2009-09-25_PERMIT FILE - M2009076 (31)

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Within the zone of younger Precambrian folding, small folds, crinkles, and sparse mineral alignments parallel the younger folds, abundant rodding and slickenside striae are oriented about at right angles to the trends of the younger folds; they formed by the shearing that accompanied the younger folding. FOLDS The gneissic rocks were deformed twice during Precambrian time. The first deformation, which took place at considerable depth in the earth's crust, was pervasive and resulted in major folds that trend sinuously north-northeast. These folds define the structural framework of the district. The second deformation, which took place at somewhat shallower depth, folded the incompetent biotite gneisses and associated rocks along axes that trend N. 55° E. and sheared the more competent microcline gneiss. Folds and shears of the younger deformation, which are superposed on the older Precambrian folds, are largely restricted to the southeast half of the Idaho Springs district (pl. 2). These effects of the younger Precambrian deformation represent part of the Idaho Springs-Ralston shear zone (Tweto and Sims, 1963, p. 998). The major folds of the older deformation are wide and largely open. Their axes trend sinuously nearly north to about N. 50° E. (pl. 2). The Idaho Springs anticline is the dominant fold in the southeastern part of the district. This anticline is one of the major folds in this part of the Front Range, for it is an asymmetric feature that marks the boundary between a large area of rocks that strike mainly northeast on the northwest side of the axis, and a large area of rocks that strike west to northwest on the southeast side of the axis. Tweto and Sims (1963) interpreted this anticline as an early manifestation of the Idaho Springs-Ralston shear zone, The anticlinal axis trends about N. 60° E. in the southern part of the area and turns to N. 45° E. in the central part; it plunges gently to moderately northeast. In the southern part of the district, the northwest limb of the Idaho Springs anticline is about 2 miles wide and dips steeply northwest; it is bounded on the northwest by the Trail Creek syncline, a relatively small, open fold (pl. 2). In the central part of the district the northwest limb of the Idaho Springs anticline is about 1.5 miles wide and is bounded on the northwest by the Pewabic Mountain syncline-northwest of which are the Bellevue Mountain anticline and the Central City anticline (pl. 2). The four folds in the northwest part of the district are relatively gentle warps in rocks that are grossly flat lying. The Central City anticline, which enlarges northward, is the dominant fold in the Central City district (Sims, 1964). Although the major older Precambrian folds are open and simple in their gross aspect, some, locally, are closed and overturned. The Bellevue Mountain anticline and Pewabic Mountain syncline, for example, are open warps near the contact between the microcline gneiss and biotite gneiss. Where these warps continue upward into the biotite gneiss, the limbs steepen and the folds overturn to the southeast (pl. 2). This feature reflects the incompetence of the biotite gneiss relative to the microcline gneiss-that is, the biotite gneiss flowed much more readily when it was deformed. In contrast to the generally open and simple character of the major folds, many small folds-a few tens of feet wide or less-are closed and have axial plane that are subparallel to the rock layering. Many of these are drag folds that have formed by slippage of successively higher layers toward the anticlines. Most of the northwest-bearing small folds are open warps and apparently formed late in the older deformation. Small folds and lineations that are related to the major older Precambrian folds are of many kinds and are ubiquitous. The axes of small folds, crinkles, and mineral alignments bear nearly north to N. 50° E., averaging about N. 25° E., parallel to the axes of the major folds. The axes of many small folds and boudinage, however, bear northwest about normal to the axes of the major folds. These northwest-bearing lineations were observed mostly in the northwestern part of the district. The effects of the younger Precambrian deformation are largely restricted to the southeast half of the district. Southeast of the boundary, which is shown on plate 2, the gneissic rocks are pervasively granulated; thin sections of most of the specimens obtained from this zone show a fine network of anastomosing, subparallel fractures. The biotite gneiss within this zone is completely folded as well as granulated, whereas the microcline gneiss is granulated but rarely folded. The younger folds are superposed on both limbs of the Idaho Springs anticline. They are small, have steeply dipping axial planes, and are distinctly asymmetric. In contrast to the sinuous northeast trend of the 14