I know there is no definitive answer to this question but please indulge me nonetheless. It seems that the Oregon Cascades are generally divided into northern, central and southern sections. The central cascades are focused on the Three Sisters area while the southern cascades are centered on the Crater Lake region. Diamond Peak lies at the nexus of these two areas.
So the question is, is the Diamond Peak area the southernmost part of the central Oregon Cascades or the northernmost part of the southern Oregon Cascades?
Here is a map of the area in question.
Any input is appreciated.