PALEOMAGNETIC TEST OF THE LOVEJOY FLOOD BASALT HYPOTHESIS

Paleomagnetic directions from basaltic lavas of the Lovejoy series at five localities in NE and central California are consistent with the hypotheses that all but the top flow of the series was rapidly erupted and at least some of them flowed great distances. These distinctive Tertiary basaltic flows were described and mapped by Turner in 1895 as a patchwork of outcrops trending southwest for 100 km across the northern Sierra Nevada, where they attain a thickness of 180 m. On the basis of megascopic and microscopic appearance, Durrell (1959) extended the Lovejoy 70 km further to the west and 120 km further to the south into the Sacramento Valley and put forward the controversial hypothesis that it was a continuous series of Upper Eocene or Lower Oligocene lavas that flowed great distances from a source somewhere east of the Sierra Nevada. K/Ar and Ar/Ar dating now shows that the series is considerably younger, 16 Ma plus or minus a few hundred thousand years (Page and others, 1995), but the rest of the hypothesis remains untested.

The stable paleomagnetic directions of all but the last-erupted flow of the Lovejoy series are southeast and upward, with a mean distinct at greater than 99% confidence from the 16 Ma reference direction for its location. Moreover, the scatter of directions about the mean is unusually low, showing that the flows were erupted within a time short compared to the time scale of secular variation of the earth’s magnetic field, that is, within a few hundred to a few thousand years. The last-erupted flow has an intermediate direction, characteristic of a geomagnetic excursion or reversal transition. We have found this same unusual direction at other localities, showing that this flow traveled at least 50 km. Statistical analysis between localities of flows with more ordinary directions shows that several of them quite probably flowed greater distances of 100 to 200 km. Taking these paleomagnetic results together with their age and geochemistry, we concur with the proposition of Wagner and others (2000) that the Lovejoy basalt series is part of the same volcano-tectonic episode that gave rise to the Columbia River Basalt Group.