EVALUATING DISPLACEMENT ALONG THE GARLOCK FAULT ZONE DURING THE PLIOCENE AND EARLY PLEISTOCENE FROM PROVENANCE OF FANGLOMERATE DEPOSITS

The question of whether intracontinental fault systems exhibit non-steady slip in space and time is central to our understanding of seismic hazard, yet the difficulty of dating displaced geomorphic and stratigraphic markers make it challenging to reconstruct slip histories. Along the Garlock Fault, an intracontinental transform fault in eastern California, bedrock markers constrain both the initiation of slip at ~10 Ma and the amount of displacement, 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 along the Garlock at longer timescales, we evaluate displaced alluvial deposits along the central portion of the fault system. Geologic mapping and characterization of fanglomerate deposits 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. A 4-km-long shutter ridge includes sediment derived from Mesquite Canyon at the base, volcanic boulders sourced from Red Mountain in the middle, and Goler Wash debris at the top. Similar relationships are mapped in other areas south of the El Paso Mountains and together provide new constraints on displacement across the El Paso, Garlock, and Savoy faults. These deposits have been translated as little as 800 m to greater than 16 km and are anticipated to yield new slip rate estimates at time intervals from the mid-Pleistocene to Pliocene. We are currently developing age estimates for these deposits using 40Ar/39Ar dating of detrital sanidine and 10Be/26Al burial dating of quartzite boulders. We expect that the long-term record preserved by the offset of these deposits will exhibit temporal variations in displacement and illuminate short-term fault behavior along the central Garlock Fault.