FAULTING STYLES, FRACTURE NETWORKS, PALEO AND PRESENT-DAY FLUID FLOW ALONG THE MARGIN OF THE QUESTA CALDERA, RED RIVER VALLEY, NEW MEXICO

The Questa Caldera has undergone significant extension accommodated by a complex array of distinctive fault zone types associated with a significant molybdenum deposit. Field-based characterization of faulted and fractured crystalline bedrock in the exceptionally well exposed caldera and its margin was completed to (1) document types of brittle structures and (2) speculate on potential effects these structures might have on the paleo to present-day ground-water flow system in relation to natural or mining-related metal and acid loading to surface waters.

Open joints are pervasive in the study area. There are distinctive joint network patterns, such as orthogonal or conjugate joint sets, with high to extreme intensities. There are three major types of fault zones, including: partially silicified, low and high angle faults with well-developed damage zones and clay and mineral-rich cores; and high angle unsilicified open faults. Conceptually, open joint networks can be thought of as providing the background permeability structure of the bedrock aquifer that is cut by discrete features such as the open faults that may act as key hydrologic heterogeneities.

The southern caldera margin runs parallel to the Red River Valley, whose incision has left an extreme topographic and presumed hydraulic gradient at a high angle to the river. Many of the faults and fault intersections run parallel to this gradient and thus have potential to provide paleo and present-day, discrete and anisotropic pathways for solute transport within the otherwise relatively low permeability bedrock.

Although brittle fracture networks and faults are highly complex, simple Darcy calculations were used to estimate potential ground-water discharges of the bedrock aquifer, caldera margin, and faults to surface waters. The calculations are calibrated to baseflow discharge and aquifer test data and provide insight into the potential discharge contributions of these features. Although the results are predictably non-unique, they show the bedrock alone could produce a significant proportion of baseflow to the Red River as could several of the fault scenarios. The method also highlights a way to integrate field-based brittle structural data with hydraulic data to narrow first order hypotheses in complex, faulted and fractured ground-water flow systems.