TECTONIC CONTROLS ON AN INTEGRATED MIDDLE TERTIARY DRAINAGE SYSTEM BELOW THE MOGOLLON RIM, CENTRAL ARIZONA

Evolution of middle Tertiary stream systems in central Arizona reflects dynamics of regional tectonism and documents Oligocene development of the Mogollon Rim. By 22 Ma an incised integrated drainage system flowed southeast in a strike valley 800 m below the Rim. Lithic conglomerates were preserved as thin braided stream deposits on the valley floor. Normal Basin and Range faults and basalt flows dated at 12-15Ma disrupted and eventually terminated the fluvial system.

Tectonic events controlled landscape evolution and stream development in the following manner. 1) Early Cenozoic Laramide uplift tilted Paleozoic and Mesozoic strata beveling the rocks down-section to the south; streams flowed northward from Laramide highlands onto the Colorado Plateau. 2) Uplift related to 25 -22 Ma core complex development incised NW-SE strike valleys; the resistant Kaibab cuesta established the early Mogollon Rim prior to 22 Ma and prevented subsequent stream flow onto the Plateau from the south. Deep, short, tributary paleovalleys carved into the Rim and filled with Paleozoic clasts. Southward, tributaries carved into uplifted crystalline rocks furnished Precambrian clasts to the fluvial system. Localized, ponded lacustrine deposition reflects downstream baselevel change. The fully integrated stream system flowed southeast into extensional basins. 3) The stream system was disrupted by middle Miocene normal faulting and volcanism related to waning stages of core complex development. The Verde Fault and related structures formed grabens that were filled with late Miocene and Pliocene sediments (Verde Formation) and volcanics. 4) Opening of the Gulf of California resulted in external drainage to sea level and canyon cutting and mesa development that mark the present landscape.