ROLE OF FAULTS IN CONTROLLING HYDROTHERMAL FLUID FLOW, SALINITY, AND HELIUM TRANSPORT ALONG THE RIO GRANDE RIFT

The Albuquerque Basin is the oldest and largest basin associated with the Cenozoic aged Rio Grande Rift (RGR). The southern terminus of this basin is characterized by high salinity and mantle helium anomalies, which are probable indications of complex mixing of mantle and crustal fluids. The role of fault controlled fluid flow in explaining observed fluxes of salinity and helium is examined in this study using mathematical modeling.

A series of E-W and N-S basin-scale, cross-sectional hydrologic models were constructed along the RGR to a depth of 15 km to incorporate deeply derived inputs. New and existing groundwater salinity, temperature, ³He/⁴He, and ¹⁴C data provide the ground truth for model calibration and sensitivity analysis. A hypothetical E-W model extending across the Albuquerque Basin was constructed with deeply penetrating sub-vertical faults bounding the RGR. Geologic maps, well bore lithologic logs, as well as gravity and seismic-surveys were used to construct the general cross-section on which the N-S model is based. This model follows the same general trend as the Rio Grande through the Albuquerque Basin and into the Socorro Basin. Multiple versions of the N-S oriented model were created based on several working hypothesis to better understand the structural and hydrologic controls at the basin boundary. One model assumes that the Tertiary dike exposed at the boundary acts as a conduit for deeply sourced fluids and primordial ³He related to the Socorro Magma Body. An alternate version assumes all the units down to the Precambrian basement rock decrease in depth significantly at the basin boundary due to the constriction of the Rio Grande Rift.

The model results illustrate the importance of deeply penetrating, moderately permeable fault zones (10^-12 to 10^-15 m²) in advective transport of groundwater, primordial ³He and mantle volatiles through the ductile boundary to shallow crustal levels. Both regional and local flow systems are evident in the model and likely account for the salinity increase in the Rio Grande. High chloride concentrations from the mantle are mixing with deep sedimentary basin brines derived from evaporites within the Permian Yeso Formation. The simulated ³He/⁴He ratios at the surface conduit exposures are within the published values measured at the basin boundary and within the RGR.