Graduate Thesis Or Dissertation
 

Basaltic volcanism and tectonics of the High Lava Plains, southeastern Oregon

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  • The High Lava Plains province (HLP) of southeastern Oregon is a Miocene to Recent volcanic upland characterized by widespread basaltic volcanism and west-migrating rhyolitic volcanism. New ⁴⁰Ar/³⁹Ar ages for HLP rhyolites demonstrate that the trend of migrating rhyolitic volcanism is robust, reflecting westward migration at a rate of -35 km/m.y. from 10 to 5 Ma, and ~15 km/m.y after 5 Ma. This pattern mirrors the trend of northeastward migrating silicic volcanism of the Snake River Plain to the Yellowstone Plateau. HLP basaltic volcanism was relatively continuous with episodes of heightened activity at ~7.6, -5.9, and 2-3 Ma. The 7.6 Ma event coincided with initiation of High Cascades volcanism suggesting a major regional tectonic event. HLP basalts are variably evolved high-alumina olivine tholeiites. Even primitive basalts are enriched relative to mid-ocean ridge basalts (MORB) in incompatible trace-elements, especially Ba, Sr, and Pb. HLP basalts are isotopically evolved relative to MORB with ⁸⁷Sr/⁸⁶Sr of 0.70305 to 0.70508 and ΕNd of +6.7 to + 1.6. Isotopic characteristics of Pliocene and Quaternary basalts are more evolved in the east than the west. Miocene basalts are of more uniform isotopic character. Helium isotopes in Quaternary basalts are constant across the HLP with ³He/⁴He of ~9 RA, reflecting either a strongly depleted MORB source or interaction with a mantle plume. The HLP and Snake River Plain are linked by divergent trends of silicic volcanism and a belt of Pliocene and younger basaltic volcanism. To explain both provinces I propose the following. At 17 Ma a small plume head was emplaced under the North American lithosphere, centered near Twin Falls, Idaho, the location predicted by plate tectonic reconstructions. Basaltic volcanism (Columbia River and Steens Basalts) resulted from emplacement of plume head material under thin lithosphere west of the craton margin and from westward flow from the plume up the lithospheric topography at the craton margin. The latter process may also have driven westward mantle flow under the HLP. Westward migrating volcanism of the HLP may also reflect greater times to incubate crustal magmatism further from the center of the plume head.
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file details JordanBrennanT2002.pdf 2017-08-08 公开 下载