The numerous porphyry copper deposits in southeastern Arizona and adjacent areas in New Mexico and Sonora are mostly within the Basin and Range tectonic province where they have been dispersed by middle to late Cenozoic tectonic extension. Reconstruction of this extension, based largely on displacements on low-angle normal faults associated with metamorphic core complexes, restores these deposits to their approximate positions at the end of Laramide orogenesis (similar to 50 Ma). This restoration places the 39 largest known deposits south and east of Phoenix, Arizona, in five linear belts, four of which trend easterly to northeasterly at high angles to the Laramide continental margin. The east to northeast trends of these four belts are interpreted as a reflection of elevated copper fertility in linear zones in the deep crust and/or upper mantle that parallel the tectonic fabric of the Paleoproterozoic Yavapai - Mazatzal orogenic belt. This geometry is consistent with proposals for copper enrichment during Paleoproterozoic subduction of oceanic lithosphere and overlying strata that were altered under reducing conditions characteristic of Paleoproterozoic seawater. Laramide mobilization of copper from these enrichment zones was associated with subduction of oceanic lithosphere and overlying strata that were altered under oxidizing conditions characteristic of late Phanerozoic seawater.

The numerous porphyry copper deposits in the Sonoran Desert region of southwestern North America are mostly within the Basin and Range tectonic province where they have been displaced and dispersed by middle to late Cenozoic tectonic extension. Reconstruction of this extension, based largely on evaluation of displacements on low-angle normal faults associated with metamorphic core complexes, restores these deposits to their approximate positions at the end of Laramide orogenesis (similar to 50 Ma). This restoration places the 39 largest known deposits in five linear belts, four of which trend easterly to northeasterly at high angles to the Laramide continental margin. The east to northeast trends of these four belts are interpreted to reflect elevated copper and molybdenum fertility in linear zones in the deep crust and/or upper mantle that parallel the tectonic fabric of the Paleoproterozoic Yavapai-Mazatzal orogenic belt.

Detrital zircon grains in the ∼1740–1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470–2490 Ma. These zircon grains are distributed over ∼40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ∼2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400–3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730–1900 Ma zircons. From this relationship we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ∼2450–2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.

Oligocene and early Miocene displacement on the Catalina–San Pedro detachment fault and its northern correlatives uncovered mylonitic fabrics that form the greater Catalina metamorphic core complex in southeastern Arizona, USA. Gently to moderately dipping mylonitic foliations in the complex are strongly lineated, with a lineation-azimuth average of 064–244° and dominantly top-southwest shear sense over the entire 115-km-long mylonite belt. Reconstruction of detachment fault displacement based on a variety of features indicates 40–60 km of displacement, with greater displacement in more southern areas. Widespread 26–28 Ma volcanism during early extensional basin genesis was followed by 24–26 Ma granitoid magmatism. Cooling of footwall mylonites continued until 22–24 Ma, as indicated by 40Ar/39Ar mica dates. Lower temperature thermochronometers suggest that footwall exhumation was still underway at ca. 20 Ma. Tectonic reconstruction places a variety of unmetamorphosed supracrustal units in the Tucson and Silver Bell Mountains above equivalent units that were metamorphosed and penetratively deformed in the Tortolita and Santa Catalina Mountains. This restored juxtaposition is interpreted as a consequence of older Laramide thrust burial of the deformed units, with northeast-directed thrusting occurring along the Wildhorse Mountain thrust in the Rincon Mountains and related but largely concealed thrusts to the northwest. Effective extensional exhumation of lower plate rocks resulted from a general lack of internal extension of the upper plate wedge. This is attributed to a stable sliding regime during the entire period of extension, with metamorphic core complex inflation by deep crustal flow leading to maintenance of wedge surface slope and detachment fault dip that favored stable sliding rather than internal wedge extension.

In 2019, the Anthropocene Working Group proposed the creation of an Anthropocene chronostratigraphic time unit to follow the Holocene Epoch. The Anthropocene time period would begin in the mid-twentieth century, coincident with rapid acceleration of multiple, ongoing anthropogenic changes to Earth’s surface and environments. Radioactive isotopes dispersed during the 1952– 1962 period of atmospheric thermonuclear-bomb tests form a proposed global marker for the beginning of the Anthropocene. This marker is proposed for purely geological rea-sons as it is reasonably precise and global in scope. These isotopes are also a marker for the initiation of a new human capacity to trigger global environmental change in a period of hours. The possibility of a global, multiyear nuclear winter following a nuclear war between North Atlantic Treaty Organization nations and Russia is suggested by recent studies of wildfires that injected sunlight-blocking smoke into the stratosphere, and by increasingly sophisticated numerical simulations of global climate following a major nuclear war. Although the proposal for an Anthropocene time period was made without consideration of the consequences of nuclear war or nuclear winter, designating the period of thermonuclear weapon tests as initiating an Anthropocene time period is supported here specifically because it indicates a new human capability for rapid and destructive environmental change on a global scale.

International System of Units to U.S. customary units Multiply By To obtain centimeter (cm) 0.3937 inch (in.) millimeter (mm) 0.03937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) meter (m) 1.094 yard (yd)

Oxygen and carbon isotope analyses of laminated Bouse marl.

The cause of Cenozoic uplift of the Colorado Plateau is one of the largest remaining problems of Cordilleran tectonics. Difficulty in discriminating between two major classes of uplift mechanisms, one related to lithosphere modification by low-angle subduction and the other related to active mantle processes following termination of subduction, is hampered by lack of evidence for the timing of uplift. The carbonate member of the Pliocene Bouse Formation in the lower Colorado River Valley southwest of the Colorado Plateau has been interpreted as estuarine, in which case its modern elevation of up to 330 m above sea level would be important evidence for late Cenozoic uplift. The carbonate member includes laminated marl and claystone interpreted previously in at least one locality as tidal, which is therefore of marine origin. We analyzed lamination mineralogy, oxygen and carbon isotopes, and thickness variations to discriminate between a tidal versus seasonal origin. Oxygen and carbon isotopic analysis of two laminated carbonate samples shows an alternating pattern of lower δ18O and δ13C associated with micrite and slightly higher δ18O and δ13C associated with siltstone, which is consistent with seasonal variation. Covariation of alternating δ18O and δ13C also indicates that post-depositional chemical alteration did not affect these samples. Furthermore, we did not identify any periodic thickness variations suggestive of tidal influence. We conclude that lamination characteristics indicate seasonal genesis in a lake rather than tidal genesis in an estuary and that the laminated Bouse Formation strata provide no constraints on the timing of Colorado Plateau uplift.

The Pliocene Bouse Formation in the lower Colorado River trough locally contains laminated marl and claystone interpreted by O’Connell et al. (2017) to represent the spring-neap tide cycle during sediment deposition in an estuary. Tidal cycles, if present, should be detectable by Fourier spectral analysis of lamination thicknesses in continuous sequences of laminated sediments. To evaluate the tidal interpretation, we attempted to photograph several laminated sequences in the southern Bouse Formation (south of Blythe, California) so that thicknesses could be measured from the photographs. Only one sequence, in lower Milpitas Wash (California), was identified where thicknesses could be determined with adequate precision from field photography. Fourier analysis of that sequence failed to identify evidence of tides. Furthermore, electron-microprobe analysis determined that laminations consist of alternating claystone and marl, which is consistent with annual changes in lake chemistry and sediment sources rather than physical changes in sediment sorting and transport during tidal cycles. Fourier analysis of data presented by O’Connell et al. (2017) of two nearby laminated Bouse sequences interpreted as tidal rhythmites also failed to identify statistically significant evidence of tides.

Earth and Mars should have been frozen worlds in their early history because of lower solar luminosity but were not, which challenges our understanding of early atmospheres and surface conditions and/or our understanding of solar evolution.This is known as the "faint young Sun problem."One resolution to the problem is that the Sun was more massive and luminous in its youth before blowing off mass.Astrophysical studies of stellar evolution and behavior, however, including recent analysis of Kepler space-telescope data, indicate that mass loss is both insufficient and occurs too early to allow for a more luminous Sun after ca. 4 Ga.Alternatively, greenhouse gases were surprisingly effective at warming young Earth and Mars.High concentrations of CO 2 with the possible addition of biogenic CH 4 are likely dominant factors promoting open-water conditions on Archean Earth.Evidence of precipitation and flowing water on young Mars, including river valleys thousands of kilometers long, is more problematic.Recent studies indicate that 3-4 Ga river valleys and delta deposits in crater lakes could have been produced in <~10 7 years.Highly transient warm periods during times of favorable orbital parameters possibly led to brief melting under otherwise icy conditions.Seasonal melting and runoff would be more likely with ~1%-10% atmospheric H 2 and CH 4 , perhaps derived from serpentinization of olivine in the martian crust and released from frozen ground by impacts and volcanism, and/or derived directly from volcanic outgassing.The recently recognized effectiveness of hydrogen and methane at absorbing infrared radiation in a thick CO 2 -dominated atmosphere, in a process known as "collision-induced absorption," is probably essential to the solution to the faint young Sun problem for Mars.

ABSTRACT The Santa Catalina and Rincon Mountains north and east of Tucson, Arizona, form one of the largest core complexes on Earth. Both ranges consist primarily of Eocene leucogranites that intrude Proterozoic and late Cretaceous granitoids, and two Oligocene plutons. Mylonitic fabrics are well developed on the southern flank of the Santa Catalina Mountains and the southwestern flank of the Rincon Mountains. The corrugated form of the two ranges reflects the grooved form of the ca. 15–30 Ma Catalina–San Pedro detachment fault exposed primarily at the foot of the ranges. Normal displacement on two younger high-angle normal faults is responsible for much of the substantial relief of the ranges. This field guide is focused on fault rocks and mylonitic fabrics in the footwalls of the detachment fault and the high-angle Pirate normal fault, and includes description and analysis of shear-zone kinematics and processes, U-Pb geochronology of leucogranites, and core-complex geomorphology.

The Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists of an arched footwall of foliated crystalline rocks bounded above by the generally outward dipping, Oligocene‐Miocene San Pedro extensional detachment fault. The southwest trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation, and inferred top‐southwest displacement on the fault. An upper plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an underappreciated influence on extensional tectonics in cratonic southwestern North America.

The Colorado River extensional corridor in southwestern North America is one of Earth's most highly extended regions of continental crust. The central part of the belt includes three imbricate, regionally northeast-dipping extensional detachment faults. The Plomosa detachment fault in the northern Plomosa Mountains in western Arizona, the middle of the three faults, dips northeastward beneath the giant Harcuvar metamorphic core complex. Approximately 1 km of lower Miocene clastic sediments, lava flows, and rock-avalanche breccias were deposited in the northern Plomosa Mountains before initiation of the Plomosa detachment fault and division of the strata into two basins with different stratal accumulations following breakup. Both the detachment-fault lower plate and upper plate were then broken and tilted by normal faults. The upper plate was fragmented into numerous fault blocks and its extension-parallel width was approximately doubled. Application of critical-taper theory to delayed basin fragmentation suggests that southwestward tilting of the land surface and underlying normal faults led to normal-fault initiation and wedge breakup. A seismic-reflection profile northeast of the northern Plomosa Mountains reveals strong, southwest-dipping reflectors that project up dip to metasedimentary tectonites in the southern Buckskin Mountains in the Harcuvar core complex. Restoration of displacement on the Buckskin and Plomosa detachment faults aligns the reflectors and tectonites with a Mesozoic shear zone in the footwall of the Plomosa detachment fault. In this restoration the combined shear zone dips northeastward rather than southwestward and projects up dip to the folds and thrusts exposed to the south and west of the northern Plomosa Mountains. This zone is interpreted as a segment of the Mesozoic Maria fold-and-thrust belt that influenced the geometry of younger detachment faults. The southwest-tilted, mylonitic lower plate of the Plomosa detachment fault includes, at its northern end, Orocopia Schist, which is a Cretaceous subduction complex that is better known from locations farther southwest and closer to the continental margin. Restoration of tectonic extension suggests that Orocopia Schist extends under the Harcuvar core complex and that a buoyant crustal root inherited from Cretaceous thrusting could not have been the cause of core-complex uplift unless the schist was emplaced by a mechanism other than subduction underplating. We propose that the rolling-hinge detachment-fault model combined with a highly mobile deep crust could account for Harcuvar core-complex genesis without a buoyant crustal root.

Geologic map of the Fairbank 7 1/2' Quadrangle Arizona Geological Survey 1955 East Sixth Street, PO Box 210184 Tucson, AZ 85721 (520) 621-2470 www.azgs.az.gov Shipman, Pearthree, Moore, and Youberg Ferguson Ferguson Ferguson Richard Cook and Haddad Shipman, Pearthree, Moore, and Youberg Spencer Youberg, Shipman, and Pearthree ¬«90 ¬«80 ¬«82 LAND TOMBSTONE HABERSTOCK HILL HUACHUCA CITY FORT HUACHUCA TOMBSTONE SE LEWIS SPRINGS FAIRBANK MCGREW SPRING Location of map area Tucson Sierra Vista Phoenix Safford Willcox Flagstaff ! _ ! ! Geologic map of the Fairbank 7 1⁄2' Quadrangle, Cochise County, Arizona Charles A. Ferguson, Todd C. Shipman, Philip A. Pearthree, Erin M. Moore, Stephen M. Richard, Jon E. Spencer, Ann Youberg, Joseph P. Cook, and David E. Haddad Arizona Geological Survey Digital Geologic Map 50 (DGM-50), version 3.0 September 2018 1:24,000 scale Citation for this map: Ferguson, C.A., Shipman, T.C., Pearthree, P.A., Moore, E.N., Richard, S.M., Spencer, J.E., Youberg, A., Cook, J.P., and Haddad, D.E., 2018, Geologic Map of the Fairbank 71⁄2' Quadrangle, Cochise County, Arizona: Arizona Geological Survey Digital Geologic Map 50 (DGM50), version 3.0, 1 sheet, layout scale 1:24,000, with text. Research supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under USGS award #04HQAG0072. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. government. The Arizona Department of Water Resources provided funding for map revisions depicted in DGM-50, version 2.0.