Abstract

The Silver Tip structure in the northern Absaroka volcanic field in Montana consists of a core of Precambrian metamorphic rocks that was differentially uplifted more than 2,500 m during Eocene time. Dikes and sills of rhyodacitic porphyry surround the Precambrian core, which has a diameter of about 4.5 km. A gravity survey along an 11 km traverse in a north-south direction across the structure shows a negative gravity anomaly of over 6 mgal. There is a density difference of 0.2 g/cc between the Precambrian rocks (2.72 g/cc) and the intrusive rhyodacite (2.51 g/cc). Ring fracturing coupled with forceful intrusion of a viscous magma is considered to be the main mechanism for the vertical displacement of the Precambrian core.

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