Tectonic significance of mildly alkaline Pliocene lavas in Klamath River Gorge, Cascade Range, Oregon

Geologic mapping in Klamath River Gorge in southern Oregon has documented a sequence of Pliocene mildly alkaline and calc-alkaline lavas unconformably underlying Pleistocene lavas of the High Cascade Series. The mildly alkaline lavas, which are separated from rocks of the older Western Cascade Series by a prominent unconformity, are olivine-phyric basalts and are distinct, both petrographically and compositionally, from the calc-alkaline basalts and andesites that typify the rocks of the Cascade Range. In thin section, the Pliocene basalts can be distinguished by the presence of abundant groundmass olivine and by the absence of hypersthene as either a groundmass or a phenocryst phase. Compositionally, these basalts are high in alkalis, TiO₂, P₂O₅, and total iron, and low in CaO and MgO when compared to Cascade calc-alkaline lavas. Similar mildly alkaline basalts have been reported from scattered localities in the central and southern Cascades and may record an extensive eruptive episode which occurred just prior to the main interval of High Cascade volcanism.

Rocks of alkaline affinities, which are characteristically found in areas of extensional tectonism, are out of place in the predominantly calc-alkaline sequences that form at compressional plate boundaries. Cascade mildly alkaline lavas may be related to a Pliocene extensional tectonic event that caused the formation of a “High Cascade Graben” contemporaneous with early High Cascade volcanism.