Structure and Evolution of the Lithosphere Beneath the Rocky Mountains: 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 southern Rocky Mountains, seismic tions in the upper mantle. 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 convection, and geological data provides insights into the from 40 to 55 km in depth. We as such, even though the crust is pre- nature 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 lithospheric mantle 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 rift, 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 magmatism and lithosphere- leosubduction zones. Proposed sutures sembly of the crust (Fig. 1). Our goal is asthenosphere interactions. are the Cheyenne belt, Lester-Farwell to differentiate the lithospheric structures One of the most profound tectonic Mountain area of northern Colorado, 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 island arc 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 upper mantle 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 Earth 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 subduction system, as shown by the ab- sence of an associated volcanic arc 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 Aspen anomaly (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 xenolith 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–Colorado Plateau 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 terranes. 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, Xenoliths 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 northern Colorado 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.