Pliocene to Holocene volcanics of the central Oregon High Cascades, largely confined to an intra-arc graben, bury older volcanic centers. Early High Cascade volcanism is recorded by late Miocene volcanics and sediments of the Deschutes Formation that accumulated in a fluvial basin east of the Cascades. Aggradation in the adjacent fluvial basin occurred episodically when pyroclastic debris choked streams. Deschutes basin aggradation commenced near 7.4 Ma and was synchronous with extension-influenced Cascade magmatism dominated by basalts and basaltic andesites. Initial subsidence of the Cascades occurred at about 5.4 Ma in response to plate-margin processes. Structural isolation of the basin from ignimbrites and volcanism-induced sedimentation resulted in widespread development of superimposed paleosols in the basin. Subsequent mafic lavas erupted near the eastern margin of the initial depression were truncated by east-stepping, down-to-the-west faults at about 5.3 Ma.

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