INVESTIGATING OLIGOCENE-MIOCENE FOLDED TUFFS AND EXTENSIONAL FAULTS ALONG THE BUCKHORN FAULT, PAHRANAGAT SHEAR ZONE, NEVADA

The Pahranagat Shear Zone (PSZ), positioned along the southern Nevada seismic belt and along the boundary between the Northern Basin and Range (NBR) and Central Basin and Range (CBR) subprovinces, consists of Cenozoic strike-slip faults with related normal faults. The Buckhorn Fault (BF), located in the center of the PSZ, and the Arrowhead Mine Fault (AMF), on the northern side of the PSZ, are two of the major left-lateral strike-slip faults in the zone. The development of a large, several kilometers across, syncline between the BF and AMF, in a region of general extension, needs explanation in order to understand the PSZ as a whole. We collected geometric and spatial data on the syncline and BF by mapping in the East Pahranagat Range at 1:12,000 scale. Map data and analyses support the following interpretations: (1) the BF had motion along it after the deposition of all the exposed Oligocene to Miocene tuffs including the youngest Kane Wash Tuff (~14.4 Ma), (2) the large syncline deforms the Tertiary tuff section as young as the Kane Wash Tuff, and (3) the BF and related normal faults cut the syncline.

The syncline may have formed in a variety of ways, some of which have been excluded. (1) The syncline is not a result of ash-flow tuffs draping over paleotopography during emplacement because the attitudes collected within the tuffs show a syncline. (2) A wedge that underwent transpression between two non-parallel faults might result in a syncline in the narrow part of the wedge. The strike of the BF (030-040º) relative to the AMF (~045º) with the syncline in the widest part of the wedge refutes the wedge formation option. (3) A fault-propagation fold should trend parallel to the fault and have offset limbs, but the strike of the fault varies along the length of the fold and the units south of the fault are not oriented properly to form the other limb. (4) The near parallelism of the BF and syncline does not support the typical en echelon fold forming geometry of 45º. Thus, the fold may have formed either as a result of an earlier deformation event or along a restraining bend.

Overall, this deformation occurred after 14.4 Ma, and the BF and related normal faults cut a large syncline that may have formed shortly prior to or as a result of motion along the BF. Recorded seismicity in the southern Nevada seismic belt suggests that the deformation continues to today.