MAPPING BASALTS ON THE MODOC PLATEAU IN NORTHEASTERN CALIFORNIA BY CORRELATING PALEOMAGNETIC DIRECTIONS

The Modoc Plateau in northeastern California is a large basalt field of Pliocene to Pleistocene age. The volcanic field is located east of the Medicine Lake Volcano (MLV) and covers roughly 2700 km². The Modoc Plateau and MLV are east of the Cascades axis and at the western margin of Basin and Range extension. Much of the Modoc Plateau has been mapped as Devils Garden basalt (Tdgb), which vary little geochemically: they are high-alumina olivine tholeiites. This geochemistry indicates a primitive mantle source without crustal contamination, originating from dry decompression melting of shallow mantle. The Modoc Plateau comprises isolated older volcanic edifices subsequently engulfed by the voluminous, low-viscosity tholeiites constructing a flat plateau-top topography disrupted by numerous low-offset Quaternary normal faults.

We continue work that characterized the “Hackamore basalts” in the western Modoc Plateau – adding new paleomagnetic sites working eastward – to probe the eruptive history of this volcanic system. Cooling lava records a snapshot of the geomagnetic field at that time. We use paleomagnetic directions to study offset on a series of basin forming faults. In three cases where field mapping indicates a single faulted lava flow offset by <30m, we find matching paleomagnetic directions on the foot and hanging walls, indicating no tilting. We further find basalts with both normal and reversed characteristic remanent magnetization directions. Our data can be further divided into directional groups spanning geomagnetic secular variation. These groups are spatially correlated, and we use them to define flows. We describe three spatially correlated polarities: a normal polarity group on the western side of the plateau near the “basalt of Plum Ridge” (0.629±0.015 Ma); a reversed polarity group to the east, near the “basalt of Badger Wells” (1.086±0.026 Ma); and a normal polarity defining the southeastern-most mapped Tdgb (2.78 – 4.61 Ma). Paleomagnetic data support that tholeiitic volcanic activity continued for several million years, from the Gauss to Brunhes polarity chrons: younger and closer in time to MLV activity than some geochronology studies found. Modoc Plateau eruptions progressed from east to west until ~0.6 Ma. Activity at MLV began at ~0.475 Ma with rhyolitic to basaltic compositions.