13: Late Cenozoic volcanic and tectonic evolution of the Great Basin and Columbia Intermontane regions
Robert L. Christiansen, Edwin H. McKee, 1978. "13: Late Cenozoic volcanic and tectonic evolution of the Great Basin and Columbia Intermontane regions", Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Robert B. Smith, Gordon P. Eaton
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The Great Basin is a tectonically youthful region that shares some features with the Columbia Intermontane region but is separated from the more mature southern part of the Basin and Range province by a zone of active seismicity and geophysical contrasts. Sedimentary, physiographic, and structural features show that during the past 17 m.y., extensional linear normal faulting has been active in the Great Basin region, and extension also is indicated by numerous dikes in the High Lava Plains and the Columbia Plateau. Cumulative tectonic extension in the Great Basin is more than 100 km. Since about 14 m.y. ago, tectonic activity in the Great Basin region has tended to become progressively more concentrated toward the margins, and extension has been taken up by a wide transform zone along the High Lava Plains. Within several tens of kilometres north of the High Lava Plains of Oregon and Idaho, cumulative extension is generally less than a few kilometres and has been nearly inactive since about 14 m.y. ago.
Volcanism in the past 17 m.y. has been characterized by basaltic and bimodal rhyolite-basalt suites. Between 17 and 14 m.y. ago, the predominant volcanism was basaltic, being somewhat alkalic and of relatively small volume in the central Great Basin, more voluminous and less alkalic northward into the plateaus of southern Oregon and the High Lava Plains, and extremely voluminous and tholeiitic in the Columbia Plateau. Since about 14 m.y. ago, basaltic and bimodal volcanism has occurred throughout the Great Basin region but generally has tended to erupt in successively narrower zones near its margins, probably in direct correspondence to the increasing concentration of normal faulting toward these margins. The High Lava Plains have been characterized during this same time by two linearly propagating volcanic systems, in which major cycles of rhyolitic volcanism have been initiated successively farther northwest and northeast. These two volcanic systems have propagated away from a region in the center of the High Lava Plains at about the same rate that faulting and volcanism in the Great Basin have been concentrated toward its margins.
A model that accounts for this evolution relates tectonic extension to the regional stress fields that result from the motions and changes in the interactions of the North American, Pacific, and Farallon lithospheric plates. In this model, geophysical and volcanic features of the region are interpreted to be due to a chain of heating events caused by this extension but conditioned by the stress and thermal history of the continental plate. Stress relief at the base of the lithosphere causes basaltic magma generation of varying amounts and at varying depths in the upper mantle, depending on the thickness and history of the overlying crust. The generation of basaltic magmas and their intrusion into and through the crust during continued extension have increased regional heat flow, lowered the rigidity of the lithosphere, caused crustal thinning, produced flowage and decreased seismic velocities in the upper mantle, caused regional uplift by thermal expansion, and produced rhyolitic magmas by localized partial melting of the lower crust.
According to the model, initial rifting occurred between 17 and 14 m.y. ago when northward migration of the Mendocino triple junction caused the continental-margin subduction zone to become short enough to allow partial coupling between two zones of transform displacement of the Pacific and North American plates. The increased coupling between these two zones caused extension in the North American plate perpendicular to the continental margin. Since about 14 m.y. ago, continued tectonic extension and basaltic magma generation have (1) caused a wide zone of oblique extension to become successively hotter and less rigid near the zone’s central axis, (2) increasingly concentrated brittle deformation and high-level magmatism outward toward the margins of the Great Basin region, and (3) produced concentrated zones of extension and crustal melting at the intersections of the resulting marginal zones with the transitional northern transform boundary of the extending region. This accounts for the symmetrically propagating volcanic systems of the High Lava Plains. The Yellowstone melting anomaly, whose locus was controlled initially by an old structural boundary, was favorably oriented to be augmented by shear melting at the base of the lithosphere; it has become self-sustaining because of the initiation of a thermal feedback cycle and the development of a root in the mantle by inward flow around a dense, sinking, unmelted residuum.