An integrated crustal study of the Basin and Range Province, Colorado Plateau region, western Arizona

The inversion method developed by Zelt and Smith (1992) has been applied to the travel-time data from the 1989 Pacific to Arizona Crustal Experiment.

The inversion technique applied to the PACE-1989 Grand Canyon profile provides a detailed picture of the crustal velocities and structure in northern Arizona. The upper crustal model along the crossline (Grand Canyon) profile consists of a thin layer ($<$2.0 km) of highly fractured sedimentary rocks with velocity variations ranging from slightly greater than 2.0 km/sec to up to 5.0 km/sec. This sedimentary layer is underlain by more homogeneous basement which was resolved to depths of about 10.0 km with velocities ranging between 5.8 km/sec to 6.2 km/sec and localized high velocity zones of $>$6.3 km/sec. The Bouguer gravity anomaly map of the area suggests that the low velocities and many gravity lows generally correlate and can be considered possible locations of Proterozoic sedimentary basins. The high velocities, on the other hand, correspond to the high density areas of intermediate to basaltic intrusions probably associated with the Late Cenozoic San Francisco volcanic field.

The combination of the densely recorded main profile, a small seismograph array deployed parallel to this profile, and the multiple shots some of which were not inline with the main profile made it possible to conduct a time-term tomographic analyses of upper crustal Pg arrivals. The results indicate high velocity zone under the San Francisco volcanic field at upper crustal depths.

The inversion of deeper reflective arrivals along the PACE-1989 Grand Canyon profile suggests an interesting crustal model for the Colorado Plateau. The new model consists of an approximately 50 km thick crust with a 40-43 km deep transition zone. The velocity in the transition zone varies from 7.4 km/sec at the top to about 7.6 km/sec at the bottom. The gravity modeling of the crustal structure derived with the seismic inversion algorithm indicates a slight thinning of the crust in the NW direction.

These results show that the upper crust of the southwestern Colorado Plateau is very complex reflecting the effects of late Proterozoic and late Tertiary tectonic events. The simpler deeper crustal structure derived is due in part to a lack of resolution, but the thick transitional layer at the base of the crust is a particularly interesting feature. (Abstract shortened by UMI.)