CENOZOIC EXTENSION IN THE GREAT BASIN: MUCH STILL UNKNOWN AND/OR DISPUTED
The most prominent example of pre-17 Ma extension, the Eocene Elko Basin, consists of a series of northwest-dipping fault zones that initiated ~47 Ma and generated half-graben basins filled with ≤1 km of sediments over a present-day area >7500 km2. Angular unconformities between 40 and 38 Ma deposits indicate further Elko extension, and this region became the Ruby Mts core complex. Magmatic heating of thick crust may be significant, but extension preceded local magmatism by 2-5 Ma, and the EB lay west of thickest crust (estimated ~45 km thick). EB is the largest area of early extension but tiny compared to the entire NBR. Other areas of major magmatism did not undergo coeval extension, e.g., the 32-27 Ma Indian Peak caldera complex, which developed on the thickest interpreted crust. Moreover, the solidified caldera belt resisted deformation so that it is the least extended part of the NBR.
Many other areas of interpreted extension are restricted to a single range with uncertain continuity along or perpendicular to strike and in total affected area. Pre-17 Ma extension seems mostly minor and resulting from inconsistent mixes of special conditions. Nevertheless, more comprehensive study of early extension is needed, especially by different groups with divergent interpretations jointly examining and challenging field interpretations.