New Slip Rates Along the Garlock Fault Zone During the Mid-Pleistocene and Pliocene from Provenance of Fanglomerate Deposits

The question of whether intracontinental transform faults exhibit non-steady slip in space and time is paramount in our understanding of transform plate boundary systems, yet reconstructing high-fidelity records of past slip is challenging. In eastern California, dextral shear along the Eastern California Shear Zone (ECSZ) passes across and may contribute to sinistral motion along the Garlock Fault Zone (GFZ). Displaced bedrock markers constrain both the initiation of slip on the Garlock at ~10 Ma and the finite left-lateral displacement along the fault system, between 54 and 68 km. These imply a long-term average slip rate of 5-7 mm yr-1. However, geodetic measurements show strain accumulation of <1-3 mm yr-1 across the fault in the present-day. Slip rates determined from the latest Pleistocene (ca. 15 ka) appear consistent with the long-term average, but Late Holocene (2-4 ka) displacements require rates of 10-14 mm yr-1. To evaluate whether this episode of rapid slip is confined to the Late Holocene or temporal variations in displacement occur at longer timescales, we reconstruct displaced alluvial deposits along the central portion of the Garlock Fault Zone (GFZ). Geologic mapping and characterization of fanglomerate deposits south of the El Paso Mountains confirm distinct provenance of different stratigraphic units that can be restored to sources both north and south of the fault system. Stratigraphic relationships exposed in these deposits are indicative of changing provenance and are related to fault activity as sediment from one source is slid in front of another. These deposits have been translated as little as 800 m to greater than 16 km along the Garlock, El Paso, and Savoy faults. We are working to determine the age of these deposits using 40Ar/39Ar dating of detrital sanidine and 10Be/26Al burial dating of quartzite boulders. These data will allow us to determine new slip rates that span time intervals ranging between ~500 ka and ~4 Ma. These data are anticipated to refine estimates of how slip is partitioned along fault strands within the GFZ and to assess whether slip along the central Garlock Fault varies during accumulated displacement along the ECSZ.