Mesozoic Structure of the Newfoundland Mountains, Utah: Horizontal Shortening and Subsequent Extension in the Hinterland of the Sevier Belt

Mesozoic structure of the Newfoundland Mountains, Utah: Horizontal shortening and subsequent extension in the hinterland of the Sevier belt

RICHARD W. ALLMENDINGER | Department of Geological Sciences, Cornell University, Ithaca, New York 14853 TERESA E. JORDAN

ABSTRACT ented at a low angle to bedding. Microfabric analysis of recrystallized The Newfoundland Mountains of northwestern Utah preserve an rocks in the contact aureole and unrecrystallized country rocks shows unusually complete record of Mesozoic deformation in the hinterland that most of the intracrystalline strain is pre-latest Jurassic in age and of the Sevier orogenic belt. Structural relations of Paleozoic miogeo- is characterized by layer-parallel shortening. Post-latest Jurassic in- clinal strata with the Newfoundland stock, which has biotite and tracrystalline strain is minor and records shortening originally ori- hornblende K/Ar ages of 153.2 ± 4.6 and 147.7 ± 4.4 m.y., respec- ented nearly vertical. The map geometries, geochronology, and tively, and dated quartz monzonite dikes (with K/Ar biotite ages of microfabric analysis suggest the following structural history (from 150.7 ± 4.5 and 143.7 ± 4.3 m.y. and a muscovite age of 149.1 ± 4.5 oldest to youngest): (1) pre-latest Jurassic regional shortening and m.y.) provide the basis for dating Mesozoic deformation in the range. thrusting and (2) syn- or post-latest Jurassic horizontal extension Geologic mapping at 1:24,000 has identified four geometric classes of during the Mesozoic. In the Cretaceous and early Tertiary, whereas faults with known or probable Mesozoic ages: (1) older-over-younger strata in the Idaho-Wyoming-northern Utah thrust belt to the east thrust faults (including the Desert Peak thrust, named herein); were shortened by 140-150 km, the rocks in the Newfoundland (2) inward-facing folds in the contact aureole of the stock; (3) low- Mountains experienced little internal strain or were being extended, angle normal faults at a high angle to bedding, intruded by even though they were also probably being translated eastward above undeformed Mesozoic dikes; and (4) younger-over-older faults ori- the deep-seated westward extension of the thrust belt décollement.

Figure 1. Generalized tectonic map of northwestern Utah showing the location of the study area in the Newfoundland Mountains. Abbreviation» : RRM = Raft River Mountains; GC = Grouse Creek Mountains; BM = Bovine Mountain; HTM = Hogup-Terrace Mountains; GM = Grassy Mountains; CI = Cra- ter Island; SI = Silver Island; PR = Pilot Range. Patterns: dashes = Archean basement; horizontal lines = upper plate of the Willard thrust; stipple = allochthonous Pennsylvanian-Permian Oquirrh Group rocks; asterisks = Mesozoic plutons.

Geological Society of America Bulletin, v. 95, p. 1280-1292, 9 figs., November 1984.

1280

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INTRODUCTION significant tectonism. On top of the Devonian strata, a major unconformity, described in greater detail below, omits most, if not all, of the Deformation of rocks in northwestern Utah is the result of tectonic Mississippian and Pennsylvanian Systems. A thick sequence of Permian events that range in age from late Paleozoic to Holocene. The Basin and sandstone and carbonate with a thin basal conglomerate overlies the Range morphology is an expression of the most recent tectonism, the unconformity. ongoing extension that probably began in the middle Cenozoic (Arm- Although Mesozoic plutons and metamorphism are well known in strong, 1972,1982; Compton and others, 1977; Zoback and others, 1981; the hinterland (Armstrong and Suppe, 1973; Armstrong, 1976; Carroon, Compton, 1983). The region also lies within the hinterland of the Meso- 1977; Miller and others, 1982; Miller, 1983b; Moore and McKee, 1983), zoic-early Cenozoic Sevier thrust belt (Armstrong, 1968; Allmendinger it has been particularly difficult to define corresponding Mesozoic struc- and Jordan, 1981), and it contains several major unconformities that tures in southern Idaho, northwest Utah, and northeastern Nevada. suggest significant Paleozoic tectonic activity as well (Jordan and Doug- Mesozoic low-angle faults with both older-over-younger and younger- lass, 1980; Dickinson and others, 1983). This paper describes the timing over-older geometries recently have been described in the Albion Range, and kinematic style of Mesozoic deformation in one part of that region. Blackpine Mountain, and the Pilot Range (Fig. 1) (Smith, 1982; Miller We focus on the Mesozoic structural history, because it provides an impor- and others, 1983; Jordan and others, 1983). As shown below, the New- tant constraint on the mechanics of foreland thrusting to the east and on foundland Mountains contain the oldest documented Mesozoic thrusts in how that thrusting is related to events and processes within the Cordillera all of northwest Utah and southern Idaho. of Nevada and California. In contrast to the hinterland, foreland deformation to the east in the The Newfoundland Mountains are located in northwestern Utah in Idaho-Wyoming-northern Utah thrust belt is much better known, in the Great Salt Lake Desert, completely isolated from other exposures of terms of both geometry and age (Armstrong and Oriel, 1965; Royse and pre-Quaternary rock (Figs. 1,2 A). The mountains lie east and southeast of others, 1975; Dixon, 1982). At the latitude of the Newfoundland Moun- the multiply deformed and metamorphosed rocks of the Pilot Range, tains, the oldest and structurally highest commonly recognized foreland Grouse Creek Mountains, and Raft River Mountains (the last two have thrust is the Willard thrust (Fig. 1) (Crittenden, 1972). If correlations with been called "metamorphic core complexes") and west of the well-known the Paris fault are correct, the Willard thrust is probably of latest Jurassic Idaho-Wyoming-northern Utah foreland thrust belt (Armstrong and and(or) earliest Cretaceous age (Armstrong and Oriel, 1965; Royse and Oriel, 1965; Royse and others, 1975). Although the Newfoundland Moun- others, 1975; compare with Dover, 1983). A structurally higher alloch- tains have not escaped the profound regional Cenozoic extension, their thon of upper Paleozoic rocks (the "Hansel allochthon") has been recog- excellent exposure of a thick, little-deformed Cambrian to Permian mio- nized (Allmendinger and Piatt, 1983), and, although a mid-Jurassic age geoclinal section and, more importantly, the presence of a Late Jurassic was tentatively proposed (Allmendinger and Jordan, 1981), it can only be stock at their northern end make them well suited to a study of Mesozoic shown to be pre-Late Cretaceous in age (Jordan and others, 1983). structural styles. In addition to our own detailed mapping (scale 1:24,000) Obscuring many of the Mesozoic features of the hinterland are the of the northern half of the range and reconnaissance mapping in the rest of profound regional extension and local metamorphism that probably com- the range, we have relied strongly on the excellent mapping of Paddock menced in the Oligocene and are continuing at present (Armstrong, 1972; (1956), work on the Newfoundland Stock by Carroon (1977), and geo- Compton and others, 1977; Wernicke, 1981; Allmendinger and others, chronology by V. Todd (cited in Carroon, 1977), W. Hoggatt-Hillhouse, 1983). During this episode, the features now recognized as the metamor- and J. Nakata (1983, personal commun.), all of the U.S. Geological phic core complexes were formed (articles in Crittenden and others, 1980). Survey. The core complexes apparently represent isolated exposures of Cenozoic On the basis of field relations, microscopic structural analysis, and extensional features that underlie much of the eastern Basin and Range. geochronology, we recognize both pre-latest Jurassic thrust faults and Cenozoic structures are difficult to recognize in the Newfoundland pre-earliest Cretaceous normal faults. The principal period of shortening Mountains, because rocks younger than Jurassic and older than Quater- in the Newfoundland Mountains predates any known thrusting in the nary are not present. Of particular importance is the question of when and Idaho-Wyoming-northern Utah thrust belt farther east. During the subse- how the rocks in the range acquired their present westward dip. As shown quent Cretaceous and early Cenozoic foreland deformation, the rocks in by the angular unconformity, a very gentle component of southward tilt the Newfoundland Mountains either experienced little internal deforma- occurred prior to the Permian. Some open folding may have taken place in tion, or they were extended parallel to bedding. Palinspastic reconstruc- the Mesozoic prior to intrusion of the Newfoundland Stock, but major tions by Royse and others (1975) suggest that, regardless of the kinematic folds with steep or overturned limbs are notably absent. The consistent east setting of rocks in the Newfoundland Mountains, they probably were dip (50° to 80°) of dike swarms suggests that 10° to 40° of westward being translated eastward above a deep-seated décollement during the tilting postdates intrusion, assuming that the dikes were initially vertical. Cretaceous and early Cenozoic. Rotation of the mountain block by that amount could well have occurred due to Cenozoic normal faulting, perhaps above a shallowly dipping ex- GEOLOGIC SETTING tensional detachment. The Newfoundland Mountains expose an unusually thick Paleozoic stratigraphie section (Fig. 2B) in a relatively simple west-dipping homo- GEOCHRONOLOGIC BASIS FOR DATING STRUCTURES cline (Paddock, 1956; Doelling, 1980; R. W. Allmendinger and T. E. Jordan, unpub. data). At the base of the section, completely within the The bases for establishing the relative sequence and, as much as contact aureole of the Newfoundland Stock, there is a sequence of Cam- possible, the absolute ages of Mesozoic structures in the Newfoundland brian limestone and dolomite containing a distinctive quartzite unit. The Mountains are the isotopic ages determined on the Newfoundland Stock overlying Ordovician to Devonian section is one of the thickest known in and the dikes in the range. As the petrology and geochemistry of the stock this part of the Great Basin (Hintze, 1973; Doelling, 1980), and it repre- and dikes have been described elsewhere (Carroon, 1977), we review sents a long period of miogeoclinal sedimentation uninterrupted by them only briefly below.

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Permian

Ps sandstone NEWFOUNDLAND -Penn.O') !v! STÒCK Pc \ conglomerate Dd Unit d

DESERT Dc Unit c PEAK o IPA A a> T. 6 N. Db T. 5 N. Unit b FIG. 3 Da Unit a co Laketown SI \ \ .v Dolomite Of h Fish Haven C Dolomite Oe Eureka Qtzt. Ocp "Crystal Pk. Dol N Swan Peak A Osp Quartzite

Ok Kanosh Shale lOOO-r FIG. 4 Garden City Ogc 'SADDLE^ |t Limestone PASSC n T 5 N. 500- CZ7 O) Cambrian E T.4N. km dolomite, limestone, 8i 0-L quartzite

FIG. c. 7 Desert Peak thrust

Figure 2. A. Simplified geologic map of the Newfoundland Dikes Mountains. Units from west to east and highest to lowest: stipple = Permian; blank = Devonian; horizontal lines = Silurian; blank = Ordo- Dikes and sills are present throughout the Newfoundland Mountains vician; vertical lines = Cambrian. Intrusives shown with "plus" (+) but are most concentrated within the Newfoundland Stock and the sur- symbol. Structure symbols: solid saw-teeth = thrust fault; hachured rounding country rocks (Fig. 3). The dikes both predate and postdate the lines = low-angle normal faults at high angles to bedding; open saw- stock. Early metadiabase dikes, composed of plagioclase, hornblende, and teeth = younger-over-older fault at low angle to bedding; heavy line = biotite, are folded, boudined, and altered in the ductilely deformed country high-angle fault. Locations of Figures 3, 4, and 7 shown. B. Strat- rocks of the contact aureole. igraphie column showing Paleozoic units of the Newfoundland Most of the poststock dikes have a composition similar to that of the Mountains. Initials to left are used in subsequent figures. Informal stock itself, and Carroon (1977) concluded that they were directly related Devonian map units: Da = Simonson Dolomite; Db and Dc = to the main phase of intrusion, on the basis of composition and their Guilmette Limestone; Dd = Pilot Shale. Ordovician nomenclature fol- orientation around the main stock. The vast majority of the dikes are lows Paddock (1956). C. Cross section through Desert Peak (location hornblende-biotite adamellite porphyries. At the north end of the range, shown in Fig. 2A) showing the four basic fault styles observed in the these dikes exhibit two distinct preferred orientations: northwest- and Newfoundland Mountains. northeast-trending, with both moderately easterly dipping. For K/Ar dating, we sampled 3 dikes, located between 1 and 8 km south of the main stock. A muscovite separate from the northernmost of the 3 dikes (loc. A in Fig. 3) produced an age of 149.1 ± 4.5 m.y. The other two dikes both yielded ages on biotite separates of earliest Creta- ceous or latest Jurassic (at localities A and B in Fig. 4, ages of 150.7 ± 4.5 and 143.7 ± 4.3 m.y., respectively, were obtained; W. Hoggatt-Hillhouse and J. Nakata, analysts, 1984, written commun.). The possibility of excess Newfoundland Stock argon affecting the age determinations of the dikes has not been tested directly, but, unlike hornblende, biotite is not generally known for prob- In general, the Newfoundland Stock has a composition of adamellite, lems of this nature. These ages are probably minimum ages, due to the although in detail the body is concentrically zoned with compositions alteration of the dikes; given the closeness of these ages to the K/Ar age of ranging from granodiorite to adamellite (Carroon, 1977). Texturally, the the stock and the similarity in composition, it is likely that the dikes and stock ranges from coarse grained with orthoclase phenocrysts at its outer the stock were related to the same intrusive episode. margins to medium to fine grained and equigranular toward the core. Minerals notable in hand specimen include orthoclase, plagioclase, biotite, MESOZOIC STRUCTURAL SEQUENCE hornblende, and quartz. The two mafic minerals are about equally abundant. Carroon noted that biotite and plagioclase are noticeably altered in In the Newfoundland Mountains, four classes of known and probable the core region of the pluton but are less so in the margin. Mesozoic structures have been recognized. Observable at both map and A smaller, much more altered outlier is present directly south of the outcrop scales, these classes of structures are (1) thrust faults, (2) structures main stock (Figs. 2, 3). It is much more varied texturally and composition- related to the intrusion of the Newfoundland Stock, (3) low-angle faults at ally, ranging from syenite to granodiorite. Mineral products of the altera- high angles to bedding, and (4) younger-over-older faults at a low angle to tion include chlorite, epidote, and diopside replacing the original mafics bedding. In general, this list is ordered from oldest to youngest, but in and sericite and clay minerals replacing the plagioclase (Carroon, 1977). detail, relations are more complex. Classes 2 and 3 may be essentially Carroon concluded that this outlier is the earliest granitic unit of the contemporaneous, and at least one fault in class 4 is older than, or the same intrusive complex, and that its low-grade alteration is due to metamor- age as, the Newfoundland Stock. phism by later, main-phase intrusion. This outlier cuts and pins the north end of the Desert Peak thrust. Just east-southeast of Desert Peak, where Thrust Faults they intrude basal Ordovician limestone, two other small outliers are also present. Their compositions are similar to that of the main body. The most prominent thrust in the range is the Desert Peak thrust, here Biotite and hornblende mineral separates from an unaltered part of named for its only exposure in the northern Newfoundland Mountains the peripheral quartz monzonite of the main body were dated by V. Todd (Fig. 2A). The thrust is visible on the north, east, and south faces of Desert and W. Hoggatt of the U.S. Geological Survey (cited in Carroon, 1977). Peak, where it repeats the Ordovician Swan Peak Quartzite, Crystal Peak The K/Ar ages obtained were 153.2 ± 4.6 m.y. for biotite and 147.7 ± 4.4 Dolomite, and Eureka Quartzite sequence (Figs. 2C, 3, 4, and 5). This m.y. for hornblende.1 Carroon attributed the slight discrepancy to 2% duplicated section amounts to -500 m of stratigraphic throw. A cross biotite and chlorite impurities in the hornblende separate but stated that section drawn perpendicular to the footwall cutoff of the Crystal Peak the biotite separate was very clean. K/Ar ages, of course, record cooling, Dolomite indicates that the minimum displacement on the thrust at this rather than crystallization, and thus the true age of the stock is likely to be locality is at least 1 km, although probably not much more (Fig. 2C). The somewhat older than the above ages. The near concordance of the biotite trace of the thrust across steep topography at Desert Peak was used to and hornblende ages, however, suggests that resetting is not a major prob- determine a strike and dip of N20°W, 35°WSW by three-point projection. lem. The stock, therefore, is pre-Tithonian in age (Harland and others, At this locality, there is a ramp in the thrust across the massive Eureka 1982). Quartzite. Both to the north and south of Desert Peak, the fault turns into a bedding plane thrust in the Swan Peak Quartzite. Noticeable duplication

10 within the Swan Peak dies out less than 1.6 km south of Desert Peak 'Constants used for all K/Ar dates: kt + \f- = .581 x 10 /yr; kfi = 4.962 x 10-10/yr; K^/K,^, = 1.167 x 10"4 mol/mol. (Fig. 4). Although the fault may well be present farther south, it is

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Recrystallized dolomite not twinned. Post-metamorphic strain not large enough for f-lamellae. RA83-3 RA83-2

Recrystallized quartz does not show undulatory extinction or deformation lamellae. Post-metamorphic strains are small. NF-2

Recrystallized quartz has undulatory extinction and elongate grain shapes (as- pect ratiosc 2:1 ) but no deformation lamellae. Strain probably due to syn-metamorphic movement on Desert Peak Thrust. NF-3

Figure 3. Geologic map of the northern Newfoundland Mountains. Fault symbols and unit abbreviations as in Figures 2A and 2B, respectively. A, B, and C show critical structure localities discussed in text. Equal-area nets and boxes summarize microfabric analyses of recrystallized rocks in the contact aureole of the Late Jurassic Newfoundland Stock. All equal-area net measurements based on 50 calcitc grains in 2 perpendicular sections from each sample; contours according to Kamb's (1959) standard deviations method, plotted with a computer program written by D. M. Miller. S = shortening axes; E = extension axes. Dashed great circles show orientation of metamorphic foliatio n. Note west-southwest-trending anticline in Cambrian section overturned toward pluton.

unrecognizable. About 1 km north of Desert Peak, the thrust repeats the K/Ar age of 149.1 ± 4.5 m.y.) is smeared out and offset by ~5 m in a Swan Peak section. thrusting sense across the fault surface (loc. A in Fig. 3), a displacement far On the north side of Desert Peak, the age of the thrust is shown by its smaller than the minimum of 1,000 m required by the offset Ordovician relations to the stock and dikes. Along the trace of the thrust, less than 100 stratigraphic units. North of locality A (Fig. 3), the thrust surface is mainly m northwest of the footwall cutoff of the Crystal Peak Dolomite, a quartz obscured by subsequent contact metamorphism related to the southern monzonite dike that cuts the altered southern outlier of the stock crosses outlier. Swan Peak and Eureka Quartzites in the hanging wall have a the fault surface at a high angle. The north-trending dike (with a muscovite lineation due to elongate, recrystallized quartz grains (Fig. 3); the eiist-west

R.I4 W. R.I3W. Structures Related to Intrusion of the Stock

DESERT PEAK In plan view, the main Newfoundland Stock is circular, with its THRUST country rock contacts exposed discontinuously around most of its perime- 16 N. ter (Fig. 3). The present maximum topographic relief on the contact is -600 m, but, because of the probable postintrusion tilting of the range, the T.5N. structural-stratigraphic relief on the contact is much greater, -1,800 m (Ordovician Eureka Quartzite to Upper? Cambrian dolomites). The characteristic structural feature of the wall rocks is the development of inward-facing (that is, toward the pluton), overturned folds with axial-planar cleavage that dips radially away from the stock. These folds are particularly well developed on the south and southeast sides of the stock. In the Cambrian units, a fold has been formed at a macroscopic (map) scale (Fig. 3); there, the fold verges northwestward and has a gentle plunge in the direction S70°W. Elsewhere, mesoscopic folds are consis- tently arrayed with axes parallel to the local contact of the stock. The scale of fold formation and overturning may be related to the structural and stratigraphic level (with larger fold amplitudes at deeper levels), even though the relative sense of shear as shown by the fold vergence is the same at all structural levels. Similar inward-facing folds have been observed around Jurassic intrusions at Crater Island, -35 km southwest of the Newfoundland Mountains (Fig. 1). The mechanical significance of the inward-facing folds is not known, although they appear to be integrally related to the processes by which the intrusive bodies were emplaced.

Low-Angle Faults at High Angles to Bedding

A set of well-exposed, nearly flat faults crops out for 13 km along the crest of the range (Figs. 2 A, 4). These faults dip very gently eastward (-5°) and cut steeply west-dipping Permian to Upper Ordovician strata at high angles (Figs. 2C, 6). In their present attitude, the faults are very low-angle normal faults; their nearly perpendicular orientation with respect to bedding and the assumed postintrusion tilting of the range suggest that the faults originally were formed with steeper east dips. They can be generally grouped in two classes: (1) dike-filled low-angle faults found north of Saddle Pass in Township 5 North and (2) calcite- and breccia-filled low- angle faults found south of Saddle Pass in Township 4 North. Although geometrically similar, these classes may be of very different ages. The dike-filled, low-angle faults are particularly well exposed at three 2 km localities: north of Desert Peak (loc. B in Fig. 3), southwest of Desert Peak (loc. A in Fig. 4), and just north of Saddle Pass (loc. B in Fig. 4). The dikes Figure 4. Geologic map of the central part of the northern New- at the last two localities have yielded K/Ar biotite ages of latest Jurassic or foundland Mountains. Unit abbreviations as in Figure 2B. Fault sym- earliest Cretaceous (Harland and others, 1982). The structural relations at bols as in Figure 2A. A and B show dike-filled, low-angle fault these localities show that, following final displacement, the faults were localities discussed in text. intruded by the dikes; the dikes themselves are undeformed, and there is little evidence for subsequent slip along the contacts between the dikes and orientation of the lineation is essentially parallel to the direction of thrust- the sedimentary rocks. These faults, which at present have the geometry of ing. No mappable offset of the southern outlier has been observed (Fig. 3). low-angle normal faults, thus were formed prior to the earliest Cretaceous. Some of the motion on the thrust thus was probably synchronous with It may be significant that the dike-filled, low-angle faults are best intrusion during the Late Jurassic. No significant shortening along the developed above the highest stratigraphic level of emplacement of the thrust has occurred since intrusion of the stock and dike 150 m.y. ago, Newfoundland Stock. At the northern locality (loc. B in Fig. 3), the fault however, and it is likely that most of the movement predates the stock, on displaces the Kanosh-Garden City contact but does not displace the the basis of the microstructure analysis described below. nearby contact between the southern outlier of the stock and the Kanosh. A much smaller thrust has also been mapped in the metamorphosed Our interpretation of the dike-filled low-angle faults is that they formed in Cambrian strata on the southeast side of the main part of the Newfound- the roof rocks during the major phase of emplacement of the Late Jurassic land Stock (Fig. 3). This thrust duplicates part of the Cambrian quartzite intrusion. Their consistent high angle with respect to bedding indicates that unit, and it has been folded around a northwestward overturned anticline they originally may have been oriented at steeper angles and were respon- that appears to be closely related to the emplacement of the stock (see sible for minor stretching in the roof rocks during pluton emplacement. below). This thrust thus probably also predates the main phase of intrusion No such age constraints are available for the calcite- and breccia- in the Late Jurassic. filled, low-angle faults at high angles to bedding. These faults generally

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Figure 5. Desert Peak fr Dm the north, showing the Desert Peak thrust duplicating the Ordovician Swan Peak, Crystal Peak, Eureka sequence.

Figure 6. Low-angle normal faults at high angles to bedding and offsetting Permian(?)-Devonian uncon Formity. Near location A in Figure 4, looking south.

show much greater brecciation than do their northern counterparts and are stratigraphic section. There are three principal occurrences: (1) within the commonly filled with calcite veins as much as 1 m thick. The geometry of Ordovician strata northwest of the Newfoundland Stock, (2) a regionally these low-angle; faults is similar to that of recently described Cenozoic extensive fault on the eastern side of the range (the "East Face fault") and extensional faults in the eastern Basin and Range (Gans and Miller, 1983; (3) within the Devonian section just below the Permian-Devonian Wernicke, 198:.; Wernicke and Burchfiel, 1982). Although the dike-filled unconformity. faults cannot be Cenozoic because of the structural relations with the dated The northernmost of these three faults has a limited outcrop area and pluton and dikes, it is entirely possible that the calcite-filled faults are either is isolated from the rest of the range by the stock. This fault cuts out the Cenozoic or Mesozoic. entire Ordovician Kanosh Shale, placing Swan Peak Quartzite on Garden City Limestone (Loc. C in Fig. 5). The amount of these last two units that Younger-over- Older Faults at Low Angles to Bedding has also been omitted is unknown. At this locality, the rocks are completely within the: contact aureole of the stock; there is neither b recciation This class of structures is perhaps the most poorly understood, and, nor a more intense foliation within either of the units along the fault although at least two faults of this class can be shown to be Mesozoic or surface. Ductile deformation and recrystallization are characteri itic of the late Paleozoic in age, the remainder may be either Mesozoic or Cenozoic entire exposure of the metasedimentary rocks. At its south end, the fault is in age. Geomet rically, all of these faults are oriented subparallel to bedding truncated by the stock (loc. C in Fig. 3); there is no displacement of the with dips no more than 5° steeper than the surrounding strata, and all omit intrusive contact between the stock and the Ordovician strata. We do not

know whether the omission of the Kanosh is due to stretching in the wall either plate near the fault, however, no brecciation is present, and only an rocks during pluton emplacement or whether there was a pre-intrusion, increase in foliation intensity can be seen in the field. Microfabric studies low-angle, younger-over-older fault. In either case, the stratal omission described below also demonstrate an increase in intracrystalline strain near must be pre- or syn- 150 m.y. B.P. the fault. The most prominent younger-over-older fault, the East Face fault, The age of the East Face fault and its relations to the other fault sets can be traced for almost 15 km from just southwest of Desert Peak to are enigmatic. Dikes were not observed crossing the fault or being trun- within 8 km of the south end of the range (Figs. 2A, 4). The dip of the fault cated by it. The low-angle faults at a high angle to bedding do not extend over that interval varies in direct relation to the dip of local bedding, past the East Face fault (Fig. 4), but we cannot determine if the latter is ranging from 80° at the north end to only 10° to 20° farther south. At its significantly younger, or if it acted as a décollement horizon for the dike- northern end, there are two splays of the fault, one separating Devonian filled faults. Lacking definitive age constraints, we regard the East Face rocks from the Silurian Laketown Dolomite, and the other between the fault as probably Mesozoic for the following reasons. (1) It is similar in Laketown and the Ordovician Fish Haven Dolomites (Fig. 4). The fault geometry to the fault on the northwest side of the Newfoundland Stock— gradually cuts downsection southward, until it encounters Eureka Quartz- both faults omit the Kanosh Shale over long distances. (2) The tectonic ite in the footwall. At that point, there is a sharp lateral ramp downsection foliation is due to pressure solution and intracrystalline strain, both of in the footwall across the Eureka-Swan Peak sequence and into the Ka- which would be enhanced by the documented Mesozoic thermal events. nosh Shale. The East Face fault remains at the stratigraphic level of the Cenozoic thermal effects apparently were negligible in the northern part of Kanosh, omitting the entire Kanosh, for 8 km southward (Fig. 4). At the the range; none of the K/Ar ages there shows any signs of resetting. south end of the fault, the Kanosh gradually reappears in the hanging wall A younger-over-older fault at a low angle to bedding near the top of (Fig. 7). The fault may extend farther south within the Ordovician sec- the Devonian section was mapped primarily on the basis of a tectonic tion, but it could not be traced with certainty because of limited exposure breccia occurring between the uppermost Devonian unit, a shaly micrite and lack of access. that may correlate with the Pilot Shale, and Unit C of massive, dark The character of deformation along the East Face fault surface varies limestone, probably the Devonian Guilmette Limestone (Fig. 4). This fault as a function of lithologies in hanging wall and footwall. Where dolomites is poorly exposed on the east side of the main ridge of the range, but it is and(or) quartzites are juxtaposed, a 1- to 5-m-wide breccia zone lacking cut by, and therefore older than, the series of dike-filled, low-angle normal significant calcite veins is typical. Where limestone and(or) shale are in faults at a high angle to bedding that displace the Devonian-Permian

0 2 km

Figure 7. Geologic map of small segment of Newfoundland Mountains south of Saddle Pass. Microstructure analyses in equal-area nets contoured as in Figure 5. In each sample, 50 grains in 2 perpendicular sections were measured. Minerals analyzed: dolomite = NF-18, NF-10, NF-12; calcite = NF-13, NF-14, NF-15; quartz = NF-11. S = shortening; E = extension. Fault with open saw-teeth is southern end of the "East Face" fault.

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unconformity eastward (loc. B in Fig. 4). A dike at this locality has a minimum K/Ar biotite age of 143.7 ± 4.3 m.y. The fault has not been Figure 8. Photomicrographs showing character of microstruc- recognized to the north, where the Pennsylvanian(?)-Permian(?) conglom- tures in metamorphosed and unmetamorphosed strata in the New- erate rests on Devonian Unit C (Fig. 4). It thus is possible that the fault foundland Mountains. In general, note 120 triple junctions between may predate the conglomerate. recrystallized grains in samples from the contact aureole and the much smaller density and width of twins in the recrystallized grains than in MICROSTRUCTURE ANALYSIS the country rock outside the contact aureole. A. Calcite from contact AND KINEMATIC EVOLUTION aureole (sample RA83-2); arrows show narrow, widely spaced twin lamellae. B. Dolomite from contact aureole (sample RA83 3); dark To establish the ages and types of strain in the Newfoundland Moun- lines in grains are cleavage planes, not twin lamellae. C. Quartz from tains block, twc general localities were sampled for microstructure analy- contact aureole (sample NF-2). D. Calcite in country rock (sample sis. The first locality is a relatively coherent block of west-dipping NF-15); arrows show dense twin lamellae. E. Dolomite in country Ordovician to Silurian strata in the southern half of the range, far removed rock (sample N1F-10); arrows show twin lamellae. F. Quartz from from the Newfoundland Stock (Fig. 7). The rocks in this block have not country rock (sample NF-11); arrows show deformation lamellae. been metamorphosed or recrystallized, and thus the microstructures there Scale bar in each photograph is 0.5 mm. See Figures 3 and 7 for should record the entire intracrystalline strain history of the range. The sample locations. second locality is at the north end of the Newfoundland Mountains, within the metamorphx aureole of the Late Jurassic pluton (Fig. 3). The formations sampled there are completely recrystallized, and any pre-latest Juras- sic microstructures have been completely annealed. By combining the data from the two sa mpling localities and taking into account the recrystallization event of known age, the pre- and post-latest Jurassic microstructural (NF-18, NF-10) or subparallel to bedding (NF-12, NF-14). Samples of the development can be defined. former group also have two pronounced extension maxima; those of the latter have only one extension maximum. Southern Sampling Locality Northern Sampling Locality We collected seven samples from the Devonian Simonson and Or- dovician Fish Haven, Eureka, Swan Peak, and Garden City Formations. Dolomite, limestone, and quartz in these units contain abundant /- lamel- We examined seven samples of recrystallized Ordovician s:rata from lae, e-lamellae, and subbasal I deformation lamellae (respectively). These the contact aureole of the Newfoundland Stock (Fig. 3). In general, the microstructures were analyzed to determine shortening and extension di- recrystallization produced equant grains with numerous 120° triple-point rections2 (Turner and Weiss, 1963; Carter and Raleigh, 1969; Ave Lal- intersections (Figs. 8A, 8B, 8C). Subgrain formation along previously lemant and Carter, 1971). The results of these analyses are shown in recrystallized grain boundaries was observed in only one sample (NF-3, Figure 7; the important observations based on the suite of samples are from the upper plate of the Desert Peak thrust). Preferred orientation of summarized below. c-axes may be significant enough in one sample (RA83-4) to affect the 1. The two samples from nearest the younger-over-older low-angle orientation of the shortening and extension maxima. Diffuse and discon- fault (NF-13, NF-14) (Fig. 7) have notably more diffuse point concentra- tinuous point concentrations were observed in only one sample (RA83-5, tions of both extension and shortening axes than do those samples either from the contact aureole of the older southern outlier). The samples, farther above or below the fault. These samples, however, exhibit shorten- collected from all exposed sides of the stock, are summarized below, on the ing subparallel to bedding and extension at a high angle to bedding. The basis of the analyses shown in Figure 3. intracrystalline strain in these samples is somewhat greater (as based on 1. All of these samples show much less intracrystalline strain than reduced spacing and increased width of twins) than in samples collected does the unmetamorphosed country rock elsewhere in the range (Fig. 8). farther away from the fault. This is consistent with field observations Calcite twins are very widely spaced (commonly only 0-5 per grain) and showing greater foliation development near the fault. so narrow as to appear as lines under the petrographic microscope. Dolo- 2. All samples show shortening maxima parallel or subparallel to mite grains (RA 83-3) are untwinned, in contrast to common twinning of bedding. In the stratigraphically highest samples, the principal shortening dolomite in the country rocks (for example, NF-18, NF-10, NF-12). direction trends north-northwest; in the lowest sample, shortening trends Quartz grains do not contain deformation lamellae, and undulatory extinc- due west. The low-angle, younger-over-older fault between the Kanosh tion is absent except near the Desert Peak thrust. Shale and the Garden City Limestone lies between samples with these two 2. The extension and shortening maxima show no consistent orienta- orientations. tion with respect to the metamorphic foliation in the rocks, suggesting that 3. Most samples have double shortening maxima (the one exception the strains recorded by the microstructures are postintrusion rather than is NF-15). The secondary maxima may be either perpendicular to bedding synintrusion. The shortening direction in all samples is similarly oriented with respect to present geographic co-ordinates, however. 3. All of the samples show shortening maxima plunging moderately 2Classically, these techniques have been considered "dynamic" analyses and in a generally easterly direction. This orientation is at a high angle to the thus have used the terms "compression" and "tension." Such assumptions are valid general westerly dip of bedding in the range. Restoring the ..0°—40° of only if strains are small and nonrotational (stress and strain are coaxial). In the postintrusion, westward tilting of the range (suggested above) to the hori- general case, however, the analyses are kinematic, and although strains are relatively small in the Newfoundland Mountains, the terms "shortening" and "extension" are zontal rotates these shortening directions to nearly vertical orientations. used here. There are no statistically significant shortening double maxima.

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E F 4. Extension maxima are variable but generally trend either north- unmetamorphosed rock occurred prior to the intrusion of the Late Jurassic south or east-west and have gentle plunges. There are no statistically stock. The dominant, pre-annealing fabric is one of layer-parallel shorten- significant extension double maxima. ing (which was not recorded by annealed samples from the northern locality), although a few samples from the southern locality also exhibit a Interpretation of Microstructure Analyses weak secondary maximum shortening at a high angle to bedding. The Paleozoic strata of the range thus were horizontally shortened in an east- A comparison of microstructures from the southern and northern west or northwest-southeast direction prior to intrusion. The principal sampling localities indicates that most of the intracrystalline strain in the extension during this stage was probably approximately vertical. The

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strains recorded in the recrystallized rocks are small and indicate a simple The unit above the unconformity throughout the Newfoundland pattern of steep to near-vertical shortening. Following intrusion, then, only Mountains is a thin (<50 m thick) conglomerate. The conglomerate unit minor shortening, probably oriented nearly vertically, was recorded by previously has been assigned to the Permian (Leonardian) by Paddock calcite grains; strain was insufficient to twin dolomite. We do not know (1956) and to the Devonian by Roberts and Tooker (1969); on the basis whether the subvertical shortening is Mesozoic or Cenozoic in age. The of relations at Crater Island, described below, we suggest that its age is microstructure results are consistent with the field observations indicating Pennsylvanian or Early Permian (Wolfcampian). The pebble tc boulder that most of the motion on the Desert Peak thrust occurred before intru- conglomerate, composed primarily of limestone and dolomite dasts de- sion as well. rived from the underlying Devonian units, is thin to medium parallel- Considering the ease with which calcite twins, it is remarkable that bedded. The conglomerate is persistent along strike, although it thins the rocks of the Newfoundland Mountains apparently "felt" no horizontal northward gradually. At the southern limit of its exposure, the conglomer- compression after the Late Jurassic. Palinspastic reconstructions of the ate is underlain by a muddy, thinly bedded limestone, Unit Dd (Fig. 2), thrust belt in Idaho, Wyoming, and northern Utah suggest that ~ 140-150 referred to by Paddock (1956) as the "Three Forks Shale" and by Hintze km of horizontal shortening occurred east of the Newfoundlands during (1973) and Doelling (1980) as the "Pilot Shale." Unit Dd contains Late the Cretaceous and early Tertiary (Royse and others, 1975; Dixon, 1982); Devonian (early Frasnian to early Famennian) brachiopods identified by the basal décollement of the thrust belt probably passed beneath the range J. T. Dutro, Jr. (1982, personal commun.) as Calvinaria sp., Warrenella, during this younger time period. The Newfoundland strata appear to have Cyrtospirifer sp., Thiemella sp., and Strophopleura. Northward, however, been rafted passively along in the upper plate of that décollement without Unit Dd gradually thins, and at the north end the conglomerate rests on experiencing either major horizontal shortening or extension. Unit Dc (Guilmette Formation). The conglomerate is overlain by sandy dolomite, dolomitic sand- REGIONAL STRATIGRAPHIC AND stone, and bioclastic dolomite. Fusulinids collected -140 m above the STRUCTURAL RELATIONS contact were identified by R. C. Douglass (1982, personal comrnun.) as Schwagerina sp. of late Wolfcampian age. The Permian sandy dolomite Given that the Newfoundland Mountains are isolated from other unit is lithologically similar to unfossiliferous dolomite and sandstone bedrock exposures, surface structural data cannot be used to indicate mapped as "unnamed sandstone" in the Grassy and Lakeside Mountains structural continuity with other areas. Stratigraphie and facies relations to the southeast (Doelling, 1964), the "Diamond Creek Sandstone''" in the between differeni: ranges are the best indicators of major, coherent struc- Hogup/Terrace Mountains to the northeast (Stifel, 1964), and the "un- tural plates. Comparison of the Newfoundland Ordovician and upper named formation" of the Cedar Mountains (south of the Grassy Moun- Paleozoic sequences with those of neighboring ranges is particularly tains) (Maurer, 1970). In those locations (Fig. 9), it overlies a 5-km section informative. of Mississippian carbonates and the Pennsylvanian-Permian Oquirrh Group (Doelling, 1980; Jordan and Douglass, 1980). The Ordovician System At Crater Island, a lithologically similar, laterally persistent conglomerate rests on a thin sequence of Chainman Shale and Diamond Peak The Ordovician of the northeastern Great Basin is an easily recogniz- Conglomerate; the top of the Diamond Peak has been dated elsewhere as able, three-part lithologie sequence (Hintze, 1973): a thick basal unit of latest Mississippian to earliest Pennsylvanian (Gordon, 1971; Smith and thin-bedded, silty limestone (Pogonip Group); a middle unit of pure quartz- Ketner, 1975). The conglomerate is overlain by a dolomitic sandstone ite (Swan Peak and Eureka Quartzites); and an upper dark dolomite unit, with minor pebble and pebbly sandstone interbeds, grading upward (Fish Haven Dolomite). The Newfoundland Mountains section has two into a unit of largely dolomite and bioclastic dolomite; this unit is similar well-developed, quartzite-bearing units, separated by a thin, dark dolomite to the Newfoundland sandy dolomite sequence described above. The low- interval. This sequence was mapped by Paddock (1956) as, from base to est fusulinid-bearing bed, ~ 110 m above the conglomerate, has Pseudofu- top, the Swan Peak Formation, Crystal Peak Dolomite, and Eureka sulinella sp., an advanced form of Schwagerina sp., and other poorly Quartzite, and we follow his usage (Fig. 2B) (R. W. Allmendinger and preserved forms, of late Wolfcampian age (R. C. Douglass, 1983, written T. E. Jordan, unpub. data). The Swan Peak Formation is a lithologically commun.). At Crater Island, thus, the conglomerate overlying a low-relief mixed unit with interbedded calcareous sandstone, limestone, and bedded erosional unconformity can be bracketed between approximately ;arliest quartzite; in contrast, the stratigraphically higher Eureka Quartzite is a Pennsylvanian and Early Permian age units, and, given the more grada- pure, massive quartzite. Similar sequences are known in neighboring tional nature of the conglomerate's upper contact than its base, it is in- ranges to the northwest (Bovine Mountain; Jordan, 1983), west (Crater ferred to be Upper Pennsylvanian or lowest Permian (Fig. 9). Island; Doelling, 1980), and southwest (Hintze, 1973) but do not occur to Similarities in stratigraphy and igneous history between the New- the east (Hintze, 1973; Doelling, 1980). foundland Mountains and Crater Island suggest that both ranges were part of the same Mesozoic allochthon. In contrast, there are major differences The Upper Paleozoic Sequence between the Newfoundland Mountains sequence and the rocks of the Hogup/Terrace and Grassy Mountains, only 20 km to the east (Fig. 9). A principal characteristic of the upper Paleozoic stratigraphy in the Pennsylvanian and Permian facies of the Oquirrh Group in the Grassy Newfoundland Mountains is a nearly bedding-parallel angular unconfor- Mountains imply some topographic relief during their deposition and indi- mity between the Devonian and Permian Systems (Figs. 2B, 4). In com- cate that a margin of the basin was nearby; however, 4 km of structural parison, —25 km to the west, Crater Island has a similar unconformable relief (that amount of section is missing at the Newfoundland Mountains) section, whereas ranges to the northwest, north, south, and east have thick developed over a horizontal distance of 20 km during the Mississippian or sequences of Mississippian carbonates and Antler-related clastics (Fig. 9). Pennsylvanian seems unlikely (Jordan and Douglass, 1980).

Grassy 8 Figure 9. Regional stratigraphie sections of Lakeside upper Paleozoic rocks in Crater Island, the New- Mountains

foundland Mountains, and the Grassy Mountains. Unnamed Locations of ranges shown in Figure 1. Note the absence of thick Oquirrh Group rocks in the two Crater Newfoundland western sections. Island Mountains

conglomerate Unnamed yDiämöncTPea~k~ Oquirrh limestone |r~T| shale i Chainman iJoana Guilmette lime siltstone I--1 sandstone

(Faulted contact) Manning Canyon

Great Blue CONCLUSIONS

The pre-Cenozoic tectonics of the northwest Utah hinterland can be Humbug evaluated in the Newfoundland Mountains because they have been little affected by the pronounced Cenozoic deformation and metamorphism Deseret typical of the metamorphic core complexes. Because all strains in the Lodgepole Guilmette Newfoundland Mountains are small, and some of the rocks were recrystal-

lized at about 150 m.y. ago, they can be separated into pre- and post- o> latest Jurassic time increments, providing one of the most complete uj o records of Mesozoic structural history in the hinterland. 1. Most of the microscopic and macroscopic shortening recorded by the rocks in the Newfoundland Mountains occurred prior to about 150 structural relations are possible. The similar strata at Crater Island occur in m.y. ago. Shortening on local thrusts was relatively small (> 1 km on the the upper plate of the Pilot Peak décollement, a pre-39-m.y.-old younger- Desert Peak thrust); the regional shortening during this time increment is over-older low-angle fault (Miller and others, 1982; Miller, 1983a). The unknown. west-tilted Newfoundland block also could lie in the upper plate of that 2. The Newfoundland Stock was intruded at some time before 150 same structure. m.y. ago (Todd, cited in Carroon, 1977). Minor extension occurred at Most importantly, this study indicates that a compressive deforma- stratigraphie levels immediately above the roof of the stock, either during tion occurred in the range, and perhaps in the entire region, prior to the or soon after intrusion. The extension was coeval with or older than dike latest Jurassic. Horizontal shortening did not continue at this structural emplacement (latest Jurassic or earliest Cretaceous). Very minor shorten- level when that deformation moved farther east during the Cretaceous, ing occurred immediately after dike emplacement, but while the rocks even though Newfoundland strata probably continued to be translated were still hot. eastward. From a regional perspective, the definition of hinterland kine- 3. During the Cretaceous and early Tertiary, the rocks exposed in matics through time will have major implications for the mechanics of the Newfoundland Mountains were probably passively rafted eastward in foreland thrust belts. the upper plate of the deep-seated foreland décollement. No horizontal shortening was recorded at their structural level; either they were un- ACKNOWLEDGMENTS strained or were slightly horizontally extended during this interval, even though, farther east, 140-150 km of shortening was occurring above the We are most grateful to David M. Miller and Max D. Crittenden, Jr., same décollement. for many discussions over the past several years concerning both the Whether the Newfoundland strata occupied a structurally higher or regional geology of northwestern Utah and the geology of the Newfound- lower position during the Mesozoic with respect to the region of alloch- land Mountains in particular. To them also are due thanks for providing thonous Oquirrh Group rocks to the east cannot be determined without base materials for mapping. We thank R. C. Douglass, J. T. Dutro, and M. subsurface data. Simplistic models of Paleozoic facies relations and basin Gordon, Jr., for providing biostratigraphic control; W. Hoggatt-Hillhouse, geometries indicate that the Newfoundland rocks may have been structur- J. Nakata, and V. Todd for the geochronology; and H. H. Doelling and ally higher, but several more complex (and probably more realistic) L. F. Hintze for providing various materials related to R. E. Paddock's

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original mapping of the range. D. M. Miller, G. Harper, L. F. Hintze, and Harland, W. B., Cox, A. V., Llewellyn, P. G., Picton, C.A.G., Smith, A. G., and Walters, F., 1982, A geologic time scale: Cambridge, England, Cambridge University Press, 128 p. L. B. Piatt kindly reviewed the manuscript. This study was supported by Hintze, L. F., 1973, Geologic history of Utah: Brigham Young University Geology Studies, v. 20, 181 5. Jordan, T. E., 1983, Structural geometry and sequence. Bovine Mountain, northwest Utah, in Miller, M., and others, National Science Foundation Grants EAR 80-18758 and EAR 82-18617. eds.. Tectonic anc stratigraphic studies in the eastern Great Basin: Geological Society of America Memoir 157, p. 215-227. Jordan, T. E., and Douglass, R. C., 1980, Paleogeography and structural development of the Late Penn:;ylvanian to Early REFERENCES CITED Permian Oquirrh Basin, northwestern Utah, in Fouch, T. C, and Magathan, E. 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Island Mountains, U"ah: Geological Society of America Abstracts with Programs, Part 5, p. 69. Compton, R. R., 1983, Displaced Miocene rocks on the west flank of the Raft River-Grouse Creek core complex, Utah, Royse, F., Warner, M. A., and Reese, D. L., 1975, Thrust belt structural geometry and related stratign phic problems, in Miller, D. M., and others, eds., Tectonic and stratigraphic studies of the eastern Great Basin: Geological Society Wyoming-ldaho-noithern Utah: Rocky Mountain Association of Geologists Symposium, p. 41-54. of America Memoir 157, p. 271-279. Smith, J. F„ Jr., 1982, Geologic map of the Strevell 15-minute quadrangle, Cassia County, Idaho: U.S. Geological Survey Compton, R. R., Todd, V. R., Zartman, R. E., and Naeser, C. W., 1977, Oligocene and Miocene metamorphism, folding, Miscellaneous Investigations Map 1-1403, scale 1:62,500. and low-angle faulting in northwestern Utah: Geological Society of America Bulletin, v. 88, p. 1237-1250. Smith, J. F„ Jr., and Ketner, K. B„ 1975, Stratigraphy of Paleozoic rocks in the Carlin-Pinon Range aret, Nevada: U.S. Crittenden, M. D., Jr., IS 72, Willard thrust and Cache allochthon, Utah: Geological Society of America Bulletin, v. 83, Geological Survey Professional Paper 867-A, p. A1-A87. p. 2871-2880. Stifel, P. B., 1964, Geology cf the Terrace and Hogup Mountains, Box Elder County, Utah [Ph.D. thesis): Salt Lake City, Crittenden, M. D., Jr., Coney, P. J., and Davis, G. H., 1980, Cordilleran metamorphic core complexes: Geological Society Utah, University of Utah, 173 p. of America Memoir 153,490 p. Turner, F. J., and Weiss, L. J., 1963, Structural analysis of metamorphic tectonites: New York, McGraw-Hill, 545 p. Dickinson, W. R., Harbaugh, D. W., Sailer, A. H., Heller, P. L., and Snyder, W. S., 1983, Detrital modes of upper Wernicke, B., 1981, Low-anj;le normal faults in the Basin and Range province: Nappe tectonics in an ext;nding orogen: Paleozoic sandstores derived from Antler orogen in Nevada: Implications for nature of Antler Orogeny: American Nature, v. 291, p. 645-648. Journal of Science v. 283, p. 481-509. Wernicke, B„ and Burchfiel, U. C., 1982, Modes of extensional tectonics: Journal of Structural Geology, v. 4, p. 105-115. Dixon, J. S., 1982, Regional structural synthesis, Wyoming salient of Western Overthrust Belt: American Association of Zoback, M. L„ Anderson, R. E., and Thompson, G. A., 1981, Cainozoic evolution of the state of streis and style of Petroleum Geologists Bulletin, v. 66, p. 1560-1580. tectonism of the Basin and Range province of the western United States: Royal Astronomical Socie ty Philosophi- Doelling, H. H., 1964, Geology of the northern lakeside Mountains and the Grassy Mountains and vicinity, Tooele and cal Transactions, v. A300, p. 407-434. Box Elder Counties Utah [Ph.D. thesis): Salt Lake City, Utah, University of Utah. 1980, Geology anc mineral resources of Box Elder County, Utah: Utah Geological and Mineral Survey Bulletin 115, 251 p. Dover, J. H., 1983, New dita on thrusts in northeastern Utah and southwestern Wyoming: Geological Society of America Abstracts with Programs, v. 15, p. 318-319. Gans, P. B., and Miller, E. L., 1983, Style of mid-Tertiary extension in east-central Nevada: Utah Geological and Mineral MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 17, 1983 Survey Special Studies 59. p. 107-139. REVISED MANUSCRIPT RECEIVED MARCH 14, 1984 Gordon, M., Jr., 1971, Biostratigraphy and age of the Carboniferous formations, in Brew, D. A., Mississippian stratigraphy MANUSCRIPT ACCEPTED MARCH 28, 1984 of the Diamond Pejik area. Eureka County, Nevada: U.S. Geological Survey Professional Paper 661, p. 34-55. CORNELL UNIVERSITY CONTRIBUTION NO 779

Printed :n U.S.A.

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