Abstract

Metasedimentary gneisses in the Idaho Springs-Central City area of the Front Range have been deformed twice in Precambrian time.

The older and major deformation was plastic folding; it was accompanied by intrusion of a series of plutons and sheets, recrystallization of the meta-sedimentary rocks, and metamorphism of the earlier members of the igneous series. It produced a major fold system consisting mainly of open, but disharmonic asymmetric and upright anticlines and synclines whose axes trend sinuously north-northeast and are spaced 1-2 miles apart. Over most of the area the fold axes are nearly horizontal or plunge at low angles, but in the southern part the axes plunge steeply northeastward. Small folds and a well-developed mineral alignment characteristically parallel the major fold axes (Bo); small-scale folds, boudinage and sparse mineral alignment are present in the Ao direction.

The younger deformation was dominantly cataclastic and restricted chiefly to a 2-mile-wide zone in the southeast part of the area. Within this zone small folds were developed locally in the relatively incompetent rock masses, and intense granulation was developed locally in the more competent units. Cataclastic products are pervasively distributed, however, through all the rocks in the zone. The younger folds are mainly terrace, monoclinal, and chevron types; the largest has a breadth of about 400 feet. These folds trend N. 55° E., are remarkably straight, and plunge at various angles, largely depending upon their position on the older, larger folds. They consistently are strongly asymmetric and show their northwest limbs raised structurally. Associated with these folds are two lineations, one (By) parallel to the fold axes and one (Ay) oriented at about 80° to the fold axes.

The younger deformation is a manifestation of extensive Precambrian shearing defined by a zone of intense cataclasis that extends both northeast and southwest of this region.

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