STRAIN PARTITIONING IN SYNTHETIC ACCOMMODATION ZONES AND SELECTED GEOTHERMAL SYSTEMS IN COLORADO

Complex fault patterns associated with accommodations zones have been described by Faulds and Varga (GSA Spec. Paper 123, 1998), and the association of geothermal systems with transverse zones in the Great Basin has been documented by Rowley (GSA Spec. Paper 123, 1998), and of geothermal systems with accommodation zones in Nevada by Faulds and Shevenell (Nevada Bureau of Mines and Geology), and others. Similar associations are clear in Colorado where thermal springs occur within en echelon offsets in range-bounding faults. There is no indication of magmatic activity associated with these thermal springs and their heat is believed to be derived from deep circulation of meteoric waters.

En echelon offsets commonly occur where tributary valleys join main valleys, and tributary waters could make a contribution to the deep circulation. A simple model of strain partitioning in a synthetic accommodation zone, however, indicates that in the “box” between the two overlapping fault segments, extensional horizontal strain perpendicular to fault strike is greatest in the two quadrants of the box that are bounded on one side by the two terminations of the fault segments. This extension may result in faulting and/or rotation, but however it is accommodated, it is likely to result in an increase in the vertical permeability for geothermal fluids. New studies of the locations of thermal springs within accommodations zones in the Upper Arkansas Valley in Colorado, combined with the results of new thermal gradient drilling indicate that the hottest areas within the accommodations zones are the areas with the greatest extensional strain, as predicted by the simple strain partitioning model. In the Mt. Princeton area, results of published geologic mapping and geophysical field camp from the Colorado School of Mines/Boise State/Imperial College (London) indicate extensions of mapped faults, and subsurface fault structures beneath glacial cover in one of the predicted high-strain zones.