TERMINATION OF THE EASTERN BUCKHORN FAULT AND TRIAXIAL STRAIN ALONG THE FAULT: IMPLICATIONS FOR THE DEVELOPMENT OF THE PAHRANAGAT SHEAR ZONE AND THE DEVELOPMENT OF THE BOUNDARY ZONE BETWEEN THE CENTRAL AND NORTHERN BASIN AND RANGE

The central part of the boundary zone between the Central Basin and Range (CBR) and the Northern Basin and Range (NBR) is formed by the left-lateral Pahranagat shear zone (PSZ), Nevada, which includes the Buckhorn fault (BF). The boundary zone is marked by differences in extension, magmatism, and geophysical signatures across it. The PSZ is a 12 km wide, 45 km long, ENE-striking, left-lateral fault zone accommodating the transfer of strain between the Caliente-Enterprise zone and the Eastern California shear zone. The BF is the central of three main strike-slip faults within the PSZ. The focus of this study is understanding deformation within the fault system, along and near the BF, and the development of the boundary zone. New 1:12,000 scale geologic map data along the eastern BF documents fault geometries and cross-cutting relationships. The BF strikes 235ﹾ, more northerly than PSZ bounding faults, and dips 88ﹾSE with a slickenline rake of 10ﹾSE. The map area exposes ~18.5 to ~15 Ma ash-flow tuffs, all of which are cut by the BF. Mapped normal and oblique-slip faults have a wide variety of strikes suggesting triaxial strain. Normal- and oblique-slip faults that strike N-S splay from the eastern end of the forming a horsetail splay in which the BF terminates eastward. Previous work shows that the BF also terminates in the western PSZ. Strain along the BF is likely greatest along the central portion and decreases as it approaches the fault tips. The BF is shorter than the PSZ bounding faults suggesting that it accommodates strain internal to the PSZ. Offset of the BF documented on the new geologic map shows that the eastern BF became inactive prior to a N-S striking fault set. A 3D model (fence diagram) supports the fault termination and triaxial strain interpretations within the zone. Also, the BF appears to have less offset than the left-lateral faults to the south and possibly the north, which are both transfer faults. Triaxial strain near the NE-striking BF within the PSZ combined with the presence of ENE-striking PSZ-bounding transfer faults suggest that the PSZ and central NBR-CBR boundary accommodates both strain transfer from normal faults to the north and south and provides 3D adjustments for differences in the shapes the NBR and CBR at the boundary. Thus, this strike-slip zone is a critical component of Basin and Range extension.