Date of Award

5-2015

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dr. Christopher Schmidt

Second Advisor

Dr. William Sauck

Third Advisor

Dr. Ronald Chase

Access Setting

Masters Thesis-Open Access

Abstract

The objective of this research was to delineate and profile the Late Cretaceous Tobacco Root batholith in southwestern Montana through the application of the gravitational method. This survey was accomplished by obtaining 232 gravity measurements in four profiles across the batholith. After correcting for known variations in the gravity field, a Bouguer anomaly map of the batholith and surrounding host rocks was created. Four residual gravity profiles and map, created by subtracting the regional gravity from the Bouguer anomaly, reveal a maximum negative gravity anomaly in the center portion of the batholith which is parallel to the trend of the batholith. The batholith consists of granitic rocks that have measured densities that are 0.20 g/cm3 smaller than the Precambrian (Archean) metamorphic rocks into which they are intruded. This density contrast produced negative gravity anomalies up to -46.7 mGal. Modeling of the profiles using measured densities indicates that the batholith extends to its greatest depth (20 km) in the subsurface. Another gravity low on the northwest, near the Jefferson River, suggests a subsurface extension of the batholith in that direction. The modeled profiles also suggest a decrease in depth and lateral extent from the center portion of the batholith to the southeast. The narrow subsurface shape on the northwest, parallel to the principal faults, the rapid thickening toward the central region, and the abrupt termination on the southeast are compatible with magma movement from the northwest.

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