Date of Award
Master of Science
Geological and Environmental Sciences
Dr. Ronald B. Chase
Dr. John D. Grace
Dr. Christopher J. Schmidt
Masters Thesis-Open Access
The eastern flank of the Bitterroot range, western Montana, consists of a 100 km-long zone of cataclasis which dips to the east. Zircons in sheared granitic plutons from three localities within this zone were dated using U-Pb methods. These zircons, which consist of anhedral cores and euhedral overgrowths, yield concordia lower-intercept ages of 48-52 million years before the present, dates which likely represent crystallization ages of the plutons. Shearing which affected these plutons, therefore, is significantly younger than the 72-76 million year age assigned for eastward movement of the Sapphire tectonic block whose detachment zone has been interpreted as the zone of cataclasis. It is proposed that the Sapphire tectonic block developed by eastward thrusting during Cretaceous time followed approximately 25 million years later by intrusive uprise of the Bitterroot dome with cataclasis along the eastern margin resulting from adjustments between its mobile infrastructure and relatively rigid suprastructure.
Arruda, "U-Pb Systematics and Ages of Rocks from the Zone of Cataclasis, Bitterroot Dome, Southwestern Montana" (1981). Master's Theses. 1785.