Abstract

The Transition Zone (TZ) of Arizona lies between the Colorado Plateau (CP) and Basin and Range (BR) Provinces, and shares some physiographic, stratigraphic, and structural aspects with each. The oldest rocks in all three provinces are Proterozoic metavolcanic and metasedimentary rocks, which record the evolution from oceanic to continental environments from >1.75 to 1.65 Ga. These rocks were strongly deformed, mostly between 1.7 and 1.6 Ga, and structurally are dominated by north- to northeast-trending folds, cleavage, high-strain zones, and shear zones. The rocks were intruded by Proterozoic granites before, during, and after deformation, and subsequently overlain in places by sedimentary and volcanic rocks of the mid- to late Proterozoic Grand Canyon Supergroup and Apache Group. All three provinces then received a similar, ~1 km-thick cover of platformal Paleozoic clastic and carbonate rocks.

With the onset of subduction beneath the southwestern edge of North America in the Triassic, the stratigraphic and structural histories of the three provinces began to diverge. Triassic and Jurassic rocks are not preserved in the TZ, but sequences of these rocks in the BR and CP have stratigraphic ties and some commonalities, indicating that the TZ originally contained similar Lower Mesozoic sequences and did not block sediment transfer between neighboring provinces at this time. In the Early to Middle Jurassic, for example, the TZ evidently contained the facies change between volcanic-dominated sequences of the BR and sediment-dominated ones of the CP. Rifting in the BR in the Late Jurassic and Early Cretaceous was accompanied by uplift of parts of the TZ and erosional removal of lower Mesozoic and upper Paleozoic strata. This uplift event is recorded by a pre-Late Cretaceous unconformity, where sedimentary rocks of the Cretaceous Interior Seaway were deposited on successively older rocks, from the Four Corners region southward to east-central Arizona. A similar uplift history, although not well documented, may have also affected the TZ of western Arizona, to explain Proterozoic clasts in Late Mesozoic conglomerates (McCoy Group) and the deposition of Late Cretaceous volcanic rocks directly on Proterozoic basement at Bagdad.

In the Late Cretaceous and early Tertiary, Laramide compression and magmatism affected all three provinces, but not equally. The BR was the most affected, being subjected to widespread intermediate to felsic magmatism, basement-involved thrusting, and associated folding and metamorphism. The TZ contains only scattered Laramide stocks and dikes, and the dominant Laramide structures are monoclines, which trend north-south, northwest, and east-west. Monoclines locally uplifted the TZ relative to the CP, such as along the north-facing Diamond Rim / Christopher Mountain monocline and the east-facing Canyon Creek monocline. During and after this uplift, large canyons were cut into the uplifted blocks, such as the Salt River paleocanyon and canyons in the western part of Grand Canyon. Gravels (Mogollon Rim Formation and correlatives) derived from the uplifted blocks were transported north and east down the canyons and deposited onto the topographically lower CP. A major drainage divide evidently existed within or southwest of the TZ, separating these north- and east-flowing drainages from