A structural analysis in the southwest Edwards Plateau area included four 15-minute quadrangles in Edwards, Kinney, and Val Verde Counties, Texas. Seven rock units of the Cretaceous Comanche Series and two units of the Cretaceous Gulf Series crop out. Subsurface Paleozoic foreland facies rocks of the southern Val Verde basin are present in the northern study area; the Devils River uplift, partly blanketed by allochthonous upper Paleozoic metamorphic rocks, underlies the southern part. A pronounced east-west-trending fault zone which cuts through the center of the study area is at least 55 mi (88 km) long and 1 to 2 mi (1.6 to 3.2 km) wide. This trend, named the Carta Valley fault zone, is characterized by a complex series of en echelon, northeast-oriented faults which are cross-cut by a lesser number of northwest-trending faults. These faults bound a series of grabens, downthrown by 200 to 300 ft (60 to 91 m), which are as large as 3 sq mi (7.7 sq km). Nine gently dipping, northwest-plunging, en echelon anticlines parallel the south side of the fault zone. The configuration

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of faults and folds strongly suggests that the surface fault zone formed by left-lateral wrench displacement in the basement rocks. Subsurface data show that the fault zone overlies a major late Paleozoic high-angle fault, downthrown to the north by over 10,000 ft (3,050 m), which marks the Devils River uplift-Val Verde basin boundary. The surface fault zone may thus be a later reflection of more pronounced wrench faulting along this basement fault during the Ouachita orogeny.

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