010-Yin-Field Guide.p+ - Earth, Planetary, and Space Sciences

A HELD TRIP GUIDE FOR MARATHON OIL COMPANY
STRUCTURAL EVOL U TION OF TH E L EWIS THRUST SYSTEM IN GLACIER
NATIONAL PAR K, WESTERN MON TAN A
AN YEN
Department of Earth and Space Sciences, University of California, Los Angeles, California 90024
May, 1993
FIELD TR IP OBJECTIVES
The purpose of the field trip is to show the style of deformation and cross-cutting
relationships in the classic Lewis thrust system, Glacier National Park, western Montana. With the
aid of detailed geologic maps and cross-sections, kinematic development of duplexes, imbricates,
extensional faults, fault-bend/propagation folds and out-of-sequence thrusts in the Lewis thrust
system is illustrated. In addition, the relationship between thrust kinematics and cross-section
balancing is discussed. The field trip guide consists of two parts: (1) an overview of the Lewis
thrust system in Glacier National Park and its evolutionary history, and (2) descriptions of field
stops.
PART I : OVER VIEW OF TH E LEWIS THRUST SYSTEM
INTRODUCTION
The Cordilleran foreland fold-and-thrust belt in the southern Canadian Rocky Mountains
and northwestern Montana consists of two tectonic elements (Fig. 1): (1) an eastern imbricatethrust belt, and (2) a western broad-fold belt. Understanding the structural origin of the broad-fold
belt is important, because it is situated between a coeval magmatic belt to the west and the
imbricate-thrust belt to the east. The magmatic and imbricate-thrust belts were speculated to have
been related during their development (Burchfiel and Davis, 1975; Price, 1981). Debates have
been centered on whether or not the crust below the broad-fold belt was significantly thickened
during the overall development of the fold-and-thrust belt. Two models may explain the geometry
of the broad folds: (1) the folds represent minor crustal shortening as expressed by their long
wavelengths and low amplitudes, and (2) the folds were related to thrusting at a deeper crustal level
and were the result of significant crustal shortening by the development of duplexes and faultbend/propagation folds (Price, 1981; Cowan and Potter, 1986; Yoos et al., 1991; Yin and Kelty,
1991a).
The Lewis thrust, first recognized by Willis (1902), is one of the major structural
components developed during the Late Cretaceous to early Tertiary in the foreland fold and thrust
belt of the southern Canadian Rockies (Bally et al., 1966; Price, 1981) and western Montana
(Mudge and Earhart, 1980). It bounds the broad-fold belt to the east and the imbricate-thrust belt to
west. The fault can be traced along strike from Steamboat Mountain in west-central Montana
(Mudge and Earhart, 1980) to the Rundle Range in southwestern Alberta, Canada (Dahlstrom et
al., 1962; Fig. 2) for at least 450 km. A significant amount of shortening in this part of the
Cordilleran foreland fold and thrust belt was accommodated along this single intracondnental
thrust. In Glacier Park near the international boundary, the Lewis thrust juxtaposes the Middle
Proterozoic Belt-Purcell Supergroup in its hanging wall over Cretaceous sedimentary rocks in its
footwall, and displaces the Belt-Purcell rocks for at least 60 km to the northeast with respective to
its footwall rocks (Price, 1962).
The first-order structural configuration of the Lewis plate has long been treated as a simple
syncline, the Akamina syncline (Dahlstrom, 1970) that extends from North Kootenay Pass of
southeastern British Columbia to Marias Pass of northwestern Montana (Ross 1959, Gordy and
others, 1977). The Syncline is part of the broad-fold belt between the imbricate-thrust belt and the
magmatic belt in western Montana. Recent studies of the Lewis thrust system in Glacier National
Park, however, reveal that complex structures including duplexes, thrust-related faultbend/propagation folds, conjugate contraction faults, and extensional faults lie underneath the little
deformed Akamina syncline (Davis and Jardine, 1984; Yin et al., 1989; Hudec and Davis, 1989;
Yin, 1991; Yin and Kelty, 1991a, 1991b; Yin and Oertel, 1993). In particular, founation of the
Akamina syncline could have been related to development of duplexes and fault-bend/propagation
folds in the Lewis thrust sheet (Yin and Kelty, 1991a; Yin and Oertel, 1993).
STRATIGRAPHY
Han2
ingWa l l
S t r
a t i
g r a
The Belt-Purcell Supergroup in the Lewis thrust sheet consists of the Altyn, Appekunny,
Grinnell, Empire, Helena, Snowslip, Shepard, Mount Shields, Bonner Quartzite, and McNamara
Formations (Fig. 3a; see Whipple et al., 1984, for detailed descriptions). The Mount Shields
Formation, Bonner Quartzite and McNamara Formation of the upper Belt-Purcell Supergroup are
only preserved in the hanging wall of the Blacktail fault, an Eocene-Oligocene normal fault that is
equivalent to the Flathead normal fault in Canada (Constenius, 1982; Bally et al., 1966).
Correlation of the Belt and Purcell Supergroups in Glacier National Park, Whitefish Range, and
adjacent parts of Canada is shown in Fig. 3b. Stratigraphy of the lower and middle Belt
Supergroup in southern Glacier National Park is shown in Fig. 3c.
The Altyn Formation, lying along the base of the Lewis thrust sheet, consists of dolomite,
dolomitic limestone, sandy dolomite, and quartz arenite. I t is overlain disconformably by the
Appekunny Formation, and is cut by the Lewis thrust below. The stratigrraphy of the Altyn
Formation is complicated by widespread intraformational bedding-subparallel faults such as the
Scenic Point fault near Two Medicine Lake in southeastern Glacier National Park.
The Appekunny Formation comprises mostly argillite and minor quartz arenite. In eastern
Glacier National Park, five laterally persistent quartz-arenite units in the formation are designated
marker bed A through marker bed E. Using the tops of marker bed C, D, and E as boundaries, the
Appekunny Formation is divided into four informal members. Similar to the Altyn Formation, the
statigraphy of the Appekunny Formation is complicated by a bedding-parallel fault, the Brave Dog
fault that lies in member 3. This fault locally cuts downsection gently to the east (Yin et al., 1989),
exhibiting a geometry of either an extensional fault or an out-of-sequence thrust.
The Grinnell Formation is composed mainly of thinly-bedded red argillite above the fault
and red quartz arenite and thickly-bedded argillite below the fault. The stratigraphy of this
formation is again complicated by a bedding-parallel fault, the Rockwell fault. The Empire
Formation consists of interbedded quartz arenite, argrillite, and minor limestone, and the Helena
Formation comprises almost entirely of limestone. The Snowslip Formation consists of sildte,
argillite, oolitic arenite, and pink quartz arenite. Resting on top of the Snowslip Formation is the
Shepard Formation that is composed mainly of carbonate rocks with minor thinly bedded quartz
arenite. Its lithology closely resembles rocks of the Altyn and Helena Formations.
The Bonner Quartzite and the McNamara Formation are only exposed in the Mt. Shield area
in the southwestern part of the park. The Bonner Quartzite consists dominantly of pinkish-gray to
pale-red, very fine- to medium-grained feldspathic arenite and less amounts of interbedded siltite
and dark-red argillite. The McNamara Formation is composed of green siltite, argillite, and
calcareous arenite and quartz arenite. Its top is not preserved in the Park.
Footwall Stratigrraphy
Stratigraphic units in the footwall of the Lewis thrust are exposed along the southern and
eastern margins of Glacier National Park. They are described by Mudge and Earhart (1983) in
detail. Briefly, these units include the Lower Cretaceous Kootenai Formation (nonmarine, greygreen and maroon mudstone), the Lower Cretaceous Blackleaf Formation (marine, grey mudstone
and interbedded sandstone), and the Upper Cretaceous Marias River Shale (marine, dark-grey
mudstone).
Kishenehn Formation
The Oligocene Kishenehn Formation (Constenius, 1982) was deposited in the hanging wall
of the Blacktail/Flathead normal fault during its movement. It is at least 3,500 m thick and consists
of light-grey to grey beds of sandstone, siltstone, mudstone in its lower part, and brick-red to
brown-red mudstone, sandstone, and conglomerate in its upper part.
GEOMETRY AND KINEMATICS OF THE LEWIS THRUST SYS I EM
The structural framework of the Lewis thrust system in Glacier Park is schematically
shown in Fig. 4. Detailed geologic mapping in Glacier National Park, Montana, reveals four
episodes of deformation in the hanging wall of the Lewis thrust.
Pre-Lewis Thrust Structures
Pre-Lewis thrust structures consist of (1) the frontal zone that is characterized by west- and
east-dipping imbricate thrusts, conjugate contraction faults, and west- and east-directed bedding-
parallel faults, and (2) the Scenic Point complex (Fig. 4). These structures are exposed along the
eastern edge of the Lewis thrust sheet. Although they are truncated from below by the Lewis thrust
(Figs. 5 and 6), their development was kinematically compatible with the emplacement of the
Lewis thrust sheet. Thus, they predate the Lewis thrust and may have formed during early stages
of the emplacement of the Lewis plate along thrust surfaces that lie below the present Lewis thrust.
The extreme structural complexity of the frontal zone may have been related to the presence of a
footwall ramp or a hanging-wall cutoff. Kinematic evolution of the frontal zone is shown in Fig. 7.
Detailed descriptions of the frontal-zone structures are presented by Yin (1991).
The Scenic Point structural complex is shown in a cross-section in Fig. 8. It is constructed
from both photos and field observations. I t is a duplex, because it consists of steeply dipping
minor thrusts that are bounded above and below by fiat faults (Dahlstrom, 1970). Note that strata
in the duplex is cut from top by an out-of-sequence roof fault. The Scenic Point complex is cut by
faults of the frontal zone, and thus, it predates the Lewis thrust.
Two models have been proposed by Boyer and Elliott (1982) for the development of
duplexes (Figs. 9a and 9b). The observed structural relationship in the Scenic Point complex
resembles the duplex geometry produced by the kinematic history shown in Fig. 9b. Note that
cross-sections such as those shown in Fig. 8 are difficult to balance if the offset strata in the
hanging wall of the roof fault are not preserved, because mass is not conserved and the amount of
the lost mass is hard to estimate.
Syn-Lewis Thrust Structures
Syn-Lewis thrust structures include (1) the late Cretaceous-early Tertiary Lewis thrust, (2)
west-dipping Brave Dog and Rising Wolf duplexes, (3) east-dipping nonnal faults, (4) Mt. Walton
fault-bend/propagation fold complex, and (5) the Akamina syncline (Fig. 4). The Akamina
syncline is a broad fold that lies directly west of the Lewis thrust and extends northwestward for
about 120 km from southern Glacier National Park, western Montana, to southeastern British
Columbia and southwestern Alberta, Canada.
The Brave Dog and Rising Wolf duplexes are best exposed in southern Glacier National
Park (Figs. 10). T he geometry of the two duplexes differ from that proposed by Boyer and Elliott
(1982; Fig. 9a), because the observed roof and floor thrusts in the Lewis thrust sheet (i.e., the
Brave Dog, Rockwell, and Lewis thrusts) do not joint each other immediately on both sides of the
duplexes cores as they suggested. In addition, the slip distribution along the roof faults of the
duplexes in the Lewis thrust sheet is also different from the Boyer and Elliott model in that the
magnitude of slip along the Brave Dog and Rockwell faults (9.0 km and 6.5 km, respectively; see
Yin et al., 1989 and Yin and Kelly, 1991a for details) are much greater than the amount of
shortening accommodated by the imbricate thrusts in the cores of the Brave Dog and Rising Wolf
duplex systems (about 3.5 to 4 km). In Boyer and Elliott's model, the development of duplex
systems is considered to be a slip-transfer mechanism from the floor thrust to the roof thrust, and
thus, the magnitude of slip along the floor and roof thrusts and the amount of shortening absorbed
by the imbricates in the core of the duplex should be equal.
During the development of a thrust system, the crustal section may undergo simple shear,
pure shear, or a combination of both. Experimental rock mechanics show that pure-shear
deformation results in a conjugate set of faults whereas simple-shear deformation results in the
foHnation of Riedel and primary shears (Fig. 11; see Yin and Kelty, 1991b for references). O n the
basis of this mechanical concept and the observation that bedding-parallel simple shear is the
dominant mode of deformation in the Lewis thrust sheet in Glacier National Park, Yin et al. (1989)
propose that the development of the Brave Dog and Rising Wolf duplexes is a result of simpleshear deformation (Fig. 12). I n particular, the imbricate thrusts in the cores of the two duplexes
were developed as primary shears in a simple-shear system. The widespread east-dipping normal
faults in the Park were also explained as a consequence of simple-shear deformation (Fig. 13; Yin
and Kelty, 1991b).
A fault-bend/propagation fold complex is exposed in western-central Glacier National Park
(Fig. 14). I t is described in details by Yin and Oertel (1993). T he complex involves rocks of the
upper Grinnell Formation, the Empire Formation, and the Helena Formation. This complex
consists of the Gunsight thrust at its base (Fig. 15a), an east-verging fault bend/propagation fold
above the Gunsight thrust (Fig. 15a), a frontal fold complex (Fig. 15b), and a west-directed roof
thrust, the Mt. Thompson fault, on the top of the frontal fold complex (Fig. 15c).
The cut-off angle between the Gunsight thrust and bedding in its footwall is about 70•
Bedding of the steep forelimb of the fault-bend/propagation fold is truncated by the Gunsight thrust
below. The cut-off angle ranges from 70
0 to 9 0
0about 120° in the northern part of the area and about 90
th h e
e
s o u t h e r n
.0 i nT Both
limbs
of the hanging wall anticline are complexly deformed. Deformation in the
p
a
r
t
.
f o l d
by W-directed
thrusts with offsets of less than 20 m and minor folds.
iforelimb
n t isecharacterized
r l i m
b
aThe shallow-dipping
n
g
lbacIdimb
e is deformed by a few E-verging overturned folds. The total
o
f
tbedding-parallel
h
shortening
e
strain of the forelimb by thrusting, folding, and penetrative strain is no
a
n
t
i
c
l
i
nmore than
e 20%, because no significant change in stratigraphic thickness across the hinge of the
ihanging wall anticline
s can be detected in the field.
The 4-km wide west-verging fold belt (Fig. 14) is bounded above by the Mt. Thompson
fault, which lies in the lower part of the Helena Formation between dominantly limestone above
and interbedded limestone and quartz arenite below. Minor W-verging folds with both amplitudes
and wavelengths of several meters are present directly below the Mount Thompson fault,
suggesting that it is W-directed. The Mount Thompson fault is different from the "passive roof
fault" of Banks and Warburton (1986), because nowhere in the study area is it directly linked with
the Gunsight fault to form the tipline of a blind thrust (cf. fig. 7 of Banks and Warburton, 1986).
It is merely an accommodation zone or decollement separating the highly folded strata below from
less folded strata above. For the same reason, the frontal fold complex does not resemble a
"triangular zone" of Price (1986).
The E-verging hanging wall anticline above the Gunsight thrust exhibits the geometry of a
fault-bend fold (Rich, 1934). It could thus have developed according to the kinematic model of
Suppe (1983). However, when the model is applied assuming constant bed thickness during
folding (eq. 12, Suppe, 1983) and using the observed initial cut-off angle (7°), the predicted
interlimb angle (2g) of the anticline and the final cut-off angle (b) are 176° and 8° for the mode I
fold and 20
0 a n d
angles,
1 6 02g = 90-120
0
a n d
b
0
forelimb
thickening and applying Jamison's fault-bend fold model (fig. 3 of Jamison, 1987), no
=
f o
7
9 0for° observed interlimb angles of 90-120° and an initial cut-off angle of 7
solution
r 0 - exists
,t
0
h
matter
forelimb thickening one assumes. I t is, however, conceivable that the fold
e
,a
n
orhow much
e
b
o from
t an initial, open mode I fault-bend fold, and that the observed interlimb angle of 90m
originated
h
o
id120°
n resulted
c o from
n later tightening of the fold. Alternatively, the thrust ramp could have been
se i s t e
initiated
as a steeper surface and later became shallow due to a simple-shear deformation in its
n
I t
w
i flattens the ramp; this process can also lead to the observed fold geometry (Fig. 16).
footwall
that
I
tf
h
An
apparent Rich-type fault-bend fold (Rich, 1934) can also be the result of faultto
h
lpropagation folding because the two types of fold share many geometric features (Jamison, 1987;
e
d
m
.Suppe andoMedwedeff, 1990; Mitra, 1990). However, applying Jamison's fault-propagation
d
e
H
lomodel (Fig.
. 2 of Jamison, 1987) together with forelimb thickening, we again found no solution for
C
o
the observed
interlimb angles of 90-120° starting from an initial cut-off angle of 7°. With 20%
w
n
s
i
e
v
forelimb
thickening
strain the predicted fold interlimb angle is about 10°, much tighter than
d
r
e
re
i,observed.
n I t seems
g
unlikely that the observed angle of 90-120
u
t0 r e s u l t e d
f r o m
n
of 10°, because,
hfold
r efrom
o an
p initial
e ninterlimb
i n angle
g
a contrary to observations, this would require
iet
f g
i W-verging
hfolding
t and thrusting in the bacldimb and E-verging features in the forelimb.
flexural
o
o
rb
The poor fit of the observed geometry with all constant-volume balanced cross-section
m
s
emodels could be due to significant volume loss during or after folding. The widespread spaced
rcleavage could well have been the result of volume loss by stress solution.
v
It should also be noted that the geometry of the entire thrust-related fold complex in weste
dcentral Glacier National Park differs from either a fault-bend fold or a fault-propagation fold by
i
npossessing (1) a passive roof fault that transports in the direction opposite to that of the basal
t
ethrust, (2) a complexly deformed fold zone in front of an asymmetric hanging wall anticline
r
l
i
m
b
a
10
between the basal thrust and the roof fault, and (3) an over-gentler thrust ramp. The development
of the Mt. Thompson roof fault caused the disharmony of folding above and below.
The structural relations discussed above provide the basis for a kinematic model that
explains the development of the fault-bend/propagation fold complex (Fig. 17). Emplacement of
the Lewis thrust sheet started with the initiation and development of the Brave Dog duplex between
the Lewis and the antiformal Brave Dog faults (Figs. 17a and 17b). The development of the Brave
Dog fault was followed by the initiation, above it, o f the Gunsight thrust in the anticlinal Grinnell
Formation (Fig. 17b). We speculate that the Gunsight thrust ramp was localized by the presence
of the anticline, produced in turn by the Brave Dog duplex. Because this is a gentle warp of only a
few degrees on either limb, less energy may have been expended by the Gunsight thrust following
the curved bedding plane than would have been needed to cut bedding along a new, straight fault
segment. The E-verging hanging wall anticline above the Gunsight fault could have been initiated
asan open fault-bend fold (Fig. 17c). Further emplacement of the Gunsight plate caused both the
development of the frontal fold complex and the tightening of the hanging wall anticline (Fig. 17d).
Associated with the latter are the development of E-directed flat faults in the backlimb and Wdirected thrusts in the forelimb. The backlimb was stretched as it passed the thrust ramp. The topto-the-west bedding-parallel Mt. Thompson fault was initiated as a consequence of the piling up of
the frontal fold complex (Fig. 17d). Further fold tightening in this complex led to the formation of
the cleavage Si in hinge zones (Fig.17d). During or after the emplacement of the Gunsight thrust
sheet the regional, west-dipping cleavage S2 developed (Fig. 17e).
The formation of the segment of the Akamina syncline in the study area was the
consequence of development of the duplexes and fault-bend/propagation folds in the Lewis thrust
sheet, because strata above the duplexes and thrust-related folds are concordant with the syncline.
The syncline is, however, disconcordant with the Lewis thrust, because the Lewis thrust in
southern Glacier National Park is hardly folded (Fig. 18). This observation contrasts strongly
with the well-established concordant relationship between the Lewis thrust and the Akamina
syncline in its hanging wall in Canada, about 100 km north of the study area (Fig. 19).
11
We propose a deformation history for this part of the Lewis plate based on crosscutting
relationships outlined above (Fig. 20). The chronological order for the foimation of major
structural elements is: (1) the Scenic Point structural complex, (2) the frontal zone, (3) the Lewis
thrust, (4) the Brave Dog Mountain duplex and east-dipping normal fault system, (5) the Rising
Wolf Mountain duplex, and (6) the Akamina syncline.
During the initial stages of development of the Scenic Point structural complex (Figs. 20a
to c), imbricate thrust faults branched off from an older basal thrust located structurally below the
present Lewis thrust. These imbricates were later cut and offset by the Scenic Point fault above.
This event was followed by development of the frontal zone (Figs. 20c to f). Field relationships in
the frontal zone (Yin, 1991) suggest that thrusts developed along two bedding-parallel
decollements, one along the Altyn-Appekunny contact and one in the Altyn Formation (Figs. 20c
and d). We assume in Figure 20d that the deeper decollement of the frontal zone shares the same
sole fault as the Scenic Point complex.
Rotation of structures in the eastern part of the frontal zone (Fig. 20e) is inferred to explain
the apparent west-dipping nonnal faults (Yin, 1991). The rotation may predate the formation of
conjugate contraction faults in the western frontal zone, because they were not rotated (Fig. 200.
After the formation of the frontal zone, the speculated pre-Lewis basal thrust and the Scenic
Point fault were gently folded. Structures in the frontal zone and the Scenic Point complex were
later cut and offset by the Lewis thrust that lies structurally higher than the early basal fault (Figs.
20f and g).
Following the formation of the Lewis thrust, the Brave Dog fault began to propagate
parallel or subparallel to bedding in the Appekunny Formation (Figs. 20g and h). Its initiation may
have been related to east-verging simple-shear deformation produced by eastward movement along
the Lewis thrust (Yin et al., 1989). The Brave Dog fault, which cuts structures in the underlying
frontal zone, became kinematically linked with the Lewis thrust through the Elk Mountain imbricate
system. The Brave Dog fault was broadly warped upward over the imbricate system as a
geometric consequence of shortening across the imbricate system (Fig. 20h). During movement of
12
the Brave Dog fault and simultaneous movement along the Lewis thrust, the east-dipping normal
fault system developed as a consequence of simple-shear deformation between the two faults (Yin
and Kelly, 1991b).
The Rockwell fault was initiated at a higher structural level above the Brave Dog fault and
propagated subparallel to bedding in the Grinnell Formation (Figs. 20h and 20i). The Rockwell
fault later became kinematically linked with the Lewis thrust through the development of the Mount
Henry imbricate system along the east side of the study area. The development of the imbricate
system beneath the Rockwell fault caused the upward warping of the low-angle fault.
Development of the Rising Wolf Mountain and the Brave Dog Mountain duplexes, together,
resulted in formation of the Akamina syncline that is defined by strata above the Rockwell fault
(Fig. 20i).
Two models have been proposed for the formation of the Akamina syncline: (1) the
formation of the syncline was related to movement along the Flathead (or Blacktail) normal fault
(Dahlstrom, 1970), and (2) the formation of the syncline was related to duplex formation and
imbricate thrusting in the footwall of the Lewis thrust (Bally et al., 1966; Gordy et al., 1977). The
result of this study suggests that neither model explains the structural relationship observed in
southern Glacier National Park. First, although the short-wavelength antiforms and synforms of
the Lewis thrust close to the Blacktail normal fault may have been related to movement along this
younger fault, the overall geometry of the long-wavelength Akamina syncline is compatible with
the development of the duplexes and the fault-bend/propagation fold in the hanging wall of the
Lewis thrust (Fig. 20h). Second, imbricate thrusting in the Mesozoic and Paleozoic strata in the
footwall of the Lewis thrust may have affected the geometry of the Lewis thrust in the south-central
part of the study area. The resultant geometry (a broad antiform) is, however, disconcordant with
the Akamina syncline as discussed above. Thus, we conclude that at least in southern Glacier
National Park the formation of the Akamina syncline was related to deformation in the hanging
wall of the Lewis thrust during emplacement of the Lewis plate. This interpretation leads to a
paradox in that the Lewis thrust is clearly folded into a synform in southwestern Alberta and
13
southeastern British Columbia, Canada, directly north of the international border as indicated by
surface (presence of thrust windows) and subsurface (well and seismic) data (Bally et al., 1966;
Dahlstrom, 1970; Gordy et al. 1977; Fig. 19).
To reconcile the discrepant observations on the geometry of the Lewis thrust north and
south of the international border, we propose a model to explain the structural relationship between
the Akamina syncline and deformation in the hanging wall and footwall of the Lewis thrust (Fig.
21). I n this model, the development of the syncline was related to duplex formation and imbricate
thrusting in two structural levels, one above and one below the Lewis thrust. T he magnitude of
shortening in the duplex and imbricate systems in the hanging wall and the footwall vary along the
structural trend. The magnitude of NE-SW horizontal shortening decreases in the footwall and
increases in the hanging wall southward. The total shortening of the hanging wall and footwall
could be approximately uniform along the structural trend in the NW-SE direction. The role of the
Lewis thrust was to transfer shortening laterally along the regional structural trend (NW-SE) from
its footwall in the Paleozoic and Mesozoic strata to its hanging wall in the Proterozoic Belt strata.
Although the defolmation in the Lewis plate north of the international border is more complicated
than what is shown in Figs. 19 and 21, the presence of thrust windows (Fig. 1) north of the
international border requires that the Lewis thrust is significantly folded (Fermor and Price, 1987).
Post-Lewis Thrust Contractional Structures
Post-Lewis thrust contractional structures include the Lone Walker fault, a high-angle
reverse fault that cuts the Lewis thrust and strikes N70°W, which is about 30- 40
0 m o r e
w e s t e r l y
than the average strike of the syn-Lewis thrust structures. The development of this fault represents
a change in compressional direction after emplacement of the Lewis plate.
Post-Lewis Thrust Extensional Structures
Post-Lewis thrust extensional structures include southwest-dipping normal faults. These
faults are part of the Eocene-Oligocene Rocky Mountain trench normal fault system.
PART I I : DESCRIPTIONS OF FIEL D STOPS
14
STOP (1) Highway 2 , —4 miles west of East Glacier:
Overview of the geology of the Lewis thrust system. From this stop, we can see the Lewis
thrust to the north that lies at the break of slope. It separates the Altyn Formation (light-yellow
cliff-forming unit) in the hanging wall and the Cretaceous sedimentary rocks (poorly exposed,
dark-grey slope-forming unit) in its footwall. We can also see the frontal-zone structures, the Mt.
Henry imbricate thrusts which is part of the Rising Wolf duplex, and the Lone Walker reverse fault
that offsets the Lewis thrust.
STOP (2) Highway 2 , -1 0 miles west of East Glacier:
Discussion on duplex development and abandonment during the evolution of the Lewis
thrust system. Overview of mechanisms for the formation of extensional faults in thrust systems.
View to the north is the Brave Dog fault that is cut by thrusts of the Mt. Henry imbricate system.
Numerous east-dipping normal faults lie directly above the Lewis thrust (Fig. 22).
STOP (3) Highway 2, Marias Pass at the Continental Divide:
Overview of the geometry of the Lewis thrust surface and its relationship to duplex systems
in its hanging wall and footwall. View to the northwest is Elk Mountains where the lower part of
the Brave Dog duplex system is exposed (Fig. 23).
STOP (4) Two Medicine Lake, southeastern Glacier National Park:
Overview of the role of out-of-sequence in the development of duplex systems. If the
lighting is right, we may see two antiformal-stack duplexes. They formed during multiple
episodes of deformation such as development of multiple roof thrusts.
STOP (5) Jackson Glacier, Going-to-the-Sun Road:
Overview of the fault-bend/propagation fold complex developed in the Lewis thrust sheet in
west-central Glacier National Park. A view southwards this point is the western steeply dipping
limb of the hanging wall anticline in this thrust-related fold complex. This fold complex together
with the Brave Dog duplex beneath it define the western limb of the broad Akamina syncline in the
park.
STOP (6) On the International Highway northeast of Chief Mountain:
15
View to the west from the Chief Mountain International Highway is the Chief Mountain
duplex (Fig. 24). Note that the roof thrust of the duplex cuts both bedding and thrusts in its
footwall, exhibiting out-of-sequence thrust geometry. This is a classic outcrop of duplex
structures. I t was first described together with the Yellow Mountain duplex to the south by Willis
(1902).
The Yellow Mountain duplex was investigated by Davis and Jardine (1984). Geologic map
and cross-section of the Yellow Mountain duplex system'ik shown in Fig. 25.
16
REFERENCES C ITED
Bally, A. W., Gordy, P. L., and Stewart, G. A., 1966, Structure, seismic data, and orogenic
evolution of southern Canadian Rockies: Bulletin of Canadian Petroleum Geologists, v. 14, p.
337-381.
Boyer, S. E., and Elliott, D., 1982, Thrust systems: American Association of Petroleum
Geologists Bulletin, v. 14, p. 337-381.
Burchfiel, B. C., and Davis, G. A., 1975, Nature and controls of Cordilleran orogenesis, western
United States: extensions of an earlier synthesis: American Journal of Sciences, v. 275, p.
363-396.
Banks, C.J., and Warburton, J., 1986, "Passive roof" duplex geometry in the frontal structures of
the Kirhar and Sulaiman mountain belt, Pakistan. Journal of Structural Geology, v. 8, p. 229238.
Constenius, K., 1982, Relationship between the Kishenehn Basin, and the Flathead listric normal
fault system and the Lewis thrust salient, in Powers, R.B., ed., Geologic Studies of the
Cordilleran Thrust Belt, Rocky Mountain Association of Geologists, p. 817-830.
Cowan, D. S., and Potter, C. J., 1986, Continent-oceans transect B-3: Juan de Fuca spreading
ridge to Montana thrust belt: Boulder, Colorado, Geological Society of America.
Dahlstrom, C. D. A., Daniel, R. E., and Henderson, G. G. L., 1962, The Lewis thrust at Fording
Mountain, British Columbia: Alberta Society of Petroleum Geologists Journal, v. 10, p. 373395.
Dahlstrom, C. D. A., 1970, Structural geology in the eastern margin of the Canadian Rocky
Mountains: Bulletin of Canadian Petroleum Geologists, v. 18, p. 332-406.
Davis, G. A., and Jardine, E. A. 1984. Preliminary studies of the geometry and kinematics of the
Lewis allochthon, Saint Mary Lake to Yellow Mountain, Glacier National Park, Montana.
Montana Geol. Soc. 1984 Field Conf. Guidebook, p. 201-209.
Gordy, P. L., Frey, F. R., and Norris, D. K., 1977, Geological guide for the C.S.P.G. 1977
Waterton-Glacier Park field conference, Canadian Society of Petroleum Geologists, 93 pp.
17
Harrison, J. E., Griggs, A. B., and Wells, J. D., 1974, Tectonic features of the Precambrian Belt
Basin and their influence on post-Belt structures: U.S. Geological Survey Professional Paper
866, 15 pp.
Harrison, J. E., Kleinfopt, M. D., and Wells, J. D., 1980, Phanerozoic thrusting in Proterozoic
Belt rocks, northwestern United States: Geology, v. 8, p. 407-411.
Harrison, J. E., Grigg, A. B., and Wells, J. D., 1986, Geologic and structural maps of the
Wallace 1
0
series map I-15609-A.
xinvestigations
2
0 M. R., and Davis, G. A., 1989, Out-of-sequence thrust faulting and duplex formation in
Hudec,
q u a d
rthea Lewis
n g thrust system, Spot Mountain, southeastern Glacier National Park, Montana:
lCanadian
e ,
Journal of Earth Sciences, v. 26, p. 2356-2364.
M o
Jamison,
n t W. R., 1987, Geometric analysis of fold development in overthrust terranes: Journal of
a n
aStructural Geology, v. 9, p. 207-219.
a E. A., 1985, Structural geology along a portion of the Lewis thrust fault [M.S. thesis]:
Jardine,
n
dLos Angeles, University of Southern California, 205 pp.
I
Kelty, T. K., 1985, The structural geology of a portion of the Lewis thrust plate, Marias Pass,
d
aGlacier National Park, Montana [M.S. thesis]: Los Angeles, University of Southern
h
oCalifornia, 226 pp.
: S., 1990, Fault-propagation folds: Geometry, kinematic evolution, and hydrocarbon traps:
Mitra,
U
.American Association of Petroleum Geologists Bulletin, v. 74, p. 921-945.
S
Mudge, M. R., 1982, A resume of the structural geology of Northern Disturbed Belt,
.
G
northwestern Montana: in Powers, R. B., ed., Geologic Studies of the Cordilleran Thrust Belt,
e
oRocky Mountain Association of Geologists, p. 91-122.
l
Mudge,
M. R., and Earhart, R. L., 1980, The Lewis thrust fault and related structures in the
o
gdisturbed belt, northwestern Montana: U.S. Geological Survey Professional Paper 1174, 18
i
pp.
c
a
l
S
u
r
v
I8
Mudge, M. R., and Earhart, R. L., 1983, Bedrock geologic map of part of the northern disturbed
belt, Lewis and Clark, Teton, Pondera, Glacier, Flathead, Cascade, and Powell Counties,
Montana: U. S. Geological Survey, Miscellaneous Investigations series, Map 1-1375.
Price, 1962, Fernie map area, east half: Alberta and British Columbia 82G, east half: Geological
Survey of Canada Paper 61-24.
Price, R. A., 1981, The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky
Mountains, in McClay, K. R., and Price, N. J., eds., Thrust and Nappe Tectonics, Geological
Society of London Special Publication 9, p. 427-446.
Price, R.A., 1986, The southeastern Canadian Cordillera; thrust faulting, tectonic wedging, and
delamination of the lithosphere: Journal of Structural Geology, v. 8, p. 239-254.
Rich, J. L., 1934, Mechanics of low-angle overthrust faulting as illustrated by Cumberland thrust
block, Virginia, Kentucky, Tennessee: American Association of Petroleum Geologists
Bulletin, v.18, p. 1584-1596.
Ross, C. P., 1959, Geology of Glacier National Park and Flathead region, northwestern Montana.
Prof. Pap. U.S. Geological Survey 296, p. 1-125.
Suppe, J. 1983, Geometry and kinematics of fault-bend folding: Am. J. Sci., v. 283, p. 684-721.
Suppe, J., and Medwedeff, D. A., 1990, Geometry and kinematics of fault-propagation folding:
Eclogae geol. Hely., v. 83, p. 409-454.
Whipple, J. W., Connor, J. J., Raup, O. B., and McGimsey, R. G., 1984, Preliminary report on
stratigraphy of the Belt Supergroup, Glacier National Park and adjacent Whitefish Range,
Montana: Montana Geological Society, 1984 Field Conference, Guidebook, p. 33-50.
Willis, B., 1902, Stratigraphy and structure, Lewis and Livingston ranges, Montana: Geological
Society of America Bulletin, v. 13, p. 305-352.
Yin, A., 1988, Structural geology of the Lewis allochthon in a transect from Head Mountain to
Peril Peak, southern Glacier National Park, Montana [Ph.D. thesis]: Los Angeles, University
of Southern California, 290 p.
19
Yin, A., Kelly , T. K., and Davis, G. A., 1989, Duplex development and abandonment during the
evolution of the Lewis thrust system: Geology, v. 17, P. 806-810.
Yin, A., and Kelly, 1 9 9 1 a . Structural evolution of the Lewis plate in Glacier National Park,
Montana: Implications for regional tectonic development: Geological Society of America
Bulletin, v. 103, p. 1073-1089.
Yin, A., and Kelty, T. K., 1991b, Development of normal faults during emplacement of a thrust
sheet: an example from the Lewis allochthon, Glacier National Park, Montana: Journal of
Structural Geology, v. 13, p. 37-47.
Yin, A., 1991, Complex Pre-Lewis thrust deformation, southeastern Glacier Park, Montana:
Mountain Geologist, v. 28, p. 91-103.
Yin, A., and Oertel, G., 1993, Kinematics and strain distribution of a thrust-related fold system in
the Lewis thrust plate, northwestern Montana (U.S.A.), Journal of Structural Geology, in
press.
Yoos, T. R., Potter, C. J., Thigpen, J. L., and Brown, L. D., 1991, The Cordilleran foreland
thrust belt in northwestern Montana and northern Idaho from COCORP and industry seismic
reflection data: American Association of Petroleum Geologists, v. 75, p. 1089-1106.
Tv T E R T I A R Y
VOLCANIC ROCKS
CALGARY
0 •
MC M E TA M OR P H I C
COMPLEX
War M E S OZOIC
GR A N I TI C ROCKS
LEWIS THRUS T
P ALE OZOIC TO
MESOZOIC
S E DIME NTARY
ROCKS
P ROTE ROZOIC BELT
S UP E RGROUP
NORTH
KOOTE NAY
PASS
HIGH—AND LOW—
ANGLE N OR M A L
FAULTS
TH R U S T
PINCHER CRE E K
0
A' LEW IS
THR US
STRIKE—SLIP FAULT
ANTICLINE AND
S Y NCLINE
OV E RTURNE D
ANTICLINE AND
S Y NCLINE
S TUDY ARE A
R +
+ ++
+++ + + + + + +
MAR IAS PASS
Pz M z
IMBRICATE —
THRUS T BELT
LEWIS THRUS T
Fig. 1. Geologic map of Cordilleran foreland fold and thrust belt in southern Canadian Rocky Mountains and
northwestern Montana between 47°N to 52°N latitude, after Bally et al. (1966), Dahlstrom (1970), Price (1981),
Harrison et al. (1974, 1980; 1986), Mudge and Earhart (1980; 1983). Location of Glacier National Park (GNP,
outlined by dashed line) and study area. Fold-and-thrust belt consists of two elements: (1) eastern imbricate-thrust
belt which occurs mostly in Paleozoic and Mesozoic sedimentary rocks, and (2) a western broad-fold belt which
occurs mostly in Proterozoic Belt Supergroup. Cross section A-A' shown in Fig. 19. AS, Akamina syncline;
GNP, Glacier National Park; H and C windows, Haig Brook and Cate Creek windows; L-C line, Lewis-Clark line;
LWF, Lone Walker fault (new name from this study); PA, Purcell anticlinorium; RMT, Eocene-Oligocene Rocky
Mountain trench normal fault system.
O L e w i s pl a t e
50
*
—
49
*
48
°
—
\
\b
'.
t tz) '
—5 0
• I
'
4
.0*•
0 • tL
. 00
" i
° • •• : -. $
•••
.
•••
•o•
TC—a n—a d—
a— - e
-.—
49
-t
°
US A
-$ Enh a s t G l a c i e r
(.4
or
s t udy a r e a
ru
s
m
— 48*
at
100 k m lf
fa
au
ul
lt
\\,
115
*
t
114
*
Fig. 2. Regional tectonic map of the Lewis thrust (after Yin et al., 1989).
113
°
1 114
H o l e - i0
n
49
E ur ek a
th e
W a ll
T.•
AN
..,
1
r
'
0
P URCE LL
LAVA
SNO W SLI PM
FM.
HELENA/
W ALLACE EMS.
I NT ERNAT I O NAL B O UNDA RY
B ear
L
.
kC. S h e pM at r d
z
r i .
.
S
.2_,,. . G .l a c i e r
' ' '
GI_ AC I R G r a:n i t r
Park -• s
'9
G lade
e •
' ' ' t
o
M -•
,, ° S t ant on-a . n J ? per Q
lef ishc< \ c > .
y .
Lak e G l a c i e r ,M t n . . . ,
/
Vie • ,
o M t n t,
-.t.( 3 1 N.,
.•
,•.-....,,.
i
. 0 ..PPAi t Ra m' 'a' 'k.\-a -t,,..,
/
y
; i; , A .
P as s if s
s
..Q
.
?• '17
i
.
'
T
o,,,
N
I
,,
a r . . 6
. . , .!
MCNAMARA
O FM.
1
93
lk M t
N
BO NNER O T Z .
K alis pell
EMPIRE F M.
0
G RI NNELL F M.
— 48 c
'
C AN AD A
APPEKUNNY(?)
PRI CHARD FM.
A
MT. SHI ELDS
L
FM.
- SHEPARD FM.
, PURCELL
LAVA
c
, SNOWSLIP FM.
1
HELENA F M.
1
- EMPIRE F M .
MT. S HI E LDS
FM.
SHEPARD FM.
P URCE LL
LA V A
SNOWSLIP F M.
HELENA FM.
EMPIRE F M.
G RI NNELL F M.
APPEKUNNY
FM.
ALT YN F M.
ar ias
Pass
Mt. St ri'elds
nows lip
Mtn.
,
r
,
1I
0
1
•
29 M iles
G RI NNELL F M.
APPEKUNNY
FM.
MONTANA
P RI CHA RD(
FM.
9
)
93
Fig. 3 a. Stratigraphic nomenclature of the Belt Supergroup in Glacier National Park; b. Stratigraphic correlation of
the Belt and Purcell Supergroups in western Montana and adjacent parts of Canada; c. Stratigraphic section of the
Belt Supergroup in southern Glacier Park.
W HITEFISH R AN GE - 1 /
Cambri an Fl athead Qu a rt z i t e
--------,....--,---,-------,----Li bby Fo rm a t i o n
Mc Namara
Fo rm a ti o n
Bonner Qu a rt z i t e
ln
o
_0 oc u) a
4
b
.)
0'
-m
o
5
,
-,,,
2
"2
1
GLAC IER N AT ION AL PAR K L
I
Eas t
W est
/
Up p e
Purca el l L a v a
r r
_ Top n o t e_x p o s e d
Mc Namara
Form ati on
cco8
a
6
P urc el l L a v a > 6
a
0
E
c .-
c: .oL0_
i
1
1
4
3
1
•-' w
g ,
.... -.0 o
- - -2
2 0
a) ,
m
o
=
o
,
0
=
.>
o
cc
o
co
,
a)
0
-J
, „c Upper m e m b e r
cti•'
- ',,-,
-0 E
' ,a
0 o L o we r m e m b e r
S heppard
Fo rm a ti o n
Ni c hol C re e k
Helena a n d W al l ac e
Formati ons
Fo rm a ti o n
P urc el l L a y > 6
.
--C
L
0)
,: :
S
oE
"; al
co
c
Phi l l i ps Fo rm a t i o n
6
5
,
a
0=)
A e
Z
Roosville
f
Form ati on
B onner Qu a rt z i t e
0
5
- c
o
—Top not e x p o s e d--o o
_
4
c Mount Shi el ds
u) a
E
Form ati on
2
S8
o
<Lav a
1
2
o c
-.- -m
0
Cambri an
S hepard Fo rm a t i o n S hepard Fo rm a ti or S hepard Fo rm a t i o r
a
m
0
SOU T H EAST BR IT ISH
C OLU M BIA 2 /
Hel ena Fo rm a t i o n Hel ena Fo rm a t i o n
a
ta
a
=
(f)
l- c
--:-
c0 71
E
-0 ,o
0
Empire Fo rm a t i o n
A ppek unny (?)
Form ati on
Formati on _
4
,r) /
E
o =
.x .
wE
a 0
< u_
a.2 Upper m e m b e r
=
il-
Lower m e m b e r
Gri nnel l
Gri nnel l
Gri nnel l
Form ati on 3 /
Empi re Fo rm a t i o n
Fo rm a ti o n
a
,...
0)
Empi re Fo rm a t i o n
Van C re e k
=
0
i-
Form ati on 3 /
4)
>•'a
E0
0=
.
5
Cre s t o n
4
Form ati on
3
2
1
c Upper p a rt
Upper p a r t
o
0- o
-o z
'
" .
c 03
c E
a
.2 „E
L o we r p a rt
Lower p a rt
-0 o"
'•- o
a-u_ _ _ _ _ _ _ _ a: u _ _ _ _ _
i
Base n o t
Base n o t
•
z
ex pos ed
ex pos ed
Al ty n Fo rm a t i o n
A l dri dge Fo rm a t i o n
_ _
Fig. 3b
Base n o t _ _
ex pos ed
—
_
Base n o t
ex pos ed
*
(slalaw) s s am pLa
0
0
0
Cs4
0
0
03
E E
0 0
0 0
•;1'
I—
c
t
)
E
0
0
t N
-
0
0
AR
GILIT
E
E
X
P
L
A
N
A
T
IO
N
L
IM
E
S
T
O
N
E
p
m
riir
1
I
ci)
0
o_
X
CC I
F— 00 - - - J K r & 2 _1 • cn
U
l, zcZI—
c >_0
0
0
•0
2 w
w
< —
CO D
0
D
0
= u- —
>
R
2 v)
X
1
2
E
LI- /0)
0 0a. - ..c
-,
c< c x> -0
Li/ _
a
l1 0
-co Z
F
_
_
E
0
0
0
0
Ic
t
E E
0
C
O
2t1 c 2
o
o
0.
cco
co
l
c
1
o
1
11 :
.0
4
1
.
:-.. Il IiI i i i i 1 1
I1::::::..I C
- I I I:".-: . tI I 1111 i I;11111.-,11:.111:g
: :•
t
:
T UAt 313011
: i:
il
i l :, .
•
I
.
1
L
1;
:,
2
2 8w
U..
2
•2
IL 0
IL
IL
—J
•--0
1w < 4)
-0 a_ c
-W 11
17)
-coI x >0
CC 0
1
: )2
, UJ —
WZ"_
E'
--EL
Z
>.
_1
-0 I
w
: : 2-w
E.
-2 0
I
>
-ci) 2
0
- _
: -
H i r l d Si Ai tal
E
0
E
0 ,
2
2
O.
>(!
CN1
O.
iio
.
II I
11
:
11
_.
1
1
1
>,
to
Z
2
2
2?" >
-I 4
<
7
,
„
D 2• co
w
- LI_ >••
a.
o
a
-<
IL
D __ I
0
t
< I
1
w
cr
I D I
0
)
11' - L-:
c
o
I
Akamn
iasyn
clin
e
A
LEWIS
THRUS T
EXPLANATION
1
QUATERNARY MORAINAL
DEPOSITS
Ot
QUATERNARY TALUS
1
UNDIFFERENTIATED
CRETACEOUS SEDIMENTARY
ROCKS
PROTEROZOIC GRINNELL
Ygr F OR M A T I ON
MEMBER 4 OF PROTEROZOIC
Y al l
4
y ap3 MEMBERA
p
EKu
N3
A
F:73MP E M NB OE FR 2 OF PROTEROZOIC
Y a P
p 2 APPEKUNNYFORMATION
p
P
1 OF PROTEROZOIC
oMEMBER
R
E
Yapl APPEKUNNY
FORMATION
p O
K
PROTEROZOIC ALTYN
M
UVat F OR
M A T I ON
T
CN O N T A C T S
A
N
DASHED WHERE APPROXIMATELY
E
Y
LOCATED, DOTTED WHERE CONCEALED
T
— F
r
R
- -O
THRUST FAULT$
I
0
N R
p
0
AND ANTICLINE
O M
r
NSTRIKE AND DIP OF BEDS
R A
O
Z
T
M
— - S TRI K E AND DIP OF
V
0OVERTURNED BEDS
A I
E
C
L O
R
N
F
T
A
U
U
R
L
N
T
E
S
D
S
Y
N
C
L
I
N
E
0i 0 . 1
1
0.5
5
I
1
km
1
I
mi l e
Fig. 5. a: Simplified geologic map of northeastern part of study area from Yin (1988) and locations of cross
sections AA', BB', CC', and DD' shown in Figure 6b. Note fault A in frontal zone changes dip direction along its
strike. The segment with west-dipping normal-fault geometry is interpreted to be result of locally east-verging
rotation of primary east-dipping thrust. See Figure 2 for location of the map. H, Mount Henry; S, Squaw
Mountain; SP, Scenic Point.
b: Geologic cross sections through lines AA', BB', CC', and DD'. Note that fault A dips to east in
section BB' and west in section CC'. Faults in frontal zone cut Scenic Point structural complex; the complex is, in
turn, cut by overlying Brave Dog fault. Absence of Scenic Point structural complex in section CC' is due to
truncation by Lewis thrust. MHIS in cross section DD', imbricates in Rising Wolf Mountain duplex.
ire
•—
•T•0
+.4
C I
4.) 0
CL)
Co
r
•
•
•
0 C s C s C P C D CS t C
CS C S C s C S
C C Cs CD
t-e) r s t
C
,
CO N SCS
tsst t s s t t
,
t
s
• ,
t
s
s
t
s
.
s
4
Co
;-•
C s C s C s C SC s
CS C s C S C D C .
rt
C s
Cs C O
t
,
t
SW
50 meters
Fig. 6. View of the frontal zone from south and sketch of structures shown in the photo.
NE
SIMPL
ES
H
E
A
R
IN
G
(3)
cd
tr
,
(3
w
CC
1c/1
W
r
A
- -W A •
b -s
.
•
1
1
• q
T0
0
0
CO
C
t .
Vr 0
r
'
n
n
i
_d•
-J
O
Z
a
<
N
I- CC
<
w
<
w
<
U-
FAU
L
TB
A
D
cc
cc
co
c
o
co <
CO
•a
0>c,81 0
/:
1
Z
E
4
•
•
=
E •04=01 m
.2 m
C
0 E
P00 II-,
•0
—
•
c.) 8
0
-0 o m
•g,10 v64 0
8 a' r4)4,4
F
.,
c.)
L
4
0
)0,
8
cE
a Et
CA 0 C I) p
[LoI ' 1 ) "
aw e l
00
4 .
>,
. •—•
b
2
• 5 8 t'A4c-.
•D
4 1
.
1
4
•,0c,„•••
0 cq.
Ini ti al St age
Fig. 9a
MAJOR T H R U S T S H E E T
S
X•••
0
B
C OM m a i M O M O N O
\
;O M
,
n o m
'
4
B
FOOTWALL I N C I P I E N T
RAMP F R A C T U R E
82
B
3
.
•
..
.
UPPER & LOWER
.
GLIDE
H OR I Z ON S
.
.
.
.
•
.
.
.
.
.
.
.
.
•
FLOOR THRUS T
Fig. 9b
Fig. 9. Kinematic models for the formation of duplex systems after Boyer and Elliott (1982).
§§
•rr,
a. p u r e - s h e a r d e f o r m a t io n
b. s im p le - s h e a r d e f o r m a t io n
Fig. 11. Fault pattern predicted by pure-shear deformation and simple-shear deformation.
SW
N
E
— - BDF
L T
a
B DF
LT
EIS
b.
RF
BDF
LT
— EIS
C.
/
/ /
L
MHI S
EIS
/
/
R F
B DF
T
d.
RF
BDF
MHI S
EIS
e
Fig. 12. Simple-shear model for the development of duplex systems, after Yin et al. (1989).
LT
Ki nem at i c M o d e l
NE
SW
Br av e D og f a u l t
Lew i s t hr us t
a.
Br av e D og f a u l t
s hear z one
-.
t•
••
••
•
\
shear z one fE- di ppi ng nor
- m al f aul t s
—
—
—
—
—
,
f
t
,
„
,
•
.
.
,
•
.
.
.
.
.
.
.
,
,
..:
• ...
.
•
.
.
.
-
.
,
.
,
-.-.1.
.
Lew i s t h r u s t
b.
Br av e D og f a u l t
—
Lew i s t h r u s t
C.
Fig. 13. Simple-shear model for the development of extensional faults in the Lewis thrust system, after Yin and
Kelty (1991b).
Fig. 14. Simplified geologic map of west-central Glacier Park and cross section BB'. E, Edwards Mountain; G,
Gunsight Mountain; J, Jackson Mountain; W, Walton Mountain; T, Mount Thompson.
—kJ).- M t . T hom pson f aul t
-r
,
-c
2,
G
u
n
s
i
g
h
t
f
a
u
l
t
71;
>
0
0 C )
C)
0
CO ' 1 '
c t
t )
t n
0
0
0
C D
C=0 c
-c
?
4.
o
0 PC I
t4-,
0
0
0
0
0
c),
0:
c) 0 ,
,—,
-,
0 :
0
c
,
0
0
0
0
0
0
0
0
0
0
00
0
0
0
0
0 0 t . - s o t r i "z t
c
0
0
0
0e, 000 —
0
o
0
- 8c>
;8
0C D
ccl) c ; I
0
00
0
,
, . .
„
c
o
,
r
,
:
9
:
Ev olution o f f o o t w a l l c u t - o f f a n g l e
01 s i m p l e shear deformation i n t he footwall
03
Fig. 16. Kinematic model for flattening of a thrust ramp due to bedding-parallel simple shear in its footwall.
NE
SW
Yg
Brave Do g duplex
Tap4.0111
Ystp3
It a p t a
rawe 0 . t duPtel
Lewis thrus t
, 2 km
a
Gunsight f a u l t
azzarza c rz z i m 4=smtmea
Lewis th r u s t
Gunsi h t f a u l t
Lewis th r u s t
Mt. Thomps on f a u l t
Brave D o g fa u l t
Lewis thr us t
Lewis thr us t
Fig. 17. Kinematic model for the development of fault-bend fold in west-central Glacier National Park, Montana.
a. Initiation and development of the Brave Dog duplex between Lewis thrust and Brave Dog fault. Yap1+2 for
members 1 and 2 of the Appekunny Formation, Yap3 for member 3 of the Appekunny Formation, Yap4-1-p for
member 4 of the Appekunny Formation and the Prichard Formation, and Yg for the Grinnell Formation.
b. Initiation of the Gunsight thmst above the antiformal Brave Dog fault.
c. Emplacement of the Gunsight thrust plate and development of the fault-bend fold as a consequence of motion
along the Gunsight fault flat-ramp.
d. Formation of the hanging wall anticline and development of the frontal fold complex. Local top-to-the-west
bedding-parallel shearing causing development of west-verging folds in the frontal fold complex and initiation of the
W-directed Mount Thompson fault. NE-SW bedding-parallel shortening in the footwall and extension in the hanging
wall as the anticlinal fault-bend fold develops further. Development of local east-dipping cleavage S1 along the
hinge zones with tightening of the west-verging folds in the frontal complex.
e. Development of west-dipping, regional cleavage S2.
/
0
CR
t7
tC)
4b
(,
/ s ur f ac e t r a c e o f
Ai
-L
Summit eMtn.
w
i
s
t
h
r
u
s
t
Br ave Dog M tn.
"PEW M tn.
2 m i l es
% 6 .0, 9
c ontour i n t e r v a l , 4 0 0
3
k m
feet
c ontour s i n f e e t a b o v e s e a l e v e l
Fig. 18. Structural contour map of the Lewis thrust showing a gently northwest-dipping, oblique thrust ramp in
southernmost part of study area. Dashed line, surface trace of the Lewis thrust; solid lines, contours of Lewis thrust
surface. Contour interval, 400 feet; contours in feet above sea level. See Figure 2 for location. Construction of
contours is based on surface irace of Lewis thrust and stratigraphic thicknesses of Altyn, Appekunny, and Grinnell
Formations.
RC
R
E
A
T
C
E
O
U
S
D
N
A
E.4
SE
NE
MBC
a M B B
MBA
ALTYN FM T
- INFERRED EARLY THRUST
MBC
s
MBB
MBA
p
s
ALTYN FM.
c
INFERRED EARLY THRUST
SCENIC POINT FAULT
SPSC
SCENIC POINT FA ULT
FZ
MBC
MBB
MBA
ALTYN FM.
INFERRED EARLY THRUST
SPSC
SCENIC POINT FA ULT
MBC
MBB
MBA
ALTYN FM.
INFERRED EARLY THRUST
SCENIC POINT FA ULT
MBC
MBB
MBA
INFERRED EARLY THRUST
SCENIC POINT FA ULT
Fig. 20. Schematic sketch showing proposed deformation history of Lewis thrust system in southern Glacier
National Park. MBA, MBB, and MBC are marker beds in Appekunny Formation. Marker bed A rests directly on
top of Altyn Formation. EMIS, imbricate system in Brave Dog Mountain duplex; FZ, frontal zone; MHIS,
imbricate system in Rising Wolf Mountain duplex; SPSC, Scenic Point structural complex.
a. Development of imbricate thrusts in Scenic Point structural complex (SPSC). Imbricates branch off
from an inferred early basal thrust. Initiation of Scenic Point fault.
b. Imbricate thrusts in Scenic Point structural complex (SPSC) are offset by Scenic Point fault to the east.
Scenic Point fault, thus, is an out-of-sequence thrust.
c. Initiation of east-directed contraction faults in eastern frontal zone (FL) along a decollement lying along
the base of marker bed A (MBA).
d. Further development of east-directed contraction faults in frontal zone along the inferred basal thrust.
Younger faults in FZ cut older decollement and Scenic Point fault and developed along the inferred basal thrust.
e. Rotation of structures in eastern frontal zone by an east-verging simple shear.
f. Development of conjugate contraction faults in western frontal zone. Folding of inferred basal thrust and
Scenic Point fault. Initiation of Lewis thrust.
g. Truncation and displacement of Scenic Point structural complex and frontal zone by Lewis thrust.
Initiation of Brave Dog fault and Elk Mountain imbricate system (EMIS) in Brave Dog Mountain duplex.
Formation of the western limb of Akamina syncline.
h. Development of Brave Dog Mountain duplex and warping of Brave Dog fault and bedding above it.
Initiation and development of east-dipping normal faults between Brave Dog fault and Lewis thrust. Truncation of
frontal zone by Brave Dog fault. Initiation of Rockwell fault and Mount Henry imbricate system (MHIS) of Rising
Wolf Mountain duplex.
i. Development of Rising Wolf Mountain duplex. Offset of Brave Dog fault by imbricate thrusts in Rising
Wolf Mountain duplex. Formation of eastern limb of Akamina syncline.
\\
MBC
MBB
MBA
WESTERN FZ
I VA I I •
NN
I
4 4 6 1
I
I
SCENIC POINT FAULT
CONJUGAT T HRUST
I SI
•
I
I
SECTION OMITTED
I M m !
L _
F
WESTERNZ
FZ
ALTYN. FM.
I
1 1 ; 1 1 1 1 1 1 . 1 " 1 1
INFERRED
EARLY THRUST
FUTURE LEWIS THRUST
/
F
U
T
FZ
U
R
E
MBC
MBB
MBA
FM.
BRAVE DOG FA ULT
SCENIC POINT FAULT
LEWIS THRUST
FUTURE ELK MTN.
IMBRICATE SYSTEM
NE
SECTION OMITTED
SECTION OMITTED
/ F U T U R E ROCKWELL FAULT
BRAVE DOG FA ULT
/
F U T U R E MT. HENRY
/ I M B R I C A T E SYSTEM
MBC
MBB
MBA
ALTYN. FM.
p
.
EMIS
LEWIS THRUST
EAST DIPPING
EXTENSIONAL
FAULT SYSTEM
SCENIC POINT FAULT
LEWIS THRUST
AKAMI NA SYNCLI NE
RISING W O LF MO UNT AI N DUPLEX
ROCKWELL FA ULT
BRAVE DO G M O UNT A I N DUP LE X
MBC
MBB
MBA
ALTYN• FM.
EMIS
MHIS
LEWIS THRUST
RANGING W A LL DUPLEX
CKWELL FA
...i!
kYE PO P FAULTi!!!!!
,
BASAL DECO LLEMET I T
Pc
....
K W I L L FA U L
T
FOOT WALL DUPLEX
BASAL DECOLLEMENT
C
B \ r
C
'
I
a
I
U pper B e l t S u p e r g r o u p
v
e
D o g a n d R oc k w el l
pl ates
F T 1 Bas al p l a t e
Pal eozoi c a n d M e s o z o i c
s edi m entar y r o c k s ( P z - M z )
P r ec am br i an b a s e m e n t
Fig. 21. Three-dimensional diagram showing structural relationship between Lewis thrust, Akamina syncline,
duplexes in both hanging wall and footwall of Lewis thrust. Shortening accommodated by duplexes decreases in
footwall and increases in hanging wall southward; Lewis thrust transferred NE-SW horizontal compression laterally
from its footwall in Paleozoic and Mesozoic strata to its hanging wall in Proterozoic Belt strata.
-
• 1, . . •; 53
,
1it ,
•(,,
;
•2
,
'
•
••!.%
(
1
3
,
)
—ksL s
Lewls thrus t
•
: it y. : f
•
•,,
Fig. 22. (a) Oblique view of east-dipping normal fault above the Lewis thrust, Summit Mountain. Yap, Appekunny
Formation; At, Alt y n Formation; K, Cretaceous units. (b) Close-up of (a).
NI
EXPVAN AT ION
Q UAT ERNARY
A LLUV I A L
DEPOSITS
Q UAT ERNARY
MO RAI NAL
DEPOSITS
CRETACEOUS
SEDIMENTARY
ROCKS
MEMBER 4 O F
PROTEROZOIC
APPEKUNNY
FORMATION
MEMBER 3 OF
PROTEROZOIC
A P PE KO NN Y
FORMATION
[yapl•i
MEMBER 2 OF
PROTEROZOIC
APPEKUNNY
FORMATION
MEMBER I OF
PROTEROZOIC
APPEKUNNY
FORMATION
CONTACTS
(DASHED W HE RE
APPROXIMATELY
LOCATED,
DOTTED WHERE
CONCEALED)
CO NT RACT I O NAL
F AULT S
—
li
I
—
A
"
A
meters
STRIKE A ND DI P
O F BEDS
A'
B RA V E DO G M O UNT A I N
3000
2500
N50
0
E
feet
Yap4
BRAVE DO G F A ULT
9000
N78
0
E
8000
7000
2000
6000
Qal
5000
1500
4000
3000
1000
2000
500
2000 f e e t
1
1000
1 km
E L K M O UNT A I N I M B RI CA T E S
Fig. 23. a: Simplified geologic map of Brave Dog Mountain area after Kelly (1985). See Figure 2 for location.
b: Geologic cross section through line AA'. Yap1-4, member 1 to 4 of Appekunny Formation.
c: View of the Brave Dog duplex from south.
Fig. 24. View of the Chief Mountain duplex.
/
Fig. 25. Geologic map and cross-section of the Yellow Mountain duplex system.
NSAW
550'14
-13
60
0'
YgI
mod-
.2 0 0 0
'
70000
6200-
------------
----
-
•
-Ir-K
-
Fig. 25 b
L-
„ s„ ,__Yau • r
, •
•
•• . • • -• : ••-wi——•
- - T
6200'
-5400'