As Siletzia accreted it also jammed the existing subduction zone , halting subduction of the Farallon plate . This terminated the Laramide orogeny that had been uplifting the Rocky Mountains , and triggered the ignimbrite sweep , a wave of large-volume silicic magmatism that swept over much of western North America between 20 and 50 Ma . This undoubtedly affected the enigmatic and controversial Challis Arc ( stretching from southeastern British Columbia to the Idaho Batholith , roughly parallel with the Olympic-Wallowa Lineament ) , but the details of this are unknown
ignimbrite sweep
Mid-Tertiary ignimbrite flare-up
Siletzia