Volcanism and extensional tectonism spread across a broad region of the western U.S. during the Cenozoic. Studies in recent years have provided much insight into the relationship between the tectonism and magmatism in the western U.S., but unsolved questions relating to the driving forces for extension remain. The Cenozoic volcanic rocks of the High Plains provide a unique perspective on such problems. These volcanic rocks are broadly similar in composition and closely contemporaneous with volcanic fields in the Rio Grande Rift and southwestward into Arizona. However, the High Plains volcanic fields lie 100-200 km east of the Rio Grande Rift in an area well beyond the effects of crustal extension. Spatial and temporal variations in composition of volcanic rocks shown from our preliminary study (e.g., isotopic changes) require significant differences in source composition and crust and mantle structure. A detailed study of this volcanic field will enable comparison with fields within the region of extension and provide essential constraints for physical models explaining relationship between magmatism and extensional tectonism.
