Reconciling Observations of the Yellowstone Hotspot with the Standard Plume Model

The Yellowstone hotspot represents the type example of plume magmatism in the continental setting. The propagation of large silicic magmatic centers along the Snake River Plain independently tracks the southwestward trajectory of North American plate motion over the last 13 My. Structural deformation associated with the hotspot track is consistent with thermal upwelling, and tomographic studies image a well-defined cylindrical conduit at least down to the mantle transition zone. Furthermore, the high 3He/4He signatures suggest a deep mantle origin for Yellowstone magmas. Yet, there are several observations of the Yellowstone region that do not fit the standard plume model for hotspot magmatism. These include: 1) prevalent coeval magmatism in and around the hotspot track that continued well after passage of the underlying plume, 2) significant bimodal magmatism that occurred throughout the Great Basin during this time, and 3) the outpouring of the Miocene Columbia River flood basalts (CRFB) well north of the hotspot track. These features have led a number of researchers to favor a shallow upper mantle origin for Yellowstone hotspot activity controlled by structural weaknesses in the continental lithosphere. Here, we propose that the observations listed above conform to the standard plume model by considering interaction of the Yellowstone plume with the descending Farallon Plate beginning at 80 Ma. Anomalous geologic activity occurred throughout the Cenozoic Era in the North American Cordillera (NAC) and must be addressed in any model for the origin of magmatism in the western US, including the Yellowstone hotspot. In particular, extensive field and geochemical studies document a pronounced eastward migration of deformation and magmatism throughout the NAC from 80 to 40 Ma. Most researchers attribute this activity to shallowing of the Farallon slab beneath NA at this time. In addition, geochemical studies in the NAC document a transition in magmatism from predominantly calc-alkaline (associated with ancient slab-derived fluids within the sub-continental lithosphere) to predominantly tholeiitic (with distinctive OIB signatures). This transition has been attributed to the eventual foundering of the shallow slab with replacement by `asthenosphere'. Here, we document that magmas with OIB affinity are observed throughout the Cenozoic in the NAC, often before a documented `transition'. We show that these magmas are primarily binary mixtures of two well-known mantle plume components EMI and FOZO. In our model, we propose that the Yellowstone starting plumehead impinged beneath the subducting Farallon Plate at 80 Ma and spread laterally while continuing to ascend. Magmas with OIB affinity erupted only after penetration of the plume through the cold, rigid Farallon slab. In this way, the CRFB, at only 10% of the eruptive volume of typical flood basalt provinces, represent partial melting of only a fraction of the original Yellowstone starting plumehead. Evidence of additional leakage of the plume is found in the Chilcotin flood basalts in BC, the Crescent Terrane volcanics in the Pacific Northwest, and kimberlites, diatremes, and widespread basaltic flows found throughout the NAC. Collectively, the magmatic features that seem to oppose the plume hypothesis can be reconciled by considering a broader context for the origin of the Yellowstone hotspot. Indeed, the `anomalous' geologic activity observed within the NAC is anticipated by the standard plume model; the frequency of hotspots observed on Earth demands that some starting plumeheads will encounter destructive plate margins and generate significant uplift, deformation, and magmatism within a broad region of the overriding lithosphere(s).