TECTONIC HISTORY AT THE MENDOCINO TRIPLE JUNCTION: QUATERNARY MAPPING OF THE GOOSE LAKE FAULT, VAN DUZEN RIVER TERRACES, AND YAGER CREEK TERRACES, HUMBOLDT COUNTY, CALIFORNIA

The Mendocino Triple Junction (MTJ), comprising the North American, Pacific, and Gorda plates, is a complex region where two distinct plate boundary systems, the Cascadia subduction zone to the north, and the San Andreas transform fault to the south, juxtapose, and are accommodated by upper plate faults. We investigated an area in the MTJ, where a fault system intersects two overlapping suites of fluvially incised terraces, to understand local tectonic history in context of the MTJ. Using 1 m resolution LiDAR imagery (USGS, 2020), we present new mapping of the Goose Lake fault system, Yager Creek terraces and Van Duzen River terraces, in Hydesville, CA. The Goose Lake fault system is a steeply dipping oblique fault with an inferred significant lateral motion component; and, as such, the fault system is atypical of faults in the southern Cascadia fold-and-thrust belt which have shallow thrust geometry. The Yager Creek and Van Duzen River terraces are uplifted fluvial strath terraces recording response to baselevel lowering, climate change, and tectonic uplift. We identified approximately seven terraces formed by Yager Creek (QYt1-7) and four terraces formed by the Van Duzen River (QDt1-4). Our mapping of the central strand of the Goose Lake fault indicates apparent right-lateral offset of terraces QYt7 and QYt1, and apparent left-lateral offset of terrace QYt3. Hence, geomorphic mapping alone appears inconclusive in determining the amount and style of lateral movement. Vertical displacement along all three strands consistently document north-facing, up to the south, fault scarps. Surface profiles across the upper terrace suites show northward tilting. Quaternary mapping assisted in targeting two terraces for Be¹⁰ cosmogenic radionuclide (CRN) exposure dating. Preliminary results from Be¹⁰ CRN exposure dating of QDt1 and QDt2 indicate a minimum age of 23 and 50 kya, respectively. If we employ these minimum terrace ages, and assume that incision rates are approximately equal to rock uplift rates, the maximum uplift rates range from 2.5—4.0 mm/yr. These are extremely high for long-term uplift rates. These rates are most likely the product of permanent deformation in the transitional region from strike-slip tectonics of the San Andreas fault zone to subduction zone forearc deformation in Cascadia.