sign in sign up
<<
Home , Aspen anomaly, Asthenosphere, Colorado Mineral Belt, Lithosphere, Lithospheric mantle, Plating (geology)

Structure and Evolution of the Beneath the : Initial Results from the CD-ROM Experiment

CD-ROM Working Group* tive weakness, expressed as a tendency most notable features on the cross sec- to be reactivated. Throughout much of tion is the dramatic lateral velocity varia- the , seismic tions in the upper . These velocity ABSTRACT refraction data have delineated a 10–15 differences could be interpreted as re- An integration of new seismic reflec- km thick, 7.0–7.5 km/s mafic lower crustal flecting temperature differences related to tion, seismic refraction, teleseismic, and layer. The base of this layer (Moho) varies modern asthenospheric , and geological data provides insights into the from 40 to 55 km in depth. We as such, even though the is pre- and evolution of the lithosphere interpret it to have formed diachronously dominantly Proterozoic, the upper man- along a transect extending from and by a combination of processes, in- tle under the Rocky Mountains would be Wyoming to New Mexico. Perhaps the cluding original arc development and interpreted to be essentially Cenozoic. major issue in interpreting the seismic subsequent magmatic underplating, and However, here we explore the hypothe- data is distinguishing lithospheric struc- to be the product of progressive evolu- sis that the under the tures that formed during Precambrian tion of the lithosphere. Rocky Mountains, although extensively growth and stabilization of the continent modified and reactivated by younger from those that record Cenozoic tecton- INTRODUCTION events, is primarily Proterozoic in age. ism. Tomographic data show that the up- The CD-ROM (Continental Dynamics This is suggested by the congruence of per mantle, to depths of >200 km, con- of the Rocky Mountains) experiment is a dipping crust and mantle boundaries tains several dipping velocity anomalies geological and geophysical study of a with major Proterozoic province bound- that project up to overlying Proterozoic transect from Wyoming to New Mexico. aries at the surface. By this hypothesis, crustal boundaries. Our integrated studies The transect obliquely crosses Phanerozoic the observed seismic velocity variations define crustal sutures that are congruent tectonic provinces (southern Rocky reflect a complex overprinting, where with the dipping mantle domains, and Mountains, Rio Grande , Great Plains) Proterozoic compositional and mechani- we interpret these crust and mantle fea- and orthogonally crosses northeast-strik- cal heterogeneities influenced Cenozoic tures as the signatures of Proterozoic pa- ing structures related to Proterozoic as- mantle and lithosphere- leosubduction zones. Proposed sutures sembly of the crust (Fig. 1). Our goal is interactions. are the Cheyenne belt, Lester-Farwell to differentiate the lithospheric structures One of the most profound tectonic Mountain area of northern , and that formed during Precambrian growth boundaries in the Rocky Mountain region Jemez lineament. The resulting thick and stabilization of the continent from is the Cheyenne belt (Fig. 1), a crustal Proterozoic lithosphere was part of North those that record Cenozoic tectonism. manifestation of the suture between America by 1.6 Ga, and has remained CD-ROM integrates a series of coordi- Archean crust and juvenile 1.8–1.7 Ga both fertile and weak as shown by re- nated seismic experiments (Keller et al., Proterozoic crust (Hills and peated deformational and magmatic reac- 1999) and geological studies to delineate Houston, 1979). New seismic reflection tivations from 1.4 Ga to present. crust and structure and pro- images of the crust (Fig. 2B) confirm that Proterozoic lithosphere of Colorado and vide a better understanding of lithospheric the Cheyenne belt dips south under the New Mexico differs from lithosphere be- evolution and geodynamical processes. Proterozoic Green Mountain arc (Condie neath the Archean core of the continent, and Shadel, 1984), consistent with north- possibly in thickness but most important GEOLOGIC AND SEISMIC verging thrusting of Proterozoic rocks by its strongly segmented nature, its long- EVIDENCE FOR THE AGE AND over Archean crust (Karlstrom and term fertility for magmatism, and its rela- STRUCTURE OF THE ROCKY Houston, 1984; Chamberlain, 1998). MOUNTAIN LITHOSPHERE However, reflection data (Morozova et Figure 1 shows the complex arrange- *CD-ROM (Continental Dynamics of the Rocky al., 2002) show that the deeper crust is Mountains) Working Group: K.E. Karlstrom (corre- ment of Precambrian crustal provinces characterized by tectonic inter-wedging sponding author, Department of and Planetary and younger tectonic elements of the Sciences, University of New Mexico, Albuquerque, similar to other sutures between old con- NM, 87108, [email protected]), S.A. Bowring, southern Rocky Mountains. Similar to the tinents and younger arcs (Cook et al., K.R. Chamberlain, K.G. Dueker, T. Eshete, E.A. Erslev, crustal signature, mantle velocities also G.L. Farmer, M. Heizler, E.D. Humphreys, R.A. Johnson, 1998) rather than subparallel, south-dip- G.R. Keller, S.A. Kelley, A. Levander, M.B. Magnani, show complex patterns between high- ping shear zones. We speculate that the J.P. Matzel, A.M. McCoy, K.C. Miller, E.A. Morozova, and low-velocity domains (Fig. 1). Figure north-dipping reflections from the F.J. Pazzaglia, C. Prodehl, H.-M. Rumpel, C.A. Shaw, 2 shows a multiscale cross section of the A.F. Sheehan, E. Shoshitaishvili, S.B. Smithson, Farwell Mountain area (Fig. 2B) project C.M. Snelson, L.M. Stevens, A.R. Tyson, and M.L. Williams. Rocky Mountain lithosphere. One of the through generally unreflective lower

4 MARCH 2002, GSA TODAY crust to coincide with a thrust-offset Moho seen in teleseismic receiver func- tion images, and with the top of a high- velocity mantle anomaly (blue anomaly of Fig. 2D) that dips north under the Archean (Dueker et al., 2001). Our pres- ent interpretation is that Proterozoic oceanic lithosphere was underthrust be- neath Archean crust during late stages of accretion of the Green Mountain arc but never developed into a self-sustaining system, as shown by the ab- sence of an associated to the north above it. This is similar to subduc- tion polarity reversal taking place as the Banda arc accretes to Australia (Snyder et al., 1996). A series of south-dipping re- flections (Lester Mountain suture) near the Farwell Mountain structure are inter- preted as a suture zone between the 1.78–1.76 Green Mountain arc and the 1.75–1.72 Rawah arc-backarc complex (Fig. 2B). Dismembered ophiolitic frag- ments crop along this boundary zone. The (Dueker et al., 2001) is an enigmatic low-velocity man- tle anomaly that lies beneath the Colorado Mineral belt. (It is imaged by regional-scale studies and occupies part of the blank area of Figure 2D.) The Colorado Mineral belt is a northeast- striking zone defined by: a Proterozoic shear zone system (McCoy, 2001); a suite of Laramide-aged plutons and related Figure 1. Geologic elements of southwestern North America showing Continental Dynamics ore deposits (Tweto and Sims, 1963); a of the Rocky Mountains (CD-ROM) reflection, refraction, and teleseismic lines. Precambrian major gravity low (Isaacson and provinces strike northeast, Laramide uplifts (gray) strike north-south, Laramide plutons (white) and Neogene volcanic fields (black) strike northeast. Locations of localities Smithson, 1976); low-crustal velocities; are shown as yellow stars. LH—Leucite Hills; SL—State Line district. Lithospheric mantle and high heat flow (Decker et al., 1988). has lower velocity toward plate margin; area of lighter color represents regions underlain by The presence of Laramide plutons here low-velocity mantle, probably containing partial melt (from Dueker et al., 2001). In the suggests that the mantle in this region Rocky Mountain– region, fingers of this hot mantle penetrate older litho- was modified during the early Cenozoic sphere along northeast-striking zones; these areas are producing basaltic melts as shown by and the high heat flow suggests contin- young volcanics along Yellowstone, St. George, and Jemez zones. ued, young heat sources. The Jemez lineament (Fig. 1) marks SEISMIC AND GEOLOGIC km/s), variable-thickness (10–15 km), the surface boundary between 1.8 and EVIDENCE FOR THE NATURE OF lower crustal layer beneath the 1.7 Ga crust of the Yavapai province (to THE LOWER CRUST Proterozoic . These velocities are the north) and 1.65 Ga crust of the This section examines seismic and ge- consistent with a dominantly mafic com- Mazatzal province (Wooden and DeWitt, ologic data from the crust, including new position. The presence and geometry of 1991; Shaw and Karlstrom, 1999). New geophysical and xenolith data, and high- this layer are well documented by both reflection data (Magnani et al., 2001, lights the importance of understanding wide-angle reflection and refraction data, Eshete et al., 2001; Fig. 2A) show south- crust-mantle interactions through time. as well as by receiver function analysis. dipping middle crustal reflections that Figure 2B shows a crustal velocity model This zone appears unreflective on all of project toward a south-dipping boundary that is based on the detailed CD-ROM re- the seismic reflection lines. between fast (south) and slow (north) fraction line (Rumpel et al., 2001; Snelson, have been recovered from mantle that extends to great depth (>200 2001). The refraction data show apprecia- the Stateline diatremes in the Proterozoic km; Fig. 2). Based on these relationships, ble topography on the Moho and a crust crust of and from we interpret the Jemez lineament to mark that varies from ~40 to 55 km thick. A highly potassic lavas from the Leucite a Proterozoic suture zone that localized notable feature is a high-velocity (7.0–7.5 Hills in the adjacent Archean crust of Cenozoic magmatism.

GSA TODAY, MARCH 2002 5 Figure 2. Synthesis of Continental Dynamics of the Rocky Mountains (CD-ROM) results. A. Line drawing of CD-ROM Jemez lineament reflection line (Magnani et al., 2001; Eshete et al., 2001) superimposed on receiver function image of Dueker et al. (2001). In receiver function images, red areas represent positive velocity gradients (velocity increases downwards) and blue areas represent negative velocity gradients. Jemez lineament separates oppositely dipping reflection systems. On basis of geologic correlations, we suggest that south-dipping reflections represent a ~1.65 Ga paleo- subduction zone and north-dipping reflections represent a ~1.4 Ga extensional shear zone system. MT— Mora thrusts; PT—Pecos thrust. B. Line drawing of CD-ROM Cheyenne belt reflection line (Morozova et al., 2002) superimposed on receiver function image of Dueker et al. (2001); dashed blue line represents high-velocity body imaged by tomography (2% contour). A— Archean lithosphere; P—Proterozoic lithosphere; Pgm—Proterozoic Green Mountain block; Prb—Proterozoic Rawah block; CB—Cheyenne belt; FM—Farwell Mountain zone; LM— Lester Mountain zone. C. Results from CD-ROM refraction experiment; interfaces at Moho and top of 7.0–7.5 km/s layer are well resolved by wide- angle reflections and refractions (Rumpel et al., 2001; Snelson, 2001); receiver function images (Dueker et al., 2001) are superimposed on refraction model and show good agreement for mafic lower crustal layer. D. Generalized geologic cross section superimposed on P-wave tomographic image of Dueker et al. (2001). Crustal structures in Cheyenne belt and Jemez lineament areas are generalized from seismic reflection data (above) with solid lines representing well-defined reflections. Locations of xenolith pipes shown as vertical lines. Dipping elements in tomographic image, combined with overlying crustal structures, are interpreted to be Proterozoic suture zones and North American litho- sphere is interpreted to extend to >200 km depth.

6 MARCH 2002, GSA TODAY GSA TODAY, MARCH 2002 7 southern Wyoming (Figs. 1 and 2). Lower helps document the importance of recur- ing reactivations of a weaker Proterozoic crustal xenoliths from the Archean side rent movements, and hence persistent crust (Koenig-Koenig, 2002). (from depths of ~30 km; 0.8–1.0 GPa) weakness, within the Colorado Mineral consist of relatively felsic hornblende-py- belt (Shaw et al., 2001). Monazite NEOGENE roxene gneisses (without garnet); they geochronology from shear zones indi- ACCOMPANIES REGIONAL typically display a weak-to-strong folia- cates two protracted, ca. 100 m.y. long, DENUDATION tion primarily defined by amphibole. orogenic episodes (1.72–1.62 Ga and A provocative hypothesis is that the These, and the mantle xenoliths from this 1.45–1.35 Ga), each consisting of numer- mantle structures that we have imaged locality, are more hydrated than the ous pulses of deformation, plus 1.1 Ga seismically may have distinct topographic Proterozoic xenoliths to the south, per- Paleozoic and Laramide movements manifestations. A combined topographic- haps compatible with a position above (Allen, 1994). Ar-Ar data (Karlstrom et al., thermochronologic study by Pazzaglia an underthrust oceanic slab (Fig. 2B). In 1997; Shaw et al., 1999) corroborate pre- and Kelley (1998) demonstrated that the contrast, the lower crustal xenoliths from vious documentation (Chamberlain and mean local relief, mean elevation, and the Proterozoic lithosphere (from depths Bowring, 1990; Bowring and Karlstrom, thermochronologically determined ex- of ~40 km; 1.2 GPa) contain little fabric 1990; Hodges and Bowring, 1995) that humation history vary systematically and include garnet, two-pyroxene gran- discrete crustal blocks throughout the across both the Cheyenne belt and ulites, and rare , consistent with southwestern United States show very Jemez lineament. Furthermore, geomor- derivation from the thick, relatively dry, different cooling histories due to differen- phic studies suggest that there is contem- high-velocity mafic layer. Proterozoic tial uplift in the Mesoproterozoic and porary uplift associated with the youthful lower crustal xenoliths record a more Neoproterozoic, controlled in part by ac- magmatism concentrated along the Jemez complex history than Archean xenoliths. cretionary structures. New fission-track lineament (Wisniewski and Pazzaglia, U-Pb zircon geochronology of Archean studies demonstrate post-Laramide differ- 2002). Here, the Canadian River has a xenoliths yields dates that are similar to ential uplift across the Colorado Mineral distinct convexity or bulge in both its the crystallization ages of rocks exposed belt (Kelley and Chapin, 2002). These long profile and terrace profiles where it at the surface (ca. 2.6–2.7 Ga). In con- crosses the Jemez lineament and has a trast, xenoliths from the Proterozoic side rate of incision about two times greater are ca. 1.65–1.7 Ga meta-igneous rocks than similar reaches upstream or down- that contain igneous and metamorphic These data confirm and extend stream of the lineament. Thus, in spite of the numerous complex processes that zircons that yield a range of ages: the hypothesis of Tweto and Sims Devonian (presumed to be the age of combine to shape landscapes, correlations eruption), ca. 500 Ma, 1370– (1963) that the Colorado Mineral such as this suggest that deep lithospheric 1420 Ma, 1640–1750 Ma (the dominant structure exerts important controls on population), and Archean (grains as old belt was a long-lived zone of today’s topography. as 3.1 Ga). The xenolith data indicate that weakness in the lithosphere. crust and mantle provinces across the DISCUSSION OF PROCESSES OF Cheyenne belt are distinct lithospheric en- STABILIZATION AND EVOLUTION tities that date back to the time of assem- OF CONTINENTAL LITHOSPHERE are stabilized by thick litho- bly (Eggler et al., 1987). data confirm and extend the hypothesis spheric mantle that extends to depths of of Tweto and Sims (1963) that the >250 km and moves through weaker REACTIVATION AND DIFFERENTIAL Colorado Mineral belt was a long-lived convecting asthenosphere. These mantle UPLIFT OF PROTEROZOIC zone of weakness in the lithosphere. “keels” resist becoming incorporated LITHOSPHERE The extent and style of Phanerozoic re- into the asthenosphere because they are Geologic studies indicate that the activation of Proterozoic lithosphere buoyant owing to the presence of Proterozoic lithosphere south of the were different between the Proterozoic strongly melt-depleted (Jordan, Cheyenne belt was repeatedly reacti- and Archean lithospheric sections. For 1988). North America is an interesting vated, whereas the Archean lithosphere example, Ancestral Rocky Mountain up- case study because it contains one of the has been relatively stable (Karlstrom and lifts formed almost exclusively south of thickest mantle keels on the planet, and Humphreys, 1998). Following protracted the Cheyenne belt. Laramide deformation western North America (e.g., from the assembly of the lithosphere from 1.78 to partially reactivated older boundaries in Canadian shield to the Pacific plate mar- 1.65 Ga, the first major reactivation event both areas, but minor analyses show gin) contains the largest mantle-velocity took place ~1.4 Ga and involved wide- a major change in style of Laramide to gradient on Earth (Grand, 1994; Van der spread bimodal magmatism and intra- faulting across the Cheyenne Lee and Nolet, 1997). Gradation from fast continental transpressional deformation belt. In the Archean lithosphere, pale- (cratonic) to slow (orogenic) upper-man- (Nyman et al., 1994). This event perva- ostrain data indicate one or two direc- tle velocity structure occurs over a re- sively affected the Proterozoic litho- tions of Laramide faulting and minimal markably short distance in the Rocky sphere but essentially terminated at the subsequent deformation. In the Mountains (Henstock et al., 1998) and Cheyenne belt. Proterozoic lithosphere, data locally indi- this is therefore an important area to In situ electron microprobe U-Pb dat- cate three stages of Laramide faulting and study the evolution of mantle structures. ing of monazite (Williams et al., 1999) three stages of Neogene faulting suggest-

8 MARCH 2002, GSA TODAY In addition to the complexity resulting during Proterozoic subduction-accretion United States indicate that these xenoliths from the Proterozoic assembly of the processes associated with assembly of were derived primarily from 1.7 Ga crust. continent, understanding the Rocky numerous small bits of juvenile litho- Thus, another major unresolved problem Mountain transect (Wyoming to New sphere, similar to the ongoing accretion is to understand the role of mafic under- Mexico) requires consideration of of Indonesian oceanic terranes to plating in forming the lower crustal layer Cenozoic modifications to the litho- Australia. Possibly, it is this spatially vari- and restructuring the Moho. sphere. Some workers have postulated able hydration that originally gave rise that the mantle had been largely re- to the compositional domains imaged in SUMMARY moved (Bird, 1988) or preserved to mod- today’s mantle. The combined geophysical and geo- erate depths (70–100 km; Livicarri and The genesis of the high-velocity lower logic data from the CD-ROM experiment Perry, 1993) by shallow-angle subduction crustal layer is not well understood, but it provide a high-resolution, multiscale im- of the Farallon slab in the Laramide. probably had a complex origin involving age of the lithosphere of the Rocky Further, many workers have postulated multiple episodes of segregation of crustal Mountain region. This image supports an upwelling of asthenosphere-to-shal- cumulates, concentration of refractory the hypothesis that the lithospheric archi- low depth (even to the base of the crust) residues of , and addition tecture of the southwestern United States following removal of the Farallon slab of underplated and/or intruded material. produced during Proterozoic assembly that caused the ignimbrite flare-up (e.g., In the Proterozoic part of the CD-ROM of juvenile terranes provided the tem- Humphreys, 1995). However, if the cross section (Fig. 2D), we suggest that plate for today’s lithospheric structure. Proterozoic lithosphere is thicker, as we the 7.0–7.5 km/s lower crustal layer may The integrated data set indicates that the suggest, another possibility is that, rather in part be a record of a series of mantle Cheyenne belt, the Farwell-Lester than complete removal, the upper mantle depletion events that extracted basaltic Mountain zone, and the Jemez lineament, was modified by a combination of melt from the lithospheric mantle and and their corresponding velocity anoma- Cenozoic events, including hydration transferred it to the vicinity of the existing lies in the mantle (to >200 km), are con- trolled by Paleoproterozoic subduction above a Laramide flat slab and litho- Moho, creating a lower crustal layer that zones that were active during collisions sphere-asthenosphere interactions that is mafic but that may also contain some of juvenile terranes. A variable-thickness, caused the ignimbrite flare-up and ultramafic material. If so, the Moho and high-velocity lower crustal layer forms Neogene magmatism and high heat flow. the lower crustal layer are younger than the base of the crust under all of the If the low-velocity upper mantle in the the assembly structures and provide a Proterozoic provinces investigated along southern Rocky Mountain region is old record of changing crustal thickness. The the CD-ROM corridor. This and the ap- and essentially intact (e.g., below the lower crustal layer is remarkably feature- preciable Moho topography are inter- Colorado Mineral belt and Jemez linea- less on regional reflection profiles and preted to be, at least in part, younger ment), then this mantle, although hot and lies below well-developed bright reflec- than the sutures and the result of under- weak and perhaps being invaded by as- tivity that we interpret to be a record of plating that took place at 1.7, 1.4, and thenosphere-derived melts, has not yet Proterozoic . Our hypothe- 1.1 Ga and more locally at several times been entrained in the convecting as- sis is that today’s thick Proterozoic crust in the Phanerozoic. Additional thenosphere. Thus, one important unre- grew in part by underplating and addi- geochronological, isotopic, and physical solved question is the depth extent of tion of mafic intrusive bodies of a variety property investigations of crustal xenolith western North American lithosphere and of ages. Based on thinning of the lower populations will be required to test this the relative contributions of modern ther- crustal layer just north of the Cheyenne hypothesis. Two provocative and testable mal differences and ancient composi- belt, the relative lack of Proterozoic over- hypotheses concerning lithospheric evo- tional heterogeneity in the present veloc- printing of Archean lower crust to the lution are: (1) the lithospheric mantle in ity structure. north, and volumetrically minor Phaner- the southern Rocky Mountains preserves Another issue is to explain the distinc- ozoic magmatism in the Archean litho- old subduction structures, is thick (>200 tive Proterozoic lithosphere. The recur- sphere, this process seems to have pref- km) and has been persistently weak, and rent reactivation of the Proterozoic litho- erentially affected the Proterozoic (2) the lowermost crust is a record of sphere suggests long-lived weakness, lithosphere. A key time for such under- progressive evolution of the lithosphere relative to Archean lithosphere. We spec- plating was ca. 1.4 Ga. Petrogenetic mod- and has grown through several under- ulate that this fundamental difference is a els suggest that the large volume of ~1.4 plating and/or intrusive events. result of the style of accretion. The Ga granitic magmatism in the middle Proterozoic orogen was rapidly assem- crust was related to melting of rocks with ACKNOWLEDGMENTS bled from oceanic terranes with no major a tholeiitic composition (Frost and The CD-ROM experiment was funded continent-continent collisions, in contrast Frost, 1997), implying that an enormous by the National Science Foundation to much of the Archean and Proterozoic volume of mafic may reside in the Continental Dynamics Program (1997– lithosphere to the north. The Proterozoic lower crust. However, only a single 1.4 2001). The refraction experiment was co- lithospheric mantle appears to be distinct Ga metamorphic zircon has been found funded by the German National Science mechanically because it is buoyant, thick, so far in the Proterozoic lower crustal Foundation. We thank Rick Carlson, Dave and strongly segmented. Thus, even prior xenoliths, and the geochronological and Snyder, Paul Morgan, and Art Snoke for to the Laramide event, this lithosphere Nd isotopic data from mafic lower crustal helpful reviews. may have been pervasively hydrated xenoliths throughout the southwestern

GSA TODAY, MARCH 2002 9 REFERENCES CITED Hodges, K.V., and Bowring, S.A., 1995, 40Ar/39Ar ther- Pazzaglia, F.J., and Kelley, S.A., 1998, Large-scale geomor- Allen, J.L., III, 1994, Stratigraphic variations, fault rocks, mochronology of isotopically zoned micas: Insights from phology and fission-track thermochronology in topographic and tectonics associated with brittle reactivation of the the southwestern USA Proterozoic orogen: Geochemica and exhumation reconstructions of the southern Rocky Homestake shear zone, [Ph.D. thesis]: Cosmochemica Acta, v. 59, p. 3205–3220. Mountains: Rocky Mountain Geology, v. 33, p. 229–257. Lexington, Kentucky, University of Kentucky, 321 p. Humphreys, E.D., 1995, Post-Laramide removal of the Rumpel, H., Snelson, C.M., Prodehl, C., Keller, G.R., and Bird, P., 1988, Formation of the Rocky Mountains, western Farallon slab, western United States: Geology, v. 23, Miller, K.C., 2001, Results of the refraction/wide angle reflec- United States: A continuum computer model: Science, p. 987–990. tion seismic experiment in the southern Rocky Mountains (CD-ROM’99): Eos (Transactions, American Geophysical v. 239, p. 1501–1507. Isaacson, L.B., and Smithson, S.B., 1976, Gravity anomalies Union), v. 82, p. F640. Bolay-Koenig, N.V., 2001, Phanerozoic deformation in the and granite emplacement in west-central Colorado: southern Rocky Mountains, USA: A kinematic analysis of mi- Geological Society of America Bulletin, v. 87, p. 22–28. Shaw, C.A., and Karlstrom, K.E., 1999, The Yavapai- Mazatzal crustal boundary in the southern Rocky Mountains: nor faults in north-central New Mexico and regional GIS Jordan, T.H., 1988, Structure and formation of the continental Rocky Mountain Geology, v. 34, p. 37–52. analyses [M.S. thesis]: Fort Collins, Colorado, Colorado State tectosphere: Journal of Petrology, v. 29, p. 11–37. University, 159 p. Shaw, C.A., Snee, L.W., Selverstone, J., and Reed, J.C., Jr., Karlstrom, K.E., and Houston, R.S., 1984, The Cheyenne 1999, 40Ar/39Ar thermochronology of Mesoproterozoic meta- Bowring, S.A., and Karlstrom, K.E., 1990, Growth and stabi- Belt: Analysis of a Proterozoic suture in southern Wyoming: morphism in the Colorado : Journal of Geology, lization of Proterozoic continental lithosphere in the south- Precambrian Research, v. 25, p. 415–446. western United States: Geology, v. 18, p. 1203–1206. v. 107, p. 49–67. Karlstrom, K.E., and Humphreys, E.D., 1998, Persistent influ- Shaw, C.A., Karlstrom, K.E., Williams, M.L., Jercinovic, M.J., Chamberlain, K.R., 1998, Medicine Bow : Timing of ence of Proterozoic accretionary boundaries in the tectonic and McCoy A., 2001, Electron microprobe monazite dating deformation and model of crustal structure produced during evolution of southwestern North America: Interaction of cra- of ca. 1710–1630 Ma and ca. 1380 Ma deformation in the continent-arc collision, ca. 1.78 Ga, southeastern Wyoming: tonic grain and mantle modification events: Rocky Mountain Homestake shear zone, Colorado: Origin and early evolution Rocky Mountain Geology, v. 33, p. 259–277. Geology, v. 33, p. 161–179. of a persistent intracontinental tectonic zone: Geology, v. 29, Chamberlain, K.R., and Bowring, S.A., 1990, Proterozoic Karlstrom, K.E., Dallmeyer, D.A., and Grambling, J.A., 1997, p. 739–742. geochronologic and isotopic boundary in NW Arizona: 40Ar/39Ar evidence for 1.4 Ga regional metamorphism in Snelson, C.M., 2001, Investigating crustal structure in western Journal of Geology, v. 98, p. 399–416. northern New Mexico: Implications for thermal evolution Washington and in the Rocky Mountains: Implications for of the lithosphere in the southwestern U.S.A.: Journal of Condie, K.C., and Shadel, C.A., 1984, An early Proterozoic seismic hazards and crustal growth [Ph.D. thesis]: El Paso, Geology, v. 105, p. 205–223. arc succession in southeastern Wyoming: Canadian Journal Texas, University of Texas, 234 p. of Earth Sciences, v. 21, p. 415–427. Keller, G.R., Karlstrom, K.E., and Farmer, G.L., 1999, Snyder, D.B., Prasetyo, H., Blundell, D.J., Pigram, C.J., Tectonic evolution in the Rocky Mountain region: 4-D Cook, F.A., van der Velden, A.J., Hall K.W., and Roberts, B.J., Barber, A.J., Richardson, A., and Tjokosaproetro, S., 1996, A imaging of the continental lithosphere: Eos (Transactions, 1998, Tectonic delamination and subcrustal imbrication of dual doubly vergent orogen in the Banda Arc continent-arc American Geophysical Union), v. 80, p. 493, 495, 498. the Precambrian lithosphere in northwestern Canada collision zone as observed on deep seismic reflection pro- mapped by LITHOPROBE: Geology, v. 26, p. 839–842. Kelley, S.A., and Chapin, C.E., 2002, Denudational histories files: Tectonics, v. 15, p. 34–53. of the Front Range and , Colorado, based on Decker, E.R., Heasler, H.P., Buelow, K.L., Baker, K.H., and Tweto, O., and Sims, P.K., 1963, Precambrian ancestry of the apatite fission-track thermochronology, in Cather, S.M., et al., Hallin, J.S., 1988, Significance of past and recent heat flow Colorado Mineral Belt: Geological Society of America eds., Tectonics, geochronology, and volcanism in the south- and radioactivity studies in the southern Rocky Mountains Bulletin, v. 74, p. 991–1014. region: Geological Society of America Bulletin, v. 100, ern Rocky Mountains and : New Mexico p. 1851–1885. Bureau of Mines and Mineral Resources Bulletin, in press. Van der Lee, S., and Nolet, G., 1997, Upper mantle S veloc- ity structure of North America: Journal of Geophysical Livaccari, R.F., and Perry, F.V., 1993, Isotopic evidence for Dueker, K., Yuan, H., and Zurek, B., 2001, Thick Proterozoic Research, v. 102, p. 22815–22838. lithosphere of the Rocky Mountain region: GSA Today, v. 11, preservation of Cordilleran lithospheric mantle during the no. 12, p. 4–9. Sevier-, western United States: Geology, Williams, M.L., Jercinovic, M.J., and Terry, M.P., 1999, Age v. 21, p. 719–722. mapping and dating of monazite on the electron microprobe: Eggler, D.H., Meen, J.K., Welt, F., Dudas, F.O., Furlong, K.P., Deconvoluting multistage tectonic histories: Geology, v. 27, Magnani, M.B., Levander, A., Miller, K.C., and Eshete, McCallum, M.E., and Carlson, R.W., 1987, p. 1023–1026. Tectonomagmatism of the Wyoming province, in Drexler, T., 2001, Seismic structure of the Jemez lineament, New J.W., and Larson, E.E., eds., Cenozoic volcanism in the south- Mexico: Evidence for heterogeneous accretion and extension Wisniewski, P.A., and Pazzaglia, F.J., 2002, Epeirogenic con- ern Rocky Mountains revisited: A tribute to Rudy C. Epis: in Proterozoic time and modern volcanism: Eos trols on the Canadian River incision and landscape evolution, Colorado School of Mines Quarterly, v. 83, p. 25–40. (Transactions, American Geophysical Union), v. 82, High Plains of northeastern New Mexico: Journal of Geology, p. F865. v. 110, no. 4, in press. Eshete, T., Miller, K.C., Levander, A., and Magnani, M.B., 2001, Seismic imaging of the Yavapai-Mazatzal province McCoy, A.M., 2001, The Proterozoic ancestry of the Wooden, J.L., and DeWitt, E., 1991, Pb isotopic evidence for boundary in northeastern New Mexico: Results from CD- Colorado Mineral belt: Ca. 1.4 Ga shear zone system in cen- the boundary between the early Proterozoic Mojave and cen- ROM and industry profiles: Geological Society of America tral Colorado [M.S. thesis]: Albuquerque, New Mexico, tral Arizona crustal provinces in western Arizona, in Abstracts with Programs, v. 33, no. 5, p. A-43. University of New Mexico, 140 p. Karlstrom K.E., ed., Proterozoic geology and ore deposits of Arizona: Arizona Geological Society Digest 19, p. 27–50. Frost, C.D., and Frost, B.R., 1997, Reduced rapakivi-type Morozova, E.A., Wan, X., Chamberlain, K.R., Smithson, S.B., granites: The tholeiitic connection: Geology, v. 25, Johnson, R.A., Karlstrom, K.E., Tyson, A.R., Morozov, I.B., Manuscript received January 4, 2002; p. 647–650. Boyd, N.K., and Foster, C.T., 2002, Geometry of Proterozoic ▲ sutures in the central Rocky Mountains from seismic reflec- accepted January 28, 2002. Grand, S.P., 1994, Mantle shear structure beneath the tion data: Cheyenne belt and Farwell Mountain structures: Americas and surrounding : Journal of Geophysical Geophysical Research Letters, in press. Research, v. 99, p. 11591–11621. Nyman, M., Karlstrom, K.E., Graubard, C., and Kirby, E., Henstock, T.J., and the Deep Probe Working Group, 1998, 1994, Mesoproterozoic contractional orogeny in western Probing the Archean and Proterozoic lithosphere of western North America: Evidence from ca. 1.4 Ga plutons: Geology, North America: GSA Today, v. 8, no. 7, p. 1–5, 16–17. v. 22, p. 901–904. Hills, F.A., and Houston, R.S., 1979, Proterozoic tectonics of the central Rocky Mountains, North America: University of Wyoming, Contributions to Geology, v. 17, p. 89–109.

10 MARCH 2002, GSA TODAY