ACTIVE DEFORMATION ALONG MULTIPLE STRANDS OF THE NORTHERN MAACAMA FAULT, MENDOCINO COUNTY, CALIFORNIA, FROM LIDAR-DERIVED TOPOGRAPHIC DATA

The Maacama fault system is one of the primary structures in the Pacific-North America transform boundary. Despite its important tectonic role and potential as a seismic source, our understanding of the structural geometry and activity on the northern reaches of the Maacama system is limited, due to a combination of heavy vegetation, rugged topography, and remoteness. A new lidar dataset of Mendocino County prompted a re-mapping of the traces of the northern Maacama fault. This new dataset is a significant improvement over extant lidar in both resolution and coverage, particularly of areas beyond the primary trace of the Maacama fault.

Scarps suggesting active fault slip are present along both the Willits strand, which runs along the western edge of Little Lake Valley, and the East Willits strand, which runs approximately parallel and ~5 kilometers east of the Willits strand. Both strands can be traced southeastward as far as Redwood Valley, ~20 km to the south. The western (Willits) strand makes a leftward restraining step at the Laughlin Range, a region of locally anomalous high elevation, while the East Willits strand follows a more linear trace to the east. The two strands merge into a single geomorphic trace in southwestern Redwood Valley, ~10 km north of the city of Ukiah. This single geomorphic trace continues south-southeastward along the ~14-km length of the Ukiah basin. Relocated seismicity, seen in both map view and in cross sections, reinforces the interpretation from lidar data that the northern Maacama fault splits into two active strands north of Ukiah at the Laughlin Range stepover. The crustal-scale geometry, in which the two strands appear to have a steep eastward dip and connect at depth, suggests that the Maacama system may be inherited from a prior tectonic regime, within the forearc of the plate boundary prior to the passage of the Mendocino Triple Junction and initiation of the Pacific/North America transform boundary.