Miocene-Pliocene(?) folds and normal faults associated with the left-lateral Buckhorn Fault, Pahranagat Shear Zone, Nevada

Abstract

Important hydrocarbon plays can occur along strike-slip faults with associated folds. We examine the mechanism of formation of a large fold adjacent to a strike-slip fault through a well-exposed case study in the Pahranagat Shear Zone (PSZ), located ~100 miles north of Las Vegas, Nevada. The PSZ consists of Cenozoic strike-slip faults with related normal faults and folds. Two of the major left-lateral strike-slip faults in this zone are the Arrowhead Mine Fault (AMF) and the Buckhorn Fault (BF) from north to south, respectively. A large, several kilometer across, syncline is located between the AMF and BF and needs explanation in order to understand the development of the PSZ as a whole. Geometric and spatial data on the syncline and BF were collected by mapping in the East Pahranagat Range at a 1:12,000 scale. Map data and analysis 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,14.4 Ma Kane Wash Tuff,(2) the large syncline deforms tuffs as young as the Kane Wash Tuff, and (3) the BF and related normal faults cut the syncline. The syncline could have formed in a variety of ways; some of which have been excluded. (1) The syncline is not a pseudo-fold resulting from ash-flow tuffs draping over paleorelief during emplacement because the attitudes of compaction foliations collected with the tuffs display a syncline the plunges and trends 17º, 032º. (2) A wedge that underwent transpression between two non-parallel strike-slip faults might result in a syncline in the narrow part of the wedge. The strike of the BF (030-040º) on the south relative to the AMF (~045º) on the north creates a westward opening wedge. The syncline is exposed in the western part of the PSZ in the widest part of the wedge. Consequently, the geometric and spatial relations refute this possibility. (3) 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, along a restraining bend or as a fault-propagation fold as the BF propagated westward. During progressive deformation that occurred after the syncline formed, the BF acted as a transfer fault with over 15 faults, three hard linked normal faults in the north and four in the south, that can transfer strain onto the BF. These normal faults can have stratigraphic offsets of up to 50m. Additionally, recorded seismicity in the southern Nevada seismic belt suggest that the deformation within the PSZ continues to today. The PSZ is located along the central part of the boundary between the Northern and Central Basin and Range sub-provinces. These sub-provinces have differences in gravity signatures, timing of initial extension and stretching factor. Three of the major left-lateral faults in the PSZ, including the BF and AMF are transfer faults. Therefore, the PSZ appears to either allow transfer of strain or accommodate differential strain between the Northern and Central sub-provinces. In summary, a progression of deformation occurred after 14.4 Ma; the BF and related normal faults cut a large syncline. The syncline may have formed shortly prior to or as a result of motion/propagation along the BF. In this case study, the large fold formed early in the progressive deformation suggesting that late-stage fault-related traps are less likely to be disrupted than the large fold.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016