Shallow velocity model of the Rio Grande rift near Socorro, New Mexico

A compressional-wave velocity model is presented for the upper few kilometres of the southern Albuquerque Basin of the Rio Grande rift in New Mexico. The velocities are determined from refractions (that is, first arrivals) on individual COCORP seismic reflection records. For the eastern third of the rift (line 1), the refractions are clear, and it is possible to define the gross structure in addition to the predominant velocity layers of 2.8, 3.7, 4.5, and 6.1 km/s. The detailed structure for much of line 1A cannot be determined. The alluvium is thicker and the data quality poorer for the western two-thirds of the rift, but it is possible to recognize some of the velocity layers seen on line 1. Sonic-log data are used to correlate the observed velocity data with the stratigraphy. The observed velocity model implies pronounced variations in thicknesses of the Tertiary, Mesozoic, and Paleozoic strata and extreme structural relief of the crystalline basement. The most conspicuous feature shown by the velocity data is a horst, an intrarift uplift of the Precambrian basement.