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The Ancestral Cascade arc formed as early as 45 Myr ago along the western edge of North America. Parts of the arc ran parallel to the modern arc in British Columbia, Washington and Oregon, whereas in the south it was located just east of the modern Sierra Nevada (Figure 1). The arc consisted of a chain of volcanic eruptions caused by subduction of the Farallon/Juan de Fuca plates. Several studies have been conducted in the past decade that seek to use these volcanics and analyze their mineral content, deposition history, and geographic location to study how this subducting slab was behaving over 45 million years. While perhaps not as well known as the Ancestral Rocky Mountains, the Ancestral Cascades give us data that can be used to understand the tectonic setting and evolution of the west coast of North America.

Figure 1. Map showing the locations of dated Ancestral Cascade deposits along the west coast. From John et al. 2012.

Map of Ancestral Cascades