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 The Laramide continental arc formed in southwestern North America at about the same time the Sierra Nevadan arc was shutting down, and the Laramide arc was active concurrent with the progress of the Laramide orogeny, from ca. 80 Ma to ca. 45 Ma. East-central Arizona offers an excellent opportunity to explore aspects of tectonics, structural geology, magmatism, and hydrothermal systems in a segment of the Laramide arc that is exceptionally well endowed with porphyry copper deposits. The structure of this region is especially complicated, with multiple generations of normal faults commonly superimposed on originally moderate-angle reverse faults with associated fault-propagation folds. A large new porphyry copper deposit, Resolution, was discovered near Superior in the mid-1990s. The discovery started a new round of development in the mining life cycle at the Resolution deposit; in the region, it contributed to copper exploration again becoming vigorous in the last decade. In the years since discovery of Resolution, important new scientific insights have been gained, including at the regional scale. Post-ore crustal extension exposed multiple levels of Laramide and older igneous and hydrothermal systems at the surface where they can be more easily mapped and sampled, and palinspastic reconstructions of post-mineral normal faulting permit the exposures to be restored to their original positions. The porphyry-related products that are observed at higher levels include local advanced argillic alteration and Cordilleran-style veins and associated mantos, such as at the Magma mine, Resolution deposit, and Old Dominion mine in the shallowest levels of the Superior-Globe-Miami area. Most porphyry copper ore bodies were developed at intermediate depths, where porphyry intrusions exhibit sericitic and potassic alteration and carbonate rocks were converted to skarn, such as in the heart of the Miami-Inspiration, Resolution, Ray, and Christmas deposits. Plutonic rocks are exposed at deeper paleodepths, where pegmatites, quartz veins, and greisen muscovite are locally observed, especially directly beneath porphyry copper orebodies, as in the Schultze and Granite Mountain plutons. Likewise, sodic-calcic alteration may be developed on the deep flanks of porphyry systems, such as adjacent to the Tea Cup pluton. Subsequent Cenozoic extension variously buried or exhumed the hypogene portions of these hydrothermal systems, leading to the development of various supergene products, both in situ and exotic.

ABSTRACT This field trip integrates economic geology with structural geology and tectonics, as well as petrology, geochemistry, and regional geology, to examine a segment of the Laramide arc that includes part of the Laramide porphyry copper province of southwestern North America. The province arguably is the second-largest porphyry copper province in the world, hosting six of the world’s 25 largest porphyry deposits on the basis of contained copper metal. The Globe–Superior–Ray–San Manuel area includes about a dozen Laramide (Late Cretaceous to early Paleocene) porphyry copper deposits and the related granodioritic to granitic plutons. These plutons and their wall rocks were tectonically dismembered and variably easterly or westerly tilted (locally >90°) during Laramide contraction and subsequent mid-Cenozoic extension. The style of both shortening and extension here remains a subject of debate. Although this trip includes one brief mine visit and examination of drill core at the Resolution deposit, it will principally focus on: (1) different parts of various plutons and the associated alteration aureoles, including review of resultant mineralization, and the original sides, roots, and deep flanks of the hydrothermal systems; and (2) structure in the adjacent wall rocks and the implications for the style and timing of deformation in absolute and relative terms to hypogene ore formation. An increased understanding of the structural geology and the alteration-mineralization zonation of the dismembered hydrothermal aureoles allows an integrated view of the original geometry and size of the porphyry systems, the relationship between porphyry copper mineralization and crustal shortening, and possible origins of deep hydrothermal alteration.

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 We describe the time-space evolution of a segment of the Laramide arc in east-central Arizona that is associated with porphyry copper mineralization, as constrained by U-Pb zircon geochronology conducted by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. Mid-Cenozoic normal faulting dismembered and tilted many of the plutons and the associated porphyry copper deposits and produced a wide range in depths of exposure. The study area reconstructs to a 75-km-long slice along the arc, with exposures from <1 to >10 km depth. The copper deposits are related to granodioritic to granitic plutons that exhibit variable magmatic sources and locally severe degrees of zircon inheritance. U-Pb zircon ages of plutons in the study area range from 75 to 61 Ma, with dioritic rocks at the older end of the range. The age range of magmatism and mineralization in a cluster of deposits near the Schultze Granite, including the Globe-Miami, Pinto Valley, and Resolution deposits, is from ca. 69–61 Ma. To the south in the Tortilla and Dripping Spring Mountains, the porphyry systems range from ca. 74 Ma at Kelvin-Riverside to ca. 69 Ma at Ray and ca. 65 Ma at Christmas. At several localities where geologic constraints exist, mineralizing plutons were emplaced following Laramide shortening. The ages of the inherited zircon cores correspond fairly closely to the ages of basement rocks in the immediate vicinity of sample sites, implying that similar basement ages and lithologies contributed to the source areas of magmas that produced Laramide porphyry deposits. The U-Pb results on hypabyssal rocks are typically 1–5 m.y. older than previous K-Ar ages, and U-Pb ages on more deeply emplaced plutonic rocks are as much as 5–10 m.y. older. These results are consistent with predictions from thermal modeling and suggest that temporal evolution of the entire Laramide arc needs revision. For this segment of the arc, magmatism was stagnant for ~15 m.y., with minimal migration over time and mineralization occurring episodically over most of that lifespan. There is no simple geographic progression in ages along or across the strike of the arc. Thus, it is difficult to call upon time-specific far-field or plate margin triggers for magmatism or mineralization. The intrusive flux of the Laramide arc appears to be similar to that of the Sierra Nevada arc during the Mesozoic during its “background” periods, rather than during episodes of flare-up. The wide compositional diversity of the Laramide arc is more akin to northeastern Nevada during the onset of extension in the mid-Cenozoic than to the Mesozoic of the Sierra Nevada.