Documenting Fracture Networks in Proterozoic Granite, Arroyo Del Tajo, New Mexico

Abstract

Recent model-based studies of two geothermal systems in New Mexico (Socorro and Truth or Consequences) suggest that the measured temperatures and chemical compositions of the fluids in these systems are best explained by deep (4–8 km) circulation of groundwater within permeable (10 -14 to 10 -12 m 2 ) fractured Proterozoic basement. A detailed investigation of joint density and connectivity in the Proterozoic Tajo Granite in the Quebradas Backcountry Byway area east of Socorro was initiated to determine if circulation of fluids in fractured Proterozoic crystalline rocks is an important geothermal process. The southernmost outcrop, which is incised by the southern tributary of Arroyo del Tajo, is the focus of this presentation. The Great Unconformity, which separates the granite from the Pennsylvanian Sandia Formation, forms the eastern boundary of the outcrop and the Quaternary Coyote fault forms the western boundary. Fractures in this 1 km (N-S) by 0.35 km (E-W) granite outcrop that are visible on Google Earth images were evaluated. In addition, 16 areas within the exposure near some of the larger fractures were examined in detail. North-northwest striking joints are common 40 to 70 m to the east of the Coyote fault and many of these fractures are filled with fluorite, barite, and quartz. Fracture density is 22 to 27/m within 4 m of the fault zone, dropping to 12/m at a distance of 18 m west of the fault zone. Most of these fractures near the fault zone are short, only 2.6 to 10.8 m long. Connectivity, which is based on the number of systematic joint intersections (nodes) in a square meter, ranges from 42 to 23/m 2 in this same area. In contrast to the relatively short fractures near the fault zone, several E-striking fractures that can be traced 60 to 270 m are common in the southern 0.27 km of the exposure. The spacing between these long, continuous fractures along the southern edge of the exposure ranges from 5.5 to 18.7 m. Northeasterly strikes are more common in the northern half of the exposure and northwesterly strikes are found throughout the outcrop. A NE-striking (65–75°) zone located approximately 0.15 km south of the northern tip of the exposure appears to cut across NW-striking (330–340°) fractures on the Google Earth images; the zone is 40–50 m wide. Field data support this observation; NE-striking iron oxide, calcite, and quartz veins cut the NW-striking fractures and cm-scale offsets across the NW-striking fractures were noted in this area. Iron-rich fluids clearly flowed through fractures in the Tajo Granite throughout the exposure, suggesting that fluids do move through fractured crystalline basement. The spatially variable fracture patterns in this large exposure of granite are related to the fact that the this fault block is rotated to the east above an east-vergent reverse fault along the eastern margin of the Laramide Sierra uplift (perhaps opening the E-striking structures) that was subsequently cut by Rio Grande rift extensional faults.