Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Mount Aetna ring dikes (Tma), one in the northwest corner of the Maysville quadrangle
and the other one in the southeast corner of the St. Elmo quadrangle. The ring shear zones
are described here because they are present as mappable zones of deformation textures
that are superimposed on the older pre-collapse country rocks. They are usually spatially
and genetically associated with ring dikes and flinty crush rock, and thus they are
important elements used to define the Mount Aetna cauldron collapse structure.
S-C mylonite fabrics are locally well developed in the ductile shear zones. C-
surfaces are the main shear planes and S-surfaces are the flattening foliations developed
obliquely to the C-surfaces (Lister and Snoke, 1984). Shannon (1988) found ten localities
around the Mount Aetna ring zone where S-C fabrics indicated the shear sense (Simpson
and Schmidt, 1983). Nine localities, including four in the Tabeguache Peak area,
indicated a shear sense compatible with cauldron subsidence (that is, cauldron block
down).
Mount Princeton pluton (~36.6 Ma) – The southeast flank of the Mount Princeton
pluton is present in a roughly 5,500 ft by 10,000 ft by 11,500 ft triangular area in the
northwest corner of the Maysville quadrangle (plate 1). The Mount Princeton pluton is
about 24 miles long and 14 miles wide and has an elliptical elongation in a N15° to 20°E
direction (figs. 3 and 4). The approximately N55°E-trending southeastern contact extends
for about six miles from near Clover Mountain in the central part of the Garfield
quadrangle, through the northwest part of the Maysville quadrangle, and into the south
part of the Mount Antero quadrangle where it is inferred to be truncated at the range-front
Sawatch fault zone.
There is a 9.5 mi long and about 1.0 mi wide, N60°E-trending zone of Mount
Princeton pluton rocks that occur outside the southeastern margin of the Mount Aetna
cauldron collapse structure (figs. 4 and 5). This zone of Mount Princeton rocks is
bounded on the northwest by the structural boundary and ring zone of the Mount Aetna
cauldron. The southeastern boundary is an intrusive contact of the Mount Princeton
pluton with older rocks, including a large area of Paleozoic sedimentary rocks south of
Mount Taylor in the Garfield quadrangle and Early Proterozoic rocks in the northwest
corner of the Maysville quadrangle. The Mount Princeton-Proterozoic contact is intruded
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