DELAMINATION ORIGIN FOR COLUMBIA RIVER FLOOD BASALTS AND WALLOWA MOUNTAIN UPLIFT IN NE OREGON

The Wallowa Mountains define the high center to a 200-km diameter circular "bulls eye" pattern of oscillating uplifts and sags, and they are comprised of the largest of the few granitic plutons in the area. The bulls eye region is the source area for most Columbia River Basalt (CRB) eruptions. Existing elevation maps of CRB flow interfaces are used to quantify the magnitude, timing and distribution of surface uplift, and teleseismic data is used to tomographically image the structure of the underlying mantle. The bull's eye area is characterized by mild pre-eruptive subsidence, syn-eruptive uplift of several hundred meters, and a persistent km-scale uplift of the Wallowa Mountains, and it overlies an anomalous volume of mantle that is devoid of partial melt compared with adjacent areas. Predictions of hotspot plume models and passive magmatic processes are inconsistent with the history, spatial pattern and magnitude of uplift. Within the context of current geological understanding, only delamination of compositionally dense plutonic roots can explain the uplift timing and magnitude and the imaged upper mantle structure. This suggests that CRB magmatism is a consequence of lithospheric processes rather than arrival of a mantle plume; it is possible that delamination was triggered by the arrival of hot mantle, and that the local intensity of CRB magmatism resulted from the combined effects of anomalously hot mantle and lithospheric delamination. As an isolated pluton uplift in oceanic terrain, this uplift provides a good "Greens function" for delamination.