Lithosperic Evolution of the North American Cordilleran Interior during the Cenozoic – Lessons from Arizona

Much of the area that makes up the state of Arizona was an elevated plateau during the first half of the Cenozoic, similar to the modern Altiplano - Puna region in the central Andes. Late Cretaceous (<73 Ma) to latest Oligocene (>25 Ma) arc related magmatic products (plutonic and volcanic) make up more than 70% of the exposed rocks in Arizona. Several high flux magmatic events are identified within this age interval: 70-64 Ma, 40-45 Ma, and 25-30 Ma, with more than 90% of magmatic volume having been produced within these short pulses. These events were similar to the younger flare-up event that produced the Altiplano-Puna Volcanic Complex in the Cordilleran interior of the central Andes and their geochemistry require the existence of the crust thicker than ~60 km until the beginning of the Miocene in Arizona. A major episode of crustal collapse and extension took place in early Miocene, and led to the unroofing of metamorphic core complexes and the initiation of the modern extension pattern. Vestiges of the earlier, thicker crust are preserved as lower crustal xenoliths (amphibolites, granulites, and eclogite facies rocks) found regionally in >25 Ma volcanics, whereas xenoliths found in younger <15 Ma volcanic rock reflect the transition to the thin modern crust.