Date of Award

8-2015

Degree Name

Master of Science

Department

Geological and Environmental Sciences

First Advisor

Dr. Christopher J. Schmidt

Second Advisor

Dr. Michael Stickney

Third Advisor

Dr. Michelle Kominz

Fourth Advisor

Dr. William Sauck

Keywords

Focal mechanisms, Southwestern Montana, crustal extension. T-axis, earthquakes

Access Setting

Masters Thesis-Open Access

Abstract

Southwestern Montana has experienced several large damaging earthquakes over the last 100 years, but many minor ones that unveil the nature of the intraplate seismicity. The region in this study is part of two distinct Late Cretaceous tectonic provinces, the Rocky Mountain Foreland Basement (RMFB) and the Cordilleran Fold and Thrust Belt (CFTB). Relationships between the two provinces and their faults show that the focal mechanisms are different. Deep focal mechanisms (between 8 and 10 km) within the RMFB can be placed on a specific fault, with many smaller events falling within the hanging wall. The hypocenters within the hanging wall are hypothesized to be related to the lowering of effective stress by the movement of hot spring water through fractured metamorphic bedrock.

T-axes within the northern CFTB cluster closely around N80°E/S80°W with few outliers. Extension in the southern RMFB generally trends roughly N10°E/S10°W. Subsidence of the Snake River Plain appears to influence the north-south extension. In the northern RMFB is a 125 km wide transition zone with T-axes oriented N45°E/S45°W. North of this transition zone, the Northern Rocky Mountain Province follows the same east-west extension as in the Basin and Range Province to the south of the Snake River Plain.

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