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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1976
Structural styles of the southern boundary of the Sapphire tectonic block Anaconda-Pintlar Wilderness Area Montana
Charles Gilbert Wiswall The University of Montana
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Recommended Citation Wiswall, Charles Gilbert, "Structural styles of the southern boundary of the Sapphire tectonic block Anaconda-Pintlar Wilderness Area Montana" (1976). Graduate Student Theses, Dissertations, & Professional Papers. 7118. https://scholarworks.umt.edu/etd/7118
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SAPPHIRE TECTONIC BLOCK ANACONDA-PINTLAR
WILDERNESS AREA, MONTANA
by
Gil Wiswall
B .A ., Colgate U n iv e rs ity , 1973
Presented in partial fulfillm ent of the requirements for the degree of
Master of Science
UNIVERSITY OF MONTANA
1976
Approved by:
hairman. Board o f Examiners
Dean^G radu hool 5
Date ^
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Wiswall, Gil, M.S., Summer, 1976 Geology
S tru c tu ra l Styles o f the Southern Boundary o f the Sapphire Tectonic Block, Anaconda-Pintlar Wilderness Area, Montana (62 pp.)
Director: Donald W. Hyndman / ic o 2> i Structural analysis of a portion of the southern boundary of the Sapphire Tectonic Block shows that rocks of the upper Belt Supergroup and early- to mid-Cambrian units have experienced three distinct deformational events. Standard structural mapping and s ta tis tic a l analysis were employed in order to determine whether the concept of the Sapphire Tectonic Block could explain the observed structural relationships in the Falls Fork drainage, Anaconda-Pintlar wilderness area, Montana. The deformational events may be divided into F] and post-F] on the basis of structural style. The F-| event is represented by a large scale, westward-verging, recumbent anticline. Deformation was by plastic flow resulting in a well-developed axial plane s c h is to s ity accompanied by strong transpo sition o f bedding. Post-F] events involve progressive deformation at shallower tectonic levels. ^ 2 superimposed concentric-style mesoscopic structures on the F-) fold. Fg was coaxial with F 2 , folding the inherited fabric into a macroscopic syncline. Continued application of stress resulted in thrusting and subsequent folding of the thrust planes. A model involving an existing step in the basement surface is employed to explain the observed sequence of deformation. F] is thought to represent decollement deformation at the base of the sliding block. The F-j fold resulted from a buttress effect of the basement step. Progressive deformation raised the F-j structure to higher tectonic levels. Post-F^ structures are consistent with the characteristic structural style and eastward transport of the Sapphire Tectonic Block.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS
Special thanks go to Dr. Don Hyndman for untiring guidance
both in the field and in the preparation of the manuscript.
Drs. Bob Weidman and Jim Talbot also provided valuable assistance.
F ield work was supported in part by a G rant-in-A id of Research
from the Society of Sigma Xi.
m
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ...... i i i
LIST OF FIGURES...... vi
LIST OF PLATES ...... v il
LIST OF TABLES ...... v ii
CHAPTER
I. INTRODUCTION...... 1
Tectonic Setting ...... 1
Regional Structure ...... 3
Stratigraphy ...... 6
Metamorphism ...... 7
Igneous Rocks ...... 8
Timing of the Sapphire Tectonic Block Movement . 10
Location and Present Study ...... 12
II. STRATIGRAPHY...... 13
Wallace Formation ...... 13
Flathead Quartzite ...... 14
U n d iffe ren tiated Cambrian ...... 15
I I I . STRUCTURAL GEOLOGY ...... 18
General Statement ...... 18
F-| F a b r ic ...... 18
Planar Structures ...... 19
IV
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F o ld in g ...... 25
Wallace Formation ...... 25
Flathead Quartzite ...... 27
U n d iffe re n tia te d Cambrian ...... 30
F-| Movement P ic t u r e ...... 30
Post-F-| Deformation ...... 33
^2 F a b r ic ...... 38
F2 E v e n t ...... 40
Thrust Faults ...... 42
Post-F-j Movement P i c t u r e ...... 45
IV. DYNAMIC INTERPRETATION ...... 48
V. SUMMARY AND CONCLUSIONS...... 55
REFERENCES CITED ...... 59
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES
F ig u re Page
1 Map o f Sapphire Tectonic Block showing s tru c tu ra l zones ...... 4
2 Location map showing present area and previous related studies ...... 11
3 Diagrammatic sketch showing va ria tio n o f Dip of F-| axial planes from east to w est ...... 20
4 Comparison o f F-i fa b ric elements in the Wallace Formation ...... 23
5 Sketch of folds in strongly transposed Wallace Form ation ...... 26
6 Complete folds in zones of less intense transposition in Wallace Formation ...... 26
7 Sketch showing calcareous layers compressed into cores o f fo ld in more competent sandy layers .... 28
8 Comparison o f bedding attitu d e s in Flathead Quartzite to attitudes of schistosity ...... 28
9 Sketches showing F] folds in the Flathead Q u a r t z i t e ...... 29
10 P-T stability limits of minerals discussed in t e x t ...... 29
11 Diagram showing fie ld s o f folding ...... 32
12 Comparison o f F-| and F2 fo ld d a t a ...... 36
13 Examples o f Fg f o l d s ...... 39
14 Orientation diagram of poles to bedding in the Georgetown th ru s t p late ...... 44
15 Diagrammatic sketch showing re la tio n s h ip o f thrusted west limb to syncline ...... 44
16 Proposed model for development of structure in Falls Fork drainage ...... 52
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF PLATES
P la te Page
I Geologic Map ...... 16
II Map of generalized structural data ...... 17
I l i a Domainal analysis ...... 34
I l l b 35
LIST OF TABLES
Table Page
1 Table summarizing terms of Donath and Parker .... 32
2 Summary of structural development ...... 58
v n
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I
INTRODUCTION
Tectonic Setting
The geology of west-central Montana is dominated by three
major tectonic features: the Idaho batholith, the Dillon block, and
the Cordilleran overthrust belt. It is generally accepted that
many of the overthrust complexes in western North America are re
lated to either uplifts of Precambrian crystalline blocks
(Osterwald, 1951; Foose, 1960; Eardley, 1963; Mudge, 1970a;
Price, 1971; Ritzma, 1971) or to igneous intrusion (Langton,
1935; Scholten and Ramspott, 1968; Armstrong, 1974; Burchfiel and
Davis, 1975; Hyndman, Talbot, and Chase, 1975). A gravitative
mechanism is most often appealed to whereby horizontal transport
and thrusting of the overlying strata (suprastructure) is accom
plished by e ith e r la te ra l spreading or downs!ope g ra v ity s lid in g
induced by a gravitational potential created by a rising core
(infrastructure).
The Idaho batholith was emplaced into the thickest (up to
13 km.) accumulation of sediments in the Belt Basin (Winston,
verbal commun., 1976). As the rising infrastructure domed the
overlying strata, portions of the suprastructure were shed radially
from the northern and northeastern borders of the batholith as noted
by Langton (1935). Langton also c ite s the contemporaneous in
jection of magma along faults and the increase in structural 1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. complexity toward the batholith as evidence in support of the re
la tio n s h ip between magma emplacement and deformation.
A coherent block of the suprastructure, the Sapphire Tectonic
Block {see Fig. 1), has recently been recognized by Hyndman, Talbot,
and Chase (1975) lying to the east of the Idaho batholith. It is
proposed that this 75 km by 100 km block of Belt, Paleozoic, and
Mesozoic sediments s lid downslope a distance o f 25 km to the east.
It acted as a distinct structural unit with characteristic de
formation defining the present boundaries of the block. Along the
west side, at the eastern border of the Idaho batholith, is a 100 km
long zone of cataclasis exhibiting a penetrative shear foliation
dipping approximately 25° to the east and having a strong down-dip
lineation (Chase, 1973). Along the north side is a 75 km long zone
o f south-dipping, high-angle reverse fa u lts showing a le f t - la t e r a l
component of strike-slip movement (Mutch, 1960; Nelson and Dobell,
1961; Weidman, 1965; Desormier, 1975). In the vicinity of the
northern Flint Creek Range, these faults swing south, transforming
to low-angle, west-dipping overthrust faults, which form the eastern
border. The Philipsburg and Georgetown thrusts are the major fa u lts
of this zone (Calkins and Emmons, 1915; Poulter, 1956, 1958;
Mutch, 1960; McGill, 1965, 1959). The southern boundary has not
been satisfactorily delineated, but is currently taken to be the
sillim anite isograd in the west and the contact with granitic plutons
in the east. The structural styles within these boundaries define
a distinct structural province.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Regional Structure
Deformation within the Sapphire Tectonic Block consists of
thrusts and open folds, which are most intense in the leading edge.
While the majority of structures show north-northwest to north-
northeast trends with northerly plunges, there is a wide range of
structural styles. Calkins and Emmons (1915) defined three structural
zones on the basis of style of folding and orientation of axial
planes of folds and fault planes. Their work has been confirmed
by subsequent studies (McGill, 1959; Mutch, 1960; Csejtey, 1963;
Hughes, 1975). In the western zone, west of the Philipsburg thrust
in the north, and Georgetown th ru st in the south, fo ld in g is
characterized by broad, open concentric-style folds having wave
lengths on the order of 8-11 km. Tighter drag folds are associated
with the high-angle reverse faults of the northern boundary
(Desormier, 1975). Within the western zone, three types of faults
are present (Hughes, 1975). Presumably the oldest are north-south-
trending, west-dipping, low-angle thrust faults related to different
competence o f s tra ta during s lid in g . Most prevalent are down-to-
th e -e a s t, north-south-trending normal fa u lts re s u ltin g from d i f
ferential movements within the block. The third type are east-
west-trending tension fractures. The middle and eastern zones lie
within the toe or leading edge of the block and there, the structural
re latio n sh ip s become much more complex and less w ell understood.
East o f the Philipsburg and Georgetown thrusts and west o f the Royal
stock-Mount Powell batholith-Goat Mountain fault, the folding is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SAPPHIRE TECTONIC
Missoula BLOCK
IDAHO BATH, Î ÊOULDER t ft U41* BAlrB ■' p r e s e n t STUDY *s‘ ^ a r e a
SUDMANITE
Granitic Rocks Western Structural Zone
Middle Structural Zone a Philipsburg Thrust b Georgetown Thrust c Royal Stock Eastern Structural Zone d Mount Powell B atho lith e Goat Mountain Thrust
Fig. 1 Map of Sapphire Tectonic Block showing structural zones. {After Hyndman, Talbot, and Chase, 1975}
of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission much tig h te r . Folds in the middle stru ctu ral zone are fo r the most part
asymmetric with west-dipping axial planes and are tighly appressed.
However, in the northern-most and southern-most extents of this zone,
the folds become more symmetrical but retain a west-dipping axial plane.
Two generations of faulting are apparent. Earlier west-dipping, low-
angle thrusts are most common. The extensive thrust planes are folded
indicating a continuum of folding and thrusting. A later set of normal
faults crosscut all structures in this zone. Folds in the eastern zone
are s im ila r in s ty le , but show asymmetry opposing th a t in the middle
zone (Calkins and Emmons, 1915; McGill, 1965). Axial planes generally
dip to the east as do some thrust planes. However the line marking
east-dipping axial planes lies west of the line marking east-dipping
thrusts (McGill, 1965). Folding in the eastern-most part of the block
is less appressed and generally more symmetrical but retains east-
dipping axial planes. Faulting in the eastern zone is much more com
plex than in the previous zones. In addition, faulting also varies
from north to south. In the north, fault planes are generally steep
and show an older set of east-dipping thrusts cut by younger normal
fa u lts . In the south, th ere are a few very f l a t thrusts showing a
west-over-east sense of movement in addition to the faulting observed
in the north (Csejtey, 1963). Age relationships are uncertain.
The extreme southeastern corner of the block is exceedingly complex
showing both north-south and east-west trending normal, high-angle
reverse, and thrust faults (Calkins and Emmons, 1915; Csejtey, 1963).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stratigraphy
Significant variability in age and thickness of sedimentary
strata is represented within the Sapphire Tectonic Block. Strati
graphie relationships are further obscured by the structural com
plexity of the block. Only a general statement is made here and
the reader is referred to other papers for more detailed accounts
on the stratigraphy (Calkins and Emmons, 1915; Poulter, 1958;
McGill, 1959; Gwinn, 1960; Kaufman, 1963; Mutch, 1960; Desormier,
1975).
The oldest rocks present in the area are those of the Pre
cambrian Belt Supergroup. No crystalline basement has been recog
nized. The Belt rocks are brought to the surface along the numerous
thrust sheets. The Belt section in the thrust plates attains thick
nesses of up to 10,000 ft. (Poulter, 1956), however, exposures
beneath the thrusts to the southeast range in thickness from 300 ft.
(Poulter, 1956) to totally absent (Winston, verbal commun., 1976)
in d icatin g pronounced thinning to the southeast.
Paleozoic and Mesozoic sediments outcrop mainly in the middle
and eastern structural zones. Ordovican, Silurian, and Triassic
s tra ta are everywhere absent. The combined thickness o f these
sedimentary units is approximately 10,000 to 20,000 f t (Calkins and
Emmons, 1915; Poulter, 1956; McGill, 1959; Mutch, 1960). The ex
posure of various portions of the entire section is structurally
controlled by thrusting and folding.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Metamorphism
Metamorphism w ith in the Sapphire Tectonic Block ranges from
lower greenschist facies to the muscovite-sil1imanite zone of the
am phibolite facies w ith overprinted contact metamorphism occurring
adjacent to most Intruslves (Mutch, 1960; Csejtey, 1963; Stuart,
1966; Presley, 1970; Flood, 1974). The western boundary of the block
shows an Increase In metamorphic grade toward the Idaho batholith
from lower greenschist facies In the east to the muscovlte-sllll-
manlte zone of the amphibolite facies In the west (Presley, 1970;
LaTour, 1974). Likewise, deeper levels of the suprastructure are
exposed from east to west (Missoula Group to Ravalli Formation,
re sp ec tively ) suggesting a re latio n sh ip between the emplacement of
the b a th o lith and metamorphism. Further, the presence o f a meta
morphic foliation (Hall, 1968; Presley, 1970; LaTour, 1974) Indicates
the coincidence of deformation and metamorphism In time.
Metamorphism In the eastern portion o f the block Is less well
known. Most workers In the area In fe r only contact metamorphism
associated with Igneous Intruslves. Workers In other parts of western
Montana have shown th a t the upper-Belt sediments have undergone
b u ria l metamorphism to the lower greenschist facies (Maxwell and
Hower, 1967; Esllnger and Savin, 1973), and the lower Belt to the
lower amphibolite facies (Norwick, 1972). Thus, It seems reasonable
to assume that the lower units of the suprastructure, having been
deposited In the deeper regions o f the B e lt embayment, should have
been metamorphosed to a t le a s t lower am phibolite fa c ie s . Further,
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Stuart (1966) has presented evidence for a region in the central
F lin t Creek Range which indicates a regional metamorphism o f the low
pressure or Abukuma type associated with isoclinal folding in the
eastern structural zone. Stuart has recognized a foliation parallel
to the axial planes of major folds. Equilibrium assemblages show
an increase from west to east in metamorphic grade from the greenschist
facies to the cordierite-amphibolite facies, which is coincident
with a west to east tightening of folds within the study area. From
th is , S tu art proposes a regional metamorphism accompanying deform ation,
deriving pressure from lithostatic and folding stress and temperature
from magma sources beneath the area.
Farther to the south in the Anaconda Range, Flood (1974) mapped
a nappe structure composed of rocks in the amphibolite facies. The * structural style and position in the stratigraphie column suggest
that this structure formed at deeper levels in the block. Flood
suggested that exposure is due to northward tiltin g of the block.
Metamorphism in both S tu a rt's and Flood's areas is compatible w ith
the gravity sliding hypothesis and it is suggested that similar re
lationships w ill be seen upon further study of other areas.
Igneous Rocks
The main phase of igneous activity in the area is represented by
numerous d io ritic to granitic epizonal plutons and stocks with
associated dikes. These rocks occur between the Idaho batholith on
the west, and the Boulder b a th o lith on the east. Because the Idaho
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. batholith, Boulder batholith, and the plutons within the Sapphire
Tectonic Block are closely spatially related, show similar petro
logic and compositional trends, and have similar radiometric ages
(Hawley, 1975; Hughes, 1975; Hyndman and others, 1975; Hyndman, Talbot
and Chase, 1975), it is suggested that they are related to the same
magma source. In addition, structural evidence shows that magmatic
emplacement occurred during or just after the deformational events
of the Sapphire Tectonic Block. The position of emplacement is
confined largely to the toe of the block (Hyndman, Talbot and Chase,
1975), and is everywhere structurally controlled (Csejtey, 1963;
Hawley, 1975; Hughes, 1975).
The plutons in the western portion of the block show eastward
spreading along low-angle thrust faults and in the cores of anti
clines (Hughes, 1975) suggesting the Idaho batholith as the source.
They g en erally do not show fo lia tio n (P resley, 1970; Hughes, 1975),
in d ica tin g emplacement a fte r tectonic movements.
Plutons along the southern boundary and in the toe (Flint Creek
Range) o f the block are o f two types. The most common are synoro-
genic plutons o f d io r ite to two-mica g ran ite composition emplaced
most commonly along thrust faults. These show well developed
foliations and evidence of shearing due to movements along the
thrust planes (Flood, 1974; Hawley, 1975). The other group, primarily
p o rp h y ritic g ra n ite s , have been emplaced p ost-o ro g en ically. These
also intrude along zones of structural weakness, often coinciding
with the synorogenic plutons (Csejtey, 1963; Hawley, 1975; Hyndman
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10
and others, 1975). The close proximity and the sim ilarity in com
positional trends of these plutons to the Boulder batholith suggest
that they are related to it.
Timing of the Sapphire Tectonic Block Movement
The beginning o f topographic r e l i e f re la te d to the emplacement
of the Idaho batholith is marked by the Beaverhead conglomerate of
mid-Cretaceous age. As the infrastructure rose to higher levels in
the crust, it began to spread eastward into the thick sedimentary
pile of the Belt embayment (Chase, 1973). This caused severe
doming of the suprastructure and mechanical weakening and meta
morphism of the infrastructure/suprastructure transition. At some
point, the suprastructure attained a gravitational potential which
exceeded the strength of the rock, and detachment occurred. As
s lid in g began, fo ld in g was succeeded by th ru s tin g , and then by
further folding. Deformation appears to have been continuous pro
ducing various structural styles depending mainly on position within
the block.
The Boulder b a th o lith and re la te d plutons were emplaced coin
cident with and continuing after deformation. Magma from the main
source for the Idaho batholith was channeled eastward along the base
of the sliding block and emplaced either within the block along
zones o f stru c tu ra l weakness or in fro n t o f the block as the Boulder
batholith. K-Ar radiometric dates for the Boulder batholith, Flint
Creek plutons, and Idaho batholith fall between 70 and 80 m.y.B.P.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. n
113 O O
mond
G ib b o n s Pots 1 N f — 4 6 O O
/ P R E S E N T ■ * STUDY
113 O O
Fig. 2 Location map showing study area in relation to pre vious related studies.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12
Because o f the close association in time o f magmatism and deformation,
the Sapphire Tectonic Block must have moved in a relatively short
period of time during the late-Cretaceous just prior to the em
placement of the Boulder batholith (Hyndman, Talbot, and Chase, 1975).
Location and Present Study
The area studied lies in the Falls Fork drainage in the Anaconda-
Pintlar wilderness area. The area covers the western part of the
Warren Peak and eastern part o f the Gibbons Pass 1 NE 7^-minute
quadrangles (see Fig. 2). The area extends south of the continental
divide for approximately 2 km. Otherwise, it is defined by the
lim its o f the basin. Access is provided by the Middle Fork o f Rock
Creek Road reached v ia Montana Highway 38. A USFS t r a i l , which
begins about 8 km from Moose Lake, leads into the area.
Field work was conducted during the summer of 1975. The area
was mapped and structural data such as bedding, schistosity, axial
planes, fold axes, bedding/cleavage lineations, and slickensides
were recorded. S ix ty rock samples were collected fo r th in section
and fabric studies. Photographs and sketches of important relation
ships were made for later reference.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I
STRATIGRAPHY
The rocks in the Falls Fork drainage consist of upper Belt
Supergroup to middle Cambrian units. Directly to the east in the
area mapped by Flood (1974), lower Belt rocks occur. Flood reports
a continuous section from the Prichard through Wallace Formations.
In the present area, the Wallace Formation, Flathead Quartzite,
and an u n d iffe re n tia te d section o f middle Cambrian rocks are present.
The Hasmark Dolomite is the youngest stratigraphie unit observed.
The Missoula Group is notably absent in the present study area
(Winston, verbal commun., 1976). In the area mapped by Poulter to
the north, the Missoula Group thins to the south. Coincident with
this trend is a rapid thickening of the Flathead Quartzite. The
stratigraphie significance of this relationship is not understood.
The area is cut by numerous dikes of granodiorite. These dikes
fo llow zones o f s tru c tu ra l weakness and are so abundant, no
attempt was made to map them. A brief description of each lithology
follow s.
Wallace Formation
The Wallace Formation is best exposed on the continental divide
around the southern and eastern perimeter of the area. It consists
of a thick, monotonous sequence of rusty-weathering impure limestone
with interbedded argillite, siltite, and quartzite. The color on
13
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14
fresh surfaces ranges from light to dark gray in the calcareous
la y e rs , and from tan to lig h t green in the argillaceous beds.
Interbeds range in thickness from 1 cm to 1 meter. The rock is
generally thinly laminated. Laminations are discontinuous due to
tight isoclinal folding and bedding transposition. The quartz-
rich layers are commonly better preserved. Although generally fine
grained, coarse radiating aggregates of actinolite are locally seen.
The lower contact is not present in the area, but has been described
by Flood (1974).
Flathead Quartzite
The best exposure of the Flathead Quartzite is on the ridge west
of Edith Lake. The lower contact with the Wallace is exposed in the
valley bottom NNW of East Pintlar Peak. This contact is stratigraphie
(presumably representing a disconformity) and grades over 5-10 meters.
The Wallace becomes less calcareous with an increasing percentage of
s ilt and sand. Within the contact zone, green micaceous, finely-
laminated argillaceous quartzite is interbedded with brown to rusty-
weathering Wallace-like beds. Above the contact, bedding becomes
slightly more massive and consists of green feldspathic quartzite.
Two pebble conglomerates are present. The lower occurs within 8
meters of the contact. It consists of red and white stretched
pebbles flo a tin g in a sandy m atrix. The pebbles are o f f a ir ly even
size, ranging from 1-4 cm. in diameter. The upper conglomerate also
shows a bimodal grain size distribution. The upper conglomerate
divides the Flathead into two distinct units. Below this conglomerate,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15
a rg illite interbeds are common, beds are on the order of several tens
of centimeters thick, and light and dark green, pink, and white
colors predominate. Above the conglomerate, the rock is more massively
bedded (up to 2 meters), white, vitreous, lacks argillite interbeds,
and the grain size is coarser. Sedimentary structures are well pre
served throughout the section, but they are more commonly visible
in the upper member. These structures include cross-beds, ripple
marks, cut and fill and horizontal laminations.
Undifferentiated Cambrian
The Silver H ill Formation and Hasmark Dolomite were identified,
but were not divided because of poor exposure and lack of visible
stru c tu ra l featu res. The S ilv e r H ill was exposed in only three
locations. It consists of interbedded argillaceous limestone and
pure limestone. The rock is banded gray and tan with the tan ar
gillaceous layers standing in relief on weathered surfaces. The
Hasmark Dolomite is cream-colored, sugary-textured, and massive.
It is homogeneous in appearance and thus, of little aid in determining
structural relationships. The lower contact with the Flathead was
not seen. In a few locations, bedding was determined to be con
cordant with nearby underlying Flathead. Therefore, the contact
is thought to be conformable and rather sharp. The upper contact
is not present in the area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. G eorgetow n Thrust
ton Pinikf
Ph^llii Lo^s
•d Cob*n Iw #
PLATE I GEOLOGIC MAP
of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission 16
LEGEND
Undiff. Cambrian
Flathead Quartzite
Wallace Formation
Thrust Fault
O th e r Foults
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17
P»nH«
PhjrllM
^ Bedding Orienlation
4 7 ^ Overturned Bedding
3 4 ^ $chistotily Orienlolion
C o b « n
® km '
PLATE II Map of generalized structural data.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I I
STRUCTURAL GEOLOGY
General Statement
Deformation in the Falls Fork drainage is characterized by
three deformational events. Post-F^ events are distinguished on
the basis of significantly different style. F^ deformation re
sulted in a very large-scale, westward-verging recumbent fold
mapped by Flood (1974) just to the east of the present area. This
fold is characterized by a well developed axial plane schistosity
which, to the east in the area mapped by Flood, is nearly horizontal.
The F^ structure becomes deformed by post-Fj deformation in the
present study area. Post-F^ is characterized by nearly symmetrical
folds which are concentric and overturned to the east. These folds
fall into the middle structural zone as previously described.
Thrusting in the area is developed on all scales from small bedding-
plane thrusts o f lim ite d e x te n t, to the Georgetown th ru st which marks
the western boundary of the area and extends approximately 65 km
to the north.
Fj Fabric
F-| deformation in the Falls Fork drainage is visible only on
the microscopic and mesoscopic scales. Since it is macroscopically
well exposed directly to the east in the area mapped by Flood (1974),
18
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structural elements are traced from the east into the present study
area. Minor structures must then be relied on in order to determine
the present structural relationships of F-j.
Geometrically, as reported by Flood, the major F-j structure is
a large recumbent anticline with a nearly horizontal axis. The
stru c tu re is w ell exposed in the upper Fishtrap Creek drainage. I t
may be observed from a distance on the south flank of Electric Peak.
Here it can be seen that the lower limb of the fold dips more
steeply than the upper limb, and the rocks become more tightly
appressed in the core. From Electric Peak, the axial surface can
be traced west for at least 5 km along the eastern flank of the con
tinental divide. The orientation of the fold varies regularly in
the upper Fishtrap Creek drainage. From northeast to southwest,
the axis trends from SE to SSW, plunging from 10® to 40®,
respectively. The axial plane dips from horizontal in the east to
45® W at the continental divide.
In the Falls Fork drainage, the axial plane progressively
steepens in dip to nearly vertical on the western edge of the area.
This is betrayed by the penetrative axial plane schistosity in the
Wallace Formation. The plunge o f minor F-| fo ld axes changes from
SE to NNE at 40° to 50®, however, this is probably due to refolding
(see Fig. 3).
Planar Structures
The lith o lo g ie layerin g (Sq) of the Wallace Formation has be
haved passively in response to F-j deformation. Bedding is recognized
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■o CD
(/)C/)
8
CD ■ D O O.C g. o Z5 ■o o APPROX SCALE
&
IT Fig. 3 Diagrammatic sketch showing variation O S- in dip of F] axial plane from east to west. "8
(/>CO o' 3 ro o 21
on the basis o f compositional v a ria tio n between sandy or s ilt y and
calcareous layers. No other sedimentary structures are preserved.
Transposition of bedding has generally been so complete that Sg
surfaces are not traceable fo r any distance. However, the degree
of transposition does vary, presumably due to position within the
major structure. That the quartz-rich layers are more competent
than the calcareous beds is indicated by the fact that these layers
are not as deformed. Similar relationships are seen in thin section.
Discontinuous lenses of elongate recrystallized quartz grains float
in a calcite matrix. The matrix shows very few preserved folds,
most o f the structures having been dismembered by shear. On the
other hand, the qurtz-rich layers occur as tightly appressed fold
hinges and attenuated limbs. The preservation of structures by
the q u a rtz -ric h layers indicates the greater competence under stress
of these beds relative to the calcareous beds. This relationship is
obvious on all scales.
In contrast to the Wallace, bedding is well preserved in the
Flathead Quartzite. Transposition has not developed on any scale,
again reflecting the more competent nature of the quartzite. Since
bedding is more than a few centimeters thick, it is rarely visible
in thin section. Where visible, Sg appears as variation in grain
size or composition.
The response of the undifferentiated Cambrian units to
deformation is similar to that of the Wallace Formation. This cannot
be observed in outcrop due to the homogeneity of the rock and poor
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exposure. In a few locations, bedding orientation was determined to
be concordant with the underlying Flathead. In thin section, the
rock proves to be heavily transposed. In most cases, bedding is
recognized by discontinuous sapropelic layers. These layers are
sheared and tightly folded as in the Wallace. It is clear that the
u n d iffe re n tia te d Cambrian units have been affe cte d by F-|, and that
deformation was sufficiently intense to reorient bedding parallel to
S ]. This had the e ffe c t o f homogenizing the rock on the mesoscopic
scale because of its nearly monomineralic composition.
The most obvious planar feature resulting from F-| deformation
is the axial plane schistosity (S-|). is mesoscopically well
developed only in the Wallace Formation. However, S-| planes are
locally present in the Flathead Quartzite near the Wallace contact.
Here S-| consists of fine laminations of parallel mica flakes giving
the rock a sheared appearance. This is evidently due to the greater
percentage of argillaceous material low in the formation near the
Wallace, and the high ductility contrast which must have existed
during F] deformation. In the Wallace, appears as a prominent
parting direction. Tightly appressed, rootless fold hinges and
attenuated limbs are commonly terminated by S-| planes. This relation
ship indicates the origin of S-|. During progressive deformation,
Sq surfaces were sim ilarly folded. These folds became isoclinal
and the limbs began to attenuate. Slip replaced flow along discrete
surfaces parallel to the F| axial planes. Figure 4 shows the re
sulting fabric. The bedding, schistosity, and F] axial planes in the
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o
S
Poles to Bedding Poles to Schistosity 2 , 4, 6 , 8 , 10% per 1% area 5, 10, 15. 20% per 1% area
N
3 0
Poles to F| Axial Planes 5, 10, 15, zO% per 1% area
Fig. 4 Comparison of F, fabric elements in the Wallace Formation.
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Wallace are essentially parallel, indicating strong transposition.
This relationship is not present in the Flathead. is generally
at a high angle to bedding, but no consistent pattern could be de
termined. Where outcrop permitted, S-j surfaces in the Flathead were
determined to be roughly parallel to those in the Wallace.
$ 1 is penetrative in all units at the microscopic scale. In the
Wallace, S] surfaces are defined by parallel flakes of biotite and/or
muscovite and elongate grains of quartz and calcite. The latter show
strong preferred orientation of crystal lattices. In the Flathead
and undifferentiated Cambrian, S] in thin section is more obvious
than in outcrop. S-j in the Flathead is defined by parallel arrange
ment of muscovite flakes, elongate quartz and feldspar grains, and
a moderate preferred lattice orientation of quartz. Indications of
flow such as cataclasis (including rotation) and recrystallization
(Donath and Parker, 1964) are common. These features include slip
surfaces cu ttin g across grain boundaries, ro ta tio n o f rounded g rain s,
and polygonal and serrate grain boundaries. Development of these
features is confined to certain zones. In the undifferentiated
Cambrian u n its , elongate c a lc ite and dolom ite grains define S-|.
Again, flow is indicated by crystal glide twins and recrystallization.
The only strain indicator observed in the area is the lower
pebble conglomerate in the Flathead. In close proximity to the
Wallace contact, S] is well developed in the conglomerate. The
pebbles have been stretched parallel to S^. Most pebbles are
elongate in the plane of S] and circular in sections perpendicular
to S]. This suggests th a t they may have been n early equidimensional
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at the time of deposition. If the assumption of original equi
dimensional ity is made, an upper lim it to the strain ratio for
can be set. The maximum ratio of the major axis to the minor axis
seen in appropriately oriented pebbles is approximately 5. The
assumption of original sphericity is obviously tenuous, and the
cited ratio is in no way intended to be a quantitative measurement.
It is cited only as a qualitative maximum.
Folding
F-j deformation has produced a variety of folds. The size and
kind of fold can be related to thickness of the layers, ductility
contrasts between layers, or position within the major structure.
Folds observed in each formation w ill be discussed in terms of these
parameters.
Wallace Formation. F^ folds in the Wallace are everywhere of
similar style and approximately equal in size. Whether folds are
continuous or dismembered is seen to depend on the degree o f trans
p ositio n (compare Figs. 5 and 6 ) . Because in s im ila r s ty le folding
the limbs are attenuated and the hinge thickened, the degree of
transposition depends on position within the fold. Where trans
position is most complete, folding is isoclinal and complete folds
are rare (see Fig. 5). Where transposition is less intense, a
variety of folds are observed. These are pictured in Figure 6 .
Although a regional pattern could not be absolutely determined, the
degree of transposition within the Wallace tends to decrease from
east to west. This is consistent with an extension of the upper
Fishtrap Creek structure into the present area.
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SO cm
Fig. 5 Sketch of folds in strongly transposed Wallace Formation.
X 1 J I cm I cm 1 j t cm
< ^ I m
Fig. 6 Complete folds in zones of less intense transposition in Wallace Formation.
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The effect of ductility contrast is graphically illustrated by
the W allace. S ilt y and sandy layers are consistently less deformed
than intervening calcareous layers. In areas of intense trans
position, fold hinges in sandy layers are less tightly appressed.
In areas where complete folds are preserved, calcareous layers are
compressed in to the cores o f folds in sandy units (see Fig. 7 ). This
re la tio n s h ip is seen repeatedly on a ll scales.
Flathead Quartzite. The difference in response between the
Wallace and Flathead illustrates on the macroscopic scale the re
lationship just mentioned. Figure 8 is a comparison o f the o rien
tation of bedding in the Flathead to S-j. In contrast to the Wallace,
bedding is not parallel to . This reflects a difference in ductility
on the regional scale. Extrapolation from mesocopic relationships
would suggest buckling of the Flathead, thereby compressing the in
competent Wallace metasediments into the core of the F-| structure.
This relationship is reported by Flood for rocks of similar
mechanical properties.
Mesoscopic folds in the Flathead are not commonly developed due
to its relatively homogeneous nature. Thickness of bedding has been
a major controlling factor during F-| deformation. Where bedding is
massive, folding is confined to certain layers. Recumbent isoclinal
folding of similar style is characteristic. Parasitic folds are also
common (see Fig. 9a,b). Where bedding is thinner, folding is be
tween layers. These structures are almost exclusively similar-style
drag-type folds showing an east over west sense of movement (see
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&
n
1C M
Fig. 7 Sketch showing calcareous layers compressed in to core of fold in more competent sandy la y e r.
3#
Poles to bedding in Flathead Poles to Si 3, 6 , 9, 12% per 1% area 5 . 1 0 , 15, 2 0 % per 1 % area
Fig. 8 Comparison of bedding attitudes in Flathead Quartzite to attitudes of schistosity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29
L 1 i 1 m
Fig. 9 Sketches showing F-j folds in Flathead Quartzite.
o CL^ r,
200 4 0 0 600 800
TEMP O C Fig. 10 P-T stability limites of minerals discussed in text (from Hyndman, 1972).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30
Fig. 9 ,c ). deformation is most obvious where argillaceous zones
are boudinaged, ptygmatically folded, and thinned parallel to S|.
The arg illite shows similar disharmonie folding. In all cases,
ductile flow is the predominant mechanism of deformation.
Undifferentiated Cambrian. No folds in these rocks could be
definitely attributed to F-j deformation. That these units are de
formed by F-| is clearly seen in thin section, as discussed above.
Movement P ictu re
The foregoing discussion, with some additional evidence, allows
the construction of a movement picture for F-| deformation. It is
stressed here that the movement picture as outlined by Sander in 1930
(Turner and Weiss, 1967) can only describe relative movements be
tween domains o f homogeneous deform ation. I t does not specify the
exact path a body has followed to its present form. The specifi
cation of path is necessarily speculative and must be a best fit of
the a v a ila b le data to a fe a s ib le model. This "dynamic in te rp re
tation" is discussed in a later chapter.
In the construction of a movement picture, it is important to
have an understanding of the external physical conditions present
during deformation. The mineral assemblages resulting from meta
morphism indicate a rather narrow range of conditions of temperature
and pressure. Metamorphism in the area is syntectonic with F^ as
illustrated by the strong parallel arrangement of elongate and platy
metamorphic minerals and deformation of prophyroblasts. Textural
evidence in d icates th a t metamorphism proceeded beyond deform ation.
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as some radiating aggregates and prismatic grains grow across the
schistosity. Flood (1974) reported the rocks in the upper Fishtrap
Creek drainage to be in the amphibolite facies of regional meta
morphism. Based on the assemblages m uscovite-biotite-quartz in
pelitic layers and tremolite-diopside-calcite-quartz in calc-silicate
units. Flood concluded that pressures of 2-4 kb and temperatures of
550® to 650® c were present during deformation (see Fig, 10).
Similar conditions are indicated in the present study area. Muscovite-
biotite-quartz is stable in the Flathead Quartzite. In the Wallace,
actionolite/tremolite-diopside-calcite-quartz is stable in carbonate-
rich layers; diopside-actinolite/tremolite-biotite-quartz in pelitic
layers. Therefore, it is clear that during F-j, metamorphism progressed
to the amphibolite facies at least up to the stratigraphie level of
the Flathead.
The observed style of F] deformation is consistent with mechanisms
envisioned by Donath and Parker (1964) to operate at these projected
conditions. Terms used in th is discussion are taken from Donath and
Parker and are summarized in Table 1. Figure 11 illu s tra te s d iffe re n t
fields of folding based on mean ductility and ductility contrast of
the deformed strata. Any single lithology is assumed to have a low
internal ductility contrast. The reader is referred to the original
paper for full treatment.
The carbonate-rich units (Wallace and u n d iff. Cambrian u n its )
have deformed in the passive field (see Fig. 11), indicating a high
to very high mean ductility. This is based on the observation that
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Table 1. Table summarizing terms of Donath and Parker, 1964
Class Type Predominant Mechanism Flexural Flexural Slip Slip between flexed layers Flexural Flow Flow within flexed layers
Passive Passive Flow Flow across layer boundaries Passive Slip S lip across layer boundaries
Quasi- Irreg. flow within and across Flexural 1 ayers
QUASI-FLEXURAL
VERY H IG H
> - h- PASSIVE _I I— u z> o H IG H z < LU FLEXURAL FLOW
MODERATE
FLEXURAL SLIP LOW
HIGH m o d e r a t e LOW
DUCTILITY CONTRAST
Fig. 11 Diagram showing fie ld s o f folding with respect to mean ductility and ductility contrast (from Donath and Parker, 1964).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33
the deformational mechanism varies from passive slip to passive flow.
On the other hand, the quartzite attained only a moderate mean
ductility because the observed mechanism is flexural flow. On the
regional scale, a high ductility contrast existed between the units
and the strata reached a high mean ductility as evidenced by the
quasi-flexural nature of the macroscopic structure. Therefore,
original mechanical anisotropy was the major controlling factor
during F-j deformation. The quartzite formed similar buckle folds,
controlling movement of the Wallace in the core.
The direction of movement has been from east to west. This is
apparent from data from both Flood's work and the present study.
The o rie n ta tio n o f F-| stru c tu res , re la tiv e movements between zones
of homogeneous deformation, and the gross geometry require an
eastern provenance for the F-j structure. This is clearly the reverse
of what is to be expected from the model outlined in the first
chapter. Speculation about the origin of the stress responsible for
the F-| structure is presented in the following chapter.
Post-F-j deformation
Deformation subsequent to F^ has clearly occurred in the area.
Minor structures fold the metamorphic schistosity (S-j) in nearly
every outcrop. The orientations of F-| axial planes and fold axes
are somewhat different from those of the F£ structures as shown in
Figure 12. And, two distinct styles of mesoscopic folding are present.
Comparison of the orientations of all fabric elements indicates
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llA
\ y P m t|« r = Phyl 1
PLATE I l i a A DOMAINAL ANALYSIS
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 0 1 ♦♦♦ ) 0 0 b0/ * * \ / *• \ / • * \ \ V / \ • • y 1 011 0IIA 111 IllA DOMAINS
Plate Illb. Poles to Fg axial planes, F2 fold axes, • schistosity, and bedding for each domain are shown. Poles to bedding in Wallace Fm. and Flathead Quartzite are differentiated. Lower hemispheres; north at top.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35
«/>
111
SYNOPTIC IIIB IVA PICT DOMAINS
Contour Inlcrvoti Sym bols for Poles to Bedding
• W allet Fm. A x io l Planes — 4, B, 1 2 .1 6 ^ * F lot head Q u orti'ile Fj Fold A xes-6.12,18. 24% pe t 1% oreo
S c h is to s ity - 5,10, 15. .20 %
Bedding—.8, 2.4,4.0. 5 6 %
Plate Illb (cont.)
/
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N
.o
12
Fi Fold Axes Poles to Fn Axial Planes 8 , 16% per 1 % area 5, 10, 15, Z0% per 1% area
N N
1 7 35
^2 Fold Axes Poles to Fo Axial Planes 6 , 12, 18, 24, 30% per 1% area 4 , 8 , 12, T6 % per 1% area
Fig. 12 Comparison o f F-| and F2 fold data
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folding about a roughly similar axis. This common axis corresponds
to that of the Falls Fork syncline (see Plate I). At first sight
the minor structures appear to be related to the regional syncline
which deforms the F-j structure. The apparent curvature of F2
axial planes could be attributed either to continued warping about
the same axis or to a later folding event. Closer examination shows
neither to be the case.
Plate III shows domains of constant orientation within the
macroscopic fold. It is apparent that the larger-scale fold controls
the orientations of the other fabric elements. In particular,
orientation of the F£ axial planes reflects the major structure.
If the minor structures are related to the large-scale fold, they
should show a constant relationship to it. Perhaps the most mis
leading aspect of the analysis is the similarity in orientation of
the axis of F 2 and the regional fold axis. This similarity suggests
contemporaneous development. However, interpretation of the dis
tribution of axes in Figure 12, c as a tight small circle distribu
tion would indicate deformation of F 2 axes about a similarly-oriented
regional axis. Given the control of fabric orientation by the
regional structure, and the demonstration of folding of the Fg axial
planes and fold axes about the regional axis (Fig. 12 c,d), it is
clear that the Falls Fork syncline is the youngest structure and
deforms the inherited fabric.
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^ 2 Fabric
As im plied above, the Fg event produced no apparent macroscopic
structure. Only small scale structures resulted. The style of de
formation is distinct from F-|. Few penetrative planar surfaces
developed during Fg movements, although lo calized zones o f crenu-
lation cleavage are observed in tight F£ folds in the Wallace Forma
tion and where S] surfaces had developed in the Flathead Quartzite.
The superimposed fabric element most useful in defining Fg is axial
surfaces ($ 2 ). Folding is predominantly by buckling.
As is the case with F^, the Wallace Formation provides the most
graphic and varied examples of F 2 structures in the area. The
Flathead and undifferentiated Cambrian units show a paucity of small-
scale structures compared with the Wallace. Various types of Fg
folds are shown in Figure 13. Recognition in outcrop is based on
folding of the metamorphic schistosity and, to a lesser degree, style.
Identification of Fg structures is facilitated by the lack of minor
structures related to F^ (Falls Fork syncline). The scale of folding
varies from several centimeters to about 15 meters. The most common
type of folds are shown in Figure 13, a-c. They are characteris
tically concentric, asymmetric drag-type folds. With the exception
of crenulations, this is the only type of Fg folds observed in the
Flathead. The trend of asymmetry is NE-SW with a west over east
sense. Other types of folds include open concentric folds, kink
folds, and en echelon folds (see Fig. 13). Fg axial planes tend to
be oriented from 5° to 60® to S-j* As noted above, Fg axes are of
relatively constant orientation.
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L 4 I j I cm I cm a. b. c.
1 L i L J I cm L J 5 m d. e . f.
lO m
9 h.
Fig. 13 Examples o f Fg fo ld s . A ll folds pictured from Wallace Formation except (e) which is from Flathead.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40
Evidence of Fg on the microscopic scale is well developed in
the Wallace Formation only. The folded schistosity seen in hand-
specimen is represented in thin section by bent grains of mica with
wavy extinction. S-j surfaces are continuous, reflecting the lack of
a penetrative schistosity or transposition associated with Fg.
Similar relationships are only locally seen in the Flathead or un
differentiated Cambrian units suggesting that the scale of deforma
tion in these rocks is larger than microscopic. Crenulations are
locally present in the Wallace and Flathead Formations. Muscovite
grains have recrystallized with good preferred orientation of
crystal lattices on alternating limbs of the kinks. Angular relation
ships between S] and $ 2 as seen in th in section are sim ilar to those
observed in outcrop.
F3 Event
Fg deformation is characterized by large-scale folding with no
associated minor structures. The structure in the study area is
dominated by a slightly overturned syncline with a subsidiary anti
cline developed on the eastern limb. This feature is concordant
with structures to the north mapped by Poulter (1956). The hinge
is w ell exposed in the v a lle y bottom approximately midway between
Johnson and P h y llis Lakes and can be traced fo r a t le a s t 1.5 km SSE
over East Pintlar Peak. The fold axis trends approximately N20 W
and plunges 30® to the north. The a x ia l surface strikes about
NlOW and dips 55® to 65® to the west.
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Because deformation produced only macroscopic structures,
it must be described in terms of preferred orientations of the
inherited fabric. These relationships are most readily observed
in the various domains discussed above (see Plate I I I ) . Because $ 2
(axial planes) is the youngest planar fabric element, it will bear
the simplest relationship to F^. For this reason, the domain
boundaries are based on the o rie n ta tio n o f $ 2. Their distribution
describes the gross structure of Fg. Local complexities in Fg
fo ld in g appear as subdomains defined by other fa b ric elements
(compare Plates I and III). These variations can be related to
either later thrusting or control exerted by earlier geometries as
discussed by Ramsay (1967, pg. 538-546).
The domains are approximately equal in size and have a consis
tent north-south trend. The poles to Sq» S-j and S 2 w ith in each
domain d efin e d iffu s e maxima. These maxima change o rie n ta tio n in a
fairly regular manner across the structure, defining great circles
about the regional fold axis (the apparent exception seen in Domain V
is the result of overturning to the east). This indicates cylindrical
folding. The uniform spread of poles to Sg (see Plate III) along
the great circle suggests the fold is approximately symmetrical
(Turner and Weiss, 1963 pg. 159). Regardless of whether the d istri
bution of F2 fold axes is interpreted as a maximum or small circle,
F3 is approximately coaxial with Fg.
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Thrust Faults
Thrust faulting in the area is well developed on various scales
from small bedding-plane thrusts to very large-scale thrusts of
regional extent. The largest thrust fault present is tentatively
correlated with the Georgetown thrust which marks the western
boundary of the area. This fault may be a subsidiary fault to the
main Georgetown th ru s t which could l i e fu rth e r to the west. The
Georgetown thrust has been mapped by several workers to the north
for over 65 km (cf. Calkins and Emmons, 1915; Poulter, 1956).
Poulter mapped rocks of the Wallace and Missoula Groups thrust upon
the upper Cambrian section. He estimated some 22,500 ft. of strati
graphie throw and 4 miles of horizontal displacement. Tracing the
Georgetown th ru st south in to the present area, a marked decrease in
both stratigraphie throw and horizontal displacement is indicated.
The thrust plane continues to cut upsection so that the Flathead
Quartzite is the overriding unit in the Falls Fork drainage. It
rests on the Falls Fork syncline and thus, superimposes younger rocks
on older (Flathead on Wallace) in the southern part of the area. In
addition, sim ilarity in lithology and thickness between the Flathead
of the syncline and the thrust plate suggest horizontal displacement
has been small. The thrust plane is folded as shown in the south
west corner of Plate I and Figure 14. The fold axis determined from
Figure 14 has an anomalously steep plunge. Reconnaissance to the
south shows a large mass of granite which may have steepened the
plunge during intrusion. However, the relative age of intrusion could
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not be determined. The generalized attitude of the thrust plane is
N25E with a variable westerly dip.
Small scale thrust faults bear similar relationships to the
geology. O rien tation s o f two small th ru s t planes are N10E/25°W and
N25E/30°W. The thrust in the valley bottom (Plate I) cuts the nose
of the Falls Fork syncline.
At firs t sight, thrusting appears to have pre-dated Fg folding
because the thrust cannot be seen to cut both limbs of the syncline
and both the thrust and Fg structure are synclinal in form. There
fore, the Fg folding appears to post-date thrusting, and either in
volved the thrust plane or resulted from thrusting. The former is
not likely based on small-scale thrust relationships. In the hinge-
zone thrust mentioned above, a portion of the west limb is dragged
over the nose of the syncline in a small-scale thrust (see Fig. 15).
This indicates that at least some thrusting was either contem
poraneous with or post-dated some folding. In addition, folding
of the thrust plate is not as tight as that of the syncline. This
is based on an exposure near Sawed Cabin Lake where a small part
of the fold in the thrust plate may be observed and on the range of
attitudes on Figure 14 and Plate II. If the thrust plate and beds
were folded together to form synclines, they should have similar
geometries. Finally, thrust plane orientations and movement d ir
ections obtained from a few isolated slickensides are consistent
with thrusting resulting from a stress similar to that which developed
Fg. Thus th ru stin g is thought to have occurred contemporaneous w ith
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Fig. 14 Orientation diagram of poles to bedding in Georgetown thrust plate. Contours: 4, 8 , 12, 16% per 1 % area.
A/
o 1 l_ KM j A P PR O X . S C A L E
F ig . 15 Diagrammatic sketch showing relationship of thrusted west limb to syncline.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45
or subsequent to e a rly fo ld in g in Fg. However, there are no un
equivocal relationships to date the two events.
Post-F] Movement P icture
Post-F-| deformations have been distinctly different from the
F] event. They are characterized by a buckling style of folding
and absence of concurrent metamorphism. Overturning to the east of
the F g structure, west over east sense of movement in the F 2 drag-
type folds, and west-dipping thrust planes point to movement from
west to east during post-F-| deformation. With respect to F^, the
later deformations may be viewed as resulting from either different
stresses, different conditions, or both.
The post-F] movements can be regarded as a progressive defor
mation resulting from a common stress system. F 2 and F3 structures
are deformed about axes which are at least sub-parallel, and possibly
coincident. The resemblance in style and mechanism of deformation
between F 2 and F3 indicate similar physical conditions of deformation.
And the response of the respective units to post-F] stresses support
this interpretation. These may be seen in Plate III. The orientation
of poles to schistosity and bedding in the Wallace Formation con
s is te n tly f a ll w ith in the same maximum in any given domain.^ In
addition, the synoptic plots of these data show a preferred orientation
The interpretation of the exception in Domain IV is uncertain. Because the data points l i e close to the hinge zone and are oriented roughly parallel to the Fo axial plane, they may represent localized development of an axial plane schistosity associated with F 3 fo ld in g
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along the great circle. This suggests passive behavior of the
Wallace during Fg and Fg fo ld in g , and th a t stress was accommodated
largely by slip along the inherited S] surface, rather than by
flexure. This is the expected response in view of the strong trans
position accompanying F-| (Turner and Weiss, 1963, pg. 499). The
maxima of poles to bedding in the Flathead Quartzite show a more
even distribution around the fold. The implication here is that this
unit has controlled the geometry of F 3 folding, responding to the
stress by buckling in a near cylindrical manner. In multiply de
formed terranes where new folding is by buckling of more competent
strata, some additional internal strain must occur if the new fold
is to be c y lin d ric a l (Ramsay, 1967, pg. 538-540). Evidence o f th is
is seen in the Falls Fork syncline where left-lateral faulting has
occurred in the hinge. These faults may have resulted from shear
stress set up along the Fg axis due to oblique application of stress
to the F-j structure. The lack of Fg structures developed in the more
competent Flathead indicates either a gradual increase in Fg stress,
or a difference in response to the imposed stress. In either case,
the further application of stress resulted in buckling of the Flathead
to form the Fg structure. The problem of thrust timing has been
discussed above.
Workers in adjacent areas have reported only portions of the
deformational events seen in the Falls Fork drainage. To the east.
Flood (1974) describes only F-| deformation. He does note regular
variation in the attitude of the F-| structure and minor refolding
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of the metamorphic schistosity. However, he feels these are related
to inhomogeneities in the F-j stress field , late stage F«| movements,
or granite emplacement. To the north, Poulter (1956) describes
folding and thrusting similar to Fg. As noted above, the Falls Fork
syncline is concordant with structures to the north along strike,
and the Georgetown th ru s t can be tie d w ith P o u lter's map. However,
he describes nothing similar to F-j or Fg. Therefore, the present
area may more completely record the structural evolution of the region
than adjacent areas.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV
DYNAMIC INTERPRETATION
In this section, the available data will be incorporated into
a model for the sequence of deformation observed in the Falls Fork
drainage. The regional setting provides several constraints which
must be considered. The area lies in the midst of the extensive
overthrust belt of western North America. It is now generally
accepted that gravity is the dominant force responsible for this
kind of deformation. Armstrong (1974) proposed high heat flow
associated with magmatic activity as a control on the style of
thrusting. Where high heat flow is available, crustal shortening
w ill be characterized by ductile thrust gliding and folding as in
the Seiver orogeny of the central Rocky Mountains. Otherwise,
b rittle compression, buckling, and shear w ill operate. Thrusting
associated w ith u p lifte d c r y s ta llin e basement is o f th is kind.
Scholten (1968) stresses the evidence for a topographic high to the
rear of the fold and thrust belt, and radial tectonic transport away
from the eastern border of the Idaho batholith. He proposes a
gravitational model whereby the emplacement of the batholith into
thick geosynclinal sediments caused an undation, imparting an easterly
dip to the sedimentary pile. The resulting gravitational potential
was relieved by radial transport away from the batholith producing
the fold and thrust belt of Idaho and Wyoming. A similar hypothesis
48
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was proposed by Hyndman, Talbot and Chase (1975) fo r the north
eastern border of the Idaho batholith, as discussed in the first
chapter. The present area lies within the Sapphire Tectonic Block.
If the Sapphire Tectonic Block hypothesis is to explain the defor
mation in the Falls Fork drainage, the following features must be
accounted for: the eastern provenance of the F^ fold, metamorphism
associated with F-|, the lack of subsequent metamorphism, and con
trasting styles of deformation.
Most troublesome is the eastern provenance of the F-j structure.
Regional relationships make it unlikely that major west-directed
movements have occurred. Along the entire length of the overthrust
belt movement is east-directed. Scholten (1968) notes that the
eastern border of the Idaho batholith is characterized by west to
east tectonic transport. Flood (1974) notes the parallelism of the
Fj axis with the Georgetown and Philipsburg thrusts. The only
possible local source for west-directed stress is the Boulder batho
lith . Although this alternative cannot be discounted, it seems un
lik e ly . Emplacement has been a t shallow cru stal le v e l, apparently
into its own volcanic ejecta (Hamilton and Meyers, 1974). Further,
only contact metamorphism resulted from emplacement and thus no
suprastructure-infrastructure discontinuity was produced. There
fore, it seems necessary to appeal to stress from the west in order
to account for formation of F,. I The timing of F-j may be constrained to mid- to late Mesozoic time.
No Precambrian or Paleozoic events of sufficient intensity to produce
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the observed s ty le or metamorphic grade accompanying F-j deformation
have been reported in the literature of the region. The Nevadan
orogeny of mid-Mesozoic age marks the beginning of major mountain
building activity in the western United States. However, this
event has not been documented in Montana (Armstrong, 1974). Ex
tensive high-grade regional metamorphism culminating in the Jurassic
has been documented in the Shuswap Complex o f southeastern B ritis h
Columbia (Hyndman, 1968). F-| c le a rly predates the la te s t Cretaceous
movement of the Sapphire Tectonic Block (Hyndman, Talbot, and Chase,
1975) since, as w ill be shown below, F 2 and F g can be correlated w ith
movements associated with the Sapphire Tectonic Block.
The Sapphire Tectonic Block is the most likely candidate to
satisfy the stress, timing, and deformational style requirements
imposed by the F-j data. Kehle (1970) proposed a gravity sliding
model in which down-dip tectonic transport of overlying strata is
accomplished by gliding on a low viscosity layer. This layer, termed
the decollement zone, deforms by simple shear*flow producing dis
harmonie isoclinal folds, bedding thrusts, penetrative cleavage,
and other features of deformation-induced crystallization. The
metamorphic grade and style of F-j folding are consistent with de
collement structure. The relationship of the Sapphire Tectonic Block
to the Idaho batholith provides a source for elevated heat flux to
aid fluid-like shear and increased metamorphic grade. The depth of
burial (calculated to be 13 km from average stratigraphie thicknesses
within the block) plus additional heat from the Idaho batholith are
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more than sufficient to significantly lower viscosities at the infra-
structure-sup rastructure boundary. Therefore, i t is proposed th a t
represents deformation in the decollement zone of the Sapphire
Tectonic Block, and the recumbent fold resulted from frontal
buttressing. The buttress effect can be related to the Dillon block
or a re la te d , unexposed basement high (see Fig. 16, b ). The complex
west-directed fold and fault pattern in the southeastern corner of
the Sapphire Tectonic Block is consistent with this interpretation.
This pattern suggests an edge effect resulting from differential
resistance to eastward movement.
It should be noted that rocks of similar metamorphic grade and
deformational style occur at other locations around the border of
the Idaho b a th o lith (Hyndman, verbal commun., 1976). This introduces
the possibility that F^ deformation occurred earlier than the Sapphire
Tectonic Block and was subsequently deformed by it. Unfortunately,
little is known about the geometry of these rocks. For this reason,
l i t t l e can be said about th is problem. Because o f the close proxim ity
of the Sapphire Tectonic Block, the sim ilarity of style to decolle
ment structure, timing constraints, and the lack of data to the con
trary, F-| in this report is assumed to be related to sliding of the
block.
The proposed model can also explain the differences in style and
metamorphic grade of the subsequent deformations. As eastward move
ment continues, a "piling up" phenomenon occurs. This results in a
magnification of early-formed buckles in the more competent layers
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C. F,
F
THRUSTING
F ig . 16 Proposed model fo r the development o f structure in the Falls Fork drainage.
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with flow of the viscous layers into the cores. This forces migration
of the structure to higher tectonic levels. Kehle (1970) states
that structures above the decollement zone form independently of
those in the zone. Resulting structures include folds and step
thrusts which form to overcome fro n ta l buttressing. The Fg structures
mark the passage from plastic shear deformation in the decollement
zone to the more b rittle regime in the deforming toe of the block.
The Fg s tru c tu re forms due to compressive stresses re s u ltin g from
frontal buttressing.
The origin of the compressive stress resulting in folding and
thrusting in the toe of gravity slide blocks is not well understood.
Several possibilities exist, the nature of which are important to
the styles of folds and thrusts formed. These possibilities include
drag on a shallowing basement, an abrupt step in the basement,
resistance by truncated strata in front of the block, or irregularities
in the basement surface. Since the nature o f the basement is almost
t o t a lly unknown, the choices are broad. In th is study, fro n ta l
buttressing is assumed to re s u lt from a step in the basement such as
the Dillon block. It is felt that this can best explain the complex
fold and fault pattern in the southeast corner of the Sapphire
Tectonic Block.
When the western limb of the syncline becomes oversteepened with
respect to the compressive stress, thrusting results. Continued
down-slope movement from the rear causes folding of the thrust
plane.
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An alternative possibility for Involvement of the F-j structure
in deformation above the decollement zone is emplacement of g ra n itic
plutons. As discussed above, contact relationships of some plutons
within the block indicate syntectonic emplacement. Flood's work
indicates that granitic emplacement occurred subsequent to F^ de
formation. The F^ structure could have been uplifted into the de
forming toe of the block by rise of granite magma. The Fg and Fg
structures would form as outlined above. No evidence distinguishes
between the p o s s ib ilitie s .
Although the model for F-j fold formation is speculative, the
style is precisely that predicted by Kehle (1970). The subsequent
deformations fit well with the regional structural style. This
style is characteristic of gravity slide tectonics as described in
the literature (e.g. Scholten, 1968). Thus it is felt that de
formations Fg, F3 and probably also F-|, in the Falls Fork drainage
can be ascribed to movement of the Sapphire Tectonic Block.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V
SUMMARY AND CONCLUSIONS
Rocks in the Falls Fork drainage include the Wallace Formation
of the Belt Supergroup, the Flathead Quartzite, and an undifferen
tiated section of the middle Cambrian up to and including the
Hasmark Dolomite. The Missoula Group is absent. These rocks have
experienced one regional metamorphism and three periods o f defor
mation. The deformational events may be divided into F-| and post-
F] based on s tru c tu ra l s ty le . The v a ria tio n in s ty le resulted from
deformation at different tectonic levels in the crust. The stress
responsible for all deformations was probably derived from a gravity
slide block off of the Idaho batholith infrastructure.
F^ deformation affected all rocks in the area. This event
resulted in a large west-verging recumbent anticline. A penetrative
axial plane schistosity developed and was accompanied by strong
bedding transposition in the less competent lithologies. The
Flathead Quartzite controlled folding geometry, deforming into
s im ila r -s ty le buckle fo ld s . Regional metamorphism to the lower
amphibolite facies accompanied this deformation. The F-j event may
represent decollement deformation at the base of a sliding block,
however other possibilities exist. The less competent Prichard
and Wallace Formations acted as slip surfaces for the overlying strata
55
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Increased heat flux from the batholith and depth of burial were
sufficient to reduce the viscosity of these units, allowing plastic
shear-flow deformation. The macroscopic structure is thought to
represent frontal buttressing at depth by a step in the basement.
Post-F-j deformation is superimposed on the F-j structure. The
d iffe re n c e in s ty le and lack o f metamorphism indicates shallower
tectonic levels. In addition, opposite directions of movement are
in d icated . The Fg event produced only mesoscopic folds which are
largely concentric in style. It is felt that this event marks the
passage of the F-j structure from the decollement zone into the over
riding strata. The Fg event occurred on the macroscopic scale only,
producing no minor structures. This deformation was coaxial with
^2» indicating progressive deformation. The inherited fabric was
folded into a syncline as a result of compressive stress set up in
the deforming toe of the block. Continued down-slope movement of
the block caused thrust faulting and subsequent folding of the fault
planes.
Several problems remain unsolved. These are: the relative ages
of Fg and thrusting, the timing of F^ with respect to the Sapphire
Tectonic Block movements and its relationship to similar structures
seen around the Idaho batholith, the eastern provenance of the F-|
structure, and the nature of the stress causing the distinct
s tru c tu ra l zones seen in the toe o f the block. In the foregoing
interpretation, an assumption of one alternative to each problem was
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made. Each assumption was based on the available data. In some
cases, however, sone possibilities seemed equally likely, as dis
cussed. In such cases, the simplest in te rp re ta tio n was chosen.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■o o Û. c g Q. Table 2. Summary of Structural Development
■ O CD
C/) C/) Probable Deformational Planar Structures Deformational Age Event Structure Formed Style
8 Jurassic or Large-scale, west-verging Ductile deforma- ë ' Mid-Cretaceous ? recumbent anticline with tion with strong well developed axial-plane transposition of schistosity (Si) and bedding, associated small-scale 3. structures 3 " CD
CD "O O Latest Cretaceous Q. Small-scale folds super Small-scale con C a imposed on F] structure centric folding O to form $2 axial planes. of S 3 1 ■ O O
CD a . Latest Cretaceous Symmetrical large-scale Large-scale con syncline overturned centric folding to east of S-j and S2 ■D CD
(/)C/) Latest Cretaceous Thrusting Small bedding-plane Brittle fracture thrusts and large- and open concen scale folded thrusts tric folding of fault surfaces
cn 00 REFERENCES CITED
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