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 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.

Abstract Extreme flooding in Arizona during the winter of 1993 resulted from a nearly optimal combination of flood-enhancing factors involving hydroclimatology, hydro-meteorology, and physiography. The floods of January and February 1993 were the result of record precipitation from the passage of an unusually high number of winter storm fronts. These fronts moved across Arizona as part of an exceptionally active storm track that was located unusually far south. The number of individual storms that entered the region and the relative position of each storm track in relation to previous storms was reflected in a complex spatial and temporal distribution of flood peaks. An analysis of the hydroclimatic context of these floods supports a general conclusion that in Arizona, front-generated winter precipitation is most often the cause of extreme floods in large watersheds, even in basins that tend to experience their greatest frequency of flooding from other types of storms. A comparison of the 1993 floods with gauged, historical, and paleoflood data from Arizona indicates that, although many individual flood peaks were quite large, they were within the range of documented extreme flooding over the past 1,000+ yr. The 1993 flood scenario provides a convincing analogue for the climatic and hydrologic processes that must have operated to generate comparably large paleofloods, that is, abnormally high rainfall totals, repeated accumulation and melting of snow, and rain on snow. Such conditions are initiated and perpetuated by a persistent winter circulation anomaly in the North Pacific Ocean that repeatedly steers alternately warm and cold storms into the region along a southerly displaced storm track. This scenario is enhanced by an active subtropical jet stream, common during El Nino-Southern Oscillation periods.

Abstract Flooding occurred throughout Arizona in January 1993 as a result of record pre-cipitation, early snow melt, and saturated soil conditions. The high flows in the Gila River and its tributaries caused failure, damage, or exposure of many natural gas pipelines crossing rivers, streams, and washes. This chapter presents a case study of erosion analysis for six El Paso Natural Gas Company (EPNG) pipelines that failed or were exposed in Arizona during January 1993. The failures were critical because these were major transmission pipelines that supplied natural gas to residential and industrial users in whole communities and groups of communities. The scour evalua-tions conducted were significant because they provided a scientific and engineering basis for emergency pipeline replacement or repair. Detailed scour and sediment transport studies were conducted to compute the design scour depth for pipeline replacement. The studies incorporated a hydrological analysis for the 100-yr design flood, surveys of channel geometry for hydraulic com-putations, and geotechnical analyses of sediment samples for grain-size distribution in the streambed. The scour part of the studies considered not only vertical scour but also lateral scour due to bank erosion, channel meander migration, and channel braiding. The scour studies were used in the design, construction, and permitting process to expedite replacement of the pipelines. For many of these pipelines, it was the first time that the engineering methods of river hydraulics and sediment transport and the science of river morphology were used in the pipeline crossing design.

Abundant pumice fragments occur in the Apache Leap Tuff of east-central Arizona, an ash-flow sheet with a maximum thickness of 600 m and a K-Ar age of 20 m.y. The amount of flattening of pumice fragments is widely variable at any particular locality, but systematic measurements show that the mean degree of flattening, defined as the “flattening ratio,” steadily increases from the top downward into the body of the sheet. Ultimately the fragments are so compacted that they lose their identity. On a logarithmic scale the plot of flattening ratios is approximately linear relative to depth of burial. The uniform downward increase in flattening combines with evidence obtained from zoning and specific gravity characteristics to show that most of the deposit is a single cooling unit. Because of the uniform trend, flattening also provides a guide to the original thickness of overlying tuff at localities at which fragments can be measured. This permits the development of stratigraphy for the seemingly uniform deposit and provides a means to estimate pre-erosion thickness of the ash-flow sheet and the amount of stratigraphic throw on faults. A mining company used flattening ratios to predict successfully the ash-flow thickness cut by a new shaft. Postemplacement crystallization and diagenetic processes have greatly reduced the initial porosity of the deposit, and present porosity values erroneously indicate a considerably higher degree of welding than is inferred from deformation of the pumice fragments. It seems that in deposits where crystallization and diagenesis have been significant, flattening ratios of pumice fragments may be a better guide than porosity to the degree of welding that occurred during cooling of the deposit. The change of flattening ratio with depth can also serve as an approximate guide to the relative viscosity of pumice during emplacement. Viscosity is determined chiefly by temperature, chemical composition, volatile content, and crystallinity. The downward change in flattening ratio in the Apache Leap Tuff is gradual, indicating a relatively high viscosity. By assuming high volatile content and low groundmass crystallinity at the time of emplacement, the high viscosity can be ascribed to the combined result of nonperalkalic chemical composition and relatively low temperature.