Probing the Structure and Evolution of the Rocky Mountains - Fig. 2

Probing the Structure and Evolution of the Rocky Mountains - Fig. 2 Figure 2. Cross-sectional synthesis of the CD-ROM transect. Generalized geologic cross section merged with S-wave tomographic image. Regional crustal structure is derived from the CD-ROM refraction experiment. More detailed crustal structure in the Cheyenne belt and Jemez lineament areas are generalized from seismic reflection data, with solid black lines in the crust representing well-defined reflections. Locations of xenolith pipes are shown as vertical lines and red stars show samples studied. Dipping elements in the tomographic image, combined with overlying crustal structures are interpreted to be Proterozoic subduction scars, and North American lithosphere is interpreted to extend to > 200 km depth.
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In spite of strong scientific interest in the region of the Southern Rocky Mountains, our knowledge of the lithospheric structure of this region was modest until a series of seismic experiments began in the mid 1990s. The Continental Dynamics of the Rocky Mountains project (CD-ROM) is the most integrated and ambitious of these projects, and its ultimate goal is to constrain and unravel aspects of Proterozoic, Laramide, and Neogene tectonism and their interactions, and the processes that have shaped the region during the past 2.0 Ga.
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The CD-ROM experiment, a coordinated set of passive and controlled source seismic experiments (Figure 1), 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 (Figure 2). Crustal provinces with different geologic histories 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 heterogeneous mantle structure, although strongly influenced by ancient compositional variations, 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 throughout the Rocky Mountains, and there is ~10-km-scale Moho topography. Both are interpreted 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.
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Continental Dynamics of the Rocky Mountains Working Group, Structure and evolution of the lithosphere beneath the Rocky Mountains: Initial results from the CD-ROM experiment, GSA Today, 12, 4-10, 2002.
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Dueker, K., H. Yuan, and B. Zurek, Thick-structured Proterozoic lithosphere of the Rocky Mountain region, GSA Today, 11, 4-9, 2001.
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Karlstrom, K. E. and G. R. Keller, The Rocky Mountain Region -- An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics: American Geophysical Union, Geophysical Monograph 154, 441p., 2005.
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Karl E. Karlstrom • University of New Mexico; Kenneth Dueker, Scott Smithson • University of Wyoming; Eugene Humphreys • University of Oregon; G. Randy Keller, Kate Miller • University of Texas at El Paso; Alan Levander • Rice University; Claus Prodehl • University of Karlsruhe, Germany; Catherine Snelson • University of Nevada at Las Vegas

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