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Precambrian



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Imaging and Physical Properties of Accreted Proterozoic Terranes in the Western U.S.

    One of the intriguing tectonic problems in the evolution of continental lithosphere is the nature of accretion of fragments or terranes of continental crust and their ultimate assimilation into the conti­nental framework.  The Proterozoic (particularly between 1.8 and 1.6 Ga) was a time of excep­tional continental growth, a record of which is preserved in the 1500 km-wide Proterozoic orogenic belt in the southwestern United States.  During September-November 1999, the University of Arizona, a participant in a major NSF-sponsored interdisciplinary project (Continental Dynamics - Rocky Mountain Project: CD-ROM), helped to collect over 200 km of high-quality vertical-component deep seismic reflection data across important Precambrian crustal boundaries in Wyoming and Colorado.  The cdrom99-map-2-x.jpg (426112 bytes)University of Arizona's primary effort, however, focused on collection of additional full-wavefield (3-component) data to record both compressional and shear waves in order to 1) improve the chances of detecting reflections from very steeply-dipping boundaries, 2) to help determine directional properties of out-of-the-plane reflections, and 3) to provide estimates of Poisson's ratios to help constrain subsurface rock compositions.

    In order to improve signal-to-noise ratios in the data, Elena Shoshitaishvili, who graduated from the U of A with a Ph.D. in Reflection Seismology, developed a new approach to harmonic noise removal that avoids some of the deleterious effects of simple notch filtering.  (This work is the subject of a manuscript submitted to Geophysics).  Notch filtering, which works well for eliminating a single problem frequency, is not effective on data that contain multiple problem frequencies.  Elena used a time-domain-based method of automatic estimation of noise frequencies and their amplitudes, followed by subtraction of these estimated anomalous harmonics from the data.  Filtering reflection data from the northern CD-ROM transect across Archean and Proterozoic terranes in southern Wyoming and northern Colorado in this way significantly improved data quality even in areas where first breaks were completely obscured by noise.  New analyses of the improved first breaks from CD-ROM multicomponent reflection data have provided more accurate near-surface velocity models which have led to better estimates of rock properties based on P- and S-wave arrivals.  This filtering also improved our ability to correlate the seismic data with geological exposures and now is providing more robust three-dimensional information about deeper events.