THE ANCESTRAL ROCKY MOUNTAINS: TECTONIC INVERSION AT A GRAND SCALE

Many of the uplifts of the Ancestral Rocky Mountains are major examples of tectonic inversion. Geophysical data show that the scale and complexity of these structures is impressive. The Wichita-Amarillo uplift, Central basin platform, and Uncomphagre uplift are the largest of these features, and the structural relief across the northern margin of the Wichita uplift along is border with the Anadarko basin is about 15 km. This feature is one of the largest structures in North America. The structures extending across Oklahoma and the Texas panhandle have been referred to as the Southern Oklahoma or Wichita aulacogen. This set of structures extends from the early Paleozoic continental margin in northeasternmost Texas ands can be interpreted to extend along this trend as far northwest as the Uncomphagre uplift. The rifting that initiated the formation of these features formed as a result of a major continental breakup in latest Precambrian/earliest Paleozoic time. We can only infer the presence of rift structures from indirect geologic evidence. However, geophysical data document that the amount of modification of the crust is impressive. In fact, little if any of the original upper continental crust remains beneath the Wichita uplift and thrust faults extent at least to mid-crustal depths. The Central basin platform in the Permian basin is also underlain by a rift structure that was inverted. In this case, the rifting occurred at ~1.1 Ga during another period of widespread rifting in North America. The global significance of these features is emphasized by recent geological and geophysical studies that have allowed us to revisit a classic analogy that was drawn between the Southern Oklahoma aulacogen (SOA) and the Dniepr-Donets basin in the southern portion of Baltica. These studies reveal that the major differences between these two major failed rifts are the degree of inversion and the nature of the magmatic modification of the crust, which are both much larger in the SOA. The deformation that formed the Ancestral Rocky Mountains is a massive inversion of these rift structures and seems clearly due to plate collision in the late Paleozoic. However, the specific tectonic process that formed these structures is a major open question in our efforts to understand the tectonic evolution of North America.