphyalongFigure dat the 1. :majority,a fromDalton 149°WSelected of Highway the. to Dalton. 151°Wtectonic corridor. Highway is units averaged The ofillustrated. gravitynorthern to provide data coverage. profiles extendsfor 150°W -100 and km filter south. our of th Alaska (Modified from Wallace and others, e map1997).short area.wavelengthA swath The of TACT gravity gravity line, and anomalies vas,-shot topog- with an interpretation of the present-day crustal structure that is additionally constrained by topography, and must yield a Bouguer gravity anomaly similar to the observed anomaly across the orogen. The resulting gravity model offers estimates on amounts of crustal overlap and erosional unroofing, while also revealing the gross geometry and tectonic affinity of crustal units in the core of the Brooks Range.

This technique utilizing kinematic and gravity modeling has been previously applied to continent-continent collision orogens elsewhere in the world (Lillie, 1991; Lillie and others, 1994). The central Brooks Range is an arc-continent collision, and offers a unique opportunity to broaden the scope of this modeling technique. Existing area-balanced structural restorations of the region by Hanks and others (1998) detail a variety of processes that may have occurred during formation of the BrooksRange.Kinematicmodelsdevelopedherecomplimentstructural reconstructions by testing the evolution of crustal structure in terms of isostatic and gravity considerations. The existing structural models provide a framework by whichredepositionof erodedmaterial,extensionalunroofing,strike-slip emplacement of crustal material, and multiple phases of collision separated by unroofing events can be modeled while maintaining local (Airy) isostasy. The results from the Dalton Highway corridor will advance understanding of other mountain belts, and will serve as a basis of comparison for similar gravity models of transects in the eastern and western Brooks Range. The gravity field over the Brooks Range is thus a guide to along-strike variations in crustal structure for this remote and inaccessible region.

The kinematic tectonic models constrained by Airy isostatic equilibrium represent a new perspective on crustal-scale evolution of the Brooks Range. An excellent summary of geodynamical models by Beaumont and others (1996) outlines the context in which kinematic modeling of mountain ranges can be viewed. Each model is designed to test specific crustal geometry and sequential evolution as collision progressed. Comparison of calculated model gravity anomalies with the observed Bouguer gravity field allows each configuration and style of deformation investigated to be rejected or accepted for further study. The gravity models generated are thus designed to improve our knowledge of tectonic events contributing to the evolution of the Brooks Range.

Quantifying Collision: The Value of Bouguer Gravity Anomalies

The development of Bouguer and free air gravity anomaliesduring collision of two continents has been modeled conceptually (Lillie, 1991) and applied to both the Appalachian-Ouachita chain (Enos, 1992) and the Alpine-Carpathian mountain belt in Europe (Lillie and others, 1994). The width of the Bouguer gravity low that develops overa collisionalorogen relates to theamount of crustaloverlap that occurs after the initial collision of crustal bodies. Once collision ceases, decay of the amplitude of the Bouguer gravity low overtimereflects theamount of erosion and isostatic reboundor "exhumation". The results from previous applications of these principles on continent-continentcollisionsare summarized in Fig. 2.

"Soft" collisions with less than 100 km crustaloverlap,such as the Western Carpathians,have a narrow Bouguer anomaly low due mainly to a crustal "keel" protruding down into the mantle (Fig.3a).Once continental crustal shortening advances beyondabout 150 km the crustal keel broadens into a crustal root that supports the topography of the mountain range. The increased crustal overlap of this "hard"collision produces a wider region of thickened crust(Fig. 3b), and consequently a broader Bouguer anomaly low is observed across orogens like the Alps. Asa collision orogenages,its topography erodes and the depressed Moho rebounds (Fig.4). The WesternCarpathians illustrate a"soft"collision that is relatively young;it has undergone only about 4 km of erosion and rebound and still retains a crustalkeel.The Ouachitas aresimilar,but are an older orogen that has experienced twice as much erosion and uplift because the crustal keel has rebounded to where it is essentially gone (Fig. 4b). A similar trend exists for "hard" collisions with the older, Southern Appalachians having experienced almost three times the amount of erosion and isostatic rebound that has occurred in the comparatively young Eastern Alps. 6

+ Width

BouguerAnomalyAmplitude Decays

YOUNG collision OLD collision Low erosion & High erosion & isostatic rebound isostatic rebound

Western Carpathians Ouachitas SOFT collision -50 km crustal overlap <100km -50 km crustal overlap -4 km erosion & -8 km erosion & crustal overlap isostatic rebound isostatic rebound Eastern Alps Southern Appalachians HARD collision -175 km crustal overlap -300 kmcrustal overlap > 150 km -10 km erosion & isostatic -28 km erosion & crustal overlap rebound isostatic rebound

Central Brooks Range ? I I

Figure 2. Previous results from kinematic modeling of continental collisions. The width of the Bouguer gravity anomaly low relates to the amount of crustal overlap, while the decay in amplitude is a function of the relative age of the orogen. 7

Ocean Closes During Subduction

Thick 'Continental Crust" Tin Oceanic Crust

a) Soft Collision Narrow Anomaly

E .00

Low Mountains Thick Sediments

Crustal Keel

b) Hard Collision

0 0 Wide Anomaly E 000 High Mountains

Figure 3. Soft versus hardcollisions.a) Soft collision(Carpathians; Ouachitas). Plate convergence stops shortly after the ocean closes, with minimal amounts of deformation and crustal thickening. The mountains are low and consist primarily of deformed sedimentary layers of the ocean and continental margin. Thick sediments and a small crustal keel protruding into the mantle create a narrow low in the Bouguer gravity anomaly. b) Hard collision(Alps; Southern Appalachians). Plate convergence continues for some time after the continents collide, causing much more deformation and crustal thickening. The broad crustal root produces a wide Bouguer gravity anomaly low. a) Immediately After Collision

High Amplitude Bouguer -M o 0 %** Anomaly Low i F /

.00

High Mountains

Thick Crust

b) Eroded to Lower Mountains

LowAmplitude Anomaly

Lower Topography; Deep Rocks Exposed

Crust Rebounds Upward

Figure 4. Erosion and isostatic rebound. a) Young mountain range. Soon after continents collide the crust may be so thick that it elevates topography as high as the Hi- malayas and Tibetan Plateau. b) Old Mountain range. The crust rebounds to near-normal continental thickness as mountains erode to smaller hills. Rocks that were once buried deeply within the Earth are thus raised to the surface in the Southern Appala- chian Mountains. The amplitude of the Bouguer gravity anomaly depends on the depth of the crustal root. The amplitude thus decreases as erosion and isostatic rebound (exhumation) proceed. GEOLOGYOF THE BROOKS RANGE

TectonicHistory ofthe Brooks Range

The Brooks Range is a Mesozoic to Cenozoic age mountain belt (Fig. 1) formed as an intraoceanic arc collided with a south-facing passive continental margin (Moore and others, 1994; Fig. 25). The southern continental margin of Alaska was likely rifted from Devonian to Early Mississippian, forming basins that were filled by northern sourced sediments of the Ellesmerian sequence during Mississippian and Early Cretaceous (Gottschalk and others, 1998). Farther south, this rifting opened the Angayucham ocean basin, a region dominated by seamounts and oceanic plateau basalts.Contraction related to the Brookian orogeny commenced in the mid-Jurassic when an island arc of Early Jurassic age was tectonically emplaced above the oceanic crust, creating the composite Angayucham terrane. Although the Angayucham terrane has largely eroded, it did encroach onto passive margin sedimentary cover in the Late Jurassic, subjecting the most distal sediments (future Coldfoot subterrane) to high pressure/low temperature (high

P/low T) metamorphicconditions.Geothermometry suggests the Coldfoot subterrane experienced peak temperatures of 550-600°C at pressures in excess of 7 kbar (Gottschalk, 1998). The Coldfoot subterrane thus was buried to depths greater than 25 km, assuming a geothermal gradient of 15-20°C/km (Blythe and others, 1998). Prograde metamorphism likely took place between Early Cretaceous and mid-Jurassic, prior to 142 Ma (Gottschalk & Snee, 1998) and possibly earlier than 171 Ma (Gottschalk and others, 1998).

Contraction of the region continued until mid-Cretaceous, with the main phase of crustal thickening and shortening taking place between 135-110 Ma. The level of detachment stepped deeper from beneath the Angayucham terrane to within the passive margin sediments that were thrust northward and are preserved as a series of north-vergent allochthons. From south to north these internally imbricated units are: the Coldfoot subterrane- a belt of schistose rocks; the Hammond subterrane-predominately metamorphosedtophyllite;and therelatively unmetamorphosed Endicott Mountains allochthon (EMA). Imbrication of these terranes occurred such that the Angayucham terrane is the highest structural unit in the central Brooks Range today. The Coldfoot subterrane sits above the -10 km thick Hammond subterrane, which in turn overlies the -8 km thick EMA (Handschy, 1998).

The Albian is a period that includes down-to-the-south extensional unroofing along reactivated thrust faults in the southern Brooks Range simultaneous with thrusting in the northern Brooks Range (Gottschalk and Oldow, 1988). The mechanism for extension is poorly understood and it is unknown whether only structurally high faults were activated to maintain critical taper, or if there was extensional collapse of the hinterland crust (Pavlis, 1989). Erosional unroofing accelerated in Albian time (Blythe and others, 1998), and debris shed from the elevated Brooks Range has since filled the foreland Colville basin and the hinterland Koyukuk basin (Fig. 1). In the foreland, the earliest sediments of the Brookian sequence were deformed during the latter stages of thin-skinned tectonics and incorporated into the EMA. The combination of thrusting and rapid unroofing by both extension and erosion led to a greenschist-facies metamorphic overprint in the Coldfoot subterrane (Gottschalk and others,1998). Exhumation was initially greatest in the Coldfoot subterrane with its focus later shifting northward where two pulses of cooling have been detected for the EMA by fission-track and40AR/39Ar analysis (Blythe and others, 1998). In both cases rapid 50°C cooling took place in 5 million years. The first pulse, dated at -100 Ma, is interpreted to correspond with the end of thrust activity in the EMA and is consistent with 6 to 10 km of deposition in the Colville basin due to Albian uplift and erosion (Moore and others, 1994). Between 60-25 Ma (Paleocene to Oligocene), elevated exhumation rates for the northern Brooks Range coincided with renewed crustal shortening and north- vergent, thick-skinnedthrustingthat resulted in formation of the Doonerak antiformal stack from Northern Alaska (NAK) crust (Blythe and others, 1996, 1998; O'Sullivan and others, 1998). The present-day surface expression of this feature, after the removal of at least 8 km overburden (O'Sullivan and others, 1998), is the northeast-southwest trending Doonerak window (Fig. 1). The window is largely surrounded by the EMA to both the north and south. Kilometer-scale denudation took place during construction of the Doonerak antiform,includinga rapid-cooling event at -25 Ma (Blythe, 1998). Since then, slow exhumation has continued at similar ratesthroughout the Brooks Range (Blythe and others, 1998). Clastic sedimentsshed from the BrooksRange sinceAlbian time have been deposited progressively farther northandeast, with the current depocenter lying north of the present northern Alaska continentalmargin(Mull, 1985; Bird and Molenaar, 1992).

Another potential component of the Brooks Range's evolution is strike-slip faulting along the east-west trending Kobuk fault zone at the southern flank of the orogen. This fault zone contains thrust and normal faults related to the episodes of crustal thickening and extension outlined previously. Within the fault system is the Malamute Fault that has undergone 80 km of dextral strike-slip motion since 112 Ma (Ave Lallement and others, 1998). The timing of this displacement is not well constrained, although it has been interpreted to be contemporaneous with thrusting in the foreland, and formation of the Doonerak window (Gottschalk and others 1998). Constraints on the Evolution of the Central Brooks Range

Geothermometry and Cooling History

Investigations using tools of K-Ar,40Ar/39Ar,apatite and zircon fission-track dating provide clues to the metamorphic and exhumation history of the Brooks Range. Several studies have determined that the Coldfoot subterrane experienced high-pressure, low-temperature metamorphism between -185-142 Ma (Wirth and Bird,1992; Blythe and others,1996; Gottschalk and Snee,1998). Peak metamorphic conditions are typically estimated to have been at -10 kbar and temperatures of --410-600°C (Patrick, 1995; Gottschalk, 1998). If the assumed geothermal gradient at the south end of the passive margin in mid- to late-Jurassic was between 15-20°C/km (Blythe and others, 1996), then the Coldfoot subterrane was buried to depths in excess of 25 km. The northernmost passive margin sediments incorporated into the EMA were exposed to maximum paleotemperatures of -300°C, suggesting a burial depth of -12 Ian (Blythe and others, 1996).

Fission track analysis has been instrumental in determining the timing and rates of tectonic and erosional exhumation of the orogen. Based on the data published by Blythe and others (1998) the following exhumation trends are noted, assuming a geothermal gradient between 15-20°C/km. Cretaceous and Early Tertiary. Exhumation was greatest in the Coldfoot subterrane where a maximum of 18.9 km was removed. Farther to the north, lower rates removed a maximum of 5.5 km from the EMA. Tertiary. Rates of erosional unroofing are interpreted to have accelerated throughout the Brooks Range between 60-55 Ma. From this period up to present-day a further -0.5 km has eroded from the Coldfoot subterrane and 2.2- 3.3 km from the EMA. I

S Doonerak window.Exhumation is interpreted to have temporarily increased between 30-25Ma,removing 4-8 km at the end of duplexformation.In the last 25my,erosion rates slightly exceeding the regional average have eroded a further 2-4 km. Overallerosion estimates.From Albian to present-day, interpreted maximum amounts of erosion are 30.75 km from the Coldfootsubterrane,32.5 km from above the Doonerakwindow,and 16.3 km from the EMA.

Geophysical Imaging of Crustal Structure

The Trans-Alaska Crustal Transect (TACT) experiment carried out by the US Geological Survey and Rice University provided the first imaging of the deep crustal structure of the Brooks Range by means of an integrated seismic reflection/refraction study. Two different interpretations of the seismic data identified an asymmetric crustal root beneath the orogen with a maximum Moho depth of -46 km and a hinterland Moho depth of -35 km (Levander and others, 1994; Fuis and others, 1995). Within the crust, prominent south-dipping reflections underneath the Doonerak window reveal the structural form of the Doonerak antiform. The basal detachment of the Doonerak antiform is observed at 10-12 km beneath the northern range front and reaches a depth of 26-28 km under the Coldfoot subterrane (Levander and others, 1994). This is perhaps the most important and reliable piece of structural information revealed by the seismic profile. The upper detachment, produced by thin-skinned tectonics during the first period of post-collisional shortening, can be identified at 3-10 km depth, but its shallow location presents resolution problems given the deep focus of the survey. I

METHODOLOGY

Descriptionof Model Units

The seismic velocity structure and crustal interpretation of the Trans-Alaska Crustal Transect(TACT)of Fuis and others(1997)provides a geophysical basis for gravity modeling (Fig.5).Using Nafe-Drake curves (Ludwig andothers,1971), the average velocity of each crustal unit is converted to a representative density value. The observed gravity and topographic profiles are taken from the 1994 Geophysics of North AmericaCD-ROM(Hittelman andothers, 1994),which provides digital gravity data on a 2.5-minute grid and topography on a 5-minutegrid.To filter out localanomalies,a swath of data from 149°W-151°W was averaged to provide a profile that approximatesthe TACT line (Figs.1 & 5). The 2D crustal structure was adjusted using the GM-SYS gravity modeling program to match the calculated Bouguer gravity anomaly with the observed Bouguer anomaly across the Brooks Range and North Slope as closely as possible without departing dramatically from the TACT interpretation of the crustal structure (Fig.5).Only the broad Bouguer low that relates to the whole crustal structureis modeled faithfully; short wavelength anomalies due to upper crustal features are largely ignored because they are too small to be resolved by kinematic modeling.

The resulting gravity model provides densities for kinematic modeling; thicknesses of the two converging crustal bodies; a guide to the gross internal divisions of the crust; and the approximate form of the Moho beneath the Brooks Range. For the calculations involved in kinematic modeling the following assumptions and simplifications are made: Crystalline crustalunits. All are given a density of 2.85 g/cm3, because minor lateral density variations thatexist within the crust are insignificantin comparison to crust/sediment and crust/mantle density contrasts. CreatedusingGM-SYS 0

-100 South North

Endicott Mountains Arctic Coastline IKoyukuk basin & 'ALbd on accreted terranes Colville:basin 0 YE

Material beneath Coldloot subtenane Crustal Keen Doonerak Duplex (unknown tectonic affinity) -45 km deep

Model Coordinate (km) V.E. = 2:1

LEGEND TO MODEL UNITS (Densities in g/cm3) Crystalline Crust Units (2.85) Sedimentary Units ® Northern Alaska (NAK) Crust Deformed Sedimentary Rocks (2.7) Triassic Passive Margin Sediments (2.5) Deformed NAK Crust Colville Basin (2.5) Oceanic NAK Crust Koyukuk Basin & Accreted Terranes (2.6) Exotic Crust Other Units Ruby Terrane Water (1.03) CNN` I Other Crustal Rock EDMantle (3.3) (Italic text denotes units in later figures)

Figure 5. TACT interpretation of crustal structure. Density structure for the Dalton Highway corridor of the central Brooks Range, based on the velocity structure of Fuis and others (1997), and gravity and topography averaged between149°W 151 °W (Fig. 1). Gravitymodel provides unit density and crustal thickness starting parameters for kinematic modeling.

C 16

NorthernAlaska (NAK) crust. The underthrust crust includes a thin cover of parautochthonous sedimentary rocks.

."Exoticcrust". Encompasses the present Angayucham subterrane, the Koyukuk subterrane and the Ruby terrane, i.e. all the units of crustal density that originally lay to the south of the NAK crust.

Material beneath the Coldfootsubterrane. Thisunit inthe southern Brooks Range is notidentifiedas a distinct model element because it has unknown tectonic affinity.Inversionof the velocitystructureyields typicalcrustaldensity for this block.

"Deformed sedimentaryrocks". This is created by allochthonous transport of late Paleozoic to early Mesozoic age passivemarginsediments and older, underlying continental margin deposits. The unit comprises, from south to north, the Coldfoot subterrane, the Hammond subterrane and the EMA.

Sedimentarybasins. The Dalton highway transect crosses the northeast comer of the Koyukuk basin where the sedimentary section is relatively shallow and terranes of oceanic affinity have accreted to Alaska. A density of 2.6 g/cm3 for this unit thus reflects a sedimentary basin combined with accreted crustal blocks, whereas the Colville basin is more appropriately modeled at 2.5 g/cm3.

Initial Model StartingGeometry

Two important changes from earlier continent-continent collision models (Lillie, 1991; Lillie and others, 1994) are represented by the starting configuration of the initial model (Fig. 6). A thin cover of water added above both crustal masses provides accommodation space for sediment deposition during the final, erosion stage of the model. Replacement of water with sediments that have less density contrast relative to the crystalline crust allows for a three to four fold deepening of Future Arctic Coast eference cola.Present-day .0 South. North rIT SedimentArctic Coast (-0.35) 27,'km III 30 km NAKhs x dsCrust = oms (0) 20 heMantle x do = (+0.45);Amc 60180 Airy Isostatic1-D mass Equilibrium: columns exert E h equalx d pressure at 75 km = 17.85 g/cm2 Zhxd=17.85g/cm2 hm x dm = omm 100 -600 V.E.=4:1 LEGEN-1.82 D -400Water -200 Model Coordinate (km) 0 .200 NAK Crust 400 600 Density-0.35 contrasts in g/cm3, relative to crystalline crust of-2.85 ExoticContinental Crust Margin Sediments O N MantleOceanic Crust&m3. height.density;thatFigure the of 6:pressure hs mantleStarting = height column;exerted geometry of sediment bydm each = for density kinematiccolumncolumn; of mantle;on ds modelinga = horizontal density Am ==.mass and of plane sediment; arrangement contrast.) (here fixedhe = of heightat 1-D 75 kmmass of crustal-depth) is column; the same. do (h = =density height ofof crust;,column; hm columns. Airy isostatic equilibrium implies d = = the basin, even with no flexural enhancement. Secondly, the initial model provides an intermediate step between previous continent-continent collision modeling and the ultimate goal of studying arc-continent collision. The exotic crust modeled here is 3 km thinner than the NAK crust, in contrast with earlier continental collision models that involved equal thickness crustal units(Lillie, 1991; Lillie and others,

1994).

The starting geometry for kinematic tectonic modeling has an ocean basin separating the exotic crust to the south from the south-facing passive margin of the NAK crust(Fig.6).A 200 km wide passive margin with sediment up to 12 km deep is based on an area-balanced reconstruction of the Brooks Range (Hanks and others, 1998).The position of the passive margin relative to the present-day Arctic coastline is constrained by the southward extent of parautochthonous sedimentary rocks. Crustal thicknesses are adaptedfrom the TACTinterpretation of crustal structure (Fig.5),withthe NAKcrystalline crust being 30 km thick and tapering southward into oceanic crust 8 km thick over a transition 100 km wide (Fig.6).The exotic crust has a full thicknessof 27 kmwith transition to zero thickness in its most northerly 200 km(Fig.6).The initial model geometry described represents the thickest exotic crust and narrowest passive margin hypothesized (Hanks and others,

1998),and as such can be considered an end-member model.

Calculation Procedure

The starting geometry, and each subsequent stage of collision, is constructed from a series of 1D mass columns that are positioned where the thickness of a block changes, or where a new unit enters the model (Fig. 6). These columns preserve Airy isostatic equilibrium spatially by exerting equal pressure at 75 km; a depth selected because it is below the deepest root that can form during collision. All columns at each time step exert the same pressure as a fixed reference column at the present-day Arctic coast, thus preserving isostatic equilibrium between each time step. The calculated structure yields a free air gravity anomaly, which is adjusted to a Bouguer anomaly using the standard corrections of -2.67 g/cm3 on land and +1.64 g/cm3 at sea (Lillie, 1999). Density contrasts relative to average crustal density of 2.85 g/cm3 are used to calculate Airy isostatic equilibrium because the contrasts of crust versus sediment and crust versus mantle are the main contributions to observed changes in gravity anomalies. Airy equilibrium assumes that the crust has no flexural rigidity; deviations of model gravity anomalies from the observed profile may be partly attributed to lithospheric flexure (Lillie, 1991).

The kinematic model shows the exotic crust moving northward in 100 km increments relative to the fixed NAK crust as the ocean basin closes through subduction (Fig. 7A-D). Except for the subducting oceanic crust, areas and thicknesses of units are held constant at each stage of collision. Oceanic crust subducted beneath the exotic crust is assumed to attain density similar to that of the upper mantle during eclogite metamorphism and thus has little effect on the gravity anomaly (Lillie, 1991). Collision occurs when the tip of theadvancing exotic crust reaches transitionalNAK crust (Fig. 7D). The sediment area is held constant, causing the contracting sediment unit to thicken rapidly immediately prior to collision; sediments are then uplifted as convergence proceeds (Fig. 7E-IT). After collision, the overriding exotic crust advances in 50 km steps, until the width of the calculated Bouguer anomaly low exceeds that of the observed anomaly (Fig. 7H). The amount of collisional shortening (crustal overlap) is estimated as the stage where thewidthsof the twoanomalies are approximately equal.

Once the amount of convergence is determined, the effect of erosion and isostatic rebound are considered to reduce the mismatch between the amplitude of the calculated and observed Bouguer lows (Fig. 8). Each ID column is considered independently, with sufficient material removed to lower topography to present- 2

A. 300 km Before Collision E. 50 km After Collision 400 400 E g- 200 Calculated 200 0 0 Z.0 Observed- E -200 -200 -400. -400 South Facing Passive Margin E. 0 20 A. -Growth of topograph' 40 1 40- y 0 South North supported bycrustalkeel 606 I -600 -400 -266 200 400 00 -400 -200 ' 200 400. B. 200 km Before Collision F. 100 km After Collision a' 400 400,F c -Z 200 200 Qa

C7 0 JJ rnE-200 -200 `fi tS -400 -400 --- Water (-1.8k 0 0Exotic-Ctu$t. NNAK AK Crust 20 20 1 - - 40 -i Mantle,(+0.45) 40

400 .-400 -200 260 0 60600 -400 -200 0' 200. 400.

C. 100 km Before Collision G. 150 km After Collision 400 400 E Width of modeled Bouguer gravity low °c ; 200 200 matches that of observed anomaly 0 0 mc77 E -200 -200 cS -400 -400 Sediments (-0.35)

20 40 0CD { Subducti g Oceanic Crust (0) 6 600 -400 -200 6 .200 400' 60600 -400 -200 200 -400 D. Time of Collision H. 200 km After Collision 400 400 cl -200 200 0 0 mcE -200 -200

cS ,400 -400 Y) I - 20ii 20 - '

40 - 1:ollision begins when the'overridingcrust 40 With Increased overlap, beginstooverlaptransitional NAK crust keel broadens Into a crustal rod 60 1- 0 t 6 -600 -400 -200 0 200 400 0000 -400 -20 0 200' 400 Model Coordinate (km) Model Coordinate'(km)- Figure 7:Initial kinematic modelDensity contrasts, expressed relative to average crustal density of 2.85 g/cm3, are thesame as inFig.6.Width of modeled Bouguer gravity anomaly low matches that of the observed profile at 150 km of collisional shortening. (V.E. = 4:1) 2

100 Observed gravity anomaly 0

After erosion, Isostatic Calculated anomaly at rebound, and deposition the end of collision of eroded material Correct width Amplitude too large -400 South North 150 km After Collision (Fig. 7g) Present-day A Coast Constrains Airy isostatic equ, brium in spare and tune lNaxknum topography - 5 km

(-0.35) E 20 (0)

o Deep low density root supports (+0.45) topography; responsible for Cnrstal root -48 km deep -400 mGal Bouguer low

-1 400 V.E. = 2:1 Model Coordinate (km)

10 km of Erosional Unroofing and Isostatic Rebound

Water replaced bysedlments Topography eroded to shed from eroded oro en present day Revels -0.35) (-0.15) 3- to 4-fold increase in basin depth due to lower density contrast of sediment E (p;':, (0)

m 0 Smaller root gives diminished Bouguer (+0.45) Cnrstal root -42 km deep gravity low, -100 mGal amplitude 60 160 6 1 26o 360 4 V.E. = 2:1 Model Coordinate (km)

Figure 8: Initial model exhumation step. Erosional unroofmg, isostatic rebound and redeposition of eroded material after collision has ended. Erosion of deformed sedimentary rocks removes lower density, upper portion of this unit, thus its overall density contrast relative to crystalline crust is reduced from -0.35 to- 0.15 (Alldensity contrasts relative to 2.85 g/cm3). Compare with TACT interpretation of crustal structure (Fig. 5). day valueswhile preserving Airy isostatic equilibrium. During the erosion process, the density contrast of the deformed sedimentary package is reduced from -0.35 g/cm3 to -0.15 g/cm3 to reflect lithification and the removal by erosion of less compacted rocks. The eroded material is redeposited to replace areas of thin water cover, creating the Colville basin to the north and the Koyukuk basin to the south of the orogen (Fig. 8). Further development of these young sedimentary basins is limited by their present elevation above sea level. Interpretation of the Bouguer gravity anomaly width thus provides an estimateof collisional shortening, while the current amplitude allows an estimate of the combinederosional unroofing and isostatic rebound(exhumation).

Assessment of the Initial Model

The initial collision model provides an assessment of the amounts of crustal overlap and exhumation. The geometry of the initial model provides an excellent starting point from which more complex models can be developed. The initial model requires -150 km of post-collisional shortening to correctly match the width of the calculated Bouguer gravity low with the observed anomaly across the orogen (Figs. 7 & 8). To approximate the present-day topography, -10 km of erosion and isostatic rebound is required, but the Bouguer low produced is still 30 mGal more negative than the observed low (Fig. 8). The primary contribution to the calculated Bouguer anomaly comes from the low-density crustal root. The -30 mGal error between modeled and observed Bouguer anomaliesis produced by the deformed sedimentary rocks that contribute up to -40 mGal. Thissuggeststhat the 10 km thick sedimentary rocks have not been exhumed deeply enough by the initial model.

The initial model produces the correct anomaly width, but it fails to meet important constraints that exist for the evolution and present-day geometry of the Brooks Range. The evolution sequence modeled implies that the maximum burial depth of any part of the former passive margin sediments is -17 km; substantially less than the 25 km depth to which the Coldfoot subterrane was buried. The shallow burial depth of 17 km requires an Early Cretaceous geothermal gradient in excess of 25 °C/km rather than 15-20°C/km as has been suggested by Blythe and others (1998). Internal deformation of the sedimentary rocks can bring the most deeply buried material to the surface, but a maximum erosion estimate of only 10 km is -20 km less than that interpreted from exhumation rates calculated by Blythe and others (1998). Topography is the firmest constraint available and itis impossible to increase erosion in the model without lowering topography below the 1.4 km average that exists along this transect.

The initial model is nonetheless a good first approximation of the degree of collision that produced the Brooks Range, suggesting -150 km of crustal overlap. It serves as an excellent end-member model against which more complex crustal geometry and evolution history constrained by area-balanced structural restorations of the Brooks Range (Hanks and others, 1998) can be compared. The model presented fails to meet constraints for crustal thickening, geothermal history and exhumation rates because it oversimplifies the evolution of the Brooks Range that involved distinct periods of thin- and thick-skinned tectonics. Adaptations required to make a more comprehensive model include:

1.Startinggeometry. A more realistic starting geometry may involve a much wider passive continental margin, or an exotic crust that looks like an oceanic arc rather than being simplified to a constant thickness. 2.Internal crustalthickening. A portion of the NAK crust must be uplifted to represent the Doonerak Duplex as a distinct event within the Brooks Range orogenesis so that a deeper crustal root forms and higher exhumation estimates are produced. 2

Sedimentburial. The sediments must be pushed, either by increased sediment thickening or by subduction, to depths of around 25 km to meet temperature and pressure constraints. Exhumation Process. Extensionalunroofing may have to be considered as a distinct exhumation event for the southern Brooks Range, rather than its effect being lumped with erosional unroofing and isostatic rebound. 25

ALTERNATIVETECTONIC MODELS

The initial model demonstrates a simple collision scheme constrained by Airy isostatic equilibrium The detailed structural restorations by Hanks and others (1998) illustrate more realistically the deformation and unroofing events that contributed to formation of the Brooks Range. For the initial kinematic model, one of these structural models provided a guide for the geometry of the passive continental margin that existed prior to collision. In this section, concepts from all three models by Hanks and others (1998) are integrated into kinematic models constrained by Airy isostasy and present-day topography. Model A develops directly from the initial model to consider Tertiary uplift of the NAK crust forming the Doonerak antiform. Model B is strongly influenced by "model B" of Hanks and others (1998), testing a wide passive margin and thin obducted crust end-member scenario. Model C is another variant on the initial model, involving an oceanic arc with an accretionary wedge and a strike-slip faulting event. In models B and C, collision involves tectonic burial of the south-facing passive continental margin, and extensional unroofing contributes to exhumation.

Model A - Introduction of Crustal Scale Duplexing

Concepts Under Test

The initial model displays a collision sequence that involves only thin-skinned tectonics. The Doonerak antiform, however, evolved during a second phase of contraction that caused uplift of basement rocks. Model A incorporates this thick- skinned phase into the initial kinematic model. The starting geometry is identical to that of the initial model, except for slight tapering of the NAK crust from north to south that reflects thinning of this unit on the TACT interpretation (Fig. 5). The NAK crust in Model A is still 30 km thick at the future Arctic Coast (400 km mark), but has a thickness of 28 km at the 100 kin mark.

Kinematic Evolution

The first phase of collision for Model A (Fig. 9A-C) involves thin-skinned tectonics and follows the progression of the initial model up to 100 km of crustal overlap (Figs. 7 & 9). The thrust detachment then steps deeper, involving rocks from the NAK crust in a late stage of thick-skinned deformation (Fig. 9D). Based on the structural model of Hanks and others (1998), the detachment beneath the Doonerak antiform dips steeply southward from the top of the NAK crust at the 150 km model coordinate. It then dips more shallowly south of the 100 km coordinate, cutting completely through the NAK crust at the 50 km coordinate. This model is consistent with structural interpretations of the region that attribute formation of the Doonerak window to crustal-scale duplexing (Oldow and others, 1987; Moore and others, 1997). Deformation of the NAK crust contributes 50 km of shortening to a model that requires -150 km of total crustal overlap to simulate the width of the observed Bouguer anomaly (Fig. 9D). Erosion and isostatic rebound removes -16 km from the highest topography at the end of collision and -1,500 km2 of deformed sedimentary rock and exotic crust from the entire orogen. Deposition of material shed from the mountains into foreland and hinterland basins completes the model (Fig. 9E).

Evaluation of Model A

The resulting model Bouguer gravity profile matches both width and amplitude of the observed anomaly (Fig 11). Inclusion of crustal-scale duplexing 2

Model anomalies equal due to preservetlon of Airy eostatic equrium through time at present-day Arctic coast

Paleocene -200 Albian \ \L

South North Triassic 0 20 km 30 40 crust tapered 60 00 -400 -200 .200 400:

B. Late Jurasslci 0 Early Cretaceous -20 Deepest seds -19 km deep Time of Collision o60 -200 0 200 400

C. Alblan

100 km crustal overlap

D. Paleocene 0 z620 50 km additional shortening to 150 km total crustal overlap $60

-1.75 km topo E. Present Day (see Fig. 10) 0 -201 16 km max. erosion YL40 1500 km2 removed antiform a60

Model Coordinate (km)

Figure 9. ModelA.Thin-skinned collisionthrough Albian time. Uplift of NAK crust basement(Doonerakantiform) occurs in a late stageofthick-skinnedtectonics. (V.E. = 2:1; see Fig. 5 forunit legend). -20

-40

-60

-80

South North Model A - Crustal Dupiexing (Fig. 9) 150 km Crustal Overlap Deep basin, negative contribution 0

E . ,I, ,- -\ . -I, 0 20 m O 40 Coldfoot subtenane missing Deepest Moho -45 km

60 - 00 160 260 360 4 Model B - Wide Passive Margin (Fig. 11) 260 km Crustal Overlap

0 E

40 Deep proximal Colville basin, Transitional crust below Deepest Moho -45 km negative contribution Coldloot subterrane 60 1 2 3 4 Model C - Oceanic Are, Strike-Slip (Fig. 12) 200 km Crustal Overlap

Model Coordinate (km) LEGEND (Density contrasts relative to crystalline crust of 2.85 g/cm3) 3-0.35) North (CoMile) Basin Fill (0) NAK Crust =0) Exotic Crust +0.45) Mantle Q(-0.25) South (Koyukuk) Basin Fill -(0) Transitional NAK Crust ®(0) Ruby Terrane -0.15) Deformed Sedimentary Rock (0) Uplifted NAK Crust (0) Other Crust Figure 10. Detailed comparison of present daystepfrom kinematic models A B and C. Each model is constrained by Airy isostatic equilibrium and present-day topography. Model A provides best calculated anomaly over the Colville basin, its hinterland crust is too thin. Model C provides most accurate surface geology, and the most suitable calculated anomaly across the Brooks Range. (V.E. =2:1) produces a Bouguer gravity anomaly closer to that observed than the initial model. Basement- involved uplift to create the Doonerak antiform increases the amount of crustal thickening, producing a crustal root -45 km deep in a position similar to the deepest Moho on the TACT interpretation (Fig. 5). A deeper crustal root prior to exhumation supports higher topography than the initial model, and thus increases the amount of erosional unroofing and isostatic rebound that can occur. The deformed sedimentary rocks are largely removed and only remain at the shallowest crustal levels, so their mass deficiency relative to the uppermost crust no longer has a significant influence on the model anomaly. Instead, the Bouguer gravity low can be attributed to the keel of crustal rock that supports the remaining topography of the Brooks Range by poking down into mantle rocks of higher density (Fig. 10).

Model A has a poor fit to observed gravity in the hinterland where the calculated values are 40 mGal too negative (Fig. 10). This error is produced by the hinterland basin that is filled -8.5 km deep by material with a -0.25 g/cm3 density contrast relative to crystalline crustal rocks. Without internal thickening of the exotic crust, isostatic and topographic constraints can only be met by the addition of a thick unit of sediment and accreted crust. A deeper hinterland basin persists in Model A as compared to the initial model because basement-uplift of the NAK crust transports lower crustal rocks northward, away from the hinterland. A wider transition from NAK crust to oceanic crust beneath the passive margin may be requiredforcrust of appropriate thickness todevelopin thehinterland. Alternatively, the detachment causing basement uplift may not cut completely through the transitional NAK crust, thus leaving a small portion of former oceanic crust undisturbed deep below the Koyukuk basin.

Model A is identical to the initial model until the transition to thick-skinned tectonics in Paleocene time. The burial history of the passive margin is therefore unchanged and the passive margin sediments are pushed no deeper than -17 km. Thus no sedimentary rocks are buried deep enough to experience high P/low T 3 metamorphism characteristic of the Coldfoot subterrane. Without a change in burial history, the increased estimate of erosion put forward in this model results in the entire sedimentary section being removed from the southern Brooks Range, exposing exotic crust where the Coldfoot subterrane should be. This error in surface geology, despite a low exhumation estimate, may indicate that the morphology of model units should be adjusted, but more likely implies that a mechanism must be introduced into the model that buries passive margin sediments deeper.

Model B - Wide Passive Margin andThin Overriding Crust

Concepts Under Test

Model B is an opposite end-member to the initial model. The passive continental margin is the widest that is likely to have existed in Triassic time. The transition from full thickness, continental NAK crust to normal thickness oceanic crust is 450 km wide. Sedimentary cover of the passive margin spans a 700 km width. Between 0 and 100 km coordinates, a rift basin is modeled in the NAK crust where the crust thins to 10 km The transitional crust beneath the majority of the passive margin sediments is 350 Ian wide and thins from 20 km at the north to 8 km at the south. Collision initiates by collapse of the passive continental margin as imbricated oceanic crust overrides the transitional crust and its overlying sediments. Shortening of sediments on such thin crust thus results in deep burial of the sediments. Obduction of 8 km thick oceanic material is the thinnest upper plate viable in collision. Tertiary contraction is considered as a separate phase of collision, preceded by an initial period of unroofing that involves both extensional and erosional events. 31

Kinematic Evolution

Convergence in model B is achieved by successive northward imbrication and deepening of the detachment level, first within the passive margin sediments, then oceanic, and finally transitional crust (Fig. 11A). This process thickens the obducted block, and moves it northward prior to collision with the NAK crust in the

Late Jurassic/Early Cretaceous (Fig. 11 B). Thickening and shortening thus assumes internal deformation of the transitional crust and buried sediments; reorganization of these units at this scale is achieved by area-balance between time steps.

Collisional shortening prior to Albian time transports the upper thrust sheet of oceanic crust and overlying sediments 200 km northward (Fig. 11 Q. The detachment remains within the transitional crust that thickens dramatically, as do the proximal passive margin sediments as they deform and metamorphose. In Albian time, topography developed by thickening of the crust and sediments undergoes extensional and erosional unroofing. Down-to-the-south normal faulting transports -1200 km2 of obducted sediment and oceanic crust in excess of 100 km from the region of highest topography, isostatically elevating the future Coldfoot subterrane (Fig 11D). Erosion calculated such that slight topography remains removes -1700 km2 and the resulting sediment partially fills the Colville basin (Fig. 11E). The combined Cretaceous exhumation processes removed up to 20 km from the central and southern Brooks Range and up to 12.5 km from the EMA. Later in the model, continental thickness crust replaces oceanic crust south of the orogen. Emplacement of this crust is not intended to simulate specific events in the tectonic evolution of Alaska south of the Brooks Range; rather, a crust of continental thickness exists in that region today, and its presence in the present-day model produces a smooth gravity gradient on the south flank of the Brooks Range (Fig. 10).

In the Tertiary, north-vergent underthrusting of the deformed transitional crust elevates the Doonerak antiform (Fig. 1 IF). After a final 60 km of contraction E 100

A. Triassic (No exotic crust) Rift Basin o, 16E 20 NAK Crust . 40 heanlc'Crust Transltional NAK Crust el 60 00 ,400 -200 0 km 200 400 B. Late 0 JurassiclEarl I Cretaceous E40 bused Future detachment is to 24 km Time of Collision 60 200 400

0 C. Albian (A) 20 a 40. 200 km crustal overlap CD O 60 -200 0 200

D. Albian (B)

> 100 km extension 1175 km2 removed 0 200' 490- E. Late Cretaceous 0 20 12.5 km erosion . 40 -1700 kne extra removed a60 J_ -200 0 200 400 -3.25 km topo F. Paleocene 0 20 60 km additional shortening o 40 260 km total crustal overlap c 60 -52 km root 200 7d* km G. Present Day (See Fig. 10) o 20 12 km final erosion 40 24.5 km total erosion o 60 -45 km root - 'Doonerak Antiform -1000 km2 extra removed 200 400 Model Coordinate (km) Figure 11. Model B. Imbricated oceanic crust and sediments are obducted onto the south-facing passive continental margin. Periods of extensional and erosional unroofing are modeled prior to upliftof theDoonerakantiform. Tectonicevents south of theBrooks Range inAlbiantime are not modeled; continental thickness crust to south of orogen after Late Cretaceous provides smooth gravity gradient(V.E. = 2:1; see Fig. 5 for unit legend). and 260 km of total crustal overlap, the wavelength of the Bouguer gravity anomaly modeled corresponds with the observed anomaly but its amplitude is too great. The final erosion step removes a further 9 to 12 km (-1000 km2) from the orogen and dramatically thins the deformed sedimentary rocks above the Doonerak antiform (Fig. 11 G). Rocks metamorphosed in excess of 20 km depth are thus returned to the surface by unroofing events as the Coldfoot subterrane.

Evaluation of Model B

The model anomaly for the present-day interpretation of crustal structure has the correct amplitude and appropriate width, but the general character of the anomaly does not match the observed anomaly well. On the southern flank of the Brooks Range the model anomaly is up to 30 mGal below the observed gravity field, and in the deepest part of the Colville basin the error is -40 mGal (Fig. 10). The present-day crustal structure derived from model B (Fig. 10) has a crustal root that extends to a depth of -45 km in a similar location to the root in the TACT interpretation (Fig. 5). Increasing the width of the passive margin and cross- sectional area of its cover for greater overlap and erosion estimates in model B compared to the initial model. Shortening and obduction of oceanic crust prior to collision substitutes for exotic crust separated from northern Alaska by an ocean basin. The estimate of 260 km crustal overlap is significantly larger than that derived from the initial model because internal shortening that thickens transitional NAK crust is contemporaneous with the first phase of crustal overlap.Model B thus supports an evolution scheme that does not involve collision of an exotic crustal block.

Obduction of oceanic crust onto the passive continental margin in the Late Jurassic/Early Cretaceous rapidly buries sediments up to -24 km, deep enough to undergo high P/low T metamorphism. This southernmost portion of the buried, deformed sediments is exhumed and rebounds to the surface in the southern Brooks Range as the Coldfoot subterrane. The unroofing process for the southern Brooks Range is enhanced by down-to-the-south normal faults that compliment erosion.

Extension accelerates exhumation soon after peak burial conditions for the deformed sediments, allowing them to uplift rapidly and gain greenschist-facies retrograde metamorphic overprints (Gottschalk, 1998). Extension in excess of 100 km removes the majority of the obducted block, thinning the hinterland crust and leaving only small fragments of oceanic crust behind as the Angayucham terrane.

Model B utilizes area balancing between time steps to simulate internal deformation of several units. Thickening of the transitional crust creates hinterland crust close to continental thickness that becomes the material underneath the Coldfoot subterrane. The transitional crust may in fact have thickened too much, causing the -30 mGal error between model coordinates 0 and 50 km (Fig. 10). With slightly less contraction, the Moho there would not be depressed to the same extent and the model Bouguer gravity anomaly would be higher. At the southern end of the Colville basin a -40 mGal error exists between model and observed Bouguer gravity anomalies. The error results in part from a modeled Moho that locally pushes NAK crust too deep, displacing higher density mantle. There is also a considerable negative contribution from the sedimentary fill of the Colville basin. At the 200 km mark the proximal basin is -3 km deep, but the correct surface geology at this location is higher density deformed sedimentary rocks (Fig. 5). Cretaceous erosion modeled in the northern Brooks Range thus removes too much section from the EMA, leading to the conclusion that the model has incorrect surface units. Model C - Oceanic Arc andStrike-Slip Involvement

Concepts Under Test

The primary goals of Model C are to look at collision where the overriding crust is represented by an oceanic arc, and to incorporate strike-slip faulting south of the orogen. This is an intermediary scenario between the "end member" models A and B. It investigates ocean basin closure and ensuing collision similar to the initial model (Fig. 7), but with an exotic crust of variable thickness. Sediments are buried deeply as the arc and its leading accretionary wedge override the passive continental margin. Extension soon after obduction adds to exhumation caused by erosional unroofing. Two periods of unroofing and isostatic rebound are modeled, separated by Tertiary contraction. The Ruby terrane is represented in model C as a crystalline crustal unit independent from the obducted exotic arc terrane. Strike-slip motion along the Kobuk-Malamute fault system introduces the Ruby terrane into the model's line of section.

The morphology of the oceanic arc and its accompanying accretionary prism are based on a survey of 50 modem and 20 ancient forearc basins by Dickinson (1995). A youngoceanic arc25 km thickis assumed(Fig. 12);that thickness is appropriate for an island arc in Airy isostaticequilibrium.Oceanic arcs tend to have underfilled forearcbasins;in this case where the arc is not exposed to sub-aerial erosion,negligible sediment cover isassumed.Using theBanda Arc(Bowin and others, 1980)as a modem day analog to the ancient arc inAlaska,a forearc basin width of-100 km fits within thetypicalrange of 25 to 125 km observed worldwide (Dickinson,1995). The accretionary wedge is modeled such that the outer arc high is 50 km south of thetrench,and occurs where the accretionary prism is twice the thickness of the oceaniccrust. The geometry of the NAKcrust and its south-facing passive continental margin are identicalto that ofthe initial model (Fig. 6). 0400 cs

E.Cret % \ Triassic Albian(A)

Pcese OBSERVED \ - L.Cret/ -20o South Paleocene North

A.Triassic rAa etionary. Wedget. 0 Y 20 30 km NAK Crust ,e 40 Meffimorphosed to Eclogite Oceanic Arc i, .(Attains Mantle Densilyf 60 -600 -400 -200 0 200 400 B. Late Jurassic) 0 Early Cretaceous 20

50 Ian crustal overlap 40 60 -200 0 200 400 C. Alblan (A) 20 100 km additional shortening 140 Sediments buried 160 km total crustal overlap below 20 km 60 200 400

0. Aiblan (B) 50 km extension -750 km2 removed 60 -1- -200 200 400 Kobijk-Wiamute Fault 0 y.-.."..,. . E. Late Cretaceous E Ruby Terrane\ - 20 10 km initial erosion F= 40 Future detachment -500 km2 removed 60 ti- -200 F. Paleocene o 20 50 km additional shortening 140 200 km total crustal overlap 60

G. Present Day (see Fig. 10) E 0 20 10 km final erosion 5.40 20 km total erosion -44 km Doonerak -1000 km2 removed CS60 0 200 400 Model Coordinate (km) Figure12. ModelC. Intraoceanic arc overrides passive marginsediments;extensional unroofmg incorporated with initial erosion; and Ruby terrane emplaced by strike-slip faulting from the east prior to Tertiary contraction. (V.E. = 2:1, see Fig. 5 for unit legend). Kinematic Evolution

The kinematic development of model C is similar to the initial model with the oceanic arc moving northward, but this time overriding the passive margin sediments (Fig. 12B). Collision proceeds until sediments of the future Coldfoot subterrane are buried to -25 km, an event that occurs after -150 km of crustal overlap (Fig. 12C). Extensional unroofing is then modeled along a detachment that cuts down from the point of highest topography, through the volcanic arc and terminates at the base of oceanic crust on the south side of the arc. Extension transports the upper portion of the obducted arc 50 km southward, reducing the overburden above the buried sediments by -750 km2 (Fig.12D). Erosional unroofing removes 10 km from the highest Cretaceous topography and 500 km2 from the orogen as a whole of erosion, with redeposition of sedimentary material in both the foreland and hinterland basins (Fig. 12E). Calculated into the model at the same time as erosion is the strike-slip emplacement of the Ruby terrane that puts the hinterland Moho at a depth of 33 km.

The second period of contraction involves thick-skinned tectonics whereby north-vergent underthrusting of the Ruby terrane uplifts NAK crust, forming the Doonerak antiform (Fig. 12F). After 50 km of additional shortening, the wavelength of the model Bouguer anomaly matches that of the observed Bouguer anomaly. Collision ends and a final erosion stage with continued basin development completes the model and its interpretation of the present-day crustal structure (Fig. 12G). A total of 200 km crustal overlap is required for model C to produce a model Bouguer gravity anomaly that closely matches the width of the observed anomaly. The final cycle of erosion from Tertiary to present-day has removed 8-10 km from the Coldfoot subterrane, 4-9 km from the EMA, and1,000 km2 from the entire orogen. The crustal keel rebounds isostatically to -44 km, a depth consistent with the TACT interpretation (Fig. 5). Evaluation of Model C

Burial of the passive margin and extensional unroofing are successful ly modeled. The southernmost 50 km of the sediments are buried 18 to 22 km, deep enough for high P/low T metamorphism. The period of extensional unroofing that occurs soon after peak burial rapidly elevates the deformed sedimentary rocks -5 km, into a retrograde metamorphic regime. The combined model of an overriding oceanicarcthat was laterextended incorporates a widely held tectonic interpretation of the Brooks Range (Moore and others, 1994). The modeled burial history is corroborated by geothermometry studies (Blythe and others, 1996) and suggests a sequence of metamorphic conditions similar to those proposed by Gottschalk (1998).

Extension and subsequent erosion remove most of the obducted arc, leaving a small block on the south flank of the Brooks Range. Arc remnants correspond to the mapped position of the Angayucham terrane. Emplacement of the Ruby terrane from the east replaces the faulted arc and oceanic crust with a unit approaching continental crustal thickness. Introduction of the Ruby terrane by strike-slip motion is a convenient means to incorporate a final Moho depth of -35 km in the hinterland, while integrating surface geology into a crustal-scale gravity model. In the model, dextral strike-slip emplacement is inferred along the Kobuk-Malamute fault system, as described by Gottschalk and others (1998). It is consistent with the pattern of terrane accretion from the southeast that brought together various exotic terranes of Alaska (Beaudoin and others, 1994). In the Yukon Flats, immediately east of the Ruby terrane, Bouguer gravity values are 30 to 40 mGals higher than at the Ruby terrane (Barnes, 1977). The greater values may indicate shallower mantle beneath the Yukon Flats, where the crust might have stretched and thinned during dextral movement along the Kobuk-Malamute fault. Model gravity anomalies toward the hinterland thus match observed values well, with the remnant arc locally producing a short wavelength, positive anomaly. A notable deviation of the model C Bouguer gravity anomaly from the observed profile occurs in the Colville basin. The calculated anomaly is 30 mGal too negative at the south end of the basin, the error decreasing towards the Arctic continental margin (Fig. 10). The model produces incorrect surface geology at the 200 km coordinate with models -2 km of basin fill where the EMA actually crops out (Fig.1). The younger, lower density sedimentary unit modeled provides a negative contribution to the model anomaly. The error is large because the model portrays NAK crust as constant thickness, whereas the TACT interpretation (Fig. 5) shows crustal thinning beneath the southern end of the Colville basin. High-density mantle would be elevated closer to the surface under thinner crust, thus increasing the calculated gravity response. Alternatively, the error could be due to isostatic disequilibrium, or flexural response of the crust (these factors will be discussed later). DISCUSSION

Assessmentof AlternativeTectonic Models

The initial model (Figs. 7 & 8) breaks collision down to its fundamental elements; crustal shortening, erosion and isostatic rebound, and sedimentation in basins flanking the orogen. It provides an initial estimate for the amount of crustal overlap and a framework for more complex models. The three alternative tectonic models constrained by Airy isostatic equilibrium test a variety of extremes for starting geometry and evolution history of the Brooks Range orogeny. Each kinematic and gravity model contributes some understanding of the Brooks Range tectonic evolution. Their individual merit lies in the degree to which the modeled Bouguer gravity anomaly, surface outcrop, and crustal structure match observations from the Brooks Range. The best features of each model can be combined to produce a composite model.

Model A demonstrates that formation of the Doonerak antiform is an important event that can be successfully incorporated into kinematic gravity modeling, matching duplex structures interpreted by theTACTseismic experiment (Fuis and others,1995).In models B and C the duplex structure undergoes internal deformation and thus more accurately depicts uplift of NAK crust than model A, where an intact block moves along adetachment.In modelC,duplexing of the NAK crustbeneaththeColdfootsubterranehelpselevateandexposethese metamorphosed rocks. Model C provides the most feasiblesolution.Its variable thickness exotic crust appropriately buries the passive continental margin sediments. Strike-slip emplacement of the Ruby terrane provides model C with continental thickness crust in the hinterland while removing thinner crust associated with the arc. Model C,however,is not a definitive solution because it has substantial error in the foreland Colville basin. 41

The processes of sediment burial beneath obducted crust and extensional unroofing in the southern Brooks Range are tested in both models B and C. Burial of distal passive margin sediments is critical for sedimentary material to reach high Plow T metamorphic conditions. The overall crustal thickness in both models during the Cretaceous, in the vicinity of the most deeply buried sediments, is insufficient for erosion and isostatic rebound alone to return the former passive margin sedimentsto the surface. For model C in particular, there is no topography and thus no potential for erosional unroofing during the Cretaceous, so extension and transportation of obducted crust to the south is a necessary exhumation process. Extension allows a large portion of the obducted block to be removed from the area of primary model focus, independent from erosional unroofing.

Model B examines a maximum end-member scenario for width of the south- facing passive continental margin. Itis unlikely that a passive margin where transitional crust spanned 450 Ian did exist prior to orogenesis because model B produces the least realistic gravity field (Fig.10). Model B does, however, demonstrates that a rift graben within the NAK crust can be accommodated into the model's evolution to produce an admissible present-day crustal structure. A major problem with model B occurs in the hinterland, where the calculated Bouguer anomaly is too negative. The anomaly can become more positive by slightly decreasing the amount of shortening and consequent thickening of the transitional crust. With a thinner crust, the Moho beneath the southern Brooks Range would be elevated and the presence of shallow, high-density mantle rocks would contribute positively to the gravity anomaly.

Model A provides a good fit to the observed Bouguer gravity anomaly in the southern Colville basin, whereas models B and C have major negative errors there (Fig. 10). In both B and C, the EMA is preserved as a thin sliver of deformed sedimentary rock between uplifted NAK crust and young basin sediments on the north. The abrupt density contrast (-0.35 g/cm3) is responsible for the negative spikes on the calculated Bouguer gravity anomalies of models B and C. In model A the EMA is 100 km wide and up to 8 km thick, providing a buffer of intermediate density between that of crust and basin fill. The density contrast of only -0.15 g/cm3 for the deformed sedimentary unit reduces the negative contribution to the gravity anomaly. Thinning of the NAK crust from north to south tested by model A enables mantle rocks to be more elevated and thus provide a suitable positive contribution to the gravity profile. The calculated Bouguer gravity anomaly for model A therefore has a smooth transition from the mountains into the Colville basin, and mimics the shape of the Moho north of the crustal keel.

Composite Model

The composite model combines the best features of models A. B, and C. It is based mainly on model C, the most feasible model, because that model displays an evolution sequence that results in: 1. A Bouguer gravity anomaly low similar in amplitude, width and general form to

the observed anomaly.

2.Surface outcrop that compares favorably with actual terranes (Fig. 1).

3.Crustal structure similar to that of the TACT interpretation (Fig. 5).

The starting geometry for the NAK crust composite model is similar to model A, because it tapers from a full thickness of 30 km down to 29 km thick at the 100 km coordinate (Fig. 13). Model A provides the best fit to observed gravity over the Colville basin (Fig. 12) due to tapering of the NAK crust and preservation of sedimentary rock in the EMA. Model C contributes the configuration of the overriding arc and its evolution history to the composite model. The only change made in the evolution of this combined model is to distribute deformation so that the deformed sedimentary rocks are thickest in the EMA at the end of collision (Fig. Location of Present-Day Arctic Coastline 29 km (0) NAK Crust 30 km 340 Subducted/ oceanic crust (+0.45) Mantle 60- -800 / /metamorphosed to eclogite/ -400 T (attains mantle density) Model1 Coordinate (km) 1 0 400 C omponentsSedimentary ExoticNAKof Starting crust crust taperedGeometry:cover is an 250oceanic -concept km wide arc from with above Model an passiveaccretionary A co ntinentalwedge margin - geometry- 50 km. wider identical than to Model Model C C ArcticFigure coastline. 13. Com positeNumbers Model in p arenthesesStarting are Geometry.density contrasts Model of constrained units relative by to Airy isostaticcrystalline equilibrium crust at 2 .85 g/cm3.relative to ( V.E.present-day = 4:1). A. 7Masslc (Starting Geometry in Fig. 13) 0 20 40 60 7600 -400 -200 0 km 200 400;

B. Late J rassici _ 0L- Early Cretaceous Y20 40Oceanic arc overrides south-facing- 50 km crustal overlap passive margin above NAK crust 60 -200 0 200 400

0 C. Alban (A) Y20 100 km additional shortening 40Passive margin sediments 150 km total crustal overlap buried -25 km deep 60 T -200 0 200 400

D. Albian (B) 0 2D 50 km extension TL 40 Extension/ unroofingallone 725 km2 removed Cokfootsubtenanetoundergo retrograTle metamorphism 60 -T 'T-i -200 0 200 40C

0 E. Late Cretaceous 20 10 km erosion X40Ruby terrane enters from east `i 500 km2 removed by strike-slip faulting along Kobuk-Malamute fault zone 60 -200A'-0 '' 200 400

0 F. Paleocene E 20 50 km additional shortening 40Contraction delaminates 200 km total crustal overlap 1t of the Ruby terrane - lower crust underthrusts NAK crust 60 -200 0 200 400

0 G. Present Day (See Fig. 15) 20 7.5 km additional erosion EL 40 Erosion lowers topography; 1000 km2 removed Isostatic rebound elevates crustal root; Colville basin deepens 1 60 T-'1 -200 6 200 400 Model Coordinate (km) LEGEND (Density contrasts relative to crystalline crust at 2.85 glcm3) Q-0.35) North (Colvft) Basin Fill -(0) NAK Crust ®(0) Ruby Terrane [fl-0.25) South (Koyukuk) Basin All M0) Uplifted NAK CrustE1+0.45) Mantle

-0.35) Passive Margin Sediments (A-D) =0) Exotic Crust E](-1.82) Water ""'1-0.15) Deformed Sedimentary Rock (E-G)

Figure 14. Composite kinematic model. Step-by-step evolution of the composite model from the starting geometry in Fig. 13 to an interpretation of present day crustal structure in Fig. 15. Progression of gravity anomalies not presented here because it is similar to that shown for model C (Fig. 12). (V.E. = 2:1) 45

0 -20 -40 -s0 -80

South North Cofdbot Srbten ne, Angayucham Terrare, brmerfy buried 21 km deep -10 tar, erosion from remains of obducted arc / Endicott Mountains Afbchthon (EMA)

0m \ Uplifted NAK crust creates Doonerak Antitorrp Crust.6f6e'Ruby ierrane delaminates. Crustal Rdot--44 km, Lower crust thrusts under N4K crust

100 200 Model Coordinate (km)

LEGEND (Density contrasts relative to crystalline crust at 2.85 gfcm3) -0.35) North (CoMfle) Basin Fill -(0) NAK Crust ®(0) Ruby Terrane

Q(-0.25) South (Koyukuk) Basin Fill M0) Uplifted NAK CrustO(+0.45) Mantle EJ-0.15) Deformed Sedimentary Rock Q(0) Exotic Crust

Figure 15.Composite model interpretation of present-day crustalstructure. The composite model is largely based on model C, but integrates concepts tested in models A and B. The Bouguer gravity anomaly produced by present-day crustal structure of the composite model fits observed data over the Brooks Range and the Colville basin. 14). To increase the thickness of the EMA at the conclusion of the composite model, the cross-sectional area of passive margin sediments prior to collision must be increased relative to model C. To achieve this, the composite model widens the passive margin sedimentary cover to 250 km (Fig. 13), thus utilizing the primary feature of model B.

To a first order, the interpretation of present-day crustal structure produced by the compositemodel explainsgravity observationsacrossthe Brooks Range (Fig. 15). End-member principles, tested by models A, B, and C are incorporated into a crustal-scalemodel of the Brooks Range and Colville basin that maintains Airy isostatic equilibrium in space and time.Estimatesof -200 km crustal overlap and up to 17.5 kin exhumation from the compositemodel are similarto those provided by model C. Relative to model C, erosion of the EMA is increased to -10 km, a result closer to estimatesof Blythe and others (1998).

Flexure, Isostasy and the Colville Basin

The composite kinematic model can account for growth of the Colville basin entirely by local (Airy) isostasy. Rifling of the NAK crust and development of the present-day passive margin at the Arctic Coast is important for the modeling process because Airy isostasy implies that the NAK crust was covered by shallow water throughout the modeled time steps. Replacement of water overlying the NAK crust with sediments shed from the eroding Brooks Range deepens the basin because the sediments have a smaller density contrast relative to the crust than does water. Consequently, a column of sediment required to maintain Airy isostatic equilibrium can be three to four times greater than water thickness at the same location. To a first order, local compensation can thus explain the geometry of the Colville basin, a foreland basin that merges into a rifted continental margin at the Arctic Ocean. 4

a) Airy (Local)Isostasy

.0 "Deep Bbuguer Anomaly Low E

High Mountains,

in 0 km effective elastic plate thickness Deep Crustal Root

b) Regional Isostasy (Flexural Rigidity)

a{ Broad, Shallower Bouguer Low E 1 High Mountains-, Flexural Flexural Basin Bulge

Shallower Crustal Root

Plate strength the topographic load

Figure 16. Schematic representation of Airy isostasy and regionalisostasy. a) With Airy isostasy, topography is supported locally by the crustal root directly below. b) In the case where the lithospheric plate has flexural rigidity, the topographic load is supported regionally and a smaller root results. Flexure downwarps the crust on either side of the mountains, forming basins in the hinterland and foreland. Models constructed assuming Airy isostasy assume the crust and underlying material have no rigidity, but in reality the lithosphere has some flexural strength. With regard to foreland basin development, the mountain building process involves loading of thrust sheets onto a lithospheric plate that flexes the foreland downward in response to the loads (Karner and Watts, 1983). Flexure thus enhances the foreland basin because topography of the mountains is compensated regionally, not just beneath the mountains themselves. A shallower crustal root results and, compared to the local (Airy) case, the Bouguer gravity anomaly is higher across the mountains and more negative over the foreland basin (Fig. 16).

A flexural study of the Brooks Range determined an effective elastic plate thickness of 65 km (Nunn and others, 1987). Under the south end of the Colville basin, effective plate thicknesses from 30 km to 90 km produced less than 1 km variation in deflection of the downgoing plate. Nunn and others (1987) also investigated the influence of varying the location of the end of the elastic plate beneath the Brooks Range. Those results also suggest that flexure would enhance the Colville basin by less than 1 km. Thus it appears that the Colville basin would deepen by <2 km if flexure were incorporated into models that assume local isostasy. Flexure is thus a second order factor in the development of the Colville basin, the majority of basin growth can be attributed to isostatic subsidence as water is replaced by basin fill.

Along-Strike Variationsin Evolutionof theBrooks Range

The Bouguer gravity anomaly over the Brooks Range can be used to investigate crustal structure for inaccessible regions east and west of the Dalton Highway. Crustal structure inferred at 145°W and 155°W helps to explain along- strike variations of the Bouguer anomaly. These two gravity models are consistent with surface geology, and compliment the evolution history presented for the central Brooks Range at150°W (Fig. 14).

The profile modeled in detail at 150°W marks a distinct change in character of the Bouguer gravity anomaly and the gross crustal structure that can be inferred from it along the trend of the Brooks Range. That profile is unusual because it has a symmetric Bouguer gravity anomaly low. Transects to the east and west have wider, asymmetric anomalies(Fig. 17)more akin to the positive-negative couple observed over collision orogens (Karner and Watts, 1983). A wider Bouguer low is observed across the NEBR than the central Brooks Range, suggesting greater crustal overlap for this younger portion of the orogen. The Brooks Range may be an "older" and "harder" collision orogen to the west of 150°W where topography is lower and the width of the Bouguer low increases. The reversal of asymmetry that begins near the Dalton Highway corridor may, however, produce an anomalously narrow but steep sided Bouguer low relative to the actual amount of crustal shortening. The section at 150°W does not necessarily represent a minimum of crustal overlap for the Brooks Range.

The "normal" asymmetric Bouguer anomaly along 155°W has a steep gradient on the south and a gentle gradient over the underthrust NAK crust. Along 145°W the Bouguer low displays "reversed" asymmetry compared to 155°W, with a gentle gradient to the south across hinterland crust interpreted to underthrust the Northeastern Brooks Range (NEBR). The hinterland crust transported from 150°W by strike-slip motion (Fig. 14) in the Late Cretaceousis similarto that along 155°W (Fig. 17). North- to northwest-vergent Tertiary convergence that produced the NEBR accounted for growth of the Doonerak antiform but had little influence farther to the west because it was oblique to the trend of the range (Moore and others, 1985). Delamination of the hinterland crust and associated uplift of the NAK crust are therefore more advanced east of the Dalton Highway. The "reversed" 01550 West "Normal" Bouguer Low Asymmetry -20 -40 0 .60 Gentle gradient across E -80 pb%od ed inNaly underthust MAX crust M -100South North Kobuk Strike-Slip Delbrmed sedimentary rocks Arctic Coast Fault thicken to the west 0 %

40-I Hinterland crustsimilar in Thickening of NAK crust unrelated to Crustal Root -42 km geometry to crust transported Tertiary contraction that formed Doonerak antiform and Northeastern Brooks Range 60,west from l5OPW try strike-slip 4 T 0 100 200 Distance (km)300 400 500 1500 West -Symmetric BouguerLow -20

-100South Kobuk-Malamute Arctic Coast% Fault Zone

E Z

100 200 Distance (km) 300 500 0145° West - "Reversed"Bouguer Low Asymmetry LEGEND moaeiea -20 l') -40 Observed [](2.5) NorthBasin Ril a-60 Gentle gradient across currently E-80 I underthrusting hinterland crust SouthBasin All -100 South North I (2.7) Northeastern Brooks Range Del. Sed. Rock 0 =(2.88) NAK Crust ®(2.85) E 20 Uplifted NAK (2.85) Exotic Crust 40 ®(2.85) Thick-skinned tertiary contract on Increases eastward,. Ruby Terrane hinterland crust may now form root Crustal Root -47 km 1 1(3.3) 60 200 Distance (km) 300 Mantle Figure 17. Along-strike variation in crustal structure of the Brooks Range. Tertiary contraction oblique to the trend of the Brooks Range has no influence at 155°W, thus the Bouguer gravity anomaly low has "normal" asymmetry. The extent of thick-skinned tectonics increases eastward, thus causing a symmetric Bouguer low at 150°W and a "reversed" asymmetric anomaly at 145°W. 51 gravity anomaly along 145°W with a gentle gradient south of the orogen suggests the deepest root may be formed by underthrust hinterland crust. Consequently uplift of the NAK crust is more extensive at 145°W than at 150°W. The reversal in asymmetry of the Bouguer gravity anomaly low from west to east may be linked to a change in the vergence of crustal underthrusting. The modeled profile at 150°W represents the zone where symmetry reverses due to uplift of the NAK crust, and a steeper gradient appears on the north side of the Bouguer low.

Global Context of theBrooks RangeOrogeny

Compared to previous applications of kinematic modeling to mountain belts around the world (Lillie, 1991; Lillie and others, 1994), the Brooks Range is best described as a relatively hard and old collision (Fig. 2). Estimates of crustal overlap are slightly inexcessof 150 km,similar tothe Eastern Alps (Fig. 2). Topography of the Brooks Range is lower than that of the Alpsas a consequenceof erosionestimates that are double those for the European orogen. The Brooks Range is an older mountain belt, but still retains a crustal root and its topography has not been lowered to the same extent as the Southern Appalachians.

A comparison has been made between the Brooks Range and the Alps in the past (Fuis and others, 1997). This study supports such analogy (Fig. 18) because 200 km crustal overlap for the Brooks Range is similar to amounts for the Alps (Lillie, 1994). The Brooks Range has lower topography, more exhumation and a shallower crustal root than the Alps because it is older and has undergone more erosion and isostatic rebound. Collisional shortening of the Alps produced the Aar massif (Schmid and others, 1996), a basement uplift structure comparable to the Doonerak antiform. Interpretation of regional seismic reflection data suggest the upper Adriatic Plate became the indenter plate during the most recent period of contraction (Ye and others, 52

1995), splitting at depth such that rigid lower crust underthrusted beneath and uplifted basement (Schmid and others, 1996). Thus, the NAK crustis similarto the European crust that was subducted during initial collision. The most recent period of contraction in the Brooks Range may besimilar tothat of the Alps with the Ruby terrane splitting at depth and its rigid lowercrustunderthrusting NAK crust. Central Brooks Range- Composite Model150°W

Angayucham Endicott Terrace Coldfoot (Deformed KoY ukuk Mountains Colville S/ Basin Subterrane Sediments) Basin N Allochthon N.

(Upper)s Doonerak Antiform NorthernAlaska :20 Ruby (Lower) Crust + Terrane - a

Alps - From Schmid and others (1996)

Penninic Nappes Molasse (Deformed \ Basin SSE Sediments) NNW

Adriatic Mantle

Figure 18: Schematic comparison of the Brooks Range to theAlps. (No vertical exaggeration; horizontal scales approximately equal). 54

CONCLUSIONS

Results Summary

A number of unresolved questions concerning the tectonic evolution of the Brooks Range were posed in the introduction to this project. The results from the composite model (Figs. 13-15) answer some of these questions, and provide insight toward understanding the mountain belt.

1.The Brooks Range has experienced -200 km of crustal overlap since the initial obduction of exotic crust onto transitional northern Alaska (NAK) crust.

2.The combined exhumation effects of erosional unroofing, extension and isostatic rebound have removed -17.5 km from the Coldfoot subterrane, -10 km from the Endicott Mountains allochthon, and-2,225 km2 from the entire profile modeled. 3.The composite model suggests the south-facing passive continental margin of the NAK crust was about 250 km wide prior to collision (Fig. 13). A narrower margin is unlikely because it would have contained insufficient sediment to account for allochthonous sedimentary rocks present in the Brooks Range today. 4.The composite model concludes with uplifted NAK crust underneath the Coldfoot subterrane in the southern Brooks Range (Fig. 15). The deformed NAK crust is supported by underthrust lower crust of the Ruby terrane that has delaminated from the upper crust. 5.Extensional unroofing appears to be an important component in construction of the Brooks Range as it allows the Coldfoot terrane to be uplifted soon after its deepest burial, thus experiencing retrograde metamorphism. Emplacement of the Ruby terrane by dextral strike-slip faulting is consistent with structures in the southern Brooks Range and the regional Bouguer gravity anomaly. A gravity model for 155°W (Fig. 17) interprets a hinterland crust similar to that transported westward from the modeled Dalton Highway profile (Fig. 14). 55

Implications of Results

Kinematic modeling of a transect across the central Brooks Range provides valuable insight into the crustal-scale evolution of the orogen. Bouguer gravity anomalies are products of both the amount of crustal convergence and the amount of exhumation in collisional mountain belts. A tectonic model estimates the Brooks Range has experienced -200 km of crustal overlap and is thus a "hard" collision. The central Brooks Range is a relatively old mountain range that has experienced up to 17.5 km of unroofing and isostatic rebound. The Brooks Range is thus similar to the Eastern Alps, a younger mountain belt that underwent a similar degree of convergence, but has not had enough time for deep exhumation to take place. The preferred tectonic scheme for the Brooks Range involves an arc-continent collision; a period of exhumation by extension and erosion; emplacement of continental thickness crust to the south of the orogen by dextral strike-slip faulting; and a final period of contraction characterized by thick-skinned tectonics where the hinterland crust splits at depth and underthrusts NAK crust. Inclusion of complex tectonic processes whilst retaining isostatic equilibrium represents a major advance in this method of kinematic modeling of continental collision.

This modeling technique could be applied further to profiles along the trend of the Brooks Range, investigating the reversal of Bouguer gravity anomaly asymmetry that exists from west to east. Relatively unconstrained gravity models presented for sections at 155°W and 145°W illustrate Tertiary convergence oblique to the trend of the Brooks Range increasing in magnitude east of the Dalton Highway. The change in vergence of crustal underthrusting in the Tertiary is a potential cause of reversed asymmetry observed along the Bouguer gravity anomaly low for 145°W. Complete kinematic models for 145°W and 155°W could quantify the amounts of collisional shortening that have produced different Bouguer anomalies along-strike. Such models 56 would facilitate a three-dimensional perspective of collision, erosion and isostatic rebound experienced by the Brooks Range. 57

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Oldow, J. S., Seidensticker, C. M., Phelps, J. C., Julien, F. E., Gottschalk, R. R., Boler, K. W., Handschy, J. W., and Ave Lallemant, H. G., 1987, Balanced cross sections through the central Brooks Range, and North Slope, Arctic Alaska: American Association of Petroleum Geologists, Special Publication, 19p., 8pl., scale 1:450,000.

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Wissinger, E. S., Levander, A. and Christensen, N. I., 1997,Seismic imagesof crustal duplexing and continental subduction in the Brooks Range: Journal of Geophysical Research, v. 102, p. 20847-20871.

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INTRODUCTION TO APPENDICES

The following appendices display the numerical inputs to GM-SYS for crustal structure of each model at each time step. The appendices are Excel spreadsheets composed of 1D mass columns that conserve Airy Isostatic equilibrium throughout the model. Listed at the start of each model are unit densities, thicknesses, depths, and areas that are constrained prior to calculations.

An explanation of model units is given in Fig. 5. Young sedimentsrefer to both sediments of the Triassic south-facing passive continental margin andTertiary Colville basin fill. Old sediments are the deformed passive margin sediments that have increased in densityafter burial, lithification, and uplift.

The fixed reference mass column for all models is located at the present-day Arctic coast,and exerts a pressureof17.85 g/cm2 at adepth of 75km. All other mass columns must exert the same pressureof 17.85g/cm2 at 75 km depth. The components of an example 1D mass column are explained in Fig. 19. 64

Model Coordinate Description

175 4.5 eroded

fLayer d/cc In (km) z (krr Ih x d Topo(old) 2.i 0.96 -0.961 2.56 Did Seds -0.1 ° 4.56 0.00 -0.68

Jplifted NAP C 5.00 4.56 0.00

IVAK Crust C) 30.00 9.56 0.00 Check I Mantle 0.4; 35.44 39.5E 15.95 h x d=17.85 g/cm2

75.0CI 75.0C 17.85 Check 2 h mantle +Z mantle = 75 km

Check 3 E of unit heights = 75 km d = unit density (excluding topo) h = unit height Z = elevation of unit top

Figure 19. Example 1D masscolumn. The shaded cells for "Uplifted NAK" and "NAK Crust" heights are constrained in this column. The height of "Old Seds" results from calculating isostatic equilibrium relative to the present-day Arctic coast reference column. Once the height of "Old Seds" is calculated, its value is linked to other cells and the heights of "Topo(old)" and "Mantle" are determined. (Positive notation for "Z" indicates depth below seal level). 65

APPENDIX 1- INITIAL MODEL

Density Constraints Thickness(km) andArea m Constraints I

Topography (young) 2.5 NAK thickness 30 Topography (Koy.) 2.6 Oceanic thickness Topography (old) 2.7 Exotic thickness 27 Topography (crust) 2.85 Initial passive margin sed. thickness 12 Water -1.82 Final Arctic coast sediment thickness 3 Youngsediments -0.35 Base depth of column 75 Old sedimentary roc -0.15 Koyukuk basin -0.25 Area of deformed sedimentary rocks 2033 NAK Crust 0 Exotic Crust 0 Constraints on columns I L Mantle 0.45

Fixed Reference Column: Layer d /cc)h (km)Z(km)h x d Present Day Arctic Coast Young seds .0 NAK Crust 0 30.00 3.00 0.00 Mantle 0.45 42.00 33.00 18.90 75. 75.00 5

300 km before collision

IINITIAL- MODEL- 400lArctic Coast I 100ITransition be!gins 0 Dceanic begins

L-aver Idd /cc Ih (km) Z (km)nt x a Ih (km)Z (km) II x d Ii (km) Z (km)hri x da Mater -1.82 i.ua 0.00 -1_a:, 1.0E 0.00 -1.9"e 1.84 0.001 -3.34

Seds -0.3 0.001 1.0E 0.0() 10.1Ei 1.84l -3.5E

NAK Crust C1 30.00 1.06 0.0C 30.00 1.0E 0.0( 8.00 12.00 0.00

Mantle 0.4 43.94 31.06 19.71 43.94 31.0E 19.77 55.00I 20.00 14. /' , /5.001 /5.01)1 W .U01 75.1)1) /5.LL 1 /.UC 75.00 75.001 17.8&

10C Normal Ocean -200 Seds beain -300 Exotic begins

Layeraver d cc I1(km) Z(km)h x d I1(km)Z(km)hnxa xd I1(km -£(In) " Inxa Mater -1.82 5.421 0.001 -9.861 5.42 0.00 -9.88 1.841 0.001 -3.34 Seds -0.3 0.00 5.42 0.001 0.00 5.42 0.00 10.16 1.841 -3.5E

Exotic C 0.00 12.001 0.OC NAK Crust CI 8.0010f5.42 0.00 8.00 5.42 0.00 8.00 12.001 0.00 Mantle 0.4 61.58 13.42 L7.71 61.58 13.42 27.71 55.00 20.001 24.7 75.1)0 75.001 17.85 75.00 75.UC -i i.oc /5.1)11 75.001 17.8&

400Seds end -500Full Exotic Layer d /cc h(km) Z(km)h x d h(km)Z(km)h x d Water -1.82 1.65 0.00 -3.01 1.65 0.00 -3.01 Seds -0.35 0.00 1.65 0.00 Exotic 0 19.00 1.65 0.00 27.00 1.65 0.00 NAK Crust 0 8.00 20.65 0.00 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 46.35 28.65 20.86 75.001 75. 75. 7. 66

200 km before collision

INITIAL MODEL 400Arctic Coast 100Transitionbegins 0Oceanic begins Layer d cch km Z(km)h x d h(km)Z(km)h x d h km Z(km)h x d Water -1.82 1.06 0.00 -1.92 1.06 0.00 -1.92 1.84 0.00 -3.34 Seds -0.35 0.00 1.06 0.00 10.16 1.84 -3.56 NAK Crust 0 30.00 1.06 0.00 30.00 1.06 0.00 8.00 12.00 0.00

Mantle 0.45 43.94 31.06 19.77 43.94 31. 66 19.77 55.00 20.00 24.75 75.00175.00117.5 75.00 75.001 7.85 75.001 75.00117851

100Normal Ocean -200Exotic begins -300Seds end Layer d /cc h(km)Z(km)h x d h km Z km hxd h kmZ km h x d

Water -1.82 5.42_0.00_-9.86 -9.86_ 1.84 0.00 -3.34 1.65 0.00 -3.01 Seds -0.35 0.00 5.42 0.00 10.16 1.84 -3.56 0.00 1.65 0.00 Exotic 0 0.00 12.00 0.00 19.00 1.65 0.00 NAK Crust 0 8.00 5.42 0.00 8.00 12.00 0.00 8.00 20.65 0.00 Mantle 0.45 61.58 13.42 27.71 55.00 20.00 24.75 46.35 28.65 20.86 75.00175.001 75.00175.0

400Full Exotic Layer d cc h(km)Z(km)h x d Water -1.82 1.65 0.00 -3.01 Exotic 0 27.00 1.65 0.00 NAK Crust 0 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 75.0 75.00 17.85

100 km before collision

INITIAL MODEL 400Arctic Coast 100Transition begin 0Oceanic begins Layer d /cc 1h(km)Z(km)hxd h km Z kmhxd h kmZ km h x d Water -1.82 1.06 0.00 -1.92 1.06 0.00 -1.92 1.84 0.00 -3.34 Seds -0.35 0.00 1.06 0.00 10.16 1.84 -3.56 NAK Crust 0 30.00 1.06 0.00 30.00 1.06 0.00 8.00 12.00 0.00 Mantle 0.45 43.94 31.06 19.77 43.94 31.06 19.77 55.00 20.00 24.75 75. 75.00 17.85 75.0 75.00 17.85175.00 75. 17.85

-100Exotic begins -200Seds end -300Full Exotic

Layer d/cc _h(km)Z(km)h x d h km Z km h x d h km Z km h x d Water -1.82 1.84 0.00 -3.34 1.65 0.00 -3.01 1.65 0.00 -3.01 Seds -0.35 10.16 1.84 -3.56 0.00 1.65 0.00 Exotic 0 0.00 12.00 0.00 19.00 1.65 0.00 27.00 1.65 0.00 NAK Crust 0 8.00 12.00 0.00 8.00 20.65 0.00 0.00 28.65 0.00 Mantle 0.45 55.00 20.00 24.75 46.35 28.65 20.86 46.35 28.65 20.86 rmm= 1 75. 00 5. 1 17.851779.55 71.061 5. 75.001 17.85 67

U rime of collision (topography created)

INITIAL MODEL 40C Arctic Coast 100 Seas begin 50 .- ILayer d cc Ih (km) IZ (km) hnxa x d Ii (km)Z (km) Inxa 1i (km) Z (km) Ih x d Tooo(seds) 2.5 0.24 -0.24 u.s

Water -1.82 1.0E 0.00 -1.92 1.06 0.00I -1.92

Seds -0.35 0.00 1.0E 0.00 9.93 O.OC1 -3.47 NAK Crust C 30.00 1.06 0.00 30.00 1.0Ei 0.00 19.00 9.93I 0.00I Mantle 0.45 43.94 31.06 19.77 43.94 31.0E 19.77 46.07 28.9; 1U.. /5.001 /5.001 1 /.651 (5.00 /5.00 1 /.65 75.00 75.001 17.85

C Thickest serfs -50 -tutSeas end

LayerLaver Id(a/cc) 1h (km)Z km hx d h km Z (km)hhxd x d Ii(km) Z (km)hn x da

Tooo(seds) 2.51 1.2C -1.20 3.00) 0.03 -0.031 0.08 Water -1.82 1.65 0.001 -3.01 Seds -0.35 19.13 0.00 -6.69 10.13 0.00 -3.55 0.00 1.65 O.OC

Exotic Crust C 0.00 19.13 0.00i 9.50 0.00I 0.00 19.00 1.65 0.00

CI 1 NAK Crust 8.00 19.13 0.0011-8.00 10.13 0.00 8.00 20.65 0.00 --.3 1 -i -- -1 - - Mantle 0.45 47.87 z /.1 21.541 56.87 18.13 25.59 46.35 28.65A 20.8E 75.00 75.00 17.65184.50175.01 22.1 75.00 75.001 17.65

200Full Exotic Layer d cc h(km)Z(km)h x d Water -1.82 1.65 0.00 -3.01 Exotic Crust 0 27.00 1.65 0.00 NAK Crust 0 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 75.001 75. 17.85

50 km after collision

INITIAL MODEL 400 Arctic Coast 150 Seds begin 10C Transition beigin laver d /cc fi (km) Z (km)n x a Ii (Km)Z (km) Ih x d Ii (km)Z (km)h x d

Topo(seds) 2.5i 1.74- -1.74 4.34

Water -1.82 1.0Ei 0.001 -1.92 1.06 0.00 -1.92 Seds -0.35 0.00 1.06 0.00 8.43 0.00 -2.95

NAK Crust C1 30.00 1.0E 0.00 30.00 1.06 0.00I 30.00 0.00 0.00 Mantle 0.45 43.94 31.ut 19.77 43.94 31.UU 1u.1/ 45.00 30.00 20.25 r--75.00 1--75.00 17.65 75.00 75.001 17.65 63.43 75.00 21.ti4

5C thickest seas U -50 Seds end

ILaver d /cc I1(km) Z (km)hn x ad 1i (km)Z (km)hxdh x d I, (km) Z (km)h x d Tooo(seds) Z. "-1Z. IL -2.70 6.75 0.03 -0.031 0.08 Water -1.82 1.65 0.00 -3.01

Seds -0.35 17.63 0.00I -6.17 10.13 0.00 -3.55 0.00 1.65 O.OC

I Exotic C 0.00I 17.63 0.00 9.50 10.13 O.OC1 19.00 1.65 0.00

NAK Crust C1 19.00 17.631 0.00 8.00 19.63 0.001 8.00 20.65 O.OC 1 Mantle 0.45 38.37 36.63 17.27 47.37 27.63 21.32 46.35 28.65 20.86 7s nr /h III; 7 /x! /h IN] fh IN . rsr 75 n(1 7s nn 17 RF 68

-150 Layer d cc h(km)Z(km)h x d Water -1.82 1.65 0.00 -3.01 Exotic 0 27.00 1.65 0.00 NAK Crust 0 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 75.00 75.00 17.85

1uu Km of crustal overlap

NITIAL MODEL 400 Arctic Coast Z00 beds be'9i 150 L.aver d(q/cc) Ii (km) Z (km) In x a Ii (km)-Z (km) Ihxd h (km) Z (km)h x d ropo(seds) 2.5 1.74 -1.74- 4.34 Nater -1.82 1.0E O.OC -1.92 1.06 0.00 -1.92 beds -0.35 0.00 1.06 O.OC 8.43 0.00 -2.95 14AK Crust C1 30.00 1.0E 0.0000030.000011.06 0.00 30.00 8.43 0.00 Mantle 0.45 43.94 31.19.77061 43.94 31.06 19.77 36.57'1 38.43 16.4E - i -- ^^-- --, - - 75.001 75.001 17.85 75.001 to.uu 17.85 75.00 75.001 17.85

100 Thickest seds 5C 0 beds end

Laver d(a/cc) lhh (km)km Z(km) lh x d h(km) IZ (km)hxdh x d Ih (km)Z km h x d ropo(seds) z. 4.20 -4.201 10.50 1.53 -1.53 3.83 Nater -1.82 1.65 0.00 -3.01

3eds -0.35 16.13I 0.00 -5.64 8.63 0.00I -3.02 0.00 1.65 0.00 lExotic C) 0.00 16.13 0.00 9.50 8.63 0.00 19.00 1.65 O.OC

I4AK Crust C 30.00 16.131 0.00 19.00 18.13 0.00I 8.00 20.65 0.00 ._ 4antle 0.45 28.87 46.131 12.99 37.87 J7.1J 11.114 46.3511 28.65 20.8E 75.001 75.00 17.85 75.001 75.00 17.85 75.00 75.001 17.85

- -100)I FullF Exotic

Layerf Id(d/cc) Ii (km) Z (km)hxdh x d Water -1.82 1.65 u.uc -3.01

Exotic Cl 27.00 1.65 0.00

NAK Crust CI 0.00I 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 75.00 ia.ut, 17.85

150 km of crustal overlap

NITIAL MODEL 400 Arctic Coast 250 beds begin 200

Laver d cc It1 (km) Z (km)h x d I1 (km)Z (km)hxdh x a In (km) Z (km)h x d Tooo(seds) 2.5 1.74 -1.74 4.34

Water -1.82 1.06i 0.00 -1.92 1.06 0.001 -1.92 beds -0.35 0.00 1.06 0.00 8.43 0.00 -2.95

Exotic C

NAK Crust CI 30.00 1.06 0.00 30.00 1.0Ei 0.00 30.00 8.43 0.00

Mantle 0.45 43.94 31.06 19.77 43.94 31.0E1 19.77 36.57 38.4371 16.4E 751)1 75 O(7 17 151 75 On 75 (101 17 H 75 11(7 75 MI 17 5! 69

150 Thickest seas 100 5E Beds end L.aver d(a/c c)h (km)Z (km)h x d h (km)Z (km) hxdnxa Ii(km)- : IZ-- (km) In x a ropo(seds) 21 4.20 -4.20 10.501 --l3.03 -3.03 7.5E O.OC -0.361 0.00 rooo(crust) 2.85 0.36 -0.3Ei 1.04

Beds -0.35 16.13 0.00I -5.64 7.12 0.00 -2.50 I IO © 1 Exotic CDi0.00 16.13 O.OC1 9.50 7.12 0.0() 18.64 0.00 0.00 NAKI. Crust C1 30.00 16.13 0.00 30.00 16.631, 0.0() 1 018.6400.00 Mantle 0.45 28.87 46.13 12.9,c 28.37 46.63 12.77 37.3E 37.64 16.81 IO.UL 75.001 17.85 /S.UU /S.UU 17.85 15.UU 75.UC 1.oa

0 -50 Full Exotic -100End oceanic NAK .ayer d(g/cc)h km Z km hxd h km Z km hxd h km Z (km h x d Water -1.82 0.86 0.00 -1.56 70,86170.00 1.56 1.65 0.00 3.01 Exotic 0 23.00 0.86 0.00 27.00 0.86 0.00 27.00 1.55 0.00 NAK Crust 0 8.00 23.86 0.00 4.0011 27.86 0.00 0.00 28.65 0.00

Mantle 0.45 43.14 31.86 19.41 43.14 31.86 19.41 46.351 28.651 20.861

75.00 75.00 7M] 75. S I 17.85 5, 75.00 1 17.85

200 km of crustal overlap

NI I IAL MODEL 400IArctic400Arctic Coastcoast 300 Seds begin 25C L.aver d(a/cc)i (km) Z (km)hxdn x a 1 (Km Z (km) hxd h (km) Z (km)h x d rooo(seds) 2.5 1.74 -1.74 4.34 Mater -1.82 1.0E 0.00 -1.92 1.06 0.00 -1.92 Beds -0.35 0.00 1.06 0.00 8.43 0.00 -2.95 4AK Crust 0 30.00 1.0E 0.00 30.00 1.06 0.00 30.00 8.43 0.0( Mantle 0.45 43.94 31.0E 19.77 43.94 31.061 19.77 36.57 38.43 16.4E 75.00 75.00 i .o:. 75.001 75.001 11.655 0.UC /5.UU 1/.u:

200 Thickest SedS 15C 100Beds end

L.aver d(L.a/cc) Ii (km) Z (km h xi h kmKrr Z kmhxd h(km)Z(km) I-ixi

ropo(seds)- 2.5 4.20 -4.20 10.5C 3.031 -3.03 7.5E 0.00 -1.8E 0.00 rooo(crust) 2.85 1.86 -1.86 5.31

Seds -0.35 16.13 0.00 -5.641 7.12I 0.00I -2.50)

I C1 1 O.OC 0.00 IExotic 9.50 7.12 0.00 17.141

C 0 1 0.00I 17.14 0.00 NAK Crust 0.00 16.13 O.OC 30.00 16.63 30.00 © 12.77 27.86 41.14 1454 Mantle 0.45 1u.ts o 46.13012.9E 28.37 46.62 -- -- -)------II ---i 15.UC 75.00 17.85 75.001 15.00 _1(.155 75.00 75.00 17.85

50I Full exotic- 0 100--'End oceanic NAK Layer dg/cc)h (km)Z (km)h x d h (km)Z (km)h x d h km Z km h x d

Topo crust 2.8511 0.9111 -0.91 2.59 II Water -1.82 0.07 1.65 0.00 -3.01 Exotic 01 22.09 0.00 0.00 27.00 0.07 0.00 27.00 1.65 0.00 NAK Crust 0 19.00 22.09 0.00 8.00 27.07 0.00 0-00 28.65 0.00

Mantle- 0.45 33.91 41.09 15.26 39.93 35.07 17.9711 46.35 28.65 20.86

75.0011 75. 7. 5 75.0011 75.00 17.5 75. 75.00 5 70

10 km erosion and rebound for 150 km after collision

INITIAL MODEL 40C Arctic Coast 25C 20C

L.ayer d/cc I1 (km)-z (km) In x a Ii (km)Z (km) Ih x d Ii(km)Z (km)h x d

i . -- 1 Topo(vounal 2.E 0.32 -0.32 0.81 0.00 -0.48 0.00 Tooo(old) 2.1 0.48 -0.48 1.31 (ounq seds -0.36 3.00 0.00 -1.05 4.01 0.00 -1.4C

old seds -0.16i 0.00I 4.01 O.OC 6.18 0.00 -0.9,:1

VAK Crust C 30.00 3.00 0.00 30.00 4.01 0.001 30.00 6.18 0.00

Mantle 0.46 42.001 33.00 18.90 40.99I 34.01 18.45 38.82 36.18- 17.47- 75.00 75.00 17.Ut 75.00 75.001 17.8 75.00 75.00 17.5

150 100 5C

L.aver dp/cc) Ihh (km)km Z (km) Ihxd Ii (km) Z (km) hxd Ih (km) Z (km)hn x ad ropo(seds) 2.7 1.05 -1 _u: 2.84 rooo(crust) 2.86 1.15 -1.15 3.11 0.3E -0.36 1.04 fDid Seds -0.15i 9.1EI O.OC -1.36 1.61 0.00 -0.241

Exotic 0 0.00 9.1E 0.00I 9.50 1.61 0.00 18.64I 0.00 0.00)

C 1 I 1 IAK Crust 30.00 9.1E O.OC 30.001 11.111 0.00 19.00 18.64 0.00

Mantle 0.45 35.82 39.1E 16.12 33.891 41.11 1 15.25 37.3E 37.64 16.81 75.00 75.00 0.00 75.00 75.00 17.651 75.00 75.00 17.5:

0 -50 -100 Layer d /cc h(km)Z km hxd h km Z (km) hxd h(km)Z(km)h x d

To o Ko. 2.6 0.35 -0.35 0.91 0.35 -0.35 0.91 0.35 -0.35 0.91 Koy. Basin -0.25 4.09 0.00 -1.02 4.09 0.00 -1.02 6.66 0.00 -1.66 Exotic 0 23.00 4.09 0.00 27.00 4.09 0.00 27.00 6.66 0.00 Nak Crust 0 8.00 27.09 0.00 4.00 31.09 0.00 0.00 33.66 0.00 Mantle 0.45 39.91 35.09 17.96 39.91 35.09 17.96 41.34 33.66 18.60

0.00175.001 .00 0.00 75.0 75.001 75.00 17.85 71

AJf'1'I ND X 2 - MODEL A

Density Constraints Thickness(km) andArea m Constraints I 1

Topography (young) 2.5 NAK thickness 30 Topography (Koy.) 2.6 NAK thickness at 100 km mark 28 Topography (old) 2.7 Oceanic thickness 8 Topography (crust) 2.85 Exotic thickness 27 Water -1.82 Initial passive margin sed. thickness 12 Young sediments -0.35 Final Arctic coast sediment thickness 3 Old sedimentary roc -0.15 Base depth of column 75 Koyukuk basin -0.25 NAK Crust 0 Area of deformed sedimentary rocks 2031 Exotic Crust 0 Mantle 0.45 Constraints on columns

Fixed Reference Column: Layer d /cch km)Z km h x d Present Day Arctic Coast Young s Ms - 3.00 0.00 -1.051 NAK Crust 0 30.00 3.00 0.00 Mantle 0.45 42.00 33.00 18.90

7 75.001 17.85

300 km before collision TRIASSIC

MODEL A 400Arctic Coast 100Transition begin 0Oceanic begins Layer d /cch km Z(km)h x d h(km)Z km h x d h(km)Z(km)h x d Water -1.82 1.06 0.00 -1.92 1.45 0.00 -2.65 1.84 0.00 -3.34 Seds -0.35 0.00 1.45 0.00 10.16 1.84 -3.56 NAK Crust 0 30.00 1.06 0.00 28.00 1.45 0.00 8.00 12.00 0.00 Mantle 0.45 43.94 31.06 19.77 45.55 29.45 20.50 55.00 20.00 24.75

75.001 75.001 17.851 75.0 75.001 17.85 75.00 75.00 17. 5

10C-1 NormalNormal OceanOcean -200ISeas beam -300 ExoticExotic beamsbegins

l.aver do3/cc) 1h (km) IZ (km hxd h(km) IZ k..:'m hxd Ii (km) Z (km) Ih x a

Nater -1.82 5.42 0.00 -9.8 6 5.42 0.00I -9.861 1.844 0.00 -3.34

Beds -0.3Ei 0.00I 5.42 O.OC1 O.OCI 5.42 0.00I 10.16 1.84- -3.5E

Exotic CI 0.00i 12.0C 0.0(

1VAK Crust CI 8.00 5.42 0.0C1 8.00 5.42 0.001 8.00 12.00 0.0(1

Mantle 0.4! 61.58 13.42 27.71 61.5E1 I3.42 --Z / . -/ 1 55.00 20.00 24.7!i fh 1u', Phtul 17 R!I75 ofI 75 OC lur 75.UU1 75.UU1 7 /.ot

-400Seds end -500Full Exotic La er d cc h(km)Z(km)h x d h(km)Z(km)h x d Water -1.82 1.65 0.00 -3.01 1.65 0.00 -3.01 Seds -0.35 0.00 1.65 0.00 Exotic 0 19.00 1.65 0.00 27.00 1.65 0.00 NAK Crust 0 8.00 20.65 0.00 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 46.35 28.65 20.86

75.0 1 75.001 17.851 75.651 5. 7.85 72

0 Time of collision LATE JURASSIC I EARLY CRETACEOUS .--I MODEL A 400 Arctic Coast 100 Seds beam 5C

L.aver d /cc hI (km) Z (km) Ih x d I1 (km)Z (km) hn x ad Ii (km)Z (km)h x d foDo(seds) 2.5 U.1L -0.10 0.25

Nater -1.82 1.06 O.OC1 -1.92 1.45 0.00 -2.65i

5eds -0.35 0.00 1.45 O.OC 10.0Ei 0.00 -3.52

IVAK Crust C 30.00 1.06 0.00I 28.00 1.45 O.OC) 18.00 10.0E 0.00 Mantle 0.45 43.94 31.06 19.77 45.55 29.45 20.51 46.94 28.06121.12-71 75.UU 75.UU 17.85 75.U0 75.M 17.& 75.00 75.00 17.85

I ""7 0 Thickest seds -5C -1uiSeds end

L.aver d/cc Ii (km)Z (km)hxdh x d Ih (km) Z (km) i x d i (km) Z (km)h x d To[)o(seds)u z.5 1.20 1 _zc 3.001 0.03 -0.03 0.0E Nater -1.82 1.65 O.OC -3.01 Seds -0.35 19.13 0.00 -6.65 10.12 0.00 -3.55 O.OC 1.65 0.00I'.

I Exotic Crustt C1 0.00I 19.13 O.OC) 9.50 10.13 0.00) 19.00 1.65 0.00 F O.OC NAK Crust C 8.00 19.13 0.001 8.00 19.621 0.0C) 8.00 20.65 Mantle 0.45 47.87 27.13 21.54 47.37 27.63 Z1.3e 46.35 28.65 20.8E iaUL 75.UU1 17.85 75.00 75.00 17.85 75.00 75.UU1 17.85

200Full Exotic Layer d cc h(km)Z km h x d Water -1.82 1.65 0.00 -3.01 NAK Crust 0 27.00 1.65 0.00 NAK Crust 0 0.00 28.65 0.00 Mantle 0.45 46.35 28.65 20.86 75.0 75.0 17.85

100 km of crustal overlap ALBIAN

MODEL A 400 Arctic coast 200 Seds begin 150

L.aver dLa/cc) I1 (km) Z (km)hxdn x a Ih (km)Z (km)I h x d h (km) Z (km) 1 x a TODO(seds) 2.5 1.51 -1.51 3.77 Nater -1.82 1.06 O.OC) -1.92 1.32 0.001 -2.41

Seds -0.35 0.00I 1.321 0.0C) 8.65i 0.00 -3.03

NAK Crust CI 30.00 1.06 0.00 28.67 1.321 0.0(1 28.33 8.65 0.00 Jlantle 0.45i 43.94 31.06 19.77 45.01 29.991 20.2t 38.01 36.99 17.11 75.00 75.00 17.85 75.uL 75.001 1 7.85 75.UU (5.00 17.85

100 Thickest seds 50 0Seds end Layer d cc h(km)Z(km)h x d h(km)Z kmLx d h km Z km h x d To o seds 2.5 3.93 -3.93 9.82 1.40 -1.40 3.49 Water -1.82 1.65 0.00 -3.01 Seds -0.35 16.40 0.00 -5.74 8.77 0.00 -3.07 0.00 1.65 0.00 Exotic 0 0.00 16.40 0.00 9.50 8.77 0.00 19.00 1.65 0.00 NAK Crust 0 28.00 16.40 0.00 18.00 18.27 0.00 8.00 20.65 0.00 Mantle 0.45 30.60 44.40 13.77 38.73 36.27 17.43 46.35 28.65 20.86

1 17.8 5 75.001 75.001 17.85 75. 75.0 17.85 75-00 0 73.

-100Full Exotic her d(g/cc)h(km)Z(km)Ex-d-::]

1 Nater 0 1.651 0.0011 -3.01 Exotic 30 27.00 1.65 0.00 VAK Crust 0 0.00 28.65 0.00 Mantle 43.943M 46.35 28.65 20.861

75.0011 011 17.851

150 km of crustal overlap PALEOCENE

MODEL A 40C Arctic Coast 250 Seds begin 200---I

Layer d(_Ncc)i1(km) Z (km)h x d Ii (km)Z (km)hxdn x a I1 (km) Z (km)hxdn x a

ToDo(seds)I 2.° 1.55 -1.5fa 3.89 Water -1.82 1.0E 0.00 -1.92 1.26 0.00 -2.25 Seds -0.3f 0.00 1.2E 0.00 8.61 0.00 -3.01 1 © 0 Uplifted NAI C 00 0.00 8.61 O.OC

I 1 I I I NAK Crust C 30.00 1.0600.00029.0001 1.2600.0C 28.678.61 0 O.OC 1 -- -, - l Mantle 0.45 43.941 31.0Ei 19.77 44.74 30.;26 20.14 's t. it 37.28 16.98 75.00 /b.UL 1 /.k$t 75.00 75.00 17.Sy 75 nC 75.00 1 7.5

150Thickest seds 100 50Seds end Layer d/cc h km Z(km)hxd h km Z (km)0h xd h (km)Z (km)lh x d To o(seds) 2.5 5.88 -5.88 14.71 3.30 -3.30 8.26 Water 1.82 1.65 0.001 -3.01 1

Seds. -0.351 14.44 0.00 -5.06 6.86 0.00 2.40 Exotic 0 0.00 14.44 0.00 9.50 6.86 0.00 19.00 1.65 0.00 Uplifted NAI 0 14.00 14.44 0.00 18.00 16.36 0.00 8.00 20.65 0.00

NAK Crust 0 28.33 28.44 0.00 1 14.00 34.36 0.00 0.00 28.65 0.00

Mantle 0.45 18.22 56.78 8.201 26.64 48.36 11.99 46.35 28.65 20.861

75. 75.00 7. 75.00 75.00 75.001 75.001 17.851

0 -50 I Full Exotic Layer d/cc)h km Z(km)hxd h(km)Z(km)h x d Water -1.82171.65 0.00 -3.01 1.65 0.00 -3.01 Exotic 0 23.00 1.65 0.00 27.00 1.65 0.00 Uplifted NA 0 4.00 24.65 0.00 0.00 28.65 0.00

Mantle 0.45 46.35 28.66t--20.86 1 46.35 28.65 20.86 075.0 75.001 17.851175.00 75.00 17.85 74

16 km erosion and rebound for 150 km after collision MIOCENE

1 - F--MODEL - A 40C Arctic Coast 25C 200 3 eroded

L.aver d/cc) Ih (km) Z (km)h x d ti (km) Z (km) Ii x d Ii (km) Z (km) I1X0 Topo(younq 2. 0.261 -0.26 0.6E 0.00 -0.39l 0.00 Tooo(old) 2.7 0.39 -0.39 1.0E

Younq seds -0.3 3.0CI 0.0CI -1.0 4.381 0.0C -1.5:f

I Did seds -0.1 0.0C1 4.38 0.0() 6.77 O.OC1 -1.02

IUplifted NAI CI 0.00 6.77 0.0CI

NAK Crust C 30.00 3.0C 0.0( 29.00 4.381 0.0Ci 28.67 6.77 O.OC Mantle 0.4, 4L.U0 33.00 18.90141.62 33.38{ 18.73E39.58 35.44 17.8C 75.00 75.00 17.851 75.001 75.00 17.8 75.00 75.00 17.ift

15C 16 eroded 100 10.5 eroded 50 fLaver dlcc Ii (km)Z (km) hnxa x d I1(km)Z (km) Ihxd Ii(km) Z (km)hh x d TOpo(Kov.) 2.E 0.50 -0.5C 1.3C Tooo(old) 2.7 1.74 -1.74 4.70 Topo(crust) 2.8 0.79 -0.79I 2.27

IKov. Basin -0.2 7.21 0.00 -1.8C

Old Seds -0.1 2.59I 0.001 -0.39I

1Exotic C1 0.0 2.59I 0.00 8.37 0.00 0.00 19.00 7.21 0.

Uplifted NAI CJ 14.02.591 I 0.00 18.00 8.37 0.001 8.00 26.21 0.00

INAK Crust CJ 28.33 16.59 0.00 14.00 26.37 0.00 0.00I 34.21 0.00 Mantle 0.4, 30.0E 44.92 13.54 34.63 40.37 15.58 40.79 34.21 18.3 75.00 75.00 17.85 75.00 75.1)0 17.8 75.00 75.00 17.&

0 -50

Layer d/cc h(km) Z(km)h x d h km Z(km) Ih x d To o Ko.) 2.6 0.35 -0.35 0.91 0.35 -0.35 0.91 Koy. Basin -0.25 6.66 0.00 -1.66 6.66 0.00 -1.66 Exotic 0 23.00 6.66 0.00 27.00 6.66 0.00 Uplifted NA 0 4.00 29.66 0.00 0.00 33.66 0.00 Mantle 0.45 41.34 33.66 18.60 41.34 33.66 18.60 75.00175.00117.85175.00175.00 17.85 APPENDIX 3 -MODEL B

DensityConstraints Thickness (km) and Area (km) Constraints

Topography (young) Nak thickness 30 Topography (Koy.) 2.6 Nak thickness beneath rift basin 10 Topography (old) 2.7 Transition NAK maximum thickness Topography (crust) 2.85 'Oceanic thickness 8 Water -1.82 Initial pasive margin sediment depth 12 Young sediments Thickest passive margin sediments 11 Old sedimentary rock -0.15 Final Arctic Coast sediment thickness 3 Koyukuk basin -0.25 Base depth of column 75 NAK Crust Oceanic NAK Crust 0 Area of rift basin sediments 1100 Mantle 0.45 Area of sediment above transition NAK 2437 Area of sediment above oceanic crust 1516 Area of deformed NAK crust 750 Area of transitional N^.K crust 4850

Fixed Reference Column: Layer d /cc)h (km) Z(km)h x d Present Day Arctic Coast Young Seds -0.35 3.00 0.00 -1.05 NAK Crust 0 30.00 3.00 0.00 Mantle 0.45 42.00 33.00 18.90 75.00 75.00 17.85

TRIASSIC STARTING GEOMETRY

MODEL B 400Arctic Coast 100Sediments begin 50Rift basin Layer d/cch km Z(km) h x d h km Z(km) h x d h(km)Z(km) h x d Water -1.82 1.06 0.00 -1.92 1.06 0.00 -1.92 1.14 0.00 -2.08 Seds -0.35 11.00 1.14 -3.85 Nak Crust 0 30.00 1.06 0.00 30.00 1.06 0.00 10.00 12.15 0.00 Mantle 0.45 43.94 31.06 19.77 43.94 31.06 19.77 52.85 22.15 23.78 75.00 75.00117.85 75.00 75.00 17.85 75.00 75.00 17.85

0 -350Oceanic begins -600Sediments end Layer d /cc h(km)Z(km) h x d h(km)Z(km) hxdh km Z km h x d Water -1.82 1.23 0.00 -2.24 1.84 0.00 -3.34 5.42 0.00 -9.86 Seds -0.35 5.13 1.23 -1.80 1016 1.84 -3.56 Nak Crust 0 20.00 6.36 0.00 8.00 12.00 0.00 8.00 5.42 0.00 Mantle 0.45 48.64 26.36 21.89 55.00 20.00 24.75 61.58 13.42 27.71

75.00 75.00 17.85 75.001 75.00 17.85175.00175.001 17.85 76

LATE JURASSIC I EARLY CRETACEOUS Time of Collision

MODEL B 400 Arctic Coast 0 -100 UCearnC N

L.aver I ; t Rn 1 dgicc)h (km) Z (km nxa h k..: ZT)-I 1hxd i (km)1'Z (km)hh xx da Mater -1.82 1.06 0.00 -1.921 1.231 0.001 -2.241 0.951 O.OC -1.72 Topo Beds 2.51 Ubduct seds -0.35

IBuried seds -0.35 5.13{ 1.231 -1.8() 7.87 0.95 -2.75i _i -- _i INAK Crust C 30.00 1.06 -u.u( - 20.00 "b.sc - 0.00 16.57 8.81 O.OC

IMantle 0.45i 43.94 31.06{ 19.77 48.64 26.3Ei 21.89 49.62 25.38 22.33 75.00 75.00 17.851 75.001 75.00 17.85 75.00 75.00 17.85

-150 -200 -250

L,aver d/cc Ih (km) Z (km) 1 x a f1 (km) IZ (km) hxd I1 (km)Z (km) Ih x d - 6 Togo seds 2.5 0.14 -0.14 0.31 1.57 -1.57 3.81 2.36 z.M 5.91

Iabduct seds -0.35 6.59 0.00 -2.31- 5.17 0.001 -1.81 4.37 0.00 -1.531 Obdiuct Oc. C 8.00 5.17 0.00 8.00 4.37 0.00I Buried seds -0.35 9.23 6.59 -3.23 10.6C 13.17 -3.71 11.97 12.37 -4.19

INAK Crust C 8.00 15.82 0.00 - _ Dceanic NAI C 8.00 Zs. i I 0.00 11.43 24.34 0.00I

IMantle 0.45i 51.181 23.82 23.033 43.23 31.77 19.45 39.24 35.76 17.6Ei 75.oc 75.001 17.85 75.00 75.001 17.85 75.00 75.00 17.85

-300 -350

L.aver d cc h (km) Z (km) 1 x a 11(km) IZ (km) Ih x d Topo seds 2.5 1.41 -1.41 3.5; Water -1.82 4.23 0.00 -7.70

Dbductseds -0.35 5.32 0.00 -1.86 3.37 4.231 -1.18

abduct Oc. C 16.00 5.32 0.00 8.00 7.6(1 0.00

Oceanic NAI C 17.71 Z1.31 0.00-

Mantle 0.45 35.9E 39.04 1b.1 t 59.40 15.6C1 26.73 75.00 75.00 17.85 75.00 75.001 17.85

ALBIAN (A) 200 km after collision

MODEL B 400 Arctic Coast 20C 15C

l..aver d(gicc) I1 (km)Z (km) h x d Ii (km) Z (km)h x d f1 (km) Z (km)hxdn x a MEWAMMiM ropo(seds) 2.5 1.69 -1.69 4.22 Nater -1.82 1.06 0.00 -1.92 1.06 0 -1-y- Beds -0.35 8.281 0.00 -2.90

IIak Crust 0 30.00 1.06-- 0.00 30.00 1.06- - 0.00 30.00 8.28 0.00 Mantle 0.45 43.94 31.06 19.77 43.94 31.06 19.77 36.72 38.28 16.52 75.00 75.001 17.85175.00 75.00 17.85 75.00 75.00 17.85

100 75 5C

L.aver Ii1 ccIhh (km)(km) .Z(km)hxdn x a h1 (Kmkm Z kmhxd It1(km) Z(km) hxd Topo(seds) I 2.5 4.11 -4.11 10.2E 5.79 -5.79 14.47 5. ut -5.06 12.64

IDbduct sedsl -0.35 1.68I 0.00 -0.59 - rBuried seds I -0.35 15.83. 0.00 -5.54 22.50 0.00 -7.87 19.94 1.68 -b.aa Trans. NAK I C 3ef NAK I C 15.00 15.83 0.001 20.00 zz.5u' " 0.001 25.00 21.62 0.00

I C 1 1 4AK Crust I 15.001 30.831 0.00 7.50 42.50 0.00

IMantle 0.45 29.17 45.83 13.131 25.00I 50.00 11.25128.38 46.62 12.77 75.00 75.00 1 7.851 75.00 75.00 17.85 75.00 10. UL 17.85 77

25 C -25

L I I Km I .aver d(x)h (km)Z (km)h x d h (km) Z (km)h x d hkmZI i) I1 x a Fopo(seds) 2.5 4.29 -4.29 iu.r; 3.0d -3.53 8.82 3.92 -3.92 9.81 Dbduct seds -0.35 2.44 1.00 -0.85 3.21 0.0 -1.11 2.81 0.00 -0.96 , , cDbduct Oc. C 4.00 2.44 0.00 8.00 3.21 0.00 8.00 z.oi 0.00 3uried seds -0.35 14.95 6.44 -5.2; a.ar i1.z1 -3.49 4.98 10.81 -1.74

Trans. NAK C 11.76 21.40 O.OC1 23.52 21.18 0.00 35.27 15.80 0.00

IAK Crust C1 12.50 33.15 0.OC1 Mantle 0.45 29.35 45.65 13.21 30.31 44.69 13.64 23.93 51.uj 10.77 -- io.ut 75.00 17.851 75.OC 75.00-1 17.85 75.00 75.00 1 i.ts5

-50 -100 -125 Layer d cchkm Z(km)h x d h km Z km h x d hkm Z(km)h x d To o sods 2.5 2.72 -2.72 6.79 3.81 -3.81 9.52 1.90 -1.90 4.75 Obduct serfs -0.35 4.02 0.00 -1.41 2.93 0.00 -1.03 1.47 0.00 -0.51 Obduct Oc. 0 8.00 4.02 0.00 16.00 2.93 0.00 8.00 1.47 0.00 Trans. NAK 0 35.27 12.02 0.00 35.27 18.93 0.00 35.27 9.47 0.00 Mantle 0.45 27.71 47.29 12.47 20.80 54.20 9.36 30.26 44.74 13.62 75.00 75.00 17.85 75.00 75.00 17.85 75.00 75.00 17.85

ALBIAN (B) 100 km minimum extension

MODEL B 5C 25 C

L.aver 7d(a/cc) Ih (km) ;Z (km) Ii x d I1 (km) Z (km)hxdi x a Ii (km)Z (km) h x d

Topo(seds) I 2.5 3.42 -3.42 8.5E 2.11 -2.11 5.29 0.80 -0.8C 2.01 r 5eds -0.35 16.51 0.00 -5.7ES 12.84i 0.00 -4.49 9.16 0.00 -3.21 Trans. NAK C 11.76 i2.ts4"'1 0.00- 23.52 9.16 0.00

Def Nak CI 25.001 16.511 0.00-11 12.50 24.6C1 0.00

Mantle 0.45; 33.49I 41.51 15.07 37.9C1 37.1CI 17.06 42.32 32.681 19.04

1 -- ---1 75.0C 75.00 17.857 75.1K; 75.00-11 17.85il75.00 75.00 1 I.t35

-25 -5U It

h (km)Z (km) hxdnxa Ii (km) Z (km) 1h x d I1(km)Z (km) Ih x d Topo(seds) 2.5 LA -1.LU 3.001 1.62 1.6e 4.Ut 2.17 -2.17 5.42 Topo(crust) 2.85 Mater -1 _n:O -2 Beds -0.35 3.7E 0.00 -1.3; 5.11 0.001 -1.79 4.57 0.00 -1.60

abduct Oc. CI 4.00 4.57 0.00

Trans. NAK C 35.27 3.78 0.00 35.27 5.11 O.OC 35.27 8.571 O.OC Mantle F=10.45 35.94 39.01E 16.17 34.62 40.3E 15.55 31.1 ti 43.84 14.02 75.00 75.00 17.85 75.OU 75.001 17.85 75.00 75.00 1 tz

-1 uU -1z" -15t

Ih (km)Z (km) Ihxd Ii (km)Z (km) Ihxd II (km)Z (km)h x d ropo (crust) 2.85 1.0E -1.un 3.09

Mater -1.82 0.01 O.OC1 -0.02 3.04 0.00 -5.54 Dbduct seds -0.35 b. 14 3.04 -2.3E

Olbduct Oc. 0 6.92 O.OCI 0.00- 8.00 9.78 O.OC Trans.Trans. NAKNAK 0 35.27 6.92 0.00 35.27 0.01 0.00I Mantle 0.45 32.81 42A S 14.7E 39.72 35.2E 17.87 57.22 17. is 25.75 75.00 75.00 1 /.CC 75 on 75.001 0.00 75.00 75.00 17.85 78

-225 h km Z km h x d Water -1.82 1.46 0.00 -2.66 Obduct seds -0.35 6.74 1.46 -2.36

Obduct Oc. 0 16.00 8.19 1 0.00 Mantle 0.45 50.81 24.19 22.86 75.00 75.00 17.85

LATE CRETACEOUS 20 km Erosion

' MODEL B 400 Arctic Coast 200 Deepest FB seds 15CI 5 eroded L.aver h (km) IZ (km)hxdn x a ti (km) Z (km)h x d Ii (km)- 1Z- (km)-_i x d Tooo(old) 2.7 0.18 -0.1800.48 Nater -1.82 1.06 0.00 -1

Young seds -0.35 3.OC1 0.00 -1 0 Oldseds- -0.15 4.79 0.0 -0.72

Def NAK C " 1AK Crust c1 30.00 1.06 0.0C7 30.00 3.OCI 0.00I 0.00 4.7900.00 - - l

l II 18.09 Mantle 0.45i 43.94 31.06 19.77 42.00133.00 18.90 40.21034.790 75.00175.001 17.851 75.00II 75.0[ 17.85 75.001 75.001 17.85 0 1 J- r- -

100 14 eroded 75 20 eroded 50 11 eroded

L.aver d( ii (km)Z (km) Ih x d Ii (km) Z (km) 1 1hn x d h- (km)k( m) Z (km)h x d 1-000(old) 2.7 0.35 -0.35 0.95 0.44 -0.44 1.18 0.22 -0.22 0.58

1 - 1.31 tNd seds -0.15i 5.58 O.OC -0.84L 7.85 0.00 -1At ts.ii"-"'.00.00 - Trans. NAK 0 -1 0.01- f7ef NAK C1 15.00 5.58 0.0C1 20.0007.85 10.00 25.00 8.72 1AK- Crust- C 15.00 20.58 0.0C1 7.50 27.85 0.00 17.74 17.841 41.28033.72 18.58 Mantle 0.45 39.42 35.58 39.65 35.35 _-75.00 - 75.001-- -- 17.85- --75.00 -) 75.00 17.85 75.00-- 75.001 17.85

25 6 eroded 01 1 eroded -25 3.5 eroded 'Layer d(g/cc)h (km)Z (km)h x d h (km)Z (km)h x d h kmt]IZ km h x d

To old 2.71 1 0.12 -0.12 0.32 0.02 -0.02 0.05 0.26 -0.26 0.71 Old seds -0.15 8.83 0.00 -1.33 8.95 0.00 -1.34 1.22 0.00 -0.18 Trans. NAK 0 11.76 8.831 0.00 23.52 8.95 0.00 35.27 1.22 0.00 Def NAK 0 12.50 20.59 0.00

AS11 Mantle 41.91 33.0911 18.86 42.54 32.46 19.14 38.50 36.50 17.33

75.00 75.00 17.85 75.00175.00 17.85 75.001 75.00 17.85

-5C 5 eroded -r5 8.5 eroded -100 6.5 eroded 1 i (km) h x d L.aver d(9/cc) 1(km) Z (km)h x d i (km) Z (km)hxdn x a; ....- Z (km) ToDo(old) 2.7 0.31 -0.31 0.83 Topo(crust) 2.85 0.30 -0.30 0.85 0.20 -0.20 0.56 abduct= seds=I -0.15- 1.43-I u.u0-- -0.21 Obduct Oc. 1.94 0.00 0.00 `i.3u "1 0.00I00.0( Trans. NAK (1 35.27 1.421 0.00 35.27 1.94 0.0C1 35.27 1.30 0.00 -- -1 -- 1 1 Mantle 0.45 38.3( 36.7C 1 --3t.za s/.tb----037.21- 17.OC 38.42 36.58 17.2; 1 -_711 75.00 75.0C-- 1 17.85 75.0 75.00 17.851 75.00 75.00 17.85 79

PALEOCENE 60 km Ruby Involved

MODEL B 400-, 225 200--i

Id(a/cc)1 (km)Z (km)hxdnxa h (km)km Z (km) 1nxa 1i(km) IZ (km) I1 x d - FoDo(vouna)I 2.5i 0.08 -0.08 0.201 0.24 -u.z4 U.b-1 roDo(old) I 2.7 tWater -1.82 1.06 0.00 -1.92- Young.. seds I -0.35 3.25 O.OC -1.14 1.43 O.OC -0.50 :)Id seds I -0.15 3.OE 1.43I -0.46

Def NAK I C NAK crustI 0I 30.00 1.06 0.00 30.00 3.25 0.00

Mantle 0.45 ' 43.94 31.06 19.77 41.75 33.,'25 18.7E) 40.4E1 34.51 1 tf.LL 75.00 75.00 17.851 75.001 75.00 17.85175.00-A 75.00 17.26

175 150 125 d/cc h(km)Z(km)h x d h km Z(km)hxd h(km)Z(km)h x d To 0 old 2.7 1.42 -1.42 3.82 3.00 -3.00 8.10 3.22 -3.22 8.69 Old serfs -0.15 4.75 0.00 -0.71 6.25 0.00 -0.94 9.11 0.00 -1.37 Def NAK 0 7.50 4.75 0.00 15.00 6.25 0.00 20.00 9.11 0.00 NAK crust 0 30.00 12.25 0.00 30.00 21.25 0.00 22.50 29.11 0.00 Mantle 0.45 32.75 42.25 14.74 23.75 51.25 10.69 23.39 51.61 10.53 75.00 75.00 17.85 75.00 75.001 17.85 75.00 75.00 17.85

10Cl 75- 50- -,

d cch km ;Z (km) hxd I1 (km)-t(Km) in x a I1 (km)Z (km) Ih x d ToDo(old) 2.7 2.43 -2.43 6.5E 1.9E -1.981 5.36 1.54 1.54 3.10 Did serfs -0.15 7.43 0.00 -1.11 5.41 0.00 -0.81 3.39 0.00 -0.51

Trans. NAK 0 11.7E 5.41 0.00 23.52 3.3E1 0.0C

Def NAK 0 25.00 -7.43 O.OC) 12.50 11.1 i 0.00

INAK crust C) 15.00 32.43 O.OC 7.50 29.67 0.00

Trans. NAK C 8.26 37.17 0.00 18.52 26.91 O.OC --- , -I IMantle 0.45i zt.51 47.43 12.41 29.57 45.43 1s.s1 31.57 43.43 14.21 75.00 75.00 17.85 75.0C 75.u0 17.85 75.00 75.001 17.85

25-i 15 -35-

d(q/cc) Ih (km) Z (km) I1 x d 1i (km) Z (km)hxd II (km) IZ- (km)hxdh x d ropo (old) 2.7 0.44I -0.44 m%,- - 0.631- -- -0.63- -l 1.7C Youna seds -0.25 1.74 0.85 -0.43I 1.50- 0.001 0.00 Dld reds -0.15 z.u;4 0.00 -0.301 0.851 0.00 u.1j

oceanic C

Trans. NAK C 35.27 2.03c 0.00I 35.27 2.59 0.0C 33.83 1.50 O.OC-- 1 Mantle 0.45 37.70---1 3/.30--- 16.97 37.14) 37.8Ei 16.71i6.t1 39.67 35.33 17.85 75.0C 75.00 17.85 75.0U 75.00 17.85 75.00 75.00 1 t.t 80

PRESENT DAY 11 km maximum final erosion

MODEL B 400 Arctic Coastline 225 2W L.aver d(a/cc) Ih (km)Z km h x d fi (km) Z (km)nh x ad I",(m Z (km) In x a ropo(young) I 2.5i 0.55 -0.55 1.38 0.75 -0.75i 1.88 New seds -0.35 1.3E 0.00 -0.48 1.37 0.0( -0.48 ".-5 (oung seds -0.35 3.00 0.00 -1_ut 3.33 1.3 -1.17 1.67 1.37 -0.58

lid seds -0.15 3.08 3.03i -0.46 _ IVAK crust C 30.00 3.00 0.0(1 30.00 4.72 0.00 30:00 6.11 0.0C-

IMantle 0.45 --12.OC I 33.00- 18.9C) 40.28I 34.72 18.13- L 38.89 36.11 17.5( MUL 75.00 17.85 73.61 75.00 17.85 73.63 75.0( 1 t.ts,,

175 1:S 150 7,5 125 1 f Layer d/cc h (km)Z km h x d h km Z km hxd h(km)Z(km)h x d To 0 old 2.7 1.14 -1.14 3.09 1.64 -1.64 4.42 1.22 -1.22 3.29 Old seds -0.15 3.52 0.00 -0.53 0.11 0.00 -0.02 0.11 0.00 -0.02 Def NAK 0 7.50 3.52 0.00 15.00 0.11 0.00 20.00 0.11 0.00 NAK crust 0 30.00 11.02 0.00 30.00 15.11 0.00 22.50 20.11 0.00 Mantle 0.45 33.98 41.02 15.29 29.89 45.11 13.45 32.39 42.61 14.58 75.00 75.00 17.85 75.00 75.00 17.85 75.00 75.00 17.85

100 ::::8 75 :::'65: 50 AS Layer d cc h kmZ km hxd h km Z km h x d h(km)Z(km)h x d Topo old 2.7 0.97 -0.97 17.85 0.80 -0.80 2.17 0.43 -0.72 1.16 To o crust 2.85 0.29 -0.29 0.82 Old seds -0.15 0.89 0.00 -0.13 0.09 0.00 -0.01 Trans. NAK 0 11.76 0.09 0.00 23.23 0.00 0.00 Def NAK 0 25.00 0.89 0.00 12.50 11.85 0.00 NAK crust 0 15.00 25.89 0.00 7.50 24.35 0.00 Trans. NAK 0 8.26 31.85 0.00 16.52 23.23 0.00 Mantle 0.45 34.11 40.89 15.35 34.89 40.11 15.70 35.25 39.75 15.86

75.00 75.00 33.071 75.00175.00 17.85 75.00 75.00 17.85

25 15 -1t

L.aver d(a/cc h (km) Z (km) I1 x d I1 (km) Z (km) 1hxd Ii(km)Z (km)hxdn x a Fopo(old) 2.7 0.64 -0.64 1.(2 0.63 -0.63 1.7C 0.52 -0.52 1.4(

(ouna seds -0.25 0.38I 0.00 -0.09 2.001 0.00 -0.5( 5 did seds -0.1 2.47 0.38 -0.37 1.14 0.00 -0.17 Dbduct seds -0.15 1.i4 1.14 -0.2E

oceanic C 2.24 2.00 0.0(

Trans. NAK C 35.27 2.84 0.00 35.27 2Z8 0.001 33.10 4.23-- 0.0(- Mantle 0.45 36.88 38.12 16.60 36.85 3t$.1 It 1 ti. btl 37.67 37.33 16.9: 75.00 75.OC 17.85 75.00 75.00 17.85 75.00 75.00-- 1-- 1.o;

-35 6.5 -60

L.aver d(a/cc I1(km)Z (km) Ii x d I1 (km) Z (km) Ih x d Total Erosion -ropo(seds) 2.7 0.431 -0.43 --1.1 i 0.35 -0.35- 0.95 Young seds -0.35 1.46 0.0C -0.51 1.1b.. 0.01 -0.41

Dceanic CI 1.50I 1.4E 0.00 Final Erosion Trans. NAK 0 33.83 2.9E 0.00 35.33 1.1 t 0.00

Mantle 0.45 38.20 36.8C WAS11 38.491 36.51 17.32 7h MI 75 1x' 17 R5 75 nrl 7-5 nr 1 1 w' 81

APPENDIX 4 - MODEL C

Density Constraints Thickness mandArea Cons =n s I I

Topography (young) 2.5 NAK thickness 30 Topography (Koy.) 2.6 Oceanic thickness' Topography (old Outer Arc crustal thickness 16 Topography (crust) 2.85 Arc thickness 25 Water -1.82 Ruby Terrane thickness 33 Youngsediments -0.35 Initial pasive margin sed. thickness 12 Old sedimentary rock -0.15 Final Arctic Coast sediment thickness Koyukuk basin -0.25 'Base depth of column 75 NAK Crust Exotic 0 krea of deformed sedimentary rocks 1016 RubyTerrane area def. sed. rock after Ruby enters 935 ,Mantle 0.45

Fixed Reference Column: Layer d cc)h km Z(km)h x d Present Day Arctic Coast Young Seds - 3.00 0.00 -1.05 NAK Crust 0 30.00 3.00 0.00 Mantle 0.45 42.00 33.00 18.90 -79-MT75.00 7.

300 km before collision TRIASSIC

MODEL C 400Arctic Coast 100Seds begin 0Oceanic begins Layer d/cch km Z km h x d h(km)Z(km)h x d h(km)Z (km)h x d Water -1.82 1.06 0.00 -1.92 1.06 0.00 -1.92 1.84 0.00 -3.34 Seds -0.35 0.00 1.06 0.00 10.16 1.84 -3.56 NAK Crust 0 30.00 1.06 0.00 30.00 1.06 0.00 8.00 12.00 0.00 Mantle 0.45 43.94 31.06 19.77 43.94 31.06 19.77 55.00 20.00 24.75

75.00175.001 7.85 75.00 75.00 17.5 75.00 75.00 17. 5

100Seds end -300Trench -350Outer Arc high Layer d /cch km Z km hxd h(km)Z(km)hxd h km Z(km)h x d Water -1.82 5.42 0.00 -9.86 5.42 0.00 -9.86 2.25 0.00 -4.00 Seds -0.35 0.00 5.42 0.00 Exotic 0 0.00 5.42 0.00 16.00 2.25 0.C+1-; NAK Crust 0 8.00 5.42 0.00 8.00 5.42 0.00 8.00 18.25 0.00 Mantle 0.45 61.58 13.42 27.71 61.58 13.42 27.71 48.75 26.25 21.94

75.00 75.001 17.85175.00 75.001 17.851 75.001 75.00 17.85

-400Forearcbasin 500Arc Layer d/cch km Z km hxd h(km)Z km h x d Water -1.82 5.42 0.00 -9.86 2.05 0.00 -3.73 Exotic 0 8.00 5.42 0.00 25.00 2.05 0.00 Mantle 0.45 61.58 13.42 27.71 47.95 27.05 21.58

75.001 75.00117.85 75.00 75.001 17.85

NOTE: Exotic crust reverts to normal oceanic structure south of 600 km mark 82

50 km after collision LATE JURASSIC / EARLY CRETACEOUS

MODEL C 400Arctic Coast 100Seds begin 50Exotic begins Layer d cch km Z km h x d h km Z (km)h x d h(km)Z km h x d Water -1.82 1.06 0.00 -1.92 1.06 0.00 -1.92 1.45 0.00 Exotic 0 0.00 1.45 0.00 Seds -0.35 0.00 1.06 0.00 5.08 1.45 -1.7 FI, NAK Crust 0 1.06 1.06 0.00 19.00 6.53 0.00 30.00. 0.00 30.00

Mantle 0.45 43.941 31.06 19.77 43.94 31.06 19.77 49.47 25.53 22.2.0 75.00 75.001 7.85 75.00175.001 17.85 75.00 75.0 7.U5

0Outer Arc high -50 -100Seds end

Layer d/cch(km)Z(km) 1h x h(km)Z(km)h x d h km Z(km)h x d Topo(crust) 2.85 0.92 -0.92 2.62 Water -1.82 2.04 0.00 -3.72 2.15 0.00 -3.91 Exotic 0 15.08 0.00 0.00 8.00 2.04 0.00 16.50 2.15 0.00 Seds -0.35 10.16 15.08 -3.56 5.08 10.04 -1.78 0.00 18.65 0.00 NAK Crust 0 8.00 25.24 0.00 8.00 15.12 0.00 8.00 18.65 0.00 Mantle 0.45 41.76 33.24 18.79 51.88 23.12 23.34 48.35 26.65 21.76

7500 75.00 17.85175.00 75C,0 1 7.85 750U 17.85

-151 Arc -250 Iormal ocean fLayer 1(g/cc) I1 (km) Z (km) Ih x d I1 (km) ;Z (km)hxdn x a Water -1.82 2.05 0.00 -3.73 5.42 0.00 -9.8E

Exotic C 25.00 2.05 0.0C1 8.O0 5.42 0.0C)

NAK Crust C 0.00I 27.05 0.0C1 Mantle 0.45 47.95 27.05 21 61.5 13.42 27.71 75.U0 75.001 17.85 75.0011 75.00- 17.55 71

150 km of crustal overlap ALBIAN (A)

VIOULL c 400 Arctic coast zoolseas begin 15C Exotic beginsbeains L.aver d/cch km Z (km)nxah x d hh (km)Z (km) Ih x d I1(km) Z (km)hh x d rooo(seds) 2.5 0.0E -0.06 0.15 Nater 1.82 1.06 0.00 -1.922 1.0E 0.001 -1.92

IExotic C 0.00)

Seds -0.35 0.0C1 1.061 0.00) 3.1fi 0.00 -1.12

1VAK Crust C 30.00 1.06 0.00 30.00 1.061 0.0() 30.00 3.1£ 0.0C Mantle 0.45 43.94 31.06 19.77 43.94 31.061 19.77 41.81 33.1£ its.tsl 75.001 75.UUI 17.551 75.00 75.001 l fist 75.00 75.001 17.55

100Outer Arc High 75Thickest Seds 50 Lay er d cch km Z(km)hxd h km Z(km)hxd h(km)Z(km)h x d To o crust 2.85 3.03 -3.03 8.64 2.13 -2.13 6.07 0.44 -0.44 1.26 Exotic 0 12.97 0.00 0.00 9.87 0.00 0.00 7.56 0.00 0.00 Seds -0.35 6.50 12.97 -2.28 8.13 9.87 -2.85 6.50 7.56 -2.28 NAK Crust 0 30.00 19.47 0.00 24.50 18.00 0.00 19.00 14.06 0.00 Mantle 0.45 25.53 49.47 11.49 32.50 42.50 14.62 41.94 33.06 18.87

75.00175.001 17.85 75.00175.001 17.85175.00175.001 17.85 83

0 -50Arc -100 Layer d cch(km)Z km h x d h km Z km h x d h km Z km h x d Water -1.821 1.00 0.00 -1.82 0.46 0.00 -0.84 2.15 0.00 -3.91

Exotic 0 16,5011 1.00 0.00 25.00 0.46 0.00 16.50 2.15 0.00 Seds -0.35 3.25 17.50 1.14 0.00 25.46 0.00

NAK Crust 0 1 8.0011 20.75 0.00 8.00 25.46 0.0011 8.0011 18.65 0.00

Mantle 0.45 46.25 28.75 20.81 41.54 33.46 18.6911 48.35 26.65 21.76

1 1 1 75.001 1 75.0011 17.851 75.001 75.001 17.85 75.00 75.00 17. 5

-150Normal Ocean rLayer d(g/cc) h km Z km hxd twater -1.82 5.42 0.00 -9.86 Exotic 0 8.00 5.42 0.00 NAK Crust 0 0.00 13.42 0.00 Mantle 0.45 61.58 13.42 75.00 75.00 17.851

50 km Extension ALBIAN (B) Q

- . MODEL C 75 Unchanoledt ` 50--I VorthI tip -of - fault- 25

ILayer Id(g/cc)d cc 11 (km) Z (km)n x a h (km) ? (km) Ih x d Ih (km) Z (km)h x d 1 i 1 III* IL hTd-Ii 7 ------1 Touo(crust) 7 2.85 2.13 -2.13t 6.07 0.44 -0.44 1.26i 01.0C 0.00 1.82 Water -1.82 I i I Exotic 0IO9.87 0.001 0.00 7.56i00.000O.OC1 8.13i 1.00IO-0.0C Seds -0.35 8.13 9.87 -2.85 6.50 7.56 -2.2E1 4.88 9.12 -1.71

NAK Crust 0 24.50 18.00 O.OC 19.00 14.06 0.0C1 13.50 14.00 0.0(

Mantle 14.62 18.81 47.5C 27.501 21.37 0.45 32.50 42.50 41.x4 33.06 IF 11 75.00 75.001 17.5t 75.07 75.UU 17.5 t ia.u 75.001 17.55

0 -50Arc Base -100lArc To h km Z km h x d h km Z kmhxd h km Z km h x d I

Water -1.82 2.64 1 0.00 -4.80 1.31 0.00 -2.3811 1.31 0.00 -2.38

Upper Block 0 0.00 2.64 0.00 8.25 1.31 0.00 12.5011 1.31 0.00 Exotic 01 8.25 2.64 0.00 12.50 9.56 0.00 8.25 13.81 0.00 Seds -0.35 3.25 10.89 1.14 0.00 22.06 0.00 NAK Cruse 0 8.00 14.14 0.00 8.00 22.06 0.00 8.00 22.06 0.00

11 Mantle 00.451 1 52.861[-22-.714 23.79 44.94 30.06 20.2.3 44.94 30.06 20.23 75.00 75.00 7.85 75.00 75.00 17.8a 75. 75.00 17.85

-151 -20C - I (km) ih (km) Z (km) n x a df-III Z (km) hxd Nater 1.112 5.37 0.00 -9.71 5.42 0.0C -9.86 JDOer Block C 8.25 5.37010.0C 8.00F-5.42F0.00I Exotic C 0.0C0 0.0C 13.62

IVAK Crust C1 0.0C 13.62 0.0C I---1 Mantle 0.4E061.35 13.62 27.62061.58013.42027.71 75.001 15.001 17.55175.00 75.00 17.tf5

NOTE: All extension takes place south of the 100 km mark

11 8

10 km Erosion LATE CRETACEOUS

MODEL C 400 Arctic Coast 200 Deepest- FB-- seas 150 L.aver d /cch (km)km Z (km)hn x ad Ii (km)Z (km)hn x da I1 (km) IZ (km) h x d Nater -1.81 1.06 0.00 -1.9'19 0.00 0.00 0.00

Young Seds -0.35 3.OC1 O.OCI -1.05 0.56 O.OCI -0.20

Exotic CI 0.00 0.56 O.OC tDid Seds -0.15 0.00) 3.00I 0.00 3.25 0.56 -0.45

I1ak Crust C 30.00 1.06 0.0C) 30.00 3.00I 0.00 30.00 3.81 0.001 i - --I "5 I-Mantle 0.45 43.94 31.06 19.77 42.001 33.00 its.au .E1.1a . 33.81 18.52 ,- 75.00 75.00 17.851 75.001 75.00 l i.oa 75.001 15.00 1,'.oc

100 10 eroded 75 5eroded 50 0.25 eroded Layer dcc h(km)Z(km)hxd h(km)Z km hxd h(km)Z(km)h x d To crust 2.85 1.27 -1.27 3.63 0.96 -0.96 2.72 0.01 -0.01 0.03 Exotic 0 4.73 0.00 0.00 6.04 0.00 0.00 7.74 0.00 0.00 Old Seds -0.15 6.50 4.73 -0.98 8.13 6.04 -1.22 6.50 7.74 -0.98 NAK Crust 0 30.00 11.23 0.00 24.50 14.18 0.00 19.00 14.24 0.00 Mantle 0.45 33.77 41.23 15.20 36.32 38.68 16.35 41.76 33.24 18.79

75.0 75.001 17.85175.001 75.001 7.85 75.00 797UO-T-T7-T.5- F 25 0N of strike-slip 0S of strike-slip Layer d/cch(km)Z(km)h x d h(km)Z km hxd h(km)Z(km)h x d Roy. Basin -0.25 4.63 0.00 -1.16 9.48 0.00 -2.37 1.50 0.00 -0.37 Exotic 0 8.13 4.63 0.00 8.25 9.48 0.00 Old Seds -0.15 4.88 12.76 -0.73 3.25 17.73 -0.49 Ruby 0 33.00 1.50 0.00 NAK Crust 0 13.50 17.63 0.00 8.00 20.98 0.00 Mantle 0.45 43.87 31.13 19.74 46.02 28.98 20.71 40.50 34.50 18.23

75.00 75-00 17.85175.00 75.001 17 85 75 00 75.001 17 85

NOTE: Area of remaining exotic crust (km2) = 9010

50 km Ruby involved PALEOCENE

MODEL C 400 25C 20C

d cc Ii (km) Z (km) Ihxd Ih (km)Z (km) hxd Ih (km) Z (km)hxdn x a Topo(young) 2.5 u.ut -0.06 0.1E 0.26 -0.26 0.65 Nater -1.82 1.06 0.00 -1.92

Young Seds -0.35 3.20 0.00 -1.122 0.3CI 0.00I -0.11

Exotic C O.OCI 0.30 0.00

Did Seds -0.15 0.001 3.20 0.00) 4.68 0.30 -0.70

IWilted NAK C 0.001 4.98 0.00

NAK Crust C 30.00 1.06 0.00IL30.00 3.20 0.00) 30.00 4.96 0.00

Mantle 0.45 43.94 31.06 19.77 41.801 33.20 18.81 40.02' 34.98 18.01 75 [X] 75 (X] 17 RE 75 on 75 n(] 17 Rf 75 (X: 75 U[] 1 7.H 175 3 km exo. 150 5.5 km exo. 125 6 km exo. d/cchkm)Z(km)hxd hkm Z(km)hxd hkm)Z(km)h x d To o crust 2.85 1.64 -1.64 4.67 3.09 -3.09 8.80 2.35 -2.35 6.71 Exotic 0 1.36 0.00 0.00 2.41 0.00 0.00 3.65 0.00 0.00 Old Seds -0.15 7.01 1.36 -1.05 9.35 2.41 -1.40 7.01 3.65 -1.05 Uplifted NAK 0 5.00 8.37 0.00 10.00 11.76 0.00 12.25 10.66 0.00 NAK Crust 0 30.00 13.37 0.00 30.00 21.76 0.00 25.00 22.91 0.00 Mantle 0.45 31.63 43.37 14.23 23.24 51.76 10.46 27.09 47.91 12.19 0.00 75.00 0.00 75.00 75.00 17.85 75.00 75.00 17. 5

100 6.5km exo. 75 7 km exo. 50 8 km exo d cchkmZkmhxd hkmZkmhxd hkmZkm h x d To o crust 2.85 2.24 -2.24 6.37 1.57 -1.57 4.48 0.36 -0.36 1.04 Exotic 0 4.26 0.00 0.00 5.43 0.00 0.00 7.64 0.00 0.00 Old Seds -0.15 4.68 4.26 -0.70 2.34 5.43 -0.35 Uplifted NAK 0 19.00 8.94 0.00 13.50 7.76 0.00 8.00 7.64 0.00 Ruby 0 0.00 27.94 0.00 11.00 21.26 0.00 22.00 15.64 0.00 NAK Crust 0 20.00 27.94 0.00 12.25 32.26 0.00 Mantle 0.45 27.06 47.94 12.18 30.49 44.51 13.72 37.36 37.64 16.81

75.001 75.001 1 .891 79.551 5. 17.85175 5.0 7.85

25 d/cchkmZkm h x d Koy. Basin -0.25 1.50 0.00 -0.38 Exotic 0 0.00 1.50 0.00 Ruby 0 33.00 1.50 0.00 Mantle 0.45 40.50 34.50 18.23

75.00 75.0 1 .85

NOTE: Area of remaining exotic crust (km2) = 900

Final Erosion MIOCENE I PRESENT

MODEL C 400 Arctic Coastline 250 225

Layer d/cchkmZkm h x d h(km) IZkm h x d h km Z km h x d To 0 oun 2.5 0.35 -0.35 0.88 0.55 -0.55 1.38 YoungSeds -0.35 3.00 0.00 -1.05 4.09 0.00 -1.43 2.97 0.00 -1.04 Old Seds -0.15 2.34 2.97 -0.35 NAK Crust 0 30.00 3.00 0.00 30.00 4.09 0.00 30.00 5.30 0.00 Mantle 0.45 42.00 33.00 18.90 40.91 34.09 18.41 39.70 35.30 17.86 75.00 75.00 17.85 75.00 .00 17.85 75.00 75. 17. 5

200 175 4.5 eroded 15C 9.5 eroded

Laver d(Wcc) Ih (km) Z (km)hn x ad h km Z k... Ih x d i (km) Z (km)hxdn x i fopo(vounu) 2.5 .75 -0.75 1.88

ropo(old) 2.7 0.9Ei -0.96 2.5ES 1.61 -1.61 4.34 (ouna Seds -0.35 1.84 0.00 -0.64

)Id Seds -0.15 4.68 1.84 -0.7C1 4.5E O.OC -0.6! 3.74i O.OC -0.5

Jplifted NAK 01 5.00 4.56 0.0(1 10.00 3.74 O.OC

IAK Crust C1 30.00 6.51 O.OC) 30.00 9.56 0.0() 30.00 13.74 O.OC IMantle 0.45 38.48 36.51 17.32 35.44 39.561 15.95i 31.2Ei 43.74 14.07 75 (Ml 75 (W 17 8F 75 nL 75 (x] 17.851 75-UL-01 /!)-UL 7 /_H5 125 8.5 eroded 100 9 eroded 75 7 eroded La er d cch km Z km h x d h(km)Z(km)h x d h(km)Z(km)h x d

To 0 old) 2.7 1.08 -1.08 2.92 0.90 -0.90 2.42 0.62 -0.62 1.67 Old Seds -0.15 3.43 0.00 -0.51 1.28 0.00 -0.19 1.72 0.00 -0.26 Uplifted NAK 0 12.25 3.43 0.00 19.00 1.28 0.00 13.50 1.72 0.00 Ruby 0 11.00 15.22 0.00 NAK Crust 0 25.00 15.68 0.00 20.00 20.28 0.00 12.25 26.22 0.00 Mantle 0.45 34.32 40.68 15.44 34.72 40.28 15.62 36.53 38.47 16.44

75. 75.55 17.5 75. 75.00 17.85 75.001 75.001 17. 5

5C 0 eroded 2;

L I I hxd -,aver d(a/cc) Z (km) n x d i (km)Z (km) n x a TODO(Koy.) it 0.35 -0.3t 0.91 rooo(ex) 2.8: 0.3E -0.3E 1.04

Youna Seds -0.2fi 2.80 O.OC1 -0.7C

IExotic C) 7.64 0.001 O.OC

IUplifted NAK 0 8.00 7.64I 0.0C

I Rubv 0 22.00 15.64I 0.00 33.00 2.80I 0.00 NAK Crust 0 0.00 37.64 0.0C Mantle 0.45 37.36 37.64 16.81 39.20 35.80 17.64 75.00 75.00 1 /.t3", 75.00 75 nc] 77.51 87

APPENDIX 5 - COMPOSITE MODEL

Thickness (km) and Area (km) Constraints DensityConstraints I

Topography (young) 2.5 NAK thickness 30 Topography (Koy.) 2.6 NAK thickness at 100 km mark 29 Topography (old) 2.7 Oceanic thickness Topography (crust) 2.85 Outer Arc crustal thickness 16 Water -1.82 Arc thickness 25 Young sediments -0.35 Ruby Terrane thickness 33 Old sedimentary roc -0.15 Initial pasive margin sed. thickness Koyukuk basin -0.25 Final Arctic Coast sediment thickness NAK Crust U Base depth of column 75 Exotic Ruby Terrane 0 krea of deformed sedimentary rocks 1270 Mantle 0.45 krea def. sed. rock after Ruby enters 1169

Fixed Reference Column: Layer d /cc)h (km Z km h x d Present Day Arctic Coast Young Seds -0.35 3.00 0.00 -1.05 NAK Crust 0 30.00 3.00 0.00 Mantle 0.45 42.00 33.00 18.90 75.00 75.00 17.85

300 km before collision TRIASSIC

COMPOSITE 400Arctic Coast 100Seds begin 0Oceanic begins Layer d/cch kmZ km h x d h km Z km h x d h(km)Z km h x d Water -1.82 1.06 0.00 -1.92 1.26 0.00 -2.29 1.84 0.00 -3.34 Seds -0.35 0.00 1.26 0.00 10.16 1.84 -3.56 NAK Crust 0 30.00 1.06 0.00 29.00 1.26 0.00 8.00 12.00 0.00 Mantle 0.45 43.94 31.06 19.77 44.74 30.26 20.14 55.00 20.00 24.75

75.00 75.001 17.85175.00 75.00 17.85 75.00 75.00 17.85

-150Seds end -300Trench -350Outer Arc high Layer d /cch(km) Z(km)h x d h(km)Z(km)h x d h km Z(km)h x d Water -1.82 5.42 0.00 -9.86 5.42 0.00 -9.86 2.25 0.00 -4.09 Seds -0.35 0.00 5.42 0.00 Exotic 0 0.00 5.42 0.00 16.00 2.25 0.00 NAK Crust 0 8.00 5.42 0.00 8.00 5.42 0.00 8.00 18.25 0.00 Mantle 0.45 61.58 13.42 27.71 61.58 13.42 27.71 48.75 26.25 21.94

75.00 75.00 17.85 75.00 75.00 17.85 75.00175.001 17.85

-400Forearcbasin 500Arc Layer d/cch (km Z(km)hxd h(km)Z (km)h x d Water -1.82 5.42 0.00 -9.86 2.05 0.00 -3.73 Exotic 0 8.00 5.42 0.00 25.00 2.05 0.00 Mantle 0.45 61.58 13.42 27.71 47.95 27.05 21.58

75.001 75.00 17.85 75.001 75.00117.85

NOTE: Exotic crust reverts to normal oceanic structure south of 600 km mark 50 km after collision LATE JURASSIC I EARLY CRETACEOUS

COMPOSITE I 5C-l Exotic begins 400 Arctic Coast I 100 Seds begin .aver p/cc)i (km)Z (km) n x a ti (km)Z (km)h x d h (km)km Z (km) Ih x d Mater -IAA 1.06 u.u. -1.uk 1.26 0.00 -2.2£ 1.55 0.001 -2.81 Exotic 0 0.00 1.551 0.0( Seds -0.35 0.010 1.26 0.0( 5.08 1.551 -1.7E NAK Crust 0 30.00 1.061 0.00 29.00 1.26 0.00 18.50 6.631 0.0( Mantle 0.45 43.94 31.0E u j. / r 44.74 30.26 20.14 49.87 25.131 22.44 75.00 75.00 1 /.ts5 /5.uu 75.001 17.85175.00 75.001 17.85

C Outer Arc high -5C -10C Seds end - I - I- I rir, 1 h (km) Z (km) 1 x d .aver Id(y.,..;)n (km)Z (km)h x d h (km) Z (km)hxdh x d Topo(crust) 2.85 0.92 -0.92 2.62 I, += Nater -1.82 1.44I 0.00 -2.63 0.95 0.00 -1.74

0 I I 0 i [Exotic j C 15.08-- i 0.00 0.01 8.00 1.44 0.00 16.50 0.95 0.00- O I 3eds -0.35 10.16 15.08 -3.56 6.78 9.44 -2.37 3.39 17.45 -1.19 I.VAK Crust C 8.00 25.240000. 8.00 16.22-' 0.00-- 8.00©020.84 0.00I Mantle 0.45 41.7E 33.24 18.7E 5u.---3 (t 24.22 22.85 46.16 28.84 20.77 75.00 75.00 17.85 75.00 /5.uu 17.85 75.00 75.00 1--; /.85

-1 biArc -25( Normal ocean

Laver d(g/cc)h (km) Z (km) hxd 1(km)Z (km) 1n x a71 Nater 1.82 2.05 0.00 -3.7; 5.42 0.001 -9.8E fExotic CI 25.00 2.05i 0.0() 8.001 5.42- -'1u.ut - 1 ---I Seds- -0.35 0.0C 1/.05--- 0.00

1 O VAK Crust CJ 0.010 27.05-- - i 0.00I Mantle 0.45 47.95 27.05 21.58 61.58 13.421/./1 75.00 75.00 17.85 75.00 75.001 17.85

150 km of crustal overlap ALBIAN (A)

COMPOSITE 400 Arctic Coast 20C Seds beain 15C Exotic begins

L.aver d(La/cc) I1 (km) Z (km) Ih x d Ii (km) Z (km)h x d Ilhh (km) 1;Z (km) Ih x d ToDo(seds) 1.5 0.14 -0.141 0.36 Mater -1.82 1.06 0.00 -1.92 1.19 0.00 -2.1E t 1Beds -0.35 0.010 1.1b 0.01 3.92 0.001 -1.37 NAK Crust C1 30.00 1.06 0.00) 29.33 1.1b 0.00 29.17 3.921 0.00 Mantle 0.45 43.94031.06 19.77 44.46 30.52 20.01 41.91 33.091 18.86 75.00 75.00 17.85 75.00 75.00 17 BE 75.00 15.001 1 /2s5n7l

100Outer Arc High 75Thickest Seds 1 ---550]1J -ayer d(cc)h (km)Z (km)hxd h(km)Z(km)h x d 111h(km)®i,h x d top-o crust 2.85 3.291 -3.29 9.37 2.52 -2.52 7.18, 0 -0.77 2.18 Exotic 0 12.71 0.001 0.0011 9.48 0.00 0.00 7.23 0.00 0.00

Se s -0.35 8.131 12.7111 -2.85 10.16 9.48 -3.56 8.13 7.23 2.85 19.64 0.00 18.50 15.36 0.00 VAK Crust 0 29.001 20.84 0.00 23.75 18.51 Mantle 0.4511 25.16 49.84 11.32 31.61 43.39 14.22 41.14 33.86

75.00 75.00 17.85 75.00 75.00 17.85 75.001 75.00117.85 -50Arc -100 Layer d cch km Z(km)h x d h(km)Z(km)h x d h km Z KEh x d

Water -1.82 0.71 1 0.00 -1.30 0.46 0.00 -0.84 2.15 0.00 -3.91 Exotic 16.50 0.46 0.00 16.50 2.15 0.00 01 0.71 0.00 25.00 Seds -0.35 4.07 17.21 -1.42 0.00 25.46 0.00 NAK Crust 0 8.00 21.28 0.00 8.00 25.46 0.00 8.00 18.65 0.00 Mantle 0.45 45.72 29.28 20.57 41.54 33.46 18.69 48.35126.65121.76

75.00 75.00 ti' 17.85 1 75.00 75.00 17.85 75.00 75.00 17.85 F

-150 Normal Ocean Layer d(g/cc)h kmZ km hxd IF]0 Water -1.82 5.42 0.00 -9.86 Exotic 0 8.00 5.42 0.00 NAK Crust 0 0.00 13.42 0.00 Mantle 0.45 61.58 13.42 27.71 75.001, 75.00 17.85

50 km Extension ALBIAN (B)

COMPOSITE 7; Unchanged 50 North tip of fault 25 .aver ldta/cc) Ih (km)Z (km) n x a 1(km) Z (km)h x d h (km) Z (km)h x d Tooo(crust) 2.851 2.52 -2.52 7.18 0.77 -0.77 z.1n Nater -1.82 0.62 0.00 -1.14

; rExotic C) 9.481 O.OCI 0.00 7.23 0.00 O.OC 8.13i 0.62 0.00 I Seds -0.3- > 10.16 9.48 -3.5E 8.131 7.23 -2.85- 6.10 8.74 -2.13 1

INAK Crust C7 23.75 `"w yx- 0.001 18.50 15.36 0.001 13.25 14.84 0.001 -, IMantle 0.41,:i 31.61 43.3£1 14.22 41.14 33.86 18.51 46.91 28.09 21.11- 75.00 75.001 17.85 75.00 75.00 17.8075.00I75.00 17.8 0 -50Arc Base -100Arc Top

h (km)Z (km)hxd h(km)Z (km) 11h x d h km Z(km)h x d water -1.82 2.35 0.00 -4.28 1.31 0.00 -2.38 1.31 0.00 -2.38 Upper Block 0.00 2.35 0.00 8.25 1.31 0.00 12.50 1.31 0.00 Exotic 00 8.25 2.35 0.00 12.50 9.56 0.00 8.25 13.81 0.00 Seds -0.35 4.07 10.60 -1.42 0.00 22.06 0.00

NAK Crust i 0 8.00 14.6711 0.00 8.00 22.06 0.00 8.00 22.06 0.00 Mantle 0.45 52.33 22.67 23.55 44.94 30.06 20.23 44.94 30.06 20.23

I 75.00 75.001 17.85 75.00 75.00 17.85 75.00 75.00 17.85

-101 -20C

Ii (km)Z (km)hxd h>q (km) km Z (km)hxdh x d Nater -1.82 5.37 0.00 -9.77 5.42 0.00 -9.86 lJpper Block C 8.25 5.37- 0.00- 1 8.00 5.42 0.00 f'Exotic C 0.00 13.62 0.0CI _Fri IAK Crust C 0.0 13.62 0.0CO 0 Mantle 0.45 61.38 13.62 27.62 61.58 13.42 27.71 /S.UU 75.00 17.8 /S.UL 75.0C 17.85

NOTE: All extension takes place south of the 100 km mark 725 km2 of exotic crust removed 10 km Erosion LATE CRETACEOUS COMPOSITE _-400 Arctic Coast 200 Deepest Young - 150 saver I(a/cc) Ih (km)Z (km)h x d h (km) Z (km) I1 x a , Ii (km) Z (km) Ih x d do= Water -1.82 1.08 0.00 1.92 0.00 0.001 0.0('1 Youna seds -0.35 3.38 0.00I -1.1n 0.42' O.OC -0.15

IExotic CI 0.00I 0.42 O.OC

Old Seds -0.15 0.00-l 3.38 0.001 4.07 0.42 -0.61 Nak Crust CI 30.00. 1.08 0.00I 29.33 3.38 0.00 29.17 4.45 "U.uu '1 Mantle 0.45 43.94I 31.06 19.77 42.29 32.71 19.03 41.351 33.65 18.61 fb.UC 75.00 17.851 75.00 75.00 1 L.tf5 75.00 75.001 17.85

100 10 eroded 75 5eroded 50 0.25 eroded Layer d/cc h(km)Z (km)h x d h(km)Z km h x d h(km)Z(km)h x d Togo crust 2.85 1.43 -1.43 4.08 1.22 -1.22 3.49 0.24 -0.24 0.68 Exotic 0 4.57 0.00 0.00 5.78 0.00 0.00 7.51 0.00 0.00 Old Seds -0.15 8.13 4.57 -1.22 10.16 5.78 -1.52 8.13 7.51 -1.22 NAK Crust 0 29.00 12.70 0.00 23.75 15.94 0.00 18.50 15.64 0.00 Mantle 0.45 33.30 41.70 14.99 35.31 39.69 15.89 40.86 34.14 18.39 75.00 75.00 17.85 75.00 75.00 17.85 75.00 75.00 17.85

25 0N of strike-slip 0S of strike-slip Layer d cch (km)Z km h x d h km)Z km hxd h km Z km h x d Koy. Basin -0.25 3.75 0.00 -0.94 8.78 0.00 -2.20 1.50 0.00 -0.37 Exotic 0 8.13 3.75 0.00 8.25 8.78 0.00 Old Seds -0.15 6.10 11.87 -0.91 4.07 17.03 -0.61 NAK Crust 0 13.25 17.97 0.00 8.00 21.10 0.00 Ruby 0 33.00 1.50 0.00 Mantle 0.45 43.78 31.22 19.70 45.90 29.10 20.66 40.50 34.50 18.23 1 75.00 75.00 17.85175.00 75.00 17.85 75.00175.00 17.85

NOTE: Area of remaining exotic crust (km2) = 900 503 km2 removed by erosion

50 km Ruby involved PALEOCENE

COMPOSITE 400 250 20C

1(a/cc) I1(km)Z (km) Ihxd Ih (km) Z (km) hxd h (kmkm Z (km)h x d Topo(voung 2.7 1.31 -1.31 3.5: water --Hi2 1.06 0.00 -1.92

Youna Seds -0.35 3.28{ 0.00I -1.15j

IDid Seds -0.15 0.00I 3.28 0.00I 10.;38 0.00 -1.5t I --i uplifted NAI 0 0.00 10.38 0.01- - fNAK Crust C 30.00 1.0Ei 0.001 29.50 3.28 0.00) 29.33 10.38 0.00 Mantle 0.45 43.94 31.0E 19.77 42.22 32.78 19.00 35.2E 39.71 15.8E 75.00 75.00 17.85 75.00 75.00 17.85 75.00 75.00 17.8! 91

175 3 km exo. 150 5.5 km exo. 125 6 km exo. d/cc)h km)Z km hxd h km Z km h x d h (km) IZ (km h x d To o crust 2.85 2.02 -2.02 5.76 2.69 -2.69 7.66 1.97 -1.97 5.60 Exotic 0 0.98 0.00 0.00 2.81 0.00 0.00 4.03 0.00 0.00 Old Seds -0.15 9.74 0.98 -1.46 7.79 2.81 -1.17 5.84 4.03 -0.88 Uplifted NA 0 5.00 10.72 0.00 10.00 10.60 0.00 11.88 9.88 0.00 NAK Crust 0 29.17 15.72 0.00 29.17 20.60 0.00 24.08 21.75 0.00 Mantle 0.45 30.11 44.89 13.55 25.23 49.77 11.35 29.16 45.84 13.12

75.00 75.00 17.85 75.00 75.00 17.851 75.00 75.00 17.85

100 6.5km exo. 7 km exo. 501 8 km exo. dlcc)h (km)Z km hxd50Z km hxd h (km)Z km h x d Toocrust 2.85 1.89 -1.89 5.39 1.42 -1.42 4.04 0.36 -0.36 1.04 Exotic 0 4.61 0.00 0.00 5.58 0.00 0.00 7.64 0.00 0.00 Old Seds -0.15 3.90 4.61 -0.58 1.95 5.58 -0.29 Uplifted NA 0 18.50 8.51 0.00 13.25 7.53 0.00 8.00 7.64 0.00 Ruby 0 0.00 27.01 0.00 11.00 20.78 0.00 22.00 15.64 0.00 NAK Crust 0 19.00 27.01 0.00 11.88 31.78 0.00 Mantle 0.45 28.99 46.01 13.05 31.34 43.66 14.10 37.36 37.64 16.81 75.00 75.00117.85 75.00175.00 17.85175.00 75.00117.85

25 d/cc h(km)Z(km)h x d Koy. Basin -0.25 1.50 0.00 -0.38 Exotic 0 0.00 1.50 0.00 Ruby 0 33.00 1.50 0.00 Mantle 0.45 40.50 34.50 18.23 75.00 75.00117.85

NOTE: Area of remaining exotic crust (km') =900

Final Erosion MIOCENE I PRESENT

COMPOSITE 400 Arctic Coastline 250--- 200 4 eroded

L.aver d cc Ii (km)z (km)hxdn x a Ii (km) IZ (km) In x a Ii (km) Z (km)h x d ODO(vounc 2.5 0.30 -0.30 0.75 oDo(old) 2.1 0.58 -0.58 1.5E

Young seds -0.35 3.00 O.OC -1.05 4.22 O.OC1 -1.48 C-Did Seds- -0.15 -1.11 0.00 1-u

IIAK Crust C 30.00 3.OC O.OC 29.50 4.22 0.0C 29.33 7.11 0.01

IMantle 0.4: 42.OCI 33.OC 18.9C 41.28 33.72 18.58 38.56 36.44 17.;35 75 01 75 001 17 R5 7.5i Of] 75 MI 17 R5 75.00 75.00 17.8!

175 7 eroded 150 7 eroded 125 6.5 eroded Layer d cch(km)Z(km)h x d h(km)Z(km)h x d h km Z km h x d To old) 2.7 0.90 -0.90 2.42 1.67 -1.67 4.50 1.06 -1.06 2.85 Old Seds -0.15 4.84 0.00 -0.73 4.63 0.00 -0.69 4.29 0.00 -0.64 Uplifted NA 0 5.00 4.84 0.00 10.00 4.63 0.00 11.88 4.29 0.00 NAK Crust 0 29.25 9.84 0.00 29.17 14.63 0.00 24.08 16.16 0.00 Mantle 0.45 35.91 39.09 16.16 31.21 43.79 14.04 34.75 40.25 15.64

75.00 75.00 17.85175.001 75.001 17.85175.00175.00 17.85 100 7.5 eroded 75 6 eroded 50 0 eroded Layer d/cc h(km)Z km h x d h km Z km h x d h (km)Z km h x d Topo(old) 2.7 0.82 -0.82 2.22 0.64 -0.64 1.74 To o ex 2.85 0.36 -0.36 1.04 Old seds -0.15 2.07 0.00 -0.31 2.30 0.00 -0.35 Exotic 0 7.64 0.00 0.00 Uplifted NA 0 18.50 2.07 0.00 13.25 2.30 0.00 8.00 7.64 0.00 Ruby 0 11.00 15.55 0.00 22.00 15.64 0.00 NAK Crust 0 19.00 20.57 0.00 11.88 26.55 0.00 0.00 37.64 0.00 Mantle 0.45 35.43 39.57 15.94 36.57 38.43 16.46 37.36 37.64 16.81

75.00 75.00 17.85 75.00175.001 17.851 75.00 75.00 17.85

25 La er d /cch kmZ km h x d To o Ko 2.6 0.35 -0.35 0.91 Koybasin -0.25 2.80 0.00 -0.70 Ruby 0 33.00 2.80 0.00 Mantle 0.45 39.20 35.80 17.64

75.001 75.001 17.85

NOTE: