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Structural Development And Tectonic Role Of The Arrowhead Mine Fault, Pahranagat Shear Zone, Nevada


The boundary between the Northern (NBR) and Central (CBR) Basin and Range sub-provinces is seismically active and a fundamental tectonic element with differences in the timing, magmatism and the onset of extension across it that may be accommodated by strike-slip faults along it. The Pahranagat shear zone (PSZ) in Lincoln County, Nevada is a 20-25 km wide zone of left-lateral faults that lies within the central part of that boundary zone. The Arrowhead Mine Fault (AMF), one of the three major faults in the PSZ, is a Cenozoic, left-lateral, strike-slip fault at which more typical Cenozoic normal faults terminate. The purpose of this research is to determine whether the normal and strike-slip faults represent two distinct periods of deformation or, alternatively, a regional transfer zone, with normal faults kinematically linked along the transfer fault. This determination requires geometric data and cross-cutting relations, which are best documented through field mapping. Consequently, 1:12,000 scale mapping with emphasis on fault cross-cutting, termination, and kinematic relationships of the AMF and normal faults in the western part of the Pahranagat Range was done. Mapped geometries show three fault sets: NNE-striking normal, NW-striking normal and NE-striking left-lateral. Two episodes of deformation are apparent from map data, a first set of NW-striking normal faults formed before the AMF due to kinematic incompatibility. Mapped geometries and associated analysis suggests that one NNE-striking normal fault set and the NE-striking left-lateral faults of the AMF formed synchronously based on kinematic compatibility of fault kinematics and orientations from slickenline and geometric data. Both the AMF and the NNE-striking normal faults that end at the AMF cut the 14 Ma Kane Wash Tuff making them both younger than 14 Ma. These age relations and the kinematically compatible geometries of the NNE-normal and NE-strike-slip faults suggest that the fault system is a transfer zone. These findings indicate that transfer faults play a significant role in structural development of the NBR-CBR boundary. This new kinematic interpretation of the AMF lays the foundation for our understanding of strike-slip faults in the boundary zone. It also indicates that extension in the boundary zone between NBR and CBR was active after 14 Ma and may still be relatively active, which is consistent with modern seismicity near it.