BASEMENT ACCRETIONARY PROCESSES - PRECURSOR TO THE TECTONIC REACTIVATION HISTORY OF THE ROCKIES AND ADJOINING REGIONS

A hypothesis explaining the tectonic patterns for the Rocky Mountain region inductively combines the accumulated research on the Precambrian history of accretion and the known reactivation history of Phanerozoic structures. The basement history of the region is one of southward accretionary growth of North America during the 1.8 to 1.6 Ga interval (the Transcontinental Proterozoic province). Available data on rock type and age allows the interpretation that several families of island arcs accreted to the Hudsonian continent and emplaced a series of sutures and boundary zones that controlled later tectonic events. After the juncture of the Wyoming and Superior Archean provinces at about 1.82 Ga, a continuing process of accretion, with several families of arcs, expanded North America southward. Accreting from the southwest were the consecutive series of terranes; the Mojave at 1.75 Ga, the Yavapai at 1.70 Ga, and the Mazatzal at 1.65 Ga. Accreting from the south were the Dawes at 1.78 Ga, the Frontier at 1.71 Ga, the Hitchcock at 1.67, and the Kansas at 1.61 Ga. Accreting from the southeast were successors to the Penokean terrane (1.83 Ga), the Southern Iowa at 1.76 Ga, the Northern Missouri, and the Central Missouri terranes. Each period of island arc accretion created a fundamental suture in the basement architecture of this portion of North America. Phanerozoic structures reactivated along these sutures include the Uncompahgre uplift, the Zuni uplift, the Cambridge arch, the North Platte arch the Central Kansas uplift and many more current structures. Each accretionary period also reactivated existing basement tectonic trends. Two strong N-S tectonic trends appear anomalous across the region; the Rocky Mountains and the Nemaha Uplift and are suggested to represent the boundary zones created by antagonism between the families of island arcs. The repetitive rejuvenation of the Rockies boundary zone is evidenced by the trend of the Ancestral Rockies, the later Laramide orogeny, and the Rio Grande Rift. Rejuvenation of the Nemaha boundary zone is evidenced by the 1.1 Ga emplacement of the Midcontinent Rift System and the series of Phanerozoic structures that together make up the Nemaha Uplift. The basement sutures have reactivated even though the interpreted stress fields are transverse to the resulting structures.