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THE ALBION-RAFT RIVER-GROUSE CREEK METAMORPHIC CORE COMPLEX: GEOLOGIC SETTING AND FIELD TRIP GUIDE

Elizabeth Miller, Ariel Strickland and Alex Konstantinou, Dept. Geological and Environmental Sciences, Stanford University

(Revised from: Elizabeth Miller and Ariel Strickland, 2007 Stanford Field Trip to ARG. Caution: Not necessarily accurate or up-to-date!!!!)

Introduction

The Albion-Raft River-Grouse Creek metamorphic core complex (ARG) is part of a chain of Cordilleran metamorphic core complexes that lie to the west of the Sevier fold- and-thrust belt (Fig. 1). The ARG is made up of three mountain ranges for which it is named (Fig. 2). The Albion Mountains and Grouse Creek Mountains are characteristic of mountain ranges in the Basin and Range Province; they have a generally north-south orientation and rise abruptly from the surrounding relatively flat topography. The Raft River Mountains have an enigmatic east-west trend and project eastward from the northern end of the Grouse Creek Mountains (Fig. 2). Late Archean crystalline basement is exposed in all three mountain ranges indicating a broad dome or structural culmination beneath this region, where probably 10-15 km minimum of relative vertical uplift (compared to surrounding regions) has occurred. In addition, significant thinning of the overlying upper part of the crust characterizes the geology of this core complex. Eocene and Oligocene granitic plutons are exposed in these ranges and based on geologic relations and cross-sections, most likely underlie much of the ARG. The ARG is unique in the Basin and Range because it is bound on both of its sides (east and west) by normal fault systems and their associated basins. In addition, earlier, shallowly-dipping high strain fabrics and mylonitic zones characterize most of the ARG. This field trip will provide a brief look at the structural style of thinning in the complex and at the relationship between plutonism, metamorphism, faulting and basin formation leading to the development of this core complex. 2

Crustal Section in the ARG core complex

The rocks exposed in the ARG complex (Fig. 3 and 3A) include (moving upward structurally and stratigraphically): ca. 2.5 Ga augen gneiss with associated minor schist and amphibolite; Late Proterozoic metasedimentary and metaigneous rocks; Paleozoic carbonate and lesser quartzite of the North American Paleozoic miogeoclinal (passive margin or shelf) succession; a thin section of Triassic sedimentary rocks; and Miocene fanglomerate, lacustrine sediments, and intercalated Miocene volcanic rocks (Fig. 4). On the perimeter of the complex to the west, east, and north are rhyolite lavas and ignimbrites and lesser basalts of the Snake River Plain suite. Oligocene granitic plutons intruded the core complex and are variably deformed. Faults which omit portions of the Paleozoic shelf section are ubiquitous throughout the ARG. Though many of the fault contacts are now subhorizontal and are themselves deformed and metamorphosed, they almost invariably place younger rocks atop older ones and are referred to as normal faults. These faults are defined based on comparison with more complete stratigraphic sections in surrounding areas but, in general, their exact nature is uncertain due to the fact that they are now mostly transposed and folded by superimposed deformation.

Precambrian to mid Paleozoic strata are everywhere deformed and variably metamorphosed. The metamorphic rocks in the lower plate of the ARG display a conspicuous sub-horizontal, penetrative foliation (S2) that represents a high degree of strain during extensional deformation (Armstrong, 1968; Wells, 1997; Egger et al., 2003; Strickland et al., 2011 a,b). Across much of the ARG, this dominant foliation developed under greenschist facies conditions and locally overprints relict fabrics that developed under amphibolite-facies conditions (S1). In pelitic rocks, staurolite, garnet and biotite that grew during the earlier metamorphism are retrogressed and enclosed by the tectonically aligned chlorite and fine-grained white micas of S2 (Wells, 1997; Egger et al., 2003). However, in the vicinity of the Eocene-Oligocene plutons, S2 developed under amphibolite facies conditions (staurolite and sillimanite grade), and evidence for earlier metamorphism and deformation is largely obliterated (Todd, 1980; Egger et al., 2003; 3

Strickland et al., 2011 a,b)). Thus peak metamorphic temperatures across much of the complex were obtained in the Cenozoic.

Core Complex Development

Superimposed on the early (and not well-understood) history of the ARG are varied episodes of Cenozoic extension-related deformation and magmatism. The ARG lies directly south of the Snake River Plain, where volcanic rocks include a 14-8 Ma bimodal suite which mostly coincides with normal faulting in the ARG, which is now well-dated on the east side of the range and occurred during deposition of a ~ 13.5- 8 Ma Miocene sedimentary and volcanic sequence (Konstantinou, in press) (Fig. 4). Compton (1983) reported basalt (14.4 ± 0.4 Ma) and rhyolite (11.7 ± 0.4 Ma) interbedded with Cenozoic fanglomerate and finer lacustrine sediments on the western side of the Grouse Creek Mountains that are in-faulted with Permian and Triassic rocks. These ages and relationships show that significant faulting occurred in the Miocene and was active in the time interval 14.4-11.7 Ma in the southern Grouse Creek Mountains. In the areas flanking the ARG, younger rhyolites (7-9 Ma ; Williams et al., 1976) exhibit only gentle dips, indicating that they have not been significantly tilted or displaced by Basin and Range-style faulting (Stanford Geological Survey, unpublished mapping, 1991).

The older Miocene volcanic rocks described by Compton occur mainly along the western side of the Grouse Creek Mountains where, as described above, they are infaulted with Paleozoic and Mesozoic sedimentary rocks and with abundant tuffs and fanglomerates dated as late Miocene (16-12 Ma) by lacustrine mollusc, ostracod, and water rush assemblages (Compton, 1983) and mammal fauna (McClelland, 1976). These Miocene sedimentary rocks are interpreted to have been shed from an unusually steep uplift due to the presence of “thick debris-flow deposits and monolithologic breccias” (Compton, 1983). The age of these syntectonic conglomerates appears to constrain the time of motion on an extensive system of down-to-the-west faults that down-drop these strata with respect to uplifted metamorphic and plutonic rocks in the adjacent mountains. Though not extensively exposed, this fault system appears to be continuous for 4 approximately 50 km along the western side of the ranges and is referred to as the Grouse Creek fault system.

Apatite fission track studies (Tc ~ 100 °C) from all three mountain ranges have yielded middle Miocene ages. In the Raft River Mountains, the apatite cooling ages range from 7 ± 1 Ma to 11 ± 1 Ma, and the ages young towards the Raft River detachment on the eastern edge of the range (Wells et al., 2000). In the Albion Mountains, apatite fission track ages range from 9 ± 3 to 17 ± 4 Ma, and in the Grouse Creek Mountains the ages range from 12 ± 3 to 15 ± 4 Ma, younging towards the Grouse Creek Fault (Egger et al., 2003). In the Raft River Mountains, the 40Ar/39Ar cooling ages of biotite and muscovite

(Tc of ~300 ºC and ~350 ºC, respectively) suggest a similar trend of younging towards the detachment fault (Wells et al., 2000).

Rocks in the footwall of these two bounding normal fault systems appear to record an episodic or continuous history of Cenozoic deformation and magmatism that predates their uplift by these fault systems. Crosscutting relationships between plutons and metamorphic fabrics help constrain the relative and absolute ages of events. The Red Butte Stocks in the southern Grouse Creek Mountains yielded a six-point Rb-Sr whole rock isochron of 24.9±0.6 Ma (Compton et al., 1977), and a U-Pb (zircon) SHRIMP-RG age of 25.3 ± 0.5 Ma (Egger et al., 2003). The Red Butte intrusions post-date an earlier period of metamorphism and deformation (D1) and clearly cut a strong subhorizontal fabric and N-S or NNE-SSW trending mineral elongation or stretching lineation (L1). Dikes and peripheral contacts of the Red Butte intrusion are themselves clearly involved in a second superimposed event (D2) characterized by a N50W-N90W stretching lineation developed throughout the ARG (L2) (Compton et al., 1977). Also, in the southern Grouse Creek Mountains, three phases of the Emigrant Pass Pluton were dated by the U-Pb SHRIMP-RG (zircon) method (Egger et al., 2003). The oldest phase is 41.3 ± 0.3 Ma and crosscuts a series of metamorphosed normal faults (Egger et al., 2003). The two younger phases (36.1 ± 0.2 Ma, and 34.3 ± 0.3 Ma) were interpreted by Egger et al. (2003) as have been intruded during development of a map-scale, N-S–trending recumbent fold that formed during vertical shortening. 5

In the northern Grouse Creek Mountains, the two mica (± garnet) Vipoint pluton intruded Archean augen gneiss and Neoproterozoic metapelitic schists and quartzites (Strickland et al., 2007, 2011). The Vipoint has a crystallization age of 29.1 ± 0.2 Ma. The whole package of rocks was deformed under amphibolite facies conditions (sillimanite was stable). The Vipoint pluton now has a prominent, shallow, WNW- plunging mineral lineation (L2) defined by aligned micas and a penetrative, west-dipping foliation that is conformable with the foliation measured in the Precambrian rocks. Sphene from the Vipoint pluton produced a U-Pb age of 20.3 ± 2.3 Ma and crystallized at a temperature of ~600 ºC based on Zr concentration thermometry (Hayden et al., 2006) (Strickland, 2010). This suggests that high-T conditions may have persisted long after the intrusion of the pluton, thus indicating slow cooling of the pluton.

In the Albion Mountains, most of the Almo pluton is an undeformed, medium- grained, biotite-muscovite-garnet bearing granite that is home to the City of Rocks National Preserve. Muscovite and garnet-bearing pegmatites in country rocks are common, both along S2 foliation planes and crosscutting all structural elements. Along its western margin, the Almo pluton intrudes a well-foliated hornblende-biotite bearing granodiorite that contains a shallow, W/NW-plunging mineral lineation. The undeformed and deformed phases of the Almo granite produced a U-Pb (zircon) SHRIMP-RG ages of 30.0 ± 0.5 Ma and 29.8 ± 0.5 Ma, respectively (Strickland et al., 20101), placing tight constrains on the timing of deformation. Similarly, zircons from a boundinaged pegmatite from the Almo pluton produced an age of 29.3 ± 3.2 Ma. These ages are overlapping within error, and support the interpretation that the Almo pluton was intruded during extensional fabric development (Miller et al., 2002).

Contact metamorphic minerals which grew in the aureoles of the Almo, Vipoint and Red Butte intrusions are aligned with and grew parallel to L2 lineation (Compton et al., 1977; Egger et al., 2003, Strickland 2010; Strickland et al. 2011a,b). Assemblages in metapelites include: garnet + staurolite + kyanite (locally sillimanite and andalusite), indicating temperature and pressure conditions of at least 550°C and ~4 kb. This pressure corresponds to a depth of at least 14 km for country rocks at the time of intrusion. This fabric appears to increase in its degree of development westward toward 6 the bounding fault contact with Miocene sedimentary rocks. Lower temperature mylonites are developed in the westernmost part of the complex beneath the bounding fault and their sense of shear indicators reveal top-to-the-west displacement (Saltzer and Hodges, 1988). The normal fault system along the western side of the range also moved in the brittle regime as evidenced by breccias, gouge, and slickensides along fault surfaces.

There are no cross-cutting intrusive relationships which could be used to date deformational structures in the Raft River Mountains but previous workers agree that metamorphic rocks across most of the eastern part of the range were deformed by top-to- the-east shear during latest E-W extension (Compton et al., 1977; Compton, 1980; Malavieille, 1987a,b; Wells et al., 1990; 2000). The temporal relationship of this event to top-to-the west structures developed further west in the Grouse Creek and Albion Mountains is unknown, though the two may be largely contemporaneous.

Because Tertiary metamorphism and deformation are so pervasive in the ARG, older structures and fabrics have been difficult to decipher and the earlier history of this region remains much more enigmatic. 40Ar/39Ar cooling ages and K-Ar dates from the Albion, Raft River and Black Pine Mountains (east of the Raft River Mountains) have led some to suggest that a major cooling event or uplift occurred in the Cretaceous (Wells, et al. 1990; Armstrong and Hills, 1967; Wells et al., 2008, 2012). Overall, it seems that Tertiary deformation, magmatism, and metamorphism have obscured earlier (pre-crustal thinning) events beyond clear recognition.

Possible Field Trip Stops in the ARG Metamorphic Core Complex

Stop 1: Clear Creek Canyon

Clear Creek Canyon is a large canyon that cuts deeply into the NE corner of the Raft River Mountains. It offers one of the largest exposures of the Archean augen gneiss in the ARG core complex, as well as some of the less common country rocks into which the gneiss originally intruded. These include metamorphosed trondhjemite, hornblendite, amphibolite, and quartz-feldspar-mica schist (Compton, 1975). The augen gneiss is 7 easily recognized by its large (several cm), pink potassium feldspar crystals. Lying unconformably above the Archean rocks is the Elba quartzite. The sedimentary (unconformity) contact between the basement rocks and the Elba quartzite is preserved almost everywhere throughout the ARG. At the contact between gneiss and the Elba Quartzite there is commonly a strip of micaceous rock which may be the remnant of an ancient soil layer that developed atop the gneiss when it was first eroded and was exposed in the Proterozoic. The age of the overlying metasedimentary rocks is not well known, due to lack of fossils and high grade metamorphicm, however recent studies based on Carbon isotopes and detrital zircons infer there metasedimentary rocks to be late Proterozoic. North of here, in the headwall of the Independence Lakes cirque, the contact between gneiss and quartzite demonstrates pronounced relief (approximately 30-65 meters) of the paleo-erosion surface.

Here we will take a short hike up to the contact and examine a flamboyant stretched pebble conglomerate (Fig. 4) that occurs locally at the base of the Elba. A small campground here will hopefully provide us with a home for the night.

Stop 2: Upper Narrows Canyon

The rocks exposed here form the type section for the Schist of the Upper Narrows (Compton, 1972; Compton et al., 1977), a psammitic muscovite-biotite-quartz schist with garnet-rich horizons that lies stratigraphically above the Elba quartzite. The schist is migmatitic, with quartzofeldspathic leucosomes and mica-rich melanosomes that define a penetrative fabric, indicating that partial melting occurred before or during deformation. A second stage of deformation crenulated the micaceous layers and folded the leucosomes, began perhaps at amphibolite facies and continued into the greenschist facies (chloritoid and muscovite) (Fig. 6). The age of these two metamorphisms are not known. Near the west end of the canyon, a ~2 meter thick layer of chlorite-white mica-garnet schist is easily accessible. The garnets here are several centimeters in size, and they have very unusual sigmoidal shapes. In thin section (Fig. 6), the garnets contain S-shaped inclusion trails that are continuous with the external foliation, indicating that they are syn-D2. Rare sillimanite is also found in thin sections, growing together with the garnet, 8 but mostly retrogressed, with continued deformation occurring under decreasing T conditions.

The dominant fabric in these rocks (S2?) includes fold axes oriented N-S to N20W and very few extension lineations oriented N50W to E-W. Structures in these rocks do not clearly reflect the strong west-vergent shear which unroofed this part of the complex, suggesting that this shear, if present, must have been localized at a higher structural level than these exposures. We will see examples of the unroofing-related deformation at later stops.

Stop 3: Dove Creek Pass

At the extreme eastern end of the Raft River Mountains, Dove Creek Pass offers an opportunity to examine several more of the metamorphic rocks that are exposed throughout the ARG. Although their original thicknesses are unknown, here they have been significantly thinned, and the rocks display a nearly horizontal foliation (D2) with a west-trending, gently-plunging lineation (L2). Shear sense is top-to-the-west (extensional). We will examine 7 units (Fig. 3). Geologic map and cross-section Fig. 7. Thin section photomicrographs Fig. 8.

• The Schist of Stevens Springs, which is typically a slope-forming unit that rarely crops out. Extensional crenulation cleavages are characteristically well- developed with millimeter to centimeter scale offset of foliation, such that the schist often breaks up into small lenses. Small porphyroblasts of garnet and staurolite can be found occasionally.

• Above the Schist of Stevens Springs is the Quartzite of Clarks Basin, which is a mica-rich quartzite with regular partings of six to nine inches. It is heavily quarried in this area for decorative flagstone known as, “Oakley stone.”

• Next we will encounter the Schist of Mahogany Peaks, a well-foliated mica schist with abundant, large garnet and staurolite porphyroblasts. The garnets and staurolites have been extensively retrogressed to chlorite and white mica

(respectively) that define the dominant foliation in these rocks (D2). These rocks 9

most clearly evidence two metamorphic and deformational events, with the younger one at lower T conditions than the first.

• The Pogonip Group lies stratigraphically above the Schist of Mahogany Peaks, and consists of marble, schist and rare quartzite (Compton, 1975). This unit forms dip slopes, and crops out as a ledgy slope former with small cliffs. The calc-silicate bearing horizons contain zoisite, vesuvianite, and large books of phlogopitic biotite.

• At the top of the hill we will find the Eureka Quartzite (a massive, pure, white quartzite) which grades into the Fish Haven Dolomite. The foliation in the dolomite varies from a well-defined planar lamination to a slabby jointing visible on a decimeter to meter scale. In many places the Fish Haven is brecciated, containing chunks of foliated dolomite.

• On the west side of this hill, the metamorphic rocks are truncated by a brittle fault and are in contact with the relatively unmetamorphosed Oquirrh limestone. At the contact is a zone of breccia several meters wide. The Oquirrh contains abundant fossils including crinoids, fusilinids, and bryozoans (Compton, 1975).

Optional stop: the town of Almo: Basement Error! Bookmark not defined.

Almo is a tiny (pop.<100) ranching community and is the closest source of food, gasoline, and telephone to City of Rocks. Almo’s claim to fame is that it was the site of a massacre of 300 settlers by Native Americans (of the Bannock tribe) in the 19th century. Revisionist historians have recently concluded, however, that the massacre did not actually take place; Almo’s historical status was dealt a crushing blow by this finding.

From this point we can look north to the high parts of the Albion Range, a series of domal uplifts including Mount Harrison (“Big Bertha Dome” of D. Miller, 1980) and the Independence Lakes Dome, which forms the high peak visible directly to the north. Cache Peak, the highest point of this dome has elevation 3151 m (10,339 feet). 10

To the west we can see City of Rocks for the first time. The City of Rocks Dome is topographically lower than the more northerly ones and is cored by exposed Oligocene (29 Ma) granite and granodiorite of the Almo Pluton. The pluton has weathered to form striking walls and spires which were a major landmark for settlers traveling west to populate California and Oregon in the 19th century. The travelers called the area, “The City of the Rocks,” and the name has remained, slightly shortened. Today, City of Rocks is a world-renowned destination for rock climbers. The geologic map for this area is shown in Figure 9.

On the ridges to the northwest of our position the younger Almo Pluton and Archean augen gneiss are in intrusive contact. It may be difficult to tell the two apart from this distance. The older gneiss has a darker appearance, with a slightly “dirty” orange-brown hue compared to the younger granite which is whiter. Look for dikes of the younger rock intruding the older one. To the east we can see the low-lying volcanic rocks of the Jim Sage Mountains. These slightly-tilted flows are mostly rhyolite (up to 1 km in cumulative thickness) and minor basalt (>100 m) associated with the Snake River Plain hotspot and are all Miocene in age (Fig. 4).

Stop 4: Almo Pluton at Twin Sisters

The Twin Sisters provide a striking example of the two granitic rocks that make up the City of Rocks. Although they appear to be twins, one sister is approximately 100 times older than the other! The more southerly sister is the latest Archean augen gneiss, while the one to the north is part of the Oligocene Almo granite. Dikes of the younger granite are visible in the older gneiss. A short walk east from the road to the nearby hills provides an opportunity to examine some very old rocks, the country rock intruded by the ca. 2.5 Ga gneiss. The majority of rock here consists of biotite and garnet-rich schists. Some amphibolite float may be found but is not evident in outcrop. This will be our campsite for tonight and tomorrow night.

Stop 5: Contact aureole of the Almo granite 11

We will drive west through City of Rocks, likely passing many groups of rock climbers. As the vehicles climb the hill out of the “City,” we will pass a hand-operated water pump which is the only source of fresh drinking water in the park.

We will drive past exposures of the Almo Pluton along a narrow dirt road that leads to the National Forest. Finger Rock is the name given to the spire-like outcrop at the sharp turn in the road. U-Pb dating of monazite from this outcrop yielded an age of 29 Ma (Jim Wright, unpublished data). We are now on the western edge of both the Albion Mountains and the Almo Pluton (Fig. 9). This stop examines the actual contact of the Almo granite with its roof rocks and allows us to look at the relationship of metamorphism to deformation at the contact.

We will stop near the National Forest boundary for a short hike up the hill. At this location, the Schist of Stevens Spring has been metamorphosed to upper amphibolite facies by the underlying intrusion of the Almo Pluton. The metamorphic assemblage present here is muscovite + biotite + garnet + staurolite + kyanite, indicating P-T conditions of approximately 550°C and 4 kb. This locality provides clear evidence that the second metamorphic/deformational event (D2) resulted in high grade assemblages and thus intrusion of the plutons occurred at comparable depths. The structural section now present above the high grade rocks of the ARG is not sufficient to account for the burial pressure indicated by the metamorphic assemblage present here, so significant thinning/uplift by faults must have occurred after intrusion of the 30 Ma Almo Pluton, not before as argued by Wells et al. (2012).

Looking to the west across Birch Creek Valley from our vantage point we see Middle Mountain, which is made up of the Precambrian rocks we have seen previously (Archean gneiss, Elba Quartzite, and Schist of the Upper Narrows) along with the Quartzite of Clarks Basin in low angle fault contact above them. The entire section on Middle Mountain has been extensively intruded by granite and deformed by top-to-the- west simple shear. Middle Mountain forms the northern portion of an extensive, 50 km long shear zone, the Middle Mountain shear zone, (top-to-the-west) which runs the length of the core complex on its west side. Extensive thinning and deformation of strata in the 12 shear zone resulted in an economic boon for the small town of Oakley; numerous quarries on Middle Mountain supply flaggy quartzite (seen on many homes and buildings in the area) which is sold as a popular decorative building material locally and elsewhere in the United States.

Previous workers, including Armstrong (1968) and Saltzer and Hodges (1988), interpreted the intrusive events of Middle Mountain and of the Almo Pluton to be temporally distinct based on the difference in texture of granitic rocks in the two areas. Recently-obtained U-Pb SHRIMP-RG analyses of zircons indicate that this is not the case. The hypidiomorphic, granular, muscovite and biotite granite of the Almo Pluton and the strongly deformed biotite granite of the Middle Mountain injection complex yielded 30.5 ± 1 Ma and 31.1 ± 0.8 Ma U-Pb zircon ages, respectively (Strickland et al., 2011a,b; Strickland, 2010). We interpret them, then, to be parts of the same intrusive complex which are separated now by younger high-angle faulting and basin development in Birch Creek Valley. The range-bounding fault responsible for this separation is covered by alluvial fan deposits and is not active today (see cross-sections, Fig. 9).

Stop 6: Evidence for Cretaceous deformation

As we drive north along the Forest Service road we will leave the contact aureole of the Almo granite as well as the high strain of the Middle Mountain shear zone. This is one of the best places to observe evidence for older deformational and metamorphic events (D1). Rocks here are typically albite-chlorite grade, but we will stop to examine an anomalous banded gneiss (migmatite?). Zircons from this outcrop were analyzed for U- Pb using the SHRIMP-RG and yielded a mixed age: intercepts at 84 ± 7 and 2524 ± 24 Ma (Strickland, unpublished data). One analysis was concordant at the lower intercept age, indicating zircon growth in the Cretaceous as opposed to Pb loss.

After a few measurements and sample collecting, we will drive a couple more miles to the end of the road. Here we have an opportunity see the much thicker and relatively undeformed Elba quartzite. We can hike up to the top of Cache Peak, which at 10,339 feet has the distinction of being the tallest peak in Idaho - south of the Snake River Plain. From here we can look down into one of two glacial cirques in Idaho - south 13 of the Snake River Plain - (the other being on Mt. Harrison just to the north), and we can look down onto City of Rocks. This hike is approximately three miles (one way) with an elevation gain of ~1600 feet. Return to the Twin Sisters for camping this night.

REFERENCES

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Armstrong, R.L., 1982, Cordilleran metamorphic core complexes - from Arizona to southern Canada: Canadian Annual Review of Earth and Planet Sciences, v. 10, p. 129-154.

Armstrong, R.L. and Hills, F.A., 1967, Rb-Sr and K-Ar geochronologic studies of mantled gneiss domes, southern Idaho: Earth and Planetary Science Letters, v.3, p. 114- 124.

Compton, R.R., 1972, Geologic map of the Yost Quadrangle, Box Elder County, Utah, and Cassia County, Idaho; Miscellaneous Geologic Investigations Map I-672: U.S. Geological Survey.

Compton, R.R., 1975, Geologic map of the Park Valley Quadrangle, Box Elder County, Utah, and Cassia County, Idaho; Miscellaneous Geologic Investigations Map I- 873: U.S. Geological Survey.

Compton, R.R., 1980, Fabrics and strains in quartzites of a metamorphic core complex, Raft River Mountains, Utah, in Crittendon, M.D. at al., eds., Cordilleran Metamorphic Core Complexes: Geological Society of America Memoir 153, p. 385-398.

Compton, R.R., 1983, Displaced Miocene rocks on the west flank of the Raft River-Grouse Creek core complex, Utah: Geological Society of America Memoir 157, p. 271-279.

Compton, R.R., Todd, V.R., Zartman, R.E., and Naeser, C.W., 1977, Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah: Geological Society of America Bulletin, v. 88, p. 1237-1250.

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Miller, D.M., 1980, Structural geology of the Albion Mountains, Idaho, in Crittendon, M.D. at al., eds., Cordilleran Metamorphic Core Complexes: Geological Society of America Memoir 153, p. 399-426.

Saltzer, S.D., and Hodges, K.V., 1988, The Middle Mountain shear zone, southern Idaho: kinematic analysis of an early tertiary high-temperature detachment: Geological Society of America Bulletin, v. 100, p. 96-103.

Strickland, A., Miller, E. L., Wooden, J., 2007, Timing of ductile deformation in the Albion-Raft River-Grouse Creek (ARG) metamorphic core complex: preliminary U- Pb SHRIMP results: GSA Cordilleran Section - 103rd Annual Meeting.

Strickland A. 2010. Polyphase deformation and metamorphism: A geochronologic perspective. Ph.D. Thesis, Stanford University.

Strickland, A., Miller, E.L. and Wooden, J., 2011a, The timing of Tertiary metamorphism and deformation in the Albion-Raft River-Grouse Creek metamorphic core complex, Utah and Idaho, Journal of Geology, v. 119, no.2, p. 185-206.

Strickland, A., Miller, E. L., Wooden, J. , Kozdon, R., and Valley, J. W. 2011b. Syn-extensional plutonism and peak metamorphism in the Albion-Raft River-Grouse Creek metamorphic core complex. American Journal of Science, 311(4), 261-261-314.

Todd, V.R., 1980, Structure and petrology of a Tertiary gneiss complex in norhtwestern Utah, in Crittendon, M.D. et al., eds., Cordilleran Metamorphic Core Complexes: Geological Society of America Memoir 153, p. 349-384. 15

Wells, M. L., 1997, Alternating contraction and extension in the hinterlands of orogenic belts: An example from the Raft River Mountains, Utah: Geological Society of America Bulletin v. 109, p.107-126.

Wells, M.A., Dallmeyer, R.D., and Allmendinger, R.W., 1990, Late Cretaceous extension in the hinterland of the Sevier thrust belt, northwestern Utah and southern Idaho: Geology, v. 18 p. 929-933.

Wells, M.L., Hoisch, T.D., Peters, M.T., Miller, D.M., Wolff, E.D. and Hanson, M.L., 1998, The Mahogany Peaks fault, a Late Cretaceous-Paleocene normal fault in the hinterland of the Sevier Orogen: Journal of Geology, v. 106, p. 623-634.

Wells, M.L., Snee, L.W., and Blythe, A.E., 2000, Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States: Journal of Geophysical Research, v. 105, p. 16,303- 16,327.

Wells, M. L., Hoisch T.D., Cruz-Uribe A.M., and Vervoort J.D. 2012. Geodynamics of synconvergent extension and tectonic mode switching: Constraints from the Sevier-Laramide orogen. Tectonics 31 (1) (2012).

Williams, P.L., Mabey, D.R., Zohdy, A.A.R., Ackerman, H., Hoover, D.B., Pierce, K.L., and Oriel, S.S., 1976, Geology and geophysics of the southern Raft River Valley geothermal area, Idaho, U.S.A., in Proceedings, United Nations Symposium on the Development and Use of Geothermal Resources, 2d, San Francisco, California, May 20-29, 1975, v. 2, p. 1273-1282. Figure1 Click here to download Figure: Fig. 1_final.pdf Volcanic rocks <29.5 Ma Age propagation of

125°W 120°W Cenozoic magmatism Colville Igneous 12-2 Ma Metamorphic complex 16-12 Ma core complex Shuswap 58-45 Ma GPS vel. vector 55-45 Ma length = 3 mm/yr Eocene-present Okanogan Cascade magmatism Stacked relative probability Mesozoic-Paleocene diagrams of timing of Cenozoic arc-batholith magmatism at six lattitude intervlas Cascade N magmatism Priest 45°N River Latt. 50-46o o

Bitterroot Absaroka Volcanics 46-43o o 55-44 Ma Lattitude 43-41 Columbia River Idaho ChallisChal Volcanics 45°N Basalts BatholithB 52-45 Ma

16.6- 15.6 Ma Pioneer 2.0-0.6 Ma 6.6-4.4 Ma

16.6- OR ID 12.7- 10.5- Snake River Plain 15 Ma 10.2-9.2 Ma 10.5 Ma 8.5 Ma 40°N 14.5- o o 43-41 Relative probability 12.8 Ma NV ARG <43.4 Ma Laramide, Rocky Mnts Ruby Sevier Basin NV UT thrust belt Approximate and Range 41-39o o eastern limit of Mz. batholith <36.0 Ma 40°N

<29.5 Ma

Snake o o Colorado 39-37 Range <23.2 Ma plateau S 0 10203040506070 Sierra Nevada Funeral <17.0 Ma Age of igneous rocks (Ma) 35°N Mnts o o Figure 1b 110°W 37-33 0100 200 400 Figure 1a Km 120°W 250 km

S C o N r S d i l l e

r a N n B f o l d Cotterel Mts

a

0.706 n

d

line

t

h r

SRP u

s 20 Km

t

b 

ARG e Albion Mts

l t R  Figure 8 20o   Jim Sage Mts Middle Figure 6 A Mnt. gneiss Raft River Valley

Middle Mt.  extend of 32-29 Ma 30o Seismic Reflection data Black batholith? A’ Almo pluton Pine Mts

 IDAHO not mapped

UTAH     

   Vipoint   pluton     o  15

 AH

 T U Raft River Mts

      



Red Butte <8.2 Ma Cotterel Basalt pluton 8.2 Ma Upper Jim Sage Lavas Grouse Creek Mts Cenozoic Basin Middle Tuffs of Jim Sage Figure 7  9.5 Ma Lower Jim Sage Lavas

Tertiary sedimentary units  42-33 Ma Cenozoic Plutons

 32 - 25 Ma Cenozoic Plutons

Emigrant Phanerozoic metasedimentary units Detachment fault, Pass plutonic  teeth on the upper Metamorphic lower plate rocks complex plate Note: Nanny Creek Archean Crystaline Basement volcanics exposed to the High-angle normal southwest of map area fault, ball on the Middle Mountain shear zone down thrown side Figure 3 Raft River Detachment Age Formation name

ORDOVICIAN POGONIP GROUP (GARDEN CITY FORMATION) (± EUREKA QUARTZITE)

Salt Lake Fm. SCHIST OF MOHOGANY PEAKS

QUARTZITE OF CLARKS BASIN

SCHIST OF STEVENS SPRINGS

QUARTZITE OF YOST 250 SCHIST OF THE UPPER NARROWS 0 m Thaynes Fm.

U. P Gerster Fm.

Unnamed unit ELBA QUARTZITE

M. P Unnamed unit

L. P

Oquirrh Group

1000

0 m Chainman Fm. Metasiltstone M Diamond Pk. Fm. Dolomite Eureka Quartzite O Garden City Fm. Calcarenite and limestone Schist and quartzite STRATIGRAPHY Green Creek A complex Orthogneiss and Schist ALBION-RAFT RIVER-GROUSE CREEK MOUNTAINS

93 10a Unit 7 10b Unit 7 (<8.2 Ma) Mean = 8.19 ± 0.14 (1.7%) 15.0 Lacustrine sequecne Lacustrine tuff with 14.0 95% conf. 14 data points lava clasts 13.0 Unit 6 (8.2 Ma) MSWD = 1.4 12.0 Upper rhyolite and basalt HUURUV\PEROVDUHı Basalt flows 11.0 Unit 5 (9.33-8.2 Ma) 10.0 Tuffs and breccias 3.0 km 9.0 8.0 Age (Ma) Unit 4 (9.46-9.33 Ma) Unit 6 7.0 Lower rhyolite Rhyolitic lavas and Rhyolite lava (n=14) 6.0 ignimbrite 5.0 Capped by basalt flows 15.0 Unit 3 (9.7-9.46 Ma) Mean = 9.33 ± 0.1 (1.2%) 14.0 Lacustrine sequence Unit 5 95% conf. 16 data points 13.0 MSWD = 0.86 Tuff with lava clasts 12.0 HUURUV\PEROVDUHı 11.0 10.0 2.5 km 9.0 Unit 2 (10.5-9.7 Ma)

Unit 4 8.0 Age (Ma) Fluvial sequence Rhyolitic lavas and 7.0 ignimbrite Rhyolite lava (n=16) 6.0 5.0 15.0 Mean = 9.46 ± 0.09 (0.9%) 14.0 Unit 1 (13.5-10.5 Ma) 95% conf. 22 data points 13.0 Alluvial Fan sequence MSWD = 1.5 12.0 HUURUV\PEROVDUHı Permian Oquirrh 11.0 formation 2.0 km 10.0 9.0 8.0 Age (Ma) 10c N 7.0 Rhyolite lava (n=23) 6.0 5.0 30 0HDQ “  ı MSWD = 3.9, 1 of 16 rej. 25 HUURUV\PEROVDUHı 20

1.5 km 15 Unit 3 10 Intercalated lacustrine Age (Ma) tuff, sandstone, marls 5 and mudstones Youngest ages (n=15) 0 30 0HDQ “  ı S. Jim Sage MSWD = 1.4, 3 of 17 rej. 25 N. Raft River Mnts HUURUV\PEROVDUHı Mean pole to plane 20 n = 20; Mean bedding n = 49; Mean bedding 15 plane: N12W, 31.6 E plane: N15E, 47.7 W 1.0 km JSR-09-6 10 *Bedding readings from all stratigrahic levels Age (Ma) Unit 2 exposed in each of the two reagions. Intercalated coarse, 5 **Contour intervals (shaded) at 25 and 50% per Youngest ages (n=14) 1% of area fluvial sandstone and 0 pebble conglomerate 30 metamorphic 0HDQ “  ı Ordovician clasts MSWD = 1.9, 0 of 9 rej. 25 10d HUURUV\PEROVDUHı 20

0.5 km 15 Unit 1 10 Breccias, Age (Ma) Upsection fanglomerates and 5 conglomerates Youngest ages (n=9) 0 30 Mean = 13.45 ± 0.28 (2.1%) ı MSWD = 0.17, 0 of 16 rej. 25

Rapid motion on Almo bounding faultRapid motion on Filling of topographic depression (lake deposits) Faulting and rotation of sequence HUURUV\PEROVDUHı 20 0.0 km Unconformity 15 Proterozoic Permian Oquirrh Formation 10 Sandy limestone SHRIMP-RG Age (Ma) *Modal composition based on ~300 point counts and siltstone analyses 5 of clasts from polished slabs. Tuff (n=16) **Symbols correspond to symbols on =Rejected by isoplot 0 stratigraphic column. 0 W Grouse Creek and Albion Mnts Raft River Mnts E Stage 3: Rapid cooling, faulting, syn-extensional 5 sedimentation and volcanism

10

15 Mean AFT Age = 13.4±1 Ma Apparent Gap: (From Egger et al., 2003) ~ 12 Ma 20 Distance (km) East of Western front of Albion Mountains 0 10 20 30 40 50 Cooling of Oligocene plutons and metamorphic rocks 25 Red Butte pluton Stage 2: Crustal melting, remobilization and diapiric rise of granitoids,

Age (Ma) accompanied by extreme thinning of cover rocks Vipoint pluton 30 Almo pluton Middle Mnt gneiss

35 Emmigrant Pass plutonic complex

Stage 1: Precursor magmatism. 40 Early Nanny Creek volcanism.

Figure 12 45 Key Apatite fission track cooling ages with Ages of volcanic rocks from 2σ error bars (Egger et al., 2003) Jim Sage and Cotterel Mountains (this study) Apatite fission track cooling ages with 2σ error bars (Wells et al., 2000) Timing constrains for normal faulting based on develop- Zircon U-Pb ages of plutons within the ment of syn-extensional basin ARG (Egger et al., 2003; Strickland et (this study) al., 2011) Fig. 6. Rolled garnets in a pelitic horizon in the Schist of the Upper Narrows, Upper Narrows. Garnets (and locally sillimanite) began growing during formation of S2 and contain isoclinal folds of an earlier foliation that can be traced by folded layers of opaque oxides. Deformation countinued after garnet growth with additional growth of chloritoid, muscovite and chlorite.

Fig. 8A Dove Creek Pass. From left, high strains in Archean orthogneiss beneath Elba Quartzite contact. Thin section shows crack-seal (anneal) textures in brittlely deformed feldspars. Upper left and top, Quartzite of Clark’s Basin right at Dove Creek Pass showing S1 and S2. S2 is axial planar to tight to recumbent folds. Thin section photomicrographs above show mimetic growth (post-tectonic) of muscovite in crenulations in fold hinge and partially annealed quartz fabrics in purer quartz- ites. Grt+Bi+Stau+wt mica schist, Bi+qtz+wt. mica Bi+wt.mica+qtz overprinted by P. shadow greenschist Grt extensional def. Top to west sense of shear

Stau rim Stau core

Bi Chl

Stau+Bi+Qtz Chl+Ms (fine grained) 5mm

Fig. 8B Dove Creek Pass. Photomicrographs of the Schist of Mohogany Peaks, samples AKRR- 11-15 and ELM RR-4. These rocks have well- preserved (but variably retrograded) M1 mineral assemblages stt+gnt, retrograded during M2 and deformed by S2. Middle not mapped Mountain Chl + Ab A' Ky(?) + Stt(?) (relict) Qal Stt Stt - in isograd Chl + Ab Ky + Sill Qal

Stt + Ky Qal Sill And +Sill 32 Stt + Ky A 32 29 Qal Qal Qal A

Syn-Cassia plutonic complex Stt +Gt metamorphism and deformation syn to post-deformation Qal Cassia plutonic complex Chl mineral assemblages Quartzite of Clarks Basin(?) and Pz marble (?) 30 Neoproterozoic metasedimentary rocks PCe, Elba Quartzite ky stt pCgg Precambrian Green Creek gneiss normal fault 31 Older deformed and intruded fault Qal mineral elongation/stretching lineations U-Pb zircon age 0 1 Mile B.

ky bi gnt sill gnt sill

E. gnt D. c. stt stt Highly attenuated Neoproterozoic units: schist of Stevens Springs quartzite of Yost schist of the Upper Narrows A Approximate eastern limit of A' Qol deformed Tertiary granite bend 8000 Ccb Ccb pCgg pCe Qal 8000 Ccb Pzm pCe Pzm pCe Qal PCe 7000 pCgg 7000 Qal pCe Qol Qal Qol pCgg 6000 pCgg Tc ? ? ? 6000 ? ? Ccb pCe ? pCgg ? 5000 ? pCgg 5000 4000 4000 3000 3000 Feet Feet Fig. 9

42