GEOPHYSICAL STUDIES OF STRUCTURAL CONTROLS INFLUENCING GROUND AND SURFACE WATER FLOW WITHIN THE SOUTHEASTERN LOWER MESILLA VALLEY, NEW MEXICO AND TEXAS

We have conducted a series of geophysical studies to better define the structural controls on the southeastern Lower Mesilla Valley where the Rio Grande begins to curve to the southeast before exiting the valley between Mt. Cristo Rey and the Franklin Mountains through the “Paso del Norte”. A combination of gravity, magnetics, conductivity, resistivity and seismic data were collected to map local and regional structural features. A unique, dike-like (~30 m long) andesite outcrop acts as a local barrier on the western edge of the Rio Grande that deflects the flow of the river towards the east. This outcrop appears to be part of a more extensive buried intrusion striking east-northeast that implies Mesozoic structural control. The intrusion is on the southern edge of the Lower Mesilla Valley and northern edge of the Sunland Paleo Valley, a separate small basin only about 70 m deep. In addition, regional gravity surveys suggest a north-northwest trending structure separates the aquifer within the Sunland Paleo Valley into two compartments with TDS values of > 10,000 mg/L to the east, and < 5,000 mg/L to the west. Other differences in groundwater geochemistry are consistent with the structural controls suggested by the geophysical data.