Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

and they are classified as catazonal and synorogenic.
Four major systems of Proterozoic faults and shear zones (NE-trending, NNWtrending,
WNW-trending, and E-trending) are recognized in Colorado (Tweto, 1980a and
1980b). Recurrent movements on many major faults and shear zones occurred during the
Proterozoic and at various times during the Phanerozoic (Tweto, 1977 and 1980a). The
NE-trending shear zones started forming during the folding and metamorphism of the
gneiss complex (Tweto, 1987). However, they were active principally during and
following intrusion of the second major Protozoic magmatic event, 1,400 Ma Berthoud
Plutonic Suite, and produced the third generation of folding in the gneissic rocks (Tweto,
1980a). The Berthoud Plutonic Suite consists of quartz monzonite to granite intrusions
that are generally discordant and non-foliated to weakly foliated and are classified as
mesozonal and anorogenic (Tweto, 1987). Evidence for intrusive activity related to the
third major Precambrian igneous event (late Middle Proterozoic), the 1,000 Ma Pikes
Peak Batholith (Tweto, 1987), is more restricted to the Front Range and is lacking in the
Maysville quadrangle and the surrounding region.
The predominant north-northwest-trending Proterozoic fault system, with
indications of both pre-1,700 m.y. and post 1,400 m.y. movements, is responsible for the
strong north-northwest geologic and structural grain of the Colorado mountain province
(Tweto, 1980a). In his detailed review of the tectonic history of west-central Colorado,
Tweto (1977) stressed the long history of recurrent movements on the north-northwesttrending
faults. Tweto (1977, p. 13) specifically stated that “the north-northwest trend of
faults of the Arkansas Valley graben suggests inheritance from Precambrian faults,
though evidence of Precambrian origin of the faults is thus far lacking”.
The third major system of west-northwest-trending Precambrian faults extends
across most of southern Colorado (Tweto, 1980b). An east segment crosses the Wet
Mountains and the northern Sangre de Cristo Range near Salida and a west segment
occurs in the Gunnison River region. The fourth major system of east-trending
Precambrian faults is predominantly present in the northern Front Range (as well as the
western San Juan Mountain region and the White River Plateau) and parallels major lithic
contacts in the basement and a linear fabric on regional gravity and magnetic maps
(Tweto, 1980a, 1980b and 1987).
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