Date of Award

Spring 1997

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Geology

Committee Director

Francis Ö. Dudas

Committee Member

Dennis A. Darby

Committee Member

John Leftwich

Call Number for Print

Special Collections LD4331.G4 I87

Abstract

Exposures of the Eocene Lowland Creek Volcanics (LCV) cover an area of 2000 km2 in southwestern Montana, and consist of basal elastic deposits, felsic tuffs, and felsic and intermediate lavas with an aggregate thickness of about 2 km. New 40Ar/39Ar dates show that volcanic activity lasted for at least 4.2 million years (52.7- 48.5 Ma), or even longer (4.5-4.7 million years: from 53.0-53.2 Ma to 48.5 Ma). During evolution of the volcanic field, early explosive volcanism was gradually replaced by extrusive activity. During the transition period (52.7-51.5 Ma), the two volcanic styles coexisted.

The evolution of the Lowland Creek volcanic field is in some features similar to that of the Challis field, the largest Eocene igneous province of the northwestern U.S. However, volcanism in the Challis occurred later, 51-45.5 Ma. The Eocene volcanics found between the two fields exhibit ages close to those of the Challis. The LCV are contemporaneous with alkalic igneous provinces of central Montana, where magmatism was initiated 54-52 Ma. The distribution of ages of Eocene igneous rocks shows that they cluster within two zones which could be related to different "root" magmatic megasystems. The western zone is presumably centered in the Challis area and includes the Challis and Absaroka fields, and a number of smaller igneous centers, where magmatism initiated about 51 Ma or later. Another system, centered in the Highwood-Bearpaw Mountains area (500-600 km from the Challis), unites the LCV and alkalic igneous centers of central Montana, with earliest volcanism at 54-52 Ma.

Contemporary geodynamic models relate the Eocene magmatism to lateral, southwestward, propagation of an asthenospheric wedge between decoupling continental and oceanic plates. This model appears to contradict geochronological evidence: there is no linear decrease in the ages of the Eocene igneous rocks in a southwestern direction, and the southwestward shift of magmatism is too rapid (500-600 km per three million years, or 17-20 cm per year) to be explained solely by plate decoupling. The Eocene magmatism could be caused by vertical influx of asthenospheric melts through the fractures in a subducted oceanic plate. It requires either disintegration of the subducted plate or reactivation of fracture zones within it.

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DOI

10.25777/scjg-2451

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