Timing and structural expression of the Nevadan orogeny, Sierra Nevada, California RICHARD A. SCHWEICKERT Department of Geological Sciences and Mackay School of Mines, University of Nevada-Reno, Reno, Nevada 89557 NICHOLAS L. BOGEN Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109 GARY H. GIRTY Geology Department, University of Kansas, Lawrence, Kansas 66045 RICHARD E. HANSON Department of Geology, School of Mines, University of Zambia, Lusaka, Zambia CHARLES MERGUERIAN Geology Department, Hofstra University, Hempstead, New York 11550 ABSTRACT style, from relatively intensely developed in whether the structures are penetrative or non- the north to very weakly developed in the penetrative. Pre-Nevadan deformation also oc- The Nevadan orogeny was a very short- south. curs locally in the western belt but cannot be lived event in the Late Jurassic that involved The regional extent and geometry of the shown at the scale of Figure 1. The nature and the deformation of a great variety of rock Nevadan structures indicate that the Nevadan style of superimposition of Late Jurassic "Neva- types and Paleozoic and Mesozoic terranes orogeny involved underthrusting of island- dan" structures on the older structures are enig- throughout the extent of the Sierra Nevada. arc rocks on the west and significant crustal matic and are complicated by the fact that both The Nevadan structures show great variation shortening in the central and eastern belts. older and younger structures of similar orienta- in style but relatively constant orientations. These features suggest that the Nevadan orog- tion have been identified in some areas. These relations can be explained by consider- eny resulted from the collision of the island The purpose of this paper is to present results ing the prior histories of the various terranes. arc (western belt) with an andean-type arc of our own structural studies that bear on the Slaty cleavages and tight folds are the (eastern belt) situated at the western edge of nature of Nevadan deformation. Over the past characteristic main-phase structures in the North America. eight years, we have been working in a variety western belt of Jurassic island-arc volcanic of areas from the central to the northern Sierra, rocks and flysch-type sedimentary rocks. A and from the western foothills eastward to the INTRODUCTION strip of phyllites and greenschists along the high country. We first outline the classical defi- eastern edge of the belt apparently represents nition of the Nevadan orogeny and early ideas similar Jurassic rocks that were deformed Prebatholithic wall rocks of the Sierra Ne- about its timing. We then summarize critical age and metamorphosed at greater depths, prob- vada underwent a complex structural history data that enable correlation of Nevadan struc- ably during underthrusting of the western prior to the emplacement of the voluminous tures in different parts of the Sierra Nevada and belt beneath the central belt. The central belt Late Jurassic and Cretaceous batholiths. Prog- describe examples of "Nevadan" structures in of Paleozoic metasedimentary and metavol- ress in unraveling the complex structural his- different terranes where we can constrain their canic rocks shows the most extreme variation tory has come slowly because the plutonic rocks ages, to illustrate the importance and influence in style of main-phase structures, from weak, obliterated much of the pre-existing rock and of the early history of the rocks. Detailed ac- spaced to crenulation cleavages in the south, separated the remnants into many discrete belts counts of pre-Nevadan structure and metamor- where polyphase deformed rocks formed a and roof pendants. phism of the older units are not presented here, structural basement, to slaty and phyllitic One of the most widely recognized deforma- but they are forthcoming. Finally, we interpret cleavages and asymmetric to isoclinal folds in tional events occurred in the Late Jurassic, dur- the significance of the Nevadan structures. One the north, where most of the Paleozoic base- ing the Nevadan orogeny (Knopf, 1929; Hinds, of our main conclusions is that pre-Jurassic ter- ment rocks lack penetrative pre-Nevadan 1934; Taliaferro, 1942; Clark, 1964; Bateman ranes and Mesozoic slate-graywacke-volcani- fabrics. Eastward-directed thrust faulting ap- and Clark, 1974; Schweickert and Cowan, clastic-rock sequences exhibited major contrasts parently was important only in the northern 1975; Saleeby and others, 1978; Schweickert, in mechanical behavior during Late Jurassic part of the range, where main-phase deforma- 1978, 1981; Saleeby, 1981; Nokleberg and orogenesis (Bogen and others, 1980). tion was most intense. The eastern belt of Kistler, 1980). Structures formed during the Jurassic and Triassic magmatic arc-volcanic Nevadan orogeny were superimposed on a great THE NEVADAN OROGENY and sedimentary rocks defines the core of a variety of rock types and pre-existing structures major synclinorium, and the rocks contain and this resulted in a wide variety of styles of The presence of folds and cleavage in penetrative slaty cleavages and asymmetric, Nevadan deformation. Figure 1 shows three Oxfordian-Kimmeridgian sedimentary and vol- tight to isoclinal folds. principal lithotectonic belts of the Sierra, re- canic rocks of the western belt, west of the Me- A late phase of Nevadan structures, con- ferred to as the western, central, and eastern lones fault zone (Fig. 1), has long been taken as sisting of northeast-trending cleavages and belts, and also shows areas containing evidence definitive evidence for Late Jurassic orogenesis minor folds, also shows a marked variation in of pre-Nevadan regional deformation, indicating in the Sierra Nevada (Knopf, 1929; Taliaferro, Geological Society of America Bulletin, v. 95, p. 967-979, 8 figs., 2 tables, August 1984. 967 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/8/967/3434921/i0016-7606-95-8-967.pdf by guest on 30 September 2021 968 SCHWEICKERT AND OTHERS 1942). These structures are locally cut by Lower volcanic rocks of the Sierra Nevada delineates a Age Constraints—A New Look Cretaceous plutons, and thus the deformation major synclinorium, with Triassic and Jurassic was regarded as later than Oxfordian age and rocks in the core of the structure flanked by Figure 2 presents data that provide the tight- earlier than Cretaceous. The occurrence of rela- progressively older sequences of rocks in the est brackets on the timing of the Nevadan oro- tively undeformed Tithonian sedimentary rocks limbs. Structural and stratigraphic relationships geny in the Sierra Nevada. Dated plutons in the Knoxville Formation to the west in the indicated to them that this synclinorium formed discussed in the text are shown in Figu e 1. In Coast Ranges led to the conclusion that the during the Nevadan orogeny. (We discuss the Figure 2, two problems are inherent. (1) The Nevadan deformation was pre-Tithonian (Talia- evidence for the synclinorium and its regional radiometric ages of stage boundaries in the Ju- ferro, 1942). In the Mount Jura area of the significance below.) rassic are poorly known (Armstrong, 1978; Har- northern Sierra Nevada (Fig. 1), Jurassic rocks Bateman and Clark (1974) recently estimated land and others, 1982). We have E.dopted as young as Callovian in age also contain a slaty the younger limit of the Nevadan orogeny to be Armstrong's (1978) suggested ages rather than cleavage, suggesting a similar Nevadan structur- about 140 m.y., on the basis of K-Ar ages of those of Van Hinte (1976) or Harland and oth- al history. plutons that cut the Nevadan cleavage, but our ers (1982), because Armstrong's tims scale Bateman and others (1963) argued that the compilation of the data suggests an older age, seems to pose fewer conflicts with geologic rela- regional pattern of metasedimentary and meta- -154 m.v. Figure 1. Regional geologic sketch map of the Sierra Nevada showing the western (W), central (C), and eastern (E) belts, and areas; of pre-Nevadan de- formation. Insert shows location of this PRE-NEVADAN DEFORMATION map on the map of California. See text for discussion of northern (N) and Pre-mid Jurassic southern (S) parts of the belts. The Mesozoic deformation western belt consists mainly of J urassic metavolcanic and metasedimentary Pre-Carboniferous deformation rocks. The central belt consists of the lower Paleozoic Shoo Fly Complex nonpenetrative and upper Paleozoic pyroclastic rocks in the north; the Calaveras Complex penetrative (horizontally ruled) and the Shoo Fly Complex (checkered pattern) occupy 0 50 km the central belt in the south. The east- 1 I I I I—I ern belt (outcrops outlined) consists of Triassic and Jurassic metavolcanic and metasedimentary rocks. Small, Irregu- lar unpatterned areas in the western metamorphic belt are Mesozoic plu- tons. Isotopically dated Cretaccous(?) metavolcanic rock:; between lat. 37°N and 38°N are not differentiated from rocks of the eastern belt. The inferred position of the axial-surface trace of the Nevadan synclinorium is shown in this region, although it is largely obli- terated by granitic rocks or concealed beneath Cretaceous(?) metavolcanic rocks. Ages of dated plutons that con- strain the age of the Nevadan orogeny are plotted; ages that provide the nar- rowest brackets are plotted in Figure 2. The foothills suture follows the Me- lones fault (MFZ) north of lat. 38°15'N and the Sonora fault (SF) south of lat. 38°15'N. Ar = Arnold; BL = Bowman Lake; MJ = Mount Jura; M = Mari- posa; P = Placerville; RRP = Ritter Range pendant; S = Sonora; SB = Sierra Buttes; BS = blueschist locality; A, B = localities discussed in tex I. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/8/967/3434921/i0016-7606-95-8-967.pdf by guest on 30 September 2021 NEVADAN OROGENY, SIERRA NEVADA, CALIFORNIA 969 the western belt thus is older than about 150 m.y., yet younger than the Oxfordian-Kimme- WESTERN CENTRAL EASTERN ridgian boundary (taken at about 158 m.y.