THE PACKARD WELL FAULT AT PALEN PASS, CALIFORNIA: AN IMPORTANT COMPONENT OF THE PALEO-EASTERN CALIFORNIA SHEAR ZONE

The Packard Well fault zone (PWFZ) is likely one of the larger-displacement NW-striking Neogene faults in SE California, with variable dextral offset estimates in the literature of up to ~24 km, and may have formed a critical component of the Late Miocene to Pliocene paleo-eastern California shear zone (ECSZ). However, attempts to understand fault linkage from better-studied ECSZ faults in the Mojave Desert to a wider paleo-ECSZ system that included faults farther E are hampered by uncertainty that PWFZ deformation was compatible with paleo-ECSZ timing and kinematics. We use geologic mapping, fault kinematic analysis, and ⁴⁰Ar/³⁹Ar dating of faulted rocks at Palen Pass, California to constrain the slip history of the PWFZ.

N-dipping strands of the PWFZ bound a 400 m-wide belt of asymmetrically folded alluvial fan conglomerate at Palen Pass. The northern fault strand thrust Mesozoic plutonic rocks over the conglomerate and preserves oblique dextral-reverse slickenlines consistent with a WNW-ESE restraining bend in a NW-SE dextral system. Sanidine ⁴⁰Ar/³⁹Ar dating of a steeply-dipping, lightly reworked ash bed in the deformed conglomerate indicates a population of young grains with a weighted mean average of 11.7 ± 0.1 Ma (2σ). A package of thin basalt flows ~5 km SW of the folded conglomerate records minor NNE-SSW shortening via conjugate strike-slip faults, and a published paleomagnetic study indicates the basalt experienced ~31° of clockwise rotation, both of which likely stem from local block rotation adjacent to the dextral PWFZ. New groundmass ⁴⁰Ar/³⁹Ar dating of this basalt yields a 7.0 ± 0.2 Ma (2σ) recoil model age.

These findings suggest that significant dextral shear occurred on the PWFZ in the Late Miocene or Pliocene, prior to deposition of undeformed late Quaternary alluvial fans. The PWFZ likely connects along strike to concealed faults to the NW (e.g., Cadiz Lake and Iron Mountains faults), and towards Blythe, California to the SE (e.g., Cibola Pass fault zone), where steep horizontal gravity gradients bound a NW-SE-elongate gravity low. Our data demonstrate that the PWFZ was kinematically and temporally compatible with a paleo-ECSZ dextral system active in the Late Miocene to Pliocene and may have played a major role in accommodating Pacific-North America dextral shear.