Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Shannon, 2005). Widmann and others (1998) reviewed the nomenclature for the
Quaternary faults along the west side of the Upper Arkansas Valley graben. This zone of
faults has been referred to as the Sawatch fault (Witkind, 1976; Kirkham and Rogers,
1981; and Ostenaa and others, 1981) and has been described as a segmented fault (Lettis
and others, 1996; and Unruh and others, 1992) or a sectioned fault (Widmann and others,
1998). The Sawatch fault has been divided into northern and southern sections with the
dividing line in the Twin Lakes area; the fault consists of more than 16 generally northtrending
faults (Widmann and others, 1998). Thus, the zone of north-northwest-trending
faults along the range front is informally referred to as the Sawatch fault zone (fig. 5;
McCalpin and Shannon, 2005).
The orientation of the Sawatch fault zone changes to a N35°E orientation
southeast of Mount Antero and produces a western flare in the south end of the Upper
Arkansas Valley graben (figs. 4 and 5). This northeast-trending structural zone along the
southeast flank of Mount Shavano has previously been included in the southern section of
the Sawatch fault (Widmann and others, 1998). Observations presented in this report
(see Structural Geology section) suggest that this northeast-trending structure is related to
a northeast-trending horst block in the Sawatch Range rift-shoulder uplift. Consequently,
we herein informally refer to this northeast-trending segment of the range-front fault zone
as the Shavano fault zone.
The southern end of the Upper Arkansas Valley graben is a structurally complex
area that Knepper (1974 and 1976) characterized as a “zone of intersecting structural
trends” related to the intersection of faults related to the Upper Arkansas Valley graben
with the faults related to the Sangre de Cristo horst and the San Luis Valley graben. He
proposed that the early rift faulting was oriented north-south and progressively opened a
north-south-trending Upper Arkansas Valley graben that was physically separate from the
San Luis Valley graben. A later stage of rift faulting with northwest trend was then
superimposed on the southern end of the Upper Arkansas Valley graben associated with
uplift of the Sangre de Cristo horst block. The northwest-trending faulting has been
suggested to be related to Neogene and Quaternary faulting, much of it after 5 to 7 m.y.
ago (Taylor, 1975; Tweto, 1979a).
Van Alstine (1968) first recognized a structural trough, on the south side of the
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