Structural Geology and Metamorphism of the Skin Gulch Area, Northern Front Range, Colorado

The Skin Gulch shear zone is one of several northeast-trending and steeply-dipping shear zones located in northern Colorado. These shear zones were all formed or reactivated during the accretion of crustal material to the southwestern (present coordinates) margin of Laurentia from 1.78 - 1.63 Ga during the Yavapai and Mazatzal orogenies. Recently, a new period of deformation ca. 1.6 Ga has been recognized in several other shear zones in northern Colorado, southern Wyoming, New Mexico, and Arizona. In order to understand the mechanism behind this ca. 1.6 Ga deformation, it is important to know the kinematic and metamorphic histories of each shear zone that was formed or reactivated during accretion. Previous to this thesis, the Skin Gulch shear zone had been recognized by other researchers but a detailed structural and metamorphic study had not been done in the area.;Here, a detailed structural and metamorphic study of the Skin Gulch shear zone is presented. Mapping resulted in the recognition of two deformational events, D1 and D2. D1 fabric elements are scarce in the field area but are preserved in areas of low D2 strain. D1 is manifested as a northwest-striking and steeply dipping S 1 foliation. One F1 fold is preserved with an axial plane parallel to S1. D2 fabric elements are the most common in the area. D2 elements come in the form of northeast-striking and steeply-dipping S2 foliations, steeply-plunging L2 lineations, and steeply-plunging F2 folds. Macroscopic and microscopic kinematic indicators suggest multiple periods of southeast-side-up motion along the Skin Gulch shear zone.;Metamorphic conditions have been estimated using pseudosection analysis. This modern technique allows for pressure and temperature conditions to be determined while taking into account the specific bulk composition of the rock. Pseudosections done for samples of pelitic units from north and south of the Skin Gulch shear zone show that the shear zone juxtaposes rocks that have experienced different metamorphic conditions. A knotted schist unit located south of the Skin Gulch shear zone yielded pressures of ~3.5 - 5 kilobars and temperatures of ~600 - 700°C. A quartzofeldspathic schist unit located north of the Skin Gulch shear zone yielded pressures of ~3.25 - 3.5 kilobars and temperatures of ~600 - 625°C. By assuming that 1 kilobar corresponds to 3.7 kilometers in depth in the crust, it is estimated that there has been between 0.92 and 6.48 km of southeast-side-up motion on the Skin Gulch shear zone which is consistent with the structural interpretation presented in chapter 3.