CENOZOIC EXTENSION IN THE GREAT BASIN: MUCH STILL UNKNOWN AND/OR DISPUTED

The timing and causes of extension in the northern Basin and Range are highly debated, with questions about contributions of thickened crust - gravitational potential energy, plate boundary reorganization, crustal heating - weakening by rollback magmatism, and dynamic topography. My non-unique interpretation is that most NBR extension began ~17-16 Ma and resulted from collapse of an elevated region of thickened crust but only when the change from subduction to transform faulting at the plate margin allowed expansion of the entire NBR. Yet numerous episodes of older extension are interpreted throughout the NBR. I discount some as incorrectly interpreting tectonism from either angular unconformities between ignimbrites with primary dips or coarse clastic deposits resulting from dam-burst floods, both common in major paleovalleys that drained the region. Continuity of ignimbrites that flowed down paleovalleys precludes major surface disruption over much of the NBR between ~45 and 19 Ma.

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.