Structural domains and their potential impact on recharge to intermontane-basin aquifers

Variations in the structures exposed in structural domains associated with faults can affect the amount of ground-water recharge to fractured rock aqu ifers supplying intermontane-basin aquifers, In the south-western Franklin Mountains, Fl Paso, Texas, an outcrop of the Ordovician Scenic Drive Format ion exposes a group of structures including faults, joints, and folds assoc iated with a left-lateral fault striking N74 degrees E and dipping 81 degre es NW, Slip along this fault has produced a structural domain with extensio nal structures (normal faults and joints) and a domain with contractional s tructures (folds and joints). These extensional and contractional domains o ccur on opposite sides of the fault. Properties of fractures within the different structural domains can influen ce ground-water recharge and migration, For example, the domain with extens ional structures contains nearly vertical joints that extend from the soil- bedrock interface to normal faults and along which precipitation can infilt rate. Since the left-lateral fault is approximately normal to the trend of the Franklin Mountains, this fault may be a conduit for ground-water flow f rom the mountains to the basin, The domain with contractional structures is limited in extent, and the fractures in this domain are poorly connected. Therefore, surface infiltration in this domain is reduced, and may not cont ribute as much to ground-water migration. Structural domains that are analo gous to this outcrop and cover larger areas can be found throughout the Fra nklin Mountains. Understanding the variations between structural domains ca n aid in siting water-supply wells and determining contaminant transport in fractured rock.