The area of study is a metroplex of nearly 2 million people astride the international border and in the center of the Rio Grande Rift. Previously recognized Quaternary faulting consists of the quantitatively studied Eastern Boundary Fault (EBF) of the Franklin Mountains (FM); the Perimental Fault at the eastern boundary of the Sierra de Juarez (SJ); small graben faults in bolson fill east of FM; and the poorly known Mesilla Valley Fault (MVF) west of FM.

Prior mapping in bedrock areas identified a number of transverse faults. The present study, using Indian 5m resolution satellite imagery and field work, expanded this investigation, and identified significant additional faults. In the FM, some visible features may be fractures.

Study of air photos and satellite imagery of the pre 600 ka bolson fill incised by the Rio Grande between the mountain ranges reveals many curvilinear features. In some places, splays occur where faults emerge from the bedrock masses.

Independent data from multiple sites along curvilinears consist of well data, cross sections, historic maps and photographs, engineering data from construction sites, geomorphic relationships, and slickensides. The curvilinears are interpreted to be small faults, and this is a new family. They illustrate usually normal and/or right-lateral components. These faults may occur from a response to the differential uplift of the resident crustal block, which resulted in North Franklin Peak being the highest structural point in Texas. These faults have interacted in a complex manner with the previously identified EBF, WBF, and MVF. Summed lengths are over 200 km.