Crustal thickness and Vp/Vs beneath the western United States: Constraints from stacking of receiver functions.

摘要

This study investigates crustal characteristics beneath the western United States. Crustal parameters, including crustal thickness (H), mean Vp/Vs, and the sharpness of the Moho as quantified by the amplitude of the P-to-S converted phases from the Moho, are measured at a total of 1000 permanent and portable broadband seismic stations using over 150,000 receiver functions recorded between 1988 and 2012. The study area is composed of a diverse set of tectonic provinces formed by a series of tectonic events, and is thus ideal for investigating crustal formation and modification by different tectonic processes. The resulting crustal thickness varies from 21 km in the Pacific Border to 57 km beneath the Rocky Mountains, with a mean thickness of 38.5+/-1.8 km. The Vp/Vs varies from 1.70 in the Great Plains to 1.90 in the Columbia River Basalt Group (north Columbia Plateau), with a mean value of 1.77+/-0.014. We explore relationship between crustal thickness, Vp/Vs, surface elevation, and the age of the crust, and find no positive relationship between crustal thickness and Vp/Vs. In addition, no clear connections between the Vp/Vs and the crustal age are found. High Vp/Vs and relatively low R values are observed at locations with crustal partial melts inferred by seismic tomography, such as Yellowstone, the eastern Snake River Plain, and the southeastern portion of the Rio Grande Rift. Moreover, secondary arrivals that arrive before the P-to-S conversion from the Moho and probably reflect the top of the so-called 7.xx layer, are observed beneath the Cheyenne Belt, boundary of Snake River Plain and Basin Range, as well as beneath the Wyoming craton, Colorado Plateau, and Delaware Basin.