The graben of the High Cascades is bounded to the east by the Green Ridge fault zone and to the west by the Horse Creek fault zone. It has a width of and a depth of . Predominantly, it has generated mafic (rich in magnesium and iron) lava, producing about of magma within the past 15,000 years. Moreover, many of the identified eruptive vents from the Quaternary include scoria cones or other mafic vents. The High Cascades graben displays a unique geochemical signature with low-K tholeiite magma and a relatively enriched mantle source, produced by extension and heat flux.
