ABSTRACT The McDowell Mountains of central Arizona contain one of the best preserved and thickest sections of pre–Apache Group Mesoproterozoic rocks in the state. The oldest formation, ash-flow tuff, has an age of ca. 1650 Ma and is overlain by a quartzite-shale-quartzite triplet. These two units are interpreted to correlate with the Red Rock Group and Mazatzal Group, respectively. Significantly, these formations are overlain by ~6 km of ash-flow tuffs and minor psammite and metabasalt. Preliminary U-Pb analysis of igneous zircons in the youngest ash-flow tuff (the Taliesin tuff) provides an age of 1546 ± 11 Ma. The oldest granite pluton, the Antiguo granite, intrudes the Taliesin tuff and has a U-Pb age of 1525.6 ± 9.5 Ma. If these preliminary results are accurate, they record volcanism, burial and metamorphism, and plutonism that occurred within the Proterozoic ‘magmatic gap’ (between ca. 1.6 and 1.45 Ga) and may be the only voluminous igneous rocks within this age range yet recognized in North America. Additionally, the stratigraphic section was deformed into km-scale folds during an episode of contraction that formed two northwest-vergent thrust faults in the northern part of the mountain range. Both thrusts were subsequently intruded by mafic hypabyssal sills and then buried to greenschist-facies depths and intruded by the coarse-grained Carefree granite at 1425 Ma.

ABSTRACT Earth fissures are tensile surface cracks exposed at Earth’s surface. In Arizona, such fissures are predominantly found in the central and southeastern regions of the state, where they form in response to subsidence driven by groundwater pumping. Growth and erosion of these fissures often occurs during large monsoon storms, resulting in slumping and collapse of the fissure walls, propagation of the fissure head, as well as the development of gully networks out from the main fissure stem. Fissure initiation and propagation threaten existing infrastructure, can cause property damage, and increase the potential for groundwater contamination from surface pollutants. It is exceedingly important that these hazards be well understood, documented, and monitored. The Arizona Geological Survey (AZGS) founded the earth fissure program in 2007 to systematically identify, map, and monitor earth fissures in Arizona. Data are released through an interactive viewer (https://uagis.maps.arcgis.com/apps/webappviewer/index.html?id = 98729f76e4644f1093d1c2cd6dabb584), which is regularly updated to show new fissures and growth of existing ones. Additionally, beginning in November 2018, repeated surveys of a series of large earth fissures in Apache Junction, Arizona, (50 km east of Phoenix) have been done using UAV-SfM (unmanned aerial vehicle–structure from motion) to better elucidate the processes controlling the short-term evolution of this geologic hazard. This field trip will take us to two fissure locations in the greater Phoenix metropolitan area. The first will be the Apache Junction earth fissure area, where we will be able to observe the large, dramatic scale of these features, as well as highlight the important role large monsoon storms have on fissure propagation and geomorphological changes. Furthermore, we will show how high-resolution topographic surveys provide a means for significant improvements to current mapping and monitoring efforts for assessing hazards related to earth fissures. The second site will be a fissure location just to the southwest of Apache Junction in Chandler Heights, Arizona, which we refer to as the “Queen Creek” earth fissure area. At this field-trip stop, we will show how fissure initiation and growth threaten human development, as well as describe the role the AZGS earth fissure program plays in identifying and monitoring these hazards.

ABSTRACT A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations strongly suggest a continental arc residue (“arclogite”), rather than a lower-plate subduction (“eclogite”), origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.

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.