TECTONICS, VOL. 5, NO. 1, PAGES 65-94, FEBRUARY1986

STRUCTURAL HISTORY OF CONTINENTAL VOLCANIC ARC ROCKS, EASTERN SIERRA NEVADA, CALIFORNIA: A CASE FOR EXTENSIONAL TECTONICS

Othmar T. Tobisch

Earth Science Department, Applied Science Building, University of California Santa Cruz

Jason B. Saleeby

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena

Richard S. Fiske

National Museum of Natural History Smithsonian Institution Washington, D.C.

Abstract. Mesozoic metavolcanic rocks and greater constrictional component than forming part of the continental volcanic the Ritter Range for rocks of comparable arc along the eastern Sierra Nevada near age. Calculations based on the strain data Mt. Goddard and in the Ritter Range show a suggest tile Mt. Goddard section has been complex history related to extensional thinned by about 50% norma] to bedding, tectonics. The rocks comprise a thick sec- much as that documentedpreviously for tion of tuffs, breccias, lava flows, rocks in the Ritter Range. Deformation sills, and ash-flow tuffs deposited in a within this part of the continental arc subaerial to subaqueous environment, with was originally thought to have formed by some subvolcanic sill-like plutons. Pb/U regional compression during the late Jur- ages of the rocks in the Mt. Goddard area assic (Nevadan) orogeny. However, our range from ca. 130-160 Ma, while rocks in study indicates that (1) parts of the the Ritter Range have a somewhat wider age deformed volcanic section are younger than range as reported previously. Repetition late 0urassic, (2) Nevadan-age breaks in of the section occurs by faulting, and deposition are not present, (3) large-scale with the exception of parts of the mid- folds expected during a regional compres- Cretaceous Minarets Caldera, all the vol- sion event are not common, and (4) the beds canic rocks show a regional slaty cleavage were tilted to a high dip prior to internal which was subsequently crenulated and/or deformation. An extensional model is pro- folded locally. The first cleavage has posed in which beds were rotated to high well-developed stretching lineations, and tilts early in the deformation as a result does not appear to have been associated of listric normal faulting. This normal with significant folding. Finite strain faulting is thought to have occurred above measurements show considerable variation a regional tumescence related to voluminous both in magnitude and symmetry. The Mt. magmatism at depth, with preservation of Goddard rocks, however, tend to show the steeply tilted Goddard and Ritter sec- slightly higher overall strain magnitude tions being facilitated by their downward transport along the margins of rising plu- Copyright 1986 tons. Flattening and steeply plunging con- by the American Geophysical Union. strictional fabrics superimposed on the tilted sections are related to strain in- Paper number 5T0776 duced by high-level inflation of magma 0278-7 407 / 86 / 005T-0776 $10.00 chambers and downward return flow of the 66 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

38 ø Modoc Plateau

Sierra Nevada Batholith Ritter Range SanFrancisco •?&••MapArea ave Desert

Mt. Goddard • •

kilometers

119 ø 118 ø

Fig. 1. Location map of the study area, which contains a 120 km long section of the continental volcanic are in the eastern S•erra Nevada, California. Shaded pattern indicates predominantly Mesozoic age metavolcanic rocks; ig- neous pattern indicates predominantly Mesozoic plutonit rocks; clear pattern indicates area of predominantly Itclocene age rocks.

keellike pendants. The main tectonic fabric dynamic evolution of magmatic arcs, or (3) shown by the continental volcanic arc rocks some combination of both. The ultimate in the eastern Sierra Nevada is largely of answer to such questions may be long in Cretaceous age, rather than Jurassic coming, but our present data allows us to (Nevadan) as originally supposed. In ad- analyze the nature of deformation from dition, the deformation, both rotation of parts of this arc and discuss certain as- beds and subsequent tectonite fabric, pects of its structural genesis. appears to be genetically related to the dynamic evolution of the magmatic arc, and Previous Work not the result of an externally imposed tectonic event. Much mapping has been carried out in the Mesozoic continental arc rocks of the east- INTRODUCTI ON ern Sierra Nevada, largely in connection with quadrangle mapping by the U.S. Geolog- General ical Survey [e.g., Moore, 1963; Rinehart and Ross, 1964; Huber and Rinehart, 1965; Mesozoic continental volcanic arc rocks Bateman, 1965; Bateman and Moore, 1965]. of the eastern Sierra Nevada extend for at This work has served as a firm basis for least 500 km from the Modoc Plateau of subsequent topical studies [e.g., Kistler, northern California to the Mojave Desert in 1966; Brook, 1977; Nokleberg, 1981; Nokle- the south (insert, Figure 1). Over the past berg and Kistler, 1980; Kistler and Swan- several years, we have been studying the son, 1981; Tobisch and Fiske, 1982; structural genesis of these rocks in order Schweickert et al., 1984a]. In spite of to address whether deformation within the these numerous investigations, the nature arc was due largely to (1) its collision and timing of deformation along the length with exotic terranes accreted to the of the arc is still poorly known, and the continental margin, or (2) inherent in the quantification of cumulative strains and Tobisch et al.' Continental Volcanic Arc, Sierra Nevada how these may vary with age in different presence of accretionary lapilli, and an parts of the arc are only just beginning absence of limestone. In addition, unit 12 to be understood [Tobisch et al., 1977; shows a complex geometry between a coarse Tobisch and Fiske, 1982; this report]. breccia and ash flow tuff, highly reminis- The structural history and strains in cent of the caldera collapse unit described Mesozoic volcanic arc rocks of the Ritter from the Ritter Range [Fiske et al., 1977; Range (Figure 1) have been studied in de- Fiske and Tobisch, 1978]. From these and tail by various workers [Kistler, 1966; previously mentioned observations, we con- Tobisch et al., 1977; Fiske and Tobisch, clude that units 9-12 (Plate 1) have been 1978; Tobisch and Fiske, 1982]. Another deposited for the most part under subaerial large enclave of comparable rocks occurs conditions, and that unit 12 may represent some 85 km to the south in the Mt. Goddard massive wall-rock slumping associated with region (Figure 1). Since the initial map- ash flow eruption in a caldera environment. ping of these rocks by Bateman [1965] and Bateman and Moore [1965], some work has Stratigraphy been done [DuBray, 1977], but the struc- tural character of the rocks has not been As shown in Plate 1, the predominant dip studied in detail. The present work inves- of bedding in all units is to the west. tigates the structural history and strains Sedimentary structures such as current found in rocks of the Mt. Goddard region. bedding, graded bedding, ripple marks, We then compare these data to structures channeling, and rip-up clasts also indicate which occur in the Mt. Ritter area and that tops face to the west. Based on field consider the implications concerning the relationships and radiometric data, we have broader structural evolution of this part divided the volcanic section into three age of the arc. groups (Figure 2): an older section repre- sented by units 4-8 with an age of 160 Ma, GEOLOGIC SETTING an intermediate section represented by units 9-12 with an age of 143 Ma, and a Rock Types and Depositional Environment younger section represented by units 1-2 with an age of 130-135 Ma. As can be seen The volcanic section in the Mt. Goddard from Figure 2, the older section is sand- area is mostly volcaniclastic consisting wiched between the intermediate and younger largely of fine-grained tuffs, lithic and sections. The exact locations of the bound- rarely accretionary lapilli tuffs, tuff- aries between these three groups are in breccias, ash flow tuffs, mafic and felsic part tentative due to the lack of detailed lava flows, lime-rich tuffs, and rare lime- age control. The ages of units 1,6, and 8- stone. Thin Mn-rich zones bearing piemon- 12, however, are either precisely known or tite are present locally, and felsic sills can be tightly constrained by known ages of are commonin parts of the section (Plate intrusive bodies which have been dated 1; cf. also Bateman and Moore [1965]). In (Plate 1; Table 2). In the northern part of this paper, we refer to the rocks by their the area, units 2 and 4 are separated by a volcanic terminology, although they have laminated phyllitic schist. This schist been subject to regional metamorphism and (unit 3, northern sector) is an intensely penetrative deformation. deformed, commonly platy rock which has Units 1-8 (Plate 1) show depositional been subsequently subjected to kinking and features of both subaqueous(graded bed- locally intense secondary deformation, and ding, cross-bedding, limestone) and subaer- probably represents a bedding parallel ial conditions (basalt flows lack pillows, fault separating units 2 and 4. While the presence of red, hematite-bearing beds, strongly laminated nature of the deformed lack of doubly graded sequences of lapilli zone diminishes to the south, the high tuff and tuff [Fiske and Matsuda, 1964; strains which characterize most of the Fiske, 1969]. The environment of deposition rocks in this area make it extremely diffi- of this part of the section is interpreted cult to determine if the fault continues as being one in which the rate of deposi- bedding-parallel to the south, dies out, or tion was more or less equal to the rate of is replaced by an unconformity tectonically subsidence, giving rise to periods of al- flattened beyond recognition. •'e prefer the ternating shallow subaqueous and low-lying first alternative and have placed unit 2 subaerial conditions. Units 9-12, however, into the younger section and units 4 and 5 are characterized by rapidly changing lat- in the older section. To the east, the eral facies, complex primary geometry, the contact between the intermediate and older 68 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

Fig. 2. Simplified diagram of Plate 1 showing the interpretation of the three main stratigraphic sections deduced from field and laboratory data. Pattern shown in southwest corner of diagram represents granitoid rocks of probable Cretaceous/Jurassic age (unit 13, Plate 1).

sections is also a fault, and is locally the analytical uncertainties of the indig- characterized by disruption in bedding enous ages. On the 206Pb/238U:207Pb/235U orientation, changes in lithology, depos- concordia diagram [Wetherill, 1956], such itional environment, and strain character- overlap is exhibited by the error polygon istics, as well as considerable topographic of a given analysis intersecting the con- expression. Since the older (160 MA) sec- cordia line. A major problem in the inter- tion lies to the west of the intermediate pretation of the Mt. Goddard data, and all (143 Ma) section, and both sections dip to mid-Paleozoic and younger zircon data sets, the west, there has clearly been fault re- is the near linearity of lower concordia. petition. We will suggest a model for gen- Such linearity could permit the disturbance erating this geometry in a later section. of the zircon isotopic system to be topo- logically expressed by downward migration Geochronology of the error polygon without resolvable divergence from the concordia line. For Fossils have yet to be found in the Mt. this reason internal concordance alone has Goddard section, but Pb/U zircon ages have been referred to as apparent concordance been determined for several key volcanic [Saleeby, 1982]. The agreement of internal- and plutonic units. The isotopic data along ly concordant ages from multiple fractions with information on analytical procedures of a given zircon population split into and uncertainties are given in Table 1. The different physical groups, or multiple locations of the geochronological sampies samples from a given map unit is referred are shown in Plate 1, and details on sample to as external or true concordance. The setting, zircon yield and preferred igneous rationale here is that the effects of known ages are given in Table 2. The discussion multistage behavior are observed to vary below focuses on the interpretation of the with the physical properties of zircon isotopic data. grains, and are also observed to be heter- Igneous ages derived from the isotopic ogeneous over map-scale units (Silver data fall in the 160 to 130 Ma range. Con- [1964], Saleeby and Sharp [1980], as exam- fident igneous age assignments are often ples). Thus one should expect external based on the condordance or equivalence of concordance as a mark of single-stage be- the Pb-U and Pb-Pb ages in a given analysis havior. (internal concordance). Such concordance is Examples of true concordance in the Mt. arrived at statistically by the overlap of Goddard data set are exhibited in samples Tobisch et al.' Continental Volcanic Arc, Sierra Nevada 69

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• v 70 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

g

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or-.- o c o co Tobisch et al.: Continental Volcanic Arc, Sierra Nevada 71

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.025 15 '-7 upper•tnercept-1120!400Me .025 _ ,5• •7FoPwPeerritffee;cCeeppf, 2i,5260;2 Mo f •-6 uper,ntercept= 1810_+ •00 Mo .021 / lower•n•ercept=1•3 i 3Ma .01 9 .15 .15 .17 .19 .21 .2:5 207pb/ 255 u Fig. 3. Concordia diagrams for discordant samples from the Mt. Goddard section [after Wetherill, 1956]. Numbers indicate samples as shown on Table 1. Con- cordia intercepts for samples 1, 6 and 7 after York [1966]. Igneous age interpretations are given in Table 2.

2,3,4, and 5. The 206Pb/238U ages of each be expected, along with downward dispersion analysis carry the greatest precision, and of the discordia arrays from concordia thus these ages are used in the assignments upper intercepts [Silver, 1964; Saleeby and of the preferred igneous ages in Table 2 Sharp, 1980]. As discussed above this is (see also Plate 1). Internal and external the opposite of what is observed. Possible discordances are observed in samples 1, 6 superpositioning of significant Pb loss and 7. In each case discordancy is observed over earlier two-stage systems is consider- to be a function of increasing grain size ed unimportant due to the linearity of the and decreasing U concentration. On observed arrays, and the internal concor- 206Pb/238U:207Pb/235U concordia didõrams dancy of the lower data points of samples 6 (Figure 3) each of the discordant arrays and 7. External concordancy of samples 2,3, disperse up from concordia lower inter- 4 and 5 argue strongly for single-stage cepts. As can be seen from Figure 3 and behavior for these systems and thus minimal Table 1, the lower intercepts of samples 6 Pb loss. On the whole the Mt. Goddard sam- and 7 correspond to internally concordant ple suite bears low U concentrations (150 data points. The discordia arrays of sam- to 400 ppm except for sample 3), which is ples 1, 6 and 7 and the relationships be- consistent with the lack of any evidence tween discordance, grain size and U concen- for significant Pb loss. tration strongly suggest the incorporation Incorporation of ancient zircon by in- of older zircon into magmas that were e- heritance from the magma source regime or rupted or crystallized at the approximate entrainment from wall rocks during magma time of the lower intercept ages. Thes• are ascent is exhibited quite clearly in sample shown as preferred igneous ages in Table 2. 6. This sample was hand and sieve split Rocks of the Mt. Goddard pendant have into a clear fraction (<45•) and two color- undergone regional thermal metamorphism and ed fractions (<45• and 45-80•). The coarser thus the possibility of Pb loss should be fraction contains faint cores. The fine considered. In terms of the discordant clear fraction yields age data which are samples (1,6 and 7) the patterns observed internally concordant. The fine colored are unlikely to be related to Pb less. fraction is discordant, and the coarse Greater discordance with increasing U con- fraction is highly discordant with an age centration and decreasing grain size would divergence of over 400 Ma. Coarse fractions 72 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

from samples 1 and 7 likewise contain stratigraphic data discussed previously rounded cores in some grains and yield the indicate that the west-facing homoclinal oldest most discordant ages, consistent sequence contains at least one structural with inheritance or entrainment. Zircon age repetition in the age (stratigraphic) se- data for a single analysis of an <80• size quence by faulting. fraction from sample 7 were reported in Chen and Moore [1982]. The reported ages Metamorphic Grade were t 206Pb/238U = 157.0 Ma, t 207Pb/235U = 158.8 Ma, and t 207Pb/206Pb = 196 Ma. The volcanic section and the older plu- Such internal discordance led these workers tons have undergone regional metamorphism to an ambiguous interpretation for the related to widespread penetrative deforma- igneous age. Examination of the new data on tion, while contact metamorphism has af- three fractions split from the original fected rocks near larger plutons. Typical population (Table 1) shows upward and down- regional metamorphic mineral assemblages ward dispersion of U-Pb ages and Pb-Pb ages include: from the ages reported by Chen and Moore [1982]. Such dispersion is the mark of a mafic rocks' plagioclase + biotite + two-stage mixing array that was homogenized chlorite + actinolite -+ (clino- in the original single fraction analysis. zoisite) Upper intercept ages are given for the discordant samples in Figure 3. Note the felsic rocks' plagioclase + quartz + white large uncertainties, which are a result of mica + epidote -+ (chlorite + magnetite + the clustering of data points near the hematite) lower intercepts. The upper intercept ages are not given specific significance in that calcareous tuff' quartz + white mica + they probably represent the overall isotop- calcite + epidote _+(tremolite) ic character of the contaminate zircon which may itself represent a multistage In addition, rare pelitic rocks bear system. almandine garnet, and the amphibole in It is significant that each of the up- mafic rocks, characterized by low zAc an- per intercepts is well back imto the Pro- gles, is probably actinolite. Zones of terozoic, like numerous upper intercept piemontite-bearing schist occur locally. ages determined for other Mesozoic metavol- Anorthite content of plagioclase determined canic amd granitoid complexes of the Sierra by universal stage measurements varies Nevada [Saleeby et al., 1985; Sams, 1985; between An18 and An31 in mafic rocks, J.B. Saleeby, unpublished data, 1985]. whereas in felsic rocks it varies from An14 Furthermore, DePaolo [1981] has resolved a to An24, averaging An16 (7 samples). The major Proterozoic sialic component within lower anorthite content in the felsic rocks Mesozoic granitoids of the Sierra Nevada by may be due to a calcium-poor rock, but the Nd and Sr isotopic studies. The wide array apparent stable occurrence of the mineral of upper intercepts given in the Figure 3 pair plagioclase-epidote in many samples concordia diagrams is not surprising. Zir- suggests this is probably not the case. con studies in high-grade para- and ortho- According to Turner [1981] the anorthite gneisses of the southernmost Sierra Nevada, content of plagioclase in amphibolite fa- which are the only direct samples available cies is >An20. The lower value in the fel- for deep crustal materials beneath the sic rocks along with the presence of actin- range, show significant isotopic variabili- olitic amphibole and piemontite, therefore, ty in Proterozoic zircon populations with suggest that PT conditions may be slightly the oldest components lying in the 1.8 to below those of the lower amphibolite fa- 2.0 Ga range [Sams, 1985]. Thus the -1.8 Ga cies. upper interc•pt of sample 6 may likewise Contact metamorpbism of volcaniclastic approximate an upper endmembercomponent in rocks is often difficult to identify in the ancient crustal materials beneath the Mt. field except in mica-rich assemblages. Our Goddard volcanic edifice. observations indicate that contact zones The first-order implications of the are pzobably relatively narrow, and where zircon data reported here are that metavol- present, rocks of suitable composition bear canic and shallow-level plutonic rocks of andalusite (sometimes the Mn-rich variety, the study area fall between 160 and 130 Ma ¾iridine) and oligoclase (An18), placing in age. The map distribution of these ages these contact zones in the hornblende horn- (Plate 1) along with the structural and fels facies [Turner, 1981]. Tobisch et al.: Continental Volcanic Arc, Sierra Nevada

N N N

N N

Fig. 4. Lower hemisphere equal area projections of structural elements meas- ured from the Mt. Goddard area (Plate 1). (a) Poles to slaty-type cleavage; (b) poles to bedding; (c) lineations formed by elcngate minerals and long axes of stretched lapilli (stretching lineation); (d) lineations formed by the intersection of bedding and slaty-type cleavage; (e) poles to axial planes of group B folds and crenulations.

Although we have separated the metamor- predominantly steeply to the west or nearly phism into regional and contact stages, it vertical (Figure 4b), and are parallel or is possible that both stages may be related subparallel to slaty cleavage. Most other to the heat associated with generation and tectonic structures also have steep orien- emplacement of the batholith. In this view, tations. Two groups of tectonic struc- regional metamorphism of the country rocks tures are present: (A) slaty cleavage, occurred while they were being penetrative- lineations (intersection and stretching ly deformed, whereas hornfelsic textures types), less commonminor folds showing indicate annealing at a late stage after axial plane slaty cleavage, and ductile deformation ceased, and where the rocks faults, all related to widespread pene- were in closest proximity to the heat trative deformation; and (B) minor folds source. This concept fits well with the showing axial plane crenulation cleavage, extensional mod•l advanced later in the lineations (mostly intersection type), paper. kinks, crenulations and crenulation cleav- age, and brittle faults, all related to STRUCTURAL CHARACTERISTICS domainal deformation. Group B structures in the Mt. Goddard area are interpreted to General postdate group A structures.

The Mt. Goddard pendant represents a Group A Structures steeply dipping "homoclinal" screen separa- ting large plutonic masses of the composite The dominant structure of this group is Sierra Nevada batholith. Bedding dips are the penetrative slaty cleavage present in 74 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada Tobisch et al.: Continental Volcanic Arc, Sierra Nevada. ?5

most exposures in the volcanic section. Its ductile faulting occur in the northwest morphology ranges from nearly continuous part of the area, and are represented by [Powell, 1979] in fine tuffs to spaced unit 3 (southern and northern sectors; disjunctive in massive rocks such as lava Plate 1). The southern sector of unit 3 flo•s, and shows characteristics comparable represents a fault zone spatially related to the cleavages developed in volcanic]as- to folds, the fault o•/ented essentially tic rocks to the north [Tobisch, 1984]. The subparallel to the axial. [,lanes of the o•ientation of the cleavage is generally mapped folds (Plate 1) and the prevailing northwest in strike with a nearly vertical slaty-type cleavage. Blocky fracturing dip (Figure 4a). within parts of this zone are interpreted Minor folds associated with the slaty as evidence for renewed movement along this cleavage are not common, but where present zone subsequent to cleavage formation. The their axial planes parallel cleavage (Fig- northern sector of unit 3 (Plate 1) is ure 5a), and their axes parallel bedding/- characterized by an intensely laminated, cleavage intersections. These lineations platy phyllitic schist discussed earlier are moderately to steeply plunging (Figure under the section on stratigraphy. 5b), generally in the northwestern quadrant (Figure 4d). Most minor folds show inter- Group B Structures limb angles ranging from open to close (Figure 5a) and rarely tight [Fleuty, These structures are distinctly domainal 1964]. Mappable folds are present in a few relative to those of group A. They have areas (e.g., northwest portion of Plate 1), been observed mostly in units 1-6 (Plate but they do not repeat substantial portions 1), and only rarely i• units 8-12. This is of the stratigraphic section. Indeed, top probably at least in part an expression of directions are nearly all west facing, and the higher strains and better cleavage the volcanic rocks essentially represent a development found i• units 1-6 which pro- composite west-dipping homoclinal sec- vided a rock geometry more conducive to tion. Although bedding shows scattered forming such structures. The dominant orientations in some areas (Plate 1 and structures are represented by kinks, minor Figure 4b), these anomolies can be related folds, crenulations, and locally crenula- to localized folding around steep axes or tion cleavage. This folding of the earlier rotation near faults or large plutons. cleavage occurs around axial planes which A strongstretching lineation definedby haveorientations here interpretedas two alignedmica or other minerals, andby discrete sets: E-ESEand N-NNE (Figure 4e). elongatevolcanic fragmentsin the lapilli Althoughthe two data sets overlap (Figure tuffs (Figure 5c) is probably the best- 4e), field relationships suggestthese sets developed linear feature in the region. The have a conjugate relationship, and we base stretching lineations plunge steeply (Fig- our interpretation on the observations that ure 4c), and are often subparallel to bed- 1) neither set is seen to refold the other, cling/cleavagelinearions (of. Figures 4c 2) where both sets occur, they form con- and 4d). jugate crenulations or folds, and 3), the Because the volcanic section is litho- two sets are symmetrically disposed around logically monotonousand bears only occa- the group A slaty-type cleavage (of. Fig- sional marker beds, documenting large-scale urea 4a and 4e). Conjugate folds with es- offset along faults formed during the duc- sentially the same orientation maxima have tile deformation is seldom possible. Two been reported from other parts of the Sier- zones which most likely represent zones of ra Nevada [e.g., Ave Lallement et al.,

Fig. 5. Photographs of structural elements. (•) Group A fold, showing slaty cleavage paralleling its axial plane. Pencil is ca. 10 cm long; (b) group A bedding/slaty cleavage intersection plunges nearly vertically parallel to pencil (15 cm long), while group B crenulation axes (seen in slaty cleavage surface) plunge moderately to the left; (c) moderately stretched lapilli showing a constrictional symmetry (nearly plane strain; see sample W, Table 3) as seen on two orthogonal joint faces. Pencil is 15 cat long. Narrow shelf is a subhorizontal joint surface, whereas other joint surfaces dip very steeply. Note axial ratios of lapilli on subhorizontal joint surface are much less than those on steeply dipping faces. 76 Tobisch et al.: Continental Volcanic Arc, Sierra Nevada

1977; Bhattacharyya and Paterson, 1985], joint surfaces at each station and the data and there is no doubt tbey represent a processed according to the metbod of regional phenomenon at least along the Elliott [1970] adapted for volcaniclastic eastern Sierra Nevada continental arc. The rocks as outlined in detail elsewhere above observations and data indicate that [Tobisch et al., 1977]. Results from each the conjugate set is geometrically related plane were then combined to obtain the to group A s laty-type cleavage and may also strain magnitude (e s) using the equation of be genetically related to it and/or to Nadai [1963, p. 68]: zones of ductile faulting as previously suggested for similar structures to the north [Iobisch and Fiske, 1976; 1982]. s 1 2 2-œ 3) 3 1 (•) As with group A structures, lack of marker horizons renders it difficult to delineate faults associated with group B where œ >œ >œ œ=ln(l+e), and e=l-lo/lo, structures. Some group B faults have been where1--11en2gth3•after andlo= length before mapped (Plate 1), and they also show a con- strain. The symmetry of the strain (•) was jugate orientation symmetrically dispersed determined using Lode's equation [Lode, about the group A slaty-type cleavage. 1926, p. 932]: These faults were not observed to be as- sociated with a crenulation or other type 2a2-œ 1-œ 3 of cleavage. v: (2) œ1-œ3 Timing of Deformation For reasons treated elsewhere [Tobisch et There is only sparse data presently al., 1977], the calculated values represent available to help constrain the age of at least a minimum strain, and are thought these structures. Since the three strati- to approximate closely the whole rock graphic sections (160 Ma, 143 Ma, and 130- strain in most specimens. 135 Ma) which comprise the volcanic sect ion The location of individual samples is contain the same structural sequence, it is shown in Plate 1, the calculated data list- clear tbat the deformation postdates the ed in Table 3, and the values of •_ and (•) youngest part of the section, which con- plotted on a Hsudiagram (Figure 6a•. As sists largely of felsic ash flow tuff (unit one can see by comparing these data, the 1, Plate 1). The maximum age of the defor- magnitude and symmetry of the strain vary mation therefore is ca. 131-+6 Ma. The La- considerably, which is at least in part a marck and Mt. Givens plutons, which are function of the rock type. tectonically undeformed and cut the vol- From these data, we conclude the follow- canic section 3 km to the north [Bateman, ing regarding the magnitude and symmetry of 1965], yield radiometric dates of around 90 the strains as well as the amount of thin- Ma [Stern et al., 1981]. Deformation then ning that the volcanic section has under- is presently constrained within the time gone: period-130-90 Ma. 1. The intermediate stratigraphic sec- tion (units 9-12, Plate 1, Figures 2 and Strair•s in the Volcanic Section 6b) showsa substantially lower strain magnitude (mean extension in X= +47%, con- We analyzed rocks for strains at 29 traction in Z= -36%) than the younger (X= stations in the field, mainly using lithic +170%, Z=-63%) or older sections (X= lapilli as strain markers (Figure 5c), but +121%, Z= -54%; see also Figures 6d-6e). We on occasion breccia fragments and vesicles interpret the lower strains in the inter- in maf•c lavas were used. The highly joint- mediate section to reflect the generally ed rock provided ample opportunity to find massive, poorly bedded nature of the rocks exposures in which joints approximately in that area, as well as the abundance of paralleled two of the principal planes of dikes and sills which are more likely to the strain ellipsoid (X>Y>Z). Slaty-type resist internal deformation than the finer- cleavage was taken to approximate the (XY) grained volcaniclastic and ash flow units plane, and other planes orthogonal to the common to the younger and older sections. cleavage and either normal (YZ) or parallel 2. The greater extension in the young- (XZ) to the stretching direction (X) ap- er and older sections is also reflected in proximated the other two principal planes. the larger component of constrictional Strain markers were measured on two such symmetry characteristically shown in those Tobisch et al.: Continental Volcanic Arc, Sierra Nevada ??

TABLE 3. Strain Data

Sample •XY •YZ •XZ • • X Y Z Number s

Mt. Goddard Area*

A - 0.53 0.62 0.95 0.72 + 67 +33 -53 B - 0.43 0.57 0.84 0.50 + 64 +22 -48 C 0.88 - 1.11 1.7 -0.59 285 -33 -61 D - 0.72 0.93 1.4 0.54 120 39 -67 E 0.38 0.35 - 1.0 -0.05 108 - 2 -51 F 0.38 0.32 - 1.0 -0.09 115 - 7 -50 G - 0.41 0.93 1.3 -0.12 156 - 5 -59 H - 0.88 1.14 1.7 0.54 160 49 -74 I - 0.32 0.80 1.1 -0.20 126 - 9 -52 J - 0.56 1.25 1.8 -0.11 265 - 8 -71 K - 0.36 0.42 0.64 0.71 41 21 -40 L 0.49 0.42 - 1.3 -0.08 151 - 3 -60 M 0.59 0.44 - 1.5 -0.15 198 - 8 -63 N 0.69 0.49 - 1.7 -0.17 245 -10 -68 0 0.50 0.46 - 1.4 -0.01 165 - 1 -61 P - 0.45 0.70 1.0 0.29 92 13 -53 Q - 0.25 0.49 0.69 0.03 65 2 -39 R - 0.49 0.73 1.1 0.35 98 19 -56 S - 0.62 0.92 1.3 0.34 123 21 -62 T - 0.44 0.60 0.88 0.49 69 21 -50 U - 0.26 0.44 0.62 0.19 53 5 -37 V - 0.21 0.32 0.52 0.31 40 7 -32 W - 0.21 0.45 0.64 -0.07 61 - 3 -35 X - 0.33 0.43 0.64 0.54 42 16 -39 Y - 0.25 0.35 0.51 0.43 39 12 -32 Z - 0.29 0.32 0.50 0.80 30 18 -32 AA - 0.16 0.33 0.59 0.19 49 4 -33 BB - 0.29 0.31 0.49 0.88 27 19 -32 CC - 0.41 0.64 0.92 0.29 81 12 -50

Mt. Ritter Areat

1 - 0.29 0.80 1.14 -0.25 130 -lO -52 2 - 0.81 1.05 1.56 0.54 128 40 -68 3 - 0.50 0.62 0.93 0.61 69 28 -52 4 - 0.40 0.45 0.70 0.78 42 27 -43 5 - 0.36 0.42 0.64 0.71 40 22 -40 6 0.25 0.70 0.84 1.39 0.47 123 35 -67 7 - 1.02 1.32 1.96 0.54 192 58 -77

ZXY,ZYZ, ZXZare the strains measuredon thoseplanes, • is the strain magnitude [cf. Tobischet al., 1977],• is the strain symmetry,a•d valuesof X, Y, andZ are percent elongations or contractions parallel to the principal axes of the strain ellipsoid where X>Y>Z (see text). •For localities, see Plate 1 Forlocalities, see Figure •, westernpart of area3. Samplesshown as dots, numbered 1-7 from south to north. 78 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

o •

o ßo 0 0 ß

0 • 0

+0

OV (• O OV (• '-i- I• •'

0 •

OV • r• -i' I•,/

0 •

, 0

'5' I•

Tobisch et al.' Continental Volcanic Arc, Sierra Nevada rocks (Plate 1 and Table 3). Hsu plots from mat-ed by rhyolite and basalt and only mi- the older section (Figure 6d) show a clear nor andesite; the depositional environment tendencytowards a plane strain (•=0), one in both areas appears to have ranged from of the deformation paths previously sug- shallow marine to low-lying land, and both gested as characteristic of orogenic belts composite sections of rocks share many of in general [Wood,1973, Figure 10; Tobisch the same structural elements in both char- et al., 1977, p.35-36]. acter and orientation. A much greater range 3. The steeply plunging bedding/slaty in protolith age is preserved in the Ritter cleavage intersections (average ca. 70 area, however, with regions 1 (upper Trias- degrees; of. Figure 4d) associated with sic/lower Jurassic) and 4 (mid-Cretaceous) group A structures suggest that beddingmay not represented in the Goddard area. Both have been already steeply inclined prior to the Ritter and Goddard pendants are compos- formation of the slaty-type cleavage. Using ite west-dipping homoclinal sequences the methodof Ramsay[1967, p. 129-132] for which show fault repetition of bomoclinal determining angular changes during deforma- sections of comparable age range (Figure tion if the mean strain ellipsoid is known, 8). Faults which repeat the Ritter sections we confirmed that the beds underwent only a are, for the most part, intruded by sills small change in orientation during the which dip steeply eastward. The fault which cleavage forming process. Sucha situation repeats part of the Goddard section (de- has also been documented in comparable scribed earlier in the section on Strati- cocks to the north in the Ritter Range graphy), however, is not intruded by sills. [Tobisch et al., 1977, p. 36]. If one dis- The structural succession in the two areas regards the scatter of bedding due to lo- at least superficially appears the same calized deformation, the mean orientations (i.e., $1aty cleavage followed by crenula- of bedding and cleavage are nearly paral- tions, etc.). Upon comparing the cross lel (of. Figures 4a and 4b). Given this and sections of Figure 8a and 8b, one is struck another generalization that the direction by the possibility that the Goddard screen of maximum contraction is normal to the is an isolated fragment of the intermediate slaty cleavage, we can gain a rough esti- interval (structural and stratigraphic) of mate of the amount the stratigraphic sec- the Ritter complex, or its lateral equiva- tion has been thinned. We have taken a WSW lent. section in the southern part of the area (Plate 1) which passes roughly through Strains Scylla and just south of Charybdis. The total thickness of the three stratigraphic The strains shown in rock units of com- sections (excluding the sill-like pluton parable age in the Ritter and Goddard areas (unit 14) in the older section) is approx- show only slight correlation of magnitude imately 6 km whenmeasured normal to strike and symmetry. In the 160 Ma section (region and the dips are compensatedfor. By using 2, Figure 7), the strains in the Goddard the mean values of contraction for each of area show a greater magnitude and more the three stratigraphic portions given constrictional component of symmetry than above,the presently• observed thickness of thosein the Ritter area (cf. Figs 6d and 6 kmhad a precleavagethickness of approx- 6c, respectively). While constrictional imately 12 km.This value of original fabrics are commonwithin faults, this thickness is a minimum,since it doesnot changecannot be attributed to faulting include thinning of the section by faulting alone, becauseboth Ritter and Goddard whichhas undoubtably taken place in this areas are highly faulted [of. Tobischet area, but which is impossible to calculate al., 1977]. Strains measured in the 130-135 because no suitable marker horizons exist. Ma section in the Goddara area (units 1-2, Plate 1) show relatively high magnitudes COMPARISON WITH AREAS TO THE NORTH and considerable scatter in symmetry (Fig- ure 6e). Rocks further along strike in the General Ritter area (region 3, Figure 7), now known to be of comparable age as units 1-2 in the Rocks in the Mt Goddard area are similar Goddard area [T.W. Stern, written communi- in manyrespects to those in the Ritter cation, 1974], showstrains which are corn- Range 85 km to the north (Figure 7). Both parable in magnitude to those of Goddard, compositesections consist dominantlyof but the latter showgreater spread in sym- pyroclastic rocks with a minor percentage metry and a greater tendency towards con- of lava flows; chemically, both are doral- striction (of. Figures 6e-6f). Given the 82 Tobisch et al.: Continental Volcanic Arc, Sierra Nevada

••ij'i•:::...... :•.x.•:•::.::::• :'.•:.x•...... • , ,• •redom•nantly Holocene to Tertiary ßI, •2• I !½•½½T½tSedimentary andVolcanic Rocks • ;•.••, •/////// Mesozmcß Metavolcamc . Rocks

/////////////

ß.• ///// .• . • //////

///////// _ • _ \ \ I \ \ \ \

\ \ \ \

-/ I I I % f \

o i i I km \\ Mt.Goddard Area \ \

Fig. 7. simplified map of the Goddard-Ritter area, showing main stratigraphic sections which are correlatable between the two areas. Age range of wall rock in the various regions is as follows' region 1:-214-160 Ma; region 2: ~160- 150 Ma; region 3' ~145-130 Ma; region 4' ~100 Ma; Pz' Paleozoic sedimentary rocks [Rinehart and Ross, 1964]. Source of radiometric ages given in Figure 8 caption. Areas showing igneous pattern indicate elongate (often deformed) plutons which are considered contemporaneous with the volcanic section (see text). Clear areas surroundedby dashed lines are approximate outlines of (for the most part) Late Mesozoic plutons showing strongly elongate forms oriented along NW strikes. Dots in region 3 of Ritter area indicate strain stations of samples 1-7, located sequentially from south (1) to north (7)(see Table 3). Figure is in part after Stern et a1.[1981, Plate 1]. above and the data from upper Triassic/ symmetry exist both parallel to and normal lower Jurassic and mid-Cretaceous sections to the arc. in the Ritter area (Figures 6g and 7, re- In summary, the strain data suggest that gion 1, and Figures 6h and 7, region 4, the Goddard area samples, while fewer in respectively), it is clear that consider- number than the Ritter suite, tend to show able variations in strain magnitude and somewhat higher overall strain magnitude Tobisch et al.' Continental Volcanic Arc, Sierra Nevada 83 and a greater co_mponentof constrictional oriented structures on rocks throughout the symmetry(mean' œs =1.03; •=_0.22) than rocks orogen. Bateman and Clark [1974] suggest in the Ritter area (mean' œs=0.82;•=0.71). the Nevadan orogeny occurred between ap- The possible causes of such strain varia- proximately 140-150 MA, while Schweickert tion in these rocks are various, among them et al. [1984b] suggest an age of 155-+3 Ma. being (1) differences in physical condi- Both these estimates are based on data tions (P,T, etc.) of deformation, (2) dif- which comes largely from the Western Meta- ferences in mechanical properties of the morphic Belt. Nokleberg and Kistler [1980] composite stratigraphic sections (3) fluc- suggest a broader time span for Mesozoic tuation of overall stress magnitude, and deformation in the roof pendants not neces- (4) proximity of subvolcanic and/or older sarily associated with the Nevadan orogeny, plutonic rocks acting as strain-resistent but they do not address the regional cause phacoids resulting in greater accumulation of the deformation. of strain in the volcanic section. Strong The collisional model, however, does similarities of metamorphic mineral assem- not account for the fact that parts of the blages and lithologic suites from the two pre-mid-Cretaceous volcanic section in both composite sections suggest that possibil- the Mt. Ritter and Mt. Goddard areas are ities (1) and (2) are probably not funda- younger than the proposed Nevadan orogeny, mental causes of the mean strain differ- and that unconformities of Nevadan age have ences shown. Possibilities (3) and (4), yet to be recognized in spite of detailed while more difficult to evaluate, are more mapping in these areas. As there are no likely important contributors, although a documented post-Nevadan collisional events definitive model is not presently possible in the Sierra Nevada, it is conceivable from the data available. that Cretaceous collision events taking place in the Franciscan Complex to the west TECTONIC FRAMEWORK may have transmitted enough •.•erõy to the Sierran orogen to form the structures in General question. Terrane accretion in the Francis- can of central California, however, can at We have considered the variation in most be correlated only with gentle folding Mesozoic volcanic stratigraphy, geochrono- and minor unconformities within the Great logy, and strains in one of the better Valley sequence [Saleeby et al., 1985]. We studied portions of the arc in order to conclude, therefore, that while there may evaluate more rigorously the larger tec- be a case for major accretion in the West- tonic framework within which such struc- ern Metamorphic Belt, it cannot be respon- tures are likely to evolve. The principal sible for the structures we observe in question that remains is, did the tilting continental arc rocks younger than Middle of the beds and the generation of penetra- Jurassic in the eastern Sierra Nevada, at tive structures within the arc occur in least in the 100 km segment under consider- response to (1) deformation related to ation. accretionary collision processes within a Another major alternative under consid- compressional framework, (2) deformation eration, that of structures forming as part related to magma emplacement within an of the dynamic evolution of the subduction extensional tectonic setting, or (3) some arc system itself, is more attractive but combination of both? also presents a number of difficulties. One of the main problems is the question of Compressional Model whether or not the downgoing slab in a subduction zone will always transmit com- To date, most workers have considered pressional strain to the arc lying in the the structures in the Sierra Nevada to have upper plate some 200 or more kilometers been generated by regional compression away. It is well known from seismic data [e.g., Bateman and Eaton, 1967; Moores, that subduction zones can vary considerably 1970; Schweickert and Cowan, 1975; Moores in dip, and some workers have classified and Day, 1984; Schweickert et al., 1984b; the diversity into two general end members, Day et al., 1985]. In the accretionary gently dipping (Chilean or C-type) and collision model, the volcanic arc and re- steeply dipping (Marianas or M-type; Uyeda lated rocks of the western foothills are and Kanamori [1979]; Uyeda [1982]). C-type considered to have collided with the con- subduction zones are characterized by tinental margin in the late Jurassic Nevad- strong mechanical coupling between the ar• orogeny. This compressional event is subducting slab and the upper plate, and thought to have imposed the northwest compressional structures develop in the 84 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada trench and forearc [Karig et al., 1979; are almost devoid of tight to isoclinal Dewey, 1980]. M-type margins, on the other folds of sufficient size that would be hand, show a weak mechanical coupling, and necessary to explain the steep dips and their forearcs are characterized by exten- homoclinal nature of the beds; (2) the sional structures [e.g., yon Hueme et al., youngest parts of the homoclinal sections 1980; Hussong and Uyeda, 1982]. M-type are strongly deformed but recently deter- margins, therefore, are not likely to pro- mined age data indicate they are post- duce much contractional strain within the Nevadan age; (3) we have not found uncon- arc. While this two-fold classification formities in the pre-mid-Cretaceous section does not consider likely complexities such despite detailed mapping, suggesting that as changes in the rate of plate advancement the regional deformation was not instigated vs retreat of the subduction hinge (i.e., until after the last beds were deposited, the process of "roll-back", Molnar and i.e., early Cretaceous; (4) strain evidence Atwater [1978]) as well as various other indicates that beds were at high tilts factors [Dewey, 1980], it will suffice for prior to the ductile deformation that the present discussion. formed the cleavage [Tobisch et al., 1977; If the subduction process is responsible this paper]; and (5) parts of the volcanic for producing a compressional framework in section are repeated by faulting resulting the continental volcanic arc under consid- in the repetition of age sequences through eration, • strong coupling between upper the homoclinal sections. and lower plates would be necessary, imply- These enigmas can be resolved if we ing a C-type. However, there is little consider a model which involved regional evidence of significant compression having extension during Cretaceous time for the operated over the Great Valley forearc area of the Sierra Nevada presently under- region. Thus, if this mechanism had in fact lain by the batholith. Although the de- operated, the regional shortening strains tailed nature and exact extent of the ex- would have been concentrated within the tensional framework is still uncertain, we thermally softened arc, while the forearc consider it likely that it is related to remained rigid or became only weakly de- lithosphere-scale tumescence arising from formed. One would expect the subhorizontal subduction zone heating and magmatism. compression to generate large-scale folds Batholith emplacement is considered an repeating substantial parts of the section, integral part of such tumescence, dynami- but such structures are not present in the cally linked to, but not the sole cause of, 100 km segment of the arc being discussed. regional extension and resulting struc- Furthermore, it is known that high-level tures. extensional structures are a dominamt fea- We envision the deformation as occur- ture along the magmatic axis in the type ring in two stages' (1) tilting of beds example of a C-type subduction arc system along listric normal faults during regional [Pitcher, 1978; Dalmayrac and Molnar, extemsion as magma and its thermal welt 1981]. In light of the above, we consider rise upward, and (2) subsequent imposition it unlikely that a model invoking regional of tectonite fabrics on the tilted section compression is viable as a deformation during eraplacement of the batholithic com- mechanism to generate structures in the ponents. Although •t is known that magmatic rocks in question. This brings us to con- arcs can go through alternating phases of sider the role of extensional tectonics and compression and tension [Dewey, 1980; Hus- the dynamic evolution of the magmatic arc song and Uyeda, 1982], which might give itself to explain the deformational pat- rise to the structural sequence we have terns observed. described, we envision the extensional framework as long lived (i.e., ca. 40 moy.) Extensional Model and that both the tilting of the beds and the internal deformation seen in the vol- General. As outlined recently [Tobisch canic sections are closely tied to the et al., 1985], applying the concept of generation and eraplacement of the batho- regional extension to explain the evolution lith. Relating deformation in the Sierra of structures observed in the continental Nevada wall rocks to some aspect of pluton volcanic arc clarifies a number of hitherto eraplacement is mot necessarily new [e.g., puzzling features. Chief among these are Kistler et al., 1971; Bateman et al., the facts that (1) the volcanic sections in 1983], and several workers have suggested both the Mt. Ritter and Mt. Goddard areas general models which envision batholithic are essentially composite homoclinal, and eraplacement taking place in an extensional Tobisch et al.: Continental Volcanic Arc, Sierra Nevada

environment with magma emplacement respon- fault separating contiguous blocks can sible for deformation of the wall rocks become cryptic. Our field data from the Mt. [Hamilton and Myers, 1967; Gastil, 1979; W. Goddard and Mt. Ritter areas are commensur- Hamilton, written communication, 1981]. ate with such a model (cf. Figure 8a and These concepts have not been explored at 8b with Figure 8c, i-iv). specific sites, however, and they have Deformational Stage 1. We consider the remained controversial, since many workers early deformational stages of the volcanic considered the plutons to be predominantly pile as being accompanied by rising of passively emplaced with wall rock struc- plutonic bodies encased in a thermal welt. tures predating pluton emplacement Regional tumescence supplemented by that [Kistler, 1966; Brook et al., 1974; Russell associated witb the rising plutons reaches and Nokleberg, 1977; Nokleberg and Kistler, the base of the volcanic edifice, and the ]. volcanic pile starts extending roughly Deformational SequenceEn.capsulated. parallel to bedding (Figures 8c, i, and Figures 8a-db show age relationships in 9a). As expansion continues, the develop- different parts of the volcanic sections in ing listric normal faults will probably the Mt. Ritter and Mt. Goddard areas, which root into a d•collement near the base of we interpret as representing the two-fold the volcanic edifice. The geometry of the sequence of events mentioned above. This listrio normal faults allows these blocks sequence is conceptually illustrated in to rotate to relatively steep dips of the Figure 8c (i-iv). In the bed-tilting phase bedding without the necessity of e•treme of the deformation, each fault block extension in the crust (Figure 9b). The contains younger strata which overlie older d•collement is thought to roughly coincide rocks in the fault block below' (e.g., in with the upper surfaces of the thermal- Figure 8c, ii, bed 2 in block II overlies plutonic welt. Unlike d•collements in some bed 3 in block I). In addition, the bed- metamorphic core complexes where the zone ding in the rotated blocks dips in the is lithologically controlled [e.g., Miller opposite direction from the dip of listric et al., 1983], the easterly dipping surface normal faults. This geometry has been of the thermal-plutonic welt may cut ob- documented in the Great Basin area by var- liquely through the volcanic section (Fig- ious workers [e.g., Proffett, 1977; ure 9b), and horizons of different age may Wernicke, 1981; Gans and Miller, 1983]. be exposed at the base of the blocks a• Subsequent ductile flattening, however, they tilt. We suggest this is the c•se in will substantially steepen the dips of both the Mt. Ritter area, where the base of the the beds and listr{c normal faults. Given juxtaposed units is successively older to the 50% flattening of the section docu- the east (Figure 8a), supporting the inter- mented earlier, a simple geometric recon- pretation that the ddcollement transgressed struction shows that beds with a tilt of the bedding at deeper levels. (for example)60 ø W and listric normal In the model introduced above the geome- faults dipping 45¸ E will be rotated to try and timing of initial deformation is dips of about 85ø W and 75¸ F., respective- related to the early members of the Creta- ly. In this situation, beds on both sides ceous batholith which crop out as a belt of the fault will dip steeply (and homo- along the western margin of the Sierra clinally) in one direction while the fault Nevada west of the tilted metavolcanic will dip steeply in the other (Figure 8c, sections [Evernden and Kistler, 1970; iv). Since the strike of the bedding in Saleeby et al., 1985]. Crystallization ages the idealized case will parallel the strike on such plutons cluster in the 125 to 100 of the listric normal fault, older parts of Ma range with a general eastward migration the section in one block will crop out on through time [Saleeby and Sharp, 1980; the "up-section" side of younger rocks in Stern et al., 1981; Chen and Moore, 1982]. the contiguous block (e.g., in Figure 8c, Early members of the western Cretaceous iv, bed 4 in block II crops out "up-sec- batholith are predominantly gabbroic to tion" of bed 1 in block III), giving the dioritic in composition, and have been appearance that older beds "overlie" young- interpreted as constituting the roots of an er beds slong a fault. This geometry andesitic volcanic chain [Saleeby and occurs where both bedding and listrio nor- Sharp, 1980; B. W. Chappell and J. B. mal faulting have been rotated to very Saleeby, unpublished petrochemical data, steep but opposing dips. Where the rocks 9811. have been subsequently subjected to strong •igure 9a depicts the Goddard-Ritter ductile flattening and metamorphism, the sections as a thick pyroclastic sequence 86 Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

/ A. EarlyCretaceous incipient andesitic stra tocone extensional basaltic faulting cinder cones Jurassic to Early Cretaceous pyroclastic , : sequence

WEST i,I I' • EAST • generallocus { of Goddard and Ritter ,onded mafic sections magmawith through dioritic time I differentiates

am Early to mid-Cretaceous andes/tic-dacitic volcano cluster thermal-pluton/c tumescence listtic normal faults steeply-tilted fault

.silicicmag•ma • I decollement

crustal m e/ting and magma mixing

migration of magmatism mid-to Late Cretaceous flattening and Cm constrictional buried caldera resurgent caldera deformation complex dissected arc

E•rl¾Cretaceous deformed gabbro, diorite

... and tonalire ..::ii • bathofith return flow high-velocity sub-ba.tholithic /\ crust ':-;':.'.'.'.'.' ......

APPROX. 20Kin

Fig. 9. Figure illustrating the regional framework within which the geometry and events shown in Figure 8 developed. See text for explanation. Evolution of volcanic and plutonic levels based on regional age patterns [Saleeby and Sharp, 1980]; Stern et al., 1981; Chen and Moore, 1982], and on models of caldera cycles [Fiske et al., 1977; Hildreth, 1981; Lipman, 1984]. Model of deep batholithic structure after Hildreth [1981], DePaolo [1981], and Saleeby et al. [1985]. Topographic relief of volcanic constructional and extensional faulting environment is exaggerated for clarity. 88 Tobisch et al.: Continental Volcanic Arc, Sierra Nevada

lying to the east of the early Cretaceous tial subhorizontal stress approximately andesitic chain. Th•s model is based on the normal to the tilted sections (Figure 8c, succession observed in the modern Chilean iv), imposing strong flattening + con- Andes [Hildreth et al., 1982; W. Hildreth, strictional fabrics on the wall rock. This personal communication, 1985] and on the ductile event was greatly facilitated by time, compositional and spatial relations heat and fluids from the invading plutons. observed across the central Sierra Nevada As a result, the thick volcanic sections batholith. The pyroclastic sequence shown were thinned overall by about 50% normal to includes rocks as old as early Mesozoic. bedding. In addition, we propose that de- Unconformities within the volcanic sequence formation was enhanced by relative downward related to Jurassic accretionary events return flow of and stretching within the have not been observed in this part of the wall rock. Strongly developed, steeply arc, and the Jurassic to early Cretaceous plunging extension lineat•ons (Figures 4c sequence, which •s considered to consist and 5c; Tobisch and Fiske [1982, Figure largely of distal deposits, is shown as a 5c]) and extensional fault movements re- continuous, subhorizontal stratigraphic lated to cleavage formation (see m•p pat- pile. terns in Tobisch et al., [1977, Figure 2a]) Subsequent magmatism in the western are interpreted as evidence for the down- Cretaceous belt was dominated by voluminous ward extension and movement of the pendants silicic (dominantly tonalitic) plutons relative to the rising plutons. which appears to have migrated eastward An additional structural consequence of from the andesitic (gabbro-diorite) chain. this flattening of the stratigraphic sec- Th•s model also predicts the presence of tions was to essentially remove substantial complex unconformities at the base of mid- parts of the overlying wall rock without Cretaceous si]icic volcanic rocks, such as recourse to massive stoping, and to rotate the Minarets Caldera in the Ritter Range the l•stric normal faults to steep dips. [Fiske et al., 1977; Fiske and Tobisch, Since our model appeals to batholith em- 1978]. Other possible remnants of such placement as a deformational agent, we unconformities are presently under investi- envision the deformation as being dia- gation in areas south of the Goddard pen- chronous, migrating eastward at about 2 to dant [Longiaru, 1985; J.G. Moore and J. 3 mm/yr with the locus of mid- to Late B. Saleeby, unpublished data, 1984). In Cretaceous arc magmatism [(Cben and Moore, addition, the extensional environment 1982]. The entire crust is considered to depicted in Figure 9 is considered an inte- have undergone considerable expansion with gral part of the arc, and not necessarily compensation of lower crustal volumes at- related to back-arc spreading. The regional tained by influxes of mantle-derived mafic trend of extensional structures is consid- magma [DePaolo, 1981; Hildreth, 1981; ered to follow the NW-NNW trend of the Saleeby et al., 1985]. The magnitude of early Cretaceous batholithic belt. Such a return flow within wallrocks bounding the regional fabric in extensional structures rising plutons is dependent on the volume and tilted sections may have provided pre- of crustal material melted and ultimately ferred pathways for subsequent late Creta- transported to high structural lev½]• •ela- ceous plutons which show a pronounced NW tive to the volume of mantle-derived mate- elongation (Figure 7). rial added to the lower crust and mixed Deformational Stage 2. The next phase with the silicic magmas. Fluid dynamic in the deformation history of the metavol- considerations suggest substantial return canic sections consisted of large silicic flow deformations within wall rocks up to plutons elongated in a NW orientation (Fig- about one magma body radius from the as- ure 7) moving into upper crustal levels and cending pluton, and smaller increments up intruding the steeply tilted section rough- to within ten magmabody radii [Grout, ly parallel to bedding. Emplacement of 1945; Marsh, 1982, 1984]. This raises the successive pulses of magma into the volcan- possibility that a significant component of ic section is thought to have occurred (at strain observed in the Goddard and Ritter least initially)as sill-or possibly pendants may be related to return flow i• blade-like bodies [cf. Fiske and Jackson, addition to that imposed by horizontal 1972] that expanded as magma was added. It compression related to magma chamber infla- is envisioned that the sill-like bodies tion. Repeated, episodic increments of were expanding at a rate greater than could these processes during successive stages of be accomodated by regional extension. Such batholith development could e•plair• the a dynamic framework could produce substan- repeated parallel deformations previously Tobisch et al.: Continental Volcanic Arc, Sierra Nevada $9 reported from the Ritter Range area graphic sections relative to uprising ply- [Tobisch and Fiske, 1982]. tons especially at middle and later stages The presence of the 100 Ma Ritter Range of deformation, when the increasingly flat- (Minarets) caldera complex [Fiske et al., tened keel-like geometr• of the composite 1977] indicates that a sequence of ignim- sections offered less resistance to sink- brite activity, caldera resurgence, and ing. Both rotation of beds and subsequent deformation extended into Late Cretaceous tectonite fabrics are post-Nevadan age and time (Figures 8a and 9c). Of significance were imposed either continuously or episo- here is the strong domainal deformation dically during Early to Late Cretaceous. observed in the 100 Ma Ritter caldera erup- In this model, deformation of the wall tive sequence [Tobisch and Fiske, 1982], rocks is considered to be related to batbo- and the tight age bracket of this deforma- lithic eraplacement. The deformation, tion provided by cross-cutting relations of therefore, is envisioned as being dia- the resurgent intrusive sequences (Fig. chronous, advancing eastward at about 2 to 8a). Strong rotational deformation of cal- 3 mm/yr with the locus of arc magmatism, dera sequences during their magmatic and but probably overlapping in time and space structural evolution is a well-documented both parallel to and normal to the arc as phenomenon[Lipman, 1984], and the struc- successive families of plutons were era- tural relations observed in the Ritter placed. The relatively constant orientation complex suggest that the caldera eruptives of structures between pendants is probably along the east side of the caldera were a functicn of the constant orientation of involved in deformation along with the the subduction zone which controlled the older tilted sections. crustal tumescence (regional extension) and hence the orientation of tilted fault CONCLUSIONS blocks and the subsequently emplaced plu- tons, From the data at our disposal, we con- The structural and stratigraphic rela- clude that generation of the majority of tions presented above for the Mt.Goddard structures and variation in strains within and Ritte• Range pendants, and the exten- the segment of the continenta.• arc under sional model for generating the dominant considerations are more likely to be re- structures in the continental volcanic arc lated to the internal dynamics of the arc carry important implications for the Neva- itself, rather than to externally imposed dan orogeny. Various workers have suggested tectonic events such as collision accre- a Nevadan event involving collision of an tion. These dynamics involved the imposi- oceanic arc with the continental margin for tion of an extensional environment on the generating compressional structuzes in the supracrustal volcanic section during mas- western Sierra Nevada [e.g., Moores, 1970; sive batholithic eraplacement throughout Schweickert and Cowan, 1975; Moores and much of the Cretaceous. Extension occurred Day, 1984; $chweickert et al., 1984b; Day normal to the belt in a general NE-SW ori- et al., 1985]. Since the continental vol- entation which generated listric normal canic rocks of the area under consideration faults striking parallel to the orogen. As in the eastern Sierra Nevada are predomi- extension continued, bedding was rotated to nantly distal deposits and were likely to steep dips along these faults, resulting in have been nearly flat lying at the time of a series of steeply tilted, elongated fault the proposed collision, large-scale folds blocks strikinõ parallel to the belt, and (i.e., longitudinal strain) would no doubt which lacked large-scale folding of any have been generated from that event. Major areal importance. Subsequent rising of folds of any areal importance, however, are magma, often eraplaced as elongate sill-like largely absent in the area. Furthermoze, bodies, worked its way into the steeply stratigraphic breaks of Nevadan age have tilted fault blocks approximately parallel not been observed within the volcanic sec- to regional bedding, expanding at a greater tion in the Goddard or F•itter pendants. rate than regional extension could accommo- This raises several possibilities: (1) the date. The overall dynamics of this magma volcanic arc in the Western Metamorphic tumescence imposed strong tectonite fabrics Belt was eraplaced into its present position on the wall rocks, and reduced the volcanic by a mechanism other than collision such as section normal to bedding by about 50% transform related strike-slip movement com- overall. This process was facilitated by mon in some volcanic arc environments heat and fluids from the plutons and from [Dewey, 1980]. In this view, Nevadan struc- downward return flow of the faulted strati- tures could b• strongly developed in the 9O Tobisch et el.: Continental Volcanic Arc, Sierra Nevada western Sierra Nevada while absent along eastward, while the volcanic section dips the continental volcanic arc; (2) Nevadan westward (cf. Figure 8a). Most compelling structures n•rtb of latitude 37030 ' may be is the chronological evidence from fossil developed primarily within east-directed and Pb/U ages, which indicates that the nappes of the western Sierra Nevada foot- faults must be present in the volcanic hills as a result of a compressional event section even if their precise placement in [Moores and Day, 1984; Day et el., 1985; the field is often difficult. It is not Ricci et el., 1985; Saleeby et el., 1985], possible to document the original listric with the advancement of such nappes limited geometry of these faults because the sub- to the forearc region of the continental stantial ductile flattening has rotated the arc. It must be reemphasized, however, that faults to high dips. silicic volcanism of the Goddard and Ritter We have collected an additional suite of pendants appears to have continued during specimens for Pb/U dating from the Ritter the Nevadan event. This may be easier to Range and are carrying out more detailed explain with the strike-slip intra-arc mapping in certain critical areas in an transport of a Jurassic arc fragment from a effort to pin down as precisely as possible different collision site into the Sierra the locality and geometry of the listric region during continued subduction and faulting we have proposed. A full report related silicic volcanism of the continen- will be forthcoming. tal arc. A final point is that strain which pro-. Mechanism of Ductile Deformation duces tectonite fabrics in metamorphic rock is perhaps too readily interpreted as re- Our proposal suggests that pluton em- suiting from regional crustal shortening placement has imposed tectonite fabrics on tectonics. In regions of voluminous magma the volcanic section. This is speculative, emplacement,the dynamicsof successively of course, and was born from the fact that injected sill-like bodies, laterally no known regional post-Nevadan event exists in the eastern Sierra Nevada which can spreading magmachambers and the return flow of less boyant wallrocks required by account for the late age of deformation in material balance during magmaascent sho•Jld the continental volcanic arc except batho- be ev81uated very carefully before the lithic eraplacement. associated metamorphic tectonites are sim- Much work needs to be done to confirm ply considered products of regional crustal our proposedmodel, not only •n documenting shortening. Indeed, voluminous magmatism age relations between plutons and wall should be considered a tectonic event of rock, but possibly more so in investigating significant regional consequences. the possible modes of emplacement of the plutons, and the effect intruding magmahas APPENDIX: SOME UNCERTAINTIES OF THE MODEL on the wall rock. There is abundant litera- ture from various orogens on diapiric era- Listric Normal Faults placement and its late-stage effects on wall rock. From many reports in the Sierra The number of these faults shown in Nevada, however, such eraplacement is not Figure 9 is muc• greater than that actually thought to be widespread, leading to the identified in the. field, and in fact their notion that the plutons were largely pas- locality is often d•fficu]t to pin down sively emp]aced. Our knowledge concerning with any precision. We have identified other types of magmaeraplacement, such as only one such fault in the Goddard area, inflation of sill-like bodies that might while we think we can identify two and produce the elongate forms so commonly perhaps a third such fault iD the •itter found in the central S•erra Nevada, and area. The fault separating blocks in the which we speculate may be a major contribu- Mt. Goddard area shows the mcst convincing tor to the deformation of the wall rocks, field characteristics as described in the has been relatively less well researched. text, and •t is relatively emsy to identi- This is an area for fertile investigation fy. Those in the Ritter Range are more in field, theoretical, and experimental cryptic. Geologic evidence, however, indi- studies (cf. for example, Marsh [1982], cates repetition of certain distinctive Fiske and Jackson [1972]), and would con- units while facing directions remain con- tribute much to supporting or rendering stant. In addition, sill• which we consider invalid the mechanisms of deformation we to have intruded one of the faults dip propose. Tobisch et al.' Continental Volcanic Arc, Sierra Nevada 91

Acknowledgements. Field and laboratory Nevada, California, Geol. Soc. Am. Bull., work has been supported in part by the U.S. 96, 1346-1347, 1985. Geological Survey, the Fluid Research Fund Brook, C. A., Stratigraphy and structure of of the SmithsoDian Institution, a Faculty the Saddelbag Lake roof pendant, Sierra Research Grant from the University of Cali- Nevada, California, Geol. Soc. A__m.Bull., fornia, Santa Cruz, and N.S.F. grant EAR 88, 321-334, 1977. 8206478 awarded to Tobisch. G½ochronolog•- Brook, C. A., W. J. Nokleberg, and R. W. cal work was supported by N.S.F. grants EAR Kistler, Nature of the angular unconform- 8018811 and EAR 8218460 awarded to Saleeby. ity between the Paleozoic metasedimen- Patience and expertise in hand pt•rification tary rocks and the Mesozoic metavolcanic and sorting of zircon fractions by Cherilyn ic rocks in the eastern Sierra Nevada, Saleeby was essential for this study. We California, Geol. Soc. A__•m.Bull., 85, are grateful to T. W. Stern of the U.S. 571-576, 1974. Geological Survey for his important contri- Chen, J. H. and J. G. Moore, Uranium-lead butions in age dating of the Ritter Range ages from the Sierra Nevada batholith, rocks, the complete report of which will be California, J. Geophys. Res., 87, 4761- published elsewhere. Jack Collender, Steve 4784, 1982. Davis, arid Joe Frey generously contributed Chen, J. H. and G. J. Wasserburg, Isotopic their time and effort to the mapping, data determination of Uranium in picomole and collecting and its analysis from part of subpicomo equantities, A.nal. Chem., 53, the area as shown in Plate 1, inset. We 2060-2067, 1981. thank Marty Morrison for running electron- Dalmayrac, B. and P. Molnar, Parallel probe analyses on andalusite and plagio- thrust and normal faulting in Peru and clase in one specimen. Reviews by Paul contraints on the state of stress, Earth Bateman, Frank Dodge, Phil Gans, Ben Page Planet. Sci. Lett., 55, 473-481, 1981. and Rich Schweickert helped clarify our Day, H. W., E. M. Moores, and A. C. Tumi- writing, for which we are grateful. nas, Structure and tectonics of the northern Sierra Nevada, Geol. Soc. Am. REFERENCES Bull., 96, 436-450, 1985. DePaolo, D. J., A neodymium and strontium Ave Lallement, H. G., C. W. Weisenberg, and isotopic study of the Mesozoic calc-alka- L. A. Standlee, Structural development of line granitic batholiths of the Sierra the Melones zone, northeastern Califor- Nevada and Peninsular ranges, California, nia, Geol. Soc. Am. Abstr. Pro,rams, J. Geophys. Res., 86, 10470-10488, 1981. 15, 372, 1977. '• -- Dewey, J.F., Episodicity, sequence, and Bateman, P. C., Geologic map of the Black- style at convergent plate boundaries, The cap Mountain quadrangle, Fresno County, Continental Crust and Its Mineral Depos- California, Map GQ-428, U.S. Geol. Surv., its, edited by D. W. Strangeway, Geol. Reston, Virginia, 1965. Assoc. Can. Spec. Pap. 20, 553-573, 1980. Bateman, P. C. and L. D. Clark, Stratigra- DuBray, E. A., Geology of the igneous and phic and structural setting of the Sierra metamorphic rocks in the Evolution- Nevada batholith of California, Pac. Goddard region of the Sierra Nevada, Geol., 8, 79-89, 1974. California, M.S. thesis, 123 pp., Stan- Bateman, P. C. and J.P. Eaton, Sierra ford Univ., Stanford, Calif., 1977. Nevada batholith, Science, 158, 1407- Elliott, D., Determination of finite strain 1417, 1967. and initial shape from deformed ellipti- Bateman, P. C. and J. G. Moore, Geologic cal objects, q•e01..So..c..Am. Bull., 81, map of the Mount Goddard quadrangle, 2221-2236, 1970. Fresno and Inyo Counties, California, Evernden, J.F., and R. W. Kistler, Chrono- Map GQ-429, U.S. Geol. Surv., Reston, logy of emplacement of Mesozoic batho- Virginia, 1965. lithic complexes in California and west- Bateman, P. C., A. J. Busacca, and W. N. ern Nevada, U.S. Geol. Surv. Prof. Pap. Sawka, Cretaceous deformation in the 623, 67 pp., 1970. western foothills of the Sierra Nevada, Fiske, R. S., Recognition and significance California, Geol. Soc. Am. Bull., 94, of pumice in marine pyroclastic rocks, 30-42, 1983. Geol. Soc. Am. Bull., 80, 1-8, 1969. Bhattacharyya, T. and S. R. Paterson, Dis- Fiske, R. S., and E. D. Jackson, Orienta- cussion: Timing and structural expres- tion and growth of Hawaiin volcanic sion of the Nevada orogeny, Sierra rifts' The effect of regional structure 9P Tobisch et al.' Continental Volcanic Arc, Sierra Nevada

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