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

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Laramide faults and lack any indication of previous movement. The flat-dipping faults differ in no major respects, other than angle of dip, from the other east-, east- northeast-, and northeast-striking faults. Further, many steep faults flatten in depth, and many flat faults steepen; this indicates that the apparently flat and apparently steep breaks do not differ genetically. For these reasons the flat-dipping and the north-trending faults are grouped with the Laramide faults in this report. Most of the Laramide faults appear to have formed after the emplacement of the quartz bostonite porphyry but before the biotite-quartz latite. In places outside the Idaho Springs district, the east-, east- northeast-, and northeast-striking faults formed in a definite sequence, but the fact that this sequence reverses locally suggests that the three sets of faults all formed over a short period of time. This apparently short period of time and the consistent movement patterns of the fault sets indicate that they are a conjugate shear system. The east-trending faults show left-lateral offset, the east-northeast-, and northeast-trending faults show right- lateral offset, and all three sets may have formed by shear stress related to compressive stress that was oriented east-northeast. The few north-trending faults formed slightly later than the main network of Laramide faults, and their origin is less certain. These faults are similar in orientation but are en echelon to the Precambrian (?) Dory Hill fault of the Central City district.. (See Sims and others, 1963, fig. 6.) Conceivably, these faults may have formed by a reactivation of the Dory Hill fault during the Laramide orogeny. The north-trending faults consistently show small left-lateral offset that took place at a late stage of mineralization. Interestingly, the northwest-trending faults in or near the Idaho Springs fault and the J. L. Emerson-Gem fault similarly show late, but right-lateral, offset. This offset is opposite to the dominant movements on the northwest-trending faults. These relations indicate that the late movements on the two fault sets were about contemporaneous. The north-trending and northwest-trending sets have the expected position and movement patterns caused by compression that was oriented north-northwest. These patterns, however, are also attributable to tensional stress that was oriented east-northeast. The tensional stress may have formed 40 subsequent to the Laramide orogeny and seems more probable than a dominant north-northwest oriented compression. JOENTS All rocks of the district contain two or more sets of joints. A study of joints in the rocks of the Idaho Springs and adjacent areas by Harrison and Moench (1961) revealed that many are related to the emplacement and cooling history of the Precambrian and Tertiary intrusive rocks; other joints may be related to the two Precambrian deformations, and still others may be related to the Laramide deformation. The joints believed to be of Laramide age provide much insight to the Laramide deformation. Detailed descriptions of the joints and the methods used to smoke them were presented by Harrison and Moench (1961) and are only summarized here. Harrison and Moench (1961) measured the attitudes of joints in outcrops throughout the area and plotted the data on Schmidt equal-area projections. The projections were then contoured-according to the method described by Billings (1954, p. 111-114) except that Harrison and Moench used the upper hemisphere - to reveal the dominant attitudes of the joint sets. The diagrams and the data on the geologic occurrence of joints were then examined to determine, as far as possible, those joint sets which may be related to the intrusive rocks, those which may be related to the two Precambrian deformation systems, and those, if any, which may be independent of these features. Some of the problems that arise in the interpretation of contour diagrams were discussed by Harrison and Moench (1961). Harrison and Moench (1961) found that certain joints are largely confined to the intrusive rocks, others vary in attitude and abundance according to variations of the Precambrian structure, and still others, termed the "regional joint system," are nearly ubiquitous and consistent in attitude in areas of contrasting lithology and structure. 18