Date of Award

6-1988

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dr. John D. Grace

Second Advisor

Dr. W. Thomas Straw

Third Advisor

Dr. Christopher J. Schmidt

Access Setting

Masters Thesis-Open Access

Abstract

Thrusting occurred in the central portion of the central Montana salient where the timing of movements is somewhat obscured by the multiple deformations from Archean time to the present. The relationship of this thrusting to the associated plutons has not been established in every case. Fission-track geochronology was employed in dating several plutons associated with the Lombard thrust fault, consequently, defining a time range in which movement along the thrust occured.

Seven plutons with known structural relationships to the Lombard thrust have ages which suggest that movement along the fault occurred 73.2 +/- 6.7 Ma. Fission-track length distributions suggest that all the plutons studied share a common thermal history. The distributions indicate that the plutons were brought to a shallow depth and cooled rapidly similar to undisturbed volcanic-type rocks. Therefore, the fission-track ages obtained in this study are very likely related to the actual crystallization of rocks.

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