USING MATHEMATICAL MODELING AND HEAT FLOW DATA TO ASSESS THE HYDROGEOLOGIC FRAMEWORK OF THE LA JENCIA AND SOCORRO SUB-BASINS OF THE RIO GRANDE RIFT, NEW MEXICO

Mathematical modeling and heat flow data are used in this study to assess the hydrogeologic framework of the Rio Grande Rift near Socorro, New Mexico. Our analysis focuses on both active and ancient groundwater flow patterns and hot spring activity within the southern La Jencia and Socorro sub-basins. These two hydrologically connected basins have undergone rift-related tectonic extension, largely by rotational movement along two basin-bounding faults, during the last 28 million years. We solve governing fluid flow and heat transfer equations using a finite element model that employs special four-node slip-elements to accommodate tectonic block motion along fault surfaces.

A suite of numerical experiments are presented which helps to resolve the hydrologic properties of the sedimentary pile, pre-Cambrian crystalline rocks, and fault zones. Present-day and paleo-heat flow data are used as the primary means of model testing. The best agreement between computed and observed heat flow data was achieved by representing faults as passive surfaces and incorporating about 3 km of moderately permeable (10-14 m2) fractured crystalline rocks into the hydrogeologic framework model. About 5% of mountain front recharge penetrates to a depth of 2.8 km below the sedimentary pile. We conclude that present-day convective thermal anomalies associated with Socorro hot springs result from inter-basin transfers of groundwater which discharge through a 'hydrologic window' in a regionally extensive confining unit (playa) created by domino-style fault block rotation.

To further test our hydrogeologic framework model, the "best fit" values of rock permeability and basement heatflow (77 mW/m2) were used to reconstruct the paleohydrology of the rift over the past 28 My. Paleohydrologic model results suggests that fault block motion shifted the regional discharge area by 15km as the hydrologic window in the regional confining unit opened. Calculated paleo groundwater flow patterns help to explain the timing of formation of the Gonzales barite prospect, potassium metasomatism of the Hells Mesa Tuff, and annealing of apatite fission tracks in Eocene Baca Formation clasts on the eastern edge of the Rio Grande Rift.