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

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REGIONAL JOINT SYSTEM Four sets of joints are almost uniformly present in all rocks except the Tertiary porphyries and are remarkably consistent in attitude despite differences in Precambrian structure. These sets, termed the "regional- joint system," are summarized in figure 4, a stereodiagram that was constructed from the regional joint maxima on Schmidt contour diagrams of joints in the Idaho Springs district. The prevalence of these joint sets throughout the Idaho Springs and surrounding areas, the angular relations between the sets, and the fact that they intersect on a nearly common line suggest that they are genetically related to one another. The regional joint system in turn shows systematic relations to the broad archlike form of the Front Range. The uplift and arching of the Front Range in Paleozoic and Laramide times (Lovering and Goddard, 1950, p. 57-63) are recognized major disturbances after the emplacement of the biotite-muscovite granite to which the regional joint system might logically be related. The Laramide arching was the more pronounced of the two disturbances. This and the fact that the geometric relation of the regional joint sets to the known shape of the Laramide arch is similar to relations shown between joints and folds elsewhere (Harrison and Moench, 1961, fig. 8) indicate that the arch and the regional joint system are genetically related. The arch trends about N. 15° W. near the Idaho Springs district; accordingly, the joint set that strikes N. 19° W. is called the "regional longitudinal set" because it is subparallel to the arch. The arch disappears southward near Cripple Creek, Colo., which indicates that it plunges gently southward. A set of cross joints related to the arch should dip steeply north and strike east-northeast; accordingly, the set that strikes N. 70° E. and dips steeply north is termed the "regional cross joint set" So-called diagonal joints are common in folded areas. Ideally, two sets are present, in each, the acute angle of intersection is bisected by the cross joints and the obtuse angle of intersection is bisected by the trend of the folds. The regional cross joint set shown in figure 4 bisects the acute angle between two intersecting sets, which are accordingly termed the "regional diagonal joint sets." The regional joint system probably formed as a result of the Laramide arching of the Front Range under compressional stresses that were oriented east-northeast. The two diagonal joint sets have the position of shear fractures for such a stress system and probably originated by shear. The cross joint set has the position of tension fractures for this stress system and may have originated by elongation or bending of the axis of the arch. Tension fractures may also form approximately parallel to the axial plane of a fold owing to the stretching of layers across the top of the fold, and they may form owing to the release of the principal stress. The regional longitudinal joint set may have originated in both ways. (See Harrison and Moench (1961) for a review of the joints expected on folds.) GEOLOGIC HISTORY SUMMARIZED In Precambrian time a thick sequence of sedimentary rocks was deeply depressed into the earth's crust; it recrystallized at high temperatures and pressures and was intensely deformed. The rocks were deformed plastically into large, mainly open folds whose axes trend north to northeast. Concurrently, small bodies of granodiorite, quartz diorite, and quartz gabbro were emplaced, recrystallized, and slightly deformed after they solidified. Small bodies of biotite-muscovite granite probably were emplaced late in this early deformation - apparently early enough to form phacoliths in folds of this deformation but too late to be deformed or recrystallized. After an interval of deep erosion, the rocks were deformed again. This younger Precambrian deformation was largely restricted to the narrow Idaho Springs-Ralston shear zone which extends at least 20 miles northeast and a few miles southwest of the Idaho Springs district. Within this zone, which is about 2 miles wide in the Idaho Springs district, pervasive granulation and folding were superposed on the previously folded rocks. Probably in late Precambrian time the through-going northwest-trending Idaho Springs fault, the J. L. Emerson-Gem fault system, and some smaller faults formed. A few relatively flat-lying faults and some steep north-trending faults may also have had their inception at this time, although their main movements came much later. • 17