Iconic Images of the Rocky Mountain Region

Submitted by RME Workshop I Participants

Results from the Colorado Plateau/Rio Grande Rift/Great Plains Seismic Transect (RISTRA 1) experiment. RISTRA 1 was a 1999-2001 950 km-long PASSCAL broadband teleseismic deployment of 54 seismographs with approximately 18 km station spacing across the southwestern U.S. The deployment followed an approximate great-circle path between Lake Powell, UT and Pecos, TX. Receiver function results (underlying grey-scale image) show crustal thickness ranging from 45 to 50 km beneath both the Colorado Plateau and the Great Plains, thinning to a minimum of 35 km centered nearly directly beneath the Rio Grande rift (RGR) axis. Tomographic inversion of teleseismic body-wave delay times (underlying S-velocity color image) show a broad low velocity region, also centered beneath the rift axis that extends from the Tularosa Basin to the Jemez Lineament. These observations suggest that the lower crust and mantle lithosphere of the RGR have deformed symmetrically about the rift axis, indicating an essentially pure shear mode of deformation characterized by symmetric stretching and necking of the crust and lithosphere. Upper mantle receiver function images show relatively flat discontinuities at 410 km and 670 km, indicating there is not a large-scale, deep-seated thermal anomaly beneath the rift, and that it is primarily an uppermost (200 km or shallower) mantle feature. However, smaller-amplitude variations in upper mantle velocity indicate unexpected localized thermal and/or or compositional variations that may be indicative of small-scale convection and lithospheric convective entrainment and/or reactivated pre-Cenozoic structures. Recent results suggest that such features may be ubiquitous in western U.S. upper mantle, and that uncovering their nature will be a prime target for EarthScope and related science efforts in the next decade. Images after Wilson et al. [2004] and Gao et al. [2004].

Gao, W., Grand, S., Baldridge, S., Wilson, D., West, M., Ni, J., Aster, R., An upper mantle seismic cross section through the Colorado Plateau, Rio Grande Rift, and Great Plains, J. Geophys. Res., 109 (B3), p.B03305, 2004.

Wilson, D., Aster, R., Ni, J., Grand, S., West, M., Gao, W., Baldridge, W.S., Semken, S., Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau, submitted to JGR, 2004.

Submitted by R. Aster, D. WIlson, J. Ni, S. Grand, and M. West

Comparison of CCP stacked receiver functions (at approximately the same scale) for the South American Andean/Sierra Pampean Mountains (30 deg S) and the North American Basin and Range/CP/Rocky Mountains (38.5 deg N) NA image from Gilbert and Sheehan [2004]; SA image from Gilbert, Beck, Zandt [2004]. Solid white lines marked M denote the Moho and dashed grey lines indicate possible active or relict structures. The Andean/SP region is underlain by a "flat subduction zone" and the Sierra Pampeanas are structures that are often cited as an active analog for the Laramide Rocky Mountains.

Submitted by G. Zandt and H. Gilbert

Site velocities and 95% uncertainties for the CORS network of continuous GPS stations during 1994-2003, from SINEX data produced by Dillinger et al. [2003]. Velocities are shown relative to the site TCUN in Tucumcari, New Mexico, and have been corrected for spherical cap motion of the North American plate. The GPS data appear to indicate about 1 mm/yr opening and 1 mm/yr sinistral strike slip across the Rio Grande rift. However, most of the site monuments were not designed for tectonic studies, and many are probably perturbed by local effects of monument wander, hydrology, and larger-scale mass loading effects. PBO backbone sites (some of which have already been installed) will be insensitive to local motion and so will improve our understanding of Rio Grande rift kinematics. However the site motions must still be corrected for mass loading phenomena, and they are too sparsely distributed (~200 km spacing)to constrain rift dynamics for geodynamical modeling. Hence some additional densification of GPS measurement will be needed to understand contemporary behavior of the Rio Grande rift.

Dillinger, W., M. Cline, R. Foote, S. Hilla, J. Ray, R. Snay, and T. Soler, Analysis of 9 Years of North American CORS Data, Eos Trans. AGU, Fall Mtg. Abstr., G31B-0701, 2003.

Submitted by Tony Lowry

Question to be addressed by Earthscope: Is there a previously unrecognized Middle Tertiary basaltic underplate under the High Plains of southeastern Colorado and northeastern New Mexico?
The tilt of the apatite fission-track partial annealing zone beneath the High Plains is in part due to high heat flow, perhaps related to middle Tertiary basaltic underplate, and in part due to exhumation driven by changes in mantle buoyancy related to the latest Eocene to early Miocene "Ignimbrite flareup."

Submitted by Shari Kelly

Surface Heat Flow vs. Lower Crust Heat Production Across Cheyenne Belt

Submitted by Lang Farmer

Graphic representation of the place-based, culturally-integrated Indigenous Physical Geology (also called Tsé na'alkaah, the Navajo term for geology) course offered at Diné College, and now being adapted for use at Arizona State University. The subject matter emphasizes the regional geology of the Colorado Plateau, and is organized in accordance with the important Navajo principle of duality: natural processes and features effected by interactions between the complementary natural systems of Earth (Nohosdzáán) and Sky (Yádilhil).  The course progresses as a cyclical journey (clockwise or "sunwise" around the circle) through systems and processes associated with the solid Earth, through near-surface interactions of Earth and Sky, to systems and processes associated with the fluid Earth and sky.

Semken, S., 2005, Sense of place and place-based introductory geoscience education for American Indian and Alaska Native undergraduates, Journal of Geoscience Education, in press.

Submitted by Steve Semken

Integration of the DEEP PROBE and CD-ROM lithospheric models. The DEEP PROBE and CD-ROM seismic refraction experiments shared a common shotpoint in central Wyoming. Together, these profiles produce a view of lithospheric structure that extends from eastern New Mexico to well into Canada. In the U.S. alone, these profiles have a total length of ~1800 km. By using gravity modeling as the medium for integrating the results of these experiments and other geological and geophysical data, a model of lithospheric structure has been constucted for the region from the south end of the CD-ROM transect to Montana. This model shows the combined effects of complex upper crustal structure, crustal thickness and variations in the mantle lithosphere.

Submitted by Randy Keller

Results of a quantitative analysis of the Western U.S. Topography (30-sec DEM ~ 1km resolution) that combine spectral analysis and eigenvalue ratio methodologies. Figure shows a) topographic flatness, b) topographic organization, c) the "k-value" or ratio of flatness to organization, and d) the topographic grain (with blue and red vectors designate all grain orientations and those orientations passing the Rayleigh test, respectively). Of interest is the correlation between spatial variation of these datasets and the crustal structures identified along the DEEP PROBE and CD-ROM profiles. The second figure shows the relative location of the various tectonic provinces in the Western U.S. in "flatness-organization" space (discussed in detail in Woodcock [1977] and in a to-be-published manuscript by Coblentz and Karlstrom), based on two ratios of the eigenvalues of the orientation tensor formed by the 3x3 matrix of the sums of cross products of the direction cosines defining the surface normal vectors within a subset of the DEM. While the ten provinces show all fall well within the "cluster" regime of the graph (so called because it describes the surface normal grouping in the steronet plots), the provinces tend to group according to their tectonic origin -- e.g., the Great Plains is distinct from the Basin and Range-Madrean Archipelago and the Yellowstone-Southern Rockies-Sierra Madre groups. We hope this approach will shed light on the relationship between subsurface tectonic processes (active and relic) and the regional topographic fabric.

Submitted by David Coblentz

Small section of an anaglyph stereo map of the Rocky Mountains that centers on the Arkansas River and the northern part of the San Luis Valley. The map was produced by down-sampling NASA/USGS SRTM data to 400 meter resolution. When the map is viewed with red/blue anaglyph glasses the topographic expression and subdivisions of the Rocky Mountains become dramatically apparent for earth science students and professionals. Whether displayed in the Geowall stereo environment or as paper anaglyph maps, these visualizations are changing the way students learn about geology.

Submitted by Mike Kelly

Active faults, seismicity, and travertine localities of NW Arizona overlain on shaded relief. Color bands represent tectonic subprovinces, and spring geochemistry is summarized in the inset Piper. The intriguing potential suggested by these data is that geochemical discriminant diagrams can identify different "lower world" source end members and mixing trends from different tectonic subprovinces (color-coding in Piper diagram matches the map subprovinces). This is the scale of the profound small scale velocity contrasts seen in tomography, raising the possibility of identifying different geochemical signatures from different mantle domains and thus helping to understand the mantle heterogeneity that will be spectacularly imaged by USArray.

Submitted by L. Crossey, K. Karlstrom, and D. Newell

The conceptual model for mantle to surface conveyance system involves both basaltic magma carrying mantle fluids to the middle crust and faults conveying them to the surface water system. In conjunction with other proposed monitoring of the Socorro magma body, we propose to monitor both wells and springs above the magma body. The frequency of microseismicity will allow direct monitoring of the connection between seismic activity and changes in composition of groundwaters.

Submitted by K. Karlstrom, D. Newell, and L. Crossey

Composite image of the upper mantle seismic structure at 100 km depth. Blue is high velocity mantle and red is low velocity mantle. The image is from the modeling of Grand [1997](1) with overlay of data from regional arrays (2). Black lines outline western U.S. physiographic provinces. Overlain on the image are 3He/4He measurements (3-28) (reported relative to air, Ra) from springs and groundwater. 3He and CO2 form a gas/fluid phase that represents direct leaks from the mantle (called xenowhiffs). Although additional data coverage is needed, regional variations in background Ra values from different tectonic provinces, when superimposed on mantle tomography, shows a striking correlation between low velocity (partially molten) mantle of the western US and elevated 3He (compare the Canadian shield to Colorado Plateau to Basin and Range). Further, spikes of 0.4 to 4 Ra correspond to fault-controlled fast pathways for mantle helium flux, such as the san Andreas fault and Walker Lane structure. Highest 3He/4He values of 3-10 Ra are close to values from direct mantle input (MORB = 8 Ra; plumes > 8 Ra) are present above Quaternary and active magmatic systems in Yellowstone, Long Valley, Jemez, and Salton trough. The combined evidence suggests that regional 3He/4He characterization in spring waters will provide evidence for nature of tectonic provinces (backgrounds), mantle leaks through the crust (fault fast pathways), and direct connections between the mantle and the surface water system. Transet time of mantle helium must be very fast (< years) given the evidence for increase in 3He/4He ratios following seismic swarms (e.g., 16).

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Submitted by D. Newell, K. Karlstrom, L. Crossey, B. Kennedy, and D. Hilton

Figure 1. (a) Geologic elements of southwestern North America showing location of teleseismic lines in red. Precambrian provinces strike northeast, Laramide uplifts strike north-south, Cenozoic volcanic fields (black) strike northeast.

(b) Tomographic image of southwestern North America at 100 km depth. Snake River Plain, Deep Probe, CD-ROM and RISTRA teleseismic lines indicated.

(c) (a) superimposed onto (b). In the Rocky Mountain-Colorado Plateau region, "fingers" of hot mantle penetrate older lithosphere along northeast-striking zones. Young basalts (< 10 Ma) are present along the Yellowstone, St. George, and Jemez zones, suggesting that these mantle domains are hot and producing basaltic melts.

Figure 2. Cross-sectional synthesis of the CD-ROM transect [modified from Karlstrom et al., 2002]. Generalized geologic cross section merged with S-wave tomographic image of Dueker et al. [2001]. Crustal structures in the Cheyenne belt and Jemez lineament areas are generalized from CD-ROM seismic reflection data, with solid lines representing well-defined reflections. Locations of xenolith pipes are shown as vertical lines. Dipping elements in the tomographic image, combined with overlying crustal structures are interpreted to be Proterozoic suture zones, and North American lithosphere is interpreted to extend to > 200 km depth (the depth extent of the tomographic image is not well resolved below 200-250 km). Crustal thickness and lower crustal mafic layer from the CD-ROM refraction experiment. Receiver function images are superimposed in red on the tomographic image and show significant upper mantle (lithospheric) layering. U-Pb zircon ages from lower crustal xenoliths show a predominance of 1.7-1.6 Ga ages in the State Line district and ~1.4 Ga in the Navajo volcanic field.

Figure 3. Cross-sectional tomographic images along lines indicated in Fig. 1b: A = Snake River Plain line, B = Deep Probe line, C = CD-ROM line, D = La Ristra line. Note presence of Cheyenne belt (CB), Farwell Mountain (FM), Colorado Mineral belt (CMB) and Jemez lineament (JL) sutures as sharp velocity gradients in multiple lines; also note dipping character of mantle velocity domains.

Figure 4. Block diagram showing results from the CD-ROM, Deep Probe, Snake River Plain, and La Ristra teleseismic lines. Major mantle velocity contrasts can be identified in multiple lines and seem to correspond (in pseudo 3-D) with surface Proterozoic province boundaries.

Submitted by Karl Karlstrom

Submitted by Michael L. Williams

(a) Results of Rayleigh Wave tomography at Yellowstone. Red and blue bands show regionalized Vs structure under the Yellowstone hotspot and eastern Snake River Plain (YVT) and surroundings (non-YVT). Width of bands gives standard error of Vs. Starting model is black line, and, for comparison, Vs in Hawaii [Preistley, Tillman, 1999] and Tectonic North America (TNA) [Grand, Helmberger, 1984] are presented.

(b) Regionalized Yellowstone Vs structure, plotted with predicted Vs profiles for two adiabats and the anhydrous peridotite solidus. Intersection of YVT profile with solidus at 105 +/- 15km suggests YVT mantle has a potential temperature of 1447C, ~150C hotter than normal convecting mantle, implying Yellowstone has nucleated off of a thermal boundary layer. Width of gray band for 1300C adiabat shows range in velocities predicted for grain sizes ranging from 1mm to 10mm. For the 1447 adiabat and dry solidus, assumed grain size is 1mm. Temperature velocity relations calculated from attenuation experiments of Jackson [2000]. Temperature of dry solidus from Hirshmann [2000].

Submitted by Derek Schutt

Yellowstone 3-D Results.

(a) Crustal thickness based on inversion of phase velocity data with PmS times as a constraint [Yuan and Dueker, pers. comm.].

(b) Mean crustal velocity. Yellow star gives most recent location of hot spot (Sour Creek Dome). Map view of transects in subfigures D and E shown.

(c) Vs at 80km depth. Blue "halo" around YVT is due to inversion regularization, and real velocity transition is probably sharper.

(d) Cross-section along YVT. White line is inferred bottom of mantle lid.

(e) Cross-section perpendicular to YVT.

Submitted by Derek Schutt