Geological Society of America Memoir, 1992
The track of the Yellowstone hot spot is represented by a systematic northeast-trending linear belt of silicic, caldera-forming volcanism that arrived at Yellowstone 2 Ma, was near American Falls, Idaho about 10 Ma, and started about 16 Ma near the Nevada-Oregon-Idaho border. From 16 to 10 Ma, particularly 16 to 14 Ma, volcanism was widely dispersed around the inferred hot-spot track in a region that now forms a moderately high volcanic plateau. From 10 to 2 Ma, silicic volcanism migrated N54°E toward Yellowstone at about 3 cm/year, leaving in its wake the topographic and structural depression of the eastern Snake River Plain(SRP). This <10-Ma hot-spot track has the same rate and direction as that predicted by motion of the Northern American plate over a thermal plume fixed in the mantle. The eastern SRP is linear, mountain-bounded, 90-km-wide trench almost entirely (?) floored by calderas that are thinly covered by basalt flows. The current hot-spot position at Yellowstone is spatially related to active faulting and uplift. Basin-and-range faults in Yellowstone-SRP region are classified into six types based on both recency of offset and height of the associated bedrock escarpment. The distribution of these fault types permits definition of three adjoining belts of faults and a pattern of waxing, culminating, and waning fault activity. The central belt, Belt II, is the most active and is characterized by faults active since 15 ka on range fronts >700 m high. Belt II has two arms forming a V that joins at Yellowstone: One arm of Belt II trends south to the Wasatch front; the other arm trends west and includes the sites of the 1959 Hebgen Lake and 1983 Borah Peak earthquakes. Fault Belt I is farthest away from the SRP and contains relatively new and reactivated faults that have not produced new bedrock escarpments higher than 200 m during the present episode of faulting. Belt III is the innermost active belt near the SRP. It contains faults that have moved since 15 to 120 ka and that have been active long enough to produce range fronts more than 500m high. A belt with inactive faults, belt IV, occurs only south of the SRP and contains range-front faults that experienced high rates of activity coincident with hot-spot volcanism in the late Tertiary on the adjacent SRP. Comparison of these belts of fault activity with historic seismic activity reveals similarities but differences in detail. That uplift migrated outward form the hot-spot track is suggested by (1) the Yellowstone crescent of high terrain that is about 0.5 km higher than the surrounding terrain, is about 350 km across at Yellowstone, wraps around Yellowstone like a bow wave, and has arms that extend 400 km southerly and westerly from its apex; (2) readily erodible rocks forming young, high mountains in parts of this crescent; (3) geodetic surveys and pale topographic reconstructions that indicate young uplift near the axis of the Yellowstone crescent; (4) the fact that on the outer slope of this crescent glaciers during the last glaciation were anomalously long compared with those of the preceding glaciation, suggesting uplift during the intervening interglaciation; (5) lateral migration of streams, apparent tilting of stream terraces away form Yellowstone, and for increasingly younger terrace pairs, migration away from Yellowstone of their divergent-convergent inflection point; and (6) a geoid dome that centers on Yellowstone and has a diameter and height similar to those of oceanic hot spots. We conclude that the neotectonic fault belts and the Yellowstone crescent of high terrain reflect heating that is associated with the hot-spot track but has been transferred outward for distances of as much as 200 km from the eastern SRP in10 m.y. The only practical mechanism for such heat transport would be flow of hot material within the asthenosphere, most likely by a thermal mantle plume rising to the base of the lithosphere and flowing outward horizontally for at least such 200-km distances. The changes in the volcanic track between 16 to 10 Ma and 10 to 2 Ma is readily explained by first the head (300-km diameter) and then the chimney (10 to km across) phases of a thermal mantle plume rising to the base of the southwest-moving North American plate. About 16 Ma, the bulbous plume head intercepted the base of the lithosphere and mushroomed out, resulting in widespread magmatism and tectonism centered near the common borders of Nevada, Oregon, and Idaho. Starting about 10 Ma near American Falls and progressing to Yellowstone, the chimney penetrated through its stagnating but sill warm head and spread outward at the base of the lithosphere, adding basaltic magma and heat to the overriding southwest-moving lithospheric plate, leaving in its wake the eastern SRP-Yellowstone track of calderas, and forming the outward-moving belts of active faulting and uplift ahead and outward from this track. We favor a mantle-plume explanation for the hot-spot track and associated tectonism and note the following problems with competing hypotheses: (1) for a rift origin, faulting and extension directions are at nearly right angles to that appropriate for a rift; (2) for a transform origin, geologic evidence requires neither a crustal flaw nor differential extension across the eastern SRP, and volcanic alignments on the SRP do not indicate a right-lateral shear across the SRP; and (3) for a meteorite impact origin, evidence expected to accompany such an impact near the Oregon-Nevada border has not been found. The southern Oregon rhyolite zone is not analogous to the eastern SRP and therefore does not disprove formation of the Yellowstone hot-spot track by a mantle plume. The postulated rise of mantle-plume head into the mantle lithosphere about 16 Ma corresponds in both time and space with the following geologic changes: (1) the start of the present pattern of basin-range extension, (2) intrusion of basalt and rhyolite along the 1,100-km-long Nevada-Oregon rift zone, (3) the main phases of flood basalt volcanism of the Columbia River and Oregon plateaus, and (4) a change from calc-alkaline volcanism of intermediate to silicic composition to basaltic and bimodal rhyolite/basalt volcanism.
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