Synthesis of Results From the CD-ROM Experiment: 4-D Image of the Lithosphere Beneath the Rocky Mountains and Implications for Understanding the Evolution of Continental Lithosphere Karl E. Karlstrom1, Steven J. Whitmeyer2, Ken Dueker3, Michael L. Williams4, Samual A. Bowing5, Alan Levander6, E. D. Humphreys7, G. Randy Keller8, and the CD-ROM Working Group9 The CD-ROM experiment has produced a new 4-D understanding of the structure and evolution of the lithosphere of the southern Rocky Mountain region. We identify relicts of at least four subduction zones that were formed during assembly of dominantly oceanic terranes in the Paleoproterozoic. Crustal provinces with different geologic his- tories correspond to distinct mantle velocity domains, with profound mantle velocity contrasts associated with the ancient sutures. Typically, the transitions between the velocity domains are tabular, dipping, extend from the base of the crust to depths of 150–200 km, and some contain dipping mantle anisotropy. The present day heteroge- neous mantle structure, although strongly influenced by ancient compositional varia- tions, has undergone different degrees of partial melting due to Cenozoic heating and/or hydration caused by transient plumes or asthenospheric convection within the wide western U.S. active plate margin. A high-velocity mafic lower crust is present through- out the Rocky Mountains, and there is ~10-km-scale Moho topography. Both are inter- preted to record progressive and ongoing differentiation of lithosphere, and a Moho that has changed position due to flux of basalt from the mantle to the crust. The mafic lower crust evolved diachronously via concentration of mafic restite during arc for- mation (pre-1.70 Ga), collision-related differentiation and granite genesis (1.70–1.62 Ga), and several episodes of basaltic underplating (1.45–1.35 Ga, ~1.1 Ga, and Ceno- zoic). Epeirogenic uplift of the western U.S. and Rocky Mountain regions, driven by mantle magmatism, continues to cause reactivation of the heterogeneous lithosphere in the Cenozoic, resulting in differential uplift of the Rocky Mountains. 1Department of Earth and Planetary Sciences, University of New 3Department of Geology and Geophysics, University of Wyoming, Mexico, Albuquerque, NM 87108 Laramie, WY 82071 2Department of Earth and Planetary Sciences, University of New 4Department of Earth Atmospheric & Planetary Sciences, Massa- Mexico, Albuquerque, NM 87108; currently at: Department of chusetts Institute of Technology, Cambridge, MA 02139 Earth and Planetary Sciences, Unversity of Tennessee, Knoxville, 5Department of Geosciences, University of Massachusetts, Amherst, TN 37996 MA 01003 6Department of Earth Science, Rice University, Houston, TX, 77005 The Rocky Mountain Region -- An Evolving Lithosphere: Tectonics, 7 Geochemistry, and Geophysics Department of Geological Sciences, University of Oregon, Eugene, Geophysical Monograph Series 154 OR 97403 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 8Department of Geological Sciences, University of Texas at El Paso, 10.1029/154GM31 El Paso, TX 79968 1 2 SYNTHESIS OF THE CD-ROM EXPERIMENT INTRODUCTION (Plate 1B; Plate 2 of Karlstrom and Keller, this volume), this complex surface geology overlies a heterogeneous mantle that The Continental Dynamics of the Rocky Mountains (CD- exhibits an overall regional trend where the stable core of the ROM) geophysical and geological transect from Wyoming to Laurentian craton (from the Great Plains northeastwards) is New Mexico (Plate 1) obliquely crosses numerous Phanero- underlain by high velocity (cold, old, thick) lithospheric man- zoic tectonic provinces (Southern Rocky Mountains, Rio tle, the Rocky Mountain/Colorado Plateau region is under- Grande rift, Great Plains) and orthogonally crosses northeast- lain by a zone of mixed, but intermediate velocity mantle, striking structures related to Proterozoic assembly of the crust. and the active western U.S. is underlain by low velocity (warm, The oldest tectonic features, formed during assembly of the young, and thinner) lithospheric mantle (Plate 2; Humphreys continent, are at high angles to the younger features, related and Dueker, 1994; Grand, 1994; van der Lee and Nolet, 1997; to the Phanerozoic plate margin. Our goal was to study the Henstock et al., 1998; Godey et al., 2003). The overall shear present-day deep continental structure and compare it to the wave velocity contrast is one of the largest velocity gradients well-understood geological history deduced from exposed resolved by surface wave analysis on Earth (van der Lee and rocks in order to differentiate the components of the present Nolet, 1997; Godey et al., 2003). The boundary zone or tran- lithospheric structure that reflect Precambrian growth and sition between the continental-scale mantle velocity domains stabilization from those that reflect Cenozoic tectonic events. (red versus blue in the tomographic image) is a wide zone This paper presents an integration of the CD-ROM seismic that includes the Great Plains-Rocky Mountain region and, experiments, xenolith studies, and geological studies of sur- when viewed at continental scale, has an overall north to north- face rocks in order to delineate crust and upper mantle struc- west trend, parallel to the Cenozoic plate margin. ture and provide a better understanding of lithospheric Within this zone are northeast-trending zones of low veloc- evolution and geodynamical processes. The goal of the paper ity mantle in the Rocky Mountain region, the Snake River Plain, is to present a synthesis of some of the important and provoca- Saint George lineament, and Jemez lineament (Karlstrom and tive results of the project, citing detailed papers in this volume Keller, this volume), that are subparallel to both NE-SW Pro- and other recent contributions. We also present composite terozoic province boundaries, and the SW- directed absolute block diagrams that integrate surface and lithospheric struc- motion of the North American plate. These 30- to 100-km- ture using results from other teleseismic experiments that scale velocity variations in the Rocky Mountain region are pro- have been conducted in the region in the last decade. found, being nearly the same magnitude as the continental scale variation. These velocity differences might be interpreted as GEOLOGIC AND SEISMIC EVIDENCE FOR primarily reflecting temperature differences, perhaps as much PROTEROZOIC SUBDUCTION SCARS as 700 °C (100 °C per 2% velocity variation; Cammanaro et al., 2004), with low velocity mantle at 100 km depths close to 1350 Plate 1A shows the complex distribution of Proterozoic °C, with partial melt present, and high velocity domains near 650 crustal provinces and younger physiographic/tectonic ele- °C, compatible with shield geotherms. To explain these large ments in the southern Rocky Mountains, including Precam- velocity variations, one hypothesis is that, even though the crust brian crustal provinces, Laramide basement uplifts, the is predominantly Proterozoic, the mantle under the Rocky Colorado Plateau, Cenozoic volcanic belts, and physiographic Mountains might be essentially Cenozoic in age (e.g. Goes and provinces. As shown in tomographic images of the western U.S. van der Lee, 2002; Wilson, 2004), with low velocity domains as upwelling of asthenosphere and high velocity domains as 9CD-ROM Working Group (* denotes graduate students): intact, or downwelling, lithosphere. Chris Andronicos, Nicholas Bolay*, Oliver Boyd*, Sam Bowring, An alternative hypothesis, presented in this paper, is that Kevin Chamberlain, Nick Christensen, Jim Crowley, Jason Cross- the lithospheric mantle under the Rocky Mountains, although white*, David Coblentz, Ken Dueker, Tefera Eshete*, Eric Erslev, extensively modified and reactivated by younger events, is Lang Farmer, Rebecca Flowers*, Otina Fox*, Matt Heizler, Gene primarily Proterozoic in age, and that Proterozoic structures Humphreys, Micah Jessup*, Roy Johnson, Karl Karlstrom, Randy are controlling some of the major velocity contrasts in the Keller, Shari A. Kelley, Eric Kirby, Alan Levander, M. Beatrice Mag- mantle. If this hypothesis is correct, large temperature, com- nani, Kevin Mahan*, Jennie Matzal*, Annie McCoy*, Grant Meyer, positional, and rheology variations are present, but the low Kate Miller, Elena Morozova, Frank Pazzaglia, Claus Prodehl, Adam Read*, Oscar Quezada*, Mousumi Roy, Hanna-Maria Rumpel, Jane velocity domains may still be traveling with North American Selverstone, Anne Sheehan, Liane Stevens*, Colin A. Shaw*, Elena lithosphere because of their buoyancy. Even though these Shoshitaishvili*, Scott Smithson, Cathy Snelson*, Mike Timmons*, domains are weak and hot and rheologically similar to Leandro Trevino*, Amanda Tyson*, Stacy Wagner*, Xin Wan*, Paul asthenosphere, they may not yet be entrained in the asthenos- Wisniewski*, Michael Williams, Huaiyu Yuan*, Brian Zurek* pheric flow. KALSTROM ETKALSTROM AL. 3 Plate 1. a) Geologic elements of southwestern North America showing locations of teleseismic lines in red. Precambrian provinces strike northeast, Laramide uplifts strike north-south, Cenozoic volcanic fields (red= Laramide; black= Neogene) strike northeast. b) Tomographic image of southwestern North Amer- ica at 100 km depth; Snake River Plain, Deep Probe, CD-ROM, and La Ristra teleseismic lines indicated. 4 SYNTHESIS OF THE CD-ROM EXPERIMENT Plate 2. Geologic elements of southwestern North America (Plate 1a) superimposed on 100 km-depth tomography (Plate 1b). In the Rocky Mountain-Colorado Plateau