2009-04-20_GENERAL DOCUMENTS - M2009076 (6)

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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 older folds, the younger folds trend N. 55° E. in a remarkably consistent pattern. Only a few folds exceed 100 feet in width, but even the small ones may be traced for long distances. Their plunge is extremely variable; it ranges from nearly horizontal just south of Idaho Springs to steeply northeast in many places to the north. The folds range in shape from structural terraces having nearly flat crests and steep northwest limbs to sharpcrested chevron folds and, locally, to nearly isoclinal folds. East-facing monoclines are locally present on the southeast limb of the Idaho Springs anticline. These shapes depend on the position of the minor folds on the Idaho Springs anticline and their origin may be explained by differential movements in which the northwest sides have been raised relative to the southeast sides (Moench and others, 1962). Some of the large younger Precanibrian folds are shown in sections on plates 2 and 3. Plate 3 was drawn from the map of the Big Five tunnel (Moench and Drake, 1966, fig. 6) and the geologic map of the district (Moench, 1964, pl. 1). The largest younger folds in the Big Five tunnel are exposed. in biotite gneiss between the Hudson( ?) and Shafter veins (pl. 3). On the tunnel level these folds are sharp-crested chevrons that have intricately crinkled opposing limbs. This is the most common shape of the younger folds in the district. The microcline gneiss, in contrast with the biotite gneiss, has been sheared on northwest-dipping fracture surfaces but has not been folded. This relationship has been observed at several places at the surface where the younger folds in the biotite gneiss were traced to the contact with the microcline gneiss but were not traced farther beyond. Small folds and lineations of many types are related to the larger younger Precambrian folds. Small • crinkles that are parallel to the younger fold axes are abundant in the folded biotite gneiss, and biotite and sillimanite have recrystallized locally along the younger fold axes. In addition, slickenside striae, rodding, and, 12