LITHOSPHERIC STRUCTURE AND EVOLUTION OF THE ROCKY MOUNTAIN TRANSECT OF THE WESTERN U.S. (CD-ROM)

AN INTEGRATED GEOLOGICAL AND GEOPHYSICAL INVESTIGATION


Funded by the Continental Dynamics Program of the National Science Foundation with additional funding provided by Deutsche Forschungsgemeinschaft


Project Summary

The Rocky Mountain region has experienced a complex geologic history that can only be unraveled by the integration of a broad range of geoscience data. This region is of great fundamental interest to studies of the dynamics and evolution of the North American continent. It is also an area where a better understanding of the scarce water resources, natural resources, and earthquake hazards depends on knowledge of the geologic structures present. The Continental Dynamics Program of the National Science Foundation has recently provided the basic funding for a large collaborative effort to study the transect shown in Figure 1. This project is known as CD-ROM (Continental Dynamics - ROcky Mountain project). About 600 km of seismic reflection data acquisition are planned in this transect along with a set of coordinated geological, geochemical, and geophysical studies. There are several data sets that would be of great value to this project but which are not covered by the NSF funding. Remote sensing data are an important example of such a data set. The Pan American Center for Earth and Environmental Studies (PACES) is funded through a cooperative agreement with the University of Texas at El Paso. Through its personnel, basic data (such as Landsat TM), and facilities PACES is playing a major role in the CD-ROM effort. However, the availability of MASTER and AIRSAR data would provide a big boost in the drive to reach the ambitious goals of CD-ROM.

The goal of CD-ROM is to understand the growth, stabilization, and reactivation of the lithosphere of the North American southwest. This area represents the juncture of two globally-unique tectonic regimes each of fundamental importance for understanding continental tectonics and together representing an unparalleled field laboratory for studies of the continental lithosphere. First, a 1500-km wide juvenile Proterozoic orogenic belt records an episode of rapid accretion of continental materials from mantle sources and their assembly to southern Laurentia between 1.8 and 1.6 Ga. Second, the present high elevations of the regional orogenic plateau in the southwest and in the southern Rocky Mountain / Rio Grande rift region in particular is the manifestation of Phanerozoic and still ongoing modification and disassembly of Proterozoic lithosphere.

Although our goals go beyond a single hypothesis, the main testable hypothesis of this investigation is that lithospheric structure produced during assembly of the southwestern United States profoundly influenced physical and chemical modification of the continental lithosphere during all subsequent tectonism, including the ongoing reorganization of small-scale asthenospheric convection and the lithospheric tectonic and magmatic response. This is suggested by the NE-trending embayments in what are generally NW-trending mantle boundaries beneath the continent and the apparent correspondence of mantle anisotropy and velocity provinces with Precambrian crustal provinces. Our investigation is designed to simultaneously understand lithospheric assembly, evolution, and present structure. The hypothesis, if verified, places profound constraints on developing theories of evolution of continental lithosphere by providing a comparison of Archean versus Proterozoic lithospheres, by delineating the process of growth of first-cycle continental lithosphere in the Proterozoic, and by analysis of the interplay between "active" and "passive" processes during lithosphere disassembly. Towards these goals, the western U.S. is an ideal place to combine crustal and mantle geophysics with geologic knowledge of crustal exposures and xenolith studies to characterize the evolution of lithospheric provinces and boundaries in four dimensions from the time of accretion to the present.

This project emulates and builds upon the highly successful interdisciplinary approach of the Canadian Lithoprobe project and complements the cooperative Canadian-U.S. Deep Probe experiment that targeted the mantle of the western U.S. This four-year investigation is focused on lithospheric boundaries within the Rocky Mountain transect which is centered on the 106th meridian and extends from southern Wyoming to central New Mexico. Following the Canadian Lithoprobe (a national geoscience initiative) model, CD-ROM brings together a critical mass of investigators from 12 institutions to document processes of assembly and modification of both crustal and mantle provinces along a transect more than 1000 km in length. Because of the challenge of simultaneously studying lithosphere assembly along NE trending boundaries and its disassembly along N to NW trending boundaries, the CD-ROM approach involves an array of geophysical studies linked together at strategic tie points and integrated with diverse geologic investigations. In particular, an integrated set of very large-scale seismic experiments will be undertaken using state-of-the-art data acquisition techniques. Satellite data, image processing, digital terrain models, GIS technology, and modern visualization schemes will be employed to create a data base suitable for integrated analysis and to view it in creative ways. These data will also aid in efforts to compile geologic data over a vast region in a cost-effective way.

The major components of this project are:

1. Crustal reflection and refraction and experiments across three key boundaries -- the Archean-Proterozoic boundary exposed at the Cheyenne Belt near the Colorado / Wyoming border and the Intra-Proterozoic boundaries exposed in the Wet Mountains of Colorado and along the Jemez lineament in northern New Mexico.

2. Teleseismic recording in northern New Mexico to image modern mantle structure.

3. Geologic and geochronologic studies of exposed rocks along the transect to understand evolution of boundaries.

4. Xenolith studies to reconstruct the compositional and physical properties of lithosphere under different provinces.

5. Modeling studies aimed at putting our results into a general framework for understanding lithosphere dynamics.

6. Integrated analysis of all available data employing modern GIS, image processing, and visualization schemes.

The CD-ROM group is attempting to provide among the most detailed images ever attained of the 3-D structure of accreted crust and mantle fragments and will place these images into a tectonic context by parallel geologic studies. This collaboration represents a significant synergism and a serious commitment to merge diverse disciplines.


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