CHARACTERISTICS OF BRITTLE DISTRIBUTED DEFORMATION ZONES IN PRECAMBRIAN CRYSTALLINE ROCKS OF THE CENTRAL COLORADO FRONT RANGE

Brittle fault zones in Precambrian crystalline rocks of the Front Range exhibit architectural styles that include localized deformation zones (LDZs – single fracture faults or faults with core materials such as gouge that is concentrated in a narrow zone with little off-fault deformation), distributed deformation zones (DDZs – zones of intense brittle fracturing where little core material is developed), and composite deformation zones (CDZs – combinations of LDZs and DDZs). Front Range DDZs are particularly interesting because they have mapped widths of over 1 kilometer, trace lengths of kilometers to 10s of kilometers, multiple-kilometer-scale splays with highly irregular shapes, and typically have displacements of only several meters to a few 100s of meters. The DDZs tend to be steeply dipping structures with a most dominant strike of northwest-southeast that parallels a commonly cited Precambrian structural fabric in the Front Range (a few northeast-southwest and north-south striking DDZs are also found). Possible Precambrian ancestry and episodic reactivation have accommodated all modes of slip and deformation, although left-lateral strike slip appears to be common. The predominant structural style is extreme and pervasive brittle fracturing surrounding a few principal slip surfaces. Despite the degree of deformation, there are remarkably few internal structures that appear to have accommodated motion. Most internal fractures are truncated against neighboring fractures, have trace lengths of only a few meters, and show little evidence of shear. Other internal features include small faults, minor breccia zones, discontinuous stringers of clay-rich gouge, Liesegang bands, and macroscopic veins. Based on a variety of alteration assemblages and residential water-well data the DDZs have apparently acted as important conduits to combined conduit-barriers for flow of hydrothermal fluids as well as present-day groundwaters. Of major significance is that these DDZs do not appear to adhere to common models for the growth of faults in the brittle crust and the processes that formed them are poorly understood. Present observations indicate that they are the result of coupled mechanical, fluid flow, reaction, and heat-related processes and ongoing work will focus on their evolution through geologic time.