Crustal structure across the Wasatch Front from detailed seismic refraction studies

Abstract

A high-resolution seismic refraction profile using large open-pit mine blasts as sources has been recorded along a 340-km profile that crosses the Basin and Range-Middle Rocky Mountains province boundary in northern Utah. Interpretation of these data and existing U.S. Geological Survey refraction lines indicate a crustal thickness of 28 km and an upper mantle P wave velocity of 7.6 km/s in the eastern Basin and Range, increasing to 40 km and 8.0 km/s, respectively, in the Middle Rocky Mountains. The increase in crustal thickness does not occur at the boundary between physiographic provinces but begins about 40 km east of the Wasatch front. The zone of increase of crustal thickness, beginning east of the Wasatch front, correlates with the eastern limit of late Cenozoic block faulting, a zone of shallow seismicity, the boundary between high and normal heat flow, and an upper mantle conductivity anomaly. A crustal low-velocity layer located between 10 and 15 km in depth is suggested to be continuous from the Basin and Range province into the Middle Rocky Mountains province. Existing data are insufficient to indicate the possible eastern limit of the crustal low-velocity layer. Shear wave velocities for intermediate crustal depths, including the low-velocity layer, are anomalously low. Poisson ratios for the low-velocity layer as high as 0.32 may be interpreted. These ratios suggest that the low-velocity layer is a zone of low rigidity.