ABSTRACT

The San Marcos arch area of south-central Texas contains great belts of normal faults that strike northeastward and are the expression of superficial crustal extension in the subsiding Gulf coastal plain. The upper Rio Grande embayment is characterized by gentle southeast-plunging folds that belong to the Sierra Madre Oriental orogenic province of northeastern Mexico and are the expression of superficial shortening of the crust. The two kinds of deformation were contemporaneous in Late Cretaceous and Early Cenozoic time. The rocks in the belt between the homogeneous folding of the Rio Grande embayment and the homogeneous faulting of the San Marcos arch display a heterogeneity of structure that is expressive of the transition of the strain field from a compressional to an extensional character. Ultramafic intrusives accompanied the normal faulting, and basaltic intrusives and low-grade metamorphism accompanied the folding.