ONGOING STUDIES OF TILTING AND UPLIFT IN THE SOUTH VIRGIN MOUNTAINS, NEVADA

Comparison of new Aluminum-in-hornblende barometric determinations from the Gold Butte Granite in the South Virgin Mts., Nevada, with published thermobarometry from the surrounding country rocks suggests a complex history of tilting of the Gold Butte crystalline block. Preliminary data from the 1.45 Ga granite suggests emplacement at pressures of ~400-700 MPa, which overlaps with data from the country rock (Fryxell and others, 1992). However, the west-to-east field gradient derived from the new data is only ~9 MPa/km, which contrasts strongly with the ~27 MPa/km from the surrounding ~1.7-1.8 Ga country rock. This preliminary data permits a significant degree of tilting between the Middle and Late Proterozoic, and suggests caution in applying the field gradient from Proterozoic rocks to constrain crustal tilting due to Cenozoic extensional unroofing. Furthermore, because the Gold Butte crystalline block has been interpreted as one of the thickest and best-exposed intact upper crustal sections in North America, is used as a laboratory for development and testing of thermochronometers, and has great potential for investigation of magmatic evolution and migration models, it is important to understand not only it's Cenozoic history, but the history of tilting, exhumation, and uplift of the block from the Proterozoic to the present day. Future work, including more detailed geobarometric studies and dating of possibly diachronous patterns of tilting of sedimentary strata across the Grand Wash Trough and South Virgin Mountains may provide useful constraints on the timing and pattern of tilting, extensional unroofing, and uplift of the Gold Butte crystalline complex.