CONSTRAINTS ON FAULT CONTINUITY AND LIMITS ON SLIP IN THE BACKARC OF NORTHEASTERN CALIFORNIA FROM AEROMAGNETIC DATA

Recently acquired aeromagnetic data from northeastern California provide constraints on tectonic style, fault continuity, and sub-volcanic geology in the region near the junction of the southern end of the Cascade arc, the western Basin and Range, and northern end of the Walker Lane belt. These data extend from the latitude of Susanville CA northward to the California-Oregon border and from the Warner Range westward to Mt. Shasta. Much of the area is characterized by short-wavelength, narrow, north to northwest-trending magnetic anomalies that reflect the underlying structural and topographic grain. These magnetic anomalies parallel the trend of the Walker Lane, are locally coincident with Quaternary faults, and project from the southeast part of the mapped area towards Medicine Lake volcano. The edges of some of these anomalies do not everywhere coincide with topographic breaks or lithologic contacts and thus may reflect structure concealed beneath the young volcanic cover. This magnetic grain is interrupted within and east of the Cascade arc by prominent semicircular magnetic highs and lows, several of which correspond to mapped Pleistocene, dominantly andesitic volcanoes which unconformably overlie a sequence of older, highly faulted andesitic rocks. Depth-filtered magnetic anomalies in places do not correspond to the surface mapped locations of the calc-alkaline vents, probably the result of complex underlying geology beneath the constructional vents. To the northeast of the Walker Lane, anomalies over the Modoc Plateau must reflect, in many places, geologic units beneath the widespread Devil’s Garden basalt given that the polarity of the magnetic anomalies over the basalt does not match its paleomagnetic direction. North- to northeast-striking magnetic anomalies in this region may represent basin and range extension or transtensional faulting within the Walker Lane, whereas a pair of long, narrow, east-west-striking magnetic highs extend across Quaternary faults without apparent lateral displacement, limiting the amount of right-lateral offset on individual Quaternary faults to less than 2 km in the region between Susanville and Fall River Valley.