The Geology of the Okanogan Region! March 22, 2018 accretion Challis arc Challis volcanics +6 detachment fault graben Laurentia metamorphic core complex Okanogan Okanogan lobe plate tectonics Quesnellia terrane accretion Challis arc Challis volcanics detachment fault graben Laurentia metamorphic core complex Okanogan Okanogan lobe plate tectonics Quesnellia terrane