ANCESTRAL CASCADES = MODERN CASCADES

The ancestral Cascade (AC) arc has been an enduring concept in interpretations of the magmatic history of western North America. The southern end of the modern Cascade (MC) arc currently coincides with the Mendocino triple junction, and using plate reconstructions, it is reasonable to infer that the Cascades formerly extended into the Sierra Nevada and western Nevada when the triple junction was farther south. However, animation of magmatic space-time patterns using ~29,000 points from NAVDAT (navdat.org) reveals little support for the AC concept; there has been a Cascade arc near the MC, and not elsewhere, for about 40 m.y.

During the Paleogene, magmatism moved southward from Montana to Nevada. At 45 Ma there were large silicic and intermediate volcanic fields in Idaho and to the east, but little magmatism near the continental margin in Oregon and Washington. By ~40 Ma the locus of silicic magmatism had moved into northeastern Nevada, whereas intermediate magmatism was established in Oregon and Washington in a belt parallel to but just west of the MC. The latter most likely represents establishment of the AC arc. This belt did not extend into California south of Mt. Shasta. In the Miocene, the silicic locus moved to central Nevada, magmatism just west of the MC continued, and central and northern California remained largely amagmatic. The first hint of AC-like magmatism in Nevada or California was mildly potassic intermediate lavas erupted 16-13 Ma in western Nevada, although these were contiguous with the southward sweep, not with the AC in Oregon. Significantly potassic magmas erupted west of this area in the late Miocene, but again this magmatism was largely separated from AC magmatism to the north by a relatively amagmatic (but poorly known) segment in northern California. This short-lived, isolated, compositionally unusual event is an unlikely candidate for a piece of the AC but resembles the voluminous intermediate volcanism that preceded large-volume ignimbrite eruptions at many of the mid-Tertiary volcanic centers of the interior of the western U.S. (including the Challis, San Juan and Mogollon-Datil volcanic fields).

We conclude that there was an AC arc, but it began about 40 Ma and has always been in the area of the MC. Evidence for an AC arc in California south of Lassen Peak is weak.