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

Discussion

TAPAS BHATTACHARYYA i _ „ . _ . ,, . . , _ ... . „ , _ _ ,., . nATi-DCAM I ^art'1 Science Board, University of California, Santa Cruz, California 95064 SCO IT R. PA [ bKSON /

Prior to commenting on several aspects of the paper by Schweickert and others, 1984, can be divided into four topics: (1) the position/exist- and others, 1984, we wish to note that we have benefited from the work of ence of the "Sonora fault," (2) the "polymetamorphism and structural these authors. We also wish to note that all of our comments concern complexity" of the Calaveras Complex and Shoo Fly Formation, (3) the statements made about the southern portion of the Western Metamorphic interpretation of "Late Phase" or Cretaceous folds and possible conjugate Belt where we are presently completing an east-west transect on scales of structures, and (4) their model for "rigid body rotation of the central belt" 1:24,000 and larger. during Nevadan deformation. We will discuss each of these in turn. Our objections to observations and interpretations by Schweickert 1. Many of the age relations and positions of contacts/faults dis- cussed by Schweickert and others, 1984, are better established in the The article discussed appeared in the Bulletin, v. 95, p. 967-979. northern Sierra or the part of the southern belt which lies north of 3£ We

Geological Society of America Bulletin, v. 96, p. 1346-1352, 1 fig., October 1985.

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think that similar relations are not nearly so well established in the south- Belt. We have observed conjugate structures in the field and in thin sec- ern portion of the western metamorphic belt. For example, the position tions in the Shoo Fly, Calaveras, greenschist-phyllite belt, and Melones and/or existence of the Sonora fault in this area is still highly questionable. fault zone. Near Briceburg, California, a particularly nice set of locally We have mapped a transect at 1:12,000 across their (Schweickert and developed conjugate kinks are superimposed on an older set of conjugate others, 1984) Sonora fault in the Merced River area and have found no folds and cleavages. Both sets of kinks fold two earlier cleavages, postdate breaks in the structures, metamorphism, or gross lithologies. From east to regional metamorphism, and do not refold one another. The kinks show west, lithologies gradually change from blocks and bands of irregular identical style and geometrical relations to those described by Paterson and banded "cherty" quartzites enclosed in an argillaceous phyllitic matrix to Weiss (1966). disrupted and commonly transposed quartzite bands in a similar matrix In this same region, there is an older set of conjugate folds that folds and, finally, to argillaceous phyllites. Limestone blocks and stringers occur only Sj and is associated with north-northwest and east-northeast-striking on both sides of their proposed fault. A pervasive, continuous cleavage axial planar crenulation cleavages. We can demonstrate that neither set of with variable orientations [oldest pre-Nevadan Calaveras cleavage(?) of crenulations folds the other or their associated cleavages and that they Schweickert and others, 1984] can be traced across these lithologies. In postdate S! and predate the kinks discussed above. These relations be- addition, our initial quantitative studies of finite strains suggest that no tween the east-northeast and north-northwest sets of structures suggest a increases in ductile strains occur within this area. It would appear that all synchroneity and pose great difficulties when interpreted as being separate of the above lithologies are part of the Calaveras Complex. Our data thus periods of folding. We consider them to be conjugate sets of structures. suggest that no fault occurs in this region. We have located zones of high These observations do not support the structural succession put forth ductile strains elsewhere, but until the spatial relations of these faults by Schweickert and others (1984) for the southern portion of the Cala- become more clear, conclusions about the structures occurring in different veras Complex. Instead of two pre-Nevadan cleavages, a weaker "Main belts seem premature. Phase" cleavage, and younger crenulations, we see four sets of structures: 2. Schweickert and others (1984, p. 973, column 2) suggest that "the (1) a very weak fissility of probably soft-sediment origin, (2) one very Calaveras Complex contains 2 sets of penetrative, pre-Nevadan struc- pervasive tectonic cleavage (Si), (3) a locally developed set of conjugate tures," "the Shoo Fly in the southern part of the belt had an extremely folds trending north-northwest and east-northeast with associated crenula- complex structural history," and (p. 977, column 3) that basement com- tion cleavages, and (4) a rarely observed set of conjugate kinks. plexes in the central belt were polymetamorphic. In addition (p. 973, 4. Schweickert and others (1984, p. 977, columns 2 and 3) suggest column 1), they suggest that "the Calaveras Complex and Shoo Fly in the that "basement complexes in the southern portion of the central belt south contain medium- to high-grade metamorphic fabrics." In our work responded as a nearly rigid block" during Nevadan deformation and that along a transect between Briceburg and El Portal, we did not observe the the Calaveras and Shoo Fly rotated on the western limb of the Nevadan structural or metamorphic complexities noted by these authors. Both the synclinorium. It is not clear to us exactly what was meant by "rigid body Shoo Fly and the Calaveras show one prominent cleavage (S[) trending rotation" of the southern part of the central belt. If they mean the rotation generally east-southeast-west-northwest. In both units, isoclinal folds of of the entire west limb of the "Nevadan Synclinorium," then the absence

bedding occur with axial planes parallel to this cleavage. In the western of strong "Nevadan" cleavage (S2) in the south is not explained by "the part of the Calaveras, near its contact with the greenschist-phyllite belt, this presence of strain hardened, high grade metamorphic basement in that cleavage shows gradual yet significant changes in orientation. To the east, area." Our thin section and quantitative strain studies reveal low-grade "chert" bands are folded with extremely variable orientations and locally regional metamorphism and moderate amounts of ductile strains related to have a spaced planar structure or fissility parallel or subparallel to their S! (comparable to the "Pre-Nevadan" of Schweickert and others, 1984). axial planes. These folds are truncated by the main Sj cleavage, and the S) If, however, the rotation implies relative rotation between the "strain has weakly crenulated the fissility. Like earlier workers (Schweickert and hardened" south and ductily deforming north on the same limb of the others, 1977), we interpret those folds and the associated planar structure "synclinorium," then, in addition to the above mentioned point, we have (fissility) as being results of soft-sediment deformation. Other interpreta- the following objections. (1) The "main phase Nevadan cleavage" in the tions are certainly possible, but this fissility is not a "pervasive" tectonic northern and the southern parts of the central belt show good parallelism cleavage. (Schweickert and others, 1984, Fig. 3 and Table 1). If there has been rigid In addition to the above structures, both the Calaveras and Shoo Fly body rotation in the south, nonparallelism is expected. (2) They have have at least one set of younger, locally developed crenulation cleavages interpreted (p. 975, column 1) the "Sonora fault" as an east-dipping thrust fault formed during the "Nevadan orogeny." The occurrence of a thrust (S2). Average orientations are N20°W, 74°SW in the Shoo Fly and N10°W, 82°SW in the Calaveras. The Calaveras also shows another set of fault as one boundary seems incompatible with the rigid body rotation of crenulation cleavage (S2A, average orientation N85°E, 74°S) that becomes the southern part of the "central belt." If the "Sonora fault" is an "east- increasingly well developed near its contact with the volcanics to the west. dipping thrust fault," the central belt rocks in the east represent rocks from For reasons discussed below, we feel that these cleavages are conjugate a deeper level and should show a higher grade of metamorphism com- structures. pared to the rocks in the west. In our studies to date, however, evidence is lacking to support their contention. Our observations also do not support the presence of high-grade, regionally metamorphosed rocks in this area. An examination of rocks

away from exposed intrusives suggests that a single episode of chlorite- to REFERENCES CITED

locally biotite-grade regional metamorphism occurred. Within contact au- Patereon, M. S., and Weiss, L. E., 1966, Experimental deformation and folding in phyllite: Geological Society of America reoles, biotite ± andalusite, as well as fibrolite, become increasingly prev- Bulletin, v. 77, p. 343-374. Schweickert, R., Saleeby, J. B„ Tobisch, O. T„ and Wright, W. H., 1977, Paleotectonic and paleogeographic significance alent. Textural relations clearly indicate that these higher-grade minerals of the Calaveras Complex, western Sierra Nevada, California, in Stewart, J. H., Stevans, C, H., and Fritsche, A. E., eds.. Paleozoic paleogeography of the western United Stales, Pacific Coast Paleogeography Symposium 1: Society postdate the regional metamorphism and occur only in contact aureoles. of Economic Paleontologists and Mineralogists, Pacific Section, p. 381-394. Schweickert, R. A., Bogen, N. L., Girty, G. H., Hanson, R. E., and Merguerian, C„ 1984, Timing and structural expression 3. The interpretations put forth by Schweickert and others (1984, p. of the Nevadan orogeny. Sierra Nevada, California: Geological Society of America Bulletin, v. 95, p. 967-979. 975 to 977) of "Late Phase" and "Cretaceous" structures critically depend MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 26, 1984 on whether conjugate sets of structures exist in the Western Metamorphic MANUSCRIPT ACCEPTED MARCH 13, 1985

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OTHMAR T. TO BISCH Earth Science Board, University of California, Santa Cruz, California 95064 RICHARD S. FISKE National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560

Schweickert and co-workers have put forth a provocative synthesis entire Sierra Nevada. If we assume that it can, then the nature of the on Sierra Nevada orogeny that undoubtedly will stimulate much response. Nevadan orogeny may not have been short lived. Perhaps it was diachro- The evolution of an. orogen which shows as much complexity as the Sierra nous, moving from place to place with time, in which case, the generation Nevada, however, is bound to remain a controversial issue. This discussion of a synclinorium affecting the entire orogen would seem unlikely. Addi- comments briefly on three topics covered in the paper by Schweickert and tionally, the deformation of the volcanic pile may be inherent in the others (1984): (1) the presence of a major synclinorium between latitudes generation of the magmatic arc itself, as we have suggested earlier (Tcbisch 37°N and 38°N, (2) the age of the Nevadan orogeny, and (3) the interpre- and Saleeby, 1983), and have little to do with the arc/continent collision tation of conjugate folds. proposed by Schweickert and others (1984). In any event, the age of the 1. In Figures 1 and 8 in Schweickert and others (1984), the axial Nevadan orogeny, as proposed by Schweickert and others (1984). may trace of a major synclinorium is shown to pass through the Ritter Range well apply in certain parts of the orogen but clearly not throughout its pendant at the latitude of Hemlock Crossing. Our work in this area does entirety. The usefulness of the term "Nevadan" needs to be re-evaluated. not support their interpretation. The thick section of metavolcanic rocks 3. Schweickert and others (1984, p. 976) stated that "conjugate folds lying east of the presumed axial trace becomes younger toward the west, as in all parts of the Sierra need not be, and probably are not, of the same indicated by top di rections and radiometric ages. West of the axial trace, age." We agree, although our original proposal (Tobisch and Fiske, 1976), however, the same direction of "younging" persists. This is shown by written before the widespread application of geochronology to structural radiometric ages fiom rocks near Hemlock Crossing (T. W. Stern, 1974, dating in the Sierra Nevada, supposed that the conjugate folds formed in written commun.) and from the age of fossils in rocks to the west at late Nevadan time. The main utility of our hypothesis concerning the Strawberry Mine (R. Imlay, 1983, written commun.). In addition, top genesis of conjugate folds and associated crenulations is that there is no directions, the westerly dip of bedding, and the sense of movement of need to call upon separate periods of deformation to explain the domainal minor folds (Z) associated with the slaty cleavage remain unchanged occurrence of crenulations that may occur in more than one statistical throughout the pendant except for uncommon reversals associated with orientation. Indeed, crenulations and conjugate crenulations are consid- local mesoscopic falding. There is, therefore, no basis to support the pres- ered as having formed in the last stages of slaty-cleavage formation ence of a synclinorium underlying the Ritter Range. (compare Tobisch and Fiske, 1976,1982) and so are genetically rela ted to 2. According to Schweickert and others (1984, p. 978), the Nevadan the strain field that generated the slaty cleavage. The Sierra Nevada coun- orogeny "was a very short-lived ... event" that "occurred ~ 155 ± 3 m.y. try rock is an excellent example of an area that contains late-stage crenula- B.P.," the major structure of which is a synclinorium that "encloses the tions and folds with axial planes in two general orientations and in which remnants of an isoclinally folded Andean-type arc." First of all, we would neither set of structure is seen to refold the other. The statement of like to underline that although tight to isoclinal minor- and meso-scale Schweickert and others (1984, p. 977) that both sets of folds in a conjugate folds occur locally within the volcanic section in the eastern belt, no structure need to form at the same time is not necessarily correct. Provided large-scale folds involving substantial parts of the volcanic section have that the orientation of a fluctuating strain field remains constant, periods of been found in the areas where we have worked. We consider the structure time may elapse between formation of left and right halves of a conjugate of the volcanic pile to be essentially a homocline dipping to the west. structure, as is implied by experimental work (Paterson and Weiss, 1966). Reference by Schweickert and others (1984) to the arc as "isoclinally Our initial suggestion that all of the conjugate folds formed at the same folded" therefore may be misleading to the uninitiated reader. Further- time throughout the Sierra Nevada is, in the light of new evidence, most more, fossil ages and radiometric age dating of volcaniclastic rocks in parts likely incorrect. In fact, two periods of conjugate folding have now been of the Ritter Range (R. Imlay, 1983, written commun.; T. Stern, 1974, documented in the phyllites near Briceburg (Bhattacharyya and Paterson, written commun.) and the Mt. Goddard area 80 km to the south (J. 1985). The proposal of successive periods of crenulation formation, based Saleeby, 1984, written commun.) indicate that parts of the section in both largely on different orientations of their axial planes (for example, north- areas are younger by 20 m.y. (that is, Upper Jurassic/Lower Cretaceous, easterly and westerly sets, Schweickert and others, 1984), is not -135 Ma) than the age proposed by Schweickert and others (1984) for the convincing, however, especially in view of the fact that neither set is seen Nevadan orogeny. In addition, our data show that strain intensities in these to refold the other. rocks are often as high as strains recorded in rocks of the older sections in the Ritter Range. If these Upper Jurassic/Lower Cretaceous rocks were REFERENCES CITED deposited after the Nevadan orogeny, surely one could find unconformities Paterson, M. S„ and Weiss, L. E., 1966, Experimental deformation and folding in phyllite: Geological Society Df America to document it, yet in the areas considered, no such relationships are Bulletin, v. 77, p. 343-374. Schweickert, R. A., Bogen, N. L., Girty, G. H., Hanson, R. E., and Merguerian, G, 1984, Timing and structural expression evident. To us, these observations are further evidence, albeit indirect, that of the Nevadan orogeny. Sierra Nevada, California: Geological Society of America Bulletin, v. 95, p. %7-979. the synclinorium does not exist in the Ritter-Goddard area. The presence, Tobisch, O. T., and Fiske, R. S., 1976, Significance of conjugate folds and crenulations in the central Sierra Nevada, California: Geological Society of America Bulletin, v. 87, p. 141-1420. in the eastern belt, of volcanic sections that have been highly deformed 1982, Repeated parallel deformation in part of the eastern Sierra Nevada, California, and its impl cations for dating structural events: Journal of Structural Geology, v. 4, p. 177-195. substantially later than the proposed time of the Nevadan orogeny raises Tobisch, O. T., and Saleeby, J., 1983, Nature and timing of late Mesozoic deformation in the central and southern Sierra many questions concerning the nature, in time and space, of the Nevadan Nevada, California: Geological Society of America Abstracts with Programs, v. 15, no. 5, p. 294. orogeny and whether, in fact, such a designation can be applied to the MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 10, 1984 MANUSCRIPT ACCEPTED MARCH 13. 1985

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RICHARD A. SCHWEICKERT Department of Geological Sciences and Mackay School of Mines, University of Nevada-Reno, Reno, Nevada 89557 NICHOLAS L. BOGEN Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109 GARY H. GIRTY Department of Geological Sciences, San Diego State University, San Diego, California 92182 RICHARD E. HANSON Department of Geology, School of Mines, University of Zambia, Lusaka, Zambia CHARLES MERGUERIAN Geology Department, Hofstra University, Hempstead, New York 11550

The discussions by Bhattacharyya and Paterson and Tobisch and lithology and structure. On the west side of the fault, phyllite and meta- Fiske raise a host of questions related to our 1984 paper, but none of their graywacke with perfect, continuous phyllitic cleavage, of greenschist grade comments takes issue with the principal points of our paper or the data (lacking biotite) occur. Locally, two penetrative cleavages can be recog- which we presented. Bhattacharyya and Paterson disagree with us on nized. On the east side of the fault, metachert and meta-argillite with a aspects of geology near the southern end of the western metamorphic belt, crude shape fabric foliation and mica schistosity, of upper greenschist to and Tobisch and Fiske disagree on interpretations relating to the Ritter lower amphibolite grade (with biotite) are found. This latter unit contains Range pendant, -50 km east of the southern end of the western metamor- little or no sandstone. The phyllite and metagraywacke contain local zones phic belt. of chaotic rocks with blocks of chert and limestone. Such phyllitic rocks are continuous with rocks near Mount Bullion that yielded a Kimmerid- DISCUSSION BY BHATTACHARYYA AND PATERSON gian fossil, and hence their structure is Kimmeridgian or younger. We have traced the metachert and meta-argillite units in outcrop to Calaveras rocks Bhattacharyya and Paterson's Discussion is based upon preliminary of identical structure and lithology 20 km north, where the schistosity and results of their strain analysis of rocks in the Merced River area, where folds predate the 157-159-m.y. Sonora dike swarm and the 164-m.y. they do not recognize evidence for polyphase deformation in Calaveras Standard pluton. The schistosity and folds in the Calaveras rocks are and Shoo Fly rocks. This is a fairly restricted part of the western metamor- therefore pre-Nevadan. Sparse crenulations are the only possible Nevadan phic belt, lacking the age constraints, which, as we discussed, exist for the structures in these rocks. region near Sonora. Significantly, they did not compare their preliminary The Sonora fault is exposed in only a few localities; one of the best data with the rocks in the Sonora area. They state that their "initial outcrops is in the town of Sonora. The fault generally is marked by a zone quantitative finite strain studies" reveal no evidence for the Sonora fault. up to a few tens of metres wide of phyllonite and mylonite with rootless We stress that Calaveras and Shoo Fly rocks are very complex structurally intrafolial folds and with a downdip stretching lineation. Adjacent to the and that it has taken us several years of study to decipher them. Our fault, the older fabrics in the Calaveras and Nevadan fabrics in the phyllitic present interpretations have advanced well beyond what we thought we rocks generally parallel the fault trace. The fault is not exposed along understood from our first field season. As to finite strain studies, it is Highway 140 or on the Merced River. important to evaluate whether the finite strains being measured result from 2. Bhattacharyya and Paterson have failed to recognize the structu- a single deformational event or from several separate deformational events. ral complexity of the Calaveras and Shoo Fly Complexes. They state Bhattacharyya and Paterson have not done this, nor will it be an easy task that along the Merced River, both the Shoo Fly and Calaveras have one to accomplish in these rocks. Although analysis of finite strain in these prominent cleavage, trending east-southeast-west-northwest, and asso- rocks is important, we believe that conventional observations of style and ciated isoclinal folds. In fact, these rocks record a complex structural overprinting are of greater importance than strain measurements for identi- history that is unlikely to be sorted out in one or two months' work. fying structural breaks and for correlating structures. In short, we do not Schweickert and Merguerian have each spent parts of three field seasons in believe that strain analysis by itself is relevant to the question of the the Merced River area and five to six field seasons in such rocks in location of the Sonora fault. the Tuolumne and Stanislaus River drainages to the north. Bhattacharyya and Paterson erroneously imply that the relations Merguerian and Schweickert (1980) and Merguerian (1981, 1982, which we described were based only on work north of lat. 38°. During the 1985) reported and documented evidence for as many as seven genera- course of our study, we collected a large amount of structural data from tions of folds and foliation in the Shoo Fly. The earliest (Dl) structures parts of the western metamorphic belt south of lat. 38°, including the predate lower to mid-Paleozoic gneissic granitoids; D2, D3, and D4 struc- Merced River region. We discuss four topics below: (1) the position/exist- tures predate the Sonora dike swarm and the Standard pluton; and D3 ence of the Sonora fault near the Merced River; (2) polymetamorphism structures are related to the Calaveras-Shoo Fly thrust. D5, D6, and D7 and structural complexity of the Calaveras and Shoo Fly Complexes; structures are crenulations and kinks that postdate the Sonora dike swarm (3) late-phase Nevadan folds, Cretaceous folds, and conjugate structures; and are probably Nevadan or younger structures. In our experience, the and (4) rotation of the central belt during the Nevadan orogeny. most obvious cleavage or foliation in an outcrop may be either S2, S3, or 1. Bhattacharyya and Paterson are unable to locate the Sonora fault S4, and without overprinting or crosscutting relations, it is impossible to along the Merced River and question its existence. Unfortunately, this fault tell which is which. was shown incorrectly in our Figures 1 and 8, which were drafted in Schweickert and others (1977), Schweickert (1979, 1981), and 1980; the Figure 1 we present now shows a more accurate location for Schweickert and Bogen (1983) noted that the Calaveras along the Merced the fault based upon mapping since 1981 at a scale of 1:24,000 by River, as in other areas, is characterized by a chaotic or mélange-like Schweickert throughout the Merced River drainage, on the Buckhorn appearance (not mentioned by Bhattacharyya and Paterson) in which Peak, Kinsley, Feliciana Mountain, and Buckingham Mountain 1W quad- large and small blocks or slabs of chert and limestone are immersed in an rangles. Throughout this region, the fault marks an abrupt change in argillaceous matrix. SI foliation in the mélange is a shape fabric foliation

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ife"

PRE-NEVADAN DEFORMATION

Pre-mid Jurassic Mesozoic deformation

Pre-Carboniferous deformation

nonpenetrative

penetrative

39"

Figure 1. Regional geologic sketch map of the Sierra Nevada, showing the western (W), central (C), and eastern (E) belts and the nature and timing of pre-Nevadan deformation (modified from Schweickert and others, 1984, Fig. 1). The trace of the Sonora fault southeast of Sonora (S) is shown more accurately than in our previous map.

and in chert blocls is a mica schistosity parallel to axial surfaces of F1 In the Calaveras, SI and S2 crosscut early intermediate and silicic folds. SI is folded by east-plunging, similar, tight to isoclinal F2 folds dikes, and these structures are identical in style to those in the Calaveras having a west-northwest-trending spaced cleavage (S2) parallel to their Complex in the Tuolumne and Stanislaus drainages, which predi.te the axial surfaces. These structures are identical in style and metamorphic Sonora dike swarm. Significantly, we have observed nine cases in the grade to D3 and 1)4 structures, respectively, in the Shoo Fly Complex. Merced River area where mafic to intermediate dikes that resemble: those

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of the Sonora dike swarm cut D1 and D2 structures. For these reasons, we phyllite and greenschist to the west, and they suffered very little Nevadan regard SI and S2 in the Calaveras and S3 and S4 in the Shoo Fly as deformation. They further state that, if the southern half of the central belt pre-Nevadan structures. The only candidates for Nevadan structures in the underwent rigid body rotation, nonparallelism of Nevadan main-phase Calaveras and Shoo Fly are sparse crenulations and local conjugate folds, cleavages in the north and south would be expected. We note that the as we discussed in our paper. regions are up to 100 km apart and that the main-phase crenulation Bhattacharyya and Paterson give no supporting evidence for their cleavage may have developed late in the rotation history of the basement interpretation that a fissility in Calaveras rocks originated by soft-sediment complexes, recording the XY plane of strain during the final increment of deformation. Although we agree that soft-sediment deformation was im- shortening. We fail to understand why they think that the existence of a portant in the Calaveras (see Schweickert and others, 1977; Schweickert, thrust fault (the Sonora fault) at the base of the Calaveras is in any way 1981), we argue that the fact that igneous dikes are locally flattened and incompatible with rigid-body rotation of the Calaveras. boudinaged within the earliest foliation indicates that the foliation is a tectonic structure. TOBISCH AND FISKE DISCUSSION Our thin-section studies reveal that Calaveras schists are character- ized by quartz + muscovite + biotite + garnet and that Shoo Fly pelites Tobisch and Fiske discuss the vexed problem of the Nevadan contain, in addition, local cordierite, even considerable distances from synclinorium and, especially, problematical age relations in the Ritter plutonic contacts. In the Shoo Fly, calc-silicate rocks contain diopside + Range pendant, where they have done detailed work. None of their points tremolite + biotite ± chlorite ± vesuvianite, and Paleozoic granitoids con- or arguments applies to any of the age constraints which we used for the tain metamorphic hastingsite + magnesian hornblende + biotite ± chlorite Nevadan orogeny—their points bear only on structural relations and tim- ± epidote ± psilomelane ± garnet. Metamorphic minerals in the few thin ing in the Ritter Range pendant. We discuss their points 1 and 2 below. sections which we have examined from the Merced River area are consis- 1. We projected the trace of the axial surface of the Nevadan syn- tent with this. Metamorphic grade may vary; however, these rocks are of clinorium through the Ritter Range pendant because the fossiliferous higher grade than the phyllitic rocks west of the Sonora fault which Lower Jurassic rocks in the eastern part of the pendant dip 60°-80° commonly have greenschist assemblages, for example, quartz + albite + west-southwest and top west. We interpret these well-dated rocks as being epidote + chlorite + stilpnomelane or quartz + actinolite + albite. part of the eastern limb; the western limb at this latitude is largely obliter- 3. This point is argued by both Bhattacharyya and Paterson and ated by the Sierra Nevada batholith. Tobisch and Fiske argue that the Tobisch and Fiske. We do not dispute the existence of conjugate structures synclinorium is not present there because rocks west of the axial surface in the western metamorphic belt, nor is this point critical to our interpreta- trace also have a westerly direction of younging. The rocks near and west tion of late-phase Nevadan structures. We presented evidence that rules of the axial surface trace of the synclinorium, however, have been inter- out the view that crenulations and conjugate crenulations in the Calaveras preted as being of Early and middle Cretaceous age by Fiske and Tobisch and Shoo Fly Complexes formed in the "last stages of slaty cleavage (1978) on the basis of a variety of unpublished radiometric age determina- formation and are genetically related to the strain field that generated the tions. They stated that the Cretaceous volcanic rocks overlie Jurassic rocks slaty cleavage" (Tobisch and Fiske). Because the crenulation cleavages are "with clear angular unconformity." We provisionally accepted their age distinctly younger than the penetrative structures in these rocks and are the interpretation and suggested that Cretaceous(?) metavolcanic rocks may only post-160-m.y. structures in the southern parts of the Calaveras and postdate and conceal the axial surface trace in this region (Schweickert and Shoo Fly, the crenulations are the only candidates for Nevadan structures. others, 1984, Fig. 1 and p. 974). Our interpretation that northwest-trending crenulations are main phase 2. The steeply dipping Lower Jurassic and older rocks in the Ritter and northeast-trending crenulations are late phase was presented as the Range pendant must have undergone tilting (or folding) to acquire their simplest hypothesis that is consistent with regional relations of Nevadan present orientation. The question is, when? Tobisch and Fiske state that structures. We noted that the examples of conjugate crenulations reported several unpublished age determinations indicate that rocks up to 20 m.y. by Tobisch and Fiske (1976) from the Calaveras and Shoo Fly may younger than our dates for the Nevadan orogeny occur in parts of the represent main-phase and/or late-phase Nevadan structures (their north- section, implying that deformation was perhaps an Early Cretaceous or ern set) cut by later, possibly Cretaceous, crenulations (their easterly set). younger event. The absence of a recognized angular unconformity be- The structures which Bhattacharyya and Paterson described near Brice- tween their supposed Upper Jurassic-Cretaceous rocks and older rocks burg are in what we mapped as the phyllite-greenschist belt, not the further suggests to them that the deformation of all rocks is Cretaceous and Calaveras, and are in accord with our observations in that area. that the Nevadan orogeny, as we defined it, may not exist in the Ritter 4. We interpreted the basement complexes of the central belt as Range pendant. We note with interest that this apparent age discrepancy having undergone mainly rotation during the Nevadan orogeny. An inde- did not deter them from proposing, in a 1982 article, the remarkable in- pendent confirmation of this interpretation is provided by paleomagnetic terpretation that the Ritter Range pendant experienced two exactly par- data of Bogen and others (1985) which indicates that rocks east of the allel and coaxial deformations ~50 m.y. apart, during the Nevadan orog- Sonora fault experienced a 25°-30° tilt to the east-northeast during the eny and during the middle Cretaceous! As we discussed in our paper Nevadan orogeny. Furthermore, uplift of these rocks implied by the pa- (p. 970-971), we have strong reservations against accepting unpublished leomagnetic determinations is supported by sandstone petrology and stra- Rb/Sr and U/Pb age determinations in metavolcanic rocks in the Ritter tigraphy of the Mariposa Formation (Bogen, 1984). Because Bhatta- Range pendant as stratigraphic ages. This is because fossiliferous Lower charyya and Paterson did not recognize that Calaveras and Shoo Fly rocks Jurassic rocks in the pendant have yielded Rb/Sr and U/Pb "ages" up to on the Merced River are highly deformed and metamorphosed, they assert 50 m.y. too young (see Huber and Rinehart, 1965; Fiske and Tobisch, that these complexes (everywhere, presumably) do not represent a strain- 1978; Kistler and Swanson, 1981). Furthermore, Tobisch and others hardened, high-grade metamorphic basement. As spelled out in point 2 (1977), Fiske and Tobisch (1978), and Kistler and Swanson (1981) all above, these complexes are more deformed and metamorphosed than the showed the supposed Upper Jurassic to Cretaceous rocks to be in fault

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contact with the fossiliferous Lower Jurassic section, indicating that struc- Geological Survey Geologic Quadrangle Map GQ-437, scale 1:62,500. Kistler, R. W„ and Swanson, S. E., 1981, Petrology and geochronology of metamorphosed volcanic rocks and a middle tural and stratigraphic relations between these rocks are not clear. It is our Cretaceous volcanic neck in the east-central Sierra Nevada, California: Journal of Geophysical Research, v. 86, p. 10489-10501. opinion that important stratigraphic problems, such as the age range and Merguerian, C., 1981, The extension of the Calaveras-Shoo Fly thrust to the southern end of the Sierra Nevada stratigraphic relations of the metavolcanic rocks in the Ritter Range pen- metamorphic belt, California: Preliminary geologic maps of the Tuolumne, Duckwall Mountain, G oveland. Jawbone Ridge, Lake Eleanor SW, SW, and NE 1/4, Buckhorn Peak, Kinsley, El Portal, and Buckingham dant remain unresolved and that a post-Nevadan age for deformation of Mountain 7.5' quadrangles, accompanied by 10-page report: San Francisco, California Division of Mines and Geology. the older volcanic section is not established. 1982, The extension of the Calaveras-Shoo Fly thrust (CSFT) to the southern end of the Sierra Nevada metamor- i phic belt, California: Geological Society of America Abstracts with Programs, v. 14, p. 215. Data from 2.>0 km along the length of the Sierra Nevada indicate 1985, Stratigraphy, structural geology, and tectonic implications of the Shoo Fly Complex and the Calaveras- virtual synchroneily of structures that we assigned to the Nevadan orog- Shoo Fly thrust, central Sierra Nevada, California [Ph.D. dissert.]: New York, Columbia University, 25!» p. Merguerian, C-, and Schweickert, R. A., 1980, Superposed mylonitic deformation of the Shoo Fly Complex in Tuolumne eny. The problematical relations noted by Tobisch and Fiske, however County, California: Geological Society of America Abstracts with Programs, v. 12, p. 120. Schweickert, R. A., 1979, Structural sequence of the Calaveras Complex between the Stanislaus and Tuolumne Rivers: they are resolved, do not invalidate our point that the Nevadan deforma- Geological Society of America Abstracts with Programs, v. 11, p. 127. tion occurred ovei a very large region during a very short time interval, 1981, Tectonic evolution of the Sierra Nevada Range, in Ernst, W. G., ed., The geotectonic develcpment of California: A symposium to honor W. W. Rubey: Englewood Cliffs, New Jersey, Prentice-Hall, p. 87-! 31. -155 + 3 m.y. age. Schweickert, R. A., and Bogen, N. L., 1983, Tectonic transect of Sierran Paleozoic through Jurassic accreted bel's: Society of Economic Paleontologists and Mineralogists, Pacific Section, 22 p. Schweickert, R. A., Saleeby, J. B., Tobisch, O. T., and Wright, W. H., 1977, Paleotectonic and paleog »graphic significance of the Calaveras Complex, western Sierra Nevada, California, in Stewart, J. H., Stevens, C. H., and REFERENCES CITED Fritsche, A. E., eds.. Paleozoic paleogeography of the western United States: Society of Economic Palecntologists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Symposium 1, p. 381-394. Bogen, N. L., 1984, Stratigraphy and sedimentary petrology of the Upper Jurassic Mariposa Formation, western Sierra Schweickert, R. A., Bogen, N. L., Girty, G. H., Hanson, R. E., and Merguerian, C., 1984, Timing and structural expression Nevada, California, in Crouch, J. K., and Bachman, S. B., eds., Tectonics and sedimentation along the California of the Nevadan orogeny. Sierra Nevada, California: Geological Society of America Bulletin, v. 95, p. 9< >7-979. margin: Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 38, p. 119-134. Tobisch, O. T., and Fiske, R. S., 1976, Significance of conjugate folds and crenulations in the central Sierra Nevada, Bogen, N. L., Kent, D. V., and Schweickert, R. A., 1985, Paleomagnetism of Jurassic rocks in the western Sierra Nevada California: Geological Society of America Bulletin, v. 87, p. 1411-1420. metamorphic belt and its bearing on the structural evolution of the Sierra Nevada block: Journal of Geophysical 1982, Repeated parallel deformation in part of the eastern Sierra Nevada, California, and its implications in the Research, v. 90 (in press). dating of structural events: Journal of Structural Geology, v. 4, p. 177-195. Fiske, R. S., and Tobisch, O-1'., 1978, Paleogeographic significance of volcanic rocks in the Ritter Range pendant, central Tobisch, O. T., Fiske, R. S., Sacks, S., and Taniguchi, D„ 1977, Strain in metamorphosed volcaniclastic rocks and its Sierra Nevada, California, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western bearing on the evolution of orogenic belts: Geological Society of America Bulletin, v. 88, p. 23-40. United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeog- raphy Symposium 2. p. 209-222. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH I, 1985 Huber, N. K., and Rinehart, C. D., 1965, Geologic map of the Devils Postpile quadrangle, Sierra Nevada, California: U.S. MANUSCRIPT ACCEPTED MARCH 13,1985

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