U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 

Geophysical Interpretations of the 

Libby Thrust Belt,

Northwestern Montana

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR
Geophysical Interpretations of the 

Libby Thrust Belt,

Northwestern Montana

By M. Dean Kleinkopf 

With a section on

Deep Folds and Faults Interpreted from Seismic Data

By Jack E. Harrison 

And a section on Interpretation of Magnetotelluric Soundings

By W.D. Stanley

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1546

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997

M A R C H 3, 18 4 

U.S. DEPARTMENT OF TH
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Kleinkopf, M. Dean (Merlin Dean), 1926–

Geophysical interpretations of the Libby thrust belt, northwestern Montana / by M.

Dean Kleinkopf ; with a section on deep folds and faults interpreted from seismic data

by Jack E. Harrison and a section on interpretation on magnetotelluric soundings by

W.D. Stanley.

p. cm. — (U.S. Geological Survey professional paper ; 1546)

Includes bibliographical references.

Supt. of Docs. no.: I 19.16: 1546

1. Thrust faults (Geology)—Montana. 2. Geology, Stratigraphic—Proterozoic.

3. Geology, Structural—Montana. 4. Geophysical prospecting—Montana.

I. Title. II. Title: Libby thrust belt, northwestern Montana. III. Series.

QE606.5.U6K52 1995

551.8′7—dc20 94–1628

CIP
CONTENTS

Abstract ........................................................................................................................... 1

Introduction..................................................................................................................... 2

Geology........................................................................................................................... 2

Aeromagnetic Anomaly Data.......................................................................................... 5

Gravity Anomaly Data .................................................................................................... 5

Geologic Interpretation of Aeromagnetic Anomalies..................................................... 6

Geologic Interpretation of Bouguer Gravity Anomalies................................................. 8

Geophysical Interpretation of Deep-Seated Geologic Features...................................... 9

Modeling of Gravity Profiles .................................................................................. 9

Deep Folds and Faults Interpreted from Seismic Data, by Jack E. Harrison.......... 13

Interpretation of Magnetotelluric Soundings, by W.D. Stanley.............................. 13

Summary and Conclusions.............................................................................................. 19

References Cited ............................................................................................................. 19

PLATES

[Plates are in pocket]

1, 2. Maps of the area of the Libby thrust belt, northwestern Montana, showing:

1. Total-intensity aeromagnetic anomalies, Bouguer gravity anomalies, and generalized geology.

2. Residual total-intensity aeromagnetic anomalies, complete Bouguer gravity anomalies, 

and structural features.

FIGURES

1, 2. Maps showing:

1. Generalized geology of Belt terrane .................................................................................................................

2. Structural features, lines of sections and profiles, and locations of magnetotelluric soundings, 

Libby thrust belt.................................................................................................................................................

3. Generalized lithologic and geophysical logs, Gibbs No.1 borehole.........................................................................

4, 5. Gravity block models of extended geologic cross sections:

4. B–B'...................................................................................................................................................................

5. J–J' ....................................................................................................................................................................

6. Diagram showing interpretation of seismic reflection profiles MT–1 and MT–2....................................................

7. Generalized resistivity log of the Gibbs No. 1 borehole ..........................................................................................

8. Magnetotelluric and geologic section across the eastern part of the Libby thrust belt and the western part 

of the Purcell anticlinorium ......................................................................................................................................

TABLE

1. Lithologies and densities assumed for gravity models of extended geologic sections B–B' and J–J', 

Libby thrust belt, northwestern Montana..................................................................................................................
GEOPHYSICAL INTERPRETATIONS OF THE 

LIBBY THRUST BELT, NORTHWESTERN MONTANA

By M. Dean Kleinkopf

ABSTRACT

Interpretations of gravity and aeromagnetic anomaly

data, as well as results from two seismic reflection profiles

and five magnetotelluric soundings, were used to study

buried structure and lithology of the Libby thrust belt of

northwestern Montana. Gravity modeling, supplemented by

structural data from the seismic reflection profiles and geologic contraints derived from projection of surface geology,

measured sections, and lateral consistency of sills, leads to

the interpretation that thrust slices of folded crystalline basement form the core of the Purcell anticlinorium and Sylvanite anticline, which consist mainly of rocks of the Middle

Proterozoic Belt Supergroup.

Gravity anomaly data show marked correlation with

major structure of the area. The Purcell anticlinorium

exhibits positive anomalies in excess of 20 mGal and the

Sylvanite anticline anomalies in excess of 10 mGal. In the

northern part of the study area, a distinct northwest-trending

high-gradient zone suggests that a buried crustal shear zone

may be present just north of Libby. The Rainy Creek and

Bobtail Creek stocks, which show distinct positive magnetic

anomalies, lie along this trend, and their emplacement may

have been influenced by the postulated zone of deformation. 

The most distinct magnetic anomalies in the principal

study area are five positive anomalies associated with Cretaceous or younger cupolas and stocks that either are exposed

or are known from drillholes to be present in the near-surface. Amplitudes range from 100 to more than 3,000 nT.

Short-wavelength anomalies are associated with outcrops of

magnetite-bearing sedimentary rocks of the Ravalli Group

of the Belt Supergroup. A strip of high magnetic gradient

correlates with the Hope fault zone. Modeling of the magnetic anomaly data was not done because of the absence of

long-wavelength magnetic anomalies having likely sources

in deeply buried Proterozoic crystalline basement. The

mafic sills of dioritic to gabbroic composition show little or

no magnetic response in outcrop, probably because the magnetite content in the sills was reduced by chemical processes

related to interaction of hydrothermal fluids with cooling

magmas of the intrusions. 

Evidence from the magnetic and gravity anomaly 

suggests that the Cabinet Mountains Wilderness is und

by a major batholith of felsic composition that extends 

north of the Dry Creek stock south almost to the Verm

River stock. Along this trend are a number of 

high-amplitude positive anomalies of 60–100 nT in ar

outcrops of the Wallace Formation of the Belt Super

and, in a few cases, the Ravalli Group. In the case o

Ravalli rocks, the highs probably are not related to out

of magnetite-bearing Ravalli group rocks that in other p

cause linear magnetic highs along ridge tops; in

because of their equidimensional shape on the map

may indicate near-surface sources related to granitic 

sions or cupolas of a larger batholith mass at a few ki

ters depth beneath the Cabinet Mountains Wilderness.

A basal surface of detachment is inferred from a 

of seismic reflections. Depth to the basal surface is 9–1

and this surface dips about 15° in inferred crystalline 

ment rocks. Strong reflections at and below 4 second

attributed to stratification in the basement such as la

gneiss or mylonite along the detachment zone for the 

tectonic folding. No seismic reflection is present fo

Belt-crystalline basement contact. Just below the Pin

thrust fault, strong reflections attributed to stratificati

crystalline basement rocks image relief in the baseme

the western flank of the Purcell anticline. 

Interpretation of the magnetotelluric data sugge

thick conductive section that is incompatible w

crystalline basement composed of granitic gneiss, 

intrusive rocks, and high-grade metasedimentary rocks 

sidered typical for the region. Thus, there are major d

ences between the deep crustal model developed from 

magnetotelluric data and the model obtained from 

gravity and seismic data. This heterogeneous type of cr

generally not low in resistivity unless partial meltin

occurred. Low resistivities may be possible in granitic 

at depths of 10–15 km and temperatures of 500°C –600

1–3 percent free water is present; however, there 

evidence in the study area for these levels of temperatu

thermally related basement conductive zone could po

be present now at depth where a low-resistivity 
2 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

ohm-m) zone forms the bottom layer in the western part of

the magnetotelluric profile. This zone corresponds approximately with the interpreted basal surface of detachment that

separates basement rocks from the Prichard Formation of the

Belt Supergroup. Some of the low resistivities observed

likely are caused by pre-Prichard crystalline rocks (metamorphic (metasedimentary) or igneous) that are conductive

because they contain large amounts of metallic minerals.

The striking variations in observed resistivities in the

Prichard Formation of from 0.6 to more than 400

ohm-meters probably reflect the percentage and continuity

of iron-sulfide-rich zones. Another possible line of thermal

evidence is the absence of magnetic sources in pre-Belt

crystalline rocks at depths of 10–15 km. The lack of magnetic anomalies may relate to shallow Curie-point temperatures, above which magnetism does not exist. 

The magnetotelluric data have limitations in structural

analysis because resistivities are mostly controlled by the

percentages of metallic minerals and not by lithology. Accuracy of the interfaces obtained from the magnetotelluric data

is not great, both because of the widely spaced sounding

locations and because of the very large contrasts in resistivity between unmineralized and mineralized zones in the Belt

Supergroup.

INTRODUCTION

Regional geophysical studies conducted by the U.S.

Geological Survey in the northern Rocky Mountains during

the past 25 years provide new insights about the geologic

framework and mineral resources of the region. In this

report, the emphasis is on interpretation of geophysical data

compiled for the Libby thrust belt in northwestern Montana

(fig. 1). Interpretations complement the results of geologic

mapping described by Harrison and Cressman (1993), who

also discussed the geology and structural framework of the

Libby thrust belt. During the course of their studies, Harrison

and Cressman constructed geologic cross sections across the

Libby thrust belt on the basis of measured sections and projections of surface geology. Depth cutoff of these sections

was 4.3 km (14,000 ft) below sea level.

In the study described herein modeling of gravity

anomaly data was done along two of the geologic cross sections to provide information about structure and lithology of

the Purcell anticlinorium and Sylvanite anticline from depths

greater than 4.3 km to at least as deep as Precambrian crystalline basement in the middle part of the crust. No magnetic

modeling was done because of the absence of magnetic

anomalies having sources likely in deeply buried crystalline

basement.

Included in this report are sections on interpretation of

seismic and magnetotelluric data. J.E. Harrison describes

interpretations of data from about 70 km of COCORP (The

Consortium for Continental Reflection Profiling) seismic

reflection surveying along two profiles that cross 

nite anticline and Purcell anticlinorium. W.D

describes analysis of five magnetotelluric soundin

profile that extends across the Libby thrust belt an

cell anticlinorium. The lines of section of the grav

and the COCORP profiles and the locations of the 

telluric soundings are shown in figure 2. Data 

audiomagnetotelluric soundings in two profiles o

vanite anticline provide information about resis

rocks in the upper few kilometers of the crust (Lon

The geology shown on plate 1 is a simplified 

the 1:125,000-scale geologic map compiled by Ha

Cressman (1993). The western parts of geologic m

Wallace and Kalispell 1°×2°quadrangles (Harriso

ers, 1986, 1992) provide the regional geologic con

Libby thrust belt. 

Magnetic and gravity anomaly data are also 

plate 1. To provide a broader perspective of the ge

setting, magnetic and gravity anomaly maps (p

compiled at a scale of 1:500,000 for the Libby thru

adjacent areas, extending from near lat 47°20' 

49°00′ N. and from near long 114°45′ W. to long 1

Previous studies of gravity and magnetic ano

applied to geologic framework and mineral resour

gations of the region are described in King a

(1970), Harrison and others (1972, 1980, 1985), 

and others (1972, 1982, 1988), Wynn and othe

Kleinkopf (1977, 1981, 1983, 1984), Kleinkopf 

(1979), Kleinkopf and Wilson (1981), Kleinkopf 

key (1982), Kleinkopf and Harrison (1982), Fou

McDonough (1984), and Harris (1985).

The author thanks the many U.S. Geologic

colleagues who contributed to this project. Jack 

offered many constructive suggestions and provi

stimulating discussions during various phases of t

and report preparation. The sections on seismic an

totelluric investigations by Jack Harrison and D

provide substantive contributions to the conclusio

paper. The geology was digitized and compiled un

vision of Stanton H. Moll; the geologic map of p

completed in final digital form by Nancy Shock. 

key and Mike Brickey collected gravity data in 

Viki Bankey compiled and edited the gravity and 

data and prepared early versions of the gravity and 

anomaly maps, and Gerda Abrams prepared later v

the gravity and magnetic anomaly maps used in t

Several colleagues offered valuable constructive 

during the technical review process.

GEOLOGY

The Libby thrust belt (fig. 1) is in the northw

of the Belt basin, which formed along the western e
GEOLOGY 

North American craton in Middle Proterozoic time

(Harrison, 1972; Harrison and Cressman, 1993). The Belt

terrane is the oldest of a series of tectonostratigraphic assemblages that make up a wedge of supracrustal rocks along the

western edge of the continental craton (Price, 1981). The

Libby thrust belt is one of a series of major north-northwest-trending structural features north of the Lewis and

Clark line, a major intraplate tectonic boundary (Reynolds

and Kleinkopf, 1977). The Libby thrust belt was characterized by Harrison and Cressman (1993) as a “ripped-apart

syncline between two anticlinal structures,” between the

Purcell anticlinorium to the east and the Sylvanite anticline

to the northwest. The Moyie thrust system is along the

western and northwestern margin of the Libby thrus

and overrides it on the southwest (Harrison and Cres

1993). The thrust belt is limited to the south by the 

fault (fig. 1), which Harrison and Cressman describe

“crustal flaw.” 

Some 15 km of Middle Proterozoic Belt Super

rocks underlies the Libby thrust belt. Rocks of the 

Supergroup grade upward from turbidites through 

ginal-marine, tidal-flat, and shallow-shelf deposits

sequence consists mostly of fine-grained sedimentary r

mainly argillite, siltite, quartzite, and carbonate. Th

chard Formation, the oldest of the Belt units, is domin

quartzite but contains beds of pyritic-pyrrhotitic arg



   yyy yyy yyy yyy





























































  





yzy



Tertiary volcanic ro

Tertiary to Jurassic

 intrusive rocks

Quaternary to Cam

 sedimentary roc

Late Proterozoic ro

 (Windermere Sy

 of Canada) 

Middle Proterozoic 

 Supergroup (Pu

 Supergroup of 

 Canada)—Stipp

 shows areas of hi

 grade metamorph

 may include som

 Belt rocks

yyyyEarly Proterozoic p

 Belt metamorph

 rocks

Contact

High–angle fault

Right–lateral fault

Á

Á

Á

Á





118° 116° 114° 112° 110°

Banff

50°

48°

46°

44°

CANADA

UNITED STATES PURCELL ANTICLINORIUM

FAULT

BELT

THRUST

MONT

LEWIS

AND CLARK

LINE

Q s e

Q s e

Q s e

TJi

Zw

Yb

Yb

Yb

Q s e

Yb

Zw

Tv

Xo

Yb

TJi

Tv

HOPE

LIBBY

Libby

EXPLANATION

Spokane

Coeur

d'Alene

Yb

Zw

TJi

TJi

ROCKY

Area of

plate 2

Q s e

Xo

Q s e

Á

MOUNTAIN

TRENCH

Anticline

Syncline

Thrust fault—Sawte

 on upper plate

Area of plate 1



Á

Xo

Tv Tv

WASHINGTON

OREGON

IDAHO

Tv

Clark Fork

0 50 100 KILOMETERS

Helena

Figure 1. Map showing generalized geology of the Belt terrane in the region of the Libby thrust belt, northwestern Montana and 

rounding area. Modified from King (1969) and Harrison and Cressman (1993). 
4 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

116° 115° 49°

Eureka

Clark Fork

Study area

5

4

Compo

section 

fig

J'

J

B

B'

J''

Composite line of

section shown in

figure 5

48°

Troy

Libby

Bonners

Ferry

IDAHO

MONTANA

MONTANA

CANADA

ROCKY

MOUNTAIN

TRE

PURCELL

NCH

ANTICLINORIUM

NO. 1 GIBBS

BOREHOLE

Line of section

shown in

figure 4 SYLVANITE

ANTICLINE

MOYIE

THRUST

SYSTEM

0 10 20 30 40 50 KILOMETERS

3

Normal fault—Bar and ball on downthrown side

Thrust fault—Sawteeth on upper plate

Strike–slip fault—Arrows show direction

 of apparent movement

Anticline—Showing direction of plunge.

 Dashed where inferred

Overturned anticline

Syncline—Showing direction of plunge.

 Dashed where inferred

Location and number of

 magnetotelluric sounding

Line of seismic reflection profile

EXPLANATION

MT–1

3 2

1

LIB

BY

THRUST

BELT

MT–2

HOPE FAULT ZONE

PINKHAM THRUST FAULT

Figure 2. Map showing major structural features in the area of the Libby thrust belt, northwestern Montana. Lines of extende

cross sections B–B' and J–J' (Harrison and Cressman, 1993) used in gravity modeling, lines of seismic reflection profiles MT–1 

and locations of magnetotelluric soundings 1 through 5 and Gibbs No. 1 borehole are also shown. 
AEROMAGNETIC AND GRAVITY ANOMALY DATA 

which is more dense than overlying rocks of the Ravalli

Group. The thickest exposures of Prichard rocks in the map

area are in the Sylvanite anticline, west of Yaak (pl. 1A),

where some 5.6 km of Prichard rocks is exposed (Harrison

and Cressman, 1993). Almost 1,000 m of mafic sills is

recorded in measured sections in the Yaak area. Succesively

overlying the Prichard Formation are the Ravalli Group, the

Helena and Wallace Formations, and the Missoula Group.

A number of short-wavelength positive magnetic

anomalies (pl. 1A) are associated with outcropping metasedimentary rocks of the Burke and Revett Formations of the

lower part of the Ravalli Group. In hand specimen, particularly those of the Burke Formation, euhedral grains of magnetite were observed in siltite (Kleinkopf and others, 1972).

The overlying Wallace Formation consists mainly of calcareous and dolomitic fine- to medium-grained clastic rocks.

Above the Wallace Formation is the Missoula Group, which

consists primarily of interbedded red and green clastic rocks

that are less dense than other Belt rocks. Rocks of the Missoula Group produce negative gravity anomalies where thick

sequences are juxtaposed against denser Belt rocks. Only

minor amounts of Cambrian sedimentary rocks are preserved in outcrop, and no other Phanerozoic sedimentary

rocks are present in the study area except for alluvial,

lake-bed, and glacial deposits and other low-density sediments of Pleistocene to Holocene age that have accumulated

in modern stream valleys. 

Rocks of the Belt Supergroup exhibit an increase in

metamorphic grade from east to west across the basin and

with depth in the stratigraphic section. Metamorphic grade

ranges from the biotite zone of the greenschist facies in the

lowest exposed part of the Prichard Formation through chlorite-sericite rocks in middle Belt to high-grade diagenesis at

the top of the Belt Supergroup (Harrison and Cressman,

1993). Because the sedimentary rocks are mostly argillite

and siltite, the average rock density generally increases

downsection with depth and with increased metamorphic

grade. Exceptions are rocks of carbonate sequences of the

Helena and Wallace Formations, which are more dense than

the underlying rocks of the Ravalli Group. Although massive

quartzite is present throughout the section, its volume as

compared to that of the total section is low, and thus its influence on gravity patterns is minimal. 

The Belt sequence in the Libby thrust belt has been

intruded by Precambrian sills of diorite to gabbro, early Cretaceous plutons ranging from granite to pyroxenite, and

Eocene plutonic rocks of quartz monzonite porphyry to

granodiorite. Middle to Late Proterozoic mafic sills are

abundant in the lower part of the Prichard Formation and are

less abundant in the Ravalli Group, Wallace Formation, and

Missoula Group. The best exposures of sills are in cirques,

particularly on the Sylvanite anticline, and on ridges (Harrison and Cressman, 1993). The sills are altered (King and

others, 1970; Bishop, 1973) and commonly exhibit little or

no magnetic expression. 

Scattered exposures of Early Cretaceous and E

plutonic igneous rocks are present in the study area. 

rocks, ranging in composition from granite and granod

to syenite and pyroxenite, have been intruded into Belt 

(Harrison and Cressman, 1993). The largest exposed pl

are the Dry Creek stock (unit Kg, pl. 1), about 1

southwest of Libby in the Cabinet Mountains, an

Vermilion River stock (unit Kg), just east of the tow

Trout Creek. Smaller exposures of granitic rocks sim

the rocks of the Dry Creek stock are present a few kilom

north, west, and south of the stock. Mafic intrusive 

plexes, principally pyroxenite and syenite (unit Kps

exposed at Vermiculite Mountain, 10 km east-northe

Libby. At Bobtail Creek, 12 km north of Libby, a plu

predominantly coarse grained, porphyritic syenite 

Kps) contains segregations of pyroxene and amphibol

AEROMAGNETIC ANOMALY DAT

Aeromagnetic anomaly data for the study area 

compiled from analog records obtained from two reg

aerial surveys flown under contract to the U.S. Geolo

Survey. The elevation of most of the study area is betw

and 2 km, and several mountain peaks are higher tha

km. Both surveys were flown east-west at a nominal al

of 2.1 km above sea level, except to clear the high peak

survey north of lat 48°30' N. was flown in 1972 by Sc

Minerals Inc., at a line spacing of 3.2 km (U.S. Geolo

Survey, 1973). The survey south of lat 48°30' N. was 

in 1968 by Lockwood, Kessler, and Bartlett, Inc., at 

spacing of 1.6 km (U.S. Geological Survey, 196

Kleinkopf and others, 1972). 

The total-intensity aeromagnetic anomaly dat

shown at scales of 1:250,000 and 1:500,000 (pl. 1A

Plate 1A is a mosaic of the aeromagnetic anomaly 

received from the contractors and is superimposed on a 

eralized version of the digital geologic map of Harriso

Cressman (1993). The residual total-intensity data 

reduced to a single datum, then continued to a commo

tude of 2.1 km above sea level and merged by fitting 

map boundaries using programs developed by 

Webring (written commun., 1981) and R.E. Sweeney 

ten commun., 1981). The merged data were then gridd

a spacing of 1 km and contoured at an interval of 20 n

2A). The International Geomagnetic Reference Field (I

for Epoch 1975 (Peddie and others, 1976), which is abo

gammas per kilometer to the northeast, was removed 

the data. 

GRAVITY ANOMALY DATA 

Gravity data for 775 stations were extracted 

regional digital data sets compiled in support of 
6 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

Geological Survey programs in geologic framework and

mineral resource appraisal. The data originate from a variety

of sources including unpublished files of the U.S. Geological

Survey, Wynn and others (1977), Kleinkopf (1981),

Kleinkopf and Wilson (1981), Bankey and others (1985),

and a U.S. Department of Defense, Defense Mapping

Agency (DMA), data base (written commun., 1989).

Measurements were made with high-sensitivity gravity

meters using four-wheel-drive vehicles and, in some cases,

foot traverses (Bankey and others, 1982, 1985; Brickey and

others, 1980). The station spacing is variable, ranging from

2–3 km along roads to greater than 5 km in roadless areas. 

Gravity observations were made at locations of known,

or recoverable, horizontal and vertical positions in terms of

longitude, latitude, and elevations. These positions include

survey bench marks, photogrammetric elevation points

shown on U.S. Geological Survey 7.5- and 15-minute topographic maps, and locations of low topographic relief at

which elevations and horizontal positions could be estimated

with confidence. The observed gravity is referenced to the

IGSN–71 (International Gravity Standardization Net) datum

(Morelli and others, 1974) by means of ties to U.S. Department of Defense bases ACIC–0442 at Missoula, Mont., and

ACIC–4006–1 at Wallace, Idaho (U.S. Department of

Defense, 1974). Gravity reduction procedures are based on

equations discussed by Cordell and others (1982). All

gravity stations were reduced to the complete Bouguer

anomaly, assuming a mean crustal density of 2.67 g/cm3,

using the 1967 gravity formula (International Association of

Geodesy, 1967). Terrain corrections were made using computer software to access digital terrain files. Terrain corrections were calculated radially around each station to a

distance of 166.7 km (Plouff, 1977). The final-processed

data, consisting of complete Bouguer gravity anomalies,

were gridded and contoured using computer routines based

on minimum curvature (Briggs, 1974; Webring, 1981).

GEOLOGIC INTERPRETATION OF 

AEROMAGNETIC ANOMALIES

The dominant anomaly patterns on the 1:250,000-scale

aeromagnetic anomaly map (pl. 1A) consist of equidimensional positive anomalies, short-wavelength positive and

negative anomalies elongated generally north to northwest,

and zones of steep magnetic gradient. Three types of geologic sources that contain relatively high percentages of

magnetite account for most of the anomalies. The sources

mainly are in outcrop but in some cases are partly or totally

buried in the near-surface. The principal magnetic anomaly

sources are igneous intrusive rocks, magnetite-bearing sedimentary rocks of the Ravalli Group, and major fault zones.

The Proterozoic basement, where drilled in western

Montana, is primarily a mixture of granite and granitic

gneiss, both of which typically are of low mag

(Harrison and others, 1972). Long-wavelength 

that may have sources in deeply buried crystalline 

rocks at about 15 km are not known to be pres

region (Harrison and others, 1980). The lack of 

magnetic anomalies in the study area is attribu

relatively nonmagnetic character of the basement 

their deep burial. Resistivites measured at crysta

ment levels may be due to high-temperature crysta

ment rocks (see discussion on magnetotellu

Correspondingly, the lack of magnetic anomalie

ascribed to a crystalline basement complex at 

10–15 km that has temperatures above the Curi

which magnetization of the rocks is lost. The ma

dioritic to gabbroic composition normally w

expected to be magnetic but show little or no 

response in outcrop. This anomalous low magne

attributed to reduction of magnetite content in th

chemical processes related to interaction between h

mal fluids and cooling magma of the intrusions 

others, 1970; Harrison and others, 1972). Gravity 

data, on the other hand, show distinct anomalies 

sources, as determined from modeling, at or near 

basement levels. 

The most distinctive magnetic anomalies in 

pal study area are five positive anomalies (number

4, and 5 on pl. 1A) associated with Cretaceous o

cupolas and stocks that are exposed or are known 

holes to be present in the near-surface. These 

range in amplitude from 100 to more than 3,00

magnetic data provide information about the s

extent of these igneous intrusions and give clues fo

other buried intrusions that have subtle expression

The most prominent of the five magnetic 

(anomaly 1, pl. 1A) is associated with a pyroxen

complex (unit Kps) that is partly exposed 

Creek–Vermiculite Mountain, about 6 km east-no

Libby. The anomaly has an amplitude exceeding 

and it correlates with the main mass of ultrama

Pyroxenite is the principal ultramafic rock, an

pyroxenite in this stock has been a major source o

lite in the United States. Various aspects of the g

the complex and adjacent areas were studied by L

Pardee (1929), Boettcher (1966), and Harrison a

man (1993), who concluded that the complex is a 

The complex consists of a biotitic core surrounded 

pyroxenite and an outer ring of magnetite pyro

large pluton of syenite and associated alkaline sye

are in the southwestern part of the complex. The 

anomaly is a combined expression of the total co

does not distinguish syenite from pyroxenite roc

and Pardee (1929) reported that the syenite locall

3–12 percent magnetite. Boettcher (1966) specu

minor fenitization of Belt rocks along the northern 

the complex may indicate a large body of carb
GEOLOGIC INTERPRETATION OF AEROMAGNETIC ANOMALIES 

depth. The Bouguer gravity anomaly data show a residual

high resulting from a change in gradient along a nosing in the

contours (pl. 1B) that is attributed to mafic rocks of the complex that are denser than the enclosing rocks. The residual

gravity high correlates with the positive magnetic anomaly

over outcrops of the complex. 

Northwest of the Rainy Creek complex, about 8 km

north-northwest of Libby, a pluton (unit Ks, pl. 1) crops out

in two places near Bobtail Creek. The associated single positive magnetic anomaly (anomaly 2, pl. 1A) has an amplitude

of 900 nT and is slightly elongated north-northwest. The pluton is mainly syenite and includes segregations of pyroxene

and amphibole (Gibson, 1948). A high nosing in the gravity

contours correlates with the positive magnetic anomaly over

the complex in much the same character as at anomaly 1.

Both the Rainy Creek and Bobtail Creek complexes are estimated to be about 100 m.y. old (Harrison and Cressman,

1993). A northwest-trending belt of high gradient in the Bouguer gravity anomaly data suggests that the two features may

be connected in the subsurface, possibly along a northwest-trending structural zone, although no surface indication

of such a zone is evident in the geologic mapping. Boettcher

(1966) stated that the syenite and ultramafic rocks at Bobtail

Creek and Rainy Creek may be comagmatic.

A low-amplitude positive magnetic platform extends to

the south of anomaly 1 across the Kootenai River for some

15 km in the form of a wide plunging nose in the magnetic

contours (pl. 1A). A small, but distinct nose is superimposed

on the magnetic platform anomaly (pl. 2A) near Fisher

Mountain. The anomaly sources may be a shallow cupola

and a large related mass of moderately magnetic rock that

gives rise to the platform anomaly. 

In the southwestern part of the map, just east of Trout

Creek, a positive magnetic anomaly (anomaly 3, pl. 1A) of

almost 300 nT is associated with outcrops of hornblende

granodiorite (unit Kg) of the Vermilion River stock. The axis

of the anomaly is oriented north-northeast and is offset

toward the northwest edge of the outcrops. Intrusive rocks

probably are present in the subsurface beneath lower Prichard rocks that crop out northwest of the outcrops of the

stock. The associated broad gravity high reflects the

high-density hornblende granodiorite rocks that are in contact with rocks of the lower part of the Prichard Formation. 

Near the southern end of the Libby thrust belt study area

and just northeast of Thompson Falls, a circular positive

anomaly (anomaly 4, pl. 1A) of more than 200 nT amplitude

indicates the presence of a buried intrusion. According to

drill-core information provided by the staff of Noranda

Exploration, Inc. (unpub. data, 1979), quartz monzonite

porphyry is present about 775 m below the ground surface

(Kleinkopf and others, 1988). The configuration of the

anomaly and the horizontal extent of the steepest gradient on

the flanks of the anomaly are consistent with depth of burial

as indicated by the core-hole information. Using

potassium-argon techniques, analysis of biotite and

potasssium feldspar from the quartz monzonite gave 

metric ages of 40–50 Ma (Marvin and others, 1984), yo

than ages for the major intrusions to the north. An asso

gravity low is discussed in the section on interpretation 

gravity anomaly data.

Another well-exposed intrusive complex, the 

Creek stock, about 15 km southwest of Libby, has less p

inent magnetic expression (anomaly 5, pl. 1A). The ex

sion is composite and consists of three small po

anomalies of less than 100 nT in areas of granitic outcr

high elevations of the eastern part of the stock. To the 

magnetic gradients decrease rapidly, and there is no ano

over exposures of the stock (Kleinkopf, 1981). The lith

of the Dry Creek stock is quartz monzonite to granod

(Gibson, 1948), and the granitic rock here probably be

to a 100-m.y.-old intrusive event (Marvin and others, 

Harrison and Cressman, 1993). The complex character 

magnetic response of the Dry Creek stock is strikingl

ferent than the distinct 300-nT positive anomalies asso

with the Vermilion River and Liver Peak stocks. Mag

susceptibilities of three samples of granodiorite from 

eastern part of the Dry Creek complex range from 0.0

0.072 International System (SI) units (Kleinkopf, 1981

generally correlate with topographic highs. The mag

highs may relate to small stocks that formed as apophy

the larger, relatively less magnetic granitic complex 

Dry Creek stock. Harrison and Cressman, 1993) rep

that all stocks and plutons in the map area have contact 

morphic aureoles. In particular, the Dry Creek stock 

1,000-m-wide hornfels zone where it intrudes rocks 

Wallace Formation and Missoula Group. The sto

sheared and foliated by faulting, particularly on the wes

(Wells and others, 1981) where the magnetic map s

little or no magnetic expression. I infer from the lack of 

netic expression that little primary magnetite was pres

the intrusion. Gibson (1948) reported no alteration o

stock but observed that magnetite formed after initial cr

lization of the magma and probably is interstitial to pyr

and other minerals. A small outcrop of granodiorite is 

12 km south of the Dry Creek stock (pl. 1A). No mag

anomaly was detected on two flight lines that passed j

the north and to the south of this outcrop, which further 

cates that the granodiorite is of low magnetiz

(Kleinkopf, 1981). 

Many of the elongated short-wavelength mag

anomalies are related to magnetite-rich horizons pre

nantly in siltite layers of the Burke and Revett Formatio

the Ravalli Group (Kleinkopf and others, 1972). 

anomalies are typically 50–100 nT in amplitude, and 

are elongated north-northwest along outcrops of R

Group rocks that are topographically high and, in 

cases, structurally controlled. Field checks in the east-c

part of the area show that positive anomalies are asso

mainly with outcrops of the Burke Formation. Hand 

ments of the Burke exhibit abundant euhedral mag
8 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

grains. Magnetic susceptibilities for three samples collected

in this area of from 0.013 to 0.035 SI units can account for

the observed anomalies. In the northern part of the area at

Mount Henry, and just to the south, two distinct positive

anomalies correlate with outcrops of Revett Formation in

high topography (Bankey and others, 1986).

The Hope fault zone and faults of the Lewis and Clark

line are expressed by parallel magnetic alignments (pl. 2A).

The Hope fault is along the southwestern margin of the

Libby thrust belt and is delineated in the magnetic anomaly

data by zones of steep gradients approximately parallel with

the fault (pl. 1A). In general, the magnetic patterns are more

variable on the northeastern side of the fault (Kleinkopf and

others, 1972). 

Southwest of the Libby thrust belt, the magnetic anomaly data show expressions of igneous intrusions and fault

zones. A few kilometers north of Wallace, Idaho, the

regional aeromagnetic anomaly map (pl. 2A) shows a

north-northeast-elongated, 200–300-nT, positive anomaly

that correlates with quartz monzonite exposures of the Gem

stock intrusive complex (Hobbs and others, 1965; Kleinkopf

and others, 1988). Also in the Wallace area, distinct

west-northwest linear magnetic trends reflect structural

alignments of the Thompson Pass, Osburn, and Placer Creek

faults, which are part of the Lewis and Clark line (Kleinkopf

and others, 1988). A large, complex positive magnetic

anomaly 25 km west of Thompson Falls correlates with outcrops of granitic intrusions (unit Ks) that are composed of

syenite. The elongate shape of the anomaly suggests that

granitic rocks are present as an extension of, or possibly as a

separate, pluton in the subsurface southwest of the outcrop.

GEOLOGIC INTERPRETATION OF 

BOUGUER GRAVITY ANOMALIES

In the area of the Libby thrust belt, there is a strong correlation of gravity anomalies with both geologic framework

and major lithologic units. The geology and principal structural features are shown, together with the Bouguer gravity

anomaly, to illustrate the gravity signatures and to provide a

reference for discussions of the geologic interpretations of

the gravity anomaly data (pls. 1B, 2B). The regional gravity

field dips gently to the east across the study area and exhibits

a complex of regional-scale positive and negative anomalies

that correspond to broad open folds west of the Rocky

Mountain trench (Harrison and others, 1980). 

The gravity anomaly patterns can be discussed in terms

of configurations of the –130- to –140-mGal contours (pls.

1B, 2B). In the middle and southern parts of the study area,

structurally deformed rocks of the Missoula Group generally

are less dense than less deformed adjacent rocks of older

parts of the Belt Supergroup. The –140-mGal contour delimits a broad negative area whose source is attributed to

low-density Missoula Group rocks that also ha

enced some reduction in density due to faulting an

ing associated with development of the Libby t

Much of the grain of the gravity anomaly patterns 

broad negative area is approximately parallel with 

of thrust and normal faults of the Libby thrust bel

2B). The broad negative gravity feature has its low

of –160 mGal just east of Thompson Falls, w

approximately coincident with the Liver Peak 

anomaly (anomaly 4, pl. 1A). This deep gravity 

mGal) forms the southern part of a horsesh

negative anomaly that is open to the north and ha

lows at both ends of the horseshoe. The distinct 

mGal) centered on the south flank of Liver Peak m

low-density felsic rocks of the intrusion. In add

deepest part of the gravity low may relate to lo

hydrothermally altered rocks associated with 

num-tungsten-mineralized rock at Liver Peak. Th

northwest extension of the horseshoe anomaly ma

structural trough of low-density Quaternary sedi

localized fracturing of Belt rocks at the intersect

Hope fault and south-trending thrust faults of 

thrust belt. Alternatively, a less magnetic 

low-density intrusive rocks at Liver Peak may be 

in the subsurface in the west and northwest areas.

North of lat 48° N., about 25 km northeas

Creek, the regional gravity low defined by the –

contour narrows and follows the north- to northeas

Fisher River where it cuts around geologic stru

low gravity values may express crossfaulting tha

the river direction here. Northwest of the 

northeast-trending gravity divide is defined by –

contours. Northwest of the divide, gravity values d

less than –144 mGal over the alluvial valley south 

which is in the center of a broad, relatively flat bot

approximately defined by the –130- to –140-mGa

Assuming that about 6 mGal of relief is associate

alluvial valley, the thickness of valley fill may exce

North of Libby, north-northwest gravity tre

late with major thrust faults of the Libby thrust 

passes north into Canada. The broad, bu

north-trending gravity low that extends south 

separates the well-defined gravity highs associate

Purcell anticlinorium on the east and the Sylvanit

on the west.

The Sylvanite anticline and the Purcell ant

exhibit prominent gravity highs that have a

exceeding 10 and 20 mGal, respectively (pls. 1B

high associated with the Purcell anticlinorium 

trends northwesterly; however, on close inspectio

patterns show that the broad high is composed of t

rate northerly trending highs arranged en echelo

northwest trend. The small anticlinal axis passing 

of the Gibbs No. 1 drillhole correlates with the sou

gravity high. The gravity high associated with t
GEOPHYSICAL INTERPRETATION OF DEEP-SEATED GEOLOGIC FEATURES 

anticlinorium is broken and offset at a location about 10 km

east of Libby. The offset is part of a west-northwest trend in

the gravity anomaly data that passes just north of Libby and

extends to the Moyie thrust fault near Troy. In addition,

discontinuous trends in the magnetic anomaly data correlate

with the gravity trends, notably just south of the Rainey

Creek intrusive complex (anomaly 1, pl. 1A). The composite

gravity high associated with the Sylvanite anticline exhibits

a distinct northerly trend and subtle, superimposed northwesterly trends. The mapped axis of the anticline is offset

southwest of the axis of the gravity high. Gravity modeling,

discussed in the next section of this paper, indicates that the

Purcell anticlinorium and the Sylvanite anticline are very

likely cored by stacks of thrust slices of dense crystalline

basement rocks that account for the large gravity highs

across these two structures. 

West of the Cabinet Mountain Wilderness, the Moyie

thrust fault is along the west side of a narrow troughlike

gravity anomaly related to the north-trending Bull River

valley (pl. 2B). At the north end of the valley, the thrust turns

and follows the northwest extension of the gravity trough

into Idaho. The deepest part of the trough is about 10 km

northwest of Troy. Three distinct lows are along the trough

associated with Bull River valley. The deepest low, which

exceeds 6 mGal amplitude, is at the north end of the valley,

an area of complex geologic structure (pl. 1B). This deep low

is separated from the next low to the south by an east-northeast-trending gravity ridge. Contour alignments and trend

projection across the Cabinet Mountain Wilderness provides

some evidence that the ridge may express a buried fault zone

that extends northeast as far as the gravity platform just north

of Libby. Although no continuous fault zone has been

mapped in the surface geology, the geologic map shows discontinous northeast-trending faults and areas of fault intersection along the projected gravity feature.

In the Cabinet Mountains Wilderness, in the central part

of the Libby thrust belt, a line of positive gravity anomalies,

uniformly about 6–10 mGal amplitude above background,

extends south-southeast from near the Kootenai River to as

far as the latitude of Trout Creek. One of the highs is centered north of the exposures of the Dry Creek stock about 15

km southwest of Libby but is much more extensive than

either outcrops of the stock or the associated magnetic anomalies described earlier in this report as well as in Kleinkopf

(1981). The second gravity high is at Snowshoe Peak, and a

possible correlative magnetic source is in the subsurface

beneath outcrops of the Wallace and Helena Formations and

the Ravalli Group. The third gravity high is at Flat Top

Mountain, about 20 km south-southeast of the second; the

highest part of this anomaly is about 5 km west of Flat Top

Mountain. Between the second and third highs, the

south-southeast trend of the highs is broken by a discontinous zone of gravity patterns that trend east-northeast toward

Fisher Mountain; crosscutting gravity contour alignments

are along the east-northeast trend. In addition, both in the

Cabinet Mountains Wilderness and in the main part 

Libby thrust belt, structural complexities and crossfa

are present along the trend (pl. 1). The anomalous g

and the structural features may reflect a zone of weakn

the crust of unknown origin. The fourth gravity high is 

16 km south-southeast at Trout Creek and correlates wi

Vermilion Creek stock of hornblende granodiorite com

tion. Kleinkopf (1981) concluded from interpretations 

gravity high over the Dry Creek stock and from the dis

tion of the magnetic anomalies that the Cabinet Mou

Wilderness may be underlain by an extensive gr

batholith that is relatively nonmagnetic. The gravity 

may reflect large near-surface intrusions along the fe

and the small positive magnetic anomalies may in

near-surface sources associated with small granitic 

sions or cupolas that may be apophyses of a po

batholithic mass at a depth of a few kilometers, as sugg

by the gravity model along geologic section J–J' (fig. 5

following section). On the basis of surface mapping and 

cations in the outcropping sedimentary rocks, Harriso

Cressman (1993), in their structure sections, showed th

Creek stock extending to the south.

GEOPHYSICAL INTERPRETATION 

DEEP-SEATED GEOLOGIC FEATUR

MODELING OF GRAVITY PROFILES

Modeling of Bouguer gravity anomaly data fo

Libby thrust belt and adjacent areas supports the inter

tion that the crystalline basement is elevated beneath th

cell anticlinorium and the Sylvanite anticline. The m

studies update previous model studies of the Purcell a

norium by myself, described in Harrison and others (1

by addition of new gravity control along the section

new subsurface lithologic information obtained from 

logs in the Atlantic Richfield–Marathon Oil Gibbs 

borehole, NE1

/4 sec. 2, T. 28 N., R. 27 W. (pl. 2). An im

tant constraint on the validity of the modeling is select

reasonable densities for the major rock types, or com

tions of rock types, depicted in the geologic cross sec

These rock types include crystalline basement, presum

be Proterozoic in age, Prichard Formation and sills

post-Prichard strata composed mainly of rocks of the R

Group, Helena and Wallace Formations, and Mis

Group. I selected values for density based on labo

measurements and on supporting evidence from wel

taken in the Gibbs No. 1 deep borehole (fig. 3). 

Densities for a given rock unit within the study ar

assumed, on the basis of the general regional consisten

rock types, to be laterally uniform. In laboratory me

ments of metamorphic rocks, Smithson (1971) found a 

sity range from 2.7 g/cm3 for granitic gneiss to 2.86 
10 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

Figure 3. Generalized lithologic and geophysical logs, Gibbs No.1 borehole, Flathead County, Montana. Neutron density log 

per centimeter per second per second. Data courtesy of Atlantic Richfield and Marathon Oil Companies. Surface section at Little 

is also shown. Modified from Harrison and Cressman (1993).
GEOPHYSICAL INTERPRETATION OF DEEP-SEATED GEOLOGIC FEATURES 

for granulite facies. I used 2.8 g/cm3 as a reasonable estimate

for the density of the crystalline basement. Density values

for 13 samples of post-Prichard Belt rocks (measured in

USGS laboratories) range from 2.58 to 2.76 g/cm3 and

average 2.67 g/cm3. Earlier laboratory measurements of Precambrian mafic sill samples yielded an average density value

of 2.91 g/cm3 (Harrison and others, 1972). The Precambrian

sills in the borehole (fig. 3) are characterized by high density,

low radioactivity, and rather high velocity (Harrison and others, 1985). The neutron-density log shows considerable variation in density of the sills, presumably related to alteration.

Density values range from about 2.76 g/cm3 to spikes on the

log exceeding 3.0 g/cm3. The information from these logs,

together with data from the lithologic log, provides reliable

thickness estimates for the sills, which total more than 1,000

m of the drilled rock. I estimated that 7.6 km or more of Prichard Formation underlies the borehole site. The assumption

was made that the density of the Prichard Formation, without

sills, is in the range of from 2.65 to 2.70 g/cm3, considering

the thick sequences of quartzite and argillite. Thus, combining these values with an verage value of about 2.91 g/cm3 for

the Precambrian sills, I obtained an average density for the

total column of Prichard Formation and sills of about 2.73

g/cm3. The lithologic units and corresponding densities

assigned to the bodies are listed in table 1. 

Gravity models were calculated along the two lines of

section, B–B' and J–J''' prepared by Harrison (Harrison and

Cressman, 1993), in an attempt to gain information about

structure and lithology along the zone of detachment and at

crystalline basement levels at depths of approximately 15

km. The sections were simplified and extended laterally

(figs. 4, 5) for purposes of the gravity modeling. Geologic

data in Idaho used to extend section B–B' are from a geologic

map of the Eastport area (Burmester, 1985). The expansion

of section J–J' in Montana is from new mapping by Harrison

(Harrison and others, 1992); the extension of section J''–J'''

is offset 13 km southward along strike in order to cross the

Purcell anticlinorium at the location of the Gibbs No. 1 borehole (fig. 2). Two-dimensional modeling of the gravity data

was done on a mainframe computer using the software SAKI

(Webring, 1982). Control for the modeled sections was provided by random-spaced gravity stations that are along or

projected into the lines of section. The resulting gravity control along the modeled sections varies from 1 to more than 5

km in spacing of stations.

The Purcell anticlinorium and the Sylvanite anticline

are appropriate for gravity model studies. Both structures

exhibit prominent positive gravity anomalies that correlate

with geologic structures at the surface. The gravity anomaly

across the Sylvanite anticline is well defined and is as much

as about 10 mGal in amplitude. The gravity expression associated with the Purcell anticlinorium has an amplitude of

15–20 mGal, and the anticlinorium is more than 50 km wide

and is remarkably continuous and linear for about 100 km to

the north into Canada. Several theories on source and mass

distribution have been suggested to account for the p

nent gravity high associated with the Purcell anticlino

In a study of electrical conductivity and gravity data

Wynn and others, 1977) modeled the anomaly as a base

uplift interpreted as the result of movement of major 

blocks. In a later study, Harrison and others (1980) 

cluded that the source of the anomaly beneath the P

anticlinorium is not a vertical uplift but very likely is a 

of thrust slices of high-density crystalline basement 

that are elevated above the regional basement surface

ing the early 1980’s, a popular theory favored among va

petroleum explorationists attributed the source of the P

anomaly to high-density lower Paleozoic carbonate 

that would provide attractive exploration targets. 

regional structural analysis, Fountain and McDon

(1984) postulated that the anomaly is caused by a high

sity body in the form of a basement ramp anticline be

the Purcell anticlinorium. Harris (1985) modeled the P

gravity anomaly and attributed the source of the anom

mafic sills in the lower part of the Prichard Formation 

a uniformly dipping crystalline basement surface. I

report, I refine, on the basis of further analysis and stud

the data, earlier interpretations in which the source 
12 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

  







        



 











 















Observed Calculated

MILLIGALS

–120

–130

–140

B B'

ELEVATION

(IN KILOMETERS)

10

2

SEA LEVEL

1

2

4

7

3

5

6

2.73

2.68

2.80

2.73

2.70

2.70

2.50

OBSERVED AND CALCULATED GRAVITY

GRAVITY MODEL

ELEVATION

(IN KILOMETERS)

10

2

SEA LEVEL

GEOLOGY

SYLVANITE

ANTICLINE

LIBBY

THRUST BELT

0 25 50 KILOMETERS

Helena and Wallace Formations

Ravalli Group

Prichard Formation and mafic sills

Basement rocks

EXPLANATION

Moyie 

Thrust

PINKHAM THRUST FAULT

DETACHMENT ZONE

Figure 4. Gravity block model of extended geologic cross section B–B' showing inferred lithology

and structure beneath the Sylvanite anticline and northwestern part of the Purcell anticlinorium, Libby

thrust belt, northwestern Montana. Numbers on gravity model refer to body number and assumed density (in grams per cubic centimeter) as shown in table 1; for example, for body 2, the density is 2.73

g/cm3.
GEOPHYSICAL INTERPRETATION OF DEEP-SEATED GEOLOGIC FEATURES 

anomalies is thrust slices of dense crystalline basement rocks

that core the Purcell anticlinorium. 

DEEP FOLDS AND FAULTS INTERPRETED 

FROM SEISMIC DATA

By Jack E. Harrison

Seismic data support the interpretation of crystalline

basement beneath Prichard as shown in figure 6. The seismic

data identify the mafic sills, which are excellent seismic

reflectors (fig. 3), and the sills clearly mimic surface structure to depths of about 8 km below ground level at which

seismic data lose definition (Harrison and others, 1985).

Seismic data were collected by others who discussed the data

with us but preferred that they not be acknowledged. Two

deep seismic lines of profile across the Sylvanite anticline

and the Libby thrust belt were prepared by COCORP (The

Consortium for Continental Reflection Profiling) in

1984–85 (Potter and others, 1986; Yoos and others, 1991).

In 1986, graphs of the unmigrated data were generously

made available for our use and are shown here (fig. 6).

Locations of the profiles MT–1 (Montana–1) and MT–2

(Montana–2) are shown on plate 1 and in figure 2, and interpretation of the profiles (generously aided by my colleague,

M.R. Reynolds) is shown in figure 6. 

Most of the shallow reflectors (≤3 seconds) relate

reasonably well to surface geology, even though the seismic

data are not migrated and therefore are not precisely in the

correct geographic position. The mafic sills in the lower part

of the Prichard Formation (fig. 3) are known seismic reflectors and are visible on both of the seismic profiles. The

Pinkham thrust fault, as identified on profile MT–2, projects

from the surface to the logical position and dip angle shown,

a position that agrees well with that obtained from other data

from about 40 km farther to the south in the Libby thrust belt.

Of less certain origin are the reflections recorded below the

Wallace Formation and the Missoula Group on profile

MT–2 near Pipe Creek. These may be reflections from sills

in the Prichard Formation, or they may be unmigrated reflections from the steep faults and bedding in the area. 

Below 3-seconds two-way travel time, the source of

reflections is more speculative. The inferred basal surface of

detachment is marked on profile MT–1 by a series of reflections and extends readily in profile MT–2 to separate various

different domains of reflections. The depth to the basal surface varies from about 9 to 18 km across the area, and the

surface dips about 15° in inferred crystalline basement rocks.

These values do not differ drastically from the depth and dip

of inferred basement rocks under the Purcell anticlinorium

as shown for the Rocky Mountains of southern British

Columbia in cross sections by Price and Fermor (1985) and

by Okulitch (1984, fig. 6). The strong reflections a

below 4 seconds (fig. 6) are, therefore, attributed to la

zones in the basement, perhaps mylonite, to the detach

zone for the older tectonic folding (?), or to strongly la

gneiss. No obvious reflection is present, at least alon

profiles, for the Belt-basement contact. The lack of a 

ment reflection is most obvious on profile MT–1, wher

reasonable thickness of the Prichard Formation require

such a contact be at or above 4 seconds. Because the l

part of the Prichard Formation in mid-Belt terrane is 

garnet zone of regional metamorphism and the Middle 

erozoic basement where drilled in western Montana is a 

ture of granite, granitic gneiss, and some mafic intr

rocks, a contact between metamorphosed Belt terran

granite or granite gneiss would produce too little contr

cause seismic reflections. A dissimilarity in basemen

types or structures is inferred from the contrast betwee

few deep seismic reflections above the Roderick Mou

thrust fault on profile MT–1 and the multiple prom

reflections below that thrust fault on profiles MT–

MT–2. None of the reflections are necessarily a

Belt-basement contact. A consequence of these observ

is that the true thickness of the present Belt Supergrou

thickest known sedimentary sequence of Middle Proter

rocks in the world, can only be determined by a

undrilled deep boreholes. 

INTERPRETATION OF MAGNETOTELLU

SOUNDINGS

By W.D. Stanley

Extensive magnetotelluric surveys have been 

ducted across the Purcell anticlinorium and Libby thrus

Phoenix Geophysics supplied the data from a profile o

soundings (fig. 2) of a proprietary survey to me for incl

in a study of regional structures. Another profile of th

prietary data, not shown in this report, is about 32 km 

of the Phoenix profile and passes through the location 

Gibbs No. 1 borehole, a convenient geological calib

point. The magnetotelluric soundings from the survey 

high quality and quite unusual in that a striking low-re

ity zone was detected on all the soundings. Earlier au

agnetotelluric studies (Wynn and others, 1977; Long, 

1988) demonstrate that in the Prichard Formation a di

tinuous zone, typically at depths of 0.9–2.5 km, is very 

ductive and has resistivities of a few ohm-meters or les

vertical distribution of mineralized zones is indicated b

low resistivities in the resistivity log from the Gibbs 

borehole (fig. 7). In the borehole, moderate amounts o

fide minerals in members F, G, and H of the Prichard F

tion cause resistivities of less than 200 ohm-m, and g
14 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA 







     



                        



    

Observed Calculated

9

2.70

5

2.67

3

2.73

2

2.73

6 2.75

7

2.69

4

2.73

1

2.80

4

2.73

8 2.64

1

2.80

OBSERVED AND CALCULATED GRAVITY

GRAVITY MODEL

GEOLOGY

MILLIGALS

125

150

J'

J

ELEVATION

(IN KILOMETERS)

2

10

ELEVATION

(IN KILOMETERS)

J''

J'

J

J'

J

2

10

SEA

LEVEL

MILLIGALS

125

150

ELEVATION

(IN KILOMETERS)

2

10

ELEVATION

(IN KILOMETERS)

2

 10

SEA

LEVEL

L I B B Y T H R U S T B E L T PURCELL ANTICLINORIUM

Gibbs No. 1

borehole

Offset 13 km

along strike

ZONE OF DETACHMENT

PINKHAM THRUST FAULT

MOYIE THRUST

SYSTEM

0 25 50 KILOMETERS

SEA

LEVEL

J''

J''

7

2.69

J'''

J'''

J'''

SEA

LEVEL
GEOPHYSICAL INTERPRETATION OF DEEP-SEATED GEOLOGIC FEATURES 

amounts of iron sulfide minerals at about 3 km (member E)

and 5 km (members A–D?) produce resistivities of 2–20

ohm-m. The low resistivities are attributed to pyrite or pyrrhotite films and disseminated mineralized zones in the

slightly carbonaceous argillitic unit. The low-resistivity

zones in the Prichard Formation and other units of the Belt

Supergroup represent easily mappable features for evaluation of mineralization in the Belt basin (Long, 1983). 

Interpretations of the five soundings provided by

Phoenix Geophysics were used to construct the resistivity

cross section shown in figure 8. The soundings are

one-dimensional in character (isotropic) in that the resistivities measured in the two coordinate directions of the field

setup are very similar. The data were interpreted using interactive forward models of four and five layers, and the cross

section was constructed by connecting the layered models

from each sounding location (fig. 8). Accuracy of the interfaces obtained from the magnetotelluric data is not great,

both because of the widely spaced sounding locations and

because of the very large contrasts in resistivity between

unmineralized and mineralized zones in the Belt Supergroup. Layer interfaces should be resolved, however, to

within 10–15 percent of their true depth, particularly the top

of the lowest resistivity layer that represents the deepest

layer in the models shown in the cross section. 

Most of the magnetotelluric data can be discussed in

terms of two factors controlling resistivity in rocks of the

Belt Supergroup—fluids and conductive mineralized rock.

The 1,200–1,600-ohm-m layer beneath soundings 1 and 2

correlates with the Missoula Group, Helena and W

Formations, and Ravalli Group (fig. 8). These resisti

are typical for older carbonate and metasedimentary 

that contain fresh pore waters, and they are characteris

most Belt rocks above the Prichard Formation (Long, 1

The 130–400-ohm-m units at the surface at soundings 

4 are associated with the more mineralized condu

Prichard Formation, as are the conductive units (

ohm-m) at soundings 1–4. The moderately low resis

(250 ohm-m) in the Ravalli Group for the magnetote

model at sounding 5 is probably related to low-grade 

tabound copper sulfide minerals, which are comm

present in the Spokane Formation, whereas the very 

resistivities of 1.5 ohm-m for the model probably refle

highly continuous iron sulfide zones in the Prichard F

tion. The thicknesses of the low-resistivity units that 

the basal layer on the magnetotelluric models could n

uniquely determined from the magnetotelluric soun

even though the data extended to frequencies of 0.000

however, estimates based on the magnetotelluric 

information suggest that conductive units must be th

than 7.7 km (with an error of about 0.1 km). The inter

ate layer of 20 ohm-m required at soundings 1 and 2 p

bly represents a part of the Prichard Formation that co

moderate amounts of iron sulfide minerals. The 

boundary of this low-resistivity layer approxim

matches the interpreted location of the Pinkham thrust 

which places highly resistive rocks of the Ravalli Grou

Helena and Wallace Formations over the Prichard Form

(fig. 8). Magnetotelluric data should be effective in 

mining the lithology and depth to basement beneath 

rocks, given a more extensive data set that would allow 

accurate interpretations.

The magnetotelluric data have limitations in stru

analysis because resistivities are controlled mostly b

percentages of metallic minerals and not by rock litho

The magnetotelluric data require that conductive r

probably of the Prichard Formation, extend to depths of 

than 10.8 km (with an error of about 1.6 km) below m

totelluric soundings 1, 2, and 3 and that a low-resistivity 

ohm-m) zone forms the bottom layer on the western p

the profile (fig. 8), corresponding approximately wi

interpreted basal surface of detachment that separates 

talline basement units from the Prichard Formation.

The lithology of the basement in the study area c

be resolved using the magnetotelluric data. Outcrop

boreholes in the region indicate basement of possibl

nitic gneiss, mafic intrusive rocks, and high-grade met

mentary rocks. The Belt basin probably formed on stre

crust of Archean age that has a Middle Proterozoic (

Ma) metamorphic overprint. This type of crust is not g

ally low in resistivity unless partial melting has occ

Feldman (1976) pointed out that low resistivities c

produced in granitic rocks at depths of 10–15 km 

temperatures of 500°C–600°C if 1–3 percent free wa



 



 Cretaceous granite

Cambrian sedimentary rocks

Missoula Group

Missoula Group and Wallace Formation

Helena and Wallace Formations

EXPLANATION

 



Ravalli Group

Prichard Formation

Basement rocks

Belt Supergroup

Figure 5 (above and facing page). Gravity block model of

extended geologic cross section J–J' showing information

about inferred lithology and structure beneath the Purcell anticlinorium, Libby thrust belt, northwestern Montana. Numbers on gravity model refer to body number and assumed

density (in grams per cubic centimeter) as shown in table 1;

for example, for body 1, the density is 2.80 g/cm3.
16 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

Yaak River Pipe Creek

NORTHWEST

Carrigan Campground West Si

SOU

0 

5

10

P

B

B

P

R

W

M M

W

W

R

P

P

R

H-W

R

P

P

B

B

M

PINKHAM

THRUST

FAULT

BASAL SURFACE

OF DETACHMENT

RODERICK

MOUNTAIN

THRUST FAULT

PROFILE MT–2

TWO-WAY TRAVEL TIME, IN SECONDS

LIBBY THRUST BELT

0 5 10 15 20 25 KILOMETERS

116° Yaak River

0

5

10 10

5

0

NORTHWEST SOUTHEAST

B

P

R

P

B

B

B

RODERICK MOUNTAIN THRUST FAULT

BASAL SURFACE OF DETACHMENT

PINKHAM

THRUST

FAULT

PROFILE MT–1

TWO-WAY TRAVEL TIME, IN SECONDS

EXPLANATION

Zone of seismic reflec

 Closer spacing of lin

 more intense reflect

Thrust fault—Showing 

 of movement. Doub

 arrow indicates back

 on fault

Geologic contact—Ap

 located or inferred

P

Figure 6. Interpretation of seismic reflection profiles MT–1 and MT–2, Libby thrust belt, northwestern Montana. Unmigrate

vided at H:V=1:1 at 6 km/sec. Surface geologic data are generalized from plate 1 of this report; 12 high-angle faults are not sho

clutter. Geologic units: P, Prichard Formation; R, Ravalli Group; W, Wallace Formation; H-W, Helena and Wallace Form

Missoula Group; S, Proterozoic mafic sills; B, crystalline basement rocks. Lines of profiles are shown in figure 2.
GEOPHYSICAL INTERPRETATION OF DEEP-SEATED GEOLOGIC FEATURES 

present, perhaps from metamorphic processes. Continued

extension of the crust beneath the Belt during sedimentation

is indicated by periodic intrusion of mafic dikes and intrusions that have continental tholeiite affinities (Harrison and

Cressman, 1993). Regional burial metamorphism reached

the garnet grade of the greenschist facies (350°C or higher)

in the lowest exposed Prichard during basin fill of at least 16

km of sediment and sills. Thus, there is no evidence in Belt

rocks for temperatures of 500°C–600°C required to produce

the type of conductors envisioned by Feldman (1976); however, a thermally related basement conductive zone could

possibly now be present at depth beneath magnetotelluric

soundings 1, 2, and 3. Another possible line of therma

dence is the absence of magnetic anomaly sources at d

of 10–15 km, presumed to be near the top of pre-Belt 

talline rocks. This absence of magnetic sources may re

shallow Curie-point temperatures, above which magn

does not exist (Mabey and others, 1978). Curie temper

are about the same order of magnitude as those suggest

Feldman (1976), 500°C–600°C. 

The extensive mafic volcanism and somewhat 

nature of the Belt-Purcell trend suggest that Belt rock

have been deposited on pre-Belt rocks in a deep contin

margin rift analogous to the Animike Basin of Minn





 





 

 





 



  

 





EXPLANA

Clastic ro

Schist a

phyllit

Mafic s

IRON SULFIDE–BEARING ZONE

PRICHARD FORMATION

Members A-D(?) Member E Member F Member H Member G

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.2 20 200 2,000 20,000 200,000 2

RESISTIVITY, IN OHM-METERS

DEPTH, IN KILOMETERS

Figure 7. Generalized resistivity log (Dual Lateralog) of Gibbs No. 1 borehole, Flathead County, Montana. Electrical data court

Atlantic Richfield and Marathon Oil Companies. Lithologic log simplified from figure 3. 
18 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

and Wisconsin (Larue, 1981) or in a linear continental rift or

aulocogen similar to the Amadeus Basin of Australia (Wells

and others, 1970). Both the Amadeus and Animike basins

contain large volumes of mafic volcanic rocks and thick

Proterozoic anoxic shales that have high metal content

(Lawler and Vadis, 1986; Wells and others, 1970). Dense

high-velocity rocks that represent basement to the gravity

and seismic methods may actually be dense, volcanogenic

metasedimentary rocks that have high metallic mineral

concentrations, similar to rocks in the Anim

Amadeus basins. In addition, I speculate that the 

ductivity of pre-Prichard crystalline basement in 

may be related to large amounts of metallic miner

the magnetotelluric data can be used to postulate 

below the Belt Supergroup may be mineralized c

basment complex, unless temperatures are gre

500°C–600°C.







 





    



 







Troy

Libby

1

2 3 4

5

Kootenai

R i v er Fisher R.

Fortine

Creek

East edge

of plate 1

Pipe

Creek

116° 115°

48°30'

0 10 20 30 40 50 KILOMETERS

Index map showing location of magnetotelluric

soundings in northwestern Montana

0 5 10 15 KILOMETERS

Missoula Group

Helena and Wallace Formations

Ravalli Group

Prichard Formation

Inferred basement rocks

Copper sulfide zone

Zone of intermittent iron sulfide minerals

Conductive zone between sills

 in Prichard Formation

Thrust fault—Double–headed arrow

 indicates backsliding

 

Contact—Inferred below 10,000 ft

Interpreted boundary between

 magnetotelluric model units—

 Numbers are model resistivities

 in ohm–meters

130

EXPLANATION

NO VERTICAL EXAGGERATION

(IN 

2

5

10

15

ELEVATION

(IN KILOMETERS)

SEA

LEVEL

Geology by J.E. Harrison

BASAL SURFACE OF DETACHMENT

PINKHAM THRUST FAULT

LIBBY THRUST BELT PURCELL ANTICLINORIUM Lake Koocanusa

Banfield 

Kootenai River

Pipe Creek

Mountain

1600

20

3

1200

20

130

5 3

400

0.6

2

1

1 

Figure 8. Magnetotelluric and geologic section across the eastern part of the Libby thrust belt and the western part of the Purc

norium. Numbers indicate magnetotelluric soundings, the locations of which are shown in figure 2.
SUMMARY AND REFERENCES CITED 

SUMMARY AND CONCLUSIONS

Regional geophysical studies conducted by the U.S.

Geological Survey in the northern Rocky Mountains during

the past 25 years provide new insights about geologic framework and mineral resources. Conclusions presented in this

report are based on integrating interpretations of these gravity, magnetic, seismic, and magnetotelluric data. 

The gravity anomaly data show a marked correlation

with major structures. The Purcell anticlinorium exhibits

positive anomalies of more than 20 mGal, and the Sylvanite

anticline exhibits positive anomalies of more than 10 mGal.

Results of gravity modeling indicate that the Purcell anticlinorium and the Sylvanite anticline are very likely cored by

stacks of thrust slices of dense crystalline basement rocks

that account for the large gravity highs across these two

structures. 

The gravity anomaly data for the Cabinet Mountains

Wilderness show a string of positive gravity anomalies

about 6–8 mGal amplitude above background. The highs

extend south-southeast in a belt from near the Kootenai

River to as far as the latitude of Trout Creek. The gravity

anomaly data and the distribution of magnetic anomalies

suggest that the Cabinet Mountains Wilderness may be

underlain by a large, uplifted granitic batholith. The gravity

highs are interpreted to reflect intrusions along this feature,

and the small positive magnetic anomalies are inferred to

indicate near-surface sources that may be associated with

small granitic intrusions or cupolas that are apophyses of the

batholithic mass at depth, as shown in the gravity model.

The principal magnetic anomaly sources are igneous

intrusive rocks, major fault zones, and magnetite-bearing

sedimentary rocks of the Ravalli Group. Although the

sources mainly are present in outcrop, in some cases they are

partly or totally buried in the near-surface. 

The most important magnetic anomalies in the

principal study area are five distinct positive anomalies

associated with Cretaceous or younger cupolas and stocks

that are exposed or are known from drillholes to be present

in the near-surface. These anomalies range in amplitude

from 100 to more than 3,000 nT. Modeling of the magnetic

anomaly data was not done because of the absence of

long-wavelength magnetic anomalies having sources likely

located in buried crystalline basement. 

The lack of detectable anomalies is attributed to the relatively nonmagnetic character and deep burial of the basement rocks or to possible elevated Curie isotherms, which

are compatible with one explanation of low resistivities

obtained from magnetotelluric measurements. The mafic

sills of dioritic to gabbroic composition show little or no

magnetic response in outcrop, probably because of a reduction of magnetite content in the sills by chemical processes

related to interaction between hydrothermal fluid

cooling magmas of the intrusions. 

Interpretation of seismic reflection data for the 

area shows an inferred basal surface of detachment at d

of 9–18 km. Strong reflections at and below 4 secon

attributed to stratification in the basement such as la

gneiss or mylonite along the detachment zone for the 

tectonic folding. No obvious reflection is present, at 

along the profiles, for the Belt-basement contact. Just b

the Pinkham thrust fault, strong reflections attribut

stratification in crystalline basement rocks image rel

the basement on the western flank of the Purcell anticl

Interpretation of magnetotelluric data shows a 

conductive basal layer that is incompatible with a cryst

basement composed of granitic gneiss, mafic intrusive 

and high-grade metasedimentary rocks. This type of c

not generally low in resistivity unless partial meltin

occurred, perhaps at depths of 10–15 km and temper

of 500°C–600°C if 1–3 percent free water is present. A 

sible explanation for a heat source at these depths is th

of magnetic sources, which may indicate shallow Curie 

peratures, as previously postulated. An alternative exp

tion is that pre-Prichard crystalline rocks are condu

simply because they contain large amounts of metallic 

erals. This is certainly the case for the Prichard Form

because surface observations of variations in resis

range from 0.6 to more than 400 ohm-m and most 

reflect the percentage of iron sulfide minerals and cont

of iron sulfide zones. Accuracy of the interfaces obt

from the magnetotelluric data is not great, both becau

the widely spaced sounding locations and because o

very large contrasts in resistivity between unmineralize

mineralized zones in the Belt Supergroup.

REFERENCES CITED

Bankey, Viki, Kleinkopf, M.D., and Hoover, Donald, 1986

physical maps of the Mount Henry roadless area, L

County, Montana: U.S. Geological Survey Miscella

Field Studies Map MF–1534–C, scale 1:50,000.

Bankey, V.L., Brickey, M.R., and Kleinkopf, M.D., 1982, Pr

facts for 1980 gravity stations in the Wallace 1°×2° qu

gle: U.S. Geological Survey Open-File Report 82–132. 

Bankey, V.L., Kleinkopf, M.D., Brickey, M.R., Mancinell

and Kulik, D.M., 1985, Gravity survey data of the Ka

1°×2° quadrangle, Montana: U.S. Geological S

Open-File Report 85–221. 

Bishop, D.T., 1973, Petrology and geochemistry of the Purce

in Boundary County, Idaho, in Belt Symposium, v. 2: 

Bureau of Mines and Geology Special Publication, p. 1

Boettcher, A.L., 1966, The Rainy Creek igneous comple

Libby, Montana: University Park, Pennsylvania 

University, Ph.D. thesis, 155 p. 
20 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

Brickey, M.R., Bankey, V.L., and Kleinkopf, M.D., 1980, Principal

facts for gravity stations in part of the Wallace 1°×2° quadrangle, Idaho and Montana: U.S. Geological Survey Open-File

Report 81–178. 

Briggs, I.C., 1974, Machine contouring using minimum curvature:

Geophysics, v. 39, p. 39–40.

Burmester, R.F., 1985, Preliminary geologic map of the Eastport

area, Idaho and Montana: U.S. Geological Survey Open-File

Report 85–517, scale 1:48,000.

Cordell, Lindrith, Keller, G.R., and Hildenbrand, T.G., 1982,

Bouguer gravity map of the Rio Grande rift, Colorado, New

Mexico, and Texas: U.S. Geological Survey Geophysical

Investigations Series Map GP–949, scale 1:1,000,000.

Fabiano, E.B., and Peddie, N.W., 1980, Magnetic declination in the

United States—Epoch 1980: U.S. Geological Survey Miscellaneous Investigation Series Map I–1283, scale 1:5,000,000.

Fabiano, E.B., Peddie, N.W., and Jones, W.J., 1976, Magnetic total

intensity in the United States—Epoch 1975.0: U.S. Geological

Survey Miscellaneous Investigations Map I–915, scale

1:5,000,000, 2 sheets.

Feldman, I.S., 1976, On the nature of conductive layers in the

Earth's crust and upper mantle, in Adam, A., ed., Geoelectrical

and geothermal studies: Budapest, Akademiai Kaido, p.

721–730.

Fountain, D.M., and McDonough, D., 1984, Crustal structure of

northwestern Montana and southern Alberta and British

Columbia based on the interpretation of gravity data: Montana

Geological Society Field Conference and Symposium, Northwestern Montana and adjacent Canada, Guidebook, p.

253–258. 

Gibson, Russell, 1948, Geology and ore deposits of the Libby quadrangle, Montana, with sections on Pleistocene glaciation by

W.C. Alden and Physiography by J.T. Pardee: U.S. Geological

Survey Bulletin 956, 131 p., 2 pl., scales 1:125,000 and

1:250,000.

Harris, D.W., 1985, Crustal structure of northwestern Montana:

Missoula, University of Montana, M.S. thesis, 59 p. 

Harrison, J.E., 1972, Precambrian Belt basin of northwestern

United States—Its geometry, sedimentation, and copper

occurrences: Geological Society of America Bulletin, v. 83,

no. 5, p. 1215–1240. 

Harrison, J.E., and Cressman, E.R., 1993, Geology of the Libby

thrust belt of northwestern Montana and its implications to

regional tectonics: U.S. Geological Survey Professional Paper

1524, scale 1:125,000.

Harrison, J.E., Cressman, E.R., and Kleinkopf, M.D., 1985,

Regional structure, the Atlantic Richfield–Marathon Oil No. 1

Gibbs borehole, and hydrocarbon resource potential west of the

Rocky Mountain trench in northwestern Montana: U.S. Geological Survey Open-File Report 85–249, 5 p.

Harrison, J.E., Cressman, E.R., and Whipple, J.W., 1992, Geologic

and structure maps of the Kalispell 1°×2° quadrangle, Montana

and Alberta and British Columbia: U.S. Geological Survey

Miscellaneous Investigations Series Map I–2267, scale

1:250,000, 2 sheets.

Harrison, J.E., Griggs, A.B., and Wells, J.D., 1986, Geologic and

structure maps of the Wallace 1°×2° quadrangle, Montana and

Idaho: U.S. Geological Survey Miscellaneous Investigations

Series Map I–1509–A, scale 1:250,000, 2 sheets.

Harrison, J.E., Kleinkopf, M.D., and Obradovich, 

Tectonic events at the intersection between the Hop

the Purcell trench, northern Idaho: U.S. Geologi

Professional Paper 719, 24 p. 

Harrison, J.E., Kleinkopf, M.D., and Wells, J.D., 1980, 

ic thrusting in Protozeroic belt rocks, northwest

States: Geology, v. 8, p. 407–411. 

Hobbs, S.W., Griggs, A.B., Wallace, R.E., and Camp

1965, Geology of the Coeur d’ Alene district

County, Idaho: U.S. Geological Survey Professi

478, 139 p.

International Association of Geodesy, 1967, Geodeti

system, 1967: International Association of Geod

Publication 3, 74 p.

King, E.R., Harrison, J.E., and Griggs, A.B., 1970

implications of aeromagnetic data in the Pend O

Idaho-Montana: U.S. Geological Survey Professi

646–D, 17 p.

King, P.B., 1969, Tectonic map of North America: U.S. 

Survey, scale 1:5,000,000, 2 sheets.

Kleinkopf, M.D., 1977, Regional geophysical studies o

ern belt basin, Montana: Geological Society o

Rocky Mountain Section, Abstracts with Programs

p. 738. 

———1981, Geophysical surveys of the Cabinet Mou

derness, Lincoln and Sanders Counties, Montana, 

resources of the Cabinet Mountains Wilderness, L

Sanders Counties, Montana: U.S. Geological Surv

1501, 77 p, scale 1:62,500.

———1983, Gravity expression of the Rocky Mountain 

Geologic and structural framework and minerals 

resources of northwest Montana: Tobacco Root 

Society Field Conference, Kalispell, Monta

Abstracts, p. 13–14.

———1984, Gravity and magnetic anomalies of the 

United States and Canada: Belt Symposium, 2nd

with Programs; Montana Bureau of Mines an

Special Publication 90, p. 85–87. 

Kleinkopf, M.D., and Bankey, V.L., 1982, Gravity 

regional structures in the northern Rocky Mountain

cal Society of America Annual Meeting, 95th, Ne

Abstracts with Programs, v. 14, no. 7, p. 531. 

Kleinkopf, M.D., and Harrison, J.E., 1982, The app

aeromagnetic and gravity anomaly maps in miner

assessments of the Wallace quadrangle, North

Mountains [abs.]: Society of Exploration Ge

Annual Meeting, 52nd Dallas, Technical Program 

and Biographies, v. 48, p. 454. 

Kleinkopf, M.D., Harrison, J.E., and Bankey, V.L., 1

showing aeromagnetic and gravity anomaly m

metallic mineral resource favorability map of th

1°×2° quadrangle, Montana and Idaho: U.S. Geol

vey Open-File Report 82–709.

———1988, Aeromagnetic and gravity anomaly m

hydrothermal veins favorability map of the Wal

quadrangle, Montana and Idaho: U.S. Geological S

cellaneous Field Studies Map MF–1354–E, 

1:250,000.

Kleinkopf, M.D., Harrrison, J.E., and Zartman, R.E., 1

magnetic and geologic map of part of northwester
SUMMARY AND REFERENCES CITED 

and northern Idaho: U.S. Geological Survey Geophysical

Investigations Map GP–830, scale 1:250,000. 

Kleinkopf, M.D., and Long, C.L., 1979, Geophysical studies in the

Wallace quadrangle, northern belt basin, Montana and Idaho

[abs.]: Northwest Mining Association Annual Convention,

85th, Spokane, Washington, Program, p. 15. 

Kleinkopf, M.D., and Wilson, D.M., 1981, Aeromagnetic and

gravity studies of the Scotchman Peak Wilderness Study Area,

Lincoln and Sanders Counties, Montana, and Bonner County,

Idaho, in Mineral resources of the Scotchman Peak Wilderness

Study Area, Lincoln and Sanders Counties, Montana, and Bonner County, Idaho: U.S. Geological Survey Bulletin 1467, 26 p. 

Larsen, E.S., and Pardee, J.T., 1929, The stock of alkaline rocks

near Libby, Montana: Journal of Geology, v. 37, no. 2, p.

97–112.

Larue, D.K., 1981, The Early Proterozoic Pre-Iron Formation

Menominee Group siliclastic sediments of the southern Lake

Superior region—Evidence for sedimentation in platform and

basinal settings: Journal of Sedimentary Petrology, v. 51, no. 2,

p. 397–414.

Lawler, T.L., and Vadis, M., 1986, Mineral potential of iron-rich

volcanogenic metasediments, Glen Township, east-central

Minnesota: Annual Institute on Lake Superior Geology, 32nd,

p. 49.

Long, C.L., 1983, Maps showing results of audio-magnetotelluric

studies in the northwestern part of the Wallace 1°×2° quadrangle, Montana and Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF–1354–B, scale 1:250,000.

———1988, Interpretations of two audio-magnetotelluric profiles

across the Sylvanite anticline, Idaho and Montana: U.S. Geological Survey Open-File Report 88–658.

Mabey, D.R., Zietz, Isidore, Eaton, G.P., and Kleinkopf, M.D.,

1978, Regional magnetic patterns in part of the Cordillera in

the western United States, in Cenozoic tectonic and regional

geophysics of the western Cordillera: Geological Society of

America Memoir 152, p. 993–106.

Marvin, R.F., Zartman, R.E., Obradovich, J.D., and Harrison, J.E.,

1984, Geochronometric and lead isotope data on samples from

the Wallace 1°×2° quadrangle, Montana and Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF–1354–G,

scale 1:250,000.

Morelli, Carlo, Gantar, C., Honkasla, Tauno, McKonnel, R.K.,

Tanner, J.G., Szabo, Bela, Uotila, U.A., and Whalen, G.T.,

1974, The international gravity standardization net 1972

(I.G.S.N.): Paris, Bureau Central de l’Association Internationale de Geodesie Special Publications 4, 194 p.

Okulitch, A.V., 1984, The role of the Shuswap Metamorphic Complex in Cordilleran tectonism—A review: Canadian Journal of

Earth Sciences, v. 21, no. 10, p. 1171–1193.

Parasnis, D.S., 1968, A supplementary comment on R. Green’s

Note, “The EAEG’S Choice of Units”: Geophysical Prospecting, v. 16, p. 392–393.

Peddie, N.W., Jones, W.J., and Fabiano, E.B., 1976, Magnetic inclination in the United States—Epoch 1975.0: U.S. Geological

Survey Miscellaneous Investigations Series Map I–912, scale

1:5,000,000, 2 sheets.

Plouff, Donald, 1977, Preliminary documentation for a Fortran

program to compute gravity terrain corrections based on topography digitized on a geographic grid: U.S. Geological Survey

Open-File Report 77–535, 43 p. 

Potter. C.J., Sanford, W.E., Yoos, T.R., Prussen, E.I., Keach, 

Oliver, J.E., Kaufman, S., and Brown, L.D., 1986, COR

deep seismic reflection traverse of the interior of the 

American Cordillera, Washington and Idaho—Implicatio

orogenic evolution: Tectonics, v. 5, p. 1007–1026.

Price, R.A., 1981, The Cordilleran foreland fold and thrust 

the southern Canadian Rocky Mountains, in McClay, K.R

Price, N.J., eds., Thrust and nappe tectonics: Geo

Society of London Special Publication 90, p. 47–48.

Price, R.A., and Fermor, P.R. 1985, Structure section of the C

leran foreland thrust and fold belt west of Calgary, A

Geological Society of Canada Special Paper 84–14. 

Reynolds, M.W., and Kleinkopf, M.D., 1977, The Lewis and 

line, Montana-Idaho—A major intraplate tectonic bou

Geological Society of America Abstracts with Program

no. 7, p. 1140–1141.

Smithson, S.B., 1971, Densities of metamorphic rocks: Ge

ics, v. 36, no. 4, p. 690–694. 

Sweeney, R.E., 1990, IGRFGRID—A program for creatio

total magnetic field (international geomagnetic reference 

grid representing the Earth's main magnetic field: U.S

logical Survey Open-File Report 90–45A, 37 p.

U.S. Department of Defense, 1974, World relative gravity ref

network, pt. 2, North America: Defense Mapping A

Aerospace Center Reference Publication 25, v. 2, with s

ment updating gravity values to the International Gravity 

dardization Net (IGSN), 1,635 p.

U.S. Geological Survey, 1969a, Aeromagnetic map of the 

and Mt. Pend Oreille quadrangles, Lincoln and Sanders 

ties, Montana, and Bonner County, Idaho: U.S. Geo

Survey Geophysical Investigations Map GP–682, 

1:62,500. 

———1969b, Aeromagnetic map of the Thompson Lakes 

rangle, Lincoln, Sanders, and Flathead Counties, Mo

U.S. Geological Survey Geophysical Investigations 

GP–683, scale 1:62,500. 

———1969c, Aeromagnetic map of the McGregor Lake

Lake area, Flathead and Lincoln Counties, Montana: U.S

logical Survey Geophyscial Investigations Map GP–684

1:62,500. 

———1969d, Aeromagnetic map of the Trout Creek quadr

Sanders and Lincoln Counties, Montana, and Shoshone 

ty, Idaho: U.S. Geological Survey Geophysical Investig

Map GP–685, scale 1:62,500. 

———1969e, Aeromagnetic map of the Thompson Falls qu

gle, Lincoln and Sanders Counties, Montana: U.S. Geo

Survey Geophysical Investigations Map GP–686, 

1:62,500. 

———1969f, Aeromagnetic map of the Hubbart Reservo

Springs area, Sanders, Flathead, and Lakes Counties, Mo

U.S. Geological Survey Geophysical Investigations 

GP–687, scale 1:62,500. 

———1969g, Aeromagnetic map of the Kingston, Kellog

part of the Fernwood quadrangles, Shoshone, Benewa

Kootenai Counties, Idaho: U.S. Geological Survey Geo

cal Investigations Map GP–688, scale 1:62,500.

———1969h, Aeromagnetic map of the Avery quadrangle, 

hone County, Idaho, and Mineral and Sanders Counties

tana: U.S. Geological Survey Geophysical Investigation

GP–689, scale 1:62,500. 
22 GEOPHYSICAL INTERPRETATIONS, LIBBY THRUST BELT, MONTANA

———1969i, Aeromagnetic map of the Haughan and St Regis

quadrangles and parts of the Simmons Peak and Illinois Peak

quadrangles, Shonsone County, Idaho, and Mineral and

Sanders Counties, Montana: U.S. Geological Survey Geophysical Investigations Map GP–690, scale 1:62,500.

———1969j, Aeromagnetic map of the Plains, Perma, Superior,

and Tarkio quadrangles, Sanders, Mineral, and Missoula,

Counties, Montana: U.S. Geological Survey Geophysical

Investigations Map GP–691, scale 1:62,500. 

———1973, Aeromagnetic map of parts of the Okanogan and

Sandpoint 1°×2Υ quadrangles, Washington-Idaho-Montana:

U.S. Geological Survey Open-File Map 294, scale 1:250,000.

Webring, M.W., 1981, MINC—A gridding program based on minimum curvature: U.S. Geological Survey Open-File Report

81–1230, 12 p. 

———1982, SAKI—A Fortran program for generalized linear

inversion of gravity and magnetic profiles: U.S. Geological

Survey Open-File Report 82–122, 29 p.

Wells, A.T., Forman, D.J., Ranford, L.C., and Cook, 

Geology of the Amadeus Basin, central Australia: 

Bureau of Mineral Resources, Geology and Geoph

letin 100, 222 p.

Wells, J.D., Lindsey, D.A., and Van Loenen, R.E., 198

of the Cabinet Mountains Wilderness, Lincoln an

Counties, Montana: U.S. Geological Survey Bullet

p. 9–19. 

Wynn, J.C., Kleinkopf, M.D., and Harrison, J.E., 

audio-frequency magnetotelluric and gravity trav

the crest of the Purcell geanticline, northwestern 

Geology, v. 5, p. 309–312. 

Yoos, T.R., Potter, C.J., Thigpen, J.L., and Brown, L.D.

Cordilleran fold and thrust belt in northwest Mo

northern Idaho from COCORP and industry seismic 

data: American Association of Petroleum Geologis

v. 75, no. 6, p. 1089–1106. 

Published in the Central Region, Denver, Colorado

Manuscript approved for publication January 11, 1994

Edited by Judith Stoeser

Graphics by Dennis Welp, Nancy Shock, R. Randall Schumann 

and Gayle Dumonceaux

Photocomposition by Gayle Dumonceaux

On-line version by Joan G. Nadeau