GSA Today
Volume 31, Issue 1

Cover image

Abert Rim, 30 km north of Lakeview, Oregon, USA. The break in slope is coincident with a discontinuity representing a regional three- to five-million–year hiatus separating two distinct styles of volcanic activity in the Cascadia backarc region. The lower slope is composed of high-K calc-alkaline lavas derived from melting of a hydrated mantle source from 30 to 20 Ma. The steeper upper slope is composed of thin flows of the tholeiitic Steens Basalt (early lavas of the Columbia River Basalt Group) derived from melting of a dry mantle source with a plume component at ca. 17 Ma.

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