Vol. 11, No. 12 A Publication of the Geological Society of America December 2001 INSIDE L Thick-Structured Proterozoic Lithosphere L Northeastern Section Meeting, p. 14 of the Rocky Mountain Region, p. 4 L Ken Dueker, Huaiyu Yuan, and Brian Zurek Rock Stars: Francis Parker Shepard, p. 20 L 2000 Presidential Address, p. 41 VDC ENGINEERED TO BE ALMOST PSYCHIC. Introducing a car so technologically advanced, it can sense the wheels. Even the engine’s output may be momentarily trouble and begin to adjust for it before the driver even reduced. Before the driver even realizes that loss of control notices that there’s a problem. It’s the 6-cylinder 212- is impending, any or all of these measures may be applied horsepower Outback VDC from Subaru. The VDC stands for automatically to help restore directional stability. It’s almost Vehicle Dynamics Control, a highly intelligent stability system as if the car has a sixth sense. that rivals those found in vehicles costing thousands more. 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Classified Advertising . .35 GSA ONLINE: www.geosociety.org 2000 Presidential Address: Grand Challenges in Earth This publication is included on GSA’s annual and Environmental Sciences: Science, Stewardship, CD-ROM, GSA Journals on Compact Disc. 41 Call GSA Member Services for details. and Service for the Twenty-First Century. Mary Lou Zoback . Printed in U.S.A. using pure soy inks. Journal Highlights . .47 Ed. note: “Dialogue” will resume in 2002. 50% Total Recovered Fiber 10% Postconsumer GSA TODAY, DECEMBER 2001 3 Thick-Structured Proterozoic Lithosphere of the Rocky Mountain Region Ken Dueker, Huaiyu Yuan, and Brian Zurek, Dept. of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006, USA ABSTRACT reveal a surprisingly thick continental lithosphere. In our A new tomographic image of the western United States transect across the Proterozoic Jemez suture, we find the shows three northeast-trending, low-velocity, upper mantle coincidence of young lithospheric volcanism, a low-velocity anomalies in the Rocky Mountain–Colorado Plateau region: mantle anomaly, and deep lithospheric layering (to 170 km). the Yellowstone, Saint George, and Jemez lineaments. Each is In our Wyoming-Colorado transects across the Archean- characterized by small compressional wave-speed anomalies Proterozoic Cheyenne suture, we find the coincidence of (±2% perturbations) that deep mantle layering and extend to 200–250 km a north-dipping, high- depth. A fundamental velocity slab, which extends question is whether they (to 200 km depth) from represent ongoing astheno- the base of an imbricated spheric convection or old Moho, directly under the lithospheric compositional Cheyenne suture. We bodies. This puzzle is suggest this slab was compounded by the trapped against the edge observation that each is of the thick, Archean-age aligned with both young Wyoming lithosphere after volcanic fields and the subduction polarity Proterozoic crustal grain flipped from south- to and/or sutures. We suggest north-directed after that the low-velocity and/or during accretion of bodies are lithospheric the first Proterozoic arc anomalies and that they along Wyoming’s south- were derived from melting ern margin 1.78–1.75 Ga. of hydrated olivine-poor Such a tectonic model for lithologies (oceanic slabs, the evolution of the their associated sediments, Cheyenne belt is con- and batholith residue) Figure 1. Image of compressional-wave velocity structure at 100 km depth sistent with observations made from combined inversion of P-wave data used to construct Fig. 2 and that were tectonically along many other shear-wave velocity model of van der Lee and Nolet (1997). High velocities Archean-Proterozoic emplaced during (blue shading) on eastern side of image reflect cold, stable tectospheric root Proterozoic suturing present beneath North American craton, and low velocities (yellow and red sutures worldwide. Overall, events. Such lithologies shading) on western side of image reflect the generally thin lithosphere and our results demonstrate would be hydrated by warm asthenosphere. In between these two “end-member” regions is average that Proterozoic crustal the water in oceanic slabs velocity (green