LARAMIDE CRUSTAL BULLDOZING BENEATH THE WESTERN ARIZONA TRANSITION ZONE

The northwest-trending Transition Zone (TZ) in Arizona is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyper-extended Basin and Range Province to the southwest. While viewed as a Late Cretaceous – Paleogene (Laramide) highland or northeast-tilted erosional slope, it is unclear if, when, or how the TZ crust might have been thickened during Laramide orogenesis. We conducted a multi-method thermochronologic investigation of Proterozoic rocks in and adjacent to the Bradshaw Mountains of the west-central Arizona TZ. Disturbed ⁴⁰Ar/³⁹Ar biotite and K-feldspar age spectra suggest that rocks in the southern TZ were at temperatures >250-300 °C prior to Cretaceous – Paleogene time. This contrasts with the northern TZ where ⁴⁰Ar/³⁹Ar biotite and K-feldspar dates are Proterozoic and the nearly flat-lying Proterozoic – Paleozoic (Great) unconformity is exposed. Completely reset zircon (U-Th-Sm)/He dates from Proterozoic rocks in the Bradshaw Mountains region range from 66 to 50 Ma. Apatite fission-track dates from the same suite of Proterozoic rocks range from 61 to 53 Ma, while those from a modern sand sample yield a unimodal age distribution with a central value of ~56 Ma. Multi-method thermochronology on individual samples and inverse thermal models suggest rapid (8-10 °C/Ma) cooling between ~65 and 50 Ma. Given the absence of any documented 65-50 Ma upper-crustal deformation in the region, we attribute TZ exhumation to northeastward bulldozing of lower crust beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (65-50 Ma) underplating and initial exhumation of the Orocopia schists to the southwest and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ~25 Ma TZ volcanic rocks.