FAULTS IN ROCK, FAULTS IN DIRT, AND ENIGMATIC BRECCIAS IN THE ESPAÑOLA BASIN, NEW MEXICO: THEIR RELATIONS TO RIFT ARCHITECTURE, GROUND WATER, AND A POSSIBLE IMPACT STRUCTURE

Brittle structures within and flanking the Española basin show great diversity in structural style, orientation, scale, age, displacement magnitude and direction, fault rocks, cements, permeability structure, and geochemistry. These parameters were characterized for a number of structures to understand their potential influence on paleo fluid flow and present-day ground-water resources. Several groups can be recognized: 1) steep ENE- and NE-striking, combined conduit-barrier faults involving Proterozoic crystalline basement and Paleozoic sedimentary rocks with localized hydrothermal alteration and likely early strike-slip and possible normal-slip reactivation; 2) NNW- to NNE-striking, steep, partial-barrier normal faults in poorly lithified Neogene basin-fill sediments with thin clay-rich cores and variable calcite and silica cements; 3) NW- to NE-striking, steep, normal and strike-slip small faults in Tertiary basaltic and volcanic rocks with likely conduit and barrier permeability structures; and 4) randomly distributed and oriented dike to pod-like autoclastic, breccia bodies in Proterozoic rocks that are atypical of fault breccias and are proximal to exposures of possible impact-related shatter cones.

Each group has distinctive geochemistry related to their origin and evolution. For example, illite dominates hydrothermally altered fault cores in crystalline rocks. In contrast, kaolinite is dominant in faults cutting both Proterozoic and Paleozoic rocks, and smectite dominates faults in basin sediments. Al and Fe are prevalent in Proterozoic basement faults, whereas Ca and Ba are prevalent in basin faults. Of particular interest is a possible iridium anomaly associated with some of the breccia bodies, consistent with an impact rather than fault-related origin.

Fault zone architecture indicates fundamental differences between the rift flank and basin faults. Crystalline rocks have fractured damage zones that could make them partial conduits for ground-water flow whereas basin faults have pervasive clay-rich cores, cements, and few fractures suggesting they could be partial barriers. Lack of evidence for major faults at the mountain front-basin interface suggests recharge to the basin is not impeded, whereas faults in the basin fill may compartmentalize the aquifer.