Salmon River Suture Zone
The Blue Mountains of eastern Oregon and western Idaho record multiple episodes of magmatism, metamorphism, and deformation. One model proposed to explain these observations involves the collision of a far travelled exotic island arc to a fringing island arc, near the North American continent to form a super-terrane off shore of the North American craton in late Jurassic, then the docking of the super-terrane to the craton in early Cretaceous. The collision of these island arcs to the continent would have created loading due to overthrusting along brittle and ductile faults/shear zones and crustal thickening in the ductile portion of the crust, increasing the depth of underlying rocks resulting regional metamorphism.
The well exposed Salmon River Belt and Salmon River suture zone of west central Idaho may record multiple metamorphic events related to this tectonic assembly. The Salmon River Belt is divided into East and West Salmon River Belts. The Western Salmon River Belt is a low-grade, chloritic schist derived from a volcaniclastic protolith, interpreted by several workers to correlate with the Wallowa Terrane of the Blue Mountain Province. The Eastern Salmon River Belt is separated from the Western Salmon River Belt by the Rapid River Thrust Fault and bounded on the east by the Western Idaho Shear Zone. The Eastern Salmon River Belt can be further subdivided into two structural plates with different tectonic histories: the Rapid River and Pollock Mountain plates (RRp and PMp, respectively). The Rapid River plate is comprised of four units, in order from structurally lowest to highest: Fiddle Creek schist, Lightening Creek schist, Berg Creek amphibolite, and the Squaw Creek schist. Previous studies within the Salmon River belt have produced metamorphic ages with large uncertainties using garnet Sm-Nd and hornblende, muscovite, biotite, potassium feldspar Ar-Ar cooling ages (Snee et al, 1995) (See Supplement: Part A). The previous garnet Sm-Nd geochronology attempted to attain ages from garnet from the PMp that displays a two stage growth history. The core of the garnet records an older metamorphic event, followed by the rim growth during later metamorphism. These garnet Sm-Nd isochron ages record a pre-144 Ma event, followed by a 128 Ma event, with final peak metamorphic equilibrium occurring at 8-9 kbar (9-11 kbar regionally as per Selverstone et al, 1992) and approximately 600° C in the Pollock Mountain plate (Getty et al, 1993). These are interpreted to record collision and amalgamation of multiple island arcs to form the Blue Mountain Province, > 144 Ma, and collision and suture of the Blue Mountain Province to North America, 128 Ma (Getty et al, 1993). Studies of deformation and magmatism conducted in the Blue Mountains of Oregon have constrained the earliest deformational event between 160 and 154 Ma.
|Figure 1- From Aliberti and Manduca, 1988, a cross section of the Salmon River Suture Zone with Sr ratios indicated at the bottom.|
|Figure 2-Google Earth image of stops near McCall, ID|
|Figure 3-Google Earth image of stops near and east of Riggins, ID along US-95 and Salmon River Road.|
|Figure 4- Google Earth image of stops near the town of Pollock. SCs and LCs contact marks the approximate contact between the Squaw Creek and Lightening Creek schists.|
|Figure 5-From Lund et al, 2008.|
STOP 1-1: Precambrian North America: (OPTIONAL) [UTM 11T 573929 4982491]
These roof pendants exposed north of McCall, ID in Ponderosa State Park are part of the Neoproterozoic sedimentary rocks that the Idaho Batholith intruded. Roof pendants are portions of the host rock that have been isolated from the main body of host rocks by erosion, and have typically undergone contact metamorphism. These should be visible along a short portion of East Side Rd when the road reaches the northernmost portion of Payette Lake, where a small river empties out.
Directions from McCall:
1. Head east on E Lake St toward N 3rd St 161 ft
2. Turn left to stay on E Lake St 0.1 mi
3. Take the 2nd left to stay on E Lake St 0.2 mi
4. E Lake St turns slightly right and becomes Hemlock St 0.1 mi
5. Turn left toward Davis Ave 0.4 mi
6. Turn left at Davis Ave 0.1 mi
7. Take the 3rd right onto Lick Creek Rd 2.1 mi
8. Slight left at East Side Rd 3.2 mi
Continue until you reach to north end of Payette Lake, and STOP 1 should be along the road to the east.
STOP 1-2: Idaho Batholith [UTM 11T 573142 4982539]
Following East Side Rd north to Warren Wagon Rd, then turn south. To the west of Warren Wagon Rd is a small portion of the Idaho Batholith. The batholith is a late Cretaceous intrusion due to subduction of the Farallon plate. The batholith is approximately 90 Ma in age. The outcrop we will see here is a deformed biotite granodiorite (I hope). Also exposed near this location is the tonalite of the Payette River Complex, which we will also see along the Salmon River later. CAMPING IN PONDEROSA STATE PARK
Ponderosa State Park to Riggins, ID
9. Head south on East Side Rd 3.2 mi
10. Continue onto Lick Creek Rd/NF-48
Continue to follow Lick Creek Rd 2.1 mi
11. Turn left at Davis Ave 246 ft
12. Take the 1st right onto Ruby St 397 ft
13. Turn left at McCall Ave N 0.5 mi
14. Turn right at Hemlock St 207 ft
15. Slight left at E Lake St 0.2 mi
16. Turn right to stay on E Lake St 0.1 mi
17. Take the 2nd right onto ID-55 N/E Lake St
Continue to follow ID-55 N 12.0 mi
18. Turn right at N Norris Ave/US-95 N
Continue to follow US-95 N 34.6 mi
19. Turn left
Once in Riggins, take Salmon River Road/Nat. For. Development Road 1614 east along the Salmon River.
STOP 2-1: Payette River Complex, along Salmon River Road. Approx 18 miles down Salmon River Road. [UTM 11T 575799 5030471]
Along Salmon River Road at French Creek, the Payette River Complex is exposed. This older phase of the Idaho Batholith is a foliated hornblende-biotite tonalite. The PRC marks the eastern boundary of the Western Idaho Shear Zone, a 20-25 km wide ductile, near vertical shear zone. Mylonitic fabrics are present in rocks affected by this Cretaceous deformational event. The WISZ extends into the Pollock Mountains Amphibolite to the west. (Lund, 2004)
This is the furthest we will go down Salmon River Road. Turn around and proceed back towards Riggins to reach other stops. Drive 5 miles along Salmon River Road west from STOP 2-2.
STOP 2-2: Western Idaho Shear Zone/Little Goose Creek Complex/Pollock Mountains Amphibolite/Kelly Mountain Schist/Looking Glass tonalite/Patridge Creek Gneiss, along Salmon River Road. [UTM 11T 569202 5027847]
This location, near the Salmon River Bridge, is complex. According to geologic mapping by K. Lund, 2004 and W. Hamilton, 1969, this area should expose Little Goose Creek Complex, an early Cretaceous orthogneiss of the Salmon River Suture intrusive Complex, and Pollock Mt. Amphibolite, a high-grade, heterogeneous mafic unit of the Eastern Salmon River Belt metamorphic suite. The Pollock Mt. Amphibolite is particularly interesting because it preserves two stages of mineral growth, which requires two metamorphic events.
STOP 2-3: Pollock Mountains Amphibolite, along Salmon River Road, near Music Bar. [UTM 11T 563292 5028758]
At this location, the Pollock Mountain Amphibolite is exposed as a garnet biotite amphibolite with occasional diopside. Also present in the heterogeneous Pollock Mountain Amphibolite are layers of biotite-poor felsic gneisses, and calc-silicate layers. The interpreted protolith for this unit is a mafic volcanic or volcanic sourced sedimentary rock, with the calc-silicate layers representing carbonate horizons or calcium-rich, supracrustal sedimentary rocks (Blake et al, 2009). The mafic rocks of the Pollock Mountain Amphibolite have been interpreted to be a back-arc basin related to subduction beneath the fringing arc of the North American continent prior to early Cretaceous docking and amalgamation of the Blue Mountains Superterrane.
|Figure 6-Basement map created from Lund, 2004 and Hamilton, 1969.|
STOP 2-4: Lightening Creek Schist, along Salmon River Road at Lake Creek Bridge [UTM 11T 561384 5027587]
The Lightening Creek schist of the RRp is a greenschist probably formed from a lava, tuff, or agglomerate. The Lightening Creek Schist is comprised of a metaconglomerate at its base and a medium to coarse grained garnet mica schist ± staurolite ± kyanite with an increasing metamorphic gradient eastward. The units composition ranges from mafic to silicic, with compositional variation occurring in layers (Hamilton, 1963). To the north of the river the orange-ish unit is the Fiddle Creek schist, which we will see later. We will park south of the bridge for STOP 2-4, then walk across the bridge to see STOP 2-5.
STOP 2-5: Lightening Creek Schist, along Salmon River Road. SAMPLE LOCATION. [UTM 11T 561062 5027769]
This particular location was recommended by D. Blake, UNC-Wilmington as a garnet-kyanite bearing location within the Lightening Creek schist of the RRp. The location is located north of the river and to the west of Lake Creek Bridge, along the dirt road, near where the river bends concavely north. A sample will be collected for garnet geochronology from this location. Anyone who volunteers to carry that sample will get a cookie.
STOP 2-6: Berg Creek Amphibolite, along Salmon River Road [UTM 11T 558580 5029003]
The Berg Creek Amphibolite, which lies between the Lightening Creek Schist and the Squaw Creek Schist is an epidote-amphibolite containing hornblende, zoisite, and garnet porphyroblasts in a fine, feldspar dominated groundmass. The whole rock geochemistry of this rock suggests basalt to basaltic-andesite composition of its protolith (Hamilton, 1963). Carbonate layers are also present throughout the unit. Notice the lineation trend of the hornblende and zoisite crystals.
STOP 2-7: Squaw Creek Schist, along Salmon River Road, near Riggins Rodeo. [UTM 11T 552664 5028802]
A phyllite to schist and the highest structural unit within the Rapid River plate, the Squaw Creek schist is of intermediate composition and widely heterogeneous, and is locally exposed as a gray phyllite, biotite schist, and a hornblende schist. It is less commonly exposed as an amphibolite, gneiss, or even marble unit. This unit has been interpreted to be a volcanic sourced sedimentary unit due to the presence of abundant interbeds. Metaperidotite pods are present near the boundary between the Squaw Creek and Lightening Creek schist to the north of Riggins. As we drive towards STOP 2-8, these pods may be visible along the road cut.
STOP 2-8: Fiddle Creek/Lightening Creek Schist contact, north of Riggins, along US-95. (OPTIONAL) [UTM 11T 553497 5037037]
The lowest structural unit in the Rapid River plate, the Fiddle Creek schist exposed north of Riggins is a mica schist with sparse garnet, hornblende, epidote, and carbonate. Like the other rocks of the Rapid River plate, the rocks are either of volcanic or volcanic sourced sedimentary origin, with intermediate to silicic composition.
STOP 2-9: Heavens Gate Lookout (a) and Heavens Gate Fault (b), west of US-95 (Conditions permitting) [a.) UTM 11T 539563 5023997 b.) UTM 11T 538196 5021828]
If snow cover does not make to road impassible, we will drive up to the Forest Ranger Lookout on Heaven’s Gate. Exposed beneath us is greenschist grade meta-volcanic breccia. The breccia clasts are clearly visible throughout the area. This has been interpreted by many as part of the Wallowa island arc terrane of the Blue Mountains Province.
STOP 2-10: Pollock Mountain along US-95, south of Riggins, ID. (OPTIONAL/SAMPLE LOCATION) [a.) UTM 11T 551307 5014333 b.) UTM 11T 552881 5009302]
This exposure of the Pollock Mountains Amphibolite shows the heterogeneity of the unit. Anywhere along US-95, (between 2-10a and 2-10b) south or Riggins where this unit is mapped is a suitable stop location.
STOP 2-11: Rapid River Thrust System/Martin Bridge Limestone (WEATHER STOP) [UTM 11T 547071 5022131]
This replacement stop is along Rapid River Road south of Riggins, 2.3 miles to fish hatchery and 0.25 miles up the Rapid River trail is the basal exposure of the Rapid River thrust with Squaw Creek schist thrust over the Martin Bridge limestone. A shallow cave in blue marble should be present which contains calcite stretching lineations, which indicate fault motion.
|1-2a: Brittany Hollon examining the outcrop.|
|1-2b: Close up of granodiorite outcrop showing hornblende , biotite, quartz, and plagioclase feldspar. Magmatic epidote in this rock provides a minimum pressure for emplacement.|
|2-1a: Steeply-dipping layers in orthogneiss.|
|2-1b: Ptygmatic folding present in the outcrop. Fractures in the outcrop are filled in by quartz.|
|2-2a: Tonalite contianing large amphibole crystals with sphene? overgrowths.|
|2-2b: Large pegmatitic dikes intruding the Payette River Complex. The dikes contain plagioclase, k-feldspar and biotite.|
|2-3: Bridge over the Salmon River (facing west toward Riggins). The area this picture was taken in contains sillimanite suggesting high temperature mineral growth.|
|2-4a: Small scale boudin train folding of plagioclase feldspar and amphibole layers.|
|2-4b: ~2cm amplitude crenulations of quartz, plagioclase, and amphibole.|
|2-4c: Map scale asymmetrical folding of calc-silicate layers. The outcrop also contains some second stage garnet growth.|
Page and text created by Matt McKay. Photos thanks to the 2010 Field Trip Crew. Picture captions by Ryan Alexander and Matt McKay. Please email questions to Matt.
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