Figure 1. Converging post-Laramide magmatic fronts (figure from Humphreys, 1995). Numbers indicate age of volcanism and double dotted lines represent the boundary of the current Basin and Range.
The mid-Tertiary ignimbrite flare-up is just one of several geologic and tectonic events that has taken place in the Cordilleran during Cenozoic time. While describing all events would require many investigations, understanding how the flare-up event fits into the Cordilleran geologic history is important. Therefore, the following is a brief summary of important geologic and tectonic events that have occurred in the Cordilleran since late Cretaceous time.
As of late Cretaceous time, the western United States had experienced accretion of terranes in the Proterozoic and Phanerozoic, at least two Phanerozoic orogenies (Antler, Sonoma), and the development of a subduction zone along the western margin in the Mesozoic (Odlow et al., 1989). In the late Cretaceous, thin-skinned deformation from the Sevier Orogeny resulted in east transport of thrust sheets across the Cordilleran region (Odlow et al., 1989). Somewhat coeval with the Sevier orogeny was the Laramide Orogeny in the late Cretaceous and early Tertiary. The Laramide orogeny consisted of thick-skinned deformation involving possible reactivation of basement-cored faults (see Erslev, 1993). Numerous theories exist to explain Laramide deformation, including basal shear generated from a shallowly subducting Farallon slab, horizontal edge forces applied at the plate margin, subduction of an oceanic plateau (see Bird, 1988 and references therein) and drag folding from a regional uplift (Yin, 1994).
With the termination of the Laramide orogeny, a complex pattern volcanism and extension occurred in the Cenozoic. Two magmatic fronts appear to converge, one from the north, and one from the south (see Humphreys, 1995 and references therein) (Figure 1). The distribution of magmatism from 25-40 Ma during the time of the flare-up (Figure 2) is concentrated in two regions: a northwestern section (Washington, Oregon, Idaho, Nevada, Utah; a small portion of California) and a southeastern region (Colorado, New Mexico, Mexico; small portions of California, Utah and Arizona) (Armstrong and Ward, 1991). Magmatic activity may have also weakened the crust to allow metamorphic core complexes to form (see Armstrong and Ward, 1991).
Figure 1. Converging post-Laramide magmatic fronts (figure from Humphreys, 1995). Numbers indicate age of volcanism and double dotted lines represent the boundary of the current Basin and Range.
Figure 2. Extent of magmatic activity from 25-40 Ma (figure from Armstrong and Ward, 1991). Red arrows show shift in magmatism during this time.
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