Segmentation of the Laramide Slab—evidence from the ...

For permission to copy, contact [email protected] 2003 Geological Society of America 655

GSA Bulletin; June 2003; v. 115; no. 6; p. 655–668; 4 figures.

Segmentation of the Laramide Slab—evidence from the southernSierra Nevada region

Jason Saleeby†

Division of Geological and Planetary Sciences, California Institute of Technology, M.S. 100-23, Pasadena, California 91125,USA

ABSTRACT

During the latest Cretaceous-early Paleo-gene Laramide orogeny, the lithosphere be-neath the southernmost Sierra Nevadabatholith and the adjacent Mojave Desertregion batholith was sheared off and dis-placed deeper into the mantle. The litho-sphere beneath the greater Sierra Nevadabatholith to the north was left intact untilmid-Miocene time, when fragments of itwere entrained as volcanic xenoliths. TheLaramide slab was evidently segmentedinto a shallow flat segment to the south anda deeper segment to the north. Shearing offof the upper mantle to the south was fol-lowed by the tectonic underplating ofschists derived from Franciscan subductioncomplex and possibly forearc basin proto-liths. The overlying batholithic crust wasdeformed, deeply denuded, and tectonicallybreached westward across its forearc re-gion while the schists were underplated.Westward breachment resulted from acombination of west-directed thrusting andextensional collapse. The westernmostbreached rocks were tectonically removedby a combination of trench linked trans-form and subduction erosion processes.Subsequent tectonic erosion by the NeogeneSan Andreas transform has left the Saliniaand the San Gabriel terranes as dispersedresidual fragments of the westwardbreached arc segment.

The shallow slab segment appears tohave been ;500 km in width, measuredalong the plate edge. To the south, the slabdescent appears to have remained deep be-neath the Peninsular Ranges batholith.Classic Laramide structures of the cratonare concentrated in a corridor that corre-sponds to the shallow slab segment as de-

†E-mail: [email protected].

fined by plate edge relations and the cor-responding trajectory of Farallon–NorthAmerican relative plate motions whenviewed on a pre-Neogene palinspastic base.The plate interior is suggested to have beendeformed first by end loading as the shal-low slab segment initially descended be-neath the plate edge, and then by greaterbasal traction components as the shallowsegment progressed beneath the cratonicregion. The subcontinental mantle litho-sphere beneath the cratonic deformationzone remained intact through Laramidetime.

Similar segmented slab topologies havebeen resolved beneath the modern Andeanorogen where the shallow segments are cor-related with the subduction of aseismicridges. A number of researchers have sug-gested that the Laramide orogeny arosefrom the subduction of a counterpart of theHess-Shatsky large igneous province of thenorthwest Pacific basin. Fragments of rockassemblages that are correlative to theHess-Shatsky province that were accretedto the Franciscan complex in Laramidetime support this view. Recognition of theresulting shallow slab segment and its tra-jectory beneath North America explains thegeographic focusing of Laramide deforma-tion, relative to the rest of the Cordilleranorogen, and the relationships between plateedge and plate interior deformationalregimes.

Keywords: tectonics, subduction, delami-nation, Cordillera, Laramide.

INTRODUCTION

The Laramide orogeny is recognized as aregional compressional event that deformedthe southwest North American craton in latestCretaceous-early Paleogene time (Miller et al.,

1992). A commonly cited plate tectonic mech-anism for the orogeny is intensified tractionand tectonic erosion of the subcontinentalmantle lithosphere due to flattening of the sub-ducted slab (Coney and Reynolds, 1977;Dickinson and Snyder, 1978; Bird, 1988). Theresponse of the craton was deformation anduplift along a north-northeast–trending corri-dor extending from southwest Arizonathrough Wyoming (Fig. 1). This intracratonaldeformation zone is for the most part inboardof the Cordilleran (Sevier) foreland fold-thrustbelt, thereby calling for special circumstancesrelative to much of Cordilleran tectonic his-tory. Slab flattening at Laramide time seem-ingly offers such a special circumstance, al-though the dynamics responsible for slabflattening are not well understood, nor is thereason for the geographic restriction of Lar-amide deformation relative to the much moreregionally extensive Cordilleran active mar-gin. Such a geographic focusing of Laramidedeformation has been suggested to have re-sulted from the subduction of a counterpart ofthe Hess-Shatsky large igneous province ofthe northwest Pacific basin, which was em-bedded in the Farallon plate (Livaccari et al.,1981; Henderson et al., 1984; Barth andSchneiderman, 1996). The analysis offeredhere finds much merit in this view, althoughthe main proposition presented here is notcontingent on this view. This proposition as-serts that the Laramide slab possessed a shal-low slab segment and that the subduction ofthis shallow segment can be correlated withboth plate edge and interior Laramidedeformation.

Geologic events along the southwest Cor-dilleran plate edge that have been attributed toLaramide tectonics include: 1) the cessation ofarc magmatism in the Sierra Nevada batholithand associated modification of the subbatho-lith geotherm by conductive cooling from be-low (Dumitru et al., 1991); 2) the low-angle

656 Geological Society of America Bulletin, June 2003

J. SALEEBY

Geological Society of America Bulletin, June 2003 657

SEGMENTATION OF THE LARAMIDE SLAB

Figure 2. Map of California region showing in generalized form a number of key tectonicand geographic features that are referred to in text. The San Andreas transform system,Transverse Ranges transrotational deformation, Neogene extension, and the eastern Cal-ifornia shear zone constitute main superposed deformations that were restored in theCalifornia region for the construction of the Figure 1 palinspastic base.

Figure 1. Map showing selected tectonic and basement features of the southwest Cordillera at the close of the Laramide orogeny on apre-Neogene palinspastic base. This map shows how the breached segment of the southwest Cordilleran batholithic belt, expressedprimarily by Salinia and the San Gabriel terrane, lies in a north-northeast–trending deformation corridor that includes classic cratonalstructures of the Laramide orogeny, and that this corridor is generally parallel to the Farallon–North American relative plate motionsfor Laramide time. The restored trace of the Garlock fault is used as a surface tracer for the position of the inflection in the slab. Thistracer and its north-northwest projection bound the northwest margin of the deformation corridor that is consistent with the slabsegmentation model for Laramide deformation. References given in text.N

thrust emplacement of Franciscan-affinitygreywacke-basalt assemblages and their meta-morphism in high-pressure greenschist–amphibolite facies (Rand-Pelona-Orocopiaschists); and 3) the spatially related deforma-tion, denudation, and westward breachment ofan ;500-km-long segment of the Cordilleranbatholith belt above the underplated schists(Silver, 1983; May, 1989; Jacobson et al.,1996; Malin et al., 1995; Barth and Schnei-derman, 1996). The disrupted segment of thebatholith belt corresponds to much of the Mo-jave Desert, Salinia and the San Gabriel ter-rane as restored to their pre-Neogene positionsalong the San Andreas fault, and the adjacentsouthern Colorado River Desert region. Thenorthern end of the disrupted segment of thebatholith belt coincides with a zone of deepdenudation of the southernmost Sierra Nevadabatholith that has rendered a structurally con-tinuous oblique crustal section (Saleeby, 1990;Pickett and Saleeby, 1993). The southern endof the disrupted segment of the batholith beltcorresponds to the northern Peninsular Rangesbatholith. For discussions below, it is usefulto symbolize these three segments of the bath-olith belt as: SNB 5 Sierra Nevada batholith;MSB 5 Mojave Desert–Salinia batholith, in-cluding the San Gabriel terrane and adjacentsouthern Colorado River Desert basementcomplexes; and PRB 5 Peninsular Rangesbatholith.

Insights regarding the deep structure of theSNB in Laramide time are augmented by pet-rologic and geochemical data on lower-crust–upper-mantle xenoliths that were entrained inmid-Miocene volcanic rocks from the SanJoaquin volcanic field, the location of whichis shown in Figure 2 (Dodge et al., 1986,1988; Ducea and Saleeby, 1996, 1998; Ducea,2001; Lee et al., 2000; Saleeby et al., 2002).Constraints on the lower-crust–upper-mantlestructure posed by the oblique crustal section,along with seismic and the xenolith data, to-gether carry important implications regardingthe topology and some of the effects of theLaramide slab. These implications and theirrelations to a spectrum of geologic featuresattributed to the Laramide orogeny are thesubject of this paper.

Much of the discussion will focus on Figure1, which is a reconstruction of the principaltectonic elements of the southwest U.S. regionat late Laramide time. Figure 2 shows someof these elements as they occur in their current

geological setting in California, as well as themajor Neogene tectonic systems that are re-stored on Figure 1. The key restorations are:1) Salinia and the San Gabriel terrane alongthe San Andreas transform system (Ross,


J. SALEEBY

Figure 3. Generalized cross section from the central Sierra Nevada region southward along oblique crustal section and across Tehachapi-Rand deformation belt into the northernmost Mojave Desert with the Rand Mountains complex restored to its pre-Neogene positionrelative to the Sierra Nevada. General location of section line shown in Figure 1. This section shows contrasting deep-crust–upper-mantle structures for central and southernmost Sierra Nevada region following Laramide orogeny, revealing a segmentation in thesubducted slab with the removal of the subbatholith mantle lithosphere and underplating of the Rand schist to the south. The subductiontrajectory of the slab was roughly at right angles to and directed into the cross-section trend. References given in text.

1984; May, 1989; Hall, 1991; Silver and Mat-tinson, 1986); 2) transrotational deformationof the Transverse Ranges (Hornafius et al.,1986); 3) displacement along the Garlock fault(Malin et al., 1995); and 4) transtensionalstrain across the Death Valley region (Snowand Wernicke, 2000). The compatibility ofthese restorations with the structure of theMojave Desert presents a problem, for whichthe answer is presumed to lie in the disputedkinematic patterns of Neogene extension anddistributed dextral shear along the Mojavesegment of the eastern California shear zone(cf. Dokka, 1986; Glazner et al., 1989, 1994;Dokka and Travis, 1990; Henry and Dokka,1992). Furthermore, it is asserted here thatLaramide tectonics left much of the MojaveDesert basement in a tectonically layeredstate, which in conjunction with only scatteredbasement exposures expressed today, greatlycomplicates the restoration of superposedNeogene deformation. The Figure 1 recon-struction also incorporates generalized pre-Cenozoic reconstructions of the greater Basinand Range province region (Coney andHarms, 1984). Major structures of the Lar-amide and Sevier deformation belts are shownafter Allmendinger (1992), Christiansen and

Yeats (1992), Miller et al. (1992), and Tickoffand Maxson (2001).

Figure 1 also shows the trace of a longitu-dinal cross section along the southernmostSNB as it extends southward across the Te-hachapi Range and the restored Rand Moun-tains of the northernmost Mojave Desert (Fig.3). This cross section displays the criticalstructural features of the Tehachapi-Rand de-formation belt, a complex structural systemthat lies along the southern Sierra-Mojave De-sert transition and that is directly related to thedenudation of the southern SNB oblique crust-al section.

CONTRASTING DEEP CRUST-UPPERMANTLE SECTIONS IN LATESTCRETACEOUS-PALEOGENE TIME

The southern ;100 km of the SNB exposesa southward-deepening oblique crustal sectionthrough Cretaceous batholith-generated crustfrom ;2 kb to ;9 kb conditions, or ;7 kmto ;35 km depths (Ague and Brimhall, 1988;Saleeby, 1990; Pickett and Saleeby, 1993;Wood and Saleeby, 1998). The oblique crustalsection, in conjunction with seismic data(Fliedner et al., 2000) and xenolith data from

the San Joaquin volcanic field, offer an indepth picture of the primary lithosphericstructure for the SNB (Ducea and Saleeby,1996, 1998; Saleeby et al., 2003). The bath-olithic crust is predominately felsic to ;35km deep. Below lies an ;10-km-thick tran-sition zone containing batholithic rocks andmafic garnet granulite residues/cumulates thatin turn grade into an ;35-km-thick sectiondominated by garnet clinopyroxenite residuesthat, like the granulites, were produced duringbatholith generation. The garnet pyroxenitesare interlayered with progressively more spi-nel and deeper garnet peridotites that extendto an ;125 km depth. This lithospheric sec-tion remained intact in the central Sierra re-gion at least until the mid-Miocene time ofxenolith entrainment. It is shown diagram-matically at the northern end of the Figure 3cross section. The garnet pyroxenite and pe-ridotite levels of the section correspond to thesubbatholith mantle lithosphere that representsthe conductively cooled mantle wedge be-neath the Cretaceous Sierran arc.

The subbatholith mantle lithosphere struc-ture exhibited for the central SNB contrastssharply from the deep structure observedalong the Tehachapi-Rand deformation belt,



shown at the southern end of the Figure 3 sec-tion. Late Neogene high-angle faults havebeen restored on Figure 3 (after Burchfiel andDavis, 1981; Malin et al., 1995) in order toelucidate the structural relations of the belt.The southernmost Sierra (Tehachapi) seismicstructure correlates with the northern Mojaveseismic structure of the Rand Mountains area.In both areas, field observations and geophys-ical data indicate that the base of the batho-lithic crust and its underlying mantle litho-sphere have been tectonically removed andreplaced by the Rand schist (Cheadle et al.,1986; Silver and Nourse, 1986; Malin et al.,1995; Wood and Saleeby, 1998). In the north-ern Mojave Desert region, the Rand thrust andlower plate schist exhibit modest structural re-lief and form the core of an antiformal win-dow. To the north in the southernmost Sierra,the schist is exposed in small windows andlarger fault-bounded slices along the Garlockfault (Sharry, 1981; Ross, 1989; Wood, 1997).Upper-plate rocks between the two areas cor-relate in age, lithology, and radiogenic iso-topes (Silver and Nourse, 1986; Saleeby et al.,1987; Pickett and Saleeby, 1994; Wood andSaleeby, 1998). Thermobarometric constraintsin both areas indicate that the level of detach-ment of the lower crust and the level of schistunderplating were ;9 kb (Sharry, 1981; Pick-ett and Saleeby, 1993; Jacobson, 1995). TheRand thrust has been seismically imaged asdipping ;208 beneath the southernmost SNBas it extends northward from its surface ex-pression along the small windows. Paleodepth(pressure) isopleth patterns within the upperplate SNB reflect a comparable dip to obliquecrustal section. Thus, the level of detachmentof the mantle lithosphere by the Rand thrustapproximated the base of the felsic batholithiclayer over a wide region.

The structural thickness of the Rand schistis unknown. Radiogenic isotopic data fromNeogene volcanic rocks of the western Mo-jave Desert region indicate a lack of continen-tal lithosphere mantle beneath the ;30-km-thick crust (Miller et al., 2000). Radiogenicand stable isotopic data of western MojaveNeogene volcanic rocks and of small intru-sions of Late Cretaceous muscovite-garnetgranites exhibit crustal components that matchthe Rand schist (Glazner and O’Neil, 1989;Miller et al., 1996). Crustal rocks that lie be-tween the schist exposures and the currentMoho were likely to have been constructed inlarge part from additional Franciscan-affinityrocks that were progressively accreted beneaththe schist. The seismic structure of the westernMojave Desert crust reflects a layered assem-blage that has characteristics that fall within

the broad range of Franciscan protoliths andmetamorphic derivatives (Cheadle et al.,1986; Li et al., 1992; Hauksson, 2000). Thefan-like seismic reflection structure and its trun-cation by the Moho beneath the Tehachapi-Rand belt (Fig. 3) is interpreted as the markof regional flattening of an underplated-duplexcomplex and the related smoothing out of theMoho subsequent to lower-crust tectonic ac-cretion (Malin et al., 1995).

The batholithic plate above the Rand thrustwas deformed during and shortly after schistunderplating. This is evident on the cross sec-tion in Figure 3 by the regional tilting of theupper-plate batholithic crust and its denuda-tion to nearly its primary Moho depths. Un-derplating of the schist and regional tiltingwere accompanied by westward oroclinalbending and penetrative plastic flow of thedeep batholithic rocks, resulting in the dis-tinctive westward protruding ‘‘tail’’ of thesouthernmost SNB (Fig. 2) (Burchfiel and Da-vis, 1981; Kanter and McWilliams, 1982;Wood and Saleeby, 1998). At mid- to upper-crust levels, the upper plate disaggregated intoa series of detachment sheets that escapedsouthward into the southernmost Sierra–northern Mojave Desert region (Wood and Sa-leeby, 1998). The combination of the tectonicremoval of the subbatholith mantle litho-sphere, and the denudation of the overlyingmid- to upper crust left the ;125-km-thickbatholithic lithosphere tapered to a thin sheetor series of sheets in the southernmost Sierra–Northern Mojave Desert region. It appears thatthe Neogene Garlock fault preferentially lo-calized itself along the tip of this tapered-offlithosphere section.

The contrast between the two subbatholithicsections shown at the opposite ends of theFigure 3 cross section calls for a fundamentaldeep-crust–upper-mantle structural break ofLaramide age of which the manifestations inthe overlying SNB were principally regionaltilting and related crustal-scale denudation. Aninflection in the underlying subducting slabthat functioned like a large lateral ramp is im-plied. This further implies that the subductingslab was segmented into a shallow segment tothe south and a deeper segment to the north.

BASEMENT TECTONICS ABOVE THESHALLOW SLAB SEGMENT

The crustal response to the shallow slabsegment is expressed by a number of geologicfeatures that include (1) the distribution of theRand and related lower plate schists, (2) thestructure of the overlying MSB as well as thesouthernmost SNB, and (3) anomalous pat-

terns in ‘‘forearc’’ sedimentation. The Randschist belongs to a family of high-pressuregreenschist-amphibolite facies rocks that werederived from protoliths encompassing Francis-can trench and Great Valley forearc basin-likeassemblages and that tectonically underliebatholithic rocks at comparable structural lev-els throughout the southern California region(Jacobson et al., 1996). Age constraints on thethrust emplacement and cooling of the schistscoincide with the latest Cretaceous, early Pa-leogene age of the Laramide orogeny (Silverand Nourse, 1986; Jacobson, 1990; Jacobsonet al., 2000; R.W. Kistler, 1996, written com-mun.). The pre-Neogene distribution of all theprincipal exposures of these schists is shownon Figure 1, which corresponds to the north-south limits of the MSB and the adjacentsouthernmost SNB. This segment of the bath-olithic belt is unique in its degree of tectonicdisruption and its related breachment west-ward across its forearc region (May, 1989;Hall, 1991; Malin et al., 1995; Barth andSchneiderman, 1996). To the north and to thesouth of the MSB, the Cretaceous forearc ba-sin survived through Laramide time. The po-sition of the forearc basin adjacent to the MSBwas occupied by Salinia following Laramidetime (Fig. 1). The northern to central areas ofSalinia contain schist exposures and regionalallochthonous relations that correlate withthose of the western Mojave Desert region(Ross, 1976, 1984; Hall, 1991; Silver andMattinson, 1986). Seismic reflection dataacross southern Salinia indicate that Salinia’sbatholithic rocks lie structurally above imbri-cated low-velocity material that most likelyrepresents subducted sediments (Trehu andWheeler, 1987). Thus, over a broad region thatextends across the southernmost Sierra to theadjacent western Mojave Desert, including thepre-Neogene positions of Salinia and the SanGabriel terrane, and continuing southeastwardinto the southern Colorado River Desert re-gion, the Cordilleran batholithic belt lies tec-tonically above Franciscan and Great Valleyforearc affinity assemblages. This profoundstructural relation developed in Laramidetime.

The forearc breachment of the MSB is alsoreflected by the westward oroclinal bend ofthe adjacent southernmost SNB and its upper-level detachment sheets (Burchfiel and Davis,1981; Wood and Saleeby, 1998). Regionallithologic belts, which are internal to the SNB,and are defined by petrochemistry and wall-rock geology, exhibit the westward deflection,both in the plastically deformed mid- to deep-crustal exposures and in the overlying detach-ment sheets. This westward deflection pattern


J. SALEEBY

in the zonation of the batholithic belt contin-ues into the MSB (Ehlig, 1981; Silver, 1983;Ross, 1984; Silver and Mattinson, 1986; May,1989; James et al., 1993; Kistler and Cham-pion, 2001), and then it ceases southward intothe PRB where the batholith’s longitudinalprimary structure as well as its transition intoits adjacent forearc basin remained intact (Sil-ver et al., 1979; Crouch and Suppe, 1993).

The deformed MSB, its tectonically under-lying schists, and the spatially related missingsegment of the forearc basin are interpreted asthe surface manifestation of the shallow seg-ment of the Laramide slab along the plateedge (Malin et al., 1995). Batholithic mag-matism ended with an abrupt regional coolingthroughout Salinia during the early Laramidetime of initial schist underplating (Naeser andRoss, 1976; Mattinson, 1978; Kistler andChampion, 2001). Magmatism waned at thesame time in conjunction with regional cool-ing in the western Mojave Desert region(Miller and Morton, 1980). Such waning inmagmatism was also marked by the emplace-ment of relatively small petrochemically dis-tinct plutons carrying isotopic signatures ofRand schist in their source (Miller et al.,1996). These western Mojave Desert and Sal-inia events were accompanied in the easternMojave Desert region by regional crustalshortening, denudation, and the developmentof basement-derived clastic wedges along theMaria-Mule Mountains deformation belt (Fig.1) (Carl et al., 1991; Foster et al., 1992; Milleret al., 1992; Karlstrom et al., 1993; Richard etal., 1994). A similar timing for the end of arcmagmatism, denudation, and cooling is re-corded in the San Gabriel terrane (Carter andSilver, 1971; Miller and Morton, 1980; Ehlig,1981; Barth and Schneiderman, 1996). Thus,the subduction of the shallow slab segment ismarked everywhere along the length of itsstrike by the termination of magmatism, de-formation, and rapid denudation of the over-lying segment of the magmatic arc.

BASEMENT TECTONICS ABOVE THEDEEP SLAB SEGMENT

The termination of arc magmatism through-out the greater SNB at the onset of Laramidetime is well documented (Evernden and Kis-tler, 1970; Stern et al., 1981; Chen and Moore,1982). It has been further suggested from themodeling of fission track data that the sub-batholith mantle lithosphere was removed byLaramide flat slab subduction at a depth ofbetween 35 and 50 km (Dumitru at al., 1991).This later assertion is clearly at odds with thexenolith data that indicate the survival of the

subbatholith lithosphere throughout Laramidetime (Fig. 3). The fission track data basis forthis lithosphere removal model also needs re-evaluation in the light of apatite (U-Th)/Hedating conducted in the same region (Houseet al., 2001). Most notable are the effects ofcooling related to Late Cretaceous-Paleogenehigh relief topography. The key element in theDumitru et al. (1991) model is conductivecooling of the Sierran crust from beneath bythe Laramide slab. In spite of the rejection ofthe conclusion that the Laramide slab removedthe mantle lithosphere from beneath the great-er SNB, the xenolith thermobarometric datarecord an apparent conductive cooling event,but for the entire mantle lithosphere (Lee etal., 2000; Saleeby et al., 2003).

The Cretaceous SNB developed by copiousarc magmatism over a time interval of ;50m.y. (Evernden and Kistler, 1970; Saleeby andSharp, 1980; Stern et al., 1981; Chen andMoore, 1982; Saleeby et al., 1987). The abun-dance of mafic batholithic rocks along thewest margin of the batholith, their ubiquitousoccurrence throughout the batholith, and iso-topic data on the large-volume felsic as wellas mafic members of the batholith, all attestto the importance of the subbatholith mantlewedge in magma genesis (Saleeby et al.,2003). Maintaining a fertile mantle wedgeover such a long period of magma genesis re-quires an influx of fertile peridotite into thewedge environment as subduction progresses.Geodynamic modeling of subduction in con-junction with global petrogenetic and geo-chemical data indicate the importance of as-thenosphere cornerflow circulation and theinflux of slab-derived hydrous fluids for man-tle wedge magmatism (cf. Billen and Gurnis,2001; Ulmer, 2001). Thermobarometric andpetrogenetic data for the sub-SNB mantle xe-noliths suggest a highly dynamic subarc man-tle wedge that had a continuous material fluxduring magma genesis (Saleeby et al., 2003).This material flux included the circulationof asthenospheric mantle, an influx of slab-derived fluids, the rendering of hydrous ba-saltic magma, and the settling of garnet-pyroxene residue batches from upper levelfelsic magma production domains. This dy-namic system was terminated abruptly at theonset of Laramide time. It is suggested thatthe single most important factor in this ter-minal event was the disruption of the astheno-sphere cornerflow pattern by the interferenceof the shallow slab segment. Without the con-tinued circulation of hot asthenosphere intothe subarc mantle wedge, basalt magma pro-duction would have ceased, leading to the ter-

mination of voluminous higher level felsicmagma production (Saleeby et al., 2003).

As shown in Figure 1, the shallow slab seg-ment initially took a highly oblique trajectorybeneath the southwest Cordilleran plate edge.The resulting damage to the subcontinentalmantle lithosphere is considered to have beenrestricted, for the most part, to the southern-most SNB and MSB regions. It is hypothe-sized that a much broader region of the sub-continental asthenosphere and its subduction-related circulation pattern were affected by theshallow slab segment. The aggregate regionaleffect was the termination of arc magmatismalong an ;1500 km segment of the plate edgestretching from the MSB northward to the Ida-ho batholith. The southern and northern endsof latest Cretaceous-early Paleocene batholith-ic belts that persisted through Laramide timealong the more northerly and southerly reach-es of the Cordilleran plate edge are shownnear the opposite margins of Figure 1. Re-establishment of the (post-Laramide) region-ally continuous magmatic arc followed an in-ward migration pattern from both ends ofthese syn-Laramide arc segments and ulti-mately coalesced in the central California re-gion by the mid-Miocene (Burchfiel et al.,1992). This migration pattern appears to haverespected the shallow slab segment corridor ofthe craton in that migration and coalesencewere inward toward, and around the edges, ofthe Colorado Plateau. This is suggested to re-flect the maximum damage zone of the as-thenosphere by the shallow slab segment.

The initial oblique trajectory of the Lar-amide slab beneath the southwest Cordilleraalso resulted in the partitioning of dextralshear into a trench-linked transform systemthat affected the SNB and the Franciscan com-plex. In the SNB, a regional dextral shear zonedeveloped along the belt of terminal batholith-ic plutons that were at solidus to hot subsoli-dus conditions during shearing (Busby-Speraand Saleeby, 1990; Tikoff and de Saint Blan-quat, 1997; Wood and Saleeby, 1998). In thesouthern SNB, the proto–Kern Canyon faultof this system exhibits progressively greaterwest-directed reverse components of shearsouthward as it roots into upper plate tecton-ites of the Rand thrust system (Fig. 1). To thenorth, the proto–Kern Canyon fault is in con-tinuity with the Sierra Crest shear zone. Otherthan this regional dextral shear zone, thegreater SNB remained structurally intact andlittle deformed. Apatite (U-Th)/He thermo-chronometry from the central region of thebatholith suggests that it existed as an orogen-ic plateau that had high relief fluvial topog-raphy during Laramide time (House et al.,



2001). These data in conjunction with regionalpatterns in igneous barometry further suggesta coherent pattern of slow denudation duringLaramide and most of Cenozoic time (Agueand Brimhall, 1988; Saleeby et al., 2003).This pattern breaks down in the southernmostSNB where tremendous uplift, oroclinal bend-ing, and detachment faulting related to schistunderplating have rendered the oblique crustalsection.

SLAB SEGMENTATION

The Andean subduction-arc system is com-monly cited as a modern analogue for Lar-amide tectonics of the southwest Cordillera.The Andean system is characterized by thesegmentation of the down-going slab into nor-mal, moderate, and shallow-dipping domains(Barazangi and Isacks, 1976; Gutscher et al.,2000). Seismic imaging of the Andean slabby Gutscher et al. (2000) reveals flat seg-ments that are as shallow as ;50 km and asdeep as ;120 km. A lateral ramp betweentwo such slab segments would yield the typeof lithosphere-scale structural patterns that areexhibited beneath the SNB-MSB transition(Fig. 3). Were it not for disruption by the SanAndreas fault, this line of reasoning could beused to predict that a similar pattern could befound beneath the MSB-PRB transition.Gutscher et al. (2000) also show a close cor-relation of shallow flat slab segments with thesubduction of oceanic plateaus or aseismicridges. The abnormal subduction geometry ofthese shallow flat slab segments is thought toarise from the buoyancy effect of the greatlythickened crustal sections relative to that ofthe abyssal lithosphere. Most shallow slabsegments associated with the subduction ofsingle plateaus or ridges are on the order of500 km in width measured along the respec-tive plate edges, which is the approximatestrike length of the disrupted segment of thebatholithic belt in the southern California re-gion (Fig. 1).

It has been suggested that a counterpart ofthe Hess-Shatsky large igneous province(Towisangna Ridge of Barth and Schneider-man, 1996) that was built on the Farallon platewas subducted in the southwest Cordilleranregion in Laramide time, and that this led tothe profound tectonic events of both the plateedge and interior (Livacarri et al., 1981; Hen-derson et al., 1984; Barth and Schneiderman,1996). Blocks of intraplate basalt and pelagiclimestone of the Permanente terrane in thecentral California Franciscan complex repre-sent fragments of this igneous province thatwere accreted in Late Cretaceous-Paleogene

time (Vallier et al., 1983; Blake et al., 1984;Sliter, 1984). It is suggested that these blocksare the remnants of a much larger massif thatwas subducted as part of the shallow slab seg-ment in the southern California region, andthat the derivative blocks were displacednorthward by dextral faulting during and fol-lowing subduction accretion (cf. McLaughlinet al., 1988). The current position of the Per-manente terrane, and the Central melange beltthat dissipated much of the trench-linkedtransform motion, are shown in Figure 2. Theorigin and accretion history of the Permanenteterrane adds considerable support for a caus-ative link between ‘‘Towisangna Ridge’’ sub-duction and Laramide deformation.

A noteworthy difference between the Lar-amide flat slab segment envisaged here andthose resolved beneath the Andes is, accord-ing to the Pacific Basin plate motion recon-structions for Laramide time (Engebretsen etal., 1985; Stock and Molnar, 1988), that atleast the initial phases of shallow slab segmentsubduction were highly oblique to the south-west Cordilleran plate edge (Fig. 1). Such isnot the case for the Andes. Some of the ap-parent consequences of this oblique subduc-tion pattern for the Laramide are discussedbelow.

SLAB SEGMENTATION ANDREGIONAL SOUTHWESTCORDILLERAN DEFORMATION

The effects of the oblique subduction of theLaramide shallow slab segment are manifestin a north-northeast–trending corridor that ex-tends from the southwest Cordilleran plateedge into the adjacent craton. Figure 1 dis-plays a close spatial correspondence betweenthe cratonic deformation zone and the dis-rupted segment of the batholith belt, particu-larly if viewed in the context of the relativemotion between the Farallon and North Amer-ican plates. A critical tracer on Figure 1 is therestored trace of the Neogene Garlock fault(after Snow and Wernicke, 2000). This faultis most pronounced along the Tehachapi-Randdeformation belt where it coincides with theinflection in the Rand thrust as it descends be-neath the ‘‘pinched out’’ terminous of theSNB oblique crustal section (Fig. 3). In thiscontext, the restored Garlock fault is used asan approximate surface tracer of the segmen-tation inflection in the Laramide slab. Relativeplate motion between the Farallon and NorthAmerican plates for early Laramide time isparallel to the trend of this tracer (Engebretsenet al., 1985). By projecting this plate motiontrajectory into the plate interior, one can see

that virtually all of the first-order Laramideplate interior structures coincide with the shal-low slab segment as defined above. The scaleof the shallow slab segment and its overlyingdeformation corridor through the forearc, arc,and distal backarc (craton) are comparable tothe spatial relations observed where the ase-ismic Juan Fernandez Ridge is descending be-neath the Andes and the adjacent Sierras Pam-peanas foreland thrust belt (Gutscher et al.,2000).

Considerable uncertainty exists for the rel-ative motions between the Farallon and NorthAmerican plates at Laramide time (Engebret-sen et al., 1985; Stock and Molnar, 1988), butthe derivative vectors that are shown on Fig-ure 1 for 1 m.y. time increments of motion inthe southern California region are taken atface value for this discussion. The cratonalzone of deformation may have resulted froma combination of end loading and basal trac-tion from the shallow slab segment. Initial endloading accompanied the shearing off of themantle lithosphere beneath the MSB. As dis-cussed below, much of the mantle lithospherebeneath the cratonal deformation zone is con-sidered to have been left intact above the shal-low slab segment. Tracking the initial 10 m.y.subduction trajectory of the shallow slab seg-ment (ca. 85–75 Ma) with the correspondingrelative motion vector puts the leading edgeof the shallow segment beneath the mid-Colorado Plateau region at ca. 75 Ma. Thetrajectory of the slab descent then began tochange to a more easterly course between 74and 69 Ma. This change in trajectory corre-sponds in time to an eastward migration and/or intensification of cratonal deformation(Brown, 1988). Arches and uplifts that lie bur-ied in the midcontinent region and that formedbetween 75 and 50 Ma (Tikoff and Maxson,2001) may also reflect this change in the sub-duction trajectory for the shallow slabsegment.

The topology and subduction trajectory ofthe Laramide slab as envisaged above helpsexplain controversies concerning the preser-vation of the subcontinental mantle litho-sphere beneath most of the southwest Cordil-lera. Isotopic data of Cenozoic volcanic rocksand mantle xenoliths (Livaccari and Per-ry,1993; Farmer et al., 1989; Miller et al.,2000; Lee et al., 2001) suggest regional post-Laramide preservation of the mantle litho-sphere in the backarc to cratonal region. Thesefindings are at odds with models of wholescale removal of the mantle lithosphere in theregion by a regionally extensive flat slab (cf.Bird, 1988; Dumitru et al., 1991). The topo-logic and kinematic model presented above


J. SALEEBY

predicts that the only region where Laramide-age removal of the mantle lithosphere is re-quired is beneath the MSB region (Fig. 1). Adeeper flat slab geometry, as imaged in partsof the Andes by Gutscher et al. (2000) and asenvisaged here for the greater SNB, wouldleave the lithosphere intact in the SNB back-arc region. In contrast, isotopic data for Mio-cene volcanic rocks of the western MojaveDesert reflect no such continental lithospherecomponent (Glazner and O’Neil, 1989; Milleret al., 2000). Miller et al. (2000) also showthat Miocene volcanic rocks that erupted inthe eastern Mojave Desert region, east of thetermination of the Garlock fault, do carry theisotopic fingerprints of the subcontinental lith-osphere mantle. These data appear to trace theapproximate downdip limit of the whole-scaleshearing off of the mantle lithosphere by theshallow slab segment. The fact that it in gen-eral spatially corresponds to the eastern ter-mination of the Garlock fault is in accord withthe interpretation of the Garlock fault as ashallow-level tracer of the slab segmentationinflection.

Discussion will now focus again on theplate edge environment. Based on the surfacedistribution of the lower plate schists and onthe westward breaching patterns of the MSB,Malin et al. (1995) suggested an ;500 kmwidth for the shallow slab segment. The pos-sible southward continuation of intra-arc tec-tonic features that may be associated with theshallow segment extends ;100 km southwardinto the otherwise intact PRB (May, 1989;Goodwin and Renne, 1991; George and Dok-ka, 1994). Most notable is the development ofhigh-strain fabrics and denudation along theeastern Peninsular Ranges mylonite zone (Fig.1). The restriction of this early Laramide agestructural zone to the northernmost PRB, andthe survival of the corresponding segment ofthe forearc basin, are taken as the mark of thesouthern bounds on the shallow slab segment.The sinistral transfer structure shown in Fig-ure 1 between the eastern Peninsular Rangesmylonite zone and the basement rooted thrustsof the San Gabriel terrane is after May (1989).The contrast between the crustal deformationexpressions of the slab segmentation inflec-tions between the SNB-MSB in the north andMSB-PRB in the south are interpreted to re-flect the dextral obliquity in early Laramideplate convergence, the time when these defor-mation systems formed.

Early Laramide ductile deformations alongthe eastern Peninsular Ranges mylonite zone,Maria-Mule Mountains deformation belt, pro-to–Kern Canyon fault, and Sierra Crest shearzone appear to have been localized by the

thermal weakening of active, or recently ac-tive, arc magmatism. This pattern may contin-ue northward along poorly exposed batholithicrocks of the northwest Nevada region (Wyldand Wright, 2001) and ultimately into thewestern Idaho shear zone (Tikoff et al., 2001).Evidence for broadly distributed dextral shearalso affecting the Laramide cratonal defor-mation zone, including east-west–oriented an-tithetic sinistral shears, has been reviewed byTikoff and Maxson (2001). This broad patternof upper plate dextral shear for Laramide timeis also manifest in the Franciscan subductioncomplex (McLaughlin et al., 1988; Jayko andBlake, 1993).

One of the outstanding problems for resolv-ing the history of Salinia is the fate of itswestern zone, which consists of Early Creta-ceous batholithic and pre-Cretaceous meta-morphic framework rocks. Petrologic and geo-chemical signatures at its northern endexposures (Silver and Mattinson, 1986; Kistlerand Champion, 2001), as well as clasts in ad-jacent Eocene conglomerates (Schott andJohnson, 2001), bear witness to the past ex-istence of such rocks. The view adopted hereis that a combination of extreme crustal atten-uation resulting from late- to post-Laramidecrustal collapse and subduction erosion, inconjunction with trench-linked transformfaulting, can account for the absence of suchrocks (Figs. 1 and 4). Several relationshipswithin the Franciscan complex bear on this is-sue. As noted above, northward translation ofthe Permanente terrane along the Central me-lange belt can account for the displacement offragments of the Towisangna Ridge from itspresumed locus of impact along the MSB seg-ment of the plate edge. Franciscan rocks ofthe Gold Beach terrane in southwest Oregon(Fig. 2) consist of Early Cretaceous westernzone-like arc rocks that are suggested to havebeen displaced northward from the westernPRB region in latest Cretaceous–early Ceno-zoic time (Jayko and Blake, 1993). Consid-ering that the northern PRB forearc is intact,excluding Neogene disruption, it seems morelikely that the Gold Beach terrane may haveoriginated as part of the MSB western zone.Franciscan rocks that bound the western mar-gin of Salinia consist of the Nacimiento block.These rocks are suggested to have originatedalong the Peninsular Ranges offshore trenchregion, and to have been displaced and ac-creted against Salinia by dextral transpressionin latest Cretaceous–early Paleogene time.(Jayko and Blake, 1993). These rocks lie in aposition that was apparently vacated by themissing western zone rocks of Salinia. Finally,as discussed below, the MSB underwent a

vigorous phase of orogenic collapse in con-junction with its disruption and forearcbreachment. Fragments of the western zone ofSalinia, as well as its forearc, may have beentectonically eroded by subduction as well asdextral shear in conjunction with orogeniccollapse.

The relationships outlined above for theFranciscan complex and Salinia at Laramidetime suggest that the westward breachmentpattern of the MSB delivered attenuated crust-al fragments of the respective arc to a trench-linked transform system that resulted fromdextral oblique convergence. Serial events intectonic erosion by strike-slip displacementand possibly subduction erosion, and subse-quent accretion of more southerly derived sub-duction complex packages, left Salinia in amuch reduced state and in a position liable forfurther disruption and dispersal along the Neo-gene San Andreas transform system.

CRUSTAL COLLAPSE ABOVE THESHALLOW SLAB SEGMENT

The tectonic collapse of orogenically thick-ened crust resulting from the mid- to Late Cre-taceous Sevier and the latest Cretaceous–earlyPaleogene Laramide orogenies has been dis-cussed by a number of researchers and linkedto mid-Tertiary extensional tectonics of theBasin and Range province (Coney and Harms,1984; Sonder et al., 1987; Wernicke et al.,1987). Hodges and Walker (1992) have dis-cussed evidence for the initiation of such oro-genic collapse in the southwest Cordillera, atleast locally, as early as latest Cretaceous time(70–75 Ma). Malin et al. (1995) suggestedthat the forearc breachment of MSB as wellas regional horizontal ductile flattening in theunderlying schists likewise reflect Laramide-age crustal collapse. This view is developedfurther below, and it is further suggested thatthe profound tectonic events that led to thereplacement of the mantle lithosphere beneaththe MSB by largely wet sediment in conjunc-tion with the late- to post-Laramide steepeningof the subducting slab promoted orogenic col-lapse and forearc breachment of the MSB.

The extreme crustal attenuation exhibitedalong the Tehachapi-Rand deformation belt(Fig. 3), and for much of the MSB is sug-gested to have in part been facilitated by rhe-ological conditions resulting from schist un-derplating. Uranium/lead age data on detritalzircon from metaclastic units of the lowerplate schists show significant Late Cretaceousbatholithic as well as Proterozoic crystallinebasement source components that match thoseof the MSB and its eastern cratonal basement



Figure 4. Series of generalized cross section models of the southwest Cordilleran plateedge in the region of the Mojave-Salinia segment of batholithic belt (MSB) showing howthe Laramide shallow slab segment deformed the forearc-arc region as subduction flat-tened; then as subduction steepened, regional extension promoted orogenic collapse andforearc breachment of the disrupted arc segment. Superpositioning of structures is notshown in continental plate for reasons of simplicity.

wallrocks (Jacobson et al., 2000). These datasuggest rapid uplift and erosion of the MSBsediment source, as well as rapid tectonictransport of the corresponding clasticwedge(s) into the metamorphic environment.Seismic data from beneath the Tehachapi-Rand deformation belt, as well as from Saliniaand the western Mojave Desert, suggest un-derplating of perhaps up to an ;30-km-thickwedge of schist-affinity material (Cheadle et

al., 1986; Trehu and Wheeler, 1987; Li et al.,1992; Malin et al., 1995; Hauksson, 2000).Exposures of the schists are dominated by me-taclastic rocks but also contain significant hy-drous metabasalts and lesser serpentinites. Thetectonic replacement of the subbatholith man-tle lithosphere by such an assemblage effec-tively replaces a dry pyroxene 1 garnet 1olivine rheology with a wet mica 1 feldspar1 quartz rheology, resulting in a tremendous

reduction in material strength (Tullis andYund, 1980; Blanpied et al., 1995; Kohlstedtet al., 1995; Wintsch et al., 1995; Tullis et al.,1996). Stable isotope and petrologic studies ofthe Franciscan Catalina schist indicate veryhigh water fluxes during progressive subduc-tion and metamorphism from 5 to 11 kb con-ditions (Bebout and Barton, 1989). The Cat-alina schist is very similar to the schists thatare underplated beneath the MSB. An analo-gous high-water flux migrating from the un-derplated schist protoliths into the overlyingquartzofeldspathic batholithic crust, as evi-denced by widespread retrograding in the up-per plate rocks, would promote substantialweakening of the upper plate as well. It is sug-gested that these extreme tectonic and rheo-logical conditions resulted in the productionof a highly weakened orogenic crustal sectionalong the plate edge that was highly suscep-tible to gravitational collapse. Malin et al.(1995) point this out and imply that this alonecan account for the collapse and forearcbreachment of the MSB. It is further suggest-ed below that the passing of the shallow slabsegment and the resulting steepening of theslab dip promoted regional extension alongthe plate edge and, in conjunction with theextreme rheological conditions noted above, aprolonged episode of vigorous orogenic col-lapse characterized medial to late Laramidetime for the MSB.

Figure 4 is a model that shows the sequenceof events that led to the uplift and collapse ofthe MSB. This model is intended to be ge-neric for the MSB region and reflects the se-quence of events affecting the northern-MSB–southern-SNB transition perhaps up to;10 m.y. earlier than for the southern MSBregion. The model is based on the concept ofthe shallow slab segment resulting from thesubduction of the Towisangna Ridge (Livac-cari et al., 1981; Hendersen et al., 1984; Barthand Schneiderman, 1996). The Figure 4 modelalso incorporates the work of McNulty andFarber (2002), which documents the earlyphases of crustal collapse of the Peruvian An-des above its corresponding shallow slab seg-ment. This work suggests that the initiation oforogenic collapse there is linked in time withthe passing of the trailing flank of the sub-ducted Nazca Ridge beneath the forearc re-gion. The corollary added here is that the sub-duction of less buoyant abyssal lithosphere, inthe wake of the subducted Nazca Ridge, in-duces a steepening in the slab that further in-duces extensional tectonics in the overlyingorogenically thickened crust.

Figure 4A shows in highly generalizedfashion the principal plate edge elements in


J. SALEEBY

the MSB region prior to arrival of the Tow-isangna Ridge. In Figure 4B, the Ridge hassubducted several hundred kilometers beneaththe plate edge. The greater buoyancy of theRidge-hosting lithosphere, relative to abyssallithosphere, results in a flattening in the sub-duction trajectory. The subarc mantle wedgeand part of the adjacent subcontinental mantlelithosphere are sheared off and displaceddowndip into the mantle with the shallow slabsegment. Mantle wedge-driven arc magma-tism is terminated. This phase of shallow slabsubduction was highly oblique to the plateedge (Fig. 1) and, as discussed above, the as-thensophere counterflow ‘‘engine’’ for mantlewedge-driven magmatism was disrupted be-hind the greater SNB resulting in its termi-nation as well.

The onset of flat slab subduction resulted inthe imbrication and destruction of the over-lying forearc basin. Fragments of the outerforearc basin as well as its bounding accre-tionary prism may have been displaced downthe shallow subduction zone and tectonicallyunderplated as the subarc mantle wedge wassheared off. High-strain deformation of theoverlying arc is shown concentrated along itswestern edge, as seen in the western Tehach-api Range, and along its active thermallyweakened eastern edge, as seen in the Maria-Mule Mountains deformation belt. The entireMSB arc segment underwent rapid uplift anderosional denudation resulting in a flood of arcdetritus into the adjacent trench. The dextralshear component of oblique subduction isshown distributed across the western edge ofthe arc and out to the trench as shown by theoroclinal bending of the southern SNB andsuch structural trends continuing into northernSalinia. To the north, dextral shear was parti-tioned into the proto–Kern Canyon fault–Sierra Crest shear system and the FranciscanCentral belt melange. Such dextral shearing inthe Franciscan complex is presumed to haveextended along the MSB and PRB segmentsof the plate edge as well.

The events depicted in Figure 4B corre-spond to early Laramide time with respect tothe plate interior events. Early Laramide de-formation of the interior is suggested to haveresulted from the endloading of the continen-tal lithosphere as the subarc mantle wedge wassheared off. Basal traction between the shal-low slab segment and the overlying continen-tal lithosphere is suggested to have become aprogressively greater driving force for plateinterior deformation as the shallow slab seg-ment propagated beneath the plate interiorregion.

Figure 4C and D depicts the passing of the

trailing flank of the Towisangna Ridge downbeyond the plate edge, and the ensuing steep-ening of the slab dip with the subduction ofnormal abyssal lithosphere. Given the relativeplate motions shown in Figure 1, a ridge thathas a down-slab dimension of ;1000 kmwould yield the time-space relations that aredepicted. There are no definitive constraintson the other dimensions of the Ridge, otherthan it is unlikely that it exceeded ;500 kmbased on the width of the deformation corridorleft by the shallow slab segment. Nor are thereany constraints on the angle that the Ridgeimpinged on the plate edge, other than the;500-km-wide deformation corridor and therelative motion vectors. Such length scales aresimilar to the major segments of the Hess-Shatsky large igneous province in the north-west Pacific basin (Iwabuchi, 1984). Suchlength scales for the Towisangna Ridgeviewed in the context of the plate kinematicsdepicted in Figures 1, 4C, and 4D, places thetrailing flank of the Ridge beneath the centralColorado Plateau region at ca. 60 Ma. Thus,the shallow slab segment still possessed thecapability of imparting a basal traction alongthe subcontinental lithosphere beneath theprincipal cratonic deformation zone.

The passing of the Towisangna Ridge’strailing flank beneath the plate edge and theattendant steepening of the slab are suggestedto have induced a suction along the slab upperplate interface that induced regional extensionand subsidence along the plate edge. Regionalextension commenced (at least locally) by ca.70 Ma along the thickened and thermally soft-ened eastern zone of the arc, and it is sug-gested to have been distributed above the en-tire MSB arc segment in conjunction withschist underplating. Underplating is suggestedto have incorporated progressively more de-nuded arc detritus with time, and possibly tec-tonically eroded fragments of the forearc andwestern zones of the arc. Such subduction ero-sion has been shown to have been an impor-tant process in attenuating the forearc crustduring shallow slab subduction beneath thePeruvian Andes (von Huene et al., 1996). Re-gional extension, subduction erosion, andtrench-linked transform fault erosion alongwestern Salinia have destroyed virtually thewestern half of the MSB as well as its forearc.This model predicts the possible existence ofthrust-wedges and/or transposed layers ofMSB as well as its forearc rocks at depth with-in the underplated schists; a possibility im-plied by seismic data (Hauksson, 2000).

The initial phases of schist underplating arethought to have contributed to crustal thick-ening, isostatic uplift, and gravitational insta-

bility of the orogen. As the slab steepened,schist underplating is suggested to have ac-celerated. Dehydration reactions and attendantfluid fluxes progressively weakened the entirecrustal column, and a ‘‘runaway’’ extensionalcollapse episode commenced. As the slab con-tinued to steepen, the upper levels of the un-derplated wedge found a trenchward trajectoryas its path of easiest escape. A return flowchannel developed in the upper wedge, similarto that modeled for accretionary wedges byCloos (1982) and Emerman and Turcotte(1983). The return flow channel exerted a trac-tion along the base of the weakened upperplate that further promoted its gravitationalcollapse and breachment toward the forearc.In some localities, this coupled upper- andlower-plate collapse process went to such anextreme as to cut out the deep crustal upperplate rocks along the ‘‘thrust’’ system and re-place them with shallow-level detachmentsheets juxtaposed immediately above theschists (Wood and Saleeby, 1998). The trans-port pattern in the return flow channel and itstraction along the base of the upper plates re-sulted in the commonly observed kinematicpatterns along the ‘‘thrust’’ exposures that arecontrary to the subduction transport pattern ofinitial schist emplacement (Ehlig, 1981; Simp-son, 1990; Yin, 2002).

The steepening of the slab and the implicitsuction at depth along the plate edge implythe inflow of asthenosphere 6 subcontinentallithosphere beneath the eastern Mojave Desertregion. Late Cenozoic volcanic-hosted lower-crust–upper-mantle xenolith suites suggest theexistence of such a complex upper-mantlestructure, as well as the presence of latestCretaceous–early Paleogene basaltic intru-sions produced by decompression partial melt-ing of ascended asthenosphere (Leventhal etal., 1995).

MARINE TRANSGRESSION ACROSSCRUSTAL COLLAPSE ZONE

The extreme crustal attenuation envisagedfor the crustal collapse process discussedabove is reflected in the latest Cretaceous tomid-Eocene transgression of marine strataacross the deeply denuded MSB (Fig. 4). Thisis extraordinary considering that the greaterSNB to the north persisted as an orogenic pla-teau that had perhaps 3 km or more regionalelevation through Laramide time (House et al.,2001). One would expect from the Figure 3cross-section relations that the initial tecto-nism and isostatic response to schist under-plating would have elevated the southernmostSNB and adjacent MSB to higher than ambi-



ent elevations. In the case of the southernmostSierra region, the adjacent Cretaceous forearcbasin sequence is missing (Goodman and Mal-in, 1992), probably as a result of uplift andoroclinal bending during schist underplating.Paleocene-Eocene strata lie unconformably onthe deeply denuded and westward-deflectedSNB of the southernmost Great Valley and ex-tend eastward along the trend of the Tehachapi-Rand deformation belt. These more easterly,locally derived, coarse clastic strata have de-positional features and structural relationssuggesting deposition in supradetachment ba-sins (Wood and Saleeby, 1998). By Eocenetime, the eastern known limits of this complexbasinal system were under marine conditions(Cox, 1987). A similar pattern of Eocene-age,coarse, locally derived marine clastic sedi-mentation characterizes the deeply denudedupper-plate basement rocks of the southernColorado River Desert region as well as cor-relative rocks of the western San Gabriel ter-rane and of adjacent southernmost Salinia(Howell, 1975). The easternmost known ex-tent of the Eocene transgression above the col-lapsed segment of the batholithic belt is de-picted by the approximate trace of the Eoceneshoreline in Figure 1. This shoreline trace con-forms to the regional facies patterns of thepre-Laramide forearc basin system both to thenorth and to the south of the collapsed MSB.

Latest Cretaceous to Eocene depositionalpatterns across the collapsed MSB suggest theexistence of a complex borderland environ-ment that underwent general regional-scaletransgression through time. Western and cen-tral Salinia record Maastrichtian to Paleocenemarine deposition of coarse, locally derived,clastic strata in a rugged normal fault con-trolled basinal setting (Grove, 1993). Such de-positional patterns are also recorded for Pa-leocene strata of the central San Gabrielterrane (Sage, 1975). At the time of the max-imum Eocene transgression, thick sequencesof marine clastic strata were shed laterallyacross the northern end of Salinia and extend-ed northward along the adjacent southernGreat Valley forearc basin. Similar strata wereshed laterally across the southern end of Sal-inia and extended southward along the north-ern reaches of the adjacent southern Californiaborderland forearc basin (Nilsen and McKee,1979). These sediment dispersal patterns mim-ic and accentuate the tectonic dispersal pat-terns of the collapsed segment of the batho-lithic belt. Possible western facies of thisdepositional system, as well as the westernzone of the batholithic belt in Salinia, aremissing and are presumed to have been dis-placed by trench-linked transform faulting and

subduction erosion. Eocene marine strata ofnorthernmost Salinia carry distinctive con-glomerate clasts that attest to the past exis-tence of rock types that typify the extremewestern zone of the southwest Cordilleranbatholithic belt (Schott and Johnson, 2001).

The latest Cretaceous–early Paleogenetransgressive marine sequences that were de-posited across the collapsed segment of thebatholithic belt should not be misconstrued asforearc basin strata. Throughout Salinia, in thesouthern Sierra–northern Mojave Desert re-gion and in the San Gabriel terrane, these stra-ta lie depositionally on westward-breached,Late Cretaceous, eastern zone batholithicrocks. To the north and south of the collapsedsegment of the batholithic belt, these stratahave sedimentary facies relations with stratadeposited in the adjacent surviving segmentsof the forearc basin system (Nilsen, 1984; Nil-sen and McKee, 1979; Kanter, 1988), butwhere these strata rest on the westwardbreached MSB, they are unique in their tec-tonic significance.

CONCLUSIONS

The deep-crust–upper-mantle structure in-herited in the southern Sierra Nevada regionfrom Mesozoic subduction and arc magma-tism contains a fundamental tectonic bound-ary that cuts across the regional trend of thebatholith belt. To the north, the subbatholithmantle lithosphere remained intact to a depthof ;125 km at least until mid-Miocene time.In the southernmost Sierra and adjacent west-ern Mojave-Salinia segment of the regionalbatholithic belt, the mantle lithosphere wassheared off by a shallow segment of the Far-allon slab and displaced deeper into the man-tle. The shallow slab segment was probablycarrying a fragment of an aseismic ridge thatwas the counterpart of the Hess-Shatsky largeigneous province of the northwest Pacific ba-sin. Its thickened mafic crustal section relativeto abyssal lithosphere is presumed to haverendered a greater buoyancy leading to slabsegmentation into the respective shallow do-main. The sheared off segment of the sub-batholith mantle lithosphere was replaced bya tectonically underplated greywacke-basaltprotolith assemblage derived from the Fran-ciscan subduction complex as well as possibleforearc basin material that was displaceddowndip into the shallow subduction zone.The overriding segment of the batholith beltwas deformed, deeply denuded, and breachedwestward into the corresponding forearc re-gion. This plate edge tectonic regime com-menced at ca. 85 Ma and persisted until ca.

60 Ma, encompassing much of the classic Lar-amide orogeny time.

Viewing the regional structures producedalong the southwest Cordilleran plate edgeduring this tectonic regime on a pre-Neogenepalinspastic base, and considering the relativemotions between the Farallon and NorthAmerican plates at Laramide time, leads to thefollowing conclusion. The Laramide slab pos-sessed a shallow flat segment along an ;500-km-long stretch of the plate edge in the south-ern California region. Classic Laramidestructures of the deformed craton lie in a cor-ridor that corresponds to the shallow flat slabsegment with respect to the trajectory ofFarallon–North American relative plate mo-tions. A direct causative link between plateedge and plate interior Laramide deformationis implied. It is suggested that end loadingduring the initial shearing off of the subbath-olith mantle lithosphere, joined by basal trac-tion along the base of the subcratonic mantlelithosphere (as the shallow slab segment wassubducted), led to the pattern of basement de-formation observed in the craton. A modernanalogue in terms of scaling of the shallowslab segment and the plate edge to plate in-terior deformation corridor is exhibited wherethe Juan Fernandez Rise is currently subduct-ing beneath the southern Andes and adjacentSierra Pampeanas foreland thrust belt. It isfurther suggested that extensional collapse ofthe deformed crust above shallow slab seg-ments may arise from the passing of the shal-low slab segment and the ensuing steepeningof the slab. This process is in its inception inthe Peruvian Andes and was taken to comple-tion along the southwest Cordillera with theregional collapse and forearc breachment ofthe corresponding arc segment.

ACKNOWLEDGMENTS

This work was funded by NSF grants EAR-9316105, EAR-9526859 and EAR-0087347. Inter-actions with M. Ducea, L.T. Silver, T. Atwater, A.Barth, M. Gurnis, R.W. Kistler, N. McQuarrie,A.M.C. Sengor, D.J. Wood, and the entire SSCDProject working group helped stimulate this synthe-sis. Drafting and technical assistance by Zorka Fos-ter, assistance in manuscript preparation by KimKlotz, and assistance in library research by Lawr-ence Leone are gratefully acknowledged.

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