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Northwest Geological Society
Society Field Trips in Pacific Northwest Geology
Field Trip Guidebook # 21
The Priest River Complex
June 2-4 2006
Led By:
Ted Doughty
Eric Cheney
Andy Buddington
This field trip guide has been re-formatted from the
original document produced by the authors. All the
original text and illustrations are reproduced here,
and nothing has been added to the document in this
process. All figures and images are reproduced at the
same size as in the original document.
NWGS Field Guides are published by the Society with
the permission of the authors, permission which is
granted for personal use and educational purposes
only. Commercial reproduction and sale of this material is prohibited. The NWGS assumes no responsibility for the accuracy of these guides, or for the author’s
authority to extend permission for their use.
Of particular note, some stops on these trips may be located on private property. Publication of
this guide does not imply that public access has been granted to private property. If there is
a possibility that the site might be on private property, you should assume that this is the case.
Always ask permission before entering private property.
Editors note:
This guide does not include road travel directions to the various field sites. Dr. Doughty
has not provided this information to date. Until this information is provided to the editors,
you will have to contact him directly (Eastern Washington University) for field directions.
Geology of the Priest River Complex
The Priest River complex is a large Eocene metamorphic core complex that extends from southern British
Columbia to east-central Washington. The eastern
boundary of the Priest River complex is defined by
the Purcell Trench fault (Rehrig et al, 1987; Doughty
and Price, 2000), which is an east-dipping and eastverging detachment fault concealed by Pleistocene,
catastrophic flood deposits that fill the Purcell Trench
valley (see below). The western edge of the Priest
River complex is marked by a diffuse, west-dipping homocline along the southern two-thirds of the
complex. At the northern end of the complex, the
infrastructure bifurcates around a southward closing
U-shaped detachment fault system; the Newport fault
(Harms and’Price, 1992). The portion of the Priest
River complex south of the Newport detachment fault
is referred to as the southern Priest River complex
in this report and it terminates to the south against
strike-slip faults of the Lewis and Clark Line (e.g.,
Reynolds, 1979). These faults link extension in the
Priest River complex with the Boehls Butte metamorphic complex and Bitterroot metamorphic complex
to the southeast (Foster et al., 2003; in press). Metamorphic rocks exposed along the eastern side of the
southern Priest River complex are composed of the
Hauser Lake Gneiss (Weis, 1968; Weissenborn and
Weis, 1976). These paragneisses were metamorphosed
at peak pressures of 8-11kb and above the breakdown
reaction of muscovite and quartz (Rhodes, 1986;
Doughty, 1995) A low pressure retrograde metamorphism accompanied mylonitization in the core of the
Priest River complex and is expressed by the growth
of andalusite after kyanite and the formation of muscovite after sillimanite and orthoclase (Rhodes, 1986).
It records the rapid, nearly isothermal decompression
that accompanied detachment faulting and tectonic
unroofing during exhumation.
Overlying the Hauser Lake Gneiss to the west, is a
thick sheet of megacrystic granitic orthogneiss (Newman Lake Gneiss) and farther to the west, a large
body of two-mica granite informally referred to as the
Mount Spokane granite (Weissenborn and Weis, 1976;
Joseph, 1990). Armstrong et al. (1987) reported poorly
defined crystallization ages of 94-143 Ma for these
igneous rocks.
In the southern Priest River complex, the rocks are
folded into a north-trending antiform that follows
the crest of the tectonically denuded core of the
complex. Cheney (1980) referred to this structure
as the Spokane dome. The Spokane dome plunges
northward beneath the Selkirk Crest, near the town
of Sandpoint, and extends to the south across the
Spokane Valley. The Spokane dome is interpreted
as a footwall flexure that formed in response to
tectonic unroofing and subsequent isostatic rebound of the infrastructure during extension (e.g.,
Spencer, 1984; Wernicke and Axen, 1988). Arched
across the Spokane dome is a 4 km thick zone of
mylonitization named the Spokane dome mylonite
zone (Rhodes and Hyndman, 1984). The Spokane
dome mylonite zone is truncated on the east by the
Purcell Trench fault, whereas the top of the mylonite zone coincides with the transition from the
infrastructure to the suprastructure along the west
side of the complex. Kinematic indicators yield a
consistent top-to-the-east sense of shear along an
azimuth of 070-074, regardless of the dip of the
mylonite zone. Rhodes and Hyndman (1984) provide detailed descriptions of these mylonitic rocks.
A characteristic feature of the Spokane dome
mylonite zone observed by others, (Rehrig et al.,
1987; Rhodes and Hyndman, 1984) is the dearth of
brittle structures, such as chloritic veins and minor
faults, that typically overprint ductile mylonitic
fabrics. This is taken as evidence that the Spokane
dome mylonite zone ceased moving before being
uplifted to the surface by the bounding detachment
faults. Similar mid-crustal mylonite zones occur
within the Valhalla complex in southeastern B.C.
and were active during the early phase of crustal
extension (Carr et al., 1987; Schaubs et al., 2002).
The timing of formation of the Priest River complex is constrained by U-Pb crystallization ages
and K-Ar and Ar40/Ar39 mica cooling ages. Synkinematic stocks and plutons, like the Silver Point
Quartz Monzonite and the Rathdrum Mountain
granite, yield 52 Ma crystallization ages (Bickford
et al. , 1985; Whitehouse et al. 1992) which demonstrate that the Spokane dome mylonite zone and
the bounding detachment faults were active during
Eocene extension.. K-Ar and Ar40/Ar39 dating of
micas across the metamorphic infrastructure reveal
a progressive west to east pattern of younging in
the cooling ages of muscovite and biotite (Miller
and Engels, 1975; Doughty and Price; 1999). This
is consistent with progressive quenching of the
metamorphic infrastructure as it was tectonically
exhumed along the east-verging Spokane dome mylonite zone, western limb of the Newport fault, and
the Purcell Trench fault (Doughty and Price, 1999).
*a complete reference list is available upon request
DAY 1: (STOPS 1-4) PRC South
of the Spokane Valley
1) Stop: Quartzites of Silver Hill
What you will see: Metasedimentary rocks of
Silver Hill along with graphitic schist, and W/Sn
bearing pegmatite.
Dubbed S.H.I.T. (Silver Hill Isolated Terrane) by
Cheney, and others (never to be published), the
rocks at Silver Hill are dominated by feldspathic
quartzites with subordinate amounts of phyllite and
phyllitic schist. The unit consists of a thick bedded sequence of fine-grained micaceous gray, tan
(orange weathering) quartzite and locally mediumgrained white, vitreous, to lavender non-micaceous
(Right) Map
showing general location
of the Priest
River Complex
relative to the
Spokane Valley
quartzite. The quartzites are generally massive and
show only the occasional heavy mineral lamination; graded bedding is sometimes suggested by
changes in mica content. Individual quartzite beds
are often separated by a thin (<2 cm) layer of schist
or phyllite. Packages of thick-bedded quartzite are
interbedded with medium to thick layers of thinly
interbedded dark gray quartzite, light gray siltite,
and dark gray phyllite. The fine-grained rocks are
recrystallized into hornfels with randomly oriented
metamorphic minerals and a weak to non-existent
foliation at high angles to bedding. The typical mineral assemblage is biotite + andalusite + orthoclase
+ sillimanite +/-muscovite +/- cordierite. The assemblage is consistent with relatively shallow (<3 kbar)
metamorphism in the contact aureole of Jurassic or
Cretaceous granodiorite stocks that occur nearby.
Silver Hill is interpreted as having been metamorphosed in the upper part of the Cretaceous magmatic
arc that existed in this part of the Cordillera prior
to formation of the Priest River Complex. Similar
plutonic rocks and associated metamorphic aureoles
occur to the east in the hanging wall of the Purcell
Trench Fault.
Lode workings near the top of Silver Hill expose
sillimanite-andalusite pegmatite bodies containing
variable amounts of Sn (cassiterite) and W (scheelite
often rimmed by wolframite). Locally, the quartzite
and siltite are interbedded with layers of graphitic
schist that are between 4 to 11 cm thick. One 4-5
meter thick graphitic schist layer is present near the
(Right) Photomicrographs from Silver Hill quartzites
main exploration shaft. Graphitic schist is unheard of
in most Precambrian rocks that crop out in this part of
North America. The protolith for the graphitic schist
is still questionable but could correlate with carbonaceous argillites of the Monk Formation (Precambrian
Windermere Supergroup) or with the lower Paleozoic
quartzite and phyllite of the Addy and Maitlen formations.
Optional Stop: Silver Hill saddle quartzite containing crossbedding and isoclinal folding.
Optional Stop: Isolated stocks of medium-grained
biotite-hornblende granodiorite and diorite intrude the
quartizite of Silver Hill in the southwestern part of the
study area. These intrusions are undeformed and were
emplaced at relatively shallow depths, based on the
textures and metamorphic minerals present in the surrounding country rocks.
2) Stop: Gneiss of Chester Creek at
Sand Road
What you will see: Quartzo-feldspathic gneiss
and micaceous schist with deformed granitic
dikes and pegmatite.
Here the Chester Creek Gneiss is comprised
of a sequence of gray-weathering, mediumgrained, feldspathic quartzite (ranging from
20-100 cm thick layers) separated by thin (<
1cm thick) bands of fine-grained schist. Pure
quartzites and amphibolites are rare. The gneiss
of Chester Creek is laced with concordant and
discordant bodies of granite pegmatite and
medium-grained, two-mica granite dikes and sills
that range from 2 cm to several meters in thickness and are tightly folded and mylonitized to
undeformed.
Mylonitized shear bands are not easy to recognize but do occur as narrow zones typically less
than 1 cm in thickness. Contact surfaces between the feldspathic quartzites and micaceous
interbeds exhibit a weak, fine-grained stretching
lineation defined by straie of minerals and rodded
quartz that is typically subparallel to recumbent
isoclinal fold hinges. Large porphyroblasts of
muscovite overgrow these weak fabrics.
(Below) Sand Road outcrop with deformed pegmatite dikes
Determining the depth of metamorphism for the
Chester Creek Gneiss is somewhat problematic.
Most of the gneiss of Chester Creek is too quartzrich to allow for the widespread growth of pelitic
index minerals. The thin micaceous interlayers
contain biotite coexisiting with muscovite in the
foliation and as larger randomly oriented porphyroblasts. Tiny bundles of fibrolite are intergrown
with the micas in a few localities. This mineral
assemblage could have equilibrated over a wide
range of pressures and temperatures in the amphibolite facies.
This outcrop is more micaceous than the typical
Gneiss of Chester Creek. The foliation, locally mylonitic, dips gently to the west and there is a weak
lineation trending about 254 degrees. Recumbent
isoclinal folds, at low angles to the lineation, occur
at the eastern end of the outcrop. A thin pegmatite
dikelet demonstrates that movement was UPDIP to
the east. This outcrop was mylonitized in the upper
part of Spokane Dome mylonite zone where mylonitization is weak and non-penetrative. Tectonic
exhumation and isostatic rebound has arched the
once east-dipping shear zone into its current westdipping attitude.
3) Stop: Savage Pizza Dike
What you will see: deformed biotite granite dike
with schlieren fabric.
Here a small dike of fine to medium grained
granite exhibits well developed schieren fabric of
dark, thin to rod-shaped or discoidal aggregates of
biotite aligned with a very weak fabric in the granite. The schlieren are subparallel to the mylonitic
lineation in the surrounding country rock (Newman Lake Gneiss and Gneiss of Chester Creek).
These textures are best explained by intrusion of
the granite into the Spokane dome mylonite zone
during shearing. Rhodes and Hyndman (1984)
and Weissenborn and Weis (1976) described
similar syntectonic intrusions that they named the
Rathdrum Mountain granite north of the Spokane
Valley, and it is probable that these intrusions are
part of the same magamtic episode. Bickford et
al.(1985) obtained a discordant lower intercept UPb zircon age of 52 Ma on the Rathdrum Mountain
granite, which approximates the most recent time
of shearing within the Spokane Dome mylonite zone.
4) Stop: Hauser Lake Gneiss at State Line
What you will see: upper amphibolite-facies migmatitic paragneiss (Hauser Lake Gneiss). Here, the Spokane dome, named by an eminent economic geologist
from the UW, can be envisioned in all its glory. Enjoy
these high-grade rocks!
The Spokane dome in core of the Priest River Complex is underlain by a thick sequence of paragneiss
named the Hauser Lake Gneiss. This monotonous
rock unit consists of thin-layered, fine¬grained, rusty
weathering, granofels, migmatitic schist, and subordinate quartzite. A typical exposure contains fibrolite
coexisting with garnet, biotite (no muscovite) and
orthoclase, with small migmatitic pods of quartz and
feldspar. Thick, concordant, sill-like masses of garnet
amphibolite are also widespread. The protolith for the
Hauser Lake Gneiss was a sedimentary sequence of
pyritic feldspathic sandstones, siltstones, and argillites with intercalated mafic igneous bodies. Numerous workers have suggest that it is Pre-Belt in age,
but the Prichard Formation of the Middle Proterozoic
Belt Supergroup, which consists of sulfide-rich, semipelites and feldspathic siltites intruded by -1450-1433
Ma mafic sills, is so like the Hauser Lake Gneiss that
current workers favor a correlation between the two.
Detrital zircons demonstrate that the Hauser Lake
Gneiss can be no older than about 1500 Ma, and the
age spectra of detrital zircons is very similar to that
reported from the Prichard Formation. Recent U-Pb
dating of amphibolites in the Hauser Lake Gneiss
demonstrates that they crystallized ca. 1450 Ma,
which is also consistent with their derivation from the
mafic sills in the Prichard Formation.
The mineral assemblage seen here is typical of the
Hauser Lake Gneiss. Metamorphism occurred in the
upper amphibolite facies at temperatures above the
second sillimanite isograd according to the following
reaction:
Muscovite + quartz —> orthoclase + aluminosilicate + water
Locally, along the western side of Saltese Flats (see
below), embayed kyanite porphyroblasts coexist with
orthoclase and show that the peak metamorphism
was above 7 kbar where partial melting occurs in
(Right) Detrital
zircon age spectra
from the Hauser
Lake Gneiss and the
Prichard Formation.
the stability field of kyanite, within bathozone 6 of
Carmichael (1978). The peak mineral assemblage is
involved in the mylonitic fabrics, demonstrating that
deformation was synchronous with peak metamorphism. Amphibolites, which are conformable with the
foliation in the metasedimentary rocks, contain hornblende and garnet, which is consistent with the condi-
(Above) the Hauser Lake Gneiss at Saltese Flats,
south of the Spokane Valley
tions derived from the pelitic rocks.
This outcrop consists of rusty weathering paragneiss that contains a crystalloblastic foliation that
is interpreted as a blastomylonitic foliation. Look
closely at some of the foliation surfaces and you
will see a mineral lineation composed of aligned
sillimanite and stretched minerals. It trends the
now familiar 72-74 degrees. The southwest end of
the outcrop contain numerous recumbent isoclinal
folds whose hinge lines are at low angle to the
lineation. These fabrics formed in the main part
of the Spokane Dome mylonite zone in the upper
amphibolite-facies. Notice how the intensity of
deformation has increased from the last stop. The
primary fabric is folded by an open northwestplunging anticline that formed late in the arching
of the Spokane dome. Cross the road to the northeast end of the outcrop and observe a cross-cutting dike of Mt. Rathdrum granite. Apparently at
this locality, mylonitization had ceased by about
52 Ma.
(Left) P/T conditions for the Hauser
Lake Gneiss, south of the Spokane Valley
Look closely (parallel to the lineation)
and you can see sigmoidal S planes
between the C planes; this world-class
example of an S-C mylonite records
top-east shearing. Note the lack of
annealing in these rocks compared to
Stop 4. This is probably due to the fact
that much of the shear occurred as the
rocks were uplifted to shallow levels
by the Purcell Trench detachment fault
and there is no opportunity for annealing.
DAY 2: (STOPS 5-10) The PRC
From Spokane to Priest River,
Idaho
5) Stop: Round Mountain
This outcrop lies within l/2 mile of the
hanging wall Belt Purcell Supergroup,
but lacks chloritic breccia that is typical of ductile-brittle detachment faults.
Look closely at the exposed foliation
surface (where the 4WD trucks have
been climbing the outcrop) and you will observe
small chlorite-filled veins, perpendicular to the lineation. These are typically found at the bottom of the
chloritic breccia zone and suggest that this outcrop
formed just beneath the zone of chloritic brecciation,
which was probably scoured out during the Pleistocene Missoula floods.
What you will see: Mylonites in the upper part of
the Purcell Trench detachment fault.
This stop is an exposure of a mylonitized equigranular granite, probably Cretaceous in age, that
was mylonitized in the Purcell Trench detachment
fault. This fault is poorly exposed but is a ductilebrittle detachment fault that offsets the Spokane
Dome mylonite zone along the east side of the
complex. As the Spokane Dome mylonite zone
arched during exhumation it became unfavorable
for continued slip and a new fault system developed along the eastern side of the complex. Continued extension along the Purcell Trench detachment fault accommodated the final uplift of the
Spokane dome to the surface. The mylonites have
a strong east-dipping mylonitic foliation (C plane)
and down-dip (72-74 degree) trending lineation.
(Below) S-C Mylonites
East is top-right
(Left) Map showing field stop locations.
coexisting with garnet, sillimanite, biotite, and rutile; muscovite
is largely absent. Look closely
and you will find small crystals of
kyanite in contact with garnet and
K-feldspar. These mineral assemblages attesting to partial melting of
these rocks in the kyanite stability
field due to the break down of muscovite+ quartz \ second sillimanite
isograd). The composition of coexisting minerals yield pressures of
9-10 kb (30-35 km) for these rocks.
One must conclude that at the time
these rocks were metamorphosed
and mylonitized, the crust was in
excess of 60 km thick (it is about
30-40 km now) and these rocks lay
in the middle of the crust.
6) Stop: Blanchard Quarry
--NO HAMMERS PLEASE!!-What you will see: Blastomylonites developed in the Hauser Lake Gneiss and a rare
exposure of all three aluminosilicates.
This stop is an exposure of mylonitized
Hauser Lake gneiss in the upper part of the
Spokane Dome mylonite zone and along
the western flank of the Spokane Dome.
The paragneiss has a dominate west-dipping
blastomylonitic foliation that formed during
peak metamorphism. Asymmetric K-feldspar
augen and extensional crenulations yield an
updip top-east sense of shear. At this locality, the Hauser Lake Gneiss contains pods of
K-feldspar and stringers of felsic minerals
Examine the kyanite in detail and
you will observe a slight pink/white
cast in some crystals. This is andalusite, the low pressure polymorph
of A12SiO5. At first glance, these rocks would appear to
contain the rare triple-point assemblage of all three aluminosilicate polymorphs. However, detailed examination shows
that the andalusite pseudomorphs the kyanite and that it is
not in equilibrium with the kyanite and sillimanite. It formed
(Below) All three aluminosilicate polymorphs at Blanchard
Quarry
after the other polymorphs during the rapid
exhumation of these rocks during Eocene
tectonic exhumation. This mineral assemblage is also found in the Clearwater core
complex, where a similar mineral paragenesis has been documented.
7) Stop: Hauser Lake Gneiss Amphibolite
What you will see: Spectacular garnet amphibolite with metamorphic coronas.
This outcrop is an incredible exposure
of a coarse-grained garnet amphibolite
interleaved with the Hauser Lake gneiss
(east end of outcrop). The Hauser Lake
gneiss contains metamorphosed mafic sills
that contain the mineral assemblage: garnet-hornblende-plagioclase-quartz-rutile
+/- biotite. Locally, clinopyroxene joins
the assemblage and is symptomatic of the
granulite facies. The spectacular coronas
in this outcrop contain symplectic intergrowths of hornblende and plagioclase
resorbing embayed garnet porphyroclasts.
These are classic decompression reactions
that record the tectonic exhumation of the
Priest River complex.
(Above) Decompression corona in amphibolite, Hauser Lake
Gneiss
Just beyond the north end of the main outcrop, a megacrystic
phase indicates that at least some of the Pend Oreille Gneiss
was derived from a tonalitic protolith. High precision TIMS
U-Pb dating reveals that these are Archean rocks (2651 Ma)
that are a rare exposure of the crystalline basement beneath
the Cordillera. The uplift of the crystalline basement here is
due to tectonic exhumation along both the Spokane Dome
mylonite zone and the eastern limb of the Newport fault. This
is the most highly denuded part of the complex.
8) Stop: Pend Oreille Gneiss
--WATCH FOR TRUCKS!!What you will see: The oldest rocks north
of the Snake River Plain in the Cordillera.
This exposure is of a biotite gneiss, named
the Pend Oreille Gneiss, that occurs in the
core of a northeast-trending antiform at the
northern end of the Spokane dome. The
antiform is subparallel to the mylonitic
lineation and is interpreted as a turtle-back
structure within the core of the Priest River
complex. This is the deepest level of the
Spokane dome and Spokane Dome mylonite zone exposed.
(Below) The Pend Oreille Gneiss
(Right) Lead-uranium dates from the Pend
Oreille Gneiss
mylonitized and it is difficulty to discern
the original relationship between the two.
Detrital zircons demonstrate that the Gold
Cup Quartzite can be no older than 1.7 Ga
and support the interpretation that there was
originally a nonconformity between the two
rock bodies.
Clean, coarse-grained sandstones do not
occur in the Middle Proterozoic Belt Supergroup, with exception of a basal quartzite
exposed along the eastern margin of the basin.
This
quartzite, known as the Neihart Quartz9) Stop: Gold Cup Quartzite
ite, has been interpreted as a sand sheet deposited
What you will see: Orthoquartzite, locally pebble-bear- on the nonconformity between the basement and
the overlying Belt Supergroup. This sand sheet
ing, that is the basal Belt Supergroup. It separates the
continues to the west beneath the allochthonous
Archean basement from the Hauser Lake Gneiss.
rocks of the Cordillera and is exposed again in
the tectonically denuded core of the Priest River
The Archean Pend Oreille Gneiss in the core of the
complex. The spectrum of detrital zircon ages
antiform is flanked on all sides by an orthoquartzite
that locally contains stretched pebbles and small flecks also support a correlation between the Gold Cup
Quartzite and Neihart Quartzite (see below).
of graphite. The contact between the two rock types is
(Right) Detrital
zircon ages from
the Gold Cup
Quartzite
10) Stop: Laclede Augen Gneiss
What you will see: Mylonitized augen gneiss with
spectacular mafic boudins.
This is an outcrop of a mylonitized augen gneiss with
a well documented crystallization age of 1576 Ma. It
is older than the Hauser Lake Gneiss and a “window”
into the nature of pre-Belt basement that underlies
the Rocky Mountains. In the late Precambrian, western Idaho was part of the supercontinent of Rodinia
which broke apart 550 Ma and formed a passive
margin in west-central Washington. 1576 Ma rocks
are absent from western Laurentia and the Laclede
Augen Gneiss and the Pend Oreille Gneiss are a
critical piercing point for identifying the continent
that broke away from Laurentia during the disintegration of Rodinia ca. 550 Ma. One possible link
is with Australia where Archean basement is overlain by a suite of granitic intrusions and volcanics
(Hitalba Suite) that are the same age as the Laclede
Augen Gneiss. Other reconstructions place Siberia
adjacent to Idaho and Washington at that time.
(Above) The “traditional” model of Rodinia adopted from
Dalziel (1997) and Torsvik et al (1996). The model posits
two rifting events, one along the present-day western margin of Laurentia sometime between 800 and 700 ma, and
a second along the present day eastern margin of Laurentia between 600 and 550 Ma.
The Laclede Augen Gneiss occurs as two folded lozenge-shaped masses within the Hauser Lake Gneiss.
It is pervasively mylonitized and the contacts between
the two rock units are structural (well exposed in the
next road cut to the east). These bodies were tectonically emplaced into the Hauser Lake Gneiss within
the lower part of the Spokane Dome mylonite zone.
Mylonitization is expressed by a strong gently eastdipping mylonitic foliation with a lineation trending
to the northeast at about 72-74 degrees. Asymmetric
augen indicate a top-east sense of shear along the
dominant mylonitic foliation. This fabric is crenulated by west-dipping and west verging macro-scale
crenulations. These are interpreted as large antithetic
extensional crenulations (see below). A mafic dike has
been boudinaged during mylonitization and back-rotated along the macro crenulations. Note that in places
the dike is also isoclinally folded. Inside the pressure
shadows between the boudins and along pull-aparts
are small masses of granitic aplite. These formed
during high-temperature mylonitization and boudinage as partial melt was injected into the pull-aparts
from the host rock (Laclede Augen Gneiss). U-Pb
dating of one of these masses yields an age of 48
Ma for this high-temperature top-east mylonitization. These new dates_show that while Cretaceous
deformation is likely along the Spokane Dome
mylonite zone, most of the fabrics seen on this trip
are Eocene in age. Look closely at the outcrop and
you will see thin east-dipping shear surfaces that
contain slickenlines and chlorite and quartz pods
along releasing bends. These surfaces record topeast shear at relatively low temperatures. Thus the
outcrop shows top-east shear during progressively
lower metamorphic conditions as the Priest River
Complex was unroofed.
Most extensional crenulations are synthetic to
the shear direction, but antithetic crenulations do
occur. Earlier workers interpreted the west-dipping crenulations as a overprinting of the Spokane
Dome mylonite zone by the west-dipping Newport
fault zone that occurs to the west. The presence,
however, of progressively shallowing top-east
kinematics in the augen gneiss argues for a simple
uplift during a single event rather than overlapping
shear in different directions.
DAY 3: (STOPS 11-14) Newport
Fault and Associated Features
11) Stop: Newport Fault Zone
What you will see: Eastern segment of the Newport
detachment fault.
The Newport Fault is a brittle-ductile detachment
fault system that records the upper levels of the
detachment fault system responsible for tectonically
exhuming the Priest River complex. Compared
to the recrystallized deep-seated shear zones seen
earlier in the trip, extension exhumed these rocks
to shallow levels during the Eocene and there is
little recrystallization and extensive overprinting by
brittle deformation.
The Newport Fault system is a complex detachment
fault system that can be subdivided into three segments. The eastern segment is. The western segment
is east-dipping and east-verging. At its southern
end, it turns to the east and becomes a gently northdipping fault zone with top-oblique slip. These two
segments merge with the Spokane Dome mylonite
zone and together form a master top-east detachment fault that was responsible for unroofing most
of the Priest River complex. The eastern segment
is west-dipping and west verging. It is slightly
younger than the other two segments and is interpreted as an antithetic fault to a predominately
top-east detachment fault system. All three segments
of the Newport Fault system display evidence for slip
along an azimuth of either 254 or 74 degrees. This
direction is identical to the direction of slip observed
in the Spokane Dome mylonite zone and supports a
linkage between the two fault systems.
This outcrop is a poorly exposed section across the
eastern segment of the Newport Fault. It juxtaposes
Prichard Formation on the west against the Hauser
Lake Gneiss and related intrusive rocks within the
core of the Priest River complex. If one starts at the
eastern end of the outcrop, the Hauser Lake Gneiss
and associated pegmatites exhibit well-formed topwest mylonitic fabrics. In places they have been
back-rotated and dip to the east. As one proceeds to
the west, the mylonitized rocks become progressively
overprinted by brecciation, chloritization, and silicification. Directly beneath the fault surface (difficult
to see in the small swale), the footwall rocks have
been transformed into microbreccia and cataclasites.
The change from ductile to brittle deformation is
common beneath detachment faults and records the
exhumation of the footwall to shallower levels during
extension.
12) Stop: Newport Fault System
What you will see: Chloride breccia in the upper
part of the Newport fault zone.
This outcrop is located along the upper part of the
southern segment of the Newport Fault system. In
this area, the 52 Ma Silver Point Quartz Monzonite
intruded the Newport Fault during slip and was
deformed within and adjacent to the fault zone. The
outcrop displays mylonitized SPQM that has been
extensively brecciated and altered into a chloride
breccia. The fault surface, now eroded, was only a
few 10’s of meters above ground level.
13) Stop: Mylonitized Silver Point
Quartz Monzonite
What you will see: Mylonites in the lower part of
the Newport fault system.
This outcrop is located near the base of the Newport
Fault system. The Silver Point Quartz Monzonite is
a protomylonite that contains a pervasive weak foliation related to movement along the Newport Fault.
In many of the boulders, zones of higher strain are
expressed by relatively thin (5-10cm) wide zones
of ultramylonite. Notice the gradational boundaries
and the warping of the protomylonitic folation into
the ultramylonites. This is a good spot to think about
what the upper and lower boundaries of the Spokane
Dome mylonite zone might look like.
14) Stop: Grottos, Tiger Formation
What you will see: Syntectonic Eocene fanglomerates in the hanging wall of the Newport fault.
This outcrop is an exposure of a west-dipping
fanglomerate (Tiger Formation) of Eocene age that
was shed off of the footwall to the western segment
of the Newport Fault. Clasts consist of Belt Supergroup quartzites, granites, and Eocene volcanics.
Apparently, the Paleozoic rocks were completely
eroded from the rising core complex by the time the
Tiger Formation was deposited in this location.
The Tiger Formation was deposited in an asymmetric 1/2 graben that formed above the listric western
segment of the Newport Fault. As slip progressed,
the hanging wall was progressively rotated to the
west forming a roll-over anticline. The Tiger Formation shows progressively steepening dips from
horizontal on the west to very steep on the east.
This records deposition during movement and rotation along the listric normal fault; i.e., the older
sediments on the east have been rotated the most.
Directly underneath the Tiger Formation lie Eocene
andesite flows and tuffs correlated with the Sanpoil
volcanics. They are the eruptive phase of the Silver
Point Quartz Monzonite. These volcanics and underlying Prichard Formation dip steeply to the west (60
degrees) and demonstrate that the Newport Fault has
a nearly horizontal attitude at depth.
The caves that make up the grottos are relicts of a
Pleistocene glacial lake that formed in front of the
retreating Cordilleran ice sheet. The fetch on the lake
created waves that eroded the grottos into the exposed cliffs of the Tiger Formation. As well, the current northward flow of the Pend Oreille River is due
to stream reversal as isostatic rebound could not keep
pace with melting of the ice sheet and for a time, the
river drainage sloped north rather than south. To date,
the Pend Oreille River has been able to keep pace as
the land surface rebounded to its former southerly
slope.