SHALLOW ANHYDROUS ASTHENOSPHERIC MELTING AND THE LOCATION OF THE LITHOSPHERE-ASTHENOSPHERE BOUNDARY BELOW SOUTHERN OREGON AND NORTHERN CALIFORNIA

An extensive geochemical dataset of young (<10.5 Ma) primitive basaltic lavas from across the northernmost Basin and Range and the central - southern Cascades is used to calculate the depths and temperatures of asthenospheric melting in this region. In conjunction with recent geophysical observations, these calculations place primary constraints on the tectono-magmatic processes driving anhydrous melting in a modern convergent margin and back-arc. The basalts have high Mg#s at low SiO₂ (<52 wt.%), low phenocryst abundances, and trace element concentrations such that we consider them to be primitive. Our new thermobarometer for melts extracted from variably depleted and metasomatized plagioclase and spinel lherzolite is employed to calculate the conditions where the primitive basalts originated in the upper mantle. The minimum depth of melting in Oregon’s High Lava Plains, a geographic sub-province of the Basin and Range, decreases towards the west, with melting occurring at ~45 km below Jordan Valley volcanic center, ~30 km below Newberry volcano, and ~25 km below Crater Lake volcano on the Cascades arc axis. To the south, the minimum depth of melting also decreases towards the west, from ~ 40 km below the Modoc Plateau to 33 km below Medicine Lake and Mt. Shasta volcanoes. Further south at Mt. Lassen, the southernmost Cascades volcanic center, the minimum depth of melting is 35 km. All basalts in this study originated at 1250-1320°C and the calculated minimum depths of asthenospheric melting are very close to the depth of the Moho as determined from a number of regional geophysical studies. These observations point to the hot nature of the mantle immediately below the Moho and suggest the mechanical lithosphere in the region is similar in thickness to the continental crust. <10.5 Ma anhydrous mantle melting in southern Oregon and northern California was likely driven by a combination of corner flow in the mantle wedge, toroidal flow around the southern edge of the subducting Juan de Fuca and Gorda plates, and crustal extension-related upwelling, not a mantle plume. Geodynamic models of mantle flow also indicate the thin mechanical lithosphere is an important factor in the generating the observed conditions in the mantle.