Structural-metamorphic chronology in a roof pendant near Oakhurst, California: Implications for the tectonics of the western

Department of Geosciences, Texas Tech University, Lubbock, Texas 79409

ABSTRACT ceous (Strand, 1967) — here considered to Despite the presence of these shear zones, compose the Oakhurst roof pendant — crop probably the most conspicuous structural Foliation, broken bedding, and two out a short distance west of Oakhurst, Cali- feature of the western metamorphic belt is a "shear zones" in pre-Cretaceous clastic fornia (Fig. 1). The pendant, surrounded by strongly penetrative, northwestward-strik- metasedimentary rocks in a roof pendant exposures of the Sierra Nevada , is ing cleavage which developed during the west of Oakhurst, California, are on trend on trend with the southernmost continuous classical (Late Jurassic) Nevadan orogeny. with the western Sierra Nevada metamor- exposures of the western metamorphic belt However, evidence of earlier (pre-Nevadan) phic belt to the northwest. In the pendant, and the contained Melones fault zone, both deformation is present in rocks of the foliations define a westward-verging, up- of which terminate against or within Calaveras—Shoo Fly assemblage (Chandra, ward-diverging fan, and lineations have batholithic rocks approximately 24 km to 1953, 1961; Baird, 1962; Hassan, 1968; orientations that plot along a great circle, or the north near Mariposa (Fig. 1). Stuart-Alexander, 1967; and others). Re- a small circle of large diameter, with a The southern part of the western meta- gional in the belt is chiefly maximum at the intersection of the circle morphic belt was divided by Clark (1960, of greenschist grade and is associated with and the synoptic foliation. Rock microtex- 1964) into eastern, central, and western the Nevadan orogeny, but metamorphism tures are characterized by porphyroblast structural "blocks," separated, respectively, of the almandine amphibolite facies in clasts, especially of hornblende, in a matrix by the Melones and Bear Mountains fault Calaveras rocks of the southern part of the of aligned cataclastic fragments and some zones, the major members of the Foothills western metamorphic belt is reported by superimposed static mineral growth. fault system. The eastern block consists Baird (1962) from the Columbia area. Three major events are interpreted. The mainly of Paleozoic chert, slate, and Recently, the western Sierra region has first, Mi, is characterized by probable syn- metasiltstone, with subordinate amounts of been interpreted in terms of plate tectonic kinematic metamorphism of the epidote- marble and metavolcanic rocks, all of the models. In general, such interpretations amphibolite facies. The second, here corre- Calaveras "formation"; the Shoo Fly for- consider the central and southern portion of lated with the classical Nevadan orogeny, mation also may be present. The central the western Sierra Nevada metamorphic involved the cataclastic development of the and western blocks contain abundant belt to represent a Mesozoic island-arc prominent, penetrative, northwest-striking Mesozoic clastic metasedimentary rocks complex (western "block" of Clark, 1964) foliation surface, Si, and the incomplete but are dominated by andesitic to basaltic sutured, or partially sutured, to the Pa- transposition of Mi hornblende lineations metavolcanic rocks of both flow and pyro- leozoic continental margin (eastern "block" (a kinematic axis: N50°E, 64°NE). During clastic origin (Clark, 1964). Farther north, of Clark, 1964) during the closure or near this event, bodies of ultramafic and gran- these structural-lithologic divisions are less closure of a rear-arc basin associated with odioritic rocks were emplaced and de- applicable. the Nevadan orogeny (Hamilton, 1969; formed along the "shear zones." Finally, Within the Melones and Bear Mountains Moores, 1972; Schweikert and Cowan, contact metamorphism, M2, chiefly of the fault zones, lithologies are chiefly of ser- 1974). In this model, the Melones and Bear albite-epidote-hornfels facies, resulted from pentinite and other ultramafic rocks, with Mountains fault zones, as well as the cen- the intrusion of batholithic rocks between associated chert and metavolcanic rocks tral block, represent the zone of suturing approximately 98 and 136 m.y. ago. —an assemblage of "ophiolitic" character. (Moores, 1972; Schweikert and Cowan, Regional implications include the follow- Sense of displacement along these faults is 1974). The northern portion of the western ing: (1) the shear zones may be southerly uncertain. Some studies suggest possi- metamorphic belt apparently is more com- remnants of the Foothills fault system, ble dominant strike-slip (Chandra, 1953; plex than that to the south. Moores (1970) probably the Melones fault zone; (2) Mj Clark, 1960; Baird, 1962; Cebull, 1972; proposed a model involving multiple arc could represent a moderately deep expres- Wetzel and Nokelberg, 1976), whereas collisions and subduction-zone "flips" be- sion of the Sonoma or Antler orogenies; others infer principally dip-slip displace- ginning in early Paleozoic time. Alternative- and (3) the kinematics of Si favor an in- ment (for example, Knopf, 1929; Russell ly, Burchfiel and Davis (1972, 1975) ex- terpretation of primarily vertical displace- and Cebull, 1974). It is possible that motion plained the development of the region as ment for the Nevadan deformation and, has been complex and that both senses of being related to the two-stage collision of a hence, lend possible support to most sub- displacement are recorded (Cebull, 1972). single Paleozoic arc with the continental duction models. If primarily or partly of dip-slip origin, the margin. The two stages are represented by faults may represent the root zone of west- the Antler and Sonoma orogenies, which INTRODUCTION erly directed, low-angle thrust sheets such were succeeded by the development of an as those exposed in the Klamath Mountains Andean continental margin in Mesozoic Metasedimentary rocks previously (Taliaferro, 1942; Clark, 1960; Davis, time. mapped as undifferentiated and pre-Creta- 1969). In a more recent discussion related to the

Geological Society of America Bulletin, v. 88, p. 1530-1534, 3 figs., October 1977. Doc. no. 71016.

1530

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Figure 1. Geologic map, interpretive cross section, and index map. Attitudes shown on geologic map are representative and do not include all those measured. Cross section is based on dip isogons (not shown) through the central portion of the pendant, but is generalized and does not represent a specific line of section. On index map, numbers indicate (1) eastern, (2) central, and (3) western "blocks" of Clark (1960, 1964); cross hachures represent areas primarily of Mesozoic rocks; and nonhachured regions, Paleozoic rocks. Pendant west of Oakhurst is in black.

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western metamorphic belt, Schweikert and Cowan (1975) explained the early Meso- zoic tectonic evolution of the western Sierra Nevada region in terms of the interaction of two partly coeval, subparallel volcanic arcs. Figure 2. Lower-hemisphere, The eastern arc developed in Late Triassic equal-area projection of 30 horn- time as a west-facing marginal arc, whereas blende lineations in central unit its western counterpart formed as an east- (contoured) and maxima for poles to foliations in (I) western (50 points), facing island-arc system beginning in late (H) central (43 points), and (HI) east- Middle Jurassic time. The two arc com- ern (35 points) units. Dashed line is a plexes collided during the Late Jurassic great circle defined by lineations (a Nevadan orogeny. The resulting suture is small circle of large diameter also fits represented by a highly sheared mélange this lineation dispersal); solid line rep- and ophiolite terrain, as well as by the faults resents the synoptic foliation plane for of the Foothills fault system. Schweikert the central unit. Contour interval for (1976) reported lawsonite blueschist rocks lineations is 5% per 1% area. Pole within a portion of this suture in the north- maxima are represented only by 20% and 24% contours per 1% area. ern Sierra Nevada. A different interpreta- tion for the western Sierra Nevada metamorphic belt is that of Wetzel and Nokleberg (1976). They suggested that the region represents a Late Jurassic oceanic ridge "telescoped" by folding and faulting actinolite (50%) and quartz (40%) with batholithic contacts where they are as- during the Nevadan orogeny. In their view, subordinate biotite (4%), diopside (3%), sociated with flow and boudinage struc- the Foothills fault system is not a suture and epidote (3%). Ultramafic and gran- tures, all apparently genetically related to zone but a "sympathetic movement zone" odioritic rocks, discussed below, crop out the emplacement of batholithic rocks rather developed in response to oblique subduc- along two discrete zones within these than to internal deformation within the tion farther west. lithologic units. The suc- metamorphic rock succession. However, The purpose of this paper is to describe cession is surrounded and locally intruded lineations are highly developed in the inte- the structural and metamorphic chronology by plutonic silicic igneous rocks of the rior of this rock succession, although main- and character recorded in rocks of the Sierra Nevada bathoKth, probably mainly ly in the hornblende schist unit. There, the Oakhurst roof pendant, an apparent south- of granodioritic composition. Bulk lineations result from alignment of long, erly remnant of the western metamorphic mineralogy of rocks composing the three needlelike hornblende crystals, which lie belt, and to consider briefly its possible sig- metamorphic units indicates a sedimentary primarily in the foliation planes and plunge nificance in the interpretation of the origin for the main rock succession. steeply northeastward. A Schmidt net pro- tectonic evolution of the western Sierra jection (Fig. 2) shows that the lineation Nevada. Mesoscopic Structural Geometry and maximum is on the synoptic Si foliation Kinematics plane for the hornblende schist unit and GEOLOGY OF THE that the overall lineation distribution de- OAKHURST PENDANT The most pronounced mesoscopic struc- fines a great circle (shown in Fig. 2) or tural feature in the pendant is a well- perhaps a small circle of large diameter. A Lithology developed, penetrative foliation (Si) which second great (or small) circle defined by strikes northwestward and dips northeast- lineations also may be present. The Oakhurst roof pendant displays ward, approximately parallel to regional That these lineations are dispersed chiefly three lithologic units (Fig. 1), the most dis- Sierra (Nevadan) foliation orientations. At along an approximate great circle but also tinctive of which is a centrally located rock most places this foliation is parallel to produce a maximum on the synoptic folia- succession composed mainly of gray-green lithologic banding. Although its strike is tion suggests that they are incompletely hornblende schist. This rock type consists consistent from place to place, dips vary transposed into the plane of foliation. typically of very fine to coarsely crystalline progressively from an average of 40°NE in Lineation dispersal along a single circle hornblende (40%) in association with the western unit, to 64°NE in the centrally implies that the distribution of linear ele- quartz (40%), diopside (10%), plagioclase located hornblende schist, to 78°NE in the ments prior to interpreted transposition (5%), epidote (3%), and biotite (2%). eastern unit (Fig. 2). Locally, dips are verti- was structurally homogeneous, possibly Numbers in parentheses are estimated aver- cal or to the southwest in the latter two aligned in a pre-Si foliation. Reasons for the ages. Flanking the hornblende schist unit on units. Thus, variation in foliation dip de- presence of a second but weaker, possibly the east and west are two units of similar fines an upward-diverging fan (Fig. 1). small circle of lineations is not clear. gross lithologic character. Both are domi- Two linear "shear zones," one in the Perhaps this weaker circle represents a sec- nantly of foliated quartzitic rocks with local hornblende schist unit and another in the ond, less dominant homogeneous domain pelitic zones, commonly of schistose ap- western unit, are approximately parallel to for the pre-Si lineations. Assuming defor- pearance. The typical mineralogie compo- the foliation (Fig. 1). These shear zones con- mation that ultimately took a form ap- sition of rocks from these two units is tain chieflly two rock types — massive to proaching that of simple shear, the quartz (55%), muscovite and sericite (8% foliated metaserpentinite and foliated, par- intersection of the major lineation-defined to 30%), biotite (10% to 30%), plagioclase tially recrystallized granodioritic rock. circle and the synoptic foliation plane for (5%), and chlorite (3%). Some zones within Foliations in these shear zone rock masses the hornblende schist unit (Fig. 2) defines the western unit display a mineral as- parallel those elsewhere in the pendant. the a kinematic axis of the deformation, semblage broadly similar to that of the Mesoscopic folds are rare in rocks of the which produced the foliation (Si) and ap- hornblende schist unit. This asemblage is of pendant. Where observed, they are close to parently transposed the lineation. In the

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I I tively deformed during the event, as indi- cated by parallelism of foliations within and Y V* AMPHIBOLE LI NEAT IONS I FOLIATIONS I 136 MY 98 MY I BATHOLITH IC INTRUSIVE ROCKS outside the zones. TRANSPOSITION The third and final event is one of static I I ! metamorphism, here termed M2. Am- STRUCTURES phibole (hornblende and actinolite) and diopside apparently recrystallized locally at the initial peak of thermal activity, but sub- EPIDOTE I DIOPSIDE II sequently the poikiloblastic growth of bio- tite, muscovite, and epidote dominated. At DI OPS I DE I EPIDOTE II many places static mineral growth was ret- AMPHIBOLE BIOTITE II rograde and at the expense of amphibole, GTZ I PLAG I AMPHIBOLE II CHLORITE II diopside, and biotite. Hence, M2 was domi- > nantly of the albite-epidote-hornfels facies. MUSC I (?) BIOTITE (?) J QTZ 11 PLAG 11 MUSC 11 T Rocks within the two shear zones also were recrystallized, as evidenced by the textures of static and retrograde mineral growth in both the granodioritic and ultramafic rocks.

M2 Intrusion of batholithic rocks in the sur- UPPER MEDIUM GRADE CATACLASTIC LOW GRADE rounding area was responsible for M2 re- crystallization, a plutonic event that is SYNKINEMATIC (?) STATIC S RETROGRADE radiogenically dated (Evernden and Kistler, 1970) as approximately between 136 m.y. -PRE-"NEVADAN"- • "NEVADAN" • " POST-"NEVADAN" (Late Jurassic) and 98 m.y. (Middle Creta- Figure 3. Generalized sequence of events as interpreted from mesoscopic and microscopic features. ceous). "Nevadan" refers to the classical Nevadan orogeny of approximately Late Jurassic time. Synthesis and Summary Oakhurst roof pendant this axis is steep, epidote; part of the amphibole is enveloped trending N50°E and plunging 64°NE. by biotite; and some biotite shows over- A synthesis of mesoscopic structural, growths of chlorite. In the shear zones, mineralogic, and microtextural interpreta- Metamorphism: Microtextural, granodioritic rocks display nonaligned se- tions for the Oakhurst pendant is shown in Minéralogie, and Structural Relationships ricite, chlorite after biotite, and, in places, Figure 3 in the form of a generalized se- post-s-plane garnet. Ultramafic rocks are quence of events. Largely geometrically Rocks of the hornblende schist unit and characterized by radial aggregates of tre- homogeneous distribution of Mj horn- those of certain atypical zones of the west- molite and anthophyllite superimposed on blende lineations (pre-Si) suggests that Mi ern unit best preserve evidence of the the earlier antigorite meshwork of the ser- was a synkinematic metamorphic event. Be- metamorphic evolution of the pendant. pentinite. This phase of apparent post-s- cause the regional foliation, Si, apparently Thin sections of such rocks display large plane recrystallization is of variable inten- developed as a result of processes partly re- hornblende porphyroblasts (actinolite in sity throughout the pendant and is nowhere sembling those of simple shear, the original the western unit), and to a lesser extent complete. (Mi) orientation of hornblende lineations is porphyroblasts of diopside, epidote, pla- The preceding mineralogic and microtex- indeterminate. Si developed during the gioclase (An40_45), quartz, and possibly tural data form the basis for the following classical (Late Jurassic) Nevadan orogeny, muscovite and biotite, all of which are interpretive chronology. The earliest meta- as evidenced by its parallelism and similar- fragmented and dragged into a well- morphic event, M1; is characterized princi- ity to the regional "Nevadan" cleavage in developed s plane. The sheared-off frag- pally by the growth of hornblende the western metamorphic belt farther north, ments are deflected around the larger clasts porphyroblasts (actinolite in the western and resulted in subvertical movement (the porphyroblasts are now porphyroblast unit) and diopside in conjunction with (N50°E, 64°NE) with associated transposi- clasts). Quartz and feldspar commonly plagioclase, biotite, muscovite, and epidote. tion. This transposition is suggested by the show strain shadows and, locally, form thin This mineral assemblage, as well as plagio- apparent migration of M! lineations into Si zones of mortar parallel to the s plane. clase composition, indicates that Mi prob- and, more generally, by the widespread Thus, a basic element of the rock fabric is a ably was of upper medium grade, reflecting parallelism of broken bedding and cleavage cataclastic texture, which is even more pro- conditions of the upper epidote amphibolite in the pendant. The interpreted direction of nounced in rocks associated with the shear facies of regional metamorphism. movement (a kinematic axis) is similar to zones. Following Mi was an event characterized that reported by Best (1963) for the region Most mineral species show varying de- by the development of a prominent micro- near Indian Gulch, approximately 40 km to grees of mineral growth that apparently scopic 5 surface, S,, the penetrative meso- the northwest (see Turner and Weiss, 1963, succeeds s-plane development. Typically, scopic foliation observed in the field. Folia- p. 514). such late growth is evidenced by poikilo- tion development was principally by cata- The shear zones, which parallel Si, prob- blastic muscovite and epidote associated clasis, which resulted in the incomplete ably developed as zones of intensified or with (1) radial sprays of sericite, (2) par- mechanical breakdown of Mi amphibole perhaps temporally prolonged tectonic mo- tially "healed" quartz and feldspar grains, (hornblende and actinolite) porphyroblasts tion during foliation production. In view of locally exhibiting incipient polygonal grain and tectonic concentration and alignment the kinematics of foliation development and boundaries, and (3) overgrowths of horn- in Si of the resulting mineral fragments. At- the upward-diverging geometry of the foli- blende, actinolite, and diopside over the tendant metamorphic recrystallization was ation fan, these shear zones could represent mineral fragments. The amphibole and slight or absent. Rocks of the two linear roots of westward-directed thrusts (with a diopside, in tum, are locally enveloped by shear zones probably were also penetra- small strike-slip component) produced in

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response to tectonic upwelling from a deep- sented evidence for subduction with a Paper 623, 42 p. er structural level (as suggested in the cross minor component of oblique (right-lateral) Hamilton, W., 1969, Mesozoic California and section of Fig. 1). During this upwelling, motion during a Nevadan collision between the underflow of Pacific mantle: Geol. Soc. the ultramafic and granodioritic rocks continental and oceanic plates. His conclu- America Bull., v. 80, p. 2409-2430. Hassan, M., 1968, Metamorphism of Jurassic within the shear zones were emplaced and sions are a result of work conducted along volcanic rocks in the Sierra foothills near deformed. Shear zone granodioritic rocks the lower Kings River, only 50 km south- Jackson and Sutter Creek [Ph.D. dissert.]: are tectonically segregated parts of the east of Oakhurst. However, Oakhurst Berkeley, Univ. California, 75 p. surrounding batholith, which probably kinematics appear not to be in agreement Knopf, A., 1929, The Mother Lode system of indicates that the early stages of batholithic with the still more recent suggestions of California: U.S. Geol. Survey Prof. Paper intrusion were synchronous with the later Wetzel and Nokleberg (1976) that the 157, 88 p. stages of the Nevadan orogeny. However, principal component of movement along Moores, E., 1970, Ultramafics and orogeny, with the plutonic magmatic episode was chiefly the Foothills fault system is right-lateral models of the U.S. Cordillera and the postorogenic and resulted in static contact strike slip. Tethys: Nature, v. 228, p. 837-842. metamorphism. 1972, Model for Jurassic island arc- ACKNOWLEDGMENTS continental margin collision in California: Geol. Soc. America Abs. with Programs, Regional Implications We thank G. T. Morahan and W. Nok- v. 4, p. 202. leberg for a critical review of the paper and Nokleberg, W., 1975, Structural analysis of a The geology of the Oakhurst pendant has E. S. T. O'Driscoll for additional com- collision between an oceanic plate and a implications that affect the tectonic in- continental plate preserved along the lower ments. terpretation of the western metamorphic Kings River in the Sierra Nevada: Geol. Soc. belt as a whole. First, the shear zones in the America Abs. with Programs, v. 7, pendant may be remnants of the eastern REFERENCES CITED p. 357-358. portion of the Foothills fault system, prob- Russell, L. R., and Cebull, S. E., 1974, Baird, A. K., 1962, Superposed deformations in ably the Melones fault zone. If so, the fault Structural-metamorphic chronology in the the central Sierra Nevada foothills, east of southern western metamorphic belt, Sierra zone (and the Foothills fault system gener- the Mother Lode: California Univ. Pubs. Nevada, and its possible regional sig- ally) extends a minimum of 35 km farther Geol. Sci., v. 42, p. 1-70. nificance: Geol. Soc. America Abs. with southward than previously mapped. Sec- Best, M. G., 1963, Petrology and structural Programs, v. 6, p. 244-245. ond, the Mi event recorded in rocks of the analysis of metamorphic rocks in the Schweikert, R. A., 1976, Lawsonite blueschist pendant apparently occurred prior to the southwestern Sierra Nevada foothills: within the Melones fault zone, northern Nevadan orogeny (that is, predates "Neva- California Univ. Pubs. Geol. Sci,, v. 42, Sierra Nevada, California: Geol. Soc. dan" cleavage) and is characterized by p. 111-158. America Abs. with Programs, v. 8, p. 409. synkinematic metamorphism of uncom- Burchfiel, B. C., and Davis, G. A., 1972, Struc- Schweikert, R. A., and Cowan, D. S., 1974, tural framework and evolution of the monly high grade for the southern portion Pre-Tithonian magmatic arcs and subduc- southern part of the Cordilleran orogen, tion zones, Sierra Nevada, California: Geol. of the western metamorphic belt. Possibly western United States: Am. Jour. Sci., Soc. America Abs. with Programs, v. 6, the event is a moderately deep manifesta- v. 272, p. 97-118. p. 251-252. tion of the Sonoma or Antler orogenies. 1975, Nature and controls of Cordilleran 1975, Early Mesozoic tectonic evolution of That two probable deformations are re- orogenesis, western United States: Exten- the western Sierra Nevada, California: corded suggests that rocks of the pendant sions of an earlier synthesis: Am. Jour. Sci., Geol. Soc. America Bull., v. 86, p. 1329- may be of Paleozoic age and correlative v. 275-A, p. 363-396. 1336. with the Calaveras "formation." Third, the Cebull, S. E., 1972, Sense of displacement along Strand, R. G., 1967, Geologic map of California, kinematics of Nevadan (S^ movement Foothills fault system: New evidence from Mariposa sheet: California Div. Mines and favor an interpretation of primarily vertical the Melones fault zone, western Sierra Geology, scale 1:250,000. Nevada, California: Geol. Soc. America displacement (with a minor lateral compo- Stuart-Alexander, D. E., 1967, Contrasting de- Bull., v. 83, p. 1185-1190. nent) for deformation within the western formation of Paleozoic and Mesozoic rocks Chandra, D. K., 1953, Geology of the Colfax and near Sierra City, northern Sierra Nevada, metamorphic belt and within the related Foresthill quadrangles, California [Ph.D. California [Ph.D. dissert.]: Stanford, Calif., Melones fault zone. Such movement is in dissert.]: Berkeley, Univ. California, 78 p. Stanford Univ., 120 p. accord with models, such as that of Davis 1961, Geology and mineral despoits of the Taliaferro, N. L., 1942, Geologic history and (1969), that portray the belt as a root zone Colfax and Foresthill quadrangles, Cal- correlation of the Jurassic of southwestern of upward-flattening thrust sheets. Finally, ifornia: California Div. Mines and Geology Oregon and California: Geol. Soc. America although this work fails to bear directly on Spec. Rept. 67, 50 p. Bull., v. 53, p. 71-112. plate tectonic models for the western Sierra, Clark, L. D., 1960, Foothills fault system, west- Turner, F. J., and Weiss, L. E., 1963, Structural it may relate indirectly to such models if the ern Sierra Nevada, California: Geol. Soc. analysis of metamorphic tectonites: San geometry of deformation in mobile belts is America Bull., v. 71, p. 483-496. Francisco, McGraw-Hill Book Co., 545 p. 1964, Stratigraphy and structure of part of assumed to reflect, or at least approximate, Wetzel, N., and Nokelberg, W., 1976, Plate the western Sierra Nevada metamorphic tectonic and structural relations for the the geometry of subduction. If this assump- belt: U.S. Geol Survey Prof. Paper 410, origin and deformation of the western tion is correct, our conclusions are compat- 70 p. metamorphic belt along the margin of the ible with those proposals that place an Davis, G. A., 1969, Tectonic correlations, central Sierra Nevada batholith: Geol. Soc. eastward-dipping subduction complex Klamath Mountains and western Sierra America Abs. with Programs, v. 8, p. 420. along the front of the western Sierra foot- Nevada, California: Geol. Soc. America hills during the Nevadan orogeny. Indeed, Bull., v. 80, p. 1095-1108. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH the kinematics of Nevadan displacements in Evernden, J. F., and Kistler, R. W., 1970, 21, 1976 the Oakhurst area are in accord with recent Chronology of emplacement of Mesozoic REVISED MANUSCRIPT RECEIVED SEPTEMBER 27, batholithic complexes in California and work by Nokleberg (1975), who has pre- 1976 western Nevada: U.S. Geol. Survey Prof. MANUSCRIPT ACCEPTED NOVEMBER 30, 1976

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