Trans-Alaska Lithosphere Investigation 

Program Prospectus

U.S. GEOLOGICAL SURVEY CIRCULAR 984

Trans-Alaska Lithosphere Investigation 

Program Prospectus

Edited by

David B. Stone

University of Alaska, Fairbanks

Robert A. Page

U.S. Geological Survey

John N. Davies

Alaska Division of Geological and Geophysical Surveys

U.S. GEOLOGICAL SURVEY CIRCULAR 984

Prepared in cooperation with the University of Alaska, Fairbanks, 

the Alaska Division of Geological and Geophysical Surveys, 

and the National Science Foundation

1986
Department of the Interior

DONALD PAUL MODEL, Secretary

U.S. Geological Survey

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Trans-Alaska Lithosphere Investigation Program Prospectus.

U.S. Geological Survey Circular 984

"Prepared in cooperation with the University of Alaska, Fairbanks, the Alaska

Division of Geological and Geophysical Surveys, and the National Science

Foundation."

Supt. of Docs. No.: I 19.4/2:984

1. Trans-Alaska Lithosphere Investigation. I. Stone, David B. II. Page,

Robert A. III. Davies, John N. IV. University of Alaska, Fairbanks. V.

Alaska. Division of Geological and Geophysical Surveys. VI. National Science

Foundation (U.S.). VII. Series.

QE47.A4T73 1986 551.1'3'09798 86-600201

Free on application to the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225

COVER

Deep seismic reflection profiling along the Trans-Alaska Lithosphere Investigation transect near Donnelly Dome, with 

the Alaska Range in the background. The five trucks in the foreground vibrate the ground in unison, sending out 

compressional seismic waves which reflect off velocity discontinuities within the upper lithosphere and return to the 

surface where they are recorded by an array of seismographs. In this survey, contracted by the U.S. Geological 

Survey in 1986, the signals were recorded by a 30-km linear array of 1,024 groups of seismographs deployed 

symmetrically about the vibrator trucks. Figure 7 (p. 14) shows an example of the data obtained.
CONTENTS

Summary - 

Introduction 

Importance of studying Alaska----- 

Transect route and geologic problemsResearch problems 

Tectonostratigraphic terranes 

"Basement" of accreted terranes 

Depth to Moho 

Orogeny and deformation of the 

litho sphere 

Subduction and magmagenesis - 

Major fault systems -- 

Neotec tonics - 

Oroclinal bending 

Earthquake hazards - -

Volcano hazards 

Metallogenesis 

Basin analysis - 

Geothermal resourcesResearch techniques - 

Seismo logical -

Page

1

2

2

5

7

7

9

10

10

11

11

11

12

12

12

12

12

Artificial sources 13 

Refraction and wide-angle

reflection profiling 

Vertical reflection

pro f i 1 ing 

Marine profiling 

Earthquake sources 

Seismic imaging 

Seismicity studies 

 13

 13

 13

 13

 13

 15

Focal mechanism studies 15

Surface wave analysis 16

Page 

Research techniques (continued)

Geophysical (nonseismic) 16

Potential-field geophysics 16

Gravity analysis 16

Magnetic analysis 17

Deep geoelectrical studies 17

Paleomagnetic studies 17

Borehole geophysical studies 17

Satellite-based studies 17

Remote sensing 17

Geodesy 18

Rock properties 18

Geological 18

Geologic mapping 18

Stratigraphic, basin analysis,

and paleontological studies 18 

Petrologic, geochemical and

isotopic tracer studies 18 

Petrologic studies of metamorphic rocks 19

Structural and fabric studies 19

Age studies 19

Metallogenesis studies 19

Neotectonic studies 19

Deep drilling 20

Investigations in progress 22

U.S. Geological Survey 22

Rice University University of

Alaska cooperative studies 23 

University of Alaska 23 

Alaska Division of Geological and

Geophysical Surveys 23 

Append ix 24

ILLUSTRATIONS

Figure 1. Global view of Alaska and Pacific and Arctic Ocean basins showing TALI

route - -- 

2. Organizational chart of TALI -

3. Map showing TALI route and tectonostratigraphic terranes of Alaska and

Siberia 

4. Map showing TALI route, tectonostratigraphic terranes, major faults,

and active volcanoes 

5. Seismic record section and P-wave velocity model for the northern

Chugach Mountains and southern Copper River Basin 

6. Map showing earthquake epicenters in the vicinity of the TALI transect 

7. Vertical seismic-reflection section from the northern

Chugach Mountains 

8. Map showing seismograph stations in the vicinity of the TALI transect 

9. Geologic map and cross section showing location of candidate site for

deep scientific hole at Middleton Island, Gulf of Alaska 

Page

3

4

9

10

14

16

21

III

Trans-Alaska Lithosphere Investigation Program Prospectus

David B. Stone, Robert A. Page and John N. Davies, Editors

SUMMARY

This prospectus presents the rationale and 

framework for a major cooperative investigation of 

the Alaskan lithosphere the Trans-Alaska Lithosphere Investigation, or TALI. TALI focuses on a 

north-south transect, extending from the active 

convergent Pacific margin to the passive rifted 

Arctic margin, following the trans-Alaska oil pipeline corridor (fig. 1). The route samples an 

exceptionally broad variety of geologic structures 

and problems along its 1,400-km length.

Goals. The goals of the Trans-Alaska Lithosphere 

Investigation are to

o Determine the structure, composition and 

evolution of the Alaskan lithosphere,

o Examine the role of accretion tectonics in the 

growth of Alaska, and

o Investigate the post-amalgamation behavior of 

Alaska and the origin of major geologic 

features mountain chains, sedimentary basins, 

regional fault zones, seismic belts, and 

volcanoes.

The primary objective of TALI is to obtain a 

continuous geologic and geophysical profile across 

Alaska by the end of this decade. Particular 

emphasis is placed on seismic-reflection and 

-refraction profiling and other geophysical methods 

to resolve the structure of the crust and underlying 

mantle and to relate surface geology to structures 

at depth inferred from geophysics.

Benefits. TALI will substantially improve knowledge of the State's geologic framework, especially 

in the subsurface, and will yield new insights into 

the geologic processes that have shaped the crust.

Improved knowledge of the geologic framework

the State's oil and

will

o Facilitate evaluation of 

mineral potential, and

o Provide a regional context in which to interpret 

detailed subsurface studies completed by 

industry and others along the transect corridor.

New insights into geologic processes will help to

o Resolve fundamental questions relating to the 

evolution of continents and to the movements 

and collisions of the diverse microplates and 

terranes that constitute the Alaskan crust,

o Elucidate the origin of Alaska's oil and mineral 

resources and the causes of the State's earthquakes and volcanoes, and

o Decipher the geologic development of western 

North America and the Pacific rim.

Organization. TALI is a cooperative effort among 

Federal and State agencies, universities, and the 

petroleum and mineral industries.

TALI is promoted and coordinated by a threeperson committee with representatives from the 

U.S. Geological Survey (USGS), the Alaska Division 

of Geological and Geophysical Surveys (ADGGS), 

and the University of Alaska, Fairbanks (UAF). This 

committee disseminates information on the 

objectives, plans, and progress of TALI and seeks to 

stimulate collaboration among potential 

investigators.

An advisory committee, consisting of scientists 

from industry, government, and academia, is being 

established to review periodically the aims and 

accomplishments of TALI, to recommend topics for 

and priorities of future work, and to foster 

participation and cooperation.

The TALI organization, consisting of the 

coordinating and advisory committees, provides 

information and recommendations to participants 

and funding sources but does not engage in funding 

activities.

Studies directly focused on TALI have been 

initiated by the USGS, in its Trans-Alaska Crustal 

Transect (TACT) project, and by Rice University 

and the UAF with support from the Department of 

Energy, the National Science Foundation, and the 

petroleum industry. The ADGGS and individual 

scientists from several universities and companies 

are collaborating in many of these studies. The 

total support for these studies in 1985 was about $2
million. Many other geologic and geophysical 

investigations that contribute to the goals of TALI 

are also being conducted by industry, universities, 

and government along or near the transect corridor.

INTRODUCTION

The importance of determining the deep 

geologic structure of Alaska has become 

increasingly apparent over the last few years. Both 

the long-term search for petroleum and mineral 

resources and the advent of the concept that Alaska 

is composed of diverse geologic terranes accreted 

to western North America demand physical 

measurements of crustal structure in Alaska. The 

exciting successes in elucidating crustal structure in 

the conterminous United States with modern 

geophysical techniques suggest that applying these 

techniques to investigation of the Alaskan 

lithosphere would yield dramatic new insights into 

the structure, composition, and evolution of the 

Alaskan crust and into fundamental tectonic 

processes that are poorly understood at present.

To stimulate investigation of the Alaskan 

lithosphere, the U.S. Geological Survey (USGS), the 

Alaska Division of Geological and Geophysical 

Surveys (ADGGS), the University of Alaska, 

Fairbanks (UAF), and Rice University are promoting 

a cooperative geophysical and geological transect of 

the crust and upper mantle, the Trans-Alaska 

Lithosphere Investigation (TALI). The primary 

route of the transect follows the north-south 

corridor of the trans-Alaska oil pipeline between 

Prudhoe Bay and Valdez (fig. 1) and extends 

offshore across the Pacific and Arctic continental 

margins.

TALI is a coordinated multidisciplinary effort 

among government, academic, and industry 

scientists and institutions. Coordination is provided 

by a modest organization consisting of two 

committees (fig. 2). A small coordinating 

committee provides day-to-day coordination. The 

editors of this prospectus are serving as the 

coordinating committee by promoting TALI and 

disseminating information on the objectives, plans, 

and progress of TALI. An advisory committee, 

consisting of scientists from industry, government, 

and academia, is being established to review 

periodically the aims and accomplishments of TALI, 

to recommend topics for and priorities of future 

work, and to foster participation and cooperation. 

Participants in TALI are directly responsible for 

securing support for their work from whatever 

source is appropriate. The TALI organization does 

not serve as a vehicle for securing or disbursing 

funds, nor does it conduct research projects.

This program prospectus outlines major 

problems to be addressed by TALI and enumerates 

relevant opportunities for research. The purpose of 

the prospectus is to define a framework within 

which a coordinated multidisciplinary program can 

evolve. We hope that it will be useful both to

investigators who wish to pursue a particular study 

and to agencies and companies that are asked to 

support a particular effort. The ultimate scope and 

accomplishments of TALI will depend on the 

interests of those who choose to participate in the 

program and their success in obtaining funds to 

support the work. No detailed timetable for the 

program is presented, but it is hoped that the 

transect can be completed by the end of this 

decade.

TALI seeks to focus a number of 

complementary geologic and geophysical 

investigations of the lithosphere on a common 

corridor. Because of the multidisciplinary 

character of the transect, the ultimate success and 

contribution of TALI depends heavily on 

collaboration and communication between the 

various disciplines and scientists and on the 

integration and synthesis of a broad variety of 

geologic and geophysical data. Not only will the 

data come from different disciplines, but they will 

be collected over a wide range of scale and density 

and will pertain to different time intervals. It is 

critical that synthesis be encouraged and supported, 

especially in the later years of TALI when the 

support for collection of new data is declining.

The USGS, University of Alaska, Rice 

University, and ADGGS are actively engaged in 

studies contributing to TALI. Individual scientists 

from several universities and companies are 

participating or collaborating. These studies are 

summarized at the end of this report. Other 

institutions are currently seeking support for future 

studies or considering future participation.

Acknowledgments Many of the ideas and 

words in this prospectus originated at a workshop 

cosponsored by the ADGGS, National Science 

Foundation, USGS, and University of Alaska in May 

1984. The workshop participants are listed in the 

Appendix. Additional ideas were contributed by 

colleagues not present at the workshop, including E. 

L. Ambos, A. W. Bally, Michael Churkin, W. D. 

Mooney, J. S. Oldow, and W. K. Wallace. Particular 

recognition is due to George Plafker, W. J. 

Nokleberg, D. L. Campbell, Arthur Grantz, and T. 

P. Miller for their contributions to this document.

IMPORTANCE OF STUDYING ALASKA

A crustal transect across Alaska has 

important implications from many points of view. 

In terms of understanding global geology and the 

large-scale dynamic interactions involved with plate 

motions, Alaska is uniquely situated between the 

actively spreading Arctic Ocean and the relatively 

northward-moving Pacific plate (fig. 1). To a first 

approximation, Alaska is simply part of the North 

American plate, but on a finer scale Alaska includes 

smaller plates or blocks that are slowly moving with 

respect to North America and whose boundaries 

currently are not fully defined. An example from 

the TALI corridor in southern Alaska is the Wrangell
Figure 1. Location of Alaska in relation to Arctic and Pacific Oceans. Also shown are Arctic 

spreading center, Aleutian megathrust, Fairweather-Queen Charlotte (F-QC) transform fault, 

and relative motion vector of Pacific plate. TALI transect corridor follows trans-Alaska 

pipeline onshore (dashed line). Approximate lateral boundaries (solid lines) of maps in figures 

4, 6 and 8 are also shown.
TALI 

committees

Participants 

Activities

Potential 

funding 

sources

TALI 

Coordinating Committee 

(ADGGS, UAF, USGS)

TALI 

Advisory Committee 

(ADGGS, Industry, 

Universities, USGS)

INFORMATION and COORDINATION

USGS

TACT

Resource

assessment

Other projects

related to TALI

USGS appropriations

Other Federal

agencies

ADGGS

Basic mapping

Mineral assessment

Other projects

related to TALI

ADGGS appropriations

State and Federal

agencies

UNIVERSITIES

Student thesis

research

Faculty research

Consortium research

(PASSCAL? COCORP?)

Federal and State

agencies

Industry

INDUSTRY

Release of selected

information in hand

Acquisition of new data

Reprocessing data

Internal funds

Cooperative

programs with

other participants

Figure 2. Organizational chart showing formal structure of TALI, operational relations among TALI 

committees and program participants, and participants' activities and potential funding sources. 

Considerable exchange between participant boxes is expected in regard to research collaboration and in 

regard to support, both direct funding and indirect support such as logistics, information exchange, data 

processing, and so forth.

block, which is bounded on the north by the Denali 

fault, on the east by the Totschunda and 

Fairweather faults, and on the south by the Aleutian 

trench and the Chugach-St. Elias fault system; its 

western boundary is ill defined.

On a continental scale, the proposed transect 

will provide a cross-sectional view across an entire 

continent, from an active convergent margin to a 

passive rifted margin, in a distance of only 1,400 

km. The new information to be obtained concerning 

the structure of the crust and upper mantle is 

critical for deciphering the evolution of Alaska; by 

analogy, it will also contribute to understanding the 

evolution of much of North America. For example, 

the TALI route traverses features that to some 

degree have equivalents in the conterminous United 

States: the Coast Ranges (Chugach Mountains), the 

Sierra Nevada (the Talkeetna Mountains and 

southern Alaska Range), the Rockies (the Brooks 

Range), and the Cretaceous basin deposits of the 

Great Plains (the Arctic Slope).

By pursuing a program of deep sounding with 

all the available tools of geology and geophysics, 

significant new information on the nature of the 

lithosphere may be forthcoming. Such studies will 

fit well with the targets of opportunity in the solid 

earth sciences as proposed to the White House by 

the Committee on Science, Engineering, and Public 

Policy (COSEPUP), and with the stated aims and 

goals of both the U.S. Geodynamics Commission and 

the International Lithosphere Program.

The concept that continental growth involves 

accretion of allochthonous (foreign) terranes driven 

by plate tectonic processes was largely spawned in 

southern Alaska and western Canada and has 

developed into a unifying theme for circum-Pacific 

geology. Although it is widely recognized that 

Alaska comprises a tectonic mosaic of accreted 

crustal terranes, the deep structure of these 

terranes is unknown, as are the details of their 

development. We do not know, for example, how 

deeply the terranes extend, on what they are 

founded, and whether they are stacked one on top of 

another. Answers to these problems are 

fundamental to understanding the geology of Alaska 

and the evolution of western North America.

From the economic point of view, a transect 

such as that proposed here has a high potential to 

add significantly to our understanding of the origin 

and mode of emplacement of the various mineral 

deposits associated with the different tectonic 

settings found within Alaska, and may also resolve 

some of the many riddles concerning the origin of 

the major oil reserves of Arctic Alaska. The 

tectonostratigraphic terrane concept already forms 

the framework for some new exploration strategies 

in western North America in particular, and the 

circum-Pacific region in general, but the detailed 

information necessary for efficient exploration 

planning is lacking. Detailed geologic surface 

mapping combined with all the available geophysical 

techniques can extend geological interpretations to
considerable depth. This is particularly important 

because large sections of the crust can be faulted 

over one another, for hundreds of kilometers. Such 

extensive low-angle faulting within the crust may 

hide large economic mineral or energy resources 

from discovery by conventional surface exploration 

techniques; younger, potentially productive rock 

sequences may be buried beneath crystalline rock 

sequences previously interpreted as "basement," as 

found, for example, in the Wind River Mountains in 

Wyoming and the southern Appalachian Mountains in 

the southeastern United States. To explore for 

subsurface deposits thus requires models of geologic 

and tectonic development that can predict possible 

target areas beneath rocks of low economic 

potential. Although our knowledge of the tectonic 

development of Alaska is at present fragmentary, it 

could improve dramatically with the information 

that a crustal transect can provide. Information of 

this sort would also apply indirectly to tectonic 

models of similar areas such as western North 

America and the rest of the circum-Pacific.

TRANSECT ROUTE AND GEOLOGIC PROBLEMS

The TALI route crosses Alaska from the 

active convergent Pacific continental margin to the 

passive Arctic margin. Over its course, the

transect samples an exceptionally wide variety of 

major geologic features active and fossil 

subduction zones, offscraped accretionary terranes, 

accreted oceanic and continental microplates, 

major structural boundaries, large-scale overthrust 

belts, sedimentary basins, and rifted margins (figs. 3 

and 4). There is no other place in North America 

where a transcontinental transect only 1,400 km 

long could examine such a diversity of major 

geologic features. The rocks along the transect 

route span the evolutionary history of the Alaskan 

crust from Paleozoic and older rocks that are 

possibly related to the North American era ton to 

the rocks of the Yakutat terrane, currently being 

accreted along the convergent Pacific margin. The 

transect therefore provides exceptional 

opportunities to elucidate the development of the 

Alaskan crust and to investigate and understand the 

types of tectonic processes that have shaped the 

geology of Alaska in the past and are continuing in 

the present.

Following the north-south oil pipeline corridor 

from Valdez to Prudhoe Bay, the entire onshore part 

of the TALI route is accessible by road. Such 

access greatly simplifies logistics and reduces costs 

of geophysical and geological investigations. TALI 

will incorporate several supplementary profiles 

intersecting the primary north-south route to

MAJOR TERRANES OF SIBERIA

CHK Chukotsk

NAY North Anyu

SAY South Anyu

ANY Anadyr

KOY Koryak

OMO Omolon

EKO Ekonay

OLY Olyutorsk

OLO Oloi

MAJOR TERRANES OF ALASKA

NS North Slope 

Kagvik

Composite terranes of the Brooks Range 

Ruby - Angayucham (composite) 

Innoko and associated terranes 

Nixon Fork 

Goodnews 

Togiak 

Wrangellia 

Peninsular 

Chugach 

Prince William 

Alexander 

Seward Peninsula 

Complex of Alaska Range terranes 

Porcupine and related terranes 

Yukon Tanana

300 KILOMETERS

Figure 3. Route of TALI transect, shown in relation to tectonostratigraphic terranes of Alaska (modified 

from D. L. Jones and others, 1981, U.S. Geological Survey Open-File Report 81-792) and Siberia (after 

D. G. Howell and others, 1985, Preliminary tectonostratigraphic terrane map of the circum-Pacific 

region, American Association of Petroleum Geologists).
200 KILOMETERS

Figure 4. Route of TALI transect (heavy dashed 

line) shown in relation to tectonostratigraphic 

terranes (modified from D. L. Jones and others, 

1981, U.S. Geological Survey Open-File Report 

81-792, and from D. L. Jones and others, 1984, 

U.S. Geological Survey Open-File Report 84-523, 

Part A), major faults, active volcanoes (stars), 

and northeast edge of Aleutian Wadati-Benioff 

zone (heavy dotted line). Roads shown by dashed 

lines.

resolve the three-dimensional aspects of structure. 

Some of the supplementary profiles will take 

advantage of the few highways that exist within the 

State (fig. 4). The transect route crosses several 

areas that have been the focus of intensive 

geological studies by the USGS, ADGGS, and others,

most notably offshore in the northern Gulf of 

Alaska and the Beaufort Sea, and onshore in the 

Cordova, Valdez, Mount Hayes, Big Delta, 

Fairbanks, Livengood, Chandalar, Wiseman, and 

Philip Smith quadrangles, and along a transect 

across the eastern Yukon-Koyukuk province and the 

Brooks Range.

In southern Alaska, the transect will examine 

the structural and seismic details of convergence 

between the Pacific and North American plates in 

the region of normal convergence at the eastern end 

of the Aleutian trench. Major questions to be 

addressed are the configuration of the continental 

crust and evidence for late Cenozoic subduction of 

oceanic crust and its sedimentary cover along the 

most active convergent margin in the continental 

United States. Knowledge about the structural 

character and history of the potentially 

petroliferous Cenozoic continental margin basin 

bordering the Gulf of Alaska and of the late 

Mesozoic and Paleogene terranes of Prince William 

Sound and the Chugach Mountains will provide 

insight into continental margin accretion 

mechanics. Northward, the route crosses the 

Mesozoic and Cenozoic accretionary sequences near 

the axis of the Alaska orocline, where detailed 

analysis of the extraordinarily well exposed 

structures could provide important information on 

the oroclinal process. The Contact fault and 

related structures represent the inner margin of an 

Eocene or younger suture between the late 

Mesozoic Chugach and Paleogene Prince William 

terranes. The well-exposed Border Ranges fault 

system, a probable subduction zone suture, 

separates the offscraped deep-sea accretionary 

terranes of the Pacific margin from the extensive 

microplate terranes of south-central Alaska. The 

subsurface configuration and nature of both of these 

major fault zones require detailed study.

Between the Border Ranges fault and the 

northern strands of the Denali fault, the transect 

crosses a remarkable collage of far-traveled exotic 

microplates and slivers set in a matrix of flysch 

basins that are believed to have agglomerated to 

south-central Alaska in late Mesozoic and later 

time. The route traverses the PeninsularWrangellia (composite) terrane, which consists of a 

late Paleozoic arc sequence locally overlain by 

rocks showing a distinctive variable lithology. The 

crustal structure of the Peninsular and Wrangellia 

terranes and the nature and configuration of the 

suture bounding the concealed contact between 

them should be investigated in detail. Additionally, 

the process and history of agglomeration of these 

terranes, as well as subsequent "continentalization" 

by continued compression, extensive plutonism, and 

some volcanism in latest Cretaceous and Tertiary 

time, are major problems for study. The possible 

relation of Eocene volcanism, major sutures, and 

strike-slip faults of this region to plate 

convergence, continental accretion, and the Alaska 

orocline is a major problem, as is the puzzling 

paucity of arc-related volcanic rocks landward of
the presumed major late Mesozoic and Tertiary 

convergent margins of Alaska.

In central Alaska, the transect will cross the 

Alaska Range and the Denali fault. The Denali 

fault is excellently exposed along the pipeline route, 

and its major splay, the Hines Creek fault, is 

exposed immediately to the west. The dual role of 

the Denali fault system as a late Mesozoic collision 

suture and as a Cenozoic strike-slip fault that 

absorbs lateral slip between the Pacific and North 

American plates is of great interest. Between the 

Denali fault and Hines Creek fault are a collage of 

narrow, fragmented late Paleozoic to Mesozoic 

terranes with oceanic affinities, including the 

Windy, McKinley, and Pingston terranes. These 

small terranes are important to study because they 

separate mainly oceanic terranes to the south, 

which are in part allochthonous with respect to 

North America, from various crystalline terranes to 

the north in the Yukon-Tanana Upland, which are 

possibly autochthonous with respect to North 

America.

The crystalline terranes of the Yukon-Tanana 

Upland, which extend northward beyond Fairbanks, 

consist of a variety of metamorphosed sedimentary, 

volcanic, and plutonic rocks with a complex 

geologic history, probably extending back to the 

Proterozoic. Along its northern margin in the 

Livengood area, the Yukon-Tanana Upland is 

bordered by a collage of crustal slices of Paleozoic 

through Jurassic age with both oceanic and 

continental margin affinities. These terranes are in 

part bounded by and in part disrupted by the Kaltag 

and Tintina fault systems, which have had large preEocene strike-slip displacements. The nature and 

extent of sutures along the margins of, and within, 

the Yukon-Tanana Upland, and the extent and 

configuration of crystalline terranes within the 

Upland, are all important problems.

The route traverses the eastern apex of the 

triangular Yukon-Koyukuk province near the 

headwaters of the Koyukuk River. The Cretaceous 

marine and nonmarine strata of this province are 

surrounded by obducted dismembered ophiolites, 

which may mark ancient continental margins. The 

origin and subsequent deformation of this basin are 

probably related to tectonic processes at other late 

Mesozoic and early Tertiary margins in Alaska. The 

transect will provide data on the nature of the 

crust, possibly oceanic, beneath this basin and the 

subsurface configuration of the major structural 

boundaries.

Along the north flank of the Yukon-Koyukuk 

basin ophiolitic rocks are obducted over rocks of the 

Brooks Range at a major tectonic boundary known 

as the Kobuk suture zone. The Brooks Range and 

North Slope of Alaska are underlain by the 

parauthochtonous Arctic continental platform, and 

by a belt of schist and marble along its southern 

margin. A critical area of investigation is the 

relationship of the metamorphic belt of the Brooks 

Range to the root zone of the extensive northwarddirected thrust sheets, including obducted

ultramafic rocks, that overlie the Arctic platform 

in the Brooks Range and its northern foothills. The 

episode of major crustal shortening was apparently 

dominantly Early Cretaceous in age. The relative 

timing of thrust episodes and uplift to events in the 

Yukon-Koyukuk basin to the south, and in the 

Colville foredeep and Canada Basin to the north, 

are important to understanding the origin of these 

regional features.

The Arctic platform or Arctic Alaska plate 

extends to the outer continental shelf in the 

Beaufort Sea, where it is overlain by about 10 km of 

virtually undeformed prograded continental terrace 

deposits. These deposits conceal a deep halfgraben, apparently part of a rifted continental 

margin. One interpretation is that this rifted 

margin may have resulted from counterclockwise 

rotation of Arctic Alaska from Arctic Canada to 

form the southern Canada Basin, but alternative 

explanations have been proposed. From the outer 

shelf, the platform bends sharply basinward, 

presumably in a zone of transitional crust, to the 

oceanic Canada Basin of the Arctic Ocean. The 

continental terrace deposits that overlie the 

transition zone are strongly rotated and slivered by 

listric gravity faults. Important problems are the 

age and geometry of rifting, the configuration of 

the crust in the transition zone, the temporal 

relation of rifting to events in the Brooks Range and 

Canada Basin, and the evolution of the sedimentary 

basins of the continental terrace and the Canada 

Basin.

RESEARCH PROBLEMS

A wide variety of research problems await to 

be addressed by TALI. Many of these problems 

were introduced in the previous section in a 

geographical context; here we discuss these and 

other research topics in a generic context. The 

construction of Alaska through the accretion of 

terranes is a topic of major importance for TALI; 

however, equally important are the evolution of the 

continent after amalgamation, the genesis of 

geologic resources, and current tectonics and 

related earthquake and volcanic activity.

Tectonostratigraphic terranes

Until recently, the geological history of 

Alaska has been interpreted under the assumption 

that Alaska is fundamentally continental in 

structure and an integral part of the North 

American continent. This assumption led to many 

complex scenarios to explain the great variations in 

rock type from one area to the next. With the 

realization in the last few years that many areas of 

Alaska are a collage of crustal fragments or 

tectonostratigraphic terranes (fig. 4), many of 

which have moved considerable distances with 

respect to each other, a whole new interpretation of
both the overall and local geology is possible.

Tectonostratigraphic terranes are defined as 

fault-bounded geologic entities of regional extent, 

each characterized by a geologic history that is 

different from the histories of contiguous 

terranes. This does not imply that the terranes are 

all exotic, simply that they are different from their 

neighbors. The recognition of the boundaries of 

terranes often has to rely on indirect observations 

such as linear magnetic and gravity anomalies. It is 

possible that terranes are stacked vertically as well 

as juxtaposed horizontally, particularly in parts of 

Interior or south-central Alaska. For example, the 

base of the Chulitna terrane is a major thrust fault 

with much younger rocks forming the lower plate. 

Over fifty terranes have been proposed in Alaska.

The transect corridor crosses some key 

sequences of terranes. One such sequence, in the 

Livengood district, probably consists of many 

rootless slivers. A task for TALI is to unravel the 

structural relations of these slivers and their history 

of emplacement. The obvious techniques to apply 

are seismic reflection and refraction and deep 

electrical sounding to determine the thickness of 

the terranes; detailed geochemical studies, 

particularly isotope geochemistry, to determine the 

genesis of crustal units; and basic geologic and 

geophysical mapping to determine the lateral 

boundaries of the terranes.

"Basement" of accreted terranes

Traditionally, Alaska has been considered as 

being built upon a "continental" basement; however, 

the whole question of the nature of continental 

crust worldwide is being reexamined. Fundamental 

differences clearly exist between continental and 

oceanic crust, but in the case of Alaska, it is 

doubtful that much of the crust is truly continental 

in nature. Much of Alaska may be composed of 

stacked terranes rafted in by the motion of oceanic 

plates, and these terranes may be stacked deeply 

enough to resemble continental crust 

geophysically. The seismic investigations that are 

ongoing, and those proposed as part of the transect 

studies, should help resolve this question and 

determine which, if any, parts of "basement Alaska" 

can be clearly labeled as belonging to continental 

North America.

An example of our lack of knowledge 

regarding the basement of Alaska is found in the 

Brooks Range and Arctic Alaska. Models for the 

basement beneath the stacked slivers of continental 

margin rocks that compose the Brooks Range range 

from subducted oceanic lithosphere to Precambrian 

crystalline basement. Widely different models can 

also be found in the literature for the Alaska Range 

and southern Alaska. True continental crust of the 

North American craton may exist in the Tindir area 

near the Canadian border. However, even this area, 

with its similarities to the Canadian shield, is being 

questioned as possibly being a terrane that was 

rafted in from further south.

Depth to Mono

Few seismic determinations have been made 

of the depth to the Mohorovicic discontinuity 

beneath Alaska, and many of the determinations are 

based on limited data. For example, estimates of 

about 50 km for the crustal thickness beneath the 

Copper River basin and the northern Chugach 

Mountains near the southern end of the transect are 

based on unreversed seismic-refraction profiles 

obtained in 1955 by the Carnegie Institution of 

Washington. Reversed seismic-refraction profiles 

shot by the U.S. Geological Survey in 1984 reveal 

that along the TALI transect corridor the northern 

Chugach Mountains are underlain to depths of at 

least 40 km by a north-dipping stacked sequence of 

paired low- and high-velocity layers that are 

interpreted as slices of subducted oceanic crust and 

upper mantle (fig. 5). Thus, the concept of a single 

simple Moho discontinuity is not applicable 

everywhere. Estimates of depth to Moho elsewhere 

along the transect route are about 40 km beneath 

the Alaska Range and about 30-35 km beneath 

Fairbanks. Precise seismic measurements are 

needed to resolve the configuration of the crust and 

to confirm and interpret the variations in crustal 

thickness across Alaska suggested by available 

seismic measurements and gravity modeling.

Until recently, seismic-refraction methods 

have provided the most precise information about 

the depth and configuration of the Moho. Seismicreflection techniques, however, are now capable of 

resolving this discontinuity. COCORP profiles in 

the contiguous United States typically resolve the 

discontinuity either as the lower boundary of a 

reflective lower crust or as a strong reflector or 

band of reflectors. Simply mapping the depth to, 

and character of, the Moho along sections of the 

transect route would contribute greatly to 

interpretations of the geologic evolution of Alaska.

Orogeny and deformation of the lithosphere

The Alaskan crust has been shaped by a 

variety of deformational processes; understanding 

the mechanics and history of these processes is one 

of the primary challenges for TALI. Among 

problems inviting study are the existence of roots 

beneath the three major mountain ranges traversed 

by the transect the Chugach, Alaska, and Brooks 

Ranges; the relative roles of isostasy and flexure in 

supporting the Alaskan topography; the thermal and 

deformational history of basins and mountain 

ranges; the mechanics of active low-angle 

subduction along the Pacific margin, continental 

accretion, thin-skinned obduction in the Brooks 

Range, and rifting along the Arctic margin. A 

broad range of geologic and geophysical techniques 

can contribute information important to these 

problems, including geobarometry, structural 

analyses, microfabric studies, seismic 

investigations, gravity and magnetic modeling, and 

heat-flow studies.
SOUTH NORTH

100 80 60 40 20 

DISTANCE FROM SHOTPOINT 8, IN KILOMETERS

Figure 5. Seismic record section and P-wave velocity model along TALI transect in northern Chugach 

Mountains and southern Copper River Basin, between about 61° and 62° N. latitude (after R. A. Page 

and others, 1986, Geology, v. 14, p. 501). _A, Record section from shotpoint 8 at north end of seismicrefraction profile. Each vertical line is a seismogram from an individual site, range of which is 

indicated by position of seismogram along horizontal axis. Traveltimes are reduced by distance divided 

by 6 km/s. Solid lines indicate refraction branches; dashed lines, reflection branches. Numbers are Pwave seismic velocities (km/s) in layers in model below. B, Velocity model determined using data from 

indicated shotpoints (SP 8-12, 19) and one shotpoint off section to north. Low-velocity zones (LVZ's) 

are hachured. Velocities and thickness of LVZ's are not well constrained. Arrows indicate positions of 

shotpoints along profile. BRF, Border Ranges fault. Location of vertical seismic-reflection section in 

figure 7 is indicated.

Subduction and magmagenesis

The TALI corridor crosses such a variety of 

tectonic settings that a large range of styles of 

magmagenesis is accessible to study. The Gulf of 

Alaska, Talkeetna Mountains, and Alaska Range 

contain plutonic belts of Tertiary, Mesozoic, and 

Paleozoic ages, respectively. The Wrangell volcanic 

pile covers over 10,000 km and is one of the 

largest in the world. The northern end of the 

Aleutian Wadati-Benioff zone, north of Cook Inlet, 

is interesting because that segment of the Aleutian 

subduction zone has no active volcanoes associated 

with it (fig. 4). It is likely that important 

constraints on the mechanics and chemistry of 

magmagenesis can be found by comparing segments 

of subduction zones without volcanoes to those that 

have them. The Gulf of Alaska is an especially 

interesting place in this regard because it

encompasses the transition from the classic 

subduction zone in the Alaska Peninsula region to an 

anomalous zone of high seismicity and no volcanoes 

in the Prince William Sound-Talkeetna Mountains 

area, to an equally anomalous but contrasting zone 

of low seismicity and major volcanism in the Copper 

River-Wrangell Mountains area. The key to the lack 

of volcanoes associated with the Prince William 

Sound segment of the subduction zone may be the 

set of conditions that give rise to its buoyancy. 

Increased buoyancy would cause a shallow angle of 

plate descent, which might lead to significant 

differences in the amounts of water and sediments 

that are subducted as well as possible differences in 

the temperature along the trajectory of 

subduction. The TALI program could provide 

critical information regarding the age, temperature, 

and density of the material being subducted in the 

central Gulf of Alaska.
Major fault systems

Faulting has obviously played a major role in 

shaping the complex geology of Alaska. The 

transect route crosses several major fault systems 

of varying character, age, and history, ranging from 

the active Aleutian megathrust along the Pacific 

margin to the reactivated strike-slip Denali, 

Tintina, and Kobuk-South Fork fault systems, to the 

Cretaceous and Tertiary normal faults of the rifted 

Arctic margin. Although many faults have been 

mapped and investigated at the surface, little is 

known about the geometry and behavior of most 

faults at depth, particularly below a few 

kilometers. Two problems of particular interest are 

the configuration and nature of faults within major 

suture zones, such as the Border Ranges, Kaltag, 

and Kobuk fault zones, and the configuration of the 

extensive thrust sheets composing the Brooks 

Range. As demonstrated elsewhere, faults 

recognized at the surface often can be traced to 

great depths within the crust using seismicreflection profiling. The reactivation of old fault 

systems is another topic of interest. A case 

example is the Denali fault system a collision 

suture in the late Mesozoic, a part of which has 

been reactivated as a strike-slip fault in late 

Cenozoic time.

Neotectonics

The current tectonic framework of southern 

Alaska is well understood relative to the Interior or 

northern Alaska. Southeast Alaska is dominated by 

transform faulting, and the directions of motion 

correspond well with the relative motion between 

the Pacific and North American plates. 

Southwestern Alaska, particularly from the Alaska 

Peninsula out into the Aleutian Arc, has a 

"textbook" subduction zone. South-central Alaska is 

more complex inasmuch as it includes the transition 

between the transform system to the east and the 

subduction zone to the west. The northern edge of 

the subducted Pacific plate, as delineated by 

earthquake hypocenters, extends several hundred 

kilometers inland from both the trench and the 

continental shelf (fig. 6). The eastern edge of the 

Aleutian segment of the under riding Pacific plate 

can be more or less located by the sudden decrease 

in earthquake activity to the east (approximately 

along dotted line, fig. 6).

Although the northwest-dipping subducting 

plate associated with the Aleutian volcanic arc is 

clearly defined by seism icity, only a few 

hypocenters indicate the subducting slab associated 

with the Wrangell volcanoes. This slab dips northnortheast at an oblique angle to the Pacific-North 

American relative motion vector. The extrapolated 

100-km depth contour on the slab lies beneath the 

active Mount Wrangell volcano. Southeast of the 

Wrangell volcanic pile is a fault-bounded block 

known as the Yakutat terrane or block which is 

presently docking against Alaska. It appears to be

moving more slowly than the Pacific plate, and thus 

is moving with respect to both Alaska and the 

Pacific plate. The transect crosses the boundary 

between the well-defined Aleutian and the poorly 

understood Wrangell Wadati-Benioff zones and 

should resolve the nature of the boundary or 

transition between these two subducted plate

71° 151° 1470 143*

69

67«i

65°

59°

Figure 6. Epicenters of earthquakes of magnitude M24.0 in vicinity of TALI transect (dashed 

line), 1900-1983 (from ADGGS files). Focal 

depth contours at 50-, 100- and 150-km levels 

are shown together with section of small circle 

(dotted line) about relative motion pole of 

Pacific and North American plates (RM-1).

10
segments.

Northern Alaska is commonly depicted as 

simply part of the North American plate, the 

Eurasian-North American plate boundary being 

somewhere in Siberia. However, the various plate 

boundaries drawn through Siberia and Sakhalin 

Island, or through Kamchatka, produce relative 

motion anomalies on global reconstructions. This 

suggests the possibility that a Bering Sea plate 

exists, which then leads to the question as to the 

location of its eastern boundary. It is possible that 

the belt of diffuse seismicity crossing the Brooks 

Range southwestward from the Mackenzie Delta 

region represents a deforming plate boundary. The 

proposed transect crosses this diffuse seismic trend, 

and it is possible that new information on the depth 

to Moho and basement structure will produce 

evidence for or against such a deformational plate 

boundary.

Oroclinal bending

In his now classic paper of 1958, S. Warren 

Carey defined the term orocline using the curved 

Alaska Range as one example. He explained the 

origin of the Alaska orocline as due to bending at 

the hinge between the Eurasian and North American 

continents as a result of the opening of the Atlantic 

Ocean. Subsequent attempts to investigate the 

orocline hypothesis have not so much disproved it as 

demonstrated other possible processes, such as the 

accretion of exotic tectonostratigraphic terranes.

The oroclinal bending concept applies not only 

to features such as the Alaska Range and the Brooks 

Range, which in Carey's model were supposed to 

have bent as part of a bending of the whole crust or 

lithosphere, but also to decoupled slices of crust. 

This mechanism has been proposed to explain the 

curvature in plan view of some of the accreted 

terranes of southern Alaska. It is supposed that 

they acquired their present curvature as the result 

of colliding with a curved backstop; the lithospheric 

plate on which they were carried was presumably 

subducted. Both of these oroclinal bending 

processes may have operated in Alaska, and if so, 

evidence may be combined in the paleomagnetic 

record and in crustal and subcrustal structures, 

particularly in the configuration of the underlying 

lithosphere.

The Brooks Range oroclines are defined in 

different ways by different authors. A somewhat 

subtle concave-northward curvature may be 

explained by compression between the Eurasian and 

North American plates. There is also the bend to 

the south if the eastern end of the Brooks Range 

structure is connected to the Rocky Mountain 

system. Many tectonic models depend on this 

connection, or the connection to the Canadian 

Arctic. Though this is not a problem that could be 

tackled directly by the present transect study, 

information on the deep structure beneath the 

Brooks Range will help constrain the possible 

models.

Earthquake hazards

In the assessment of seismic risk, often least 

understood are the basic seismotectonic and 

geologic facts on which estimates of the earthquake 

hazard are founded. These facts include 

considerations such as the geometry of the WadatiBenioff zone, the activity of various fault segments, 

and the delineation of seismotectonic zones. Our 

understanding of these fundamental geological facts 

evolves slowly, in concert with the development of 

information from many disciplines; rarely can 

fundamental advances be made in the course of the 

risk analysis for a given project because the scope 

of investigation, resources, and time frame are all 

too limited. Therefore projects such as TALI can 

make important contributions to the assessment of 

earthquake hazards in Alaska, both because of the 

new information that will be developed and because 

of the synthesis of geological and geophysical 

information that it will encourage. In southern 

coastal Alaska, the TALI corridor includes the 

presently active plate boundary where basic 

questions exist as to the depth and lateral extent of 

the Pacific plate beneath Alaska, especially under 

the Wrangell volcanoes. In south-central Alaska, 

the corridor crosses several suture zones and 

transform faults, such as the Border Ranges and 

Denali fault systems. In the Interior, it crosses the 

Fairbanks and Rampart seismic zones, areas of 

persistent seismic activity that are not yet 

associated with any specific structures. In northern 

Alaska, the corridor crosses the Kaltag-YukonPorcupine and Kobuk-South Fork fault systems and 

the many thrust sheets of the Brooks Range. In 

each of these areas, the potential is high for 

developing new information on the activity or 

capability of specific fault systems and for making 

a major advance in our understanding of the basic 

seismotectonic framework of Alaska.

Volcano hazards

The Wrangell Mountains in south-central 

Alaska are a cluster of largely post-Pliocene calcalkaline andesitic volcanoes. The edifices are 

closely spaced, reach elevations up to 5,000 m and 

have had extremely high eruption rates as compared 

to other arc-type volcanoes around the Pacific 

rim. The Wrangell volcanoes are among the largest 

volcanoes in the world to be found at a convergent 

plate margin. A Wadati-Benioff zone associated 

with the Wrangell Mountains has recently been 

defined, suggesting that post-Pliocene volcanism is 

linked to a subducted plate segment that is possibly 

attached to the leading edge of the Yakutat block 

(terrane). The Wrangell volcanic pile of late 

Tertiary through Quaternary age is not related to 

the underlying Paleozic and Mesozoic basement (the 

Wrangellia terrane). A major geophysical anomaly 

associated with the Wrangell volcanoes is a regional 

gravity low in excess of 150 mGal amplitude, the 

origin of which is not well understood.

11
Fundamental tectonic and volcanologic 

questions related to the unique Wrangell volcanoes, 

which could be answered by nearby TALI traverses, 

include structure of the crust, origin of the mass 

deficiency beneath the Wrangells, extent and 

configuration of the subducting plate, thickness and 

relationship of the Wrangell volcanic rocks to the 

Wrangellia terrane, and unusually high effusive 

rates of Wrangell volcanism. Related to the latter 

point are obvious volcanic hazard concerns that 

need to be addressed. For example, a late 

Quaternary volcanic debris flow from Mount 

Wrangell reached the Copper River near the 

Richardson Highway a distance of about 45 km. 

The Wrangell Mountains may be the source of some 

of the very large ash deposits found in Alaska. 

Detailed geochemical studies including isotope 

geochemistry could shed some light on the source 

regions of the unusual Wrangell lavas.

Metallogenesis

It has long been known that certain kinds of 

mineral deposits are associated with specific 

geologic settings, but it is only in recent years that 

some of the cause-and-effect relationships have 

been discovered. An example is the direct 

observation of mineral-rich hot springs in the rift 

valleys associated with submarine spreading 

centers, and the recognition of economic mineral 

deposits that originally developed in such settings. 

In general, it appears that major metal ore deposits 

are generated near, or are related to spreading 

centers and subduction zones. Since it is now 

recognized that much of Alaska is made up of 

fragments of crust rafted in from elsewhere, and 

that many of these fragments have been associated 

with subduction zones before and(or) during the 

accretion process, the location of these fossil zones 

would be an aid to mineral exploration. The 

converse of this is the identification of the tectonic 

settings from studies of mineral deposits. An 

example is the hypothesis of a failed rift for the 

generation of the Brooks Range mineralized belt. In 

either case, the identification of ancient tectonic 

boundaries within the jumble of Alaskan terranes 

could significantly enhance mineral exploration 

strategies.

Basin analysis

Nearly every field of geology and geophysics 

can contribute substantially to the geologic analysis 

of basins; hence the multidisciplinary studies 

planned for TALI will aid greatly in our 

understanding of Alaskan basins. In particular, the 

plans for TALI that include detailed geologic 

mapping as well as radiometric and biostratigraphic 

dating will give new insight into basin history, 

stratigraphy, and structure. One profile through a 

basin cannot adequately characterize the detailed 

stratigraphy and structure of parts of the basin that 

are remote from wells and outcrops. However,

regional crustal features that control the location 

and configuration of basins may be sufficiently well 

characterized by a single line or section. Study of 

seismic-reflection data together with available 

outcrop and well data can illuminate the 

relationship between the regional crustal structure 

and the broader aspects of basin stratigraphy and 

structure, such as regional faults, unconformities, 

and major changes in depositional style. The 

evolution and thermal history of a basin can be 

analyzed by combining seismic-reflection data and 

information about heat flow and the metamorphic 

pathways of rocks that floor or border the basin.

Geothermal resources

The southern part of the TALI corridor 

traverses the Copper River basin, an area which 

includes a region near Mount Wrangell volcano that 

has a moderate to high potential for geothermal 

energy recovery. The rate of volcanic activity in 

the Wrangell Mountains has been about five times 

greater than that reported for other circum-Pacific 

volcanoes. It is probable that a large amount of 

heat has been transferred into the upper crust. 

Mount Wrangell is still active, and Mount Drum was 

active less than 200,000 years ago. Residual heat 

reservoirs may be quite near the surface in the 

vicinity of these volcanoes. The basic geological 

and geophysical studies planned for the USGS TACT 

project will help to constrain models for the 

development of the Wrangell volcanoes. Other 

studies are needed to constrain models of the heat 

budget for the Wrangell area. It is possible that a 

significant energy source could be developed here.

RESEARCH TECHNIQUES

The challenging problems in Alaskan geology 

and tectonic evolution are best addressed by 

application of a broad range of complementary 

research techniques drawn from different earthscience disciplines. Because investigation of 

Alaskan geology in its third dimension depth is a 

major thrust of TALI, geophysical methods, and 

particularly seismic techniques, will play a vital 

role.

Seismological

Seismological techniques provide valuable 

constraints on earth structure and rock composition 

when used in conjunction with geologic and 

laboratory techniques, well data, and other deep 

sounding techniques, including gravity, 

aeromagnetic, and magnetotelluric methods. For 

example, the geologic nature of a buried seismic 

discontinuity recognized in a seismic record can be 

interpreted if the discontinuity can be traced to the 

surface. As another example, knowledge of rock 

velocity in combination with resistivity provides a 

powerful constraint on rock composition.

12
A major development that could benefit TALI 

is the Program for Array Seismic Studies of the 

Continental Lithosphere (PASSCAL) initiated by 

IRIS (Incorporated Research Institutes in 

Seismology). PASSCAL is a collaborative effort of 

more than 40 universities and research institutions 

to design, build, and operate a 1,000-element 

seismic array to investigate the structure, physical 

properties, and earthquake physics of the 

continental lithosphere. The elements of this 

proposed array would be portable digital seismic 

recorders that would obtain data over a broad band 

of frequencies with a high dynamic range. An array 

of such recorders would undoubtedly facilitate and 

enhance many of the seismological investigations 

that might be proposed as part of TALI; however, 

several critical seismic studies can be implemented 

with existing instrumentation.

Artificial sources 

Refraction and wide-angle reflection profiling

Seismic-refraction and wide-angle seismicreflection profiling yield estimates of lateral 

velocity variations and depths to velocity 

discontinuities within the crust and upper mantle. 

The technique is capable of resolving structural 

variations on the order of a few kilometers and 

velocity variations of a few percent. Spread lengths 

of 100 to 200 km and relatively large explosions 

(1,000 kg) are required to determine structure in the 

upper and lower crust, but longer spreads and even 

larger explosions are generally necessary to 

investigate the Moho and upper mantle. This 

technique is therefore suited for investigating 

seismic structure of sufficiently large 

tectonostratigraphic terranes where velocity 

contrasts within and between terranes exceed a few 

percent. The lower crustal structure of the 

terranes, including the deep structure of mountain 

ranges and variations in depths of Moho, is 

addressable by seismic-refraction techniques once 

the upper crustal structure is known. The seismicrefraction technique is already producing exciting 

results from southern Alaska. Initial USGS seismicrefraction profiles in the Chugach Mountains reveal 

a complex lithosphere comprising thin accretionary 

terranes overlying stacked slices of inferred 

subduction assemblages of oceanic crust and upper 

mantle (fig. 5). Experiments that can be performed 

with large explosions include acquisition of new 

profiles, extension and densification of existing 

profiles, and amplitude studies to determine seismic 

attenuation and velocity gradients.

Vertical reflection profiling

Vertical seismic-reflection profiling is the 

principal geophysical tool of the petroleum industry 

for exploring sedimentary basins with fine 

resolution of stratigraphy and geological structure. 

In the last decade, seismic-reflection profiling of

the continental crust has become an important 

means for exploring deep crustal structure with 

greater resolution than any other geophysical 

technique. Vertical reflection profiling can reveal 

the configuration of both simple and complex 

crustal structure with high resolution. Reflection 

profiling, especially when large offsets are used, 

also provides information on seismic velocities. 

Where deep crustal reflections can be traced to 

shallow depths and correlated with surface geologic 

features, the method is exceptionally powerful.

Reflection profiling will surely provide 

critical information on Alaska's complex nearsurface geology and its heterogeneous crust of 

accreted terranes. A realistic goal for TALI is a 

complete end-to-end seismic-reflection profiling 

study of the trans-Alaska oil pipeline route. To 

stimulate interest in this goal, the USGS in 1986 

contracted acquisition of two demonstration 

profiles in southern Alaska one in the Chugach 

Mountains and southern Copper River Basin, 

crossing the Border Ranges suture zone, and one in 

the Alaska Range, crossing the Denali fault. Both 

profiles successfully imaged crustal reflectors. 

Field records south of the Border Ranges fault, for 

example, reveal prominent horizontal and gently 

dipping, anastomosing reflections in the depth range 

15 to 35 km (fig. 7). In combination, the refraction 

and reflection data provide powerful constraints on 

interpretations of the structure of the upper 

lithosphere.

Marine profiling

Marine studies including seismic-reflection, 

seismic-refraction, and dredge sampling could 

provide essential constraints on the structure of the 

Alaskan continental margins. In particular, seismic 

profiling could yield information on the 

configuration of the North American-Pacific plate 

boundary at depth, the structures developed within 

the overriding plate, and the geometry of the 

subducting oceanic slab. Seismic-refraction data 

complement vertical reflection data by providing 

more detailed velocity information and by sampling 

the earth over different ray paths and at different 

frequencies. When combined with test well data 

and seafloor sampling, seismic data can yield 

information on the timing of deformational events 

and the nature of the reflective seismic horizons. 

Single-ship, large-source-array, and two-ship marine 

studies are particularly powerful ways to obtain 

seismic profiles.

Earthquake sources 

Seismic imaging

It is important to supplement knowledge of 

crustal structure obtained by seismic refraction and 

reflection with information on the deep structure of 

the lithosphere and asthenosphere. Techniques 

using the arrival-time residuals of seismic waves

13
NORTH

12

5 KILOMETERS

14
from teleseismic and regional earthquakes are the 

most useful for investigating the earth to depths 

significantly below the Moho. Deep imaging of the 

earth's crust and upper mantle using these 

techniques has provided fundamental information 

about tectonic processes related to hot spots, 

subduction zones, rift zones, plate boundaries, and 

intraplate volcanic centers. Three-dimensional 

inversion and tomographic techniques can be used to 

delineate heterogeneities in the crust and mantle 

associated with a subducting plate.

The resolution of teleseismic-residual 

techniques for delineating velocity anomalies is 

about 5 km horizontally and 10 km vertically, 

depending upon the density and configuration of the 

seismic array used for recording the teleseisms. 

Velocity variations as small as 0.5 percent can be 

measured. Using intermediate-period seismometers 

in the arrays, shear waves can be recorded so that 

shear-wave traveltime residuals can also be 

modeled. Modeling shear-wave velocity anomalies 

simultaneously with P-wave anomalies allows a 

more definitive interpretation of the physical and 

thermal properites of the modeled volume. In 

addition to teleseisms, the large number of deep 

earthquakes occurring in southern Alaska itself can 

be used to model the subduction zone there, as has 

been done in Japan. P- and S-wave amplitude data 

from teleseisms and regional earthquakes can 

support estimates of the spatial variation of seismic 

attenuation to provide additional constraints on the 

structure and physical properties in the deep crust 

and upper mantle.

Seism icity studies

Precise earthquake hypocenters can be used to 

delineate buried active faults. Such structures 

often are not amenable to investigation by seismic 

refraction or reflection because they either are too 

deeply buried or juxtapose rocks of similar 

velocities or densities. The distribution of 

seism icity also provides important clues in regard to 

the Theological properties and stress conditions at 

depth within the lithosphere.

Figure 7. Vertical seismic-reflection section 

from the northern Chugach Mountains, 

southern Alaska. See figure 5 for location. 

Strongly reflective character of section 

between 5 and 10.5 s is consistent with 

layered sequence of alternating low and high 

velocities inferred from refraction/wideangle reflection survey (fig. 5). Horizontal 

axis, distance along profile. Vertical axis, 

two-way traveltime; 1 s approximately 

equals 3 km. Section is partial brute stack 

produced in field. Seismometer group 

interval, 30 m; vibrator interval, 120 m. 

Fold of data increases from 1 at edges of 

record to about 30 in center.

Hypocenters of earthquakes large enough to 

be recorded at teleseismic distances can be 

determined to accuracies of several kilometers, 

provided the traveltimes are precisely measured and 

the velocities of seismic waves to the recording 

stations are adequately known. Frequently, the 

location accuracy of large historic earthquakes can 

be improved by relocating the events using specially 

calibrated traveltimes. Shocks recorded by dense 

regional seismograph arrays can be located to 

accuracies approaching 1 km if the regional velocity 

structure is adequately known. Hypocentral 

resolution, or accuracy of relative locations among 

adjacent shocks, typically exceeds absolute 

accuracy, because it is limited only by errors in 

measuring arrival times. Thus, the configuration 

and orientation of a buried seismogenic feature may 

be determined more accurately then its actual 

location.

Seism icity studies can contribute to the 

solution of several important problems. Among 

them are the extent, continuity, and configuration 

of subducted lithosphere beneath southern Alaska 

and, in particular, between the Aleutian and 

Wrangell Wadati-Benioff zones; the styles of 

faulting and rates of seismic deformation on the 

Aleutian and Wrangell megathrusts and within the 

overriding and subducting plates in southern coastal 

Alaska; the relation of seism icity to mapped or 

geophysically inferred geologic structures and 

lithology; the reactivation of ancient faults and 

sutures; and the seismotectonic framework of 

Alaska. The solution of many of these problems 

requires the temporary deployment of seismograph 

arrays either to supplement clusters of existing 

seismograph stations (fig. 8) or to provide 

seismograph coverage where none currently exists.

Focal mechanism studies

Focal mechanism studies include both 

traditional double-couple interpretations of P-wave 

first motions and S-wave polarizations, and seismicmoment tensor solutions which have become 

routinely possible with the advent of a worldwide 

set of digital seismograms. Each of these 

techniques provides information that is grossly 

interpretable in terms of the state of stress in the 

vicinity of the earthquake source. For all 

earthquakes, except very shallow ones, focal 

mechanism studies provide the only possible 

measurement of the state of stress in the vicinity of 

the focal region.

Focal mechanism solutions might suggest the 

nature of the northeastern termination of the 

Aleutian Wadati-Benioff zone whether the 

subducted slab is torn at that point or merely 

becomes seismically quiet. For shallow earthquake 

concentrations along the transect route, focal 

mechanism studies in combination with precise 

hypocentral determinations could resolve the 

orientation and sense of motion of buried active 

faults.

15
Surface wave analysis

Measurement of phase- and group-velocity 

dispersion of Rayleigh waves provides a simple 

method to determine regional shear-wave velocity 

structure from earthquake-generated waves 

recorded at a few stations. The theory of 

interpretation relies upon simple plane-layered 

earth structures in which the layer thicknesses and 

compressional velocities are known. For this 

reason, the method is most powerful when applied 

to a region for which seismic-refraction data are 

available.

Relying upon natural sources, the experiment 

requires a relatively long period of continuous 

recording (6 months to 1 year) to obtain an adequate 

data set. The dimensions of the larger Alaskan 

terranes, approximately 200 km, are adequate for 

measurements of waves with periods of 15 to 200 

seconds using a 5- to 10-station array. Generally 

speaking, this bandwidth is suitable for determining 

shear velocity structure from the mid-crust to the 

low-velocity channel of the asthenosphere. A large 

number of stations reduces error in the dispersion 

measurements and thus improves confidence in the 

interpretations. A reasonable TALI objective would 

be to employ five or more instruments with 

bandwidth from 1 to 100 seconds for 6 to 12 months 

across each large terrane. This would allow 

determination of the average elastic properties for 

each terrane, properties that are not available from 

methods based on active seismic sources.

Geophysical (nonseismic)

Other geophysical studies will play an 

important role in support of the TALI seismological 

studies. These include potential-field methods, such 

as gravity and magnetic modeling; geoelectrical 

methods, such as magnetotelluric soundings; 

borehole-based methods, such as heat-flow 

measurements and in situ stress measurements; 

paleomagnetic methods to determine paleolatitude 

of major portions of the Alaskan crust; and aircraftand satellite-based methods, such as remote sensing 

to help delineate structural and geological blocks, 

and satellite-based geodetic measurements to 

detect present-day relative motions between these 

blocks.

Potential-field geophysics 

Gravity analysis

Local variations (anomalies) in the earth's 

gravity field can be used to discriminate geologic 

bodies that differ in density from their neighbors 

and to infer their geometries and contrasts in 

density. Thus, gravity is useful in studying such 

features as igneous intrusions, sedimentary basins, 

faults, geological contacts, and crustal roots 

associated with foldbelts or subduction zones. 

Gravity data are also critical for assessing whether

71°, 151° 147° 143°

69°i

67

61°

59°

65

63°

200 KILOMETERS 

_I

Figure 8. Location of seismograph stations in 

vicinity of TALI transect, 1984-1985. USGS 

stations are represented by hexagons, UAF 

stations by squares, and NOAA stations by 

diamonds. Note paucity of stations north of 

65° N. latitude.

the topography along the transect is isostatically or 

dynamically supported. The USGS is compiling 

existing gravity data in digital form to facilitate 

production of complete Bouguer gravity anomaly 

maps along the transect route, and for use in 

interpreting the geologic and other complementary 

geophysical data. Limited additional gravity data

16
are being collected in conjunction with the ongoing 

USGS seismic profiling and in conjunction with 

several State, Federal, and university field 

projects. It is hoped that industry data will also 

become available. In general, additional gravity 

coverage is needed within the TALI corridor; it may 

be obtainable with recently developed airborne 

gravity techniques.

Opportunities exist for modeling not only the 

geometries and density of geologic bodies and the 

configuration of geologic boundaries within the 

TALI corridor, but also the rheologic response of 

the lithosphere to tectonic processes, including 

subduction, obduction, formation of batholiths and 

volcanic fields, and oroclinal bending. An important 

data base for large-scale rheological modeling is 

that derived from satellite altimetry.

Magnetic analysis

Although magnetic data are available for most 

of the State and all of the TALI corridor, they are 

commonly in various formats and have been 

collected in various ways. The USGS and the 

ADGGS have compiled the available data, but the 

data still need refining and gaps need to be filled.

Opportunities exist for analysis of existing 

magnetic field data along the TALI corridor and 

also for the collection and analysis of new data in 

certain areas. These opportunities include modeling 

of magnetic structures via semi-inverse and 

filtering techniques, estimation of Curietemperature depths, and synthesis of magnetic 

results with those from geologic, seismic, and other 

geophysical methods.

Deep geoelectrical studies

Deep geoelectrical studies include 

magnetotellurics, active-source time-domain 

sounding, and direct-current resistivity sounding. 

These methods are capable of determining 

electrical resistivity at depths ranging from a few 

meters to several hundred kilometers. Resistivity 

of crustal and mantle rocks is related to rock type, 

temperature, and water content; it thus may help to 

identify geological features that are hard to detect 

otherwise, such as deep crustal conductors that may 

characterize mylonitized zones or tectonically 

mobilized zones. As is common with geophysical 

observations, the properties determined by 

geoelectrical methods seldom have a unique 

interpretation, but they can be used to constrain, or 

their interpretation can be constrained by, the 

results of other geophysical techniques.

Magnetotelluric soundings are especially 

useful when done in conjunction with seismicreflection and -refraction profiles, as they can 

penetrate to similar depths. Geoelectrical data can 

in places indicate major faults and depths of 

sedimentary basins, differentiate between 

sediments of different geologic ages, and resolve 

spatial variations in the physical properties of the

crust; thus they are useful in constraining and 

augmenting seismic interpretations. Opportunities 

exist to conduct geoelectric sounding along the 

TALI corridor and to apply appropriate 

interpretative techniques.

Paleomagnetic studies

Paleomagnetism uses remanent magnetism, 

acquired as the rocks were laid down or cooled from 

a molten state, to reconstruct paleogeography. 

Available data indicate that several major terranes 

of southern Alaska originated at equatorial 

paleolatitudes. There is not yet enough information 

available to resolve the rates or style of northward 

motion, or its timing, although as more data 

accumulate, the constraints on possible models get 

tighter. Paleomagnetic measurements can also aid 

in determining whether terranes identified on 

geologic grounds were ever significantly displaced 

from one another. For Arctic Alaska, however, a 

pervasive overprint has masked the paleomagnetic 

record and made this determination difficult. 

Nevertheless, studies of this overprint itself, in the 

Brooks Range and elsewhere, may give valuable 

information relating to the thermal and tectonic 

history of the terranes of the TALI corridor.

Borehole geophysical studies

A number of boreholes may become available 

for research purposes during TALI, such as 

petroleum or mining exploration holes, seismic shot 

holes, holes drilled in connection with other 

scientific programs (for example, permafrost 

studies), or holes dedicated for specific TALI 

objectives. These boreholes will provide 

opportunities for heat-flow measurements, which 

would permit preparation of a heat-flow map for 

the TALI corridor and nearby portions of Alaska and 

Canada; in situ stress measurements via 

hydrofracturing techniques or emplacement of 

strain gauge arrays; and measurement of 

geoelectric, density, seismic, and other geophysical 

properties, and lithologic descriptions of the rock 

masses penetrated by the borehole.

Satellite-based studies 

Remote sensing

Airborne and satellite remote sensing 

techniques use electromagnetic radiation in the 

visible, near-visible, and gamma-ray bands collected 

from airborne or satellite sensors. The data are 

processed to produce images which reflect tectonic 

elements (for example, faults), geochemical 

signatures (for example, alteration patterns), and 

physical properties (for example, the thermal 

inertia of the rocks). Opportunities exist to produce 

remote sensing images of the TALI corridor and to 

integrate them with geologic and other types of 

data.

17
Geodesy

New developments in space and satellite 

geodesy allow relative distance measurements at 

the earth's surface at the decimeter level at 

present, and perhaps at the centimeter level within 

the lifetime of TALI (5-6 years). These techniques 

are being applied to Alaskan geodynamic problems 

initially near the active plate boundary along the 

Pacific margin, with emphasis on seismic gaps. In 

the future, the techniques will be used to 

investigate internal plate deformations. Starting in 

1984, six sites in Alaska were occupied by very long 

baseline interferometry (VLBI) stations, three of 

which focused on the Pacific-North American plate 

margin. Simultaneous measurements in Japan, 

Kwajalein, Hawaii, California and Canada were 

made to establish the first baseline data for the 

relative motion between the Pacific and North 

American plates and for stability within the North 

American plate. These measurements will bear on 

subjects of central interest to TALI: (1) plate 

boundary processes in southern Alaska; (2) relative 

motion between terranes bounded by active faults in 

the interior of Alaska; (3) isostatic equilibrium and 

vertical crustal deformation; (4) strains associated 

with ongoing flexure of the lithosphere; and (5) 

delineation of stress provinces through combination 

of space geodetic measurements with geologic and 

volcanic stress indicators and with other direct and 

indirect stress measurement methods.

Rock properties

Information on the properties of rocks from 

the TALI corridor is needed for use in modeling and 

interpreting geophysical data along the corridor. 

Rock properties measured by the geophysical teams 

involved in TALI should be incorporated into a 

widely accessible data base. Important parameters 

to include are rock type and mineralogy, porosity, 

pore fluids, chemistry, density, magnetic 

susceptibility, electrical resistivity, and P- and Swave seismic velocities and velocity anisotropies. 

Many measurements of these various rock 

properties exist, but they are scattered throughout 

industry, State and Federal agencies, and 

universities. Existing information needs to be 

compiled in an accessible data base to which new 

data could be added.

Geological

TALI presents many geological research 

opportunities for advancing the knowledge of the 

stratigraphy, structure, tectonics, and geologic 

history of Alaska. Geological studies will provide 

the framework for deciphering the deep crustal 

structure of Alaska and will elucidate the nature 

and history of accretionary tectonic processes that 

have shaped the Alaskan lithosphere. Geological 

data and interpretations are essential for providing 

the foundation for interpreting data obtained from

seismic and potential-field geophysical methods, 

and from other geophysical and geochemical 

techniques that will be incorporated in TALI.

Deciphering the complex geologic history of 

the TALI corridor demands application of a broad 

suite of geological and geochemical methods. 

Maximum progress requires coordination and 

integration of various types of geological and 

geochemical studies, many of which should proceed 

in parallel. For example, structural and fabric 

studies of metamorphic rocks should be integrated 

with petrologic and isotopic studies to determine 

the temperatures, pressures or depths, and ages of 

simultaneous periods of metamorphism and 

deformation.

Geologic mapping

Geological mapping is the most basic 

ingredient of the TALI program in the sense that all 

other earth-science disciplines and techniques 

require knowledge about the nature and distribution 

of the various bedrock units that occur along the 

transect route. The principal components of 

geological mapping should include: (1) compilation 

of existing mapping within 50 to 100 km of the TALI 

route at scales of 1:250,000 or larger; (2) 

systematic mapping along the TALI route at a scale 

of 1:63,360; (3) larger scale mapping of geologically 

complex or critical areas, such as major sutures; (4) 

collection and compilation of data for topical maps, 

such as metamorphic isograd and facies maps, 

structural maps, and maps of active faults and 

Quaternary tectonic deformation; and (5) regional 

maps along the route with emphasis on synthesis.

Stratigraphic, basin analysis, and paleontological 

studies

Several types of basic information are needed 

to elucidate the nature, origin, and geologic history 

of sedimentary rocks exposed along or near the 

TALI route. This knowledge will further 

understanding of the timing of amalgamation of 

accreted terranes and the long-term history of 

vertical tectonic movements in and near 

sedimentary basins. The types of information 

needed include: (1) sedimentary facies 

distribution; (2) sedimentary petrology; (3) 

diagenetic and thermal history, and organic 

maturation data; (4) biostratigraphy, paleoecology, 

and paleontologic province data; (5) structural 

framework of sedimentary basins; and (6) 

paleocurrent and provenance data.

Petrologic, geochemical, and isotopic tracer 

studies

These three disciplines will provide significant 

information on the temperature-pressure histories 

of the various exposed crystalline terranes. They 

can provide information on the existence of ancient 

magmatic arcs in Alaska, on the distribution of

18
accreted oceanic crust, and on plutonism associated 

with accretion of allochthonous terranes onto 

nuclear Alaska. Petrologic, geochemical, and 

isotopic investigations will focus on the various 

igneous and metamorphosed igneous rocks to 

determine the mineral phases, the abundances of 

major, minor, and trace elements, and the isotopic 

composition of selected elements, such as strontium 

and lead. The most important goals of such 

investigations will be: (1) to infer the original 

plate-tectonic environment of the volcanic rocks, 

such as mid-ocean ridge, intraoceanic arc, or 

intraplate platform; (2) to determine the pressuretemperature conditions of generation of various 

plutonic rocks; and (3) to determine the nature and 

origin of compositionally distinctive mafic or 

intermediate rock suites, such as the mafic complex 

of the Border Ranges and associated intermediate 

intrusive rocks of the Chugach Range. Together, 

such results may place constraints on the nature of 

the lower and intermediate crust that are not 

obtainable by other means.

Petrologic studies of metamorphic rocks

Petrologic studies will address the pressuretemperature and deformational history of 

metamorphosed crystalline rocks exposed along the 

TALI route. Amalgamation and accretion of 

terranes should leave a weak to strong metamorphic 

imprint on deep crustal rocks. Recently developed 

techniques can yield relatively precise information 

on the temperature, total pressure or depth, and 

fluid pressures that occurred in each event; and 

isotope geochronology can commonly date these 

events. Petrologic studies of metamorphic rocks, 

including determination of prograde and retrograde 

mineral assemblages, microprobe analyses of 

mineral phases, and studies of fluid inclusions and 

vein fillings will elucidate the pressure-temperature 

conditions of metamorphism. In conjunction with 

age-annealing and temperature studies by fissiontrack and AT- AT methods, the various 

petrologic results may be used to construct the 

pressure-temperature paths, temperature gradients, 

and histories of uplift of metamorphic rocks.

Structural and fabric studies

Structural and fabric studies will yield 

important information on the deformational history 

of rock sequences. Such studies can provide 

constraints on timing and geometry in the 

amalgamation of allochthonous terranes and in their 

accretion to the margin of nuclear Alaska. 

Important topics for investigation include: (1) the 

structural history of individual terranes or geologic 

regions; (2) the geometry of the deformation(s) 

exhibited by various rock sequences or masses in 

each terrane or geologic region; (3) the kinematics 

or movements of each terrane or geologic region 

during migration and accretion; and (4) the 

movements of individual minerals and correlation of

these movements with those deduced from 

geometric analyses of structures.

Age studies

Dating of rocks is requisite to deciphering the 

complex geologic record. Dating studies will 

provide information on (1) the ages of igneous, 

metamorphic, and some sedimentary rocks 

encountered along the transect; (2) the plate 

tectonic origins and modifications of igneous rocks; 

and (3) the timings of metamorphisms and 

deformations that occurred during discrete tectonic 

events, such as terrane accretion. Essential dating 

techniques include K-Ar, U-Pb zircon, Nd-Sm whole 

rock, Rb-Sr whole rock and mineral isochron, and 

fission track analyses.

Metallogenesis studies

Currently an extensive study is being 

completed by the U.S. Geological Survey and the 

Alaska Division of Geological and Geophysical 

Surveys on the genesis of metallic mineral deposits 

and their relation to accreted terranes of Alaska. 

This study is being done for the Alaskan volume of 

the Decade of North American Geology under the 

sponsorship of the Geological Society of America. 

The ADGGS, USGS, and U.S. Bureau of Mines are 

also studying the metallogenesis of mineral deposits 

in the Dalton Highway corridor in the southern 

Brooks Range. TALI will present a unique 

opportunity to conduct additional detailed studies of 

mineral deposits in each metallogenic province 

exposed along the transect route, and to relate the 

formation and modification of mineral deposits to 

the origin, migration and accretion, and 

metamorphism of the enclosing terranes. Detailed 

studies of deformed and metamorphosed mineral 

deposits should also be integrated with metamorphic 

petrologic and structural studies to help determine 

temperatures, pressures, and timings of tectonic 

events. These studies will provide valuable 

information on the mineral resource potential of 

Alaska.

Neotec tonic studies

Alaska is the most tectonically active area of 

the United States, and studies of neotectonics will 

provide valuable clues about ongoing geologic and 

tectonic processes. Three major areas of 

neotectonic studies could contribute significantly to 

TALI. First, rates and styles of Holocene tectonic 

deformation could be determined in a variety of 

geologic domains along the TALI route. Current 

strain is being monitored by periodic observations of 

a geodetic network across the strike-slip Denali 

fault. In southern coastal Alaska, studies of 

displaced shorelines and geodetic surveys have 

documented horizontal and vertical deformation 

associated with the thrusting during the great (Mw 

9.2) Prince William Sound earthquake of 1964.

19
Information on the earlier Holocene vertical 

displacement history of the Prince William Sound 

area has come from studies of radiocarbon-dated 

uplifted terraces and drowned shorelines. Little is 

known about Holocene deformation elsewhere along 

the corridor. Second, late Tertiary and early 

Quaternary tectonic deformation could be 

documented by studies of erosional surfaces that 

could include determination of: (1) the existence of 

single or multiple surfaces; (2) relative heights of 

erosional surfaces above mean sea level; (3) 

warping, rotation, or faulting of erosional surfaces 

during the Holocene; or (4) deposition of alluvial 

fans, rock glaciers, fluvial sediments, or marine 

sediments on erosional surfaces. Third, more 

extensive local seismograph networks, augmented 

by geodetic strain and tilt instrumentation, could be 

established along active faults, as now exist along 

various major faults in parts of central and southern 

California. The data from such networks could 

document current rates of seismic slip and senses of 

offset along the faults. Additionally, geologic 

studies of trenches dug across major, known or 

suspected active faults, such as the Denali fault, 

could provide information on their paleoseismic 

history.

Deep Drilling

The spectacular results of the Soviet deep 

drilling program at the Kola research well are an 

ample demonstration of the value of deep 

continental drilling. It is essential that a few deep 

wells be drilled to investigate geologic processes 

and in situ physical properties and, secondarily, to 

provide some "ground truth" tie points for 

geophysical interpretations of critical deep geologic 

sections. Continental scientific drilling is a major 

element of the NSF's Continental Lithosphere 

Program, and appears on its way to being 

established as a major program in its own right, 

much in the same mold as the ocean drilling 

program under JOIDES. A consortium of 

universities DOSECC (Deep Observation and 

Sampling of the Earth's Continental Crust, Inc.)-- 

was formed in 1984 to design and manage the NSF 

continental scientific drilling program.

One obvious target in southern Alaska is a 

deep hole at the south end of the transect to 

examine geologic processes and physical conditions 

within an active subduction zone. A candidate site 

for a hole is on Middleton Island, near the eastern 

end of the Aleutian arc (fig. 9).

Middleton Island sits on the upper plate of the 

Aleutian megathrust, at the edge of the continental 

shelf, 50 km landward of the Aleutian trench. A 

petroleum well, drilled 3 km southeast of the island 

to a depth of nearly 4 km, indicates a normally 

stacked sequence that bottoms in early or middle 

Eocene strata. Elevated wave-cut terraces on the 

island record a sequence of at least six major 

episodes of tectonic uplift in the past 4,200 years, 

the most recent of which accompanied the

magnitude 9.2 (Mw) earthquake of 1964. This uplift 

is the consequence of near-orthogonal plate 

convergence at a rate of 5.8 cm/yr.

A single 24-fold seismic-reflection profile 

across the continental margin, passing 15 km 

northeast of Middleton Island, shows late Tertiary 

and Quaternary marine sedimentary deposits 

subducted subhorizontally, with no apparent 

disturbance, beneath the Tertiary rocks of the 

continental slope. Based on extrapolations from the 

seismic-reflection profile and from regional 

earthquake data, the top of the subducted plate may 

be shallower than 10 km below Middleton Island. If 

so, this may be the only onshore location in the 

world where it is feasible to drill into an active 

zone of subduction.

A broad range of problems could be addressed 

with a deep hole beneath Middleton Island, among 

which are the pressure of pore fluid in the 

subduction zone and its role in faulting and 

subduction, the state of stress in the vicinity of the 

megathrust and in the overlying plate, and the 

physical properties of materials in the megathrust. 

Sampling within the subduction zone would 

document the physical and chemical alteration of 

marine sedimentary deposits subducted about I 

million years ago. A deep hole would also provide a 

quiet observatory for monitoring seismicity and 

acoustic emissions in the vicinity of the megathrust 

as well as changes of pore pressure, strain, fluid 

chemistry and temperature associated with the 

accumulation and release of strain during the 

earthquake process.

Additional marine seismic data in the vicinity 

of Middleton Island are needed to provide more 

detailed information on the structure and 

stratigraphy of the overriding and subducting plates 

in the vicinity of the island.

Figure 9. Possible site for deep scientific hole 

at Middleton Island in Gulf of Alaska 

(modified from George Plafker and others, 

1982, Geological Society of America Map 

MC-28P). Proposed well would pierce upper 

plate of Aleutian megathrust and penetrate 

subduction zone beneath upper plate. _A, 

Locations of Tenneco Middleton Island No. 1 

well and U.S. Geological Survey 

multichannel seismic-reflection line 425. 

B, Structure section with hypocenters from 

U.S. Geological Survey Preliminary 

Determination of Hypocenters, 1967-1977. 

Large triangles are earthquakes located with 

50 or more stations; small, with fewer than 

50 stations. Geologic units: umT upper 

and middle Tertiary rocks (siltstone, 

sandstone organic shale, locally abundant 

submarine volcanic rocks and sandy 

mudstone); IT lower Tertiary rocks (marine 

and continental clastic rocks).

20
146* 152' 148° 144'

B

60° -

AREA OF

THIS 

FIGURE

62°

X umT

60°

100 KILOMETERS

Middleton Island No. 1 well 

(projected)

umT umT -AX'

21
INVESTIGATIONS IN PROGRESS

There are many investigations already in 

progress along the TALI corridor. Most of the large 

investigations are promoted by one or more of the 

following groups: U.S. Geological Survey (USGS), 

Alaska Division of Geological and Geophysical 

Surveys (ADGGS), University of Alaska (UAF), and 

Rice University (RU). Numerous scientists from 

academia, industry, and government are cooperating 

in these studies. The following paragraphs describe 

major ongoing projects related to TALI; a complete 

summary describing every ongoing project is beyond 

the scope of the present effort.

U.S. Geological Survey

A major component of TALI is the TransAlaska Crustal Transect (TACT), a multiyear 

project being conducted by the U.S. Geological 

Survey. TACT is an integrated geologic and 

geophysical investigation of the structure, 

composition, and evolution of the Alaskan crust 

along the pipeline corridor and its seaward 

extensions across the continental margins. The 

investigation features coordinated application of 

complementary disciplines and methods, and 

multidisciplinary interpretation and synthesis of 

geologic and geophysical data.

TACT is planned as a phased project, in which 

a broad range of geologic and geophysical studies 

will be focused sequentially on segments of the 

transect. As proposed, the project would include 

extensive seismic-refraction and seismic-reflection 

profiling, both onshore and offshore; geologic 

mapping and topical geologic studies; gravity and 

magnetics studies; magneto telluric sounding; heat 

flow studies; and seismic imaging of the crust and 

upper mantle primarily using earthquake sources.

TACT was initiated during 1984 in southern 

Alaska. Field studies focused on the northern 

Chugach Mountains and the Copper River Basin, 

between Valdez and the Alaska Range. Seismicrefraction/wide-angle seismic-reflection profiling 

was completed along 260 km of the transect route 

from a point about 60 km east of Valdez northward 

to the Denali fault in the Alaska Range. In 

addition, two cross profiles were obtained, a 130-km 

line along the axis of the Chugach Mountains east of 

Valdez and a 155-km profile extending 

southwestward from Gulkana across the southwest 

sector of the Copper River Basin. Geologic studies, 

in cooperation with other groups, were concentrated 

in the Chugach Mountains and western Wrangell 

Mountains. Bedrock mapping at a scale of 1:63,360 

or larger was completed for a strip at least 20 km 

wide and roughly 100 km long centered on the 

transect route from east of Valdez to the Copper 

River Basin and along the western margin of the 

Wrangell Mountains, and for four critical 15-minute 

quadrangles (Valdez B-3, B-4, C-3, and C-4) 

spanning the suture zone between the Chugach and

Peninsular-Wrangellia (composite) terranes. Topical 

geologic studies conducted in the vicinity of the 

transect and the auxiliary Chugach refraction 

profile included the pressure-temperature history of 

regionally metamorphosed rocks; the petrology and 

petrogenesis of ultramafic, mafic, and volcanic 

rocks; isotopic studies; fission-track analyses; local 

stratigraphic and paleontologic studies; structural 

analysis; and detailed mapping along sutures and 

major faults. An aeromagnetic survey was flown at 

1-mile (1.6 km) spacing over the western Wrangell 

Mountains to fill a hole in the existing aeromagnetic 

data base. In addition to the field investigations, 

existing gravity measurements in the Gulkana and 

Valdez 1-degree quadrangles have been terraincorrected, and complete Bouger gravity maps for 

these quadrangles are being prepared.

The types of geologic, seismic, and 

geophysical studies initiated in 1984 were continued 

in 1985 with the focus shifted to Prince William 

Sound and the Alaska Range. The seismic 

refraction/wide-angle reflection profile along the 

transect was extended 65 km southward to the 

Copper River delta. Two additional transverse lines 

were shot: a 135-km line along Montague, 

Hinchinbrook, and Hawkins Islands in the Prince 

William terrane and a 110-km line from Gulkana to 

Mentasta Pass across the Copper River Basin and 

the inferred boundary between the Peninsular and 

Wrangellia terranes. Geologic mapping and detailed 

topical studies were conducted in the vicinity of the 

transect route in the Chugach Mountains and the 

Alaska Range. In subsequent years, these studies 

will progress northward with the goal of completing 

the land investigations by the end of the decade.

The scope of TACT has been expanded in 1986 

to include deep seismic-reflection profiling and 

reconnaissance magnetotelluric sounding. The 

initial reflection targets were an 80-km profile 

across the suture zone between the Chugach and 

Peninsular-Wrangellia (composite) terranes and a 

50-km profile across the Denali fault in the Alaska 

Range. Marine investigations, heat flow 

measurements, and seismic imaging of the crust and 

upper mantle from earthquake sources are 

dependent upon future funding.

More than 20 scientists from academia, 

industry, and other governmental agencies (both 

State and Federal) are cooperating with or 

participating in the TACT geological and 

seismological studies under a variety of 

arrangements.

Numerous USGS projects are complementing 

the TACT studies and contributing to the transect 

effort. Examples include the monitoring and 

investigation of earthquakes in southern coastal 

Alaska, petrologic and geochemical studies of the 

origin and age of the Wrangell Lavas, geologic 

mapping and mineral resource appraisal of the 

eastern Alaska Range and the Livengood 

quadrangle, a geologic transect across the Koyokuk 

basin, and the statewide study of 

tectonostratigraphic terranes.

22
Rice University University of Alaska Cooperative 

Studies

Rice University and the University of Alaska 

(Fairbanks) have been cooperating on studies in the 

Brooks Range area as part of an Alaskan transect 

program. The project has been funded by industry 

as an industrial associate program with considerable 

additional support provided by the ADGGS.

To date, Rice University has concentrated on 

investigating the detailed structural history of an 

area known as the Doonerak window area and the 

structure of the schist belt.

The University of Alaska efforts have 

concentrated on interpreting the available gravity 

and earthquake data for Arctic Alaska. The gravity 

data are mainly for the Dalton Highway (obtained 

from the USGS) and for several long traverses along 

roughly parallel north-south river valleys (ADGGS 

data). These gravity data are being modeled using 

ADGGS geologic cross-sections and maps, and 

aeromagnetic and rock physical property data. 

Earthquakes for Arctic Alaska have been relocated 

using different velocity models. An apparent belt 

of low-level seismicity stretches from interior 

Alaska to the Mackenzie delta area. The 

northeasterly trend may be a function of station 

distribution, and the events may be more 

widespread throughout the Brooks Range; however, 

the existence of shallow seismic activity in this 

area presumably represents crustal deformation or 

readjustment. It is hoped that at least a minimal 

network of seismometers can be permanently 

installed in Arctic Alaska so that these small events 

can be better located and their significance 

established.

University of Alaska

The University of Alaska is also working on a 

number of TALI-related projects. Among these are 

the operation of a seismic network (in cooperation 

with ADGGS and the USGS); paleomagnetic studies 

of the travel histories of terranes, particularly the 

Wrangellia terrane and other southern terranes; 

goethermal potential studies in the Copper River 

Basin; and various mapping projects (both by faculty 

and for student thesis research) along portions of 

the whole TALI route. The K-Ar geochronology 

laboratory (currently operated cooperatively with 

ADGGS) is also involved in dating and thermal 

history studies in cooperation with many of the 

individual projects.

Alaska Division of Geological and Geophysical 

Surveys

The Alaska Division of Geological and 

Geophysical Surveys has a number of cooperative 

projects with the USGS, UAF, and RU located in 

and near the TALI corridor that will contribute to 

the geophysical and geological data base on which 

TALI can draw. Each of these ADGGS projects is

generally directed at assessing the petroleum, 

mineral, or hydrological resources of Alaska. Some 

of these projects are regional or statewide in 

geographical extent.

Three active ADGGS projects will provide 

both regional and detailed local structural and 

stratigraphic information that can be projected into 

the geophysical profiles. A major ADGGS mapping 

project in the south central Brooks Range is run in 

cooperation with the USGS, UAF, RU, and the U.S. 

Bureau of Mines (USBM). Nineteen l:63,360-scale 

quadrangles have already been studied as part of 

this project. The studies include bedrock and 

surficial mapping, gravity and magnetic modeling, 

structural and lithostratigraphic terrane analysis, 

stratigraphy, paleontology, geochronology (U-Pb, 

Rb-Sr, Sm-Nd, and K-Ar), whole rock and 

exploration geochemistry, metallogeny, and 

resource evaluation. Results of these studies are 

already producing new interpretations of the 

regional tectonics and crustal evolution of the 

Brooks Range that will be important considerations 

in any syntheses of TALI results. ADGGS has also 

supported a small but separate UAF study of skarns 

in this area. A second ADGGS l:63,360-^cale 

mapping project is focused in the Killik River, 

Chandler Lake, and Philip Smith quadrangles, which 

transect the haul road in the north central Brooks 

Range. The project includes several UAF thesis 

studies. This project is generating information on 

the timing of Brooks Range thrusting and 

subsequent uplift as well as structural, 

biostratigraphic, and lithostratigraphic data. Basin 

analyses of these data are improving our 

understanding of the evolution of the North Slope 

petroleum province. A third major ADGGS project 

that is contributing to that understanding and to the 

geologic data base for TALI is focused on the 

eastern North Slope where petroleum potential 

evaluations are required for possible oil lease 

sales. This project, which is run in cooperation with 

the USGS, UAF, USBM, and the U.S. Bureau of Land 

Management, involves bedrock mapping and 

structural and stratigraphic studies. The results of 

surface studies are to be correlated with subsurface 

geology and structure in order to analyze oil basins 

in the eastern North Slope. Closely coordinated 

with this project is a regional study of the surficial 

geology.

Completed projects in the Fairbanks and 

Livengood areas provide bedrock mapping, 

geochemical and mineral analyses, and 

interpretation of the structural geology. Long-term 

mapping and structural geology and petrology 

projects in the Chugach Mountains are contributing 

to an understanding of the regional tectonics and 

crustal evolution in the southern part of the TALI 

transect, including ideas on the significance of the 

boundary between the Peninsular and Wrangellian 

terranes. Finally, some mapping and interpertation 

has been carried out by DGGS geologists in the 

Valdez quadrangle.

23
In addition to the various geological mapping 

projects outlined above, ADGGS has several 

geophysicists who are working nearly full-time in 

interpreting North Slope seismic, gravity, and

magnetic data. Their expertise, particularly in 

permafrost areas, will be invaluable in interpreting 

the seismic data acquired by the TALI and TACT 

groups.

APPENDIX

List of Participants

TALI Workshop

Anchorage, Alaska

May 29, 1984

Hans G. Ave Lallemant 

Fred Barker 

David F. Barnes 

Selena Billington 

David L. Campbell 

Mark P. Cloos 

John N. Davies 

John Dillon 

Oscar J. Ferrians 

Gary S. Fuis 

Robert M. Hamilton 

William P. Harbert 

Tekla Harms 

Henry S. Hasegawa 

Ernest Hauser 

James F. Hays 

N. Ross Hill 

Travis Hudson 

H. M. lyer 

Klaus H. Jacob 

Susan M. Karl 

Juergen J. Kienle

Jo Ann Kimzey 

Alan R. Levander 

Dorie Malven 

Jill McCarthy 

Paul Metz 

Thomas P. Miller 

Charles G. Mull 

Warren J. Nokleberg 

D. K. Norris 

Jack K. Odum 

Jack E. Oliver 

Robert A. Page 

Garnet Pessel 

George Plafker 

Lawrence D. Porter 

I. G. Sohn 

David B. Stone 

John F. Sweeney 

Robert Thompson 

Donald L. Turner 

Charles T. Young 

Lynn A. Yehle

24 687-049/48002
UNIVERSITY OF ALASKA

U.S. GEOLOGICAL SURVEY

ALASKA DIVISION OF GEOLOGICAL AND GEOPHYSICAL SURVEYS

NATIONAL SCIENCE FOUNDATION