TEMPORAL ASSOCIATION OF MIDDLE MIOCENE BASIN AND RANGE EXTENSION AND COLUMBIA RIVER FLOOD BASALT VOLCANISM: IMPLICATIONS FOR TECTONIC MODELS OF THE YELLOWSTONE HOTSPOT

Basin and Range extension took place over a large area between the Colorado Plateau and the Sierra Nevada during the middle Miocene. This period of rapid, large-magnitude extension is best documented in southern Nevada, but it affected much of what is now Nevada and Utah between about 16-17 and 10-12 Ma. The onset of middle Miocene extension is coeval with voluminous ca. 16-17 Ma eruptions of basaltic lava in the Pacific Northwest that mark the onset of Yellowstone hotspot volcanism. Mafic volcanism and the onset of extension are thus almost perfectly correlated in time (16-17 Ma) but not space. The close timing is unlikely to be a coincidence—either one caused the other, or both arise from the same underlying process. The voluminous mafic lavas are often attributed to the head of a deep mantle plume impinging on the lithosphere, but the kinematic consequences of Basin and Range expansion are more easily explained by changes in the Pacific – North American plate boundary. A deep mantle plume origin for the mafic lavas is problematic in this context because it fails to create favorable boundary conditions for large-scale westward motion of the Sierra Nevada, while plate-boundary control on the timing of extension is problematic because it does not provide a mechanism for rapidly generating large volumes of mafic lava outside of the extending area. Proposed alternative Yellowstone hotspot models may offer a way to resolve this problem. In one, a long-lived Yellowstone hotspot contributed to formation of the Eocene Siletzia terrane of coastal Oregon, was suppressed by the subducting Farallon plate, and then abruptly brought back into contact with the crust during the Miocene plate-boundary reorganization that also led to Basin and Range extension. In another model, both the Miocene flood basalts and subsequent hotspot track result from upper-mantle processes driven by the subducting Farallon plate, and could have begun when a major middle Miocene plate boundary reorganization led to fragmentation and retreat of the downgoing slab. In either case, northward extension of the Basin and Range into Oregon would promote fracturing of the crust and provide pathways for rapid eruption of flood basalt lavas.