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Structure and Evolution of the LithosphereBeneath the Rocky Mountains: InitialResults from the CD-ROM Experiment, p. 4

A New Symbol for a Great Vision, p. 26

Vol. 12, No. 3 A Publication of the Geological Society of America March 2002


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science articleStructure and Evolution of the Lithosphere Beneath theRocky Mountains: Initial Results from the CD-ROM ExperimentCD-Rom Working Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Dialogue: A Collective Vision for the Future:GeoJournals, an Online Aggregate of Fully InterlinkedGeoscience Society Journals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

GSA Foundation Update . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

In the Spotlight: The Paleontological SocietyLooking tothe Future While Studying the Past . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14Rock Stars: Arthur Holmes: An Ingenious Geoscientist . . . . . . . . . . . . . . . . . . 16

Upcoming Deadlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

What are Those Shlemon Mentor Programs atSection Meetings All About? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Congressional Science Fellow Report: Geology from the Hill:A Challenging Beginning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

2002 Section Meetings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Eleventh Annual Biggs Award Announced . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Announcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Journal Highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

A New Symbol for a Great Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

GeoVentures: Iceland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

Classified Advertising . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

On the cover: Longs Peak, Colorado. Photo by K. Karlstrom. See Structure andEvolution of the Lithosphere Beneath the Rocky Mountains: Initial Results from theCD-ROM Experiment, p. 410.

ContentsVol. 12, No. 3 March 2002

GSA TODAY, MARCH 2002 3

Notice something new on this page? Hint: Look to the left a couple ofinches! This month, GSA unveils its new logo. Read the whole story inA New Symbol for a Great Vision on pages 26 and 27.


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4 MARCH 2002, GSA TODAY

CD-Rom Working Group*

ABSTRACTAn integration of new seismic reflec-

tion, seismic refraction, teleseismic, andgeological data provides insights into thenature and evolution of the lithospherealong a transect extending from

Wyoming to New Mexico. Perhaps themajor issue in interpreting the seismic

data is distinguishing lithospheric struc-tures that formed during Precambriangrowth and stabilization of the continentfrom those that record Cenozoic tecton-ism. Tomographic data show that the up-per mantle, to depths of >200 km, con-tains several dipping velocity anomaliesthat project up to overlying Proterozoiccrustal boundaries. Our integrated studiesdefine crustal sutures that are congruent

with the dipping mantle domains, andwe interpret these crust and mantle fea-tures as the signatures of Proterozoic pa-leosubduction zones. Proposed suturesare the Cheyenne belt, Lester-FarwellMountain area of northern Colorado, and

Jemez lineament. The resulting thickProterozoic lithosphere was part of North

America by 1.6 Ga, and has remainedboth fertile and weak as shown by re-peated deformational and magmatic reac-tivations from 1.4 Ga to present.Proterozoic lithosphere of Colorado andNew Mexico differs from lithosphere be-neath the Archean core of the continent,possibly in thickness but most importantby its strongly segmented nature, its long-

term fertility for magmatism, and its rela-

tive weakness, expressed as a tendencyto be reactivated. Throughout much ofthe southern Rocky Mountains, seismicrefraction data have delineated a 1015km thick, 7.07.5 km/s mafic lower crustallayer. The base of this layer (Moho) variesfrom 40 to 55 km in depth. Weinterpret it to have formed diachronouslyand by a combination of processes, in-cluding original arc development andsubsequent magmatic underplating, and

to be the product of progressive evolu-tion of the lithosphere.

INTRODUCTIONThe CD-ROM (Continental Dynamics

of the Rocky Mountains) experiment is ageological and geophysical study of atransect from Wyoming to New Mexico.The transect obliquely crosses Phanerozoictectonic provinces (southern RockyMountains, Rio Grande rift, Great Plains)and orthogonally crosses northeast-strik-ing structures related to Proterozoic as-sembly of the crust (Fig. 1). Our goal isto differentiate the lithospheric structuresthat formed during Precambrian growthand stabilization of the continent fromthose that record Cenozoic tectonism.CD-ROM integrates a series of coordi-nated seismic experiments (Keller et al.,1999) and geological studies to delineatecrust and upper mantle structure and pro-

vide a better understanding of lithosphericevolution and geodynamical processes.

GEOLOGIC AND SEISMICEVIDENCE FOR THE AGE AND

STRUCTURE OF THE ROCKYMOUNTAIN LITHOSPHERE

Figure 1 shows the complex arrange-ment of Precambrian crustal provincesand younger tectonic elements of thesouthern Rocky Mountains. Similar to thecrustal signature, mantle velocities alsoshow complex patterns between high-and low-velocity domains (Fig. 1). Figure2 shows a multiscale cross section of theRocky Mountain lithosphere. One of the

most notable features on the cross sec-tion is the dramatic lateral velocity varia-tions in the upper mantle. These velocitydifferences could be interpreted as re-flecting temperature differences related tomodern asthenospheric convection, andas such, even though the crust is pre-dominantly Proterozoic, the upper man-tle under the Rocky Mountains would beinterpreted to be essentially Cenozoic.However, here we explore the hypothe-

sis that the lithospheric mantle under theRocky Mountains, although extensivelymodified and reactivated by youngerevents, is primarily Proterozoic in age.This is suggested by the congruence ofdipping crust and mantle boundaries

with major Proterozoic province bound-aries at the surface. By this hypothesis,the observed seismic velocity variationsreflect a complex overprinting, whereProterozoic compositional and mechani-cal heterogeneities influenced Cenozoicmantle magmatism and lithosphere-as-

thenosphere interactions.One of the most profound tectonicboundaries in the Rocky Mountain regionis the Cheyenne belt (Fig. 1), a crustalmanifestation of the suture between

Archean crust and juvenile 1.81.7 GaProterozoic island arc crust (Hills andHouston, 1979). New seismic reflectionimages of the crust (Fig. 2B) confirm thatthe Cheyenne belt dips south under theProterozoic Green Mountain arc (Condieand Shadel, 1984), consistent with north-

verging thrusting of Proterozoic rocksover Archean crust (Karlstrom andHouston, 1984; Chamberlain, 1998).However, reflection data (Morozova etal., 2002) show that the deeper crust ischaracterized by tectonic inter-wedgingsimilar to other sutures between old con-tinents and younger arcs (Cook et al.,1998) rather than subparallel, south-dip-ping shear zones. We speculate that thenorth-dipping reflections from theFarwell Mountain area (Fig. 2B) projectthrough generally unreflective lower

Structure and Evolution of the LithosphereBeneath the Rocky Mountains: Initial Resultsfrom the CD-ROM Experiment

*CD-Rom (Continental Dynamics of the RockyMountains) Working Group: K.E. Karlstrom (corre-sponding author, Department of Earth and PlanetarySciences, University of New Mexico, Albuquerque,NM, 87108, [email protected]), S.A. Bowring,K.R. Chamberlain, K.G. Dueker, T. Eshete, E.A. Erslev,G.L. Farmer, M. Heizler, E.D. Humphreys, R.A. Johnson,G.R. Keller, S.A. Kelley, A. Levander, M.B. Magnani,

J.P. Matzel, A.M. McCoy, K.C. Miller, E.A. Morozova,F.J. Pazzaglia, C. Prodehl, H.-M. Rumpel, C.A. Shaw,A.F. Sheehan, E. Shoshitaishvili, S.B. Smithson,C.M. Snelson, L.M. Stevens, A.R. Tyson, and M.L. Williams.


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crust to coincide with a thrust-offsetMoho seen in teleseismic receiver func-tion images, and with the top of a high-

velocity mantle anomaly (blue anomalyof Fig. 2D) that dips north under the

Archean (Dueker et al., 2001). Our pres-ent interpretation is that Proterozoicoceanic lithosphere was underthrust be-

neath Archean crust during late stages ofaccretion of the Green Mountain arc butnever developed into a self-sustainingsubduction system, as shown by the ab-sence of an associated volcanic arc to thenorth above it. This is similar to subduc-tion polarity reversal taking place as theBanda arc accretes to Australia (Snyder etal., 1996). A series of south-dipping re-flections (Lester Mountain suture) nearthe Farwell Mountain structure are inter-preted as a suture zone between the1.781.76 Green Mountain arc and the

1.751.72 Rawah arc-backarc complex(Fig. 2B). Dismembered ophiolitic frag-ments crop along this boundary zone.

The Aspen anomaly (Dueker et al.,2001) is an enigmatic low-velocity man-tle anomaly that lies beneath theColorado Mineral belt. (It is imaged byregional-scale studies and occupies partof the blank area of Figure 2D.) TheColorado Mineral belt is a northeast-striking zone defined by: a Proterozoicshear zone system (McCoy, 2001); a suiteof Laramide-aged plutons and related

ore deposits (Tweto and Sims, 1963); amajor gravity low (Isaacson andSmithson, 1976); low-crustal velocities;and high heat flow (Decker et al., 1988).The presence of Laramide plutons heresuggests that the mantle in this region

was modified during the early Cenozoicand the high heat flow suggests contin-ued, young heat sources.

The Jemez lineament (Fig. 1) marksthe surface boundary between 1.8 and1.7 Ga crust of the Yavapai province (tothe north) and 1.65 Ga crust of theMazatzal province (Wooden and DeWitt,1991; Shaw and Karlstrom, 1999). Newreflection data (Magnani et al., 2001,Eshete et al., 2001; Fig. 2A) show south-dipping middle crustal reflections thatproject toward a south-dipping boundarybetween fast (south) and slow (north)mantle that extends to great depth (>200km; Fig. 2). Based on these relationships,

we interpret the Jemez lineament to marka Proterozoic suture zone that localizedCenozoic magmatism.

SEISMIC AND GEOLOGICEVIDENCE FOR THE NATURE OFTHE LOWER CRUST

This section examines seismic and ge-

ologic data from the crust, including newgeophysical and xenolith data, and high-lights the importance of understandingcrust-mantle interactions through time.Figure 2B shows a crustal velocity modelthat is based on the detailed CD-ROM re-fraction line (Rumpel et al., 2001; Snelson,2001). The refraction data show apprecia-ble topography on the Moho and a crustthat varies from ~40 to 55 km thick. Anotable feature is a high-velocity (7.07.5

km/s), variable-thickness (1015 km),lower crustal layer beneath theProterozoic terranes. These velocities areconsistent with a dominantly mafic com-

position. The presence and geometry ofthis layer are well documented by both

wide-angle reflection and refraction data,as well as by receiver function analysis.This zone appears unreflective on all ofthe seismic reflection lines.Xenoliths have been recovered from

the Stateline diatremes in the Proterozoiccrust of northern Colorado and fromhighly potassic lavas from the LeuciteHills in the adjacent Archean crust of

letti

Lin

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CD ROMSeismic Lines

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ARCHEAN

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Northern Boundary

pre-1.7 Ga

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Boundary

1.65Ga Deformation

Souther

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LH

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location of fig. 2

N

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suture zoneteeth onupper plate

Figure 1. Geologic elements of southwestern North America showing Continental Dynamicsof the Rocky Mountains (CD-ROM) reflection, refraction, and teleseismic lines. Precambrian

provinces strike northeast, Laramide uplifts (gray) strike north-south, Laramide plutons(white) and Neogene volcanic fields (black) strike northeast. Locations of xenolith localitiesare shown as yellow stars. LHLeucite Hills; SLState Line district. Lithospheric mantlehas lower velocity toward plate margin; area of lighter color represents regions underlain bylow-velocity mantle, probably containing partial melt (from Dueker et al., 2001). In theRocky MountainColorado Plateau region, fingers of this hot mantle penetrate older litho-sphere along northeast-striking zones; these areas are producing basaltic melts as shown byyoung volcanics along Yellowstone, St. George, and Jemez zones.


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0

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rustal Ve

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0

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Wyoming Craton

ArcheanSalida-unnisonBlock1780-730 Ma Reflecti mic Line

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southern Wyoming (Figs. 1 and 2). Lowercrustal xenoliths from the Archean side(from depths of ~30 km; 0.81.0 GPa)consist of relatively felsic hornblende-py-roxene gneisses (without garnet); theytypically display a weak-to-strong folia-tion primarily defined by amphibole.These, and the mantle xenoliths from this

locality, are more hydrated than theProterozoic xenoliths to the south, per-haps compatible with a position abovean underthrust oceanic slab (Fig. 2B). Incontrast, the lower crustal xenoliths fromthe Proterozoic lithosphere (from depthsof ~40 km; 1.2 GPa) contain little fabricand include garnet, two-pyroxene gran-ulites, and rare eclogites, consistent withderivation from the thick, relatively dry,high-velocity mafic layer. Proterozoiclower crustal xenoliths record a morecomplex history than Archean xenoliths.

U-Pb zircon geochronology of Archeanxenoliths yields dates that are similar tothe crystallization ages of rocks exposedat the surface (ca. 2.62.7 Ga). In con-trast, xenoliths from the Proterozoic sideare ca. 1.651.7 Ga meta-igneous rocksthat contain igneous and metamorphiczircons that yield a range of ages:Devonian (presumed to be the age ofkimberlite eruption), ca. 500 Ma, 13701420 Ma, 16401750 Ma (the dominantpopulation), and Archean (grains as oldas 3.1 Ga). The xenolith data indicate that

crust and mantle provinces across theCheyenne belt are distinct lithospheric en-tities that date back to the time of assem-bly (Eggler et al., 1987).

REACTIVATION AND DIFFERENTIALUPLIFT OF PROTEROZOICLITHOSPHERE

Geologic studies indicate that theProterozoic lithosphere south of theCheyenne belt was repeatedly reacti-

vated, whereas the Archean lithospherehas been relatively stable (Karlstrom andHumphreys, 1998). Following protracted

assembly of the lithosphere from 1.78 to1.65 Ga, the first major reactivation eventtook place ~1.4 Ga and involved wide-spread bimodal magmatism and intra-continental transpressional deformation(Nyman et al., 1994). This event perva-sively affected the Proterozoic litho-sphere but essentially terminated at theCheyenne belt.

In situ electron microprobe U-Pb dat-ing of monazite (Williams et al., 1999)

helps document the importance of recur-rent movements, and hence persistent

weakness, within the Colorado Mineralbelt (Shaw et al., 2001). Monazitegeochronology from shear zones indi-cates two protracted, ca. 100 m.y. long,orogenic episodes (1.721.62 Ga and1.451.35 Ga), each consisting of numer-

ous pulses of deformation, plus 1.1 GaPaleozoic and Laramide movements(Allen, 1994). Ar-Ar data (Karlstrom et al.,1997; Shaw et al., 1999) corroborate pre-

vious documentation (Chamberlain andBowring, 1990; Bowring and Karlstrom,1990; Hodges and Bowring, 1995) thatdiscrete crustal blocks throughout thesouthwestern United States show verydifferent cooling histories due to differen-tial uplift in the Mesoproterozoic andNeoproterozoic, controlled in part by ac-cretionary structures. New fission-track

studies demonstrate post-Laramide differ-ential uplift across the Colorado Mineralbelt (Kelley and Chapin, 2002). These

data confirm and extend the hypothesisof Tweto and Sims (1963) that theColorado Mineral belt was a long-livedzone of weakness in the lithosphere.

The extent and style of Phanerozoic re-activation of Proterozoic lithosphere

were different between the Proterozoicand Archean lithospheric sections. Forexample, Ancestral Rocky Mountain up-lifts formed almost exclusively south ofthe Cheyenne belt. Laramide deformationpartially reactivated older boundaries inboth areas, but minor fault analyses showa major change in style of Laramide toHolocene faulting across the Cheyennebelt. In the Archean lithosphere, pale-ostrain data indicate one or two direc-tions of Laramide faulting and minimalsubsequent deformation. In theProterozoic lithosphere, data locally indi-cate three stages of Laramide faulting andthree stages of Neogene faulting suggest-

ing reactivations of a weaker Proterozoiccrust (Koenig and Erslev, 2002).

NEOGENE TECTONICSACCOMPANIES REGIONALDENUDATIONA provocative hypothesis is that the

mantle structures that we have imaged

seismically may have distinct topographicmanifestations. A combined topographic-thermochronologic study by Pazzagliaand Kelley (1998) demonstrated that themean local relief, mean elevation, andthermochronologically determined ex-humation history vary systematicallyacross both the Cheyenne belt and

Jemez lineament. Furthermore, geomor-phic studies suggest that there is contem-porary uplift associated with the youthfulmagmatism concentrated along the Jemezlineament (Wisniewski and Pazzaglia,

2002). Here, the Canadian River has adistinct convexity or bulge in both itslong profile and terrace profiles where itcrosses the Jemez lineament and has arate of incision about two times greaterthan similar reaches upstream or down-stream of the lineament. Thus, in spite ofthe numerous complex processes thatcombine to shape landscapes, correlationssuch as this suggest that deep lithosphericstructure exerts important controls ontodays topography.

DISCUSSION OF PROCESSES OF

STABILIZATION AND EVOLUTIONOF CONTINENTAL LITHOSPHERE

Cratons are stabilized by thick litho-spheric mantle that extends to depths of>250 km and moves through weakerconvecting asthenosphere. These mantlekeels resist becoming incorporatedinto the asthenosphere because they arebuoyant owing to the presence ofstrongly melt-depleted peridotite (Jordan,1988). North America is an interestingcase study because it contains one of thethickest mantle keels on the planet, and

western North America (e.g., from theCanadian shield to the Pacific plate mar-gin) contains the largest mantle-velocitygradient on Earth (Grand, 1994; Van derLee and Nolet, 1997). Gradation from fast(cratonic) to slow (orogenic) upper-man-tle velocity structure occurs over a re-markably short distance in the RockyMountains (Henstock et al., 1998) andthis is therefore an important area tostudy the evolution of mantle structures.

These data confirm and extend

the hypothesis of Tweto and Sims

(1963) that the Colorado Mineral

belt was a long-lived zone of

weakness in the lithosphere.


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In addition to the complexity resultingfrom the Proterozoic assembly of thecontinent, understanding the RockyMountain transect (Wyoming to NewMexico) requires consideration ofCenozoic modifications to the litho-sphere. Some workers have postulatedthat the mantle had been largely re-

moved (Bird, 1988) or preserved to mod-erate depths (70100 km; Livicarri andPerry, 1993) by shallow-angle subductionof the Farallon slab in the Laramide.Further, many workers have postulatedan upwelling of asthenosphere-to-shal-low depth (even to the base of the crust)following removal of the Farallon slabthat caused the ignimbrite flare-up (e.g.,Humphreys, 1995). However, if theProterozoic lithosphere is thicker, as wesuggest, another possibility is that, ratherthan complete removal, the upper mantle

was modified by a combination ofCenozoic events, including hydrationabove a Laramide flat slab and litho-sphere-asthenosphere interactions thatcaused the ignimbrite flare-up andNeogene magmatism and high heat flow.If the low-velocity upper mantle in thesouthern Rocky Mountain region is oldand essentially intact (e.g., below theColorado Mineral belt and Jemez linea-ment), then this mantle, although hot and

weak and perhaps being invaded by as-thenosphere-derived melts, has not yet

been entrained in the convecting as-thenosphere. Thus, one important unre-solved question is the depth extent of

western North American lithosphere andthe relative contributions of modern ther-mal differences and ancient composi-tional heterogeneity in the present veloc-ity structure.Another issue is to explain the distinc-

tive Proterozoic lithosphere. The recur-rent reactivation of the Proterozoic litho-sphere suggests long-lived weakness,relative to Archean lithosphere. We spec-ulate that this fundamental difference is aresult of the style of accretion. TheProterozoic orogen was rapidly assem-bled from oceanic terranes with no majorcontinent-continent collisions, in contrastto much of the Archean and Proterozoiclithosphere to the north. The Proterozoiclithospheric mantle appears to be distinctmechanically because it is buoyant, thick,and strongly segmented. Thus, even priorto the Laramide event, this lithospheremay have been pervasively hydrated

during Proterozoic subduction-accretionprocesses associated with assembly ofnumerous small bits of juvenile litho-sphere, similar to the ongoing accretionof Indonesian oceanic terranes to

Australia. Possibly, it is this spatially vari-able hydration that originally gave riseto the compositional domains imaged in

todays mantle.The genesis of the high-velocity lower

crustal layer is not well understood, but itprobably had a complex origin involvingmultiple episodes of segregation of crustalcumulates, concentration of refractoryresidues of partial melting, and additionof underplated and/or intruded material.In the Proterozoic part of the CD-ROMcross section (Fig. 2D), we suggest thatthe 7.07.5 km/s lower crustal layer mayin part be a record of a series of mantledepletion events that extracted basaltic

melt from the lithospheric mantle andtransferred it to the vicinity of the existingMoho, creating a lower crustal layer thatis mafic but that may also contain someultramafic material. If so, the Moho andthe lower crustal layer are younger thanthe assembly structures and provide arecord of changing crustal thickness. Thelower crustal layer is remarkably feature-less on regional reflection profiles andlies below well-developed bright reflec-tivity that we interpret to be a record ofProterozoic plate tectonics. Our hypothe-

sis is that todays thick Proterozoic crustgrew in part by underplating and addi-tion of mafic intrusive bodies of a varietyof ages. Based on thinning of the lowercrustal layer just north of the Cheyennebelt, the relative lack of Proterozoic over-printing of Archean lower crust to thenorth, and volumetrically minor Phaner-ozoic magmatism in the Archean litho-sphere, this process seems to have pref-erentially affected the Proterozoiclithosphere. A key time for such under-plating was ca. 1.4 Ga. Petrogenetic mod-els suggest that the large volume of ~1.4Ga granitic magmatism in the middlecrust was related to melting of rocks witha tholeiitic basalt composition (Frost andFrost, 1997), implying that an enormous

volume of mafic rock may reside in thelower crust. However, only a single 1.4Ga metamorphic zircon has been foundso far in the Proterozoic lower crustalxenoliths, and the geochronological andNd isotopic data from mafic lower crustalxenoliths throughout the southwestern

United States indicate that these xenolithswere derived primarily from 1.7 Ga crust.Thus, another major unresolved problemis to understand the role of mafic under-plating in forming the lower crustal layerand restructuring the Moho.

SUMMARY

The combined geophysical and geo-logic data from the CD-ROM experimentprovide a high-resolution, multiscale im-age of the lithosphere of the RockyMountain region. This image supportsthe hypothesis that the lithospheric archi-tecture of the southwestern United Statesproduced during Proterozoic assemblyof juvenile terranes provided the tem-plate for todays lithospheric structure.The integrated data set indicates that theCheyenne belt, the Farwell-LesterMountain zone, and the Jemez lineament,and their corresponding velocity anoma-lies in the mantle (to >200 km), are con-trolled by Paleoproterozoic subductionzones that were active during collisionsof juvenile terranes. A variable-thickness,high-velocity lower crustal layer formsthe base of the crust under all of theProterozoic provinces investigated alongthe CD-ROM corridor. This and the ap-preciable Moho topography are inter-preted to be, at least in part, youngerthan the sutures and the result of under-plating that took place at 1.7, 1.4, and1.1 Ga and more locally at several times

in the Phanerozoic. Additionalgeochronological, isotopic, and physicalproperty investigations of crustal xenolithpopulations will be required to test thishypothesis. Two provocative and testablehypotheses concerning lithospheric evo-lution are: (1) the lithospheric mantle inthe southern Rocky Mountains preservesold subduction structures, is thick (>200km) and has been persistently weak, and(2) the lowermost crust is a record ofprogressive evolution of the lithosphereand has grown through several under-

plating and/or intrusive events.

ACKNOWLEDGMENTSThe CD-ROM experiment was funded

by the National Science FoundationContinental Dynamics Program (19972001). The refraction experiment was co-funded by the German National ScienceFoundation. We thank Rick Carlson, DaveSnyder, Paul Morgan, and Art Snoke forhelpful reviews.


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Manuscript received January 4, 2002;

accepted January 28, 2002.

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