Date of Award:

8-2012

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

John W. Shervais

Committee

John W. Shervais

Committee

Barry B. Hanan

Committee

Carol M. Dehler

Committee

Anthony R. Lowry

Committee

James P. Evans

Committee

Keith D. Putrika

Abstract

Among the terrestrial planets, only Earth has volcanism and plutonism that is related to plate tectonics. At the bottom of the oceans, mid-ocean ridges form where tectonic plates drift apart. More visible, and explosively dangerous, is volcanism related to the subduction of oceanic plates back into the Earth’s mantle. Less clear is the origin of the third major type of volcanism, the so-called intra-plate volcanism, which is thought to represent thermally driven upwellings from the lower mantle. This dissertation investigates two of these unique volcano-tectonic settings which occur in the western United States.

Subduction zones are regions of intense earthquake activity and explosive volcanism. The sinking of oceanic lithosphere in subduction zones provides most of the force needed to drive the plates and cause mid-ocean ridges to spread, with the result that plate tectonics and subduction zones are both surficial and interior expressions of Earth’s dominant tectonic mode. Subduction zones deliver raw materials to the “subduction factory,” where oceanic lithosphere, sediments, and seawater re-equilibrate with the mantle, triggering melting and incidentally creating continental crust. What is not recycled in the upper few hundred kilometers of a subduction zone sinks to the core-mantle boundary, where this residue may be reheated for a billion years or so until it is resurrected as a mantle plume.

Mantle plumes or “hotspots” are thought to play a crucial role in the Earth’s thermal and tectonic evolution and are one of the major processes affecting the interior of the Earth. They have long been implicated in the rifting and breakup of continents, and plume-derived melts play a significant role in the creation and modification of sub-continental mantle lithosphere. Hot spots are zones of anomalous volcanism that cannot be directly associated with plate tectonic processes. They are typically thought to be supported by columnar upwellings of hot material or “plumes,” that arise from the deep mantle and punch through the mobile, convecting, shallow mantle to reach the surface. The term ‘hotspot’ is often applied to any long-lived volcanic center that is not part of the global network of mid-ocean ridges and island arcs. This concept of stationary heat sources in the mantle was first introduced in the 1960’s, with more evidence gathered thereafter. Although direct evidence for the existence of mantle plumes is sparse, their occurrence is consistent with our present understanding of mantle dynamics.

This dissertation investigates the Coast Range Ophiolite, California and the Snake River Plain, Idaho, which represent two distinct provinces within the North American Cordillera and ultimately concluded that the Coast Range Ophiolite and the Snake River Plain formed in response to a subduction zone and hotspot setting, respectively. The objectives of this research were to integrate the chemical and physical structure of these provinces, combine mantle geochemistry with geophysics, and trace each province back to their respective mantle provenance. The methods used to attain these objectives include major and trace element chemistry, which represents first-order data needed to understand fractional crystallization, magma recharge and mixing, assimilation of lower crust and/or pre-existing mafic intrusions, melting in the upper mantle at different depths, and variations in the composition of the mantle source region. Isotope data were also required to sort out the processes responsible for the major and trace element chemistry of eruptive products, distinguish the magma source components, and to constrain the time periods and dynamics of magmatic processes.

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This work made publicly available electronically on July 30, 2012.

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