Timing and Structural Expression of the Nevadan Orogeny, Sierra Nevada, California

Timing and structural expression of the Nevadan orogeny, Sierra Nevada, California

RICHARD A. SCHWEICKERT Department of Geological Sciences and Mackay School of Mines, University of Nevada-Reno, Reno, Nevada 89557 NICHOLAS L. BOGEN Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109 GARY H. GIRTY Geology Department, University of Kansas, Lawrence, Kansas 66045 RICHARD E. HANSON Department of Geology, School of Mines, University of Zambia, Lusaka, Zambia CHARLES MERGUERIAN Geology Department, Hofstra University, Hempstead, New York 11550

ABSTRACT style, from relatively intensely developed in whether the structures are penetrative or non- the north to very weakly developed in the penetrative. Pre-Nevadan deformation also oc- The Nevadan orogeny was a very short- south. curs locally in the western belt but cannot be lived event in the Late Jurassic that involved The regional extent and geometry of the shown at the scale of Figure 1. The nature and the deformation of a great variety of rock Nevadan structures indicate that the Nevadan style of superimposition of Late Jurassic "Neva- types and Paleozoic and Mesozoic terranes orogeny involved underthrusting of island- dan" structures on the older structures are enig- throughout the extent of the Sierra Nevada. arc rocks on the west and significant crustal matic and are complicated by the fact that both The Nevadan structures show great variation shortening in the central and eastern belts. older and younger structures of similar orienta- in style but relatively constant orientations. These features suggest that the Nevadan orog- tion have been identified in some areas. These relations can be explained by consider- eny resulted from the collision of the island The purpose of this paper is to present results ing the prior histories of the various terranes. arc (western belt) with an andean-type arc of our own structural studies that bear on the Slaty cleavages and tight folds are the (eastern belt) situated at the western edge of nature of Nevadan deformation. Over the past characteristic main-phase structures in the North America. eight years, we have been working in a variety western belt of Jurassic island-arc volcanic of areas from the central to the northern Sierra, rocks and flysch-type sedimentary rocks. A and from the western foothills eastward to the INTRODUCTION strip of phyllites and greenschists along the high country. We first outline the classical defi- eastern edge of the belt apparently represents nition of the Nevadan orogeny and early ideas similar Jurassic rocks that were deformed Prebatholithic wall rocks of the Sierra Ne- about its timing. We then summarize critical age and metamorphosed at greater depths, prob- vada underwent a complex structural history data that enable correlation of Nevadan struc- ably during underthrusting of the western prior to the emplacement of the voluminous tures in different parts of the Sierra Nevada and belt beneath the central belt. The central belt Late Jurassic and Cretaceous batholiths. Prog- describe examples of "Nevadan" structures in of Paleozoic metasedimentary and metavol- ress in unraveling the complex structural his- different terranes where we can constrain their canic rocks shows the most extreme variation tory has come slowly because the plutonic rocks ages, to illustrate the importance and influence in style of main-phase structures, from weak, obliterated much of the pre-existing rock and of the early history of the rocks. Detailed ac- spaced to crenulation cleavages in the south, separated the remnants into many discrete belts counts of pre-Nevadan structure and metamor- where polyphase deformed rocks formed a and roof pendants. phism of the older units are not presented here, structural basement, to slaty and phyllitic One of the most widely recognized deforma- but they are forthcoming. Finally, we interpret cleavages and asymmetric to isoclinal folds in tional events occurred in the Late Jurassic, dur- the significance of the Nevadan structures. One the north, where most of the Paleozoic base- ing the Nevadan orogeny (Knopf, 1929; Hinds, of our main conclusions is that pre-Jurassic ter- ment rocks lack penetrative pre-Nevadan 1934; Taliaferro, 1942; Clark, 1964; Bateman ranes and Mesozoic slate-graywacke-volcani- fabrics. Eastward-directed thrust faulting ap- and Clark, 1974; Schweickert and Cowan, clastic-rock sequences exhibited major contrasts parently was important only in the northern 1975; Saleeby and others, 1978; Schweickert, in mechanical behavior during Late Jurassic part of the range, where main-phase deforma- 1978, 1981; Saleeby, 1981; Nokleberg and orogenesis (Bogen and others, 1980). tion was most intense. The eastern belt of Kistler, 1980). Structures formed during the Jurassic and Triassic magmatic arc-volcanic Nevadan orogeny were superimposed on a great THE NEVADAN OROGENY and sedimentary rocks defines the core of a variety of rock types and pre-existing structures major synclinorium, and the rocks contain and this resulted in a wide variety of styles of The presence of folds and cleavage in penetrative slaty cleavages and asymmetric, Nevadan deformation. Figure 1 shows three Oxfordian-Kimmeridgian sedimentary and vol- tight to isoclinal folds. principal lithotectonic belts of the Sierra, re- canic rocks of the western belt, west of the Me- A late phase of Nevadan structures, con- ferred to as the western, central, and eastern lones fault zone (Fig. 1), has long been taken as sisting of northeast-trending cleavages and belts, and also shows areas containing evidence definitive evidence for Late Jurassic orogenesis minor folds, also shows a marked variation in of pre-Nevadan regional deformation, indicating in the Sierra Nevada (Knopf, 1929; Taliaferro,

Geological Society of America Bulletin, v. 95, p. 967-979, 8 figs., 2 tables, August 1984.

967

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1942). These structures are locally cut by Lower volcanic rocks of the Sierra Nevada delineates a Age Constraints—A New Look Cretaceous plutons, and thus the deformation major synclinorium, with Triassic and Jurassic was regarded as later than Oxfordian age and rocks in the core of the structure flanked by Figure 2 presents data that provide the tight- earlier than Cretaceous. The occurrence of rela- progressively older sequences of rocks in the est brackets on the timing of the Nevadan oro- tively undeformed Tithonian sedimentary rocks limbs. Structural and stratigraphic relationships geny in the Sierra Nevada. Dated plutons in the Knoxville Formation to the west in the indicated to them that this synclinorium formed discussed in the text are shown in Figu e 1. In Coast Ranges led to the conclusion that the during the Nevadan orogeny. (We discuss the Figure 2, two problems are inherent. (1) The Nevadan deformation was pre-Tithonian (Talia- evidence for the synclinorium and its regional radiometric ages of stage boundaries in the Ju- ferro, 1942). In the Mount Jura area of the significance below.) rassic are poorly known (Armstrong, 1978; Har- northern Sierra Nevada (Fig. 1), Jurassic rocks Bateman and Clark (1974) recently estimated land and others, 1982). We have E.dopted as young as Callovian in age also contain a slaty the younger limit of the Nevadan orogeny to be Armstrong's (1978) suggested ages rather than cleavage, suggesting a similar Nevadan structur- about 140 m.y., on the basis of K-Ar ages of those of Van Hinte (1976) or Harland and oth- al history. plutons that cut the Nevadan cleavage, but our ers (1982), because Armstrong's tims scale Bateman and others (1963) argued that the compilation of the data suggests an older age, seems to pose fewer conflicts with geologic rela- regional pattern of metasedimentary and meta- -154 m.v. Figure 1. Regional geologic sketch map of the Sierra Nevada showing the western (W), central (C), and eastern (E) belts, and areas; of pre-Nevadan deformation. Insert shows location of this PRE-NEVADAN DEFORMATION map on the map of California. See text for discussion of northern (N) and Pre-mid Jurassic southern (S) parts of the belts. The Mesozoic deformation western belt consists mainly of J urassic metavolcanic and metasedimentary Pre-Carboniferous deformation rocks. The central belt consists of the lower Paleozoic Shoo Fly Complex nonpenetrative and upper Paleozoic pyroclastic rocks in the north; the Calaveras Complex penetrative (horizontally ruled) and the Shoo Fly Complex (checkered pattern) occupy 0 50 km the central belt in the south. The east- 1 I I I I—I ern belt (outcrops outlined) consists of Triassic and Jurassic metavolcanic and metasedimentary rocks. Small, Irregu- lar unpatterned areas in the western metamorphic belt are Mesozoic plutons. Isotopically dated Cretaccous(?) metavolcanic rock:; between lat. 37°N and 38°N are not differentiated from rocks of the eastern belt. The inferred position of the axial-surface trace of the Nevadan synclinorium is shown in this region, although it is largely obliterated by granitic rocks or concealed beneath Cretaceous(?) metavolcanic rocks. Ages of dated plutons that constrain the age of the Nevadan orogeny are plotted; ages that provide the nar- rowest brackets are plotted in Figure 2. The foothills suture follows the Me- lones fault (MFZ) north of lat. 38°15'N and the Sonora fault (SF) south of lat. 38°15'N. Ar = Arnold; BL = Bowman Lake; MJ = Mount Jura; M = Mari- posa; P = Placerville; RRP = Ritter Range pendant; S = Sonora; SB = Sierra Buttes; BS = blueschist locality; A, B = localities discussed in tex I.

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the western belt thus is older than about 150 m.y., yet younger than the Oxfordian-Kimme- WESTERN CENTRAL EASTERN ridgian boundary (taken at about 158 m.y. B.P). K N S Farther north, along the Feather River, Trias- 130- N s N s -130 ki sic cherts west of the Melones fault zone (Irwin

Q_—- and others, 1978; Hietanen, 1973). contain a Guad. northwest-trending slaty cleavage. Hietanen's

Co 140- -140 mapping showed that these rocks were force- fully intruded by tonalitic plutons, one of which has yielded a 146-m.y. K-Ar hornblende age Tith. Ind. , Yuba . (Fig. 1) (Evernden and Kistler, 1970). -150- Ar. -150 In the central belt (Fig. 1), age constraints are 1 1 , Hay. , , Hoy. , ^ Kim. as follows. In the northern part, a strong 1 northwest-trending slaty to phyllitic cleavage is trfÖxf." Mairiposa \ Fm. 1 1 developed in lower and upper Paleozoic rocks. 160" 1 ' dikes ' -160 co 1 We have traced the same cleavage into the Call. ' Sland. ' ' 1 Lower and Middle Jurassic (Toarcian to Bajo- Mt. ' ^ Bath. cian) Sailor Canyon Formation in the eastern

^ 170" Jura Ritter -170 belt (Girty and others, 1980). Farther north, a similar cleavage with the same orientation af- ^ Baj. fects Callovian and possibly younger rocks in the Mount Jura area (McMath, 1966) (this age bracket is shown in Fig. 2). The strong cleavage in rocks of all ages therefore appears to be Figure 2. Age data bearing on the ages of Nevadan structures. Only features providing Nevadan. tightest age brackets are shown. Shaded band shows approximate interval of Nevadan oro- In the southern part of the central belt, near geny. See text for discussion and Figure 1 for locations of data points. Western belt: Yuba = lat. 38°N, a swarm of mafic dikes emplaced pluton on Yuba River near lat. 39°30'N, K-Ar age; Guad. = Guadelupe pluton southwest of within the Calaveras and Shoo Fly Complexes Mariposa, K-Ar age; Mariposa Fm.: fossil age. Central belt: Hay. = Haypress Creek granodior- (Schweickert, 1981; Schweickert and Bogen, ite (see eastern belt); Ar. = pluton near Arnold, K-Ar age; dikes = mafic dike swarm cutting 1983) postdates the 170-m.y.-old Parrotts Ferry Calaveras and Shoo Fly Complexes, K-Ar ages; Stand. = Standard pluton near Sonora, K-Ar pluton (Sharp and Saleeby, 1979) and the 164- and U-Pb ages. Eastern belt: Hay. = Haypress Creek granodiorite east of Sierra Buttes, K-Ar m.y.-old Standard pluton (Stern and others, age; Ind. = Independence dike swarm southeast of Ritter Range pendant, U-Pb ages; Mt. Jura, 1981) near Sonora and a pluton with a 161 -m.y. fossil ages; Ritter sequence, fossil ages. Sources of radiometric ages: Evernden and Kistler K-Ar hornblende age (Evernden and Kistler, (1970); Chen and Moore (1979); Sharp and Saleeby (1979); Sharp (1980); Stern and others 1970) east of Placerville (Fig. 1). Sharp (1980) (1981). reported K-Ar ages of about 157 to 159 m.y. on some of these dikes. The dikes are locally folded tions in the Sierra Nevada than does Van Hinte's probably cannot be resolved any more closely and possess a northwest-trending Nevadan time scale, and because Harland and others' than by about ±5 m.y. cleavage. Near Arnold (Fig. 1), a granodiorite (1982) interpretation contains no more recent Throughout the western belt, fossiliferous pluton with a 152-m.y. K-Ar hornblende age data than that of Armstrong (1978). Even if the slate and graywacke of Late Jurassic age (the (Evernden and Kistler, 1970) cuts both the dike time scale of Harland and others (1982) were late Oxfordian to early Kimmeridgian Mariposa swarm and its associated younger cleavage. In used, our conclusions about timing would be Formation and its equivalents) are cleaved and the central belt, Nevadan structures thus can be unchanged. Nevertheless, stage boundaries may tightly folded. In the southern part of the west- bracketed between about 159 m.y. and 152 m.y. have uncertainties of as much as 5 m.y. (2) In ern belt, west of Mariposa, the 139-m.y.-old In the eastern belt, Mesozoic volcanic and most cases, ages of plutons shown in Figures 1 Guadelupe pluton (Evernden and Kistler, 1970) sedimentary rocks are widespread. In the north, 1 and 2 are based on K-Ar hornblende ages ; be- cuts the slaty cleavage in the Mariposa Forma- on Mount Jura (Fig. 1), as noted above, rocks as cause these represent cooling ages, the K-Ar ages tion (Bateman and Clark, 1974; Best, 1963). young as Callovian (early Late Jurassic) are in- are probably somewhat younger than ages of In the northern part of the belt on the Middle volved in the Nevadan folding (McMath, 1966). intrusion. In addition, radiometric ages typically and South Forks of the Yuba River, Evernden East of the Sierra Buttes, the Haypress Creek have associated errors of about 1% to 3% (al- and Kistler (1970) obtained a 150-m.y. K-Ar granodiorite pluton, which yielded a K-Ar though these are rarely reported). In rocks about hornblende age from a granodiorite pluton that hornblende age of 154 m.y. (Hay in Fig. 2) 150 m.y. old, these errors could mean an uncer- crosscuts slaty cleavage in rocks mapped by (Evernden and Kistler, 1970), cuts Jurassic or tainty of 3 to 9 m.y. This indicates that the tim- Clark (1976) that contain Upper Jurassic older metavolcaniclastic rocks bearing the same ing of events bracketed by dated plutons (Kimmeridgian) fossils. Curtis and others (1958) cleavage, placing a tight upper age limit on the reported a 146-m.y. K-Ar hornblende age on the Nevadan deformation. Penryn pluton near the edge of the Great Valley In the southern part of the eastern belt, Ne- 'All K-Ar ages have been recalculated using the revised decay constants recommended by Steiger and (not plotted in Fig. 2), which also cuts Upper vadan cleavages probably occur in the Ritter Jager (1978). Jurassic slates. The penetrative deformation in Range pendant (Nokleberg and Kistler, 1980;

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Tobisch and Fiske, 1982), but the age con- TABLE I. MAIN-PHASE NEVADAN CLEAVAGES PLOTTED [N FIGURE 3 straints are somewhat controversial. Lower Ju- Stratigraphie unit Number of Average orientation Source rassic pelecypods (Rinehart and others, 1959) points occur in part of the section that contains a slaty cleavage (Tobisch and Fiske, 1982). Isotopic WESTERN North A. Mariposa Formation 27 N20°W. 83°NE Creely (1965) and new dala BELT dating of these rocks has given results that are South B. Mariposa Formation 211 N35°W. 85°NE New dala inconsistent with this fossil age. Fiske and To- C. Unnamed phyllite and greenschist 31 N26°W, 90° New data bisch (1978) reported U-Pb zircon ages of 153 CENTRAL North D. Shoo Fly Complex 95 N30°W, 90° New dala and 158 m.y. from ash-flow tuffs that overlie BELT E. Upper Paleozoic pyroclastic rocks S3 N18°W, 72°NE New dala and underlie the fossiliferous strata. The samples F. Bowman Lake batholith 21 N23°W, 72°SW New dala from which the zircon was dated also yielded a South G. Calaveras Complex 41 N27°W, 82°NE New data H Shoo Fly Complex 48 N32°W, 78°NE New dala Rb/Sr whole-rock isochron age of 163 ± 5 m.y., according to Kistler and Swanson (1981). These EASTERN North I. Sailor Canyon Formation 11 N18°W, 78°NE New data BELT isotopic systems produced ages that are 17 to 50 South J. Jurassic-Triassic melavclcanic rocks 88 N23°W. 73°SW Russell and Nokleberf (1977) m.y. younger than the range of possible Early Jurassic ages of the fossils (180 to 212 m.y., Note: vertical planes have two dois on perimeter. • - Average fi pole. according to the time scale of Armstrong, 1978).

Figure 3. Summary stereoplots of main-phase Nevadan cleavages in the western, central!, and eastern belts. Table 1 lists data pertaining to these plots. All plots show poles to dominant cleavages. Western belt: A. Slates of Mariposa Formation or its equivalents. B. Slates of Mariposa Formation. C. Phyllite and greenschist between Melones and Sonora faults. Central belt: D. Shoo Fly Complex near Bowman Lake. E. Upper Paleozoic pyroclastic rocks between Sierra Buttes and Bowman Lake. F. Paleozoic Bowman Lake batholith. G. Upper Paleozoic Calaveras Complex. H. Shoo Fly Complex. Eastern belt: I. Lower Jurassic Sailor Canyon Formation. J. Triassic or Jurassic metavolcanic rocks of Ritter Range pendant. Squares are average orientation. See Table 1 for average orientations and sources of data.

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Kistler and Swanson (1981) postulated struc- scale but conflicts with Van Hinte's (1976) time of probable Jurassic age on the east (phyllite belt tural complications not recognized by Tobisch scale, which places the Oxfordian-Kimmerid- in Fig. 1). The phyllite and greenschist were and Fiske (1982) to explain the isotopic data. gian boundary at about 143 m.y. B.P., 5 to 10 included in the Calaveras Complex by Alternatively, we suspect that the isotopic sys- m.y. later than the ages of plutons cutting Ne- Schweickert (1981) and by many previous tems did not remain closed during the Nevadan vadan cleavages. workers, but our recent work has identified lith- orogeny and/or during later thermal events pro- ologic and metamorphic contrasts between these duced by younger intrusions. STYLES OF rocks and the Calaveras chert-argillite rocks to Southeast of the Ritter Range, dikes of the NEVADAN STRUCTURES the east and their similarity in lithology to Juras- Independence dike swarm (not shown in Fig. 1) sic rocks west of the Melones fault (Schweickert locally crosscut the cleavage in metavolcanic Figure 3 shows summary stereoplots of orien- and Bogen, 1983). Clark (1964) also reported rocks that are similar to those in the Ritter tations of cleavage (Table 1) that we have at- an Upper Jurassic (Kimmeridgian) fossil from Range. Chen and Moore (1979) reported U-Pb tributed to the Nevadan orogeny in regions locality A, in Figure 1. We here consider the ages of 148 m.y. on several dikes in the swarm, discussed above. In nearly all cases, the cleavage probable Jurassic phyllite and greenschist to be thereby bracketing possible Nevadan deforma- parallels the axial surfaces of minor folds. The part of the western belt. A previously unnamed tion between Early Jurassic and latest Jurassic most remarkable feature is the general consis- ductile thrust fault (here called the Sonora fault), (148 m.y. B.P.) tency of orientation of cleavages and axial sur- which we have mapped from near Sonora to the Another problem in the Ritter Range is that faces of minor folds throughout the entire range. southeastern edge of the metamorphic belt, sep- metavolcanic rocks with Cretaceous Rb-Sr and We refer to the northwest-trending structures as arates the phyllite and greenschist from the U-Pb ages (Kistler and Swanson, 1981; Fiske "main-phase" structures, because, in most re- chert-argillite rocks of the Calaveras Complex and Tobisch, 1978) also possess slaty cleavages gions, we have identified a younger generation and thus forms the eastern boundary of the (Tobisch and Fiske, 1982). This could suggest of Nevadan structures (discussed below) that de- western belt in its southern part (see also that in this region all cleavages that are present forms the "main-phase" structures. Schweickert and Bogen, 1983). in both Jurassic and Cretaceous rocks formed The phyllite and greenschist are significant, during Cretaceous time. Alternatively, Tobisch Western Belt because they locally contain evidence of two and Fiske (1982) argued that the Jurassic rocks generations of penetrative, isoclinal folding of the Ritter Range underwent two parallel de- Main-phase structures in the western belt are, (Figs. 4C, 4D), and in many places the domi- formations, first, during the Nevadan orogeny by definition, Nevadan structures. They are best nant schistosity is a composite of S| x S2. These and, second, during the Cretaceous. We note known as slaty cleavage and minor folds in rocks are more strongly recrystallized than are that their interpretation is critically dependent slates and graywackes of the Upper Jurassic the slates west of the Melones fault (as noted by upon the validity of isotopic ages of metavol- Mariposa Formation and its equivalents. Where Eric and others, 1955, and by Baird, 1962), lack canic rocks in the Ritter Range. We nonetheless we have studied the Mariposa Formation in Tuo- primary sedimentary or igneous textures, and adopt Tobisch and Fiske's (1982) interpretation lumne and Mariposa Counties in the southern contain very strong downdip stretching line- of Nevadan structures in the Ritter Range and part of the belt, the dominant structure is a ations that parallel hinge lines of F| and F2 acknowledge that in some parts of the Sierra northwest-trending, penetrative, slaty cleavage folds. We regard the two generations of penetra- care must be taken to distinguish possible Cre- in mudstones and a spaced cleavage in gray- tive structures as main-phase Nevadan struc- taceous structures from cleavages and folds wacke (Schweickert and Bogen, 1983). Best tures, although they probably are slightly older formed during the Nevadan orogeny. (1963) mapped a similar slaty cleavage in the than slaty cleavage in the Mariposa Formation. An over-all consistency is found in the data Mariposa Formation farther south. Cleavage in This is the case because the phyllitic schistosity is from all three belts, as shown by Figure 2. The graywacke is an expression of flattening of rela- locally cut by a northwest-trending crenulation tightest age brackets come from the central belt, tively incompetent volcanic-rock fragments, so- cleavage that is parallel to main-phase cleavages where the Nevadan structures apparently lution of quartz grains, and metamorphic in the slates. These phyllitic rocks thus under- formed in the interval 154 to 159 m.y. B.P. If the crystallization of very fine-grained chlorite and went more intense polyphase deformation and younger age limit on the Mariposa Formation is white mica. Cleavage is parallel to the axial sur- slightly higher-grade metamorphism during the estimated at about 156 m.y. B.P. (Fig. 2), an faces of mesoscopic folds (Figs. 4A, 4B), which Nevadan orogeny than did rocks to the east or interval of about 156 to 154 m.y. B.P. is implied. are relatively uncommon. Metavolcanic units west. We suggest that during the main phase of This result is well within the overlap of probable that underlie and interfinger with the Mariposa the Nevadan orogeny the phyllitic and slaty errors of older and younger age brackets shown generally contain much weaker cleavage and in rocks underwent progressive deformation that in Figure 2 and suggests that the Nevadan de- some places appear to lack cleavage entirely. involved deeper, more ductile, and locally poly- formation occurred within an interval of only a Locally, in areas near the Melones fault, slaty phase deformation and underthrusting of the few million years. In this paper, we take 155 ± 3 cleavage in the Mariposa is deformed by phyllitic rocks than did the slates and that the m.y. as a conservative estimate of the age of northwest-trending, tight folds, possibly also of northwest-trending crenulation cleavage devel- Nevadan deformation. Late Jurassic age. oped in the phyllites as the rocks to the west acquired their slaty cleavage. Fabric data for the Age relations of Figure 2 are also noteworthy, The Melones fault, north of lat. 38°15'N, rocks of both subunits are summarized in Figure because they place a constraint on the age of separates the western belt from Paleozoic rocks 3 and Table 1. the Oxfordian-Kimmeridgian boundary. This of the central belt. South of lat. 38°15'N, the boundary, contained in the Mariposa Formation Melones fault generally forms the boundary be- The southern part of the Melones fault, where (Imlay, 1961), is clearly older than the Nevadan tween slate and greenstone of the Mariposa it separates the slaty rocks from the phyllite and orogeny and thus is older than about 155 m.y. Formation and underlying units on the west greenschist, imbricates the Jurassic rocks of the This is consistent with Armstrong's (1978) time (slate belt in Fig. 1), and phyllite and greenschist western belt and juxtaposes rocks with contrast-

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WESTERN BELT-SOUTH

H Im 0.5m 7.5m

CENTRAL BELT

NORTH NORTH SOUTH

<~mafic dikes

\Shoo Fly quartziie

10cm 5cm

Figure 4. Sketches of typical main-phase Nevadan structures. A-D from southern part of western belt; E-G from central belt. A. Folds of contact between Mariposa and Penon Blanco Formations, south of Sonora. Thinly bedded turbidites overlie massive tuff. B. Asymmetric, similar fold in thinly bedded turbidites of Mariposa Formation, south of Sonora. C. Profile view of disharmonic flow folds of an earlier fabric in greenschist in very narrow part of phyllite belt, 20 km south of Placerville (not shown in Fig. 1). The early fabric is axial planar to local early isoclinal folds that are not present in this outcrop. D. Second-phase ductile disharmonic folds of S] in marble, with dikes of pyritic greenschist. In left part of outcrop there is a long-limbed first-phase isoclinal fold with the Si foliation parallel to its axial surface. To the right of the early

isocline, F2 folds deform the St axial-surface foliation. Phyllite belt, 10 km south of Sonora. E. Similar folds in thinly bedded, quartzose turbidites of Shoo Fly Complex near Bowman Lake. F. Fold in rhythmically bedded chert, Devonian Sierra Buttes Formation, near Sierra Buttes. Spaced cleavage parallels the axial planes of folds. G. Rare fold of mafic dikes that intrude Shoo Fly Complex 15 km northeast of Sonora.

ing structural styles. This portion of the Melones that is parallel to the main-phase cleavage in the fault zone indicate that it was a dip-slip fault. fault was mapped by Eric and others (1955), slates. As noted previously, northwest-trending The fact that more highly deformed anc meta- Clark (1964), Morgan (1976), and Evans and folds of the slaty cleavage also occur locally morphosed rocks occur in its hanging wall Bo wen (1977). Our detailed structural studies near the fault. A detailed description of the Mel- suggests that it was a reverse or thrust fault. It along this part of the fault indicate that the Late ones fault is beyond the scope of this paper but may have been rotated to its present steep dip Jurassic fault trace is typically a 5-m-wide zone is the subject of a forthcoming paper (R. A. when folds and cleavage developed in the Juras- of ductile transportation of layering of rocks on Schweickert and others, unpub. data). sic rocks. We view folding and formation of both sides. Stretching lineations in this zone The narrowness and the ductile nature of the cleavage in conjunction with thrusting on the plunge steeply downdip (R. A. Schweickert and Jurassic Melones fault indicate that it most Melones fault as aspects of Nevadan deforma- others, unpub. data). In some instances, the slaty likely was a deep-seated, ductile shear zone. In a tion in the western belt. Nokleberg and Kistler cleavage in the rocks to the west is more intense ductile shear zone, stretching lineations in the (1980) expressed a similar view, except that they or is penciled near the fault (see Fig. 7 below), foliation may indicate the transport direction regarded the Melones as a right-lateral strike-slip and the rocks acquire a phyllitic sheen. In other across the zone (Ramsay and Graham, 1970; fault, despite the steeply plunging stretching cases, the ductile fault fabric is cut by a cleavage Ramsay, 1980). If so, lineations in the Melones lineations.

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The structure of the northern part of the Me- markedly domainal in many areas. Brittle faults Calaveras (Merguerian and Schweickert, 1980; lones fault is poorly known, but Middle Jurassic commonly parallel the cleavage and axial sur- Merguerian, 1981). Nevadan structures over- or older blueschists that occur along the Yuba faces of folds. print polydeformed Shoo Fly rocks as a River (Fig, 1) contain a definite Nevadan struc- North of the Bowman Lake region (Fig. 1), northwest-trending spaced or fracture cleavage tural overprint (Cebull and Russell, 1979; main-phase Nevadan structures become more that is axial-planar to asymmetric folds. The Schweickert and others, 1980), suggesting that intense, and cleavage surfaces acquire a marked Nevadan cleavage is not penetrative. Late Jurassic movements also occurred along phyllitic sheen. Sedimentary layering in rocks is The dike swarm in the Calaveras Complex the northern part of the fault. strongly transposed into the Nevadan structural also extends into the Shoo Fly. The dikes rarely orientation. are mildly deformed by open folds with Central Belt In general, the slaty cleavage and tight folding northwest-trending axial surfaces and show local of rocks in the northern part of the central belt offset of the dike margins along northwest- The central belt (Fig. 1) contains two resemble Nevadan structures in the Jurassic trending spaced cleavages (Fig. 4G). markedly different bedrock complexes. In the rocks of the western belt, but important differ- north, the lower Paleozoic Shoo Fly Complex ences are that, from south to north, a transition Eastern Belt occupies most of the belt and is overlain by a occurs from cleavages that are more domainal Devonian to Permian pyroclastic sequence and are accompanied by brittle faulting, to pene- Mesozoic volcanic and sedimentary rocks of (Schweickert, 1981; Harwood, 1983). The rocks trative slaty cleavages, and, finally, to phyllitic the eastern belt in all reported cases contain generally are not above chlorite grade, and they cleavages, indicating that the main-phase Neva- penetrative slaty cleavages and asymmetric, tight generally show no evidence of pre-Nevadan dan deformation was more intense in the north. to isoclinal folds (Kistler, 1966a; McMath, metamorphism. South of Placerville (lat. Southern Part. The Calaveras Complex con- 1966; Schweickert, 1976; Brook, 1977; Durrell 38°45'N), the upper Paleozoic-(?)lower Meso- tains two sets of penetrative, pre-Nevadan struc- and d'Allura, 1977; Russell and Nokleberg, zoic Calaveras Complex occupies the major tures that are earlier than 170-m.y.-old plutons 1977; Fiske and Tobisch, 1978; Tobisch and part of the belt but is flanked to the east by a and a mafic dike swarm mentioned above Fiske, 1982; Girty and others, 1980; Nokleberg narrow strip of Shoo Fly rocks (Fig. 1). Unlike (Schweickert, 1979, 1981; Schweickert and and Kistler, 1980; Nokleberg, 1981; Harwood, the Shoo Fly in the north, the Calaveras Com- Bogen, 1983). Figure 5 shows that in some re- 1983). Although intense contact-metamorphic plex and Shoo Fly in the south contain medium- gions (for example, north of Sonora) (Fig. 1) the recrystallization has overprinted the Nevadan to high-grade metamorphic fabrics (Schweick- dominant schistosity trends easterly and is axial- fabrics in the roof pendants, style and intensity ert, 1979; Merguerian and Schweickert, 1980; planar to tight folds that deform an earlier tec- of development of folds and cleavages in the Merguerian, 1981, 1982). Owing to these pro- tonic foliation. The earlier foliation itself is eastern belt most nearly resemble those of the found differences in pre-Nevadan structural and axial-planar to Fj isoclinal folds and is com- Mariposa Formation in the western belt. To- metamorphic history, marked contrasts in the monly marked by flattening of chert clasts in bisch and others (1977) and Tobisch and Fiske styles of Nevada structural fabrics occur in the pebbly mudstone and by growth of metamor- (1982) documented large amounts (as much as central belt. phic minerals. Probable Nevadan structures are 84%) of flattening strain associated with these Northern Part. In the northern part of this expressed in the following ways. (1) Upper Ju- penetrative structures in the Ritter Range belt, the Shoo Fly Complex and the overlying rassic mafic dikes are locally folded and warped pendant. Paleozoic pyroclastic sequence did not possess about northwest-trending axial surfaces; (2) the Minor structures and map relations of rocks regionally extensive, penetrative structural fab- schists separating the dikes possess weak to in the eastern belt indicate that these rocks rics prior to the Nevadan orogeny. The Shoo Fly prominent crenulation cleavages that parallel generally occupy the core of a major synclino- contains at least one generation of pre-Nevadan axial surfaces of folded dikes; and (3) the dikes rium (Schweickert, 1981). For instance, the folds, and, in the western part of the belt, pre- occasionally contain spaced cleavages. Sporadic western limb of the synclinorium is best pre- Devonian cleavages (Bond and Schweickert, lenses of talc schist (metaserpentinite) that are served between lat. 39° and 40°N and is formed 1981; Schweickert and Hanson, 1982), but in interlayered with the east-trending pre-Nevadan by steeply east-dipping and eastward-younging most areas, these rocks do not contain pre- foliation contain a penetrative, northwest-trend- Jurassic and Paleozoic sequences. McMath Nevadan cleavages or lineations (Girty and oth- ing Nevadan foliation, as shown in Figure 5. (1966) and Harwood (1981a, 1981b) identified ers, 1980; Varga and Moores, 1981). Except for These rocks had such high ductility and low two localities (MJ and B in Fig. 1) where the local slump folds, pre-Nevadan structures are strength that they acquired penetrative fabrics, axial region and both limbs of the synclinorium not present in the Paleozoic pyroclastic sequence whereas the enclosing schists developed sporadic are preserved. crenulations and the dikes rarely developed where we have studied it. Harwood (1983), Between lat. 37° and 38°N, Upper Triassic spaced (or fracture) cleavages. Cleavages none- however, reported pre-Late Triassic folds and and Lower Jurassic metavolcanic and metased- theless have parallel orientations in all three cleavage near locality B in Figure 1. imentary rocks (Kistler and Swanson, 1981; rock types. We believe that the penetrative, Main-phase Nevadan structures in both the Rinehart and others, 1959) dip vertically to northwest-trending foliation in the talc schists is Shoo Fly and the overlying pyroclastic sequence steeply southwest and young to the southwest a flattening foliation, as are other Nevadan folia- generally consist of slaty cleavage in mudstone (Huber and Rinehart, 1965; Rinehart and Ross, tions (compare with Aune, in Strand and and argillite, and spaced cleavage in sandstone, 1964; Kistler, 1966b; Fiske and Tobisch, 1978; Koenig, 1965). chert, pyroclastic rocks, and hypabyssal intru- Keith and Seitz, 1981; Tobisch and Fiske, sive rocks. Cleavages are parallel to the axial The Shoo Fly in the southern part of the belt 1982). These rocks unconformably overlie un- planes of associated folds that are tight and had an extremely complex structural history. At fossiliferous upper Paleozoic(?) rocks to the east asymmetric and that have variable orientations least two generations of penetrative structures in and are unconformably overlain to the west by of hinge lines (Figs. 4E, 4F). The cleavage is the Shoo Fly predate the oldest structures in the probably Cretaceous metavolcanic rocks (Fiske

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CENTRAL BELT- S

4 X 1000 FT

S3 NE VAD AN Penetrative in talc schist)

.MAFIC DIKES (post-!70) Figure 5. Structural map of the Calaveras Complex on Rose and Knight Creeks 10 km north of Sonora (Fig. 1), showing ^ÔC"- S2 AXIAL-PLANE FOLIATION superposition of brittle Nevadan structures (S3) on pre-Nevadan metamorphic fabrics (Si and S2). Note that the Nevadan cleavage FORM LINES is penetrative in lenses of talc schist.

and Tobisch, 1978; Kistler and Swanson, 1981; ville thrust) that carried the inverted western parts of the prebatholithic bedrock except the Tobisch and Fiske, 1982). We interpret these limb of the Nevadan synclinorium over the core southern part of the central belt. The'e, poly- relations to indicate that the eastern limb of the of the structure (see also Speed and Moores, metamorphic rocks of the Calaveras and Shoo synclinorium is preserved between lat. 370 and 1981). This relationship suggests that the thrust- Fly Complexes contain only sporadic, weakly 38°N, and thus in Figures 1 and 7 (see below) ing occurred as a result of the shortening that developed Nevadan crenulation cleavage and a we have inferred that the axial-surface trace of produced the synclinorium itself. few open folds. In addition, the major pre- the synclinorium, although concealed or obliter- In the western belt near lat. 39°N, Xenophon- Nevadan east-trending dike swarm that occurs ated by younger rocks, lies west of the exposures tos and Bond (1978), Tuminas (1980), and in the Calaveras and Shoo Fly rocks shows very of Jurassic and Triassic metamorphic rocks in Moores and Day (1983) mapped west-dipping little evidence of shortening, indicating that these this region. Minor folds and slaty cleavages thrust faults bounding several structural blocks bedrock complexes functioned as nearly rigid (Fig. 3) apparently parallel the axial surface of and apparently again indicating eastward over- blocks with only minor amounts of internal the synclinorium; there are probably several thrusting (see Fig. 8 below). According to Speed strain during the main-phase Nevadan deforma- orders of parasitic folds developed. and Moores (1981) and Moores and Day tion. These rocks nevertheless are flanked by (1983), these thrusts are folded by northwest- Mesozoic rocks both east and west that under- East-Vergent Thrust Faults trending Nevadan folds. went very strong penetrative deformation. The Nevadan structural contrast is most extreme Available evidence indicates that, north of lat. Summary of Main-Phase Structures across the Sonora fault (Fig. 1), west cf which 39°N, thrust faults involve rocks of all three Mesozoic phyllite and greenschist underwent belts. In the Mount Jura area (Fig. 1), McMath Figure 8 (see below) shows that during the two phases of intense Nevadan deformation, and (1966) documented the existence of an impor- main-phase Nevadan deformation, intense pene- east of which previously metamorphosed Pa- tant northeast-directed thrust fault (the Taylors- trative cleavages were widely developed in all leozoic chert-argillite rocks show only sporadic

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TABLE 2. LATE-PHASE NEVADAN CLEAVAGES PLOTTED IN FIGURE 6 cases, dated Upper Jurassic or Lower Creta- ceous plutons. The structures are strongly Slratigraphic unit Number of Average orientation Source points developed in the northern part of the range and appear to die out gradually as they are traced to WESTERN North No data BELT South A. Mariposa Formation 12 N30°E, 90° New data the south. Figure 6 shows stereoplots of late- B. Unnamed phyllite and greenschist 15 N16°E,80°SE New data phase structures. In the northern part of the central and eastern CENTRAL North C. Shoo Fly Complex 146 N25°E, 80°SE New data BELT belts, the late-phase structures are N20°-40°E- D. Upper Paleozoic pyroclastic 76 N20°E, 83°SE New data trending, domainal, spaced to slaty cleavages; South E. Calaveras Complex 9 N17°E, 90° New data F. Shoo Fly Complex 98 N30°E, 90° New data counterclockwise asymmetric folds with N20°- 40°E-trending axial surfaces; and minor faults EASTERN North G. Sailor Canyon Formation 13 BELT with the same trend (Girty and others, 1980; South No data Varga, 1980). Many of the more conspicuous mesoscopic folds in the region are late-phase Note: vertical planes have two dots on perimeter. • = Average ii pole. folds. These structures are cut by the Haypress Creek pluton east of Sierra Buttes (Fig. 1) and therefore definitely are dated as Nevadan. crenulations and kinks. These metamorphic structural relations of the east-vergent thrusts to Weaker structures of similar orientation occur rocks formed a rigid basement during the Neva- the Melones fault are far from clear, the style of widely in the southern parts of the western and dan deformation. Jurassic faulting seems to differ significantly central belts, where they again postdate and de- South of lat. 39°N, the Melones and Sonora from that to the south of lat. 39°N. form the main-phase structures. The Mariposa faults are interpreted as west-vergent Nevadan Formation contains domainal, spaced cleavages thrust faults, along which Jurassic rocks were Late-Phase Nevadan Structures trending N20°-40°E, which are axial-planar to imbricated and thrust beneath Paleozoic base- open, counterclockwise, asymmetric folds of the ment rocks of the central belt. North of lat. Cleavage and Folding. In each of the areas main-phase cleavage (Bogen, 1979). Such folds 39°N, several east-vergent thrusts occur in addi- where we completed detailed structural studies, actually fold the Melones fault in the Sonora tion to the west-vergent Melones fault, indicat- we have discovered evidence of a late phase of area, as shown in Figure 7. If these folds and ing a more complex pattern of imbrication Nevadan structures (Table 2) that deforms the cleavages with northeast orientations are the during the Nevadan orogeny. Although the main-phase cleavage and predates, in some same age as those in the northern Sierra (see

Figure 6. Stereoplots of late-phase Nevadan structures. All plots show poles to cleavage. Table 2 contains data relevant to these plots. Western belt (no data available from northern part): A. Mariposa Formation, south of Sonora. B. Phyllite and greenschist, south of Sonora. Central belt: C. Shoo Fly Complex near Bowman Lake. D. Paleozoic pyroclastic rocks near Bowman Lake. £. Calaveras Complex northeast of Sonora. F. Shoo Fly Complex northeast of Sonora. Eastern belt (no data available from southern part): G. Sailor Canyon Formation. Squares are average orientations. See Table 2 for average orientations and sources of data.

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It is possible that Cretaceous deforrriational MELONES Figure 7. Geologic sketch events that are better known in high-country FAULT map of the Melones fault roof pendants like the Ritter Range (Fig. 1) ductile shear zone where it crosses Sullivan (Nokleberg and Kistler, 1980; Tobisch and Creek, 10 km southwest of Fiske, 1982) may have produced some of the Sonora (Fig. 1). Note that northwest- and northeast-trending crenulations trace of Melones fault is in the Calaveras and Shoo Fly, but we consider folded by northeast-trendiing this less likely because no evidence of strong late Nevadan folds. Key: Cretaceous deformation has yet been produced 1. Mariposa Formation. in the western metamorphic belt. In fact, we con- Dashes show orientations of sider it highly unlikely that the Nevadan orog- main-phase slaty cleavage. eny left no imprint in the basement rocks, and Dotted lines are form lines of that relatively minor Cretaceous deformation, bedding. 2. Phyllite and instead, produced the crenulation we observed. greenschist. Black lenses are It should be noted that our interpretation of marble. Line pattern shows axial surface crenulation in the Calaveras and Shoo Fly S orientation of phyllitic schiis- Complexes differs from that of Tobissh and tosity. Shorter line segments Fiske (1976). They described conjugate crenula- show traces of an earlier foli- tions in the Shoo Fly Complex (their area C) » J ation. Note ductile shear zone and offered the hypothesis that these conjugate within phyllite and green- 2. folds of foliation in the Shoo Fly correla te with schist. conjugate folds in the eastern belt (their areas A!

and A2) and in the phyllites of the western belt (their area B). They interpreted these structures to indicate a minor compressional event with the discussion below), they indicate that the Me- sized a point made by Nokleberg and Kistler direction of maximum compression oriented lones was inactive by the late phase of the Ne- (1980) that northwest-striking slaty cleavages parallel to the length of the range during a vadan orogeny. N20°-40°E-striking crenulation cannot be assumed to be Nevadan cleavages relaxation of compressive forces at the end of the cleavages, spaced cleavages, and kink folds without corroborating evidence. In this papier, Nevadan orogeny. occur sporadically in both the Calaveras and we have included only data on slaty cleavages We disagree with their interpretation for sev- Shoo Fly Complexes (Fig. 6). Tobisch and Fiske for which reliable age brackets are available. eral reasons. First, not all of the conjugate folds (1976) discussed crenulations, some of which The west-trending, later cleavages noted here are described by Tobisch and Fiske (1976) deform a may be related to this generation, in the southern probably Cretaceous, but, owing to the prob- Nevadan cleavage. As discussed above, the Cal- parts of the western, central, and eastern belts lems of correlation discussed below, no more averas and Shoo Fly Complexes contain not one (Fig. 1). We have not included their data in definitive statement can be made. but several pre-Nevadan structural fabrics and Figure 6, however, because of uncertainties we believe the foliation in their Figure 4C is about correlation of their structures (as discussed Problems of Correlation of Crenulation pre-Nevadan. In addition, because the Shoo Fly below). Cleavages in Basement Rocks in particular had such a complex structural history, conjugate or box folds cannot automati- Cretaceous(?) Folds and Cleavages Major problems concern the identity and ages cally be assumed to be Late Jurassic or younger. of crenulation cleavages in the Sierra Nevada. We have observed box folds produced by the In all areas that we have studied, the struc- Although it is possible to date crenulation cleav- interference of different generations of pre- tures previously described are overprinted by ages in some parts of the basement complexes: of Nevadan folds in the Shoo Fly (R. A. Schweick- still later, domainal, west-trending, spaced the central belt, this is not true for all occurren- ert and C. Merguerian, unpub. data), but cleavages or crenulation cleavages. In some ces. We tentatively correlate northwest- and nowhere in the Shoo Fly have we observed con- cases, such as east of Bowman Lake (Fig. 1), northeast-trending crenulations in the Calaveras jugate folds involving the crenulations that we these structures can be traced into Jurassic or and Shoo Fly Complexes with main-phase a nd have provisionally assigned to the Nevadan older plutons. Although these structures have late-phase Nevadan structures for two reasons. orogeny. not been studied in detail, we have locally noted (1) Age relations indicate that they are the only This points up a second problem with To- open folds, the axial planes of which are paral- structures in these rocks that could have formed bisch and Fiske's (1976) hypothesis. Conjugate leled by these cleavages. Similar west-trending during the Nevadan orogeny. The much more folds in all parts of the Sierra need not be, and structures have been recognized in many high- prominent schistosities and foliations in these probably are not, of the same age. Indeed, con- country roof pendants, where, according to rocks are clearly pre-Nevadan. (2) In the west- jugate folds in part of Tobisch and Fiske's

Nokleberg and Kistler (1980) and Nokleberg ern metamorphic belt of the Sierra Nevada, (1976) area A2 apparently occur in mid- (1981), they are of mid-Cretaceous age. Tobisch Nevadan deformation was much more intense Cretaceous rocks (Fiske and Tobisch, 1978; To- and Fiske (1982) reported evidence from the than any later Cretaceous deformations (Talia- bisch and Fiske, 1982) and thus are not Ritter Range pendant (Fig. 1) for Cretaceous ferro, 1942). We have not used orientation as Nevaclan. slaty cleavage that is statistically parallel to evidence for assigning them to the Nevaclan A third problem is that it is commonly very probable Nevadan cleavage. They re-empha- deformation. difficult to deduce exact age relationships be-

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tween the axial planes of conjugate or box folds. It is critical to Tobisch and Fiske's (1976) hypothesis to establish that both limbs of conjugate folds developed at the same time. We cannot, however, deduce from their photograph of a conjugate fold in the Shoo Fly (Tobisch and Fiske, 1976, Fig. 4C) what the age relationships are. This may be a fairly common problem, compounded by the fact that, as they noted, there are very few examples of conjugate folds to study. We suggest two possible alternative interpre- tations for the conjugate folds that Tobisch and Fiske (1976) described in the southern part of the Shoo Fly. (1) The conjugate folds may be pre-Nevadan box folds produced by interference of two unrelated structures. (2) The crenulations are Jurassic or younger, as they suggested, but result from the interference of northerly trending main-phase and/or late-phase Nevadan crenulations with a westerly trending set of crenulations. The westerly set may be part of a generation of Cretaceous crenulations described by Nokleberg and Kistler (1980). A similar interpretation was suggested by Nokleberg and Kistler (1980) for the westerly set of crenulations that Tobisch and Fiske (1976) described from the Lower Jurassic rocks of the Ritter Range pendant (Fig. 1). Owing to the impossibility of dating precisely the crenulations that Tobisch and Fiske (1976) reported in the Shoo Fly, there is at present no basis for choosing between their interpretation and either of our two alternatives, but we believe that all are possibilities that should be borne in mind while studying crenulation cleavages and box folds in basement rocks like those of the Calaveras and Shoo Fly Complexes.

DISCUSSION

The main-phase Nevadan deformation af- fected all parts of the Sierra Nevada north of lat. Figure 8. Summary sketch map schematically showing areas of penetrative and nonpenetra- 37°N (see also Nokleberg and Kistler, 1980). tive main-phase Nevadan structures and late-phase structures. Orientations of patterns approx- South of lat. 37°N, problems are met in distin- imate structural orientations. The western (W), central (C), and eastern (E) belts are shown, guishing Nevadan deformation from intense together with the inferred axial-surface trace of the Nevadan synclinorium. The map does not Cretaceous deformation (J. B. Saleeby, 1982, show variations in intensity of slaty cleavage that occur in the western belt. Faults shown are personal commun.). Although detailed strain believed to have been active during the Nevadan orogeny. Compare with Figure 1 and note analyses have been done only in parts of the that areas lacking penetrative Nevadan fabrics are those characterized by one or more strong central Sierra in the Ritter Range pendant (To- pre-Nevadan fabrics. bisch and others, 1977; Tobisch and Fiske, 1982), qualitative observations suggest that the cantly, east-directed Nevadan thrust faults are the polymetamorphic basement complexes in intensity of the main-phase deformation was known only in the northern parts of the range, the southern part of the central belt responded as greatest in the northern Sierra in each of the suggesting that in this region shortening was not a nearly rigid block. In this area, the bulk of the three main belts. In the northern Sierra, all entirely accommodated by flattening and flow- strain due to the Nevadan deformation is inter- rocks, including thick lower and upper Paleo- age but was in part taken up on east-directed preted as rotation of the Calaveras and Shoo Fly zoic sequences in addition to Mesozoic se- thrusts. Complexes on the western limb of the Nevadan quences, contain moderate to strong cleavages Major structural contrasts occur in the south- synclinorium without penetrative deformation. and asymmetric minor folds (Fig. 8). Signifi- ern parts of the three belts (Fig. 8). For example, In contrast, slaty cleavage and in some places

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tight to isoclinal folds formed in Jurassic and which had pre-Nevadan structural fabrics and gether with regional relations discussed by Triassic sequences at the same latitude, both to polyphase metamorphic histories, probably many other geologists, are best attributed to the west and to the east. Along the projected lacked significant ductility contrast and pore a collisional event. Age constraints discussed axial surface of the synclinorium, however, be- fluids. These rocks had essentially been "strain here indicate that deformation occurred about tween lat. 37°50'and 38°30'N, Mesozoic rocks hardened" and therefore were less susceptible to 155 ± 3 m.y. B.P. If the Mariposa Formation apparently are not preserved at the present level penetrative deformation during Late Jurassic is regarded as synorogenic flysch, however, of exposure; the few small pendants are com- orogenesis. the Nevadan orogeny may have begun is early posed of strongly deformed metasedimentary as Oxfordian (N. L. Bogen, unpub. data; rocks of probable Paleozoic age. Mesozoic rocks Significance of the Nevadan Orogeny Schweickert and Bogen, 1983). at this latitude are east of the axial surface of the The structural geology of the Sierra Nevada is synclinorium and apparently occupy the core of The Nevadan orogeny was a very short-lived consistent with the hypothesis that the Nevadan a tight, subsidiary synclinal fold (Brook, 1977) but intense deformational event that occurred orogeny occurred as a result of the Late Jurassic (Figs. 1 and 8). The Mesozoic rocks in this re- throughout the extent of the Sierra Nevada and collision of an island arc, now located in the gion are flanked to the west (and underlain?) by adjacent regions. The major Nevadan structural western foothills, with the basement of an the rigid basement rocks of the Calaveras and feature of the Sierra Nevada, as noted by Bate- Andean-type arc situated at the western edge of Shoo Fly Complexes. Perhaps the presence of man and co-workers (Bateman and others, North America (Schweickert and Cowan, the mechanically competent basement rocks in 1963; Bateman and Wahrhaftig, 1966; Bateman 1975). If this hypothesis is correct, it implies that this region produced a structural culmination in and Clark, 1974), is a synclinorium the axial- ancient, deeply eroded crustal collision zones the axis of the synclinorium, accounting for the surface trace of which trends N25°-30°W and may be remarkably foreshortened. lack of exposure of Mesozoic rocks along the extends some 400 km along the length of the The significance of the late-phase structures is axial trace of the synclinorium at this latitude. Sierra Nevada (Figs. 1, 8). This is a very large- at present unknown. They probably represent The mechanical properties and prior histories scale fold that probably involves the entire oblique northwest-southeast shortening follow- of rock units evidently played major roles in thickness of the crust and is analogous to major ing the main-phase deformation, as suggested by determining their responses to main-phase anticlinoria and synclinoria developed in the Tobisch and Fiske (1976) for their conjugate Nevadan deformation. For example, the Upper northern Appalachians (Cady, 1969). crenulations, and could indicate left-oblique Jurassic graywacke and mudstone of the west- The Nevadan synclinorium encloses the rem- convergence in the late stages of the N;vadan ern belt were deformed soon after they were nants of an isoclinally folded Andean-type arc, orogeny. It is noteworthy that the late-phase deposited, and they probably contained appre- represented by the Upper Triassic and Lower, structures we have described are most intensely ciable pore water during the deformation. These Middle, and Upper Jurassic volcaniclastic and developed in the northern part of the range. rocks probably underwent tectonic compaction pyroclastic rocks of the eastern belt (Schweick- and dewatering during the main-phase Nevadan ert, 1978, 1981). Age relations summarized in ACKNOWLEDGMENTS deformation. The very strongly deformed green- this paper indicate that these rocks were de- schist and phyllite probably represent progres- formed during the Nevadan orogeny prior to the We gratefully acknowledge the continuing sive deformation of Jurassic graywacke and emplacement of the Upper Jurassic and Cre- support of the National Science Foundation greenstone at deeper levels, during underthrust- taceous batholiths. The geometry and the re- (Grants NSF-EAR-76-10979, NSF-EAR-78- ing. The Triassic and Lower to Middle Jurassic gionally consistent orientation of the regional 14779, and NSF-EAR-78-23567) for our work rocks in the eastern belt, composed primarily of main-phase Nevadan cleavage and of the syn- in Paleozoic terranes in the Sierra and that of the marine volcaniclastic rocks, intermediate to si- clinorium itself both argue for a major east- U.S. Geological Survey (Contracts USGS-14- licic pyroclastic rocks with abundant pumice, northeast-directed compressional event near the 08-0001-17724 and USGS-14-08-0001- .8376) and some subaerial ash flows, probably also suf- end of the Jurassic. for our work in the western belt and along the fered a large degree of tectonic compaction and The geology of the southern part of the west- Melones fault. dewatering during the deformation. ern belt, with evidence for ductile thrusting and Discussions and field trips with G. Bond, In the northern Sierra Nevada, Paleozoic progressive deformation of Jurassic graywacke, D. S. Cowan, T. Engelder, D. Harwood. D. L. rocks also acquired penetrative fabrics, but the shale, and greenstone, that produced slaty rocks Jones, D. Kent, J. Saleeby, W. Sharp, W. response overall to Nevadan deformation in in the west and polyphase-folded phyllite and Snyder, O. T. Tobisch, and W. H. Wright, III, much of the region was more brittle than that of greenschist adjacent to the Sonora fault, strongly were especially helpful in forming our opinions the Mesozoic rocks to the west and east. This suggests that the phyllitic rocks represent parts of about the Nevadan orogeny and Sierra Nevada may reflect an advanced state of lithification and the western belt that were more deeply under- geology in general. Gerard Bond, Teriy En- compaction and small amounts of pore fluids in thrust than the slaty rocks. gelder, and Othmar Tobisch reviewed an early the Paleozoic rocks. Although these Paleozoic We believe that the features of the Nevadan version of the manuscript. Mike Carr, Warren rocks generally lacked pre-Nevadan fabrics, the orogeny discussed here, including its short dura- Nokleberg, Ben Page, Jason Saleeby, and Oth- nearly penetrative Nevadan fabrics in them tion, its regional extent, and its main-phase mar Tobisch provided thorough Geological So- developed chiefly because the main-phase Ne- structures involving east-northwest shortening, ciety of America reviews that improved the vadan deformation was more intense in the possibly of the entire crust, and progressive un- clarity of this paper. None of the above, how- north. derthrusting of an island-arc sequence in the ever, is responsible for any errors of fact or in- In the central Sierra, the Calaveras Complex western belt (Schweickert and Cowan, 1975; terpretation that we may have made. K. Coles, and the southern part of the Shoo Fly Complex, Saleeby, 1981, 1982; Schweickert, 1981), to- B. Devlin, E. Nelson, P. Nickeson, D. Stoner,

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and T. Wilson provided able assistance at var- and Walters, R„ 1982, A geologic time scale: Cambridge, England. Saleeby. J. B„ 1982, Polygenetic ophiolite bell of the California Sierra Nevada: Cambridge University Press, Earth Science Series, 131 p. Geochronological and tectonostratigraphic development: Journal of ious times in the field. Harwood. D. S., 1981a, Late Paleozoic pyroclastic rocks in eastern Placer Geophysical Research, v. 87. p. 1803 1824. County, Sierra Nevada, California [abs.]: Geological Society of America Saleeby. J. B„ Goodin. S. E., Sharp. W. D.. and Busby. C. J„ 1978. Early This work was completed while the authors Abstracts with Programs, v. 13, p. 59. Mesozoic paleotectonic-paleogeographic reconstruction of the southern were at Lamont-Doherty Geological Observa- 1981b, Geologic map of the Granite Chief Wilderness Study Area and Sierra Nevada region, in Howell, D. G., and McDougall, K. A., eds., adjacent parts of the Sierra Nevada, California: U.S. Geological Survey Mesozoic paleogeography of the western United States: Society of Eco- tory of Columbia University. Miscellaneous Field Studies Map MF 1273-A, scale 1:62,500. nomic Paleontologists and Mineralogists, Pacific Section. Pacific Coast 1983, Stratigraphy of upper Paleozoic volcanic rocks and regional Paleogeography Symposium 2, p. 311 336. unconformities in the northern Sierra terrane, California: Geological Schweickert. R. A., 1976, Shallow-level plulonic complexes in the eastern Society of America Bulletin, v. 94, p. 413-422. Sierra Nevada, California, and their tectonic implications: Geological REFERENCES CITED Hietanerc, A., 1973. Geology of the Pulga and Bucks Lake quadrangles, Butte Society of America Special Paper 176, 58 p. and Plumas Counties, California: U.S. Geological Survey Professional 1978, Triassic and Jurassic paleogeography of the Sierra Nevada and Armstrong, R. L.. 1978, Pre-Cenozoic Phanerozoic time scale- Computer file Paper 731, 66 p. adjacent regions, California and western Nevada, in Howell, D. G., and of critical dates and consequences of new and in-progress decay- Hinds. N.E.A., 1934, The Jurassic age of the last granitoid intrusives in the McDougall. K. A., eds.. Mesozoic paleogeography of the western constant revisions, in Cohee, G. V.. Glaessner. M. F.. and Hedberg. Klamath Mountains and Sierra Nevada, California: American Journal United Slates: Society of Economic Paleontologists and Mineralogists, H. D., eds.. Contributions to the geologic time scale: American Associa- of Science, v. 227, p. 182 192. Pacific Section. Pacific Coast Paleogeography Symposium 2, tion of Petroleum Geologists Studies in Geology, No. 6, p. 73- 91. Huber, N. K.. and Rinehart. C. D.. 1965, Geologic map of the Devils Postpile p. 36! 384. Baird, A. K., 1962, Superposed deformation in the central Sierra Nevada quadrangle. Sierra Nevada, California: U.S. Geological Survey Geologic 1979, Structural sequence of ihe Calaveras Complex between the foothills east of the Mother Lode: University of California Publications Quadrangle Map GQ-437, scale 1:62,500. Stanislaus and Tuolumne Rivers [abs.]: Geological Society of America in Geological Sciences, v. 42, p. I 70. Imlay, R. W.. 1961, Late Jurassic ammonites from the western Sierra Nevada, Abstracts with Programs, v. 11, p. 127. Bateman, P. C. and Clark, L D„ 1974, Stratigraphic and structural setting of California: U.S. Geological Survey Professional Paper 374D, 1981, Tectonic evolution of the Sierra Nevada Range, in Ernst, W. G., the Sierra Nevada batholith, California: Pacific Geology, v. 8, p. 79 89. p. D1 D30. ed.. The geoteetonie development of California: A symposium in honor Bateman. P. C., and Wahrhaftig, C. W„ 1966, Geology of the Sierra Nevada: Irwin, W. P.. Jones. D. L., and Kaplan, T. A„ 1978, Radiolarians from pre- of W. W. Rubey: Englewood Cliffs, New Jersey, Prentice-Hall, California Division of Mines and Geology Bulletin 190, p. 105 172. Nevadan rocks of the Klamath Mountains, California and Oregon, in p. 87 131. Bateman. P. C.. Clark, L. D., Huber, N. K„ Moore. J. G.. and Rinehart, C. D.. Howell. D. G„ and McDougall. K. A., eds., Mesozoic paleogeography Schweickert, R. A., and Bogen, N. L., 1983. Tectonic transect of Sierran 1963, The Sierra Nevada batholith A synthesis of recent work across of the western United States: Society of Economic Paleontologists and Paleozoic through Jurassic accreted belts: Society of Economic Paleon- the centra! part: U.S. Geological Survev Professional Paper 4I4D, Mineralogists, Pacific Section, Pacific Coast Paleogeography Sympo- tologists and Mineralogists, Pacific Section. 22 p. p. DI-D46. sium 2. p. 303 310. Schweickert, R. A., and Cowan. D. S., 1975, Early Mesozoic tectonic evolution Best, M. G., 1963. Petrology and structural analysis of metamorphic rocks in Keith, W. J., and Seitz, J. F., 1981, Geologic map of the Hoover Wilderness of the western Sierra Nevada. California: Geological Society of America the southwestern Sierra Nevada foothills: University of California Pub- and adjacent study area, Mono and Tuolumne Counties, California: Bulletin, v. 86, p. 1329 1336. lications in Geological Sciences, v. 42. p. 111 157. U.S. Geological Survey Miscellaneous Field Studies Map MF-110IA, Schweickert, R. A., and Hanson. R. E„ 1982, Geologic transect of the northern Bogen, N. L., 1979, Four sets of structures in the Upper Jurassic Mariposa scale 1:62,500. Sierra Nevada along the north fork of the Yuba River: Peninsula Geo- Formation. Tuolumne County, California [abs.]: Geological Society of Kistler. R. W., 1966a, Structure and metamorphism in the Mono Craters logical Society Second Annual Field Trip. 18 p. America Abstracts with Programs, v. 11. p. 70. quadrangle, Sierra Nevada. California: U.S. Geological Survey Bulletin Schweickert, R. A., Armstrong. R. L., and Harakal, J. E., 1980. Lawsonite Bogen. N. L., Girty, G. H„ Hanson, R. E.. Merguerian. C.. and Schweickert, 1221E, 53 p. blueschisi in the northern Sierra Nevada, California: Geology, v. 8, R. A., 1980, Contrasting styles of deformation during the Nevadan oro- 1966b, Geologic map of the Mono Craters quadrangle. Mono and p. 27 31. geny. Sierra Nevada. California (abs.]: Geological Society of America Tuolumne Counties, California: U.S. Geological Survey Geologic Sharp, W. D., 1980, Ophiolite accretion in the northern Sierra [abs.]; American Abstracts with Programs, v. 12, p. 98. Quadrangle Map GQ-462, scale 1:62,500. Geophysical Union Transactions, v. 61, p. 1122. Bond. G. C., and Schweickert, R. A., 1981, Significance of pre-Devonian Kistler. R. W„ and Swanson, S. E„ 1981. Petrology and geochronology of Sharp. W. D., and Saleeby, J.. 1979, The Calaveras Formation and syntectonic melange in the Shoo Fly Complex, northern Sierra Nevada, California metamorphosed volcanic rocks and a middle Cretaceous volcanic neck mid-Jurassic plutons between the Stanislaus and Tuolumne Rivers, [abs.]: Geological Society of America Abstracts with Programs, v. 13. in the east-central Sierra Nevada, California: Journal of Geophysical California [abs.]: Geological Society of America Abstracts with Pro- p. 46. Research, v. 86, p. 10489 10501. grams, v. 11, p. 127. Brook. C. A., 1977, Stratigraphy and structure of the Saddlebag Lake roof Knopf, A.. 1929, The Mother Lode system of California: U.S. Geological Speed, R. C, and Moores. E. M., 1981, Geologic cross-section of the Sierra pendant. Sierra Nevada. California: Geological Society of America Bul- Survey Professional Paper 157, 88 p. Nevada and the Great Basin along 40°N lat„ northeastern California letin. v. 88. p. 321 331. McMath. V. E., 1966, Geology of the Taylorsville area, northern Sierra and northern Nevada: Geological Society of America Map and Chart Cady, W. M., 1969. Regional tectonic synthesis of northwestern New England Nevada, California: California Division of Mines and Geology Bulletin Series, no. MC-28L. and adjacent Quebec: Geological Society of America Memoir 120. 190. p. 173 183. Steiger, R. H., and Jager, E.. 1978, Subcommission on geochronology: Conven- 181 p. Merguerian. C., 1981, Tectonic significance of the Calaveras Shoo Fly thrust tion in the use of decay constants in geochronology and cosmochronol- Cebull. S. E.. and Russell. L. R.. 1979, Role of Meiones fault zone in the (CSFT), Tuolumne County, California (abs.J: Geological Society of ogy, in Cohee, G. V., Glaessner, M. F., and Hedberg, H.. eds., structural chronology of the North Yuba River area, western Sierra America Abstracts with Programs, v. 13, p. 96. Contributions to the geologic time scale: American Association of Pe- Nevada, California: Geological Society of America Bulletin. Part I, 1982. The extension of the Calaveras Shoo Fly thrust (CSFT) to the troleum Geologists. Studies in Geology No. 6, p. 67 71. v. 90. p. 225 227: Part N. v. 90. p. 528 544. southern end of the Sierra Nevada metamorphic belt (SNMB), Califor- Stern, T. W.. Bateman. P. C„ Morgan, B. A., Newell, M. F., and Peck, D. L.. Chen. J., and Moore, J. C., 1979. Late Jurassic Independence dike swarm in nia (abs.]: Geological Society of America Abstracts with Programs, 1981. Isotopic U-Pb ages of zircon from the granitoids of the central eastern California: Geology, v. 3. p. 129 133. v. 14. p. 215. Sierra Nevada, California: U.S. Geological Survey Professional Paper Clark, L. D., 1964, Stratigraphy and structure of part of the western Sierra Merguerian. C. and Schweickert, R. A.. 1980, Superposed mylonitic 1185, 17 p. Nevada metamorphic belt. California: U.S. Geological Survey Profes- deformation of the Shoo Fly Complex in Tuolumne County, California Strand. R. G.. and Koenig, J. B., 1965, Sacramenio sheet. Geologic map of sional Paper 410, 70 p. [abs.]: Geological Society of America Abstracts with Programs, v. 12, California: California Division of Mines and Geology, scale 1:250,000. 1976. Stratigraphy of the north half of the western Sierra Nevada p. 120. Taliaferro, N. L., 1942, Geologic history and correlation of the Jurassic of metamorphic belt, California: U.S. Geological Survey Professional Moores, E. M„ and Day. H. W., 1983, An overthrust model for Mesozoic southwestern Oregon and California: Geological Society of America Paper 923. 26 p. Sierran tectonics [abs.]: Geological Society of America Abstracts with Bulletin, v. 53, p. 71 112. Creely. R. S.. 1965. Geology of the Oroville quadrangle. California: California Programs, v. 15, p. 293. Tobisch, O. T.. and Fiske, R. S.. 1976. Significance of conjugate folds and Division of Mines and Geology Bulletin 184, 86 p. Morgan. B. A.. 1976, Geology of Chinese Camp and Moccasin quadrangles. crenulations in the central Sierra Nevada, California: Geological Society Curtis. G. H.. Evernden. J. F.. and Ltpson, J. L, 1958, Age determinations of Tuolumne County, California: U.S. Geological Survey Miscellaneous of America Bulletin, v. 87. p. 1411 1420. some granitic rocks in California by the potassium-argon method: Cali- Field Studies Map MF-840, scale 1:24.000. - - 1982. Repeated parallel deformation in part of the eastern Sierra fornia Division of Mines Special Report 54, 16 p. Nokleberg, W. J., 1981, Stratigraphy and structure of the Strawberry Mine roof Nevada. California and its implications in the dating of structural events: Durrell. C.. and d'Allura, J., 1977, Upper Paleozoic section in eastern Plumas pendant, central Sierra Nevada. California: U.S. Geological Survey Journal of Structural Geology, v. 4, p. 177 195. and Sierra Counties, northern Sierra Nevada. California: Geological Professional Paper 1154. 18 p. Tobisch, O. T., Fiske, R. S., Sacks. S„ and Taniguchi, D., 1977, Strain in Society of America Bulletin, v. 88. p. 844 852. Nokleberg. W. J., and Kistler. R. W.. 1980, Paleozoic and Mesozoic deforma- metamorphosed volcamclastic rocks and its bearing on the evolution of Eric. J. H., Stromquisi, A. A., and Swinney, C. M.. 1955. Geology and mineral tions in the central Sierra Nevada. California: U.S. Geological Survey orogenic belts: Geological Society of America Bulletin, v. 88, p. 23 40. deposits of the Angels Camp and Sonora quadrangles. Calaveras and Professional Paper 1145, 24 p. Tuminas, C. A.. 1980. Structural relations in the eastern part of the Smartville Tuolumne Counties. California: California Division of Mines and Ramsay. J. G.. 1980, Shear zone geometry: A review: Journal of Structural ophiolite block, northern Sierra Nevada. California [abs.]: Geological Geology Special Report 41. 55 p. Geology, v. 2, p. 83 99. Society of America Abstracts with Programs, v. 12, p. 156. Evans. J. R„ and Bowen. O. E.. 1977. Geology of the southern Mother Lode, Ramsay. J. G., and Graham. R. H„ 1970, Strain variation in shear belts: Van Hinte, J., 1976, A Jurassic time scale: American Association of Petroleum Tuolumne and Mariposa Counties, California: California Division of Canadian Journal of Earth Sciences, v. 7. p. 786 813. Geologists Bulletin, v. 60, p. 489 497. Mines and Geology Map Sheet 36. scale 1:24,000. Rinehart. C. C., and Ross, D. C.. 1964, Geology and mineral deposits of the Varga, R. J., 1980, Structural and tectonic evolution of ihe early Paleozoic Evernden, J. F., and Kistler. R. W., 1970, Chronology of emplacement of Mount Morrison quadrangle. Sierra Nevada, California, with a section Shoo Fly Formation, northern Sierra Nevada range, California [Ph.D. Mesozoic batholithic complexes in California and western Nevada: U.S. on a gravity study of Long Valley by L. C. Pakiser: U.S. Geological thesis]: Davis, California, University of California. 248 p. Geological Survev Professional Paper 623. 42 p. Survey Professional Paper 385, 106 p. Varga, R. .1., and Moores, E. M„ 1981, Age, origin, and regional significance of Fiske. R. S.. and Tobisch. O. T.. 1978, Paleogeographic significance of vol- Rinehart, D. D.. Ross, D. C..and Huber, N. K.. 1959. Paleozoic and Mesozoic an unconformity that predates island-arc volcanism in the northern canic rocks of the Ritter Range pendant, central Sierra Nevada, Califor- fossils in a thick stratigraphic section in the eastern Sierra Nevada, Sierra Nevada: Geology, v. 9, p. 512 518. nia. in Howell. D. G.. and McDougall. K. A., eds.. Mesozoic California: Geological Society ol America Bulletin, v. 70, p. 941 945. Xenophontos, C. and Bond, G. C.. 1978. Petrology, sedimentation, and paleo- paleogeography of the western Unived Stales: Society of Economic Russell, S„ and Nokleberg, W. J., 1977, Superimposition and timing of defor- geography of the Smartville ophiolite, in Howell. D. G.. and McDou- Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleo- mations in the Mount Morrison pendam and in the central Sierra gall, K. A., eds., Mesozoic paleogeography of the western United Slates: geography Symposium 2, p. 209 222. Nevada. California: Geological Society of America Bulletin, v. 88, Society of Economic Paleontologists and Mineralogists, Pacific Section, Girt>. G. H.. Schweickert. R. A., and Hanson. R. E„ 1980. Structural fabrics of p. 335 345. Pacific Coast Paleogeography Symposium 2, p. 291 302. the Shoo Fly Complex near Bowman Lake, northern Sierra Nevada. Saleeby, J.. 1981. Ocean floor accretion and volcanoplutonic arc evolution of California [abs.J: Geological Society of America Abstracts with Pro- the Mesozoic Sierra Nevada, in Ernst, W. G., ed.. The geoteetonie Mancascknt RK I.ii'H)RV twSoaf-rv JAMMKV I J. 1981 grams, v. 12. p. 107. development of California: Rubey Volume 1: Englewood Cliffs, New Rt visH) Manuscript Rk fivh> May 20. 1983 Harland. W. B.. Cox. A. V.. Llewellyn, P. G.. Pickton. C.A.G.. Smith. A. G„ Jersey, Prentice-Hall. p. 132 181 Manuscrih ACCRKHO-. Juni 3, 1983

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