PROGRESSIVE FRACTURING AND HYDROTHERMAL ALTERATION AT A GEOMETRIC SEGMENT BOUNDARY, CHALK CLIFFS, CENTRAL COLORADO

Located approximately 200 km southwest of Denver, the Chalk Cliffs display an intensely fractured and hydrothermally altered quartz monzonite. They are situated along a geometric segment boundary linking two segments of the Sawatch Normal Fault, a northwest trending system that forms the western boundary of the northern Rio Grande Rift. While most studies of geometric segment boundaries are inhibited by poor exposure, the excellent exposures provided by the Chalk Cliffs, combined with the intimate association between fractures and hydrothermal alteration, make the Cliffs a natural laboratory to study the deformation history of this segment boundary.

Using the various hydrothermal products as signatures of deformational events, previous researchers have outlined several episodes of segment boundary development. Preliminary results from a fracture investigation completed at the Chalk Cliffs during the summer of 2002 indicate that the boundary evolution is more complex than previously reported. Using standard structural data and data collected from circular scanlines and windows, we have outlined a new scheme to define the evolutionary history. Initial deformation was accompanied by the precipitation of a fine-grained, pale-green chlorite fill along sub-vertical fractures. A second episode of darker chlorite then became stable and pervasively altered the quartz monzonite within subhorizontal shear zones. A quartz + sulfide fluid flowed locally through some of these shear zones after most of the chlorite alteration and before the laumontite, which became stable towards the end of the chlorite-stable phase. Laumontite alteration and/ or coeval hematite precipitation exist(s) adjacent to fractures of variable orientation but the most common occurrence is along north-south trending subvertical fractures. Calcite was the last main phase to stabilize. Both platy calcite and rhombahedral calcite exist as fracture fill in northwest-trending, steeply dipping fractures. In the eastern end of the Cliffs, either epidote or rhodochrosite locally exists in some calcite veins. Future work includes testing the statistical significance of the correlations, calculating paleostresses using slickenline data, and analyzing the contributions from fractures on bulk permeability.