COMPARISONS BETWEEN CRUSTAL STRUCTURE IN THE BASIN AND RANGE PROVINCE OF SOUTHERN NEVADA, CALIFORNIA, AND WESTERN ARIZONA AND THE RIO GRANDE AND KENYA RIFTS

The Basin and Range province is an unusually wide continental rift zone, and its structure and evolution have been the topic of much study and debate, underscoring the need for additional geophysical constraints. Several large seismic experiments (SSCD, DELTA FORCE, and PACE) have revealed interesting relations with the adjacent Colorado plateau and Sierra Nevada Mountains. They also show that while the crustal thickness is surprisingly uniform in the Basin and Range area, there are some intriguing variations. The velocity models derived from these experiments coupled with analysis of gravity data reveal some intracrustal features that can be correlated with extensional regimes. In particular, the lower crust in the seismic models thickens in regions where the extension is greater, and the regions with thickened lower crust also correlate with long wavelength gravity highs. However, the velocity/density structure indicates no significant excess of mafic material in the crust. Thus, these results suggest that the mechanism for maintaining crustal thickness during extension involves some combination of lower crustal flow and magmatic additions of material of intermediate to felsic composition. In contrast to the Basin and Range province, the classic Rio Grande and Kenya rifts are examples of narrow rifts. In both of these rifts, the crust thins in a pattern that correlates with geologic observations of extension. Seismic data in the Rio Grande rift are of relatively low resolution but reveal no evidence of significant magmatic modification of the crust. Although the Socorro magma chamber is an extremely interesting feature, it is only a few 100 m thick. In Kenya, the KRISP seismic project results indicate the presence of ~ 10 km of crustal thickening due to underplating beneath the Kenya dome. North of this dome, the crust thins and the amount of crustal extension increases, while modest crustal thinning and underplating is associated with the rift south of this dome.