ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT New detrital zircon data from deformed metasedimentary rocks of the Mazatzal Group in the northern Mazatzal Mountains, Arizona, indicate that formation of a regional fold-and-thrust belt occurred after ca. 1570 Ma. Regional correlations with pelites within the syncline at Four Peaks and deformed and intruded sediments in the upper Salt River Canyon allow us to revise the timing of deformation to ca. 1470–1444 Ma, contemporaneous with the Picuris orogeny in New Mexico. Fold- and thrust-style deformation of the Mazatzal Group was previously interpreted to be Paleoproterozoic and was a hallmark of the ca. 1650 Ma Mazatzal orogeny in the southwestern United States. However, recognition that protoliths of the deformed rocks formed in the Mesoproterozoic requires reconsideration of the age and regional tectonic significance of the orogenic event in its type locality. Our new findings are incompatible with published tectonic models invoking a regional ca. 1650 Ma Mazatzal orogeny and localized, pluton-enhanced deformation across the region ca. 1450 Ma. This field trip visits and reviews three localities across the Tonto Basin of central Arizona: (1) the northern Mazatzal Mountains; (2) Four Peaks of the southern Mazatzal Mountains; and (3) exposures of the early Mesoproterozoic Yankee Joe Group in the upper Salt River Canyon. At each location, deformation previously attributed to ca. 1650 Ma is, instead, demonstrably younger and represents a different episode of regional orogenesis. Thus, the nomenclature and tectonic significance of ca. 1650 Ma versus 1450 Ma regional orogenic events must be reconsidered and revised to reflect our present data and understanding, with implications for the tectonic evolution of Proterozoic rocks of southwestern North America.