A dominantly basaltic late Miocene and early Pliocene (about 5 to 10 m.y. old) lava field lies directly east of the Cascade Range on the northwesternmost edge of the Basin and Range province. More than 12,400 km2 of Modoc County, California, and Klamath and Lake Counties, Oregon, termed the Devils Garden, is underlain by basalt with a total volume of about 850 km3.
The basalt of Devils Garden is diktytaxitic olivine tholeiite, characterized by high Al content; low K, Rb, and Cs content; and high K/Rb and K/Cs ratios. In these respects, it resembles mid-oceanic-ridge basalt (MORB), although it differs in other respects, such as high concentration of Ba and Sr, low K/Ba, and higher 87Sr/86Sr (0.7036–0.7039) ratios. Chemical characteristics indicate that little or no contamination by sialic crustal material has occurred. The basalts originated in the upper mantle and were erupted through crust thinned by tectonic extension behind the Cascade Range volcanic arc.