THE MESOPROTEROZOIC OROGENIC FRONT IN ARIZONA AND A RE-EVALUATION OF THE SLATE CREEK SHEAR ZONE

Several widely separated Proterozoic metamorphic sequences across south-central Arizona are all cut by cryptic northwest-vergent thrust faults in a narrow band trending northeastward across the state from the Gila Bend Mountains to Payson, a distance of more than 230 km. New preliminary radiometric ages from the McDowell Mountains within this belt constrain the age of thrust faulting to between ca. 1526 and 1425 Ma. All of these sequences, and many of the thrusts, are intruded by conspicuous felsic and mafic sills found only within this narrow band and have obscured the presence of the thrusts themselves. This close association suggests a causal relationship between magmatism and thrust faulting, and it is here proposed that the widespread bimodal sills represent a previously unrecognized pulse of Mesoproterozoic magmatism that was spatially and temporally related to crustal contraction. The association of thrust faulting, magmatism, and metamorphism within this narrow band and within this age range define the location of the Picuris orogenic front in Arizona. The presence of weakly deformed granites in the same area that cluster between ca. 1660 and 1625 Ma indicate that the same fold-and-thrust belt was active during the Mazatzal orogeny. The nearby Slate Creek shear zone has been previously described as a crustal-scale suture between two blocks with different histories. New detailed mapping, however, shows that the stratigraphy of the Proterozoic Alder Group is not displaced significantly across the Slate Creek shear zone as previous workers had proposed. The SCSZ is, thus, not a boundary separating blocks with different histories. The available U-Pb ages of igneous rocks to the northwest of the orogenic front are all considerably older than those to the southeast. With this new insight, the orogenic front forms a much sharper boundary than does the SCSZ and forms a major crustal-scale suture across southwest Laurentia that was active during both the ca. 1.6 Ga Mazatzal orogeny and the ca. 1.4 Ga Picuris orogeny.