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.