ABSTRACT that formed above subduction zones. The California Arc: Recent geological and geophysical data Understanding the petrology and tectonic show that a significant fraction of the crust framework of these granitic batholiths (~33 km) in the central Sierra Nevada has stirred great geologic controversies Thick Granitic batholith is granitic, requiring that the and continues to pose several major batholith be underlain by a significant problems in modern geology, such as Batholiths, residual mass prior to Cenozoic exten- quantifying the rates and processes of sion. Although batholith residua are com- crustal growth versus recycling in arc en- monly thought to be granulites, xenolith vironments (e.g., Hamilton, 1988). One Eclogitic Residues, data indicate that eclogite facies residues of our major limitations in deciphering were an important part of the California large-scale arc magmatic features is the arc at depth. The arc was continuously limited knowledge of their vertical di- Lithospheric-Scale active for >140 m.y., yet most surface mension. How deep do they extend, and/or shallow crustal magmatism took what is their composition at depth, and Thrusting, place via two short-lived episodes: one in how thick is the crust beneath arcs? How the Late Jurassic (160–150 Ma), and a sec- much of the crustal thickening is tectonic ond, more voluminous one in the Late versus magmatic? Are batholiths riding on and Magmatic Cretaceous (100–85 Ma). These magmatic major thrust faults or emplaced along flare-ups cannot be explained solely by major strike-slip faults? Is magmatism in increases in convergence rates and mag- major arcs steady state, and how do mag- Flare-Ups matic additions from the mantle. Isotopic matic rates correlate with plate conver- data on xenoliths and midcrustal expo- gence rates? sures suggest that North American lower Mesozoic arc rocks of the western crustal and lithospheric mantle was un- North American Cordillera (Anderson, derthrusted beneath accreted rocks in the 1990) are exposed throughout California arc area. The Late Cretaceous flare-up is (Fig. 1) and once formed a continuous proposed to be the result of this major belt that has since been dismembered by west dipping–lithospheric scale thrusting, Cenozoic tectonism. This paper presents an event that preceded flare-up by an updated view on the composition, ~15–25 m.y. I suggest that the central part structure, and tectonic evolution of the of the arc shut off at ~80 Ma because the magmatic arc of California. source became melt-drained and not be- Mihai Ducea, University of Arizona, cause of refrigeration from a shallowly SETTING Department of Geosciences, subducting slab. The California arc formed as a product Tucson, AZ 85721, USA, of the prolonged subduction of ocean [email protected] INTRODUCTION floor beneath the southwestern edge of Cordilleran batholiths are extensive the North America plate (Dickinson, belts of intermediate calc-alkalic plutons 1981). The arc was active between 220 Figure 1. Map of central and southern California (after Jennings, 1977) showing geologic features discussed in text. Mesozoic granitic and related metamorphic rocks are in red. In blue are mainly Jurassic Franciscan formation rocks. Darker red areas show location of Mesozoic arc-related amphibolite-granulite terrains. 4 NOVEMBER 2001, GSA TODAY and 80 Ma. The igneous crystallization deep crust in the southern Sierra Nevada. complex transition from lower crust to depths of the presently exposed rocks Second, the three exposed sections of the upper mantle located on thickened conti- vary from 0 to ~30 km (Ague and midcrustal arc rocks have intermediate nental crust (Hildreth and Moorbath, Brimhall, 1988; Saleeby, 1990). Together, compositions and are dominated by 1988). The residual mass could be (1) a the segments of the California arc gener- Mesozoic igneous and meta-igneous restite (i.e., the solid left after partial melt- ated ~0.7 million km3 of granitic material. rocks. The southern Sierra Nevada is par- ing and extraction of melt from the deep Figure 1 shows three key features that, ticularly important because it represents a crust), (2) a cumulate (i.e., resulting from together, provide information on the na- tilted exposure through the batholith crystal fractionation in deep-seated ture of the crust beneath the arc. showing that arc-related granitoids domi- magma chambers), or (3) both. In this 1. The granitoid plutons are mostly upper nate the shallow to ~30-km-deep section paper, the nongenetic term residue is crustal exposures of tonalites and gran- of the crust. The bulk chemistry of these used to include restite and cumulate. odiorites, with only minor (<3% of ex- deeper exposures corresponds to a low- Simple mass balance calculations using posed rocks) mafic intrusions. The silica tonalite (Saleeby, 1990). major elements indicate that the ratio of central and southern Sierra Nevada This evidence suggests that ~25–30 km residue to melt in an arc column is ~1 to 2 batholith represents the main and most of the present-day Sierra Nevada crust if the bulk material is basaltic or andesitic studied part of the arc and consists of (Ducea, 1998). Thus, accepting a grani- >90% magmatic products. toid thickness of 30–35 km and assuming 2. Three deeper crustal exposures of the a 40 km depth for the transition from arc—the western Salinian block granulitic to eclogitic residues (Wolf and (Compton, 1960), the Tehachapi com- This evidence suggests Wyllie, 1993), there may have been plex, southernmost Sierra Nevada 30–50 km of eclogite facies residues be- (Ross, 1985), and the Cucamonga com- that ~25–30 km of the neath the batholith, depending on the plex in the San Gabriel Mountains present-day Sierra Nevada bulk chemistry of the system. (Barth and May, 1992)—expose mid- crustal plutons and upper amphibolite crust comprises rocks that RESIDUE BENEATH THE BATHOLITH to granulite facies framework rocks Exposed Deeper Crust that equilibrated at as much as ~30 km are mainly granitoids. The three midcrustal arc exposures in beneath the arc. California consist of deformed sequences 3. Miocene volcanic rocks from the cen- of metamorphosed intrusions inter- tral Sierra Nevada in the San Joaquin spersed with a commonly migmatized volcanic field (Fig. 1; Dodge et al., comprises rocks that are mainly grani- metasedimentary framework, all of which 1986) host xenoliths representing sam- toids. Adding the average erosion depth are intruded by younger, weakly de- ples of the deepest crust and upper of ~6 km in the Sierra Nevada (Ague and formed, or undeformed plutons. mantle beneath the batholith. Brimhall, 1988) to the estimates from Cretaceous U-Pb zircon ages (e.g., seismic data, one can conclude that the Mattinson and James, 1986; Sams and A VERY THICK (30–35 km) GRANITIC batholith must have been ~30–35 km Saleeby, 1988) indicate that the metamor- BATHOLITH thick. The calculated batholith thickness phism and magmatism in these exposures In two classic papers, Bateman and has important implications for the com- is Cordilleran arc–related. The metamor- Wahrhaftig (1966) argued for and position of the deeper arc crust. Calc- phosed framework consists of mostly Hamilton and Myers (1967) argued alkaline intermediate rocks cannot be di- upper amphibolite and subordinate against thick granitoids underlying the rectly extracted by mantle melting granulite facies rocks that record peak currently exposed Sierra Nevada (Wyllie, 1984), thus requiring a second pressures of 7–9 kbar (Pickett and batholith. Various geophysical data such stage of fractionation or partial melting of Saleeby, 1993; Barth and May, 1992; as gravity, heat flow, and various seismic mantle-derived rocks and significant Compton, 1960). The rocks are heavily results have not been able to resolve this crustal residues. If melting of the down- foliated, lineated, and partly migmatized. controversy convincingly. going oceanic crust (Drummond and The intimate spatial association of amphi- We now have two lines of evidence Defant, 1990) was responsible for arc bolite and granulite facies rocks is that argue strongly for a significant (35 magmas in California, the residue could thought to reflect dehydration melting km) thickness of granitoids (rocks that have been “disposed of” via subduction. reactions of amphibolite protoliths (e.g., ε Hansen and Stuk, 1993). Several intru- have >60% SiO2) in the California arc. However, the negative Nd values of most First, a recent seismic refraction study California arc magmas (published values sions of large (>100 km2), undeformed carried out across the Sierra Nevada range between +5 and −12 with an aver- bodies of pyroxene tonalites and gran- shows that crustal rocks have Vp of 6–6.3 age of ~–6) are incompatible with such a odiorites crosscut the foliated framework. km/s throughout the ~33 km of seismo- model because the oceanic lithosphere The mineral assemblages of the unde- ε formed plutons indicate that they were logically defined crust, and are underlain has a fairly constant Nd of ~+10. by mantle peridotite (Fliedner and Therefore, most of the second-stage melt- emplaced at pressures similar to those Ruppert, 1996). On this evidence, ing and/or fractionation and generation recorded by the metamorphic assemblage. Fliedner et al. (2000) concluded that of a residue must have taken place in the Significantly, although there is widespread tonalites and granodiorites extend to the lower lithosphere beneath the arc in a evidence for migmatization, it appears GSA TODAY, NOVEMBER 2001 5 (Rapp and Watson; 1995, Wolf and Wyllie, 1993). These rocks are interpreted here to be an important part of the lower crustal residue beneath the arc. This is based on the large range of equilibration pressures for the Sierran garnet pyroxenites, the great thickness of batholithic crust, and mass balance con- straints calling for a thick residue and hence a significantly thickened crust during arc formation. Trace element patterns are consistent with the garnet pyroxenite xenoliths being batholith residues.