MAPPING LAYERS IN THE FRANCISCAN COMPLEX AND FAULTS OF THE SAN ANDREAS SYSTEM WITH NEW AEROMAGNETIC DATA IN THE NORTHERN COAST RANGES, CALIFORNIA

We use new aeromagnetic data to map structure, delineate faults, and infer cumulative offsets across the San Andreas fault system between the Mendocino Triple Junction and Clear Lake. Difficult access, poor outcrops, abundant landslides, dense vegetation, and rugged terrain make it hard to use traditional geologic mapping to define the geometry of faults within the San Andreas system and to subdivide the predominantly Franciscan Complex rocks that crop out in this region. These important steps are needed to assess seismic hazard and to map the transition from transform to subduction zone tectonics.

New aeromagnetic data help provide these important steps. The survey was flown along flight lines spaced 800 m apart at a nominal altitude of 300 m above terrain, a significant improvement upon existing flight line data (up to 10 km apart). The new survey extends north from Ukiah to Laytonville and from 20 km offshore on the west to the San Joaquin Valley-Coast Ranges margin on the east.

The new data reveal two dominant trends of magnetic anomalies. Between the San Andreas and Maacama faults narrow, elongate, N50W trending magnetic anomalies within the Coastal Belt of the Franciscan Complex predominate. Several anomalies continue to the south of the survey where they have been interpreted to reflect folded coherent bodies of magnetic Franciscan greywacke. New magnetic data show that some of these interpreted folds are nearly 50 km long. This pattern is truncated ~5 km E of the neotectonic strand of the San Andreas fault, indicating an older strand of the fault. On the E, the anomalies are shorter and are truncated by the Maacama fault, with no clear offset equivalent on the E side of the fault.

East of the Maacama fault magnetic anomalies trend more northerly, parallel to the main strike slip faults. Ophiolitic rocks likely cause these higher-amplitude anomalies, some of which are strung out along the Bartlett Springs fault. Two pairs of offset magnetic anomalies argue for about 8-9 km of cumulative offset. The new data also corroborate the presence of a 100 km long magnetic belt that coincides with the McCreary Glade seismicity lineament. The lineament may reflect an older, reactivated basement structure. The absence of correlative magnetic anomalies across the Maacama fault suggests that it took advantage of an older basement structure.