Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

quartz monzonite ring dikes (Tma). The fault crosses Browns Creek and continues
northeastward where it intersects the range-front Sawatch fault zone at Raspberry Gulch.
On the north side of Browns Creek, geological relations between the Mount Antero
leucogranite and the California leucogranite intrusions indicate that the McCoy Creek
fault has significant down-to-the-northwest sense of offset (Shannon, 1986 and 1988).
The northeast-trending fault extends to the southwest along McCoy Creek into the
Garfield quadrangle. In the area of the North Fork, the fault has significant down-to-the
northwest offset indicated by the offset of the North Fork leucogranite intrusion
(Shannon, 1988).
The second major northeast-trending fault in the northwest part of the quadrangle
is the range-front Shavano fault zone (figs. 4 and 5). This fault zone has an overall trend
of N35°E and defines the main break in slope at the range front. The strong structural and
spatial relationship between the northeast-trending faults and the set of northeast-trending
rhyolite dikes suggest these fractures were controlling the emplacement of the Oligocene
dikes. The group of rhyolite dikes (Trp and Tr) that occur in the Proterozoic rocks have
an overall orientation of N25°E with 50°NW dip. Two of the dikes were intruded along
the Shavano fault zone and local brecciation indicates continued movement of the fault
zone after they were emplaced.
The relations described here indicate the presence of a major northeast-trending
horst block, herein informally referred to as the Jones Peak horst, which makes up the
exposed bedrock area in the northwest part of the Maysville quadrangle (fig. 5). The
Jones Peak horst block is about 48,000 ft long and about 13,000 ft wide. It appears to be
terminated on the northeast at the north-northwest-trending range-front Sawatch fault
zone. The Jones Peak horst extends southwest into the Garfield quadrangle where the
continuation of the McCoy Creek fault south of the North fork is uncertain. Estimated
offset on the northeast margin of the Jones Peak horst is a minimum of about 600 ft in the
North Fork in the Garfield quadrangle, and possibly up to 2,300 ft on Mount White in the
Mount Antero quadrangle. No datum allows the determination of offset on the southeast
margin of the Jones Peak horst, but a minimum of 4,000 ft of offset on the basement
rocks is suggested by the elevation difference from the top of Mount Shavano to the
Tertiary and Quaternary sedimentary rocks southeast of the Shavano fault zone. The
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