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Northwest Geological Society

Society Field Trips in Pacific Northwest Geology

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.

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

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) Map showing general location of the Priest River Complex relative to the Spokane Valley

(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.

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.

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.

(Below) Sand Road outcrop with deformed pegmatite dikes

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

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.

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

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

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-

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.

(Above) the Hauser Lake Gneiss at Saltese Flats, south of the Spokane Valley

(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. 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.

DAY 2: (STOPS 5-10) The PRC From Spokane to Priest River, Idaho

5) Stop: Round Mountain

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. 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

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

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.

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.

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


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