QUANTIFYING MODIFICATIONS IN HOLOCENE ALLUVIAL FAN MORPHOLOGY TO DETERMINE PALEOSEISMICITY OF THE PANAMINT VALLEY TRANSTENSIONAL RELAY (ECSZ)

Mapping offset alluvial fan surfaces is a common approach to determine the timing and strike lengths of Holocene surface ruptures. However, obtaining high resolution spatial or temporal paleoseismic records from mapping alluvial fans can be challenging due to difficulties in confidently subdividing or correlating deposits across large distances. Here, we apply quantifiable and repeatable observations of fan morphology and weathering to develop a local high-resolution alluvial fan chronology, to determine the timing and kinematics of late Holocene complex fault ruptures in the Panamint Valley, Eastern California Shear Zone (ECSZ). We produce a 1:4000 scale tectonogeomorphic mapping in the 40 km² transtensional relay zone between the Ash Hill and Panamint Valley faults using NCALM lidar DEMs and aerial imagery, and develop a fan chronology consisting of ten generations of late Pleistocene to Holocene alluvial deposits, based on changes in bar and swale morphology, weathering, and desert pavement development. We further date offset deposits using post-infrared feldspar infrared-stimulated luminescence and quantify rupture kinematics offsets at over 250 piercing points in alluvial surfaces using newly generated high resolution (5 cm) drone-based structure from motion digital surface models.

Our high resolution fan mapping and geochronologic dating show three key results: 1) deformation in the transtensional relay is distributed among 100+ fault strands located between the Ash Hill and Panamint Valley faults, occurring in parallel and en echelon arrays 5-7 km in length, with fault spacings up to 100s of meters; 2) the relay zone has accommodated four earthquakes over the past ~4ka, and the timing of these events overlap with ruptures on the adjacent Ash Hill and Panamint Valley faults; 3) displacement magnitude per event ranges from 0.6 – 1.0 m of right lateral slip and 0 – 0.2 m of dip slip, with slip kinematics similar to that of the Ash Hill fault. These results suggest that the complex faulting in the transtensional relay accommodates strain transfer between the Ash Hill and Panamint faults over multiple earthquake cycles. These temporal and kinematic relationships may support a geometric link at depth or the reoccupation of subsurface structures capable of transferring strain over larger distances.