Origin and Evolution of the Sierra Nevada and Walker Lane themed issue Introduction: Origin and Evolution of the Sierra Nevada and Walker Lane Keith D. Putirka1,* and Cathy J. Busby2,* 1California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, California 93720, USA 2Department of Earth Science, University of California, Santa Barbara, California 93106, USA BACKGROUND AND HISTORY Figure 1. A map showing an outline of the Sierra Nevada This Geosphere themed issue is an outgrowth and approximate boundaries of of our Penrose Conference: Origin and Uplift the Walker Lane belt. The out- of the Sierra Nevada, California, which was line of the Walker Lane (and held in Bridgeport, California, August 16–20, its southern extension into the 2010. The theme is here expanded to include Eastern California Shear Zone) the Walker Lane (Fig. 1), since a large number is modifi ed from Faulds et al. of our Penrose abstracts were oriented to that (2005) and Oldow and Cashman topic, and because that region is no less a part (2009); we draw the western Sierra of the Sierran story than the high peaks them- boundary to coincide with the Walker selves. A fundamental question for the confer- Sierra Nevada range front; the N evada Lane ence and themed issue is “How did the Sierra Walker Lane belt is then drawn Nevada form?” The question can mean many to include the region of the things to disparate disciplines. One might refer Basin and Range province where & to the age and origin of the rocks that form the basins and ranges trend more Sierra Nevada batholith, or instead to the time N–S, rather than NE–SW. Our Eastern at which such rocks were uplifted to form the outline for the Sierra Nevada California topographic crest of the eastern Sierra. One includes the topographically con- Shear might also speak to the origin of canyons and tiguous part of the range, and Zone peaks formed by erosion as much as uplift, or to so includes the Lassen Volcanic the time at which the Sierra’s varied present-day Center, which is the geologically ecological zones were established. The answers southern terminus of the modern Cascades province. The base map is from Andrew Birrell to these questions can be quite different, but are (http://birrell.org/andrew/reliefMaps/image.php?zone = west&type = jpg&scale = 50). not necessarily independent, as insights from one may lend insight to another. Finally, the complete story of the Sierra also cannot be understanding of the forces that carve Sierran are the early tectonic speculations on the origin told without the tectonic forces that act on the peaks and canyons. Perhaps less well known of what we now refer to as the “Sierran block” Sierran crust, which involves the evolution of are 19th-century speculations on the nature and the adjacent Walker Lane structural belt. the San Andreas Fault system and the opening of range uplift. Ransome (1898, p. 71), for The Walker Lane belt has been in the geologic of the Gulf of California. example, mapped lava fl ows in the west-central literature since Locke et al. (1940) used the term What makes the Sierra Nevada mountain Sierra Nevada; noting contrasts in Neogene and to describe a region of the western Great Basin range of particular interest is the rich history modern canyon slopes, he suggested that “the where extensional faults have a signifi cant dex- of geologic studies. Pioneers of Sierran geol- elevation of the crest of the Sierra Nevada… tral shear component. But, six years earlier, ogy have provided a well-constructed platform has been…produced by a simple block-tilting Gianella and Callaghan (1934) not only drew on which later scientists could build, with the without perceptible warping”—a view that the outlines of the Walker Lane, but also noted Range of Light illuminating processes as diverse has been subsequently accepted and quantifi ed the similarity of horizontal components of strain as glaciation, structural geology, petrology, and (Huber, 1981; Wakabayashi and Sawyer, 2001). between the western Great Basin and what tectonics. Some aspects of this storied history Early workers also recognized that Eocene and was then referred to as the San Andreas “rift.” are well known, such as John Muir’s early work younger strata in the Sierra Nevada were largely Gianella and Callaghan (1934, p. 22) speculated on Sierran glaciers and glacial erosion (Muir, preserved in “paleochannels” that transported “that the underlying causes of movement in at 1871, 1911; followed by Matthes, 1929, and material from east to west (parallel to modern least the western part of the Great Basin may Blackwelder, 1931), which initiated our current rivers), although their headwaters were inferred be related to those in California” (referring to to lie at the present-day crest (Lindgren, 1911), the San Andreas), and suggested that the Walker *Emails: Putirka: [email protected]; Busby: which we now know did not form until 10 Ma Lane may represent “a tectonic line” that sepa- [email protected]. or later (Busby and Putirka, 2009). Also of note rates the San Andreas and Great Basin structural Geosphere; December 2011; v. 7; no. 6; p. 1269–1272; doi: 10.1130/GES00761.1; 1 fi gure. For permission to copy, contact [email protected] 1269 © 2011 Geological Society of America Putirka and Busby regimes. This view of a connection between but important questions. For example, the cur- including the southern Sierra, however, indi- Walker Lane motion and stress and strain along rent standard model clearly implies that the cate a distinct pulse of volcanism at 10 Ma the San Andreas Fault system has survived the petrologic nature of batholith development throughout the range. More perplexingly, as plate tectonic view of the San Andreas (Atwater, determines the rates and timing of future uplift. even Farmer et al. (2002) note, Pliocene lavas in 1970), and has been verifi ed by Global Position- Earlier views of Sierra Nevada uplift posited an the southern Sierra still carry the geochemical ing Studies, which show that the Walker Lane isostatic model, whereby the high elevations in signature of enriched mantle lithosphere, so an takes up 20%–22% of Pacifi c–North American the eastern Sierra are supported by great thick- enriched mantle lithosphere must have existed displacement (Bennett et al., 1999; Dixon et al., nesses of crust (>60 km; Bateman and Eaton, below the Sierra at the time of the Pliocene 1995, 2000). Perhaps even more impressive 1967). The discovery by seismologists that the eruptions. is that a structural geologist today could use crust is ≤40 km in thickness, and is of greater • Did delamination of ancient lithosphere nearly the precise same language as Gianella thickness in the lower-elevation western part of occur in the Pliocene or the Miocene, or earlier? and Callaghan did in 1934, some three decades the range (Fliedner and Ruppert, 1996), shows If delamination (and asthenosphere upwelling) prior to the plate tectonic revolution. that Airy isostasy cannot apply to the Sierra. is a Pliocene event, what is the driving force Since these early works, the Sierra Nevada Warm mantle asthenosphere is thus posited to of mantle melting so as to yield volcanism at and adjacent Walker Lane regions have played provide buoyancy to support its high elevations 15–10 Ma, which occurs throughout the range? center stage in studies of batholith construc- (Wernicke et al., 1996). If this view is correct, If asthenosphere upwelling predated the Plio- tion (Kistler and Peterman, 1973), the origin the “high Sierra” exists only because of the cene, what structural controls led to volcanism of low-angle normal faults (Proffett, 1977) formation, and removal, of dense crystalline from the Pliocene to the present? and associated “chaos” formed by huge gravi- residues of magmas parental to the batholith. Much new research is now focused on the tational slides (e.g., Troxel and Wright, 1987), Absent the formation of dense residues, the spatial scope and temporal extent of what has the evolution of pull-apart basins (Burchfi el lithosphere “drip” could not have formed, and been termed the “Ancestral Cascades.” Chris- and Stewart , 1966), the tectonic signifi cance of asthenosphere upwelling would not occur, pro- tiansen and Yeats (1992) outlined a region of ophiolites and ultramafi c rocks (Moores, 1970), viding no source of buoyancy. Miocene-age andesitic volcanism that extends the origin of metamorphic core complexes Not unrelated are potentially competing from central California into the Basin and (Wright et al., 1974), and volcanic processes models for Sierra Nevada uplift; some argue Range province south to Las Vegas (Colgan such as bimodal volcanism and magma mixing that the range has mostly been high since the late et al., 2011). Based on space-time distributions (Bacon, 1982; Bacon and Metz, 1984). It may Cretaceous (House et al., 1999), while others of volcanic rocks using NAVDAT, Glazner be fair to say that the Sierra Nevada is a type argue that range uplift has occurred mostly and Farmer (2008) indicate that what has been example of many such processes, including since 10 or 5 Ma (Huber, 1981; Wakabayashi referred to as Ancestral Cascades are rocks that range uplift and microplate formation, and the and Sawyer , 2001). These models might not be may instead be related to larger Cordilleran spa- crustal extension processes that yield continen- mutually exclusive: Clark et al. (2005) suggest tial patterns of volcanic migration and so have tal rift basins and mountain ranges. at least three pulses of uplift, occurring in the little connection to the modern Cascades; they late Cretaceous, again at 32 Ma, and another at thus suggest that the term “Ancestral Cascades” CURRENT ISSUES IN SIERRA 3.5–0 Ma.