UPPER MANTLE ANELASTICITY AND TECTONIC EVOLUTION OF THE WESTERN UNITED-STATES FROM SURFACE-WAVE ATTENUATION

Rayleigh wave phase and group velocities and attenuation coefficients in the period range 18 to 120 s have been determined for three regions along nine paths across the western United States using a two-station technique. The attenuation coefficients were found to increase from east to west between the Rocky Mountains and the Pacific coast. Rayleigh wave group and phase velocities were inverted, using a differential procedure, to obtain shear wave velocity models for three regions. These velocity models were then used in an inversion process where Q-beta-1 as a function of depth was obtained from observations of Rayleigh wave attenuation. The inversion results show that Q-beta values in the upper mantle of the western United States are lowest for the coastal regions and westernmost Basin and Range and highest for the Rocky Mountains and western Great Plains. Intermediate Q-beta values appear to occur in the upper mantle beneath the Basin and Range province and the Columbia and Colorado plateaus, although uncertainties in the data prevent a clear separation between that region and coastal regions. Low values of Q-beta occur in the upper crust, higher values in the lower crust, and highest values in the uppermost 20 km of the mantle of all these regions. These overlie an upper mantle low-Q zone in which Q-beta values decrease from east to west, lowest values (15-20) being similar to those observed above descending slabs in the western Pacific. The regional pattern of upper mantle shear wave Q values is similar to that observed for upper crustal shear wave Q values inferred from surface wave studies and for Lg coda Q values. The patterns of upper crustal and upper mantle Q variations correlate with the temporal sequence of tectonic and magmatic activity in the western United States, lowest Q values in both depth ranges occurring where tectonic activity has been most recent. Partial melting and/or enhanced dislocation motion in the mantle and variations in the volume of interstitial hydrothermal fluids in cracks in the upper crust may both be ultimately due to processes which occurred during plate consumption and interaction near the western margin of the United States.