Production and loss of high-density batholithic root - southern Sierra Nevada, California

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1 Production and loss of high-density batholithic root - southern Sierra Nevada, California Saleeby, J.* Division of Geological and Planetary Sciences, California Institute of Technology, Mail Stop , Pasadena, CA Ducea, M. Department of Geosciences, University of Arizona, 1040 E. Fourth St., Tucson, AZ Clemens-Knott, D. Department of Geological Sciences, California State University, Fullerton, CA * jason@gps.caltech.edu ABSTRACT Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 10 kb liquids typical of Cretaceous Sierra Nevada batholith granitoids are generated and garnet+clinopyroxene dominate the residue assemblage. Upper mantle-lower crustal xenolith suites that were entrained in mid-miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by garnet clinopyroxenites which are shown by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage. The xenolith data in conjunction with an exposed oblique crustal section in the southernmost Sierra, as well as seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 45 km depths, a thick (~ 35 km) underlying garnet clinopyroxenite residue sequence, and underlying garnet peridotite lithospheric mantle to ~ 125 km depths. The peridotite section appears to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate lower crust dominated by mantle wedge derived mafic intrusions. Genesis of the composite batholith over an ~ 50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere. Sierra Nevada batholith magmatism ended by ~ 80 Ma in conjunction with the onset of the Laramide orogeny. The greater Sierra Nevada batholith mantle lithosphere was cooled to a 1

2 conductive geotherm from beneath by flat slab subduction. Disruption of the cornerflow circulation of asthenosphere into the wedge environment by the Laramide slab is suggested to have been the prime factor in the termination of magma genesis. In the southernmost Sierranorthern Mojave Desert region the sub-batholithic mantle lithosphere, including the garnet clinopyroxenite residue sequence, was mechanically delaminated by a shallow segment of the Laramide slab, and was replaced by underthrust oceanic assemblages. These relations record a segmentation of the Laramide slab beneath the southernmost Sierra, similar to that observed beneath the modern Andes. Despite these Laramide events the greater Sierra Nevada mantle lithosphere for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene-Quaternary lavas to garnet absent, spinel and plagioclase peridotites, whose thermobarometry define an asthenosphere adiabat, as well as seismic data indicate that much of the remaining sub-sierran lithosphere was removed in Mio-Pliocene time. Such removal is suggested to have arisen from a convective instability related to high-magnitude extension in the adjacent Basin and Range province, and to have worked in conjunction with the recent phase of Sierran uplift and a change in regional volcanism to more primitive compositions. In both the Mio-Pliocene and Late Cretaceous lithosphere removal events the base of the felsic batholith was the preferred locus of separation. This is suggested to have resulted from the sharp rheological contrast between weak quartzofeldspathic deep batholithic rocks, and the stronger and much denser residue sequence. Such phenomena may be of global importance in continent edge arcs, and responsible for the common Moho depths of 30 to 40 km observed in the derivative batholithic belts. This is also suggested to help fractionate and isolate the Earth's felsic crust in the long term. INTRODUCTION Over the past decade there has been an increase in awareness that during the formation of felsic magmatic arcs a substantial mass of olivine-poor, high-density residues should accumulate at depth. Key to this realization are melting experiments of hydrous mafic assemblages at pressures in excess of ~10 kb which render tonalitic to granodioritic melts and garnet clinopyroxene residues (Wolf and Wyllie, 1993,1994; Rapp and Watson, 1995). The likelihood that such melt-residue relations are a first-order process in magmatic arcs is supported by REE 2

3 studies showing that felsic batholithic rocks are commonly in equilibrium with a garnet-rich residue (Dodge et al, 1982; Gromet and Silver, 1987; Ross, 1989; Kay and Kay, 1991). Such REE data has been interpreted to imply an eclogitic source for magmatic arcs. The eclogitic residue rock that we consider here is fundamentally different, however, having formed by crystal-liquid equilibria as opposed to solid state metamorphic reactions. In this paper we review evidence for the development of a thick (~35 km) high-density root complex beneath the Cretaceous Sierra Nevada batholith (SNB) (Ducea and Saleeby, 1998b; Ducea, 2001). Such a high-density root complex contrasts markedly with the low-density crustal root originally envisaged for the SNB which was invoked as a mechanism to support the elevation of the modern range (Bateman and Eaton, 1967), but which has since been shown not to exist by seismic data (Wernicke et al 1996; Jones and Phinney, 1998; Ruppert et al, 1998; Fliedner et al, 2000). The high-density root complex consists primarily of garnet and pyroxenes, mainly clinopyroxene. It is referred to below as the GPR (garnet-pyroxene residues). The Cretaceous SNB may be treated as the type Cordilleran batholithic belt, having formed along the North American plate edge during subduction of the Farallon plate. The resolution of this GPR root beneath the SNB carries important implications for the existence of similar roots beneath other major segments of the Cordilleran batholithic belt, and by analogy other Phanerozoic, and Phanerozoic-like, batholithic belts. The accumulation of substantial masses of garnet-clinopyroxene residues beneath magmatic arcs has lead a number of workers to suggest that such lower crustal cumulates ultimately sink into the mantle (Kay and Kay, 1991, 1993; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Ducea and Saleeby, 1998a). Recent analysis of subcontinental lithospheric mantle (SCLM) chemical structure and rheology suggests that Phanerozoic SCLM upon cooling to stable conductive geotherms will be gravitationally unstable relative to the asthenosphere, and will tend to sink or delaminate (O Reilly et al, 2001). The presence of a substantial thickness of garnet-clinopyroxene rocks within such SCLM should add to the gravitational instability. In this paper we review evidence for the sinking of the GPR and adjacent peridotitic lithosphere into the underlying mantle (Ducea and Saleeby, 1998b). Based primarily on Re-Os isotopic data Lee at al (2000) discuss the likelihood of delamination processes having affected the SCLM beneath the SNB. They did not consider the GPR in their analysis, considering them as crustal rocks. 3

4 Another aim of this paper is to clarify the definition of the SCLM that evolved beneath the SNB, and to explore how its loss fits into the petrogenetic and tectonic development of the region. As we track the evolution of the SCLM beneath the Sierra Nevada region it will become clear that it has changed dramatically through time. For this reason we distinguish it from the subbatholithic mantle lithosphere (SBML) which was produced in Cretaceous time and which included elements of the ancient (Proerozoic) North American SCLM, tectonically accreted oceanic lithosphere mantle, and asthenosphere. We pose the question that if the production and loss of the GPR is a global process in arc evolution then perhaps it is an effective multi-stage mechanism for the fractionation and longterm isolation of felsic crust. This process may carry important implications for the long-term geochemical evolution of the mantle. Our findings suggest that as the GPR sank into the mantle, large-ion lithophile (LIL) element-rich accessory phases melted and supplied enriched silicic veins to asthenospheric peridotites which appear to have ascended and replaced the GPR (Ducea and Saleeby, 1998b). This provides a mechanism for re-enriching at least the shallow mantle in LIL elements through geologic time. The central and southern SNB have offered a rich array of findings by virtue of superimposed latest Cretaceous and Cenozoic processes. Regional tectonic events which disrupted the southernmost SNB in latest Cretaceous-Paleocene time (Laramide orogeny) resulted in an oblique crustal section which extends virtually to the base of the felsic batholith (Saleeby, 1990; Pickett and Saleeby, 1993; Malin et al, 1995). Furthermore, mid-miocene to Quaternary low-volume volcanic centers in the southern SNB and adjacent Basin and Range province offer a time series in sampling of the sub-snb mantle by their accidental entrainment of lower crust and upper mantle xenoliths (Ducea and Saleeby, 1996). Petrogenetic and age data on the oblique batholithic section and on the xenoliths, when integrated with geophysical data, lead us to a three-dimensional view of the evolution of the SNB and its mantle underpinnings and how it evolved over the past ~100 m.y. THE SIERRA NEVADA BATHOLITH IN THREE DIMENSIONS Our discussion focuses on the southern half of the SNB, whose geomorphic expression is truncated in the south by the Neogene Garlock fault (Fig. 1). This sinistral fault developed 4

5 preferentially along the Late Cretaceous-Paleocene Tehachapi-Rand deformation belt, a complex structural system which is directly linked to the depth of exposure of the southernmost SNB (Malin et al, 1995; Saleeby, in review). Igneous barometry (Ague and Brimhall, 1988), the preservation of the mid-cretaceous supra-batholithic silicic volcanic complexes ( Saleeby et al, 1990; Fiske and Tobisch, 1994), and Early Cretaceous ring dike complexes (Clemens-Knott and Saleeby, 1999) indicate shallow levels of exposure southward to latitude ~36º N. Further south there is a steep gradient to ~10 kb depths in the western Tehachapi Range (Pickett and Saleeby, 1993; Ague, 1997). This gradient is offset along the axis of the southernmost Sierra by the Late Cretaceous proto-kern Canyon fault (Fig. 1). However, structural continuity between the shallow and deep level exposures is preserved along the regional stirke of the southern SNB which renders an oblique crustal section (Saleeby, 1990). The principal tilt direction for the oblique section is approximated by the Figure 5 section line shown on Figure 1. The Cretaceous SNB was generated and emplaced into a metamorphic framework that possessed a suture between Paleozoic oceanic lithosphere to the west and North American Proterozoic lithosphere to the east (Saleeby, 1981; 1990; Kistler, 1990). Correspondingly batholithic petrochemical patterns show generation of the western domains of the batholith primarily within a depleted mantle regime, and axial to eastern domains within a continental lithospheric domain (Kistler and Peterman, 1973; DePaolo, 1981; Saleeby et al, 1987; Kistler, 1990; Chen and Tilton, 1991; Clemens-Knott et al, 1991; Coleman et al, 1992; Pickett and Saleeby, 1994; Sisson et al, 1996). The lithologic expression of this transverse compositional gradient is a western batholith domain rich in hydrous mafic to ultramafic cumulates with differentiates rarely more felsic than tonalite; and axial to eastern domains having predominately granodioritic compositions (Moore, 1959; Saleeby, 1981, 1990; Clemens- Knott, 1992). Regardless of this compositional gradient, hydrous mafic cumulates and dikes ranging from melagabbro to diorite are ubiquitous throughout the entire SNB (Saleeby, 1981; Saleeby et al, 1987; Coleman et al, 1992; Sisson et al, 1996; Coleman and Glazner, 1998). The southern SNB also displays a pronounced transverse age gradient with mafic magmatism commencing ~130 Ma in the west and felsic magmatism ending in the east by ~ 80 Ma (Evernden and Kistler, 1970; Saleeby and Sharp, 1980; Stern et al, 1981; Chen and Moore, 1982; Saleeby et al, 1987, 1990). During this 50 m.y. history of batholith generation, the greatest volume of material was 5

6 emplaced between 100 and 84 Ma. The SNB also contains Jurassic and Triassic plutons along its northwest and southeast margins. These lower volume plutons are separated in time from the Cretaceous composite batholith by regional deformations and a magmatic lull (Saleeby, 1981), and thus for the purpose of this paper are treated as part of the pre-batholithic framework. There are three key features of the depth and compositional zonation patterns discussed above that are pertinent to our focus. 1) Direct observations of the oblique crustal section indicate that the axial and presumably eastern domains of the SNB are predominantly felsic to depths corresponding to ~10 kb. These observations are consistent with seismic refraction data across shallower-level exposures (1 to 3 kb) of the batholith to the north which show lowvelocity felsic crust extending down to km (Moho) depths (Jones and Phinney, 1998; Ruppert et al, 1998; Fliedner et al, 2000). 2) Observations in the deep-level exposures show that hydrous mafic cumulates ranging from melagabbro to quartz diorite underwent local partial remelting to yield tonalitic leucosomes and garnet rich residues. Localized remelting of the mafic cumulates occurred broadly within the same batholithic growth cycle in which the cumulates formed (Saleeby et al, 1987; and unpublished data). 3) Hydrous mafic cumulates and dikes that are ubiquitous throughout the SNB attest to the mantle wedge source for the batholithic magma systems irregardless of any acquired crustal components. We will return to these features in the synthesis of a primary lithospheric column for the SNB in the context of our volcanic-hosted xenolith data, and regional geophysical data. AGE AND PETROGENETIC SETTINGS OF VOLCANIC-HOSTED XENOLITHS Volcanic-hosted xenolith localities are shown on Figure 1. They fall mainly into two age/compositional groups which are differentiated on Figure 1 ( Domenick et al, 1983; Dodge et al, 1986, 1988; Mukhopodhyay and Manton, 1984; Beard and Glazner, 1995; Ducea and Saleeby; 1996, 1998a). The first group is from mid-miocene volcanic centers and is characterized by abundant garnet pyroxenites, garnet-and spinel bearing peridotites, and mafic granulites. The second group is from Pliocene-Quaternary centers and is devoid of garnetbearing lithologies being characterized more by spinel and plagioclase peridotites. We will focus first on the mid-miocene group. 6

7 Mid-Miocene xenolith-bearing localities are concentrated in areas characterized by relatively shallow-level exposures of the SNB, greater than 100 km north of the tilted crustal section. This location suggests ascent through a relatively complete batholithic column in a region that escaped the Laramide tectonics which rendered the deep batholithic exposures of the tilted crustal section. Age and petrogenetic data for the mid-miocene suite are discussed in Ducea (1998, 2001) and Ducea and Saleeby (1998b). In summary Sm/Nd mineral isochron ages on garnet clinopyroxenites and granulites fall uniquely within the range of zircon U/Pb ages for the central and southern SNB. Time-corrected (100 Ma, median age for southern SNB) C Nd, and initial Sr and Pb radiogenic isotopic ratios likewise cluster about the maxima for those determined for the central and southern SNB. Oxygen isotope data for batholithic rocks and garnet pyroxenites cluster together as well. Concentration data on REE for both SNB tonalites and granodiorites can be correlated to REE data for garnet clinopyroxenites by partition coefficients as complimentary melt-residue assemblages (Ducea, 1998, 2001). The age, isotopic, and REE data all indicate a cogenetic relationship between the garnet pyroxenite and granulite xenoliths, and the overlying batholith which is consistent with the experimental data of Wolf and Wyllie (1993,1994) and Rapp and Watson (1995). Figure 2 summarizes thermobarometric data for both the mid-miocene and Pliocene- Quaternary suites of xenoliths (after Ducea and Saleeby, 1996, and unpub. data; Lee et al, 2000). We focus first on garnet pyroxenites and granulites of the mid-miocene suite. Garnet clinopyroxenites dominate the data array clustering at pressures of ~ 15 to 25 kb; and along with garnet websterites they extend to pressures as high as ~ 33 kb. Granulites, typically containing garnet lie between 8 and 13 kb and overlap with the two lowest pressure garnet pyroxenites. The hydrous basalt solidus is also plotted on Figure 2 (Wolf and Wyllie, 1994; Rapp and Watson, 1995; Vielzuef and Schmidt, 2001). Note that the garnet clinopyroxenite data cluster to the right of the solidus above the plagioclase out boundary, and the granulites cluster between this boundary and the garnet in boundary. The granulite cluster along with the lower edge of the garnet pyroxenite cluster resemble a re-entrant that forms in the solidus at decreasing water contents. The Figure 2 inset shows that the re-entrant forms as the solidus shifts to higher temperatures as water content decreases (after Vielzuef and Schmidt, 2001). Considering the relationships shown for the garnet pyroxenite and granulite clusters on Figure 2, and the parallel 7

8 and complimentary geochemical relations to the SNB that were discussed above, we assert that the P-T data on the pyroxenite and granulite xenoliths reflect crystal-liquid equilibria in the source region of the SNB, as opposed to metamorphic equilibration conditions. Peridotite xenoliths from mid-miocene localities are less abundant than pyroxenites,, and consist of garnet and spinel peridotites with pressures between ~ 13 and 42 kb(mukhopadhyay and Manton, 1994; Ducea and Saleeby, 1996 and unpub. data; Lee et al, 2000). Samples for which detailed thermobarometry have been performed are shown on Figure 2. These samples bear orthopyroxene temperature zonation patterns which along with the corresponding pressures record an early geotherm close to the wet peridotite solidus, followed by cooling to a lithospheric geotherm. The lower P peridotites whose high-t history began near the garnet-spinel boundary are spinel bearing. The P-T relations of Figure 2 along with the frequency distributions of the xenolith suites (Ducea and Saleeby, 1996) define a vertical lithologic sequence consisting of a basal garnet peridotite lithospheric mantle with inclusions of garnet pyroxenite, overlain by a garnet pyroxenite section with inclusions of garnet and shallower spinel peridotite, in turn overlain by garnet granulites with inclusions of garnet pyroxenite (Fig. 3). We adhere to a seismic velocity definition for the Moho thereby placing most of the GPR within the SBML. Below we will return to the subject of the petrologic Moho within this lithologic sequence. The high T history of the peridotites as well as Os isotopic data (Lee et al, 2000) suggest an influx of asthenosphere and cooling to a lithospheric geotherm in the construction of the deeper levels of this lithospheric sequence. We will return to this subject as well. We focus briefly here on the Pliocene-Quaternary suite of xenoliths. As noted above this suite is totally devoid of garnet-bearing lithologies, thus making thermobarometric determination more challenging and less precise. Nevertheless, phase relations between plagioclase and spinel peridotites coupled with the Ca-in-olivine barometer and pyroxene thermometry yield an asthenosphere adiabat P-T trajectory (Fig. 2) (Ducea and Saleeby, 1996). We interpret the Pliocene-Quaternary suite of mantle xenoliths as modern asthenosphere that has replaced the sub-batholithic lithosphere represented by the mid-miocene suite (Ducea and Saleeby, 1998a). We will further substantiate this interpretation below. A PRIMARY LITHOSPHERIC COLUMN FOR THE SIERRA NEVADA BATHOLITH 8

9 Petrogenetic and thermobarometric data for the mid-miocene xenolith suite are integrated with geological relations of the oblique crustal section in the construction of a primary lithospheric column for the SNB (Fig. 3). We have focused this column for the ~100 Ma time slice of batholith petrogenesis. The map trace of the oblique crustal section is best defined by ~100 Ma batholithic rocks, and the most intact remnant of a supra-batholithic volcanic complex (Ritter Range caldera) is likewise ~100 Ma in age. The procedure used for the crustal part of the section was to use down-structure viewing techniques along the trace of the Figure 5 section line (Fig. 1), which reaches sub-volcanic levels at ~ 2kb; and then generalize the structure from 2 kb to volcanic levels based on the structure of the Ritter Range area. The deeper (SBML) structure of the section is conjectural and based on the convergence of rock-types and equilibration conditions between the deepest exposed levels of the batholith and the xenolith suite, and on the relative abundances and equilibration conditions of the xenoliths (Fig. 2). An outstanding feature of the Figure 3 reconstruction is the predominance of felsic batholithic rocks to between 30 and 35 km depths. These direct observations are in agreement with deep seismic data across the shallow-level exposures of the SNB to the north of the oblique crustal section (Jones and Phinney, 1998; Ruppert et al, 1998; Fliedner et al, 2000). Fliedner et al (2000) also note that the seismic velocities measured for the deep felsic batholith are slightly low, unless there is a strong regionally consistent anisotropy along the trend of the batholith. There is in fact such a pervasive anisotropy along the trend of the batholith, expressed most strongly at ~ 3 kb and greater levels (Saleeby, 1990). This consists of the NW trending and steeply-dipping tubular shapes of plutons and their internal contacts, solidus to hot subsolidus flow fabrics, and the pervassive structure of high-grade metamorphic pendants. Furthermore, much of the pendant material consist of quartzites, marbles and steeply-dipping quartz-rich schistose rocks which further contribute to low seismic velocities and the regional anisotropy. The steep regional primary fabric of the SNB continues downward to between 8 and 10 kb levels. It is clear from the oblique crustal section that the crust has been completely reconstituted by batholith production. The felsic batholith grades into a mafic granulitic layer at ~10 kb conditions. Within the transition zone there are noritic and pyroxene diorite-tonalite intrusions which resemble granulites, but are mantle-derived magmatic rocks that had variably fractionated prior to entering 9

10 the lower crust. These deep intrusive rocks are also joined by layers and pods of hydrous mafic and ultramafic cumulates. Based on direct observations of the deep-level rocks and the phase relations summarized in Figure 2 the garnet granulites are for the most part residues left from the partial melting of hydrous mafic to intermediate composition rocks at roughly 10 kb conditions. The granulitic layer is relatively thin (~ 5 km) and grades downwards into garnet-rich residues of the feldspar-free GPR, which is ~ 35 km thick. The transition from feldspathic to feldspar free residues/cumulates would correspond to the seismic Moho upon cooling of the section to a lithospheric geotherm. Petrogenetically the GPR is crustal rock, having evolved in a deep crustal magma genesis domain and having the isotopic fingerprints of crustal rocks. Accordingly, the base of the GPR corresponds to the petrologic Moho. The GPR is envisaged to have developed sequentially in large batches as volumetrically comparable batches of felsic magma formed, gathered and ascended. Throughout this partial melting and melt extraction process variably fractionated wedge-derived mafic magmas continued to invade the felsic magma source regime thereby supplying additional heat, fluids and labile components to the system. Those invading magmas that penetrated through the felsic magma source regime were entrained within and commingled with the respective felsic magma batches. This complex dynamic structure would yield a broad transition zone between mantle and crustal seismic velocities like that imaged beneath the active Andean arc (Graber and Asch, 1999). The granulitic and GPR zones of the derivative lithospheric column would in essence correspond to the melt-drained mixingassimilation-storage-homogenization (MASH) zone of Hildreth and Moorebath (1988),although direct observations and isotopic data on the deep-level exposures, as well as xenolith isotopic data indicate that homogenization was typically far from complete (Saleeby et al, 1987; Pickett and Saleeby, 1994; Ducea and Saleeby 1998b). Following cessation of magmatism and cooling to a lithospheric geotherm the GPR section and its underlying peridotite section together constituted the sub-batholithic mantle lithosphere (SBML). Focusing now on the peridotite section we assert that these rocks represent the vestiges of the mantle wedge beneath the SNB. Based on Os isotopic data, and the thermal history summarized in Figure 2, Lee et al (2000) interpret peridotite xenoliths from depths of 45 km to be asthenospheric mantle that replaced SCLM during a Cretaceous to early Cenozoic delamination event. These workers did not consider the GPR in their analysis, having lumped it 10

11 into the overlying crust. We offer an alternative interpretation that we feel better explains the overall petrogenetic and geodynamic framework. Recent geodynamic modeling of subduction zones (Billen and Gurnis, 2002) and petrophysical considerations (Debari et al, 1987; Debari and Coleman, 1989; Jull and Kelemen, 2001) imply a weak low viscosity mantle wedge in active subduction-arc systems as well as high Moho temperatures (800 to 900º C at ~11 kb). One of the reported results of the weak low viscosity wedge is a suction invigoration of corner flow patterns which tap asthenospheric reservoirs and pull such materials into the wedge environment. A vigorous influx of non-depleted asthenospheric mantle, with its ensuing depletion during arc magma genesis, seems required in order to sustain arc magmatism for 10 s of m. y. without exhausting the source regime. Accordingly we interpret the bulk of the peridotite section to represent the last remnants of asthenospheric mantle that circulated into the wedge environment. The approach of the high-t orthopyroxene core data array to the wet peridotite solidus (Fig. 2) is interpreted as the mark of slab-derived fluid saturation of the mantle wedge during wedge magma genesis. We will return to the cooling history of the wedge below. We consider the peridotite section to have undergone substantial chemical change in the wedge environment. As noted by Lee et al (2000) high Mg numbers of the peridotites indicate a melt extraction event(s). Nd, Sr, and Os isotopic data on the peridotites (Mukhopadhyay and Manton, 1994;Lee et al, 2000; Zeng et al, 2001a) indicate extreme heterogeneity with some values lying on the asthenospheric mantle array, and a scatter of values dispersing to time integrated LIL enrichments of Sr i as high as 0.708, and nd as low as Possible sources for such enrichments are fluids derived from the slab including subducted sediments (cf. Elliott et al, 1997), inclusions of ancient SCLM, and the melting of labile accessory phases that were included in GPR drips that detached from the residue pile and foundered into, and possibly through, the peridotitic wedge (cf. Ducea and Saleeby, 1998c). We suggest that garnet clinopyroxenite and websterite lenses shown on Figure 3 deep within the peridotite section represent the vestiges of such drips that stagnated within the wedge. We will return to this mantle re-enrichment process below. From the Figure 3 reconstruction it is further suggested that SNB genesis occurred in conjunction with the complete reconstitution of the Sierran lithosphere. We now return to the crustal sequence depicted in Figure 3 to briefly discuss some of the batholith petrogenetic issues. Assuming the correct application of the Wolf and Wyllie 11

12 (1993,1994) and Rapp and Watson (1995) melting experiments in the production of the GPR from a hydrous mafic source we now consider the ultimate source for these materials. Field, petrographic and isotopic observations of the deep-level exposures indicate that mantle-derived hydrous cumulates at least locally undergo such a melting process geologically soon after their initial accumulation. Thus a potential source for the large volume production of such felsic meltgarnet rich residue sequences is the underplating and thickening of such cumulates as wedgederived magmas ascend to the base of the crust and fractionate. If such accumulation is rapid enough, and if high sub-arc Moho temperatures as discussed above are applicable, a large mass of such cumulates could revert back through the hydrous mafic solidus. On Figure 4 we present theoretical data showing the dominance of garnet and clinopyroxene as residual phases in felsic melt production from a hydrous arc basalt source in the pressure range of 1 to 1.5 GPa. An additional factor in this analysis that also comes from the direct observations is the progressive advection of heat into the source cumulate pile during its remelting by continued mantle wedge magmatism. Such magmas include hydrous basalt to basaltic-andesite dikes, and noritic to pyroxene tonalitic plutons which were fractionated to various degrees by the time they ascended into the crust. The derivative rocks are observed at all structured levels of the SNB, cutting hydrous mafic cumulate sequences, dispersed as the remnants of dikes that commingled and partially hybridized as inclusions in felsic batholithic rocks, and locally as shallow level (subvolcanic?) dike and sill complexes (Mack et al, 1979; Reid et al, 1983; Saleeby et al, 1987; Saleeby, 1990; Coleman et al, 1992; Clemens-Knott, 1992; Sisson et al, 1996; Clemens-Knott and Saleeby, 1999). Geological relations and isotopic data on the batholith, its metamorphic framework, and the GPR xenolith suite lead us to further consider a polygenetic origin for the hydrous rocks in the batholith source regime. Potentially important regionally extensive framework rocks include: 1) Paleozoic ophiolitic mafic crust and overlying Jurassic arc volcanic sequences of the western metamorphic belt that may have been thrust eastward deep into the pre-batholithic framework in early Mesozoic time (Saleeby, 1981); and 2) Proterozoic deep continental crust that may have been emplaced westward into the source regime prior to and/or during batholith generation in the root zone of the Cordilleran foreland thrust system (Saleeby, 1981, 1999; Ducea, 2001). The westernmost branch of this regional thrust system is shown on Figure 1. The first of these two 12

13 possibilities is difficult to assess, although small inclusions of meta-harzburgite of probable western Sierra ophiolite belt heritage exist within the western domain of the deep batholithic exposures. A Proterozoic lower continental crustal component is recorded in the eastern domain batholith as well as some GPR xenolith samples by Sr, Nd and Pb isotopic variation patterns (Chen and Tilton, 1991; Ducea and Saleeby, 1996; Zeng et al, 2001b). Furthermore, the shallowest level spinel peridotite xenolith (~ 12 kb) from the sub-batholithic suite(not shown on Fig. 2) yields ancient SCLM Os isotopic signatures and evidence of heating to sub-arc solidus conditions (Lee et al, 2000). This may be interpreted as a remnant of the ancient SCLM that was tectonically transported into the source regime along with Proterozoic deep crustal rocks, or alternatively ambient SCLM of the eastern domain of the mantle wedge. Both east and west directed thrust systems were also capable of introducing metasedimentary admixtures to the source which are shown to be a moderate to subordinate component by both batholith and xenolith isotopic data (Saleeby et al, 1987; Kistler, 1990; Clemens-Knott, 1996; Ducea and Saleeby, 1998b). Finally, nowhere in the metamorphic pendants of the southern SNB are there direct remnants of melted oceanic mafic basement or Proterozoic continental basement. Highgrade pendant rocks consist totally of supercrustal protoliths. Thus the possible contribution of these basement elements to the batholtih source regime operated at a structural level beneath that represented by the pendants. Considering the above discussion in the context of: 1) the transverse isotopic and bulk compositional gradients of the SNB, 2) the ubiquitous occurrence of Cretaceous age hydrous mafic cumulates and dikes throughout the SNB; and 3) the volumetrically significant hydrous mafic cumulates that occur in the western domain of the batholith lead us to conclude the following. Wedge-derived hydrous mafic magmatism was the primary magma source for the SNB. The eastern continental domain of the pre-batholithic framework contributed to the source regime by the emplacement of deep crustal Proterozoic basement into the root zone of the eastdirected foreland thrust belt. Oceanic crustal rocks may have been contributed to the source regime by pre-batholithic underthrusting from the west. The continuous flux of wedge-derived hydrous mafic magma progressively thickened the arc basement, which progressively remelted to form felsic melts and the GPR. The common remnants of commingled mafic dikes within the felsic batholith attests to the continuous influx of mafic magmas throughout the multi-stage 13

14 generation of the felsic magma batches. Following the ~80 Ma termination of batholithic magmatism the SBML was cooled to a lithospheric geotherm with the preservation of the structure depicted in Figure 3. LOSS OF THE SUB-BATHOLITHIC HIGH-DENSITY ROOT In a recent review of continental lithospheric structure and composition O Reilly et al (2001) assert that relative to Archean and Proterozoic lithospheric domains Phanerozoic SCLM is more prone to delaminate and sink through the asthenosphere. The primary driving force invoked in this analysis is the negative buoyancy of Phanerozoic SCLM, due to its relatively non-depleted state. Various workers have suggested SCLM delamination events for a variety of times and tectonic settings for the southwest U.S. (Bird, 1979, 1988; Ducea and Saleeby, 1998a; Lee et al, 2000). The supporting evidence, as well as the tectonic settings, of these presumed delamination events very greatly. The dynamic processes which potentially govern such delamination events are varied as well. For example, Bird (1979, 1988) discusses mechanical delamination in which high-density lithospheric mantle is peeled away along a surface of relative weakness, such as the Moho, or is sheared away by flat-slab subduction. In contrast convective instabilities have been invoked as a mechanism to remove lithosphere and lower crust (Conrad and Monar, 1997; Houseman and Monar, 1997; Jull and Kelemen, 2001). Key to this analysis are density perturbations resulting from temperature contrasts which are convectively removed. Jull and Kelemen (2001) have added to this analysis upper lithosphere density contrasts introduced by the growth of mafic-ultramafic cumulate sequences in the roots of magmatic arcs. In their analysis they point out the compounding effect of high temperature and high initiating regional strain rates in instigating the instability and removed event. In the discussion below we rely on elements of all these delamination mechanisms to explain geological and geophysical relations which suggest at least two distinctly different SBML removed events for the southern SNB. We distinguish between mechanical delamination and convective removal, but assert that the regionally planar contact zone between the felsic batholith and the GPR was for both mechanisms the preferred locus of separation. In Figure 3 we have developed a model for the primary lithospheric structure for the Cretaceous SNB. For our discussions below we consider the essential features of this model to be: 1) the predominance of felsic batholithic rocks to a depth of ~ 35 km; 2) a high-density 14

15 residual sequence consisting of mainly garnet and clinopyroxene(gpr) between ~ 45 and 75 km deep; and 3) underlying lithospheric peridotite to a depth of ~ 125 km. In the discussions below we use Figure 3 as a template for lithospheric structure at the time of the ~ 80 Ma end of magmatism for the SNB. Integration of field observations, Pliocene-Quaternary volcanic-hosted xenolith studies, and geophysical data for the southern Sierra Nevada region reveals that much, or all, of the SBML was removed in at least two distinct events in contrasting regional tectonic settings: 1) latest Cretaceous-Paleogene (Laramide) subduction tectonics for the southernmost Sierra region; and 2) late Neogene high magnitude (Basin and Range) extensional tectonics for the greater SNB region to the north. Laramide Delamination in the Southernmost Sierra Nevada Region The Laramide orogeny is recognized as a regional compressional event which deformed the southwest North American craton in latest Cretaceous-early Paleogene time (Coney, 1976). A widely accepted plate tectonic mechanism for the Laramide orogeny is intensified traction and tectonic erosion along the base of the continental lithosphere resulting from flattening of the subducted oceanic slab (Coney and Reynolds, 1977; Dickinson and Snyder, 1978; Bird, 1988). Geologic events along the southwest North American plate edge that have been attributed to Laramide tectonics include the end of magmatism in the SNB and associated modification of the sub-batholithic geotherm, and the emplacement of ensimatic metamorphic units (Rand-Pelona- Orocopia schists) beneath the batholithic belt in the southernmost Sierra Nevada-Mojave Desert region (Dumitru et al, 1991; Malin et al, 1995; Jacobson et al, 1996, 2000; Saleeby, in review). Constraints on the lower crust upper mantle structure posed by the oblique crustal section of the southern SNB, along with seismic data and the Miocene volcanic-hosted xenolith data carry important implications on the topology and some of the physical affects of the Laramide slab. Our model of the primary lithospheric column for the SNB (Fig. 3) reveal an intact lithospheric structure to a depth of ~125 km. This primary structure survived in the central Sierra Nevada region at least until mid-miocene time, when it was sampled by deep-seated volcanism. We do not consider accidental sampling of a tectonically telescoped section as a viable alternative to the sampling of an intact section. Cumulate and/or annealed textures of high-grade mineral assemblages greatly predominate over retrograde tectonitic fabrics in the deep-level 15

16 xenolith suites. Our conclusion for these observations is that flat slab subduction did not disrupt the sub-batholithic lithosphere of the central Sierra Nevada region. The intact lithospheric structure exhibited for the central Sierra Nevada batholith contrasts sharply from that observed along the Tehachapi-Rand deformation belt of the southernmost Sierra region. This is readily visualized by inspection of the longitudinal cross section of Figure 4. Here we have palinspastically restored offset on late Neogene high-angle faults in order to elucidate the first order structural relations of the deformation belt (Malin et al (1995). The southernmost Sierra (Tehachapi) seismic structure correlates with the northern Mojave seismic structure adjacent to the Rand Mountains(Cheadle et al, 1986). In both areas field observations and geophysical data indicate that the base of the batholithic crust and its underlying SCLM have been tectonically removed by the thrust emplacement of the ensimatic Rand schist (Cheadle et al, 1986; Silver and Nourse, 1986; Malin et al, 1995; Wood and Saleeby, 1998; Miller et al, 2000). In the northern Mojave Desert region the Rand thrust exhibits low dips and modest structural relief. The thrust is exposed in the core of an antiformal window in the Rand Mountains, and to the north in the southernmost Sierra as small half windows and larger fault-bounded slices along the Garlock fault. Upper plate rocks between the two areas correlate in age, lithology and radiogenic isotopes (Silver and Nourse, 1986; Saleeby et al, 1987; Pickett and Saleeby, 1994; Wood and Saleeby, 1998). Thermobarometric constraints in both areas indicate that the level of detachment of the lower crust (Rand thrust) was between 8 and 10kb (Sharry, 1981; Pickett and Saleeby, 1993; Jacobson, 1995). The Rand thrust has been seismically imaged dipping ~20º beneath the southernmost SNB as it extends northward from its surface expression along the half windows. Paleo-depth (pressure) isopleth patterns within the upper plate SNB reflect a comparable dip to the tilted crustal section. The implication here is that the level of mechanical delamination of the lower batholithic crust by the Rand thrust approximated the base of the felsic batholithic layer over a very wide region. Denudation of the deep batholithic rocks during crustal tilting occurred primarily by the southward and westward escape of mid-to upper crustal detachment sheets (Malin et al, 1995; Wood and Saleeby, 1998). Mid-to upper crust disruption was also enhanced by syn-thrusting dextral displacement along the proto-kern Canyon fault which at its deepest exposed levels roots into the thrust system (Wood and Saleeby, 1998; Saleeby, in review). 16

17 We suggest that the Laramide mechanical delamination of the lower batholithic crust and underlying SBML was facilitated by the removal of GPR rocks that were the southward continuation of that which remained intact beneath the greater SNB to the north. In addition to structural continuity along the oblique crustal section from southern deep levels to northern shallow levels, the overall petrologic and isotopic character of the batholith is continuous, and REE data for the deep batholithic rocks are consistent with a garnet-rich residue in their source (Ross, 1989). Geochronologic data indicate that emplacement of the Rand schist beneath the unrooted batholith of the Tehachapi-Rand deformation belt began between 80 and 85 Ma (Silver and Nourse, 1986; Saleeby and R. W. Kistler, unpub. data). This age range also corresponds to the end of magmatism throughout the entire SNB (Stern et al, 1981; Chen and Moore, 1982; Saleeby et al, 1987). The Rand schist is recognized as one member of a family of similar schists which tectonically underlie Cordilleran batholithic rocks at comparable structural levels throughout the southern California region (Jacobson et al, 1996). Age constraints on the emplacement of these schists coincide with the latest Cretaceous-early Paleogene age of the Laramide orogeny (Silver and Nourse, 1986; Jacobson, 1990; Jacobson et al, 2000). It thus follows that the Laramide slab did subduct at a low angle in the southern California region, which resulted in the mechanical delamination of the SBML. How then did the Laramide event manifest in the greater SNB to the north? The termination of arc magmatism in the SNB at the onset of Laramide time is a welldocumented relation. It has been further suggested from the modeling of fission track data that the base of the central SNB was removed by Laramide flat slab subduction at a depth of between 35 and 50 km (Dumitru, 1990; Dumitru et al, 1991). This later assertion is clearly at odds with the xenolith data (Fig. 3). Furthermore, the fission track data basis for the model needs reevaluation in the light of more recent He dating performed in the same region (House et al, 1997, 2001). Most notably are the effects of cooling by early unroofing with the development of high relief topography as shown by the He data. The key element in Dumitru s (1990) model is conductive cooling of the Sierran lithosphere from beneath by the Laramide slab. In spite of our rejection of Dumitru s (1990) conclusion that the Laramide slab removed the SBML from beneath the central SNB, the xenolith data appears to reflect such a conductive cooling event. As shown in Figure 2 the peridotite section beneath the GPR shows a pronounced cooling trajectory 17

18 from an array that resembles the wet fertile peridotite solidus to an array that corresponds to a cool SCLM geotherm. We interpret this event as a result of Laramide flat slab conductive cooling from beneath. A recent analysis of flat slab geometries reveal segmentations into shallow and deep flat slab domains (Gutscher et al, 2000). We suggest that the greater SNB rested above a deep (~125 km) flat slab segment during Laramide time, in which case the conductive cooling model used by Dumitru (1990) and Dumitru et al (1991) are applicable to upper crustal levels at a 100 m.y. time scale. We now turn our attention to the early Laramide termination in SNB magmatism. The Cretaceous SNB developed by copious arc magmatism over a time interval of ~ 50 m.y. (Evernden and Kistler, 1970; Saleeby and Sharp, 1980; Stern et al, 1981; Chen and Moore, 1982; Saleeby et al, 1987, 1990) The abundance of mafic batholithic rocks along the west margin of the batholith, their ubiquitous occurrence throughout the batholith, and isotopic data throughout the batholith attest to the importance of the sub-batholithic mantle wedge in magma genesis (ie, Kistler and Peterman, 1973; Saleeby and Sharp, 1980; De Paolo, 1981; Saleeby et al, 1987; Kistler, 1990; Sisson et al, 1996; Coleman and Glazner, 1998). Maintaining a fertile mantle wedge for 10 s of m. y. of magma genesis requires an influx of fertile mantle into the wedge environment during subduction. As discussed above this presumably occurs by corner flow during which fertile asthenosphere is drawn into the mantle wedge environment; a flow pattern which arises from dynamic relations that extend down-dip from the magma source region (Moffatt, 1964; Billen and Gurnis, 2002). We suggest that the cessation of SNB magmatism was indeed caused by Laramide flat slab subduction, but for the greater SNB region the causative process was disruption of the corner flow pattern. Recent geodynamic modeling (Billen and Gurnis, 2001) suggests that as the subducting slab flattens, presumably due to the buoyancy effects of the impingement of younger oceanic lithosphere, the mantle wedge becomes stronger and more viscous. Upper and lower plate coupling increase and corner flow ceases. In the case of Laramide orogeny the result for the greater SNB was the termination of magmatism followed by conductive cooling from beneath by the slab. The modern Andean subduction-arc system is commonly cited as an actualistic model for the Laramide tectonics of SW North America. An outstanding feature of the Andean system is the segmentation of the downgoing slab into normal, moderate and shallow-dipping domains (Barazangic and Isacks, 1976; Gutscher et al, 2000). As suggested by Malin et al (1995) and 18

19 Saleeby (in review) we assert that the contrast in deep crustal structure between the greater SNB and the southernmost SNB and adjacent Mojave Desert reflects a segmentation in the Laramide slab. Imaging of the sub-andean slab by Gutscher et al (2000) shows segments as shallow as ~ 50 km, and as deep as ~120 km. Such segmentation, or warping of the Laramide slab under the southern Sierra Nevada into two such domains could render the along strike variations in deep crust-upper mantle structure observed between the greater SNB and the southernmost Sierra- Mojave region. Whether the 8-10 kb level of Laramide lower crust delamination and schist emplacement in the southern segment represent the slab interface, or the accumulated affect of tectonic erosion by the slab and underplating of oceanic materials is uncertain. In this regard, the Rand thrust as shown on Figure 5, dipping northward beneath the SNB represents the remnants of the slab segmentation structure. It resembles a large lateral ramp in the Laramide subduction thrust system. This segmentation structure left the GPR and underlying mantle wedge of the greater SNB intact throughout Laramide time during the mechanical delamination of equivalent rocks to the south. Late Neogene Convective Removal in the Greater Sierra Nevada Batholith Region The mid-miocene and Pliocene-Quaternary xenolith suites contrast profoundly in composition, petrogenesis and physical conditions of equilibration (Fig. 2). Most notable is the lack of any GPR samples in the Pliocene-Quaternary suite. The geographic region carrying the younger suite is to the east of that of the older suite, yet within the confines of the SNB, mainly its eastern domain (Fig. 1). Age, isotopic and REE data referenced above for the eastern domain are in accord with GPR rocks existing beneath it in the past. The contrasts between the two xenolith suites are too extreme and consistent to be attributed to accidental sampling bias. Another contrast between the two suites is the presence of common glass inclusions in the Pliocene-Quaternary suite, which are lacking in the mid-miocene suite except for one important exception that is discussed below. Furthermore, all attempts to date the younger suite yield virtually modern ages (Ducea, 1998). These findings further support the modern asthenospheric origin for the younger peridotites as suggested by the P-T trajectory shown on Figure 2. Figure 6 summarizes the results of some recent geophysical studies in the southern Sierra Nevada region. Regional refraction studies show a crustal thickness of 30 to 35 km across the main part of the SNB, as exposed at 1 to 3 kb levels, and westward thickening to an ~40 km deep 19

20 sag beneath the western domain of the SNB (Ruppert et al, 1998; Fliedner et al, 2000). The refraction studies also show Vp of ~ 6 km/sec extending down to the Moho under the main part of the SNB, which is consistent with the oblique section exposures indicating a felsic batholith to ~35 km depths. Velocities as high as 6.4 km/sec are resolved beneath the western mafic domain. The refraction data also reveal low Pn velocities and along with regional passive experiments indicate a lack of a mantle lithosphere lid along the eastern Sierra-Owens Valley region (Jones and Phinney, 1988; Ji and Helmberger, 1999; Melborne and Helmberger, 2001). Geophysical data as well as observations of glass inclusions in Pliocene-Quaternary xenoliths reveal the presence of partial melt in the shallow upper mantle of the region (Ducea and Saleeby, 1996; Park et al, 1996; Jones and Phinney, 1998). The modern lithosphere-asthenosphere boundary virtually corresponds to the Moho beneath much of the southern Sierra Nevada region. Jones and Phinney (1988) further discuss evidence for a tectonized and lithologically interlayered Moho in this region which is consistent with the occurrence of highly tectonized felsic batholithic rocks in the Pliocene-Quaternary xenolith suite (Ducea and Saleeby, 1996). Teleseismic data further reveal a high-velocity mass within the upper mantle beneath the western Sierra (Zandt and Carrigan, 1993; Jones et al, 1994; Ruppert et al, 1998). Jones et al (1994) refer to this mass as a mantle drip, a term that we adopt and which implies formation by convective instability. According to the tomogram published in Ruppert et al, (1998), the drip extends downwards from near Moho depths and attains its strongest definition between 100 and 200 km depths. The velocity contrast between much of the drip and the adjacent mantle is sufficiently high to warrant its interpretation as eclogitic. The drip is separated from the Sierran crust by a lens of low Pn mantle, and is bounded to the east by a shaft-like domain of low velocity mantle which merges with the low Pn mantle rocks beneath the Sierran Moho (Fig. 2). In the analysis that follows we consider the mantle drip to be the remnants of the convectively removed SBML. Calculated densities of Sierran garnet pyroxenite xenoliths are on average 0.2 g/cc denser than garnet and spinel peridotites over a pressure range extending up to 10 GPa (Ducea and Saleeby, 1998a). Such a density contrast is undoubtedly greater for peridotites containing inclusions of melt. We assert that a minimum 35 km thick layer of GPR would be gravitationally unstable relative to the underlying lithospheric perodotite, as well as the deeper asthenosphere, 20

21 and should ultimately detach and sink into the mantle. Such a density contrast is significantly greater than those compiled by O Reilly et al (2001) for lithospheric peridotites of vastly different ages and for shallow asthenosphere peridotites; contrasts implying the gravitational instability of cooled Phanerozoic lithosphere. Our analysis of the xenolith data leads us to conclude that nearly half of the SBML consisted of GPR rocks, and the remainder of relatively non-depleted garnet peridotite. It follows that the SBML was highly susceptible to detachment and sinking relative to the underlying asthenosphere. We must ask how did the high-density GPR and the rest of the SBML survive through much of Cenozoic time, and what triggered its removal between mid-miocene and Pliocene time? In a recent geodynamic analysis of convective instabilities in the lower crust Jull and Kelemen (2001) argue for high Moho temperatures, and/or high regional strain rates for the initiation of such detachment processes. We suggest that conductive cooling of SBML by the Laramide slab and the ensuing benign tectonic setting through mid-cenozoic time left the SBML in a gravitationally metastable state. We further suggest that subsequent high-magnitude extension, that is well documented immediately east of the southern SNB, was linked to the regional strain rate field that triggered the dynamics of the instability. Such extension progressed and accelerated from mid-miocene to Pliocene time (Snow and Wernicke, 2000). Critical to this analysis is the interpretation set forth by Jones et al (1994), and Jones and Phinney (1998) that high magnetude extension in the adjacent Basin and Range was kinematically linked to the eastern Sierra via a deep crustal shear zone (Fig. 6). High velocity rocks along the lower plate of the shear zone are suggested to have in part been extended by the emplacement of Pliocene- Quaternary mafic intrusives derived from ascending asthenosphere. The time interval of this high magnetude extension also corresponds to a change in the volcanism of the region to more primitive compositions (Ducea and Saleeby, 1998a). A distinct pulse of mafic potassic volcanism concentrated at 3 to 4 Ma has been suggested by Manley et al (2000) to represent decompression partial melting of mantle rocks rising along an adiabat while replacing delaminated SBML of the Sierra Nevada. The rate at which the SBML drip descended, the critical length scales, and the dynamic response of the ascending asthenosphere and residual crust are poorly understood. Most models of convective instability of the lower crust-upper mantle suggest significant dynamic responses 21

22 at 10 m.y. time scales. We can offer some possible constraints on the temporal relations for the Sierra Nevada case. One late Miocene xenolith location in the central Sierra (8.6 Ma, Manley et al, 2000) is aberrant in that it contains primarily high temperature spinel peridotites and is lacking GPR lithologies. This, as well as spinel peridotite xenoliths from a Pliocene location contain silicic glass inclusions with trace element characteristics suggestive of derivation from partially melted labile accessory phases from GPR rocks (Ducea and Saleeby, 1998a). The implications are that GPR detachment locally began by 8.6 Ma, and if the analysis of Pliocene volcanism for the region offered by Manley et al (2000) is accepted, a broad domain of asthenospheric mantle had ascended to the base of the felsic crust by 3 to 4 Ma. Thus in latest Miocene time we envisage an upper mantle setting beneath the eastern and axial SNB similar to that discribed for the eastern Sino-Korean craton (Yuan, 1996) with the lithosphere variably invaded by asthenosphere. By mid-pliocene time the SBML of the Sierra had for the most part been replaced by asthenosphere. A rough comparison of the centroid of the in situ SBML (~75 km depth, Fig. 6), to the centroid of the highest velocity core of the mantle drip (200 km depth) analyzed in the above temporal context leads to a hypothetical descent rate for the SBLM of ~ 25mm/yr, comparable to plate tectonic transport rates. Another aspect of the drip dynamics that we take the liberty of diagrammatically depicting in Figure 6 is the more rapid descent of the GPR layer as compaired to the underlying wedge peridotite section. The penetration of the GPR drip core into its peridotite envelope yield a density structure that resembles the tomogram published by Ruppert et al (1998). The westward offset of the mantle drip from the axial region of the SNB as well as its overall morphology pose some interesting possibilities. Modeling of convective instabilities (cf. Jull and Keleman, 2001) suggest that morphologic pertubations in the unstable layer can instigate and concentrate the growth of a drip-like structure. We suggest that the overall more mafic composition of the western domain of the SNB including its SBML may have served as such a pertubation, resulting in the net westward flow of the GPR and its ultimate dripping through and dragging along its underlying peridotite section. The deep felsic crust concievably experienced a traction from this flow regime and responded by shear flow thickening to the observed sag above the point of drip gathering and ultimate detachment. The tectonized Moho resolved by Jones and Phinney (1998) beneath the eastern and axial Sierra may reflect the source area for deep felsic 22

23 material that has moved westward into the sag. The surface reponse to this dynamic pattern was the maintenance, and possible further subsidence, of low elevations in the southwest Sierra and adjacent Great Valley. A wave of geologically recent uplift followed in the wake of the westward flowing and descending SBML as asthenosphere ascended beneath the residual crust of the eastern and axial Sierra. As this process progressed it is suggested that the SBML drip was deflected southwestward by an asthenosphere counterflow pattern similar to that which has been modeled by Chase (1979). Such a flow trajectory is also consistent with the regional mantle seismic anisotropy with fast directions oriented generally E-W (Fliedner et al, 2000). Finally, out of section effects should be noted. Most accounts of the sub-sierran drip structure recognize it as having a cylindrical shape. Perhaps the drip gathered with along strike as well as across strike components of flow, relative to the regional strike of the SNB. In this regard it should be pointed out that the map location of the drip structure corresponds to a broad embayment of the active Great Valley sedimentary basin into the crystalline rock exposures of the western Sierra Nevada (Fig. 1). The asthenospheric mantle that has replaced the SBML has a number of features suggesting that it has been locally hybridized or contaminated by lithospheric remnants. Jones and Phinney (1998) point out the physical and likely compositioned heterogeneity of the shallow asthenospheric mantle under the eastern Sierra region. Nevertheless, the P-T trajectory shown in Figure 2 for Pliocene-Quaternary mantle xenoliths lie along an asthenosphere adiabat (Ducea and Saleeby, 1996). However, Lee et al (2000) report a few additional samples from one of the Quaternary locations whose P-T data are displaced slightly to the low T side of the adiabat suggesting that asthenosphere ascent was not purely adiabatic, and/or there was entrainment of pre-existing lithospheric peridotite. Such complexities are further suggested by geochemical data. Nd, Sr and Os isotopic data on the Pliocene-Quaternary mantle xenoliths show wide isotopic heterogeneity ranging from modern asthenospheric to Proterozoic SCLM values (Beard and Glazner, 1995; Lee et al, 2000; Zeng et al, 2001a). Likewise Pliocene-Quaternary basalts erupted in the region reveal such a heterogeneous mantle source regime (Van Kooten, 1981; Menzies et al, 1983; Beard and Glazner, 1995; Ducea, 1998). We assert, however, that the primofacto definition of asthenosphere lies in its physical state which is clearly defined for the Pliocene to Recent upper mantle beneath the southern SNB region by the asthenospheric adiabat 23

24 given on Figure 2, and the seismic velocity studies (Ruppert et al, 1998; Jones and Phinney, 1998; Ji and Helmberger, 1999; Fliedner et al, 2001; Melbourne and Helmberger, 2001). In terms of its chemical heterogeneity we assert that there is an array of possible processes that may have worked together, or independently, to render the observed patterns. These include: 1) the entrainment and commingling of displaced SBML; 2) metasomatism by partial melts of labile accessory phases of the displaced GPR; and 3) metasomatism by melts and/or fluids derived from the remnants of sediments that were subducted during Mesozoic or early Cenozoic time. On Figure 6 we diagrammatically depict the entrainment of wedge peridotite enclaves into the ascending asthenosphere. DISCUSSION The analysis offered above for late Cenozoic volcanic-hosted xenoliths and geophysical data for the southern Sierra Nevada region, in conjunction with insights offered by the oblique crustal section offers a rich array of insights in topics ranging from the generation of the SNB and its lithospheric underpinnings to the Recent uplift of the range. We close with a discussion of a number of these topics. Arc Magmatism Direct observations of the oblique crustal section of the southern SNB indicate that the crust has been completely reconstituted by batholith production to depths of at least 30 km (Saleeby, 1990). Seismic studies performed across the shallow-level exposures to the north are consistent with this view (Jones and Phinney, 1998; Rupper et al, 1998; Fliedner et al, 2000). The xenolith data further indicate that the entire SBML has been reconstituted in conjunction with batholith production (Figs. 2 and 3). Nowhere in our sampling of the SNB lithospheric column is there evidence of a regionally extensive high-grade metamorphic terrane underlying the felsic batholith. Even the subordinate granulites are best explained as residues and/or cumulates produced by crystal-liquid equilibria related to batholith formation. Such large-scale reconstitution of a young continent-like lithospheric section raises questions regarding the relative proportions of juvenile mantle-derived components verses recycled continental components. 24

25 Isotopic studies of the SNB typically call for subequal amounts of juvenile mantlederived material and recycled crustal material for batholith production (Kistler and Peterman, 1973; DePaolo, 1981; Saleeby et al, 1987; Kistler, 1990; Chen and Tilton, 1991). The involvement of enriched SCLM in the mantle wedge source reduces the relative amount of crustal recycling needed to satisfy the isotopic data (Coleman et al, 1992; Sisson et al, 1996; Coleman and Glazner, 1998). However, Sr-Nd and Pb-Pb isotopic variation patterns for the felsic batholith and GPR indicate a non-trivial component of lower continental crustal materials in the axial and eastern domains (Chen and Tilton, 1991: Ducea and Saleeby, 1998b; Ducea, 2001; Zeng et al, 2001b). Furthermore, Sr-O isotopic variation patterns for parts of both the axial and adjacent transitional western domain indicate a nontrivial component of continent-derived sediment in the material budget (DePaolo, 1981; Saleeby et al 1987; Kistler, 1990; Clemens- Knott, 1992). Thus for the felsic SNB there can be little question that recycled continental crustal components were an important part of the material budget. In terms of the bulk composition of the axial and eastern domains of the SNB, the reconstructed lithospheric section of Figure 3 in conjunction with elemental abundance data offer some constraints. Table 1 shows averaged major element abundances of Sierran garnet clinopyroxenite xenoliths (Ducea, 1998), and granitoids of the central SNB (Bateman and Dodge, 1970). Inspection of the Figure 3 lithospheric section reveals subequal amounts of the GPR and overlying felsic batholith. The computed average of such a 1:1 mixture yields a high-mg basaltic-andesite. A slightly more mafic bulk composition is conceivable by factoring in the subordinate granulite layer in the mixture, and by considering the possible early-stage loss of GPR drips into and through the underlying peridotitic wedge (Fig. 6). If we consider the entire Cretaceous SNB, however, the bulk composition becomes substantially more mafic. The western domain of the SNB, including a substantial tract of plutonic rocks known only from basement cores and geophysical data beneath the Great Valley (Saleeby and Williams,1978; Saleeby, 1981; Oliver and Robbins, 1982) is routinely ignored in the literature when various workers consider the mass budget of the SNB. Between Latitudes 36º and 37º N there is a well-exposed belt of intrusives that are typical of those found beneath the Valley (westernmost domain) whose structure and petrology offer insights into Early Cretaceous SNB magmatism. Here a series of well-exposed ring dike complexes that developed over a ~ 5 25

26 m.y. time intervals show similar petrogenitic patterns which are typical of the entire exposure belt, which may contain remnants of other disrupted ring dike complexes (Mack et al, 1979; Saleeby and Sharp, 1980: Clemens-Knott and Saleeby, 1999). Each starts with the emplacement of voluminous olivine-bearing hydrous mafic and ultramafic cumulates followed by well defined crescent to ring-shaped dikes of norite and pyroxene diorite-tonalite. They appear to be the roots of long-lived basalt-andesite volcanic centers (cf. Clemens-Knott and Saleeby, 1999; Clemens- Knott et al, 2000). Sedimentary petrofacies data for the Lower Cretaceous interval of the Great Valley forearc sequence reveal copious detritus derived form such a mafic arc (Dickinson and Rich, 1972). The Early Cretaceous SNB was primarily mafic, similar to intro-oceanic arcs, with the principal controlling factors being its generation within a depleted mantle source regime (Kistler and Peterman, 1973; Saleeby and Sharp, 1980; Clemens-Knott et al, 1991; Clemens- Knott, 1992), and its ascent through oceanic lithosphere that had been sutured against North American lithosphere and trapped above the Mesozoic subduction zone (Saleeby, 1981). The western extent of such mafic SNB rocks beneath the Great Valley is unknown. Such rocks are abundantly represented in basement cores from the axial regions of the northern Great Valley where they occur in association with gravity-magnetic anomalies (Saleeby and Williams, 1978), and in the southwest Sierra Foothills they are shown to be the principal source of such anomalies (Oliver and Robbins, 1982). High-velocity mid-crustal rocks that extend across the axial southern Great Valley could be in large part composed of such rocks (Fig. 6). Recognizing the basaltic composition of the western SNB is not only important for considering the bulk composition of the entire batholith and its material budget, but also in recognizing the broader significance of virtually all the principal components of the mafic domain occurring also in small proportions at all observed structural levels of the felsic batholith to the east. We assert that copious mantle wedge-derived hydrous mafic magmatism was the principal material and energy source for the entire SNB. The eastern domain of the wedge environment probably contained remnants of ancient SCLM, but for the most part the wedge was kept fertile by the influx of asthenospheric mantle induced by corner flow circulation over the subducting slab (Moffatt, 1964; Billen and Gurnis, 2002), and by fluids liberated from the subducting slab (cf. Elliott et al, 1997). The high temperature history of the SBML peridotite xenoliths suggest initial setting of the thermobarometers along the water saturated peridotite 26

27 solidus (Fig. 2). We interpret this as the mark of water saturated partial melting in the wedge source for the mafic magmas. The P-T data array for the GPR likewise indicate water saturated or near saturated melting in the higher-level polygenetic source for the felsic batholith. In the west the wedge-derived magmas passed through accreted oceanic lithosphere, and for the most part built basalt-andesite volcanic centers and underlying ultramafic-mafic plutonic complexes. To the east these magmas encountered progressively more continental lithosphere and crust, both as the result of the ambient material east of the pre-batholithic suture, and as a result of the westward rooting of the Foreland thrust belt. Experimental and theoretical data presented above imply that GPR production in the east entailed higher felsic melt fractions than to the west, as a function of higher proportions of felsic materials in the lower crust source regime. In the west metamorphosed peridotite, gabbro and basalt with subordinate turbidites and pelagic deposits were the principal host rocks for the ascending mafic magmas. The net result was the production of a much more mafic primary lithospheric column for the western SNB than that to the east. Gabbronorite and hydrous ultramafic-mafic cumulates are common constituents of the western domain of the SNB (Mack et al, 1979; Saleeby and Sharp, 1980; Clemens-Knott, 1992; Clemens-Knott and Saleeby, 1999). Jull and Keleman (2001) have discussed the potential for the convective instability of a thick accumulation of such arc rocks relative to peridotitic mantle rocks. In their analysis they did not factor in the potential for deep-level remelting of such rocks with the production of garnet-rich residues. They do discuss the solid-state conversion of deeplevel accumulations of such rocks to eclogite. However, the course grain size of such deepgabbroids, a limited fluid budget, a declining thermal gradient, as well as likely low strain rates would work against pervassive eclogite conversion in such an environment (cf. Hacker, 1996; Austheim, 1998; Leech 2001). The felsic products of dehydration melting of such continent component-absent hydrous gabbroids are well represented by the belt of Early Cretaceous tonalitic plutons which constitute much of the currently exposed western domain. Such rocks probably render the felsic crust seismic velocities detected for some of the deeper parts of the western domain (Ruppert et al, 1998: Fliedner et al, 2000). We thus consider the western domain of the SNB to have carried the highest integrated crustal density and the highest SBML density of the entire SNB lithospheric section. We feel that this explains why the convective removal of the entire SBML instigated beneath the western Sierra region (Fig. 6). 27

28 Loss of the Lower Crust Considerable discussion has been focused on delamination or convective removal of the SCLM (Bird, 1979, 1988; Conrad and Molnar, 1997; Houseman and Molnar, 1997), and there seems to be a consensus that the Phanerozoic SCLM in particular is susceptible to such attrition in geologic time (O Reilly et al, 2001). The mechanisms and importance of lower crustal loss seems to have gained less of a consensus (Dewey et al, 1993; Kay and Kay, 1991, 1993; Lee et al, 2000; Jull and Kelemen, 2001; Leech, 2001). Most discussions of lower crustal loss focus on the transition of crustal rocks to eclogite facies, particularly in collisional orogenic belts. Concerns have been raised over the extent of the eclogite conversion due to sufficient fluid activities, kinetic effects of protolith grain size distributions, and the involvement of retrograde reactions to lower density assemblages. Overstepping of eclogite facies reaction boundaries in exhumed ultra-high pressure metamorphic terranes of as much as 1 GPa have been documented as well (Austrheim, 1998). We have little to offer that bears directly on these concerns. We do assert, however, that we have provided evidence for a lower crustal removal mechanism that is significantly different from the mechanisms referred to above. The production of the GPR in the source regime of a long-lived magmatic arc is conceivably a much more effective mechanism for the production of a high density crustal root than the conversion of crustal rock sequences to eclogite facies assemblages by crustal thickening. The segregation of garnet + pyroxene residues from felsic magmas by crystal-liquid equilibria is a much more effective mechanism for producing very high density crustal affinity rocks than the eclogite solid state transition. Segregation and migration of the felsic melt upwards is conceivably a near complete process, which is consistent with our GPR xenolith observations of a lack of felsic inclusions. Furthermore, the dilution of the hydrous mafic source by pre-existing felsic crustal components results in higher felsic melt fractions with the same high-density residue assemblage, at given pressure conditions. This is born out in the types of theoretical calculations shown in Figure 4 as well as by experimental data (Carroll and Wyllie, 1990). These observations and considerations are in sharp contrast to what is observed and what is to be expected in orogenically thickened continental crust. In such settings a diverse suite of protolith assemblages are introduced into the deep crustal metamorphic environment thereby producing an array of densities for which only the mafic endmember metamorphic products could match the high-density GPR assemblage. The 28

29 aforementioned concerns of complete eclogite transformation further undermine the density increase upon tectonic thickening. Finally, the crystal-liquid fractionation process of GPR production differentiates the crust into a well-defined upper felsic batholithic layer and an underlying high-density GPR layer. Orogenic thickening will be less effective in its mechanical segregation, and in conjunction with the varied protolith and incomplete transformation effects will yield a broad integrated gradient in increasing lower crustal densities with possible density inversions packaged in. We reiterate that the removal of the GPR and underlying peridotite section was at essentially the same structural level by both mechanical delamination during Laramide flat slab subduction in the southernmost Sierra, and by convective instability during the onset of late Neogene Basin and Range extension in the greater SNB region to the north. We suggest that the pronounced depth-dependent density contrast between the GPR and the overlying felsic batholith in conjunction with the greater strength of the garnet+pyroxene aggregate (Ji and Martingnole, 1994; Kolle and Blacic, 1983) as compared to the probable fluid present deep felsic crust (Tullis and Yund, 1977) provided an ideal separation horizon upon the onset of suitable superimposed strain rates. The reconstructed SNB lithospheric column of Figure 3 shows a pronounced transition between 30 and 40 km depth from felsic batholithic assemblages through mafic granulites to the GPR. Christensen and Mooney (1995) report that most inactive Phanerozoic continental arcs exhibit a typical 30 to 40 km depth to the Moho. We think that this can be explained with the simple phase relations exhibited in Figure 2 by both the experimental data and the xenolith thermobarometric data. For a section of batholithic crust and underlying mantle that remains intact and not deeply exhumed the plagioclase out boundary in the residue sequence will correspond to a seismic Moho at a depth of between 30 to 40 km. Separation of the underlying mantle lithosphere will fall in this same 30 to 40 km depth interval during disruption by extensional or thrust tectonics or where thermal conditions drive convective instabilities. Such localization of separaton accentuates the fractionation and survival of large-volume felsic batholiths through geologic time. Sierran Uplift 29

30 The long-standing interpretation for the support of the 3000 to 4000 meter high Sierra has been the existence of a low-density Airy crustal root for the batholith (Bateman and Eaton, 1967). Phase relationships for the production of felsic batholiths, xenolith data and recent geophysical studies indicate that this interpretation is incorrect (Figs. 2, 3 and 6). A contrast of ~ 0.2 g/cc is representative of the approximate density difference between the GPR and typical mantle peridotite. Using a mantle Pratt compensation model for support of the modern range (cf. Jones et al, 1994), the convective replacement of the ~ 35 km thick GPR and its underlying peridotitic lithosphere by asthenospheric mantle would yield an uplift response of ~ 3000 m. The late Miocene to Pliocene age suggested for the loss of the GPR corresponds in time with the onset of the current phase of Sierran uplift (Huber, 1981). The dynamic response of Sierran uplift to the loss of its SBML and the ascent of asthenospheric mantle are at a 5 m.y. time scale. In contrast conductive cooling of continental lithosphere occurs at a 100 m. y. time scale. Heat flow measurements for the Sierra Nevada are at the extreme low range for continental regions ( Saltus and Lachenbruch, 1991). It is suggested that the modern surface heat flow signal is that of the Laramide slab cooling event that is recorded in the cooling trajectories of the now removed SBML (Fig. 2). Furthermore, seismicity in the southern Sierra Nevada extends nearly to the Moho suggesting a relatively cool, brittle felsic crustal section (Wong and Savage, 1983; Miller and Mooney, 1995). This suggests that the thermal signal from the Pliocene SBML removal event has yet to propagate very far into the deep Sierran crust. This is consistent with the two different time scales at which convective removal dynamics verses heat conduction work. CONCLUSIONS Recent experimental studies on the melting of hydrous mafic to intermediate composition assemblages in the 10 to 30 kb range indicate that felsic magma production renders a residue rich in garnet and clinopyroxene. Isotopic and petrogenetic studies of garnet clinopyroxenite xenoliths entrained in mid-miocene volcanic centers from the central SNB indicate that they are coeval and cogenetic with the overlying felsic batholith, and that they represent residues left from the partial melting of such hydrous assemblages. The deepest-level exposures of the SNB are part of a southward deepening oblique crustal section which reaches ~10 kb conditions. The SNB is structurally continuous and dominately felsic to these deep levels, and its structure 30

31 indicates complete reconstitution of the crust by composite batholith formation. Within the deeplevel exposures a number of hydrous mafic cumulate bodies exhibit local remelting with the production of tonalitic melts and garnet rich residues. Furthermore, such hydrous mafic cumulates are a ubiquitous part of the batholith, at all structural levels, in various proportions. From these observations, it is suggested that large volume felsic magmas of the SNB, and conceivably other Cordilleran-type batholithic belts, are produced by a two-stage process initiating with mantle wedge-derived hydrous mafic magmatism above subducting oceanic lithosphere. As hydrous mafic cumulates form and thicken at lower crust-upper mantle levels, along with enclaves of pre-existing crustal rocks, they ultimately undergo remelting to generate felsic batholithic magmas and a garnet clinopyroxenite residue. Garnet clinopyroxenite xenolith thermobarometric data suggest that a ~ 35 km thick residue root formed beneath the SNB. This root is petrogenetically crustal rock but seismically mantle rock. Upon cooling the high-density root formed the upper 45 km of SBML section which was floored by mantle wedge peridotite. The peridotites rendered xenoliths with equilibration pressures corresponding to depths as much as ~125 km. The mantle xenolith data along with the oblique crutal section data further reveal that the entire lithosphere of the Sierra Nevada was reconstituted in conjunction with batholith production. Garnet clinopyroxenite constituted a significant component of the Sierran SBML both as a concentrated upper layer and as enclaves in the underlying wedge peridotite section. These garnet-rich rocks are on the order of 0.2 g/cc denser than typical mantle peridotite. Consequently their high concentration in the SBML rendered it gravitationally unstable relative to asthenospheric mantle. At least two SBML removal events occurred in the southern Sierra Nevada region. First, in early Laramide orogeny time (latest Cretaceous) it was mechanically delaminated in the southernmost Sierra and adjacent Mojave Desert region by shallow-level flat slab subduction. Then in late Neogene time, in the greater Sierra Nevada to the north it was convectively removed in association with high-magnitude Basin and Range extension immediately to the east. The SBML was replaced by underthrust oceanic assemblages during Laramide delamination in the south, and by asthenospheric mantle during late Neogene convective removal to the north. 31

32 Garnet pyroxenite and deeper level peridotites of the SBML survived in the central Sierra Nevada region until their mid-miocene entrainment in their host lavas. This in conjunction with the removal of such rocks further south by Laramide tectonics suggests that the Laramide slab was segmented into a shallow segment to the south and a deeper segment to the north. The main expression of Laramide flat slab subduction to the north appears to be the termination of arc magmatism and conductive cooling of the subarc mantle wedge. It is suggested that the flat slab disrupted the corner flow pattern which kept the wedge environment fertile, either mechanically and/or by cooling the wedge and increasing its viscosity. Termination of batholithic magmatism over the shallow slab segment to the south entailed the mechanical disruption and removal of the entire magma source regime. The transition from felsic batholithic materials to high density residues between 35 and 45 km depths provides a primary rheological boundary for delamination to occur along. Both SBML removal events in the southern Sierra Nevada region occurred along this boundary despite the constrasting attendant tectonic environments. Many batholithic belts around the world render 30 to 40 km crustal thickness. It is suggested that the processes described above for the production and loss of the high density residue for the SNB are globally important for continent edge arcs, and that such processes working over geologic time both chemically and mechanically frationate and isolate felsic crust at the earth s surface. The sinking of garnet-rich residues from deep crustal magma chambers into the mantle also provides a mechanism for returning crustal chemical components and isotopic signitures to the mantle. ACKNOWLEDGMENTS Support for this work was provided by NSF grants EAR , EAR , and EAR (Saleeby) and EAR (Ducea). Discussions and field excursions with L.T. Silver, R. W. Kistler, D.L. Anderson, M. Gurnis, D. Helmberger, C.T. Lee, M.B. Wolf, P.J. Wyllie, G. Zandt, L. Zeng,, and the entire Southern Sierra Continental Dynamics Project working group helped stimulate this work. Drafting and technical assistance from Zorka Foster is gratefully acknowledged. 32

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