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

Saleeby, J.*Division of Geological and Planetary Sciences, California Institute of Technology, Mail Stop100-23, Pasadena, CA 91125

Ducea, M.Department of Geosciences, University of Arizona, 1040 E. Fourth St., Tucson, AZ 85721

Clemens-Knott, D.Department of Geological Sciences, California State University, Fullerton, CA 92634

*Email: 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

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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 Sierra-

northern 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

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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”.

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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 sub-

batholithic 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 long-

term 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

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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

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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 low-

velocity felsic crust extending down to 30-35 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 garnet-

bearing lithologies being characterized more by spinel and plagioclase peridotites. We will focus

first on the mid-Miocene group.

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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) CNd, 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

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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

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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

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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 mixing-

assimilation-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

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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 Sri as high as 0.708, and ∈nd as low as –10.5. 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

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(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 melt-

garnet rich residue sequences is the underplating and thickening of such cumulates as wedge-

derived 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

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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. High-

grade 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 east-

directed 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

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

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

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).

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 well-

documented 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 re-

evaluation 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

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

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

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,

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

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

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

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.

25

Isotopic studies of the SNB typically call for subequal amounts of juvenile mantle-

derived 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

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

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 deep-

level accumulations of such rocks to eclogite. However, the course grain size of such deep-

gabbroids, 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).

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

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

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

31

indicates complete reconstitution of the crust by composite batholith formation. Within the deep-

level 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.

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-9526859, EAR-9815024, and

EAR-0087347 (Saleeby) and EAR-0087125 (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.

33

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