CRYPTIC FAULTING AND CRUSTAL SCALE FLUID INTERCONNECTIONS IN THE GREAT BASIN EXTENSIONAL PROVINCE, NEVADA AND UTAH; IMPLICATIONS FROM DEEP MT RESISTIVITY SURVEYING

In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp which are interpret to approach low dip angles with depth. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust in this province. In this paper, recent magnetotelluric (MT) profiling results are presented which reveal families of structures commonly dominated by high-angle conductive structures interpreted to reflect crustal scale faults. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to be residual from basaltic underplating. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. Dense MT array surveying in the Dixie Valley thermal area of western Nevada shows that the main displacement fault lies nearly 2 km basinward from the topographic scarp, with shallow basement between as confirmed by drilling. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to the lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. These results demonstrate that a variety of fault structures exist in active extensional environments presumably controlled by localized rheology and stress features.