Abstract

The Little Chief Granite porphyry stock of Tertiary age, which is exposed over an area of 11 mi2 (28.5 km2) and through an elevation range of 6,300 ft (1,920 m), has the form of a steep-sided dome, with contacts that dip mainly from vertical to 60° outward. Along the eastern side of the stock, the contact dips vary locally from 25° outward to 35° inward. The stock is estimated to have a relatively flat floor and is made up of two intrusive phases: a concordant south phase, which preserved ∼3,000 ft (900 m) of overburden, and a crosscutting north phase, whose roof lay very close to the estimated position of the ground surface at the time of intrusion of the stock.

Magma for the stock apparently moved upward to levels now located near modern sea level in a narrow, east-trending dike system. It began to spread laterally and upward to form a stock when it reached the almost horizontal contact between crystalline basement rocks and overlying, relatively incompetent, carbonate-rich sedimentary rocks. A contact phase of the granite, which occurs as inclusions, was formed by reaction with dolomitic wall rocks at this time. By the time the magma chamber was 1,000 ft (300 m) thick, extension of the rock had formed a system of westward-dipping normal faults along which rhyolite porphyry dikes were intruded. Continued upward movement of the magma resulted in the formation of a nearly rectangular trapdoor of sedimentary rocks bounded by vertical faults. The trapdoor opened to the west with vertical throws of more than 3,000 ft (900 m) on the western side of the stock and a north-trending hinge line slightly east of the stock. The trapdoor faults offset the earlier normal faults and associated dike swarm. Simultaneously with formation of the trapdoor, a regional system of vertical, north-trending faults formed south and west of the stock; these faults converge on the trapdoor from the south and also offset the earlier normal faults. The magma continued to rise, doming and stretching the wall rocks within the trapdoor, until the roof of the south phase of the stock reached its present position at a modern elevation of about 11,000 ft (3,350 m). The north phase magma continued rising to about 13,000 ft (3,960 m) in elevation, probably venting to the ground surface and, in the process, breaking the trapdoor along an east-trending tear fault that extends east of the stock.

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