Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 1
Presentation Time: 8:20 AM


IHINGER, Phillip D., WATKINS, James M., BERNHARDT, Jesse E. and JOHNSON, Breck R., Geology, Univ of Wisconsin - Eau Claire, 105 Garfield Ave, Eau Claire, WI 54701,

The western edge of the North American continent has experienced an unusual history of prolonged magmatism and deformation over the last 80 million years. In particular, significant calc-alkaline magmatism and structural deformation are found well inboard of that found at typical continental arc margins. Researchers have documented other curious features in the magmatic and deformational record of the distended province, including: 1) ‘thin-skinned’ deformation of the Sevier orogeny; 2) basement uplift associated with the ‘thick-skinned’ Laramide orogeny; 3) extension in the hinterland contemporaneous with Laramide deformation in the foreland; 4) early Tertiary production of two-mica granites; 5) coeval eruption (same time and same place) of arc-like magmas and magmas with OIB trace element and isotopic signatures throughout the province from 50 My to today; 6) basin and range extension during the mid-Tertiary with concurrent production of significant bimodal basalt-rhyolite volcanism; 7) massive outpouring of the Columbia River flood basalts in the Miocene; 8) divergence of the ‘Yellowstone’ hotspot track at ~15 Ma to form the coeval ‘Newberry’ track; and 9) continued active mafic and felsic volcanism. Many of these features have been attributed to the shallowing of the subducting Farallon plate beneath western North America between 80 and 50 My, and substantial evidence supports this hypothesis. Subduction of progressively younger oceanic crust and/or a warm oceanic plateau, and accelerated westward motion of the North American plate are plausible mechanisms already proposed to generate this shallowing, but they fail to account for features 3-9 listed above. We posit that impingement of the positively buoyant starting plume head of the Yellowstone hot spot with the underside of the negatively buoyant, subducting Farallon plate beginning at ~80 My resulted in massive uplift of the slab, the overlying mantle wedge, and the continental lithosphere. Increased heat flow, uplift-induced pressure release, and fluids released from devolatization led to large-scale melting of the sub-continental lithosphere and crust. Our model is consistent with geophysical measurements of the present-day Cordilleran province, including a seismically anomalous upper mantle, high heat flow, and broad regional uplift.