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

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Timing and Structural Expression of the Nevadan Orogeny, Sierra Nevada, California: Discussions and Reply 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/10/1349/3445091/i0016-7606-96-10-1349.pdf by guest on 29 September 2021 DISCUSSIONS AND REPLY 1347 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.
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