CRETACEOUS UNROOFING AND SUBSEQUENT LONG-TERM EROSIONAL STABILITY OF THE NORTHEASTERN ARIZONA TRANSITION ZONE: THE MOGOLLON AXIS

Apatite (U-Th)/He thermochronometry from a sample transect collected lengthwise along the Arizona Transition Zone (a province that lies between the SW margin of the Colorado Plateau and the NE margin of the Basin and Range province in Arizona) exhibits a range of Early Cretaceous to early Eocene cooling ages, with a dominant mid- to Late Cretaceous period of unroofing across the NE margin of the zone adjacent to the Mogollon Rim. Thermal modeling utilizing the new data along with pre-existing apatite fission-track data sets and geologic relationships suggest several key events in the thermal history of the zone, including (1) kilometer-scale denudation coincident with the development of the Mogollon highlands during the Sevier orogeny, but which was complete by mid-Cretaceous time; (2) more protracted unroofing lasting from Late Cretaceous through early Paleogene time toward the southwest; (3) a lack of significant aggradation or further denudation subsequent to Laramide and mid-Tertiary unroofing in the adjacent Colorado Plateau, and (4) late Paleogene denudation at the southwestern border of the zone, which appears similar to denudation patterns across much of the northwestern Mojave-Mogollon highlands. These thermal histories appear to define a stable, fulcrum-like “Mogollon axis” in the NE part of the Transition Zone, active from Late Cretaceous time to the present. This relatively narrow zone experienced only minor amounts of aggradation and denudation over the last c. 80-100 Ma, also recorded by unconformities at the bases of thin Cretaceous, Oligocene and Miocene deposits preserved along the Mogollon Rim. On either side of the axis, kilometer-scale deposition and denudation occurred, including erosion of the Mojave-Mogollon highlands, widespread deposition of thick Late Cretaceous/Paleogene sediments on the Colorado Plateau, mid-Tertiary erosion of the SW margin of the plateau, regional drainage reversal from NE to SW at some point in early to mid-Tertiary time, and massive denudation of the interior of the Colorado Plateau in Late Tertiary time.