IS THE UPLIFT AND TECTONIC HISTORY OF THE BLUE MOUNTAINS, OREGON, CONSISTENT WITH A MODEL OF PLUME-INDUCED DELAMINATION?

Recent work has led to speculation on whether a mantle plume, residing beneath accreted arc terranes of the Blue Mountains, Oregon, may have provided a trigger for delamination associated with the Columbia River flood-basalt eruptions (Hales et al., 2005). Such a model is consistent with the time-line of source melting as recorded in the basalt stratigraphy, and with recent thermo-mechanical experiments of plume impingement near cratonic boundaries (Burov et al., in press). These experiments generate a topographic and structural profile that mimics that displayed along a N-S cross-section above the Yellowstone plume head. The profile generates higher elevations with broad folds near the edge of the plume head (Blue Mountains), and lower elevations above the plume center (Owyhee Plateau) due to relaxation of lithospheric stresses after 1-4 m.yrs. Low surface elevations exist across the cratonic boundary, where the numerical experiments generate a flat plateau, similar to that north of Lewiston, Idaho. Here, the Lewiston Basin is predicted by the model experiments, which produce a pronounced subsidence near the cratonic margin caused by subduction-like down-thrusting of the cratonic mantle. This compressional environment may help to explain the “Lewiston structure,” an E-W trending thrust fault with folds in the hanging wall forming a spectacular 500-m-high escarpment lying adjacent and parallel to the cratonic boundary, consistent with the model experiments. Other E-W folds in the Pacific Northwest have been attributed to Tertiary rotation; however, the Lewiston structure lies near the pole of rotation so that compression there cannot be so easily attributed to that mechanism. Farther south, the Wallowa Mountains form the central uplift to a circular feature with a bull's eye pattern, having a diameter of 230 km. This feature, which has been attributed to drip-like delamination near the center portion of the Chief Joseph dike swarm (Hales et al., 2005), is remarkably similar in size, shape, and tectonomagmatic setting to coronae on Venus, which have been attributed to delamination at the frontal edge of mantle-plume heads (e.g., Stofan and Smrekar, 2005). Although many surficial features appear to be compatible with plume-induced delamination, a definitive relationship must await future investigations.