Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

of the quadrangle. The north-northwest- and northeast-trending faults cutting the Mount
Princeton pluton continue northward into the Mount Antero quadrangle where they cut
the early Oligocene Mount Aetna ring dikes and the Oligocene Mount Antero
leucogranite intrusions (Shannon, 1988). Thus, some of the faults have significant offsets
that are younger than about 29.8 Ma (average age of leucogranites).
The north-northwest-trending faults are the southern end of a zone of similar
north-northwest-trending faults that define the western margin of a long, rift-shoulder
horst block that is informally referred to as the Collegiate Peaks horst (figs. 4 and 5;
Shannon and others, 1987c; Shannon, 1988). The down-to-the-west sense of offset across
the western margin of the horst block was estimated to be about 2,600 ft on the basis of
offset of internal textural igneous-stratigraphic horizons in the Mount Princeton pluton
and other lithologic discontinuities. Additional support for differential uplift of the horst
block is indicated by the distribution of peaks higher than 14,000 feet and the resetting of
apatite fission-track ages. The down-to-the-west sense of apparent offset on the northnorthwest-trending
fault (which offsets the Proterozoic Xag/Xhig-Xgdf contact in the
northwest quadrant of the Maysville quadrangle) is compatible with it being related to the
western margin of the Collegiate Peaks horst. The north-northwest-trending faults in the
northwest quadrant of the quadrangle are of similar orientation to faults in the southwest
quadrant, some of which are interpreted to be Laramide faults. It is suggested that the
north-northwest trend of the Collegiate Peaks horst may be influenced by Laramide
basement structures (and possibly even earlier Late Paleozoic and Proterozoic structures)
that have been superimposed on the Oligocene intrusions.
The northeast-trending faults in the northwest part of the Maysville quadrangle
also fit into a larger regional picture of complex faulting in the Sawatch Range riftshoulder
uplift. The northeast-trending fault crossing the very northwest corner of the
Maysville quadrangle, herein informally referred to as the McCoy Creek fault, is situated
between Mount Shavano and Tabeguache Peak and extends into the southwest corner of
the Mount Antero quadrangle. This fault is oblique to the southeast margin of the Mount
Aetna cauldron ring zone defined by various ring zone features including Mount Aetna
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