ABSTRACT Metamorphosed strata of the Ritter Range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels (<8 km) in the Late Cretaceous Sierra Nevada magmatic arc. Paleozoic to Middle Jurassic strata of the pendant have been penetratively shortened 30%–50%, forming slates, phyllites, and schists with bed- ding-inclined cleavage and associated linear structures. The cleavage, secondary cleavages, and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the Farallon and North American plates. Microstructural relations show that cleavage development ac- companied greenschist to amphibolite facies metamorphism. Metamorphic hornblende, muscovite, and biotite in the wall rocks show weak to strong preferred orientation and have 40 Ar/ 39 Ar plateau ages ranging from about 85 to 80 Ma. Hornblende, and possibly mus- covite, are expected to retain 40 Ar at or near estimated peak metamorphic temperatures of ~500 °C, thus their 40 Ar/ 39 Ar ages closely cor- respond to the time of cleavage formation. Granitic plutons adjacent to the Ritter Range pendant have yielded U-Pb and other ages that cluster from 91 to 82 Ma. We conclude that bedding-inclined cleavage and metamor- phism in the Ritter Range wall rocks devel- oped in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons. Keywords: emplacement, geochronology, gran- ite, Sierra Nevada batholith, structures, trans- pression. INTRODUCTION The nature of strain regimes in active mag- matic arcs is controversial. Some workers have maintained that arc-perpendicular extension is the dominant regime in magmatic arcs during pluton emplacement (e.g., Hamilton, 1981, 1988, 1995; Smellie, 1995) and that such a strain regime provides a solution to the room problem; i.e., the mechanism by which space is made in the crust for plutons (e.g., Hutton, 1988). Others propose that arc magmas are commonly em- placed during arc-perpendicular shortening but that vertically oriented mass-transfer mecha- nisms such as stoping and wall-rock return flow alleviate the room problem (e.g., Paterson and Miller, 1998, and references therein). A third pos- sibility is that plutons are emplaced in sites of lo- calized extension formed at releasing bends or step-overs along intrabatholithic strike-slip faults in an environment that is transpressive on the re- gional scale (e.g., Saleeby, 1981; Guineberteau et al., 1987; Tikoff and Tessier, 1992). Such a strain regime may develop in magmatic arcs characterized by oblique plate convergence (e.g., Fitch, 1972; Beck, 1986; Glazner, 1991). Ancient magmatic arcs such as the Sierra Nevada batholith offer the potential to examine cleavages and other structures that formed penecontemporaneously with emplacement of plutons and thereby provide insight into strain regimes in active arcs. The central Sierra Nevada is an excellent area in which to study the dynam- ics of pluton emplacement and its relation to wall-rock cleavage because the batholith and its wall rocks are superbly exposed and extensively mapped (Bateman, 1992). Large volumes of granitic magma were emplaced in this region from about 120 to 80 Ma. During this period, the locus of magmatism migrated eastward at an av- erage rate of 2.7 mm/yr (Chen and Moore, 1982). Thus the Cretaceous Sierra Nevada batholith has an overall structure consisting of northwest-strik- ing belts of approximately coeval plutons that be- come younger to the east. The level of the Late Cretaceous batholith currently exposed in the central Sierra varies from west to east, with gen- erally shallower, upper crustal levels in the east. Hornblende geobarometry yields ~5 kilobars for plutons in the west, and ~1–2 kilobars in the east (Ague and Brimhall, 1988). The Ritter Range pendant is a 10-km-scale remnant of metamorphosed volcaniclastic, vol- canic, and sedimentary rock within the eastern part of the central Sierra Nevada batholith (Fig. 1). Paleozoic to Middle Jurassic strata of the pendant have been penetratively shortened 30%–50%, forming slates, phyllites, and schists with weakly to strongly developed cleavage and linear structures (Tobisch and Fiske, 1982). Cleavage development in these rocks has previ- ously been attributed to one or more periods of pre-Cretaceous deformation, including the Late Jurassic Nevadan orogeny (e.g., Nokleberg and Kistler, 1980; Nokleberg, 1983). These age as- signments imply that the principal wall-rock structures and strains significantly predate em- placement of the Late Cretaceous granitoids that bound the Ritter Range pendant on three sides. Tobisch and Fiske (1982), however, proposed a mid-Cretaceous or younger strain event based on the presence of a weakly developed cleavage in a felsic dike with a U-Pb age of 100 Ma within the Triassic to Jurassic strata. Recent structural and geochronological studies indicate that local strain fields in the central Sierra Nevada varied from extensional to contractional to transpressional during Cretaceous batholith em- placement. West of the Ritter Range, extensional deformation is recorded in the Courtright shear zone along the western margin of the Mt. Givens granodiorite during its emplacement at about 1059 Development of Cretaceous transpressional cleavage synchronous with batholith emplacement, central Sierra Nevada, California Warren D. Sharp* Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Othmar T. Tobisch Earth Sciences Department, University of California, Santa Cruz, California 95064, USA Paul R. Renne Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA GSA Bulletin; July 2000; v. 112; no. 7; p. 1059–1066; 7 figures. Data Repository Footnote: Data Repository item 200073 contains additional material related to this article. *E-mail: wsharp@bgc.org.