Grenville-Age, Polyphase Deformation of Mid-Proterozoic Basement, NW Van Horn Mountains, Trans-Pecos, Texas

Abstract

Psammitic, pelitic, and mafic schists in a basement horst in the NW Van Horn Mountains of west Texas preserve evidence of a multiphase, synmetamorphic, Grenville-age ductile deformation. Three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding phases have been identified. The second phase of deformation produced the dominant foliation and foliatipn-intersection lineation observed throughout the area. This deformation is inferred to have resulted from a regional-scale folding, possibly associated with nappe formation, at or near peak metamorphic conditions. Mineral assemblages indicate amphibolite facies, and Fe-Mg exchange thermometry provide maximum temperatures of corresponding to mid-amphibolite facies conditions. Three subsequent fold generations () formed under progressively lower metamorphic conditions; produced a weakly developed foliation ( ) whereas neither of the two later folding phases formed a penetrative fabric. Fluids or heat associated with the widespread intrusion of late syn- to post-orogenic pegmatites facilitated a static recrystallization event which altered previously formed deformational fabrics. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. The Grenville-age, progressive deformational history recorded in the basement rocks of the Van Horn area is similar to that noted in portions of the Llano Uplift of central Texas and in the Adirondack Mountains of New York and suggests that the Grenville orogeny was characterized by oblique convergence along the length of the North American craton.