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Robert J. Fleck

Publications and source records attributed to Robert J. Fleck.

At least 19 recordsLinked to original sources
USGS2021

Geochronologic, isotopic, and geochemical data from pre-Cretaceous plutonic rocks in the Lane Mountain area, San Bernardino County, California

Pre-Cretaceous, predominantly dioritic plutonic rocks in the Lane Mountain area, California, intrude metasedimentary and metavolcanic rocks considered part of the El Paso terrane. New geochronologic (uranium-lead zircon), geochemical, and isotopic data provide a reliable basis for dividing these pre-Cretaceous plutonic rocks into two mappable suites of Permian–Triassic and Late Jurassic ages. The 26 Permian–Triassic samples included in this report have a mean age of ~248 mega-annum (Ma), range in composition from monzodiorite to quartz monzonite and granodiorite, and have a mean initial 87 Sr/ 86 Sr ratio (Sri) of ~0.7045. The 22 Late Jurassic samples have a mean age of ~149 Ma, range in composition from gabbro to granite, and have a mean Sr i of ~0.7055. Accurate mapping of these two plutonic suites and their detailed field relations with the associated metamorphic rocks is essential for resolving the geologic history and regional tectonic significance of the Lane Mountain area. The sub-0.706 Sr i values of both plutonic suites at Lane Mountain are consistent with previous suggestions that the El Paso terrane is allochthonous and did not develop on Precambrian continental lithosphere. Both suites are considered parts of northwest-trending magmatic arcs interpreted to have formed above east-dipping subduction zones along the evolving North American continental margin, and both arcs are interpreted to cross a major east-west-trending boundary between the El Paso terrane and rocks considered part of ancestral North America in the San Bernardino Mountains area to the south. The El Paso terrane thus appears to have been attached to the San Bernardino Mountains area at least since Permian–Triassic time, although the boundary probably has been modified by Cenozoic faulting.

California↗
USGS2019

Geochronologic, isotopic, and geochemical data from igneous rocks in the Lane Mountain area, San Bernardino County, California

We present new geochronologic, isotopic, and geochemical data for selected igneous rocks in the Lane Mountain area, California. We determined SHRIMP-RG U-Pb zircon ages for the following units: (1) Larrea complex (~253 Ma and ~149–146 Ma); (2) Daisy granodiorite (~151 Ma); (3) Jack Spring quartz monzonite (~85–82 Ma); (4) unnamed porphyritic dikes and stocks (~80–73 Ma); and (5) Lane Mountain volcanics (~22 Ma). These results confirm and refine previous age determinations for the Larrea complex, Jack Spring quartz monzonite, and Lane Mountain volcanics, and provide the first reliable ages for the Daisy granodiorite and the unnamed porphyritic rocks. Sri of the plutonic rocks increases from ~0.704 to ~0.705–0.706 to ~0.708–0.709 for samples dated as ~250, ~150, and ~85–70 Ma, respectively, which indicates progressively greater involvement of Precambrian continental lithosphere in magma generation through time. The Sri and geochemical data suggest that the most significant change in magmatic source occurred between ~150 and 85 Ma.

California↗
USGS2019

Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada

The purpose of this report is to summarize geochemical, petrographic, and geochronologic data for samples, principally those of unmineralized Tertiary volcanic rocks, from the Tonopah, Divide, and Goldfield mining districts of west-central Nevada (fig. 1). Much of the data presented here for the Tonopah and Divide districts are for samples collected by Bonham and Garside (1979) during geologic mapping in and around those districts, whereas much of that for samples from the Goldfield district were obtained by Ashley (1974; 1979; 1990a). Additional data were derived from samples collected between 2012–2015, as part of the Mineral Resources Program funded project titled: “Magmatic-tectonic history and component sources of major precious metal deposits in the southern Walker Lane”. A small amount of additional geochemical data for samples from each of the districts were compiled from other sources. Individual sample collectors are identified by appropriate entries in the “Collector” field (appendix 1) and published sources of geochemical data are defined by entries in the “Chem_Src” data field (appendix 1).

California, Nevada↗
USGS2019

Geology and paleontology of the late Miocene Wilson Grove Formation at Bloomfield Quarry, Sonoma County, California

An extensive fauna of at least 77 taxa is reported from the basal Wilson Grove Formation in a small quarry just north of the town of Bloomfield, Sonoma County, California. The fauna represents intertidal to shallow subtidal water depths and water temperatures interpreted from the fauna, consistent with the latitude of the fossil locality (37° north) during the late Miocene. The fauna from Bloomfield Quarry is unusually large and diverse from such a small area. It consists of thousands of specimens of 4 brachiopod, 42 mollusk (28 bivalves and 14 gastropods), 6 arthropod (1 crab, 1 shrimp, and 4 barnacles), and 25 vertebrate (3 sharks, 1 ray, 8 bony fishes, 9 marine mammals, and 4 birds) taxa. Unusual in the fauna is the abundant and diverse brachiopod fauna, the diverse barnacle fauna, which was described previously, and the extensive and diverse vertebrate fauna. Most significant among the vertebrates is the walrus fauna, which is the most diverse assemblage of walrus yet reported worldwide from a single locality. A single strontium (Sr) isotope age determination of about 8 million years (megaannum, Ma) from a pectinid mollusk is consistent with a new age determination of the overlying, informally named Roblar tuff as described by Sarna-Wojcicki in 1992 (6.203±0.011 Ma) and previously reported age determinations (recalculated here) from basalt (9.27±0.06 Ma) underlying these deposits. The Roblar tuff at Bloomfield Quarry can be correlated with other sites, including the Delgada Fan offshore northern California and the Coalinga anticline in California’s Central Valley. These age determinations conform with the “Jacalitos” California provincial molluscan stage age, the Hemphillian North American Land Mammal age determined from the fossils, and is part of the International Tortonian Stage of the Miocene.

California↗
USGS2019

Characterization of the rhyolite of Bodie Hills and 40Ar/39Ar intercalibration with Ar mineral standards

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40 Ar/ 39 Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor ( R BHs/GA1550 ) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R TCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; R FCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; R ACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes.

Chemical Geology↗
USGS2018

Geologic map and database of the Chocolate Mountain Aerial Gunnery Range, Riverside and Imperial Counties, California

The northwest-trending Chocolate Mountains are situated along the northeastern margin of the southern Salton Trough. The Chocolate Mountain Aerial Gunnery Range occupies most of the 75-km-long part of the Chocolate Mountains that lies between Salt Creek to the north and California State Highway 78 to the south. Mapping studies in the Chocolate Mountains within the gunnery range are few and this study was conducted in cooperation with the U.S. Navy (Naval Facilities Engineering Command Southwest, San Diego, California) and U.S. Marine Corps (Range Management Department, Marine Corps Air Station, Yuma, Arizona). Crystalline basement rocks in the Chocolate Mountains range in age from early Proterozoic to middle Cenozoic. Early and middle Proterozoic metamorphosed sedimentary and plutonic rocks include sillimanite-biotite-quartz feldspar gneiss, layered biotite-quartz-feldspar gneiss, biotite-quartz-feldspar augen gneiss, and largely undeformed late Proterozoic anorthosite and syenite. These rock types, which crop out as dispersed domains in the Chocolate Mountains, are remnants—along with more extensive domains observed in the Eastern Transverse Ranges to the north and in the San Gabriel Mountains to the northwest—of an originally more continuous assemblage that has been dextrally displaced along strands of the San Andreas Fault System.

California↗
USGS2018

Late Neogene–Quaternary tephrochronology, stratigraphy, and paleoclimate of Death Valley, California, USA

Sedimentary deposits in midlatitude continental basins often preserve a paleoclimate record complementary to marine-based records. However, deriving that paleoclimate record depends on having well-exposed deposits and establishing a sufficiently robust geochronology. After decades of research, we have been able to correlate 77 tephra beds exposed in multiple stratigraphic sections in the Death Valley area, California, United States. These correlations identify 25 different tephra beds that erupted from at least five different volcanic centers from older than 3.58 Ma to ca. 32 ka. We have informally named and determined the ages for seven previously unrecognized beds: ca. 3.54 Ma tuff of Curry canyon, ca. 3.45 Ma tuff of Furnace Creek, ca. 3.1 Ma tuff of Kit Fox Hills, ca. 3.1 Ma tuff of Mesquite Flat, ca. 3.15 Ma tuff of Texas Spring, 3.117 ± 0.011 Ma tuff of Echo Canyon, and the ca. 1.3 Ma Amargosa ash bed. Several of these tephra beds are found as far northeast as central Utah and could be important marker beds in western North America. Our tephrochronologic data, combined with magnetic polarity data and 40 Ar/ 39 Ar age determinations, redefine Neogene sedimentary deposits exposed across 175 km 2 of the Death Valley area. The alluvial/lacustrine Furnace Creek Formation is a time-transgressive sedimentary sequence ranging from ca. 6.0 to 2.5 Ma in age. The ca. 2.5–1.7 Ma Funeral Formation is typically exposed as a proximal alluvial-fan facies overlying the Furnace Creek Formation. We have correlated deposits in the Kit Fox Hills, Salt Creek, Nova Basin, and southern Death Valley with the informally named ca. 1.3–0.5 Ma Mormon Point formation. In addition, our correlation of the late Pleistocene Wilson Creek ash bed 15 in the Lake Rogers deposits represents the first unambiguous sequences deposited during the Last Glacial Maximum (marine isotope stage [MIS] 2) in Death Valley. Based on this new stratigraphic framework, we show that the Pliocene and Pleistocene climate in Death Valley is consistent with the well-established marine tropical/subtropical record. Pluvial lakes in Death Valley and Searles Valley began to form ca. 3.5–3.4 Ma in the late Pliocene during MIS MG5. Initiation of lakes in these two hydrologically separated valleys at the same time at the beginning of a cooling trend in the marine climate record suggests a link to a cooler, wetter (glacial) regional climate in North America. The Death Valley lake persisted until ca. 3.30 Ma, at the peak of the M2 glaciation, after which there is no evidence of Pliocene lacustrine deposition, even at the peak of the Northern Hemisphere Glaciation (ca. 2.75 Ma). If pluvial lakes in the Pliocene are an indirect record of glacial climate conditions, as they are for the Pleistocene, then a glacial climate was present in western North America for ∼200,000 yr during the Pliocene, encompassing MIS MG5–M2. Pleistocene pluvial lakes in Death Valley that formed ca. 1.98–1.78 Ma, 1.3–1.0 Ma, and ca. 0.6 Ma (MIS 16) are consistent with other regional climate records that indicate a regional glacial climate; however, Death Valley was relatively dry at ca. 0.77 Ma (MIS 19), when large lakes existed in other basins. The limited extent of the MIS 2 marsh/shallow lake in the Lake Rogers basin of northern Death Valley reflects the well-known regional glacial climate at that time; however, Death Valley received relatively lower inflow and rainfall in comparison.

Geological Society of America Bulletin↗
USGS2017

Geologic map of the Beacon Rock quadrangle, Skamania County, Washington

The Beacon Rock 7.5′ quadrangle is located approximately 50 km east of Portland, Oregon, on the north side of the Columbia River Gorge, a scenic canyon carved through the axis of the Cascade Range by the Columbia River. Although approximately 75,000 people live within the gorge, much of the region remains little developed and is encompassed by the 292,500-acre Columbia River Gorge National Scenic Area, managed by a consortium of government agencies “to pro­tect and provide for the enhancement of the scenic, cultural, recreational and natural resources of the Gorge and to protect and support the economy of the Columbia River Gorge area.” As the only low-elevation corridor through the Cascade Range, the gorge is a critical regional transportation and utilities corridor (Wang and Chaker, 2004). Major state and national highways and rail lines run along both shores of the Columbia River, which also provides important water access to ports in the agricultural interior of the Pacific Northwest. Transmission lines carry power from hydroelectric facilities in the gorge and farther east to the growing urban areas of western Oregon and Washington, and natural-gas pipelines transect the corridor (Wang and Chaker, 2004). These lifelines are highly vulnerable to disruption by earthquakes, landslides, and floods. A major purpose of the work described here is to identify and map geologic hazards, such as faults and landslide-prone areas, to provide more accurate assessments of the risks associated with these features. The steep canyon walls of the map area reveal exten­sive outcrops of Miocene flood-basalt flows of the Columbia River Basalt Group capped by fluvial deposits of the ances­tral Columbia River, Pliocene lavas erupted from the axis of the Cascade arc to the east, and volcanic rocks erupted from numerous local vents. The Columbia River Basalt Group unconformably rests on a sequence of late Oligocene and early Miocene rocks of the ancestral Cascade volcanic arc, which underlies most of the map area. The resistant flood-basalt flows form some of the famous landforms in the map area, such as Hamilton Mountain. Extensive landslide complexes have devel­oped where the basalt flows were emplaced on weak volcani­clastic rocks.

Washington↗
USGS2017

Paleomagnetism and 40Ar/39Ar geochronology of the Plio-Pleistocene Boring Volcanic Field: Implications for the geomagnetic polarity time scale and paleosecular variation

Paleomagnetic directions and 40 Ar/ 39 Ar ages have been determined for samples of lava flows from the same outcrops, where possible, for 84 eruptive units ranging in age from 3200 ka to 60 ka within the Boring Volcanic Field (BVF) of the Pacific Northwest, USA. This study expands upon our previous results for the BVF, and compares the combined results with the current geomagnetic polarity time scale (GPTS). Lava flows with transitional directions were found within the BVF at the Matuyama-Brunhes and Jaramillo-Matuyama polarity boundaries, and replicate ages corresponding to these and other boundaries have been newly ascertained. Although the BVF data generally agree with GPTS chronozone boundaries, they indicate that onset of the Gauss-Matuyama transition and Olduvai subchron occurred significantly earlier than given in the current time scale calibration. Additional comparisons show that the BVF results are consistent with recent statistical models of geomagnetic paleosecular variation.

Physics of the Earth and Planetary Interiors↗
USGS2017

Analysis of the age and paleomagnetic orientation of the Broadwell Mesa Basalt, Bristol Mountains, CA

To add to the regional paleomagnetic data documenting block rotation in eastern California, we determined the age and paleomagnetic rotation of the Broadwell Mesa basalt, a basalt in the Bristol Mountains, CA as part of an effort to constrain the timing and rotation of blocks adjacent to the fault. The east-striking sinistral Broadwell Mesa fault cuts and separates the basalt into two outcrops. An 40Ar/39Ar date from the northern outcrop yields an age of 5.46 ± 0.04 Ma. Two sites consisting of 40 paleomagnetic cores from the basalt indicate the basalt is reversely magnetized and that there has been no significant rotation (< 11º) between the two basalt outcrops.

California↗
USGS2016

Geologic and geophysical maps and volcanic history of the Kelton Pass SE and Monument Peak SW Quadrangles, Box Elder County, Utah

The Kelton Pass SE and Monument Peak SW 7.5' quadrangles are located in Box Elder County, northwestern Utah (figure 1; plate 1). The northern boundary of the map area is 8.5 miles (13.7 km) south of the Utah-Idaho border, and the southern boundary reaches the edge of mud flats at the north end of Great Salt Lake. Elevations range from 4218 feet (1286 m) along the mud flats to 5078 feet (1548 m) in the Wildcat Hills. Deep Creek forms a prominent drainage between the Wildcat Hills and Cedar Hill. The closest towns are the ranching communities of Snowville, Utah (10 miles [16 km] to the northeast) (figure 1), and Park Valley, Utah (10 miles [16 km] to the west). The Kelton Pass SE and Monument Peak SW 7.5' quadrangles are located entirely within southern Curlew Valley, which drains south into Great Salt Lake, and extends north of the area shown on figure 1 into Idaho. Curlew Valley is bounded on the west by the Raft River Mountains and on the east by the Hansel Mountains (figure 1). Sedimentary and volcanic bedrock exposures within the quadrangles form the Wildcat Hills, Cedar Hill, and informally named Middle Shield (figure 1). Exposed rocks and deposits are Permian to Holocene in age, and include the Permian quartz sandstone and orthoquartzite of the Oquirrh Formation (Pos), tuffaceous sedimentary rocks of the Miocene Salt Lake Formation (Ts), Pliocene basaltic lava flows (Tb) and dacite (Tdw), Pleistocene rhyolite (Qrw) and basalt (Qb), and Pleistocene and Holocene surficial deposits of alluvial, lacustrine, and eolian origin. Structurally, the map area is situated in the northeastern Basin and Range Province, and is inferred to lie within the hanging wall of the late Miocene detachment faults exposed in the Raft River Mountains to the northwest (e.g., Wells, 1992, 2009; figure 1). This mapping project was undertaken to produce a comprehensive, large-scale geologic map of the Wildcat Hills, as well as to improve understanding of the volcanic and tectonic evolution of southern Curlew Valley. The resultant publication includes a geologic map of the Kelton Pass SE and Monument Peak SW quadrangles (plate 1), two interpretive geologic cross sections (plate 2), new geophysical data and interpretations, and new geochronology data for volcanic units within and near the quadrangles.

Utah↗
USGS2015

Gold-silver mining districts, alteration zones, and paleolandforms in the Miocene Bodie Hills Volcanic Field, California and Nevada

The Bodie Hills is a ~40 by ~30 kilometer volcanic field that straddles the California-Nevada state boundary between Mono Lake and the East Walker River. Three precious metal mining districts and nine alteration zones are delineated in Tertiary-Quaternary volcanic and Mesozoic granitic and metamorphic rocks that comprise the volcanic field. Cumulative production from the mining districts, Bodie, Aurora, and Masonic, is 3.4 million ounces of gold and 28 million ounces of silver. Small amounts of mercury were produced from the Potato Peak, Paramount-Bald Peak, and Cinnabar Canyon-US 395 alteration zones; a native sulfur resource in the Cinnabar Canyon-US 395 alteration zone has been identified by drilling. There are no known mineral resources in the other six alteration zones, Red Wash-East Walker River, East Brawley Peak, Sawtooth Ridge, Aurora Canyon, Four Corners, and Spring Peak. The mining districts and alteration zones formed between 13.4 and 8.1 Ma in predominantly ~15&ndash;9 Ma volcanic rocks of the Bodie Hills volcanic field. Ages of hydrothermal minerals in the districts and zones are the same as, or somewhat younger than, the ages of volcanic host rocks. Based on volcanic stratigraphy, geochronology, remnant paleosurfaces, and paleopotentiometric surfaces in mining districts and alteration zones, present landforms in the Bodie Hills volcanic field reflect incremental construction of stratovolcanoes and large- to small-volume flow-domes, magmatic inflation, and fault displacements. Landform evolution began with construction of the 15&ndash;13 Ma Masonic and 13&ndash;12 Ma Aurora volcanic centers in the northwestern and northeastern parts of the field, respectively. Smaller volcanoes erupted at ~11&ndash;10 Ma in, between, and south of these centers as erosional detritus accumulated north of the field in Fletcher Valley. Distally sourced, 9.7&ndash;9.3 Ma Eureka Valley Tuff filled drainages and depressions among older volcanoes and was partly covered by nearly synchronous eruptives during construction of four large 10&ndash;8 Ma volcanoes, in the southern part of the field. The lack of significant internal fault displacement, distribution of Eureka Valley Tuff, and elevation estimates derived from floras, suggest that the Bodie Hills volcanic field attained present elevations mostly through volcano construction and magmatic inflation, and that maximum paleoelevations (>8,500 ft) at the end of large volume eruptions at ~8 Ma are similar to present elevations.

California, Nevada↗
USGS2015

Geochronology of Cenozoic rocks in the Bodie Hills, California and Nevada

The purpose of this report is to present geochronologic data for unaltered volcanic rocks, hydrothermally altered volcanic rocks, and mineral deposits of the Miocene Bodie Hills and Pliocene to Pleistocene Aurora volcanic fields of east-central California and west-central Nevada. Most of the data presented here were derived from samples collected between 2000&ndash;13, but some of the geochronologic data, compiled from a variety of sources, pertain to samples collected during prior investigations. New data presented here (tables 1 and 2; Appendixes 1&ndash;3) were acquired in three U.S. Geological Survey (USGS) 40 Ar/ 39 Ar labs by three different geochronologists: Robert J. Fleck (Menlo Park, CA), Lawrence W. Snee (Denver, CO), and Michael A. Cosca (Denver, CO). Analytical methods and data derived from each of these labs are presented separately. The middle to late Miocene Bodie Hills volcanic field (BHVF) is a large (>700 km 2 ), long-lived (~9 million years [m.y.]), episodic eruptive complex (John and others, 2012) in the southern segment of the ancestral Cascades arc (du Bray and others, written commun., 2015) north of Mono Lake and east of Bridgeport, California (fig. 1). The field is near the west edge of the Walker Lane and the northwest edge of the Mina deflection where structures related to these shear zones may have localized magmatism. The Walker Lane (fig. 1) is a broad, northwest-striking zone of right-lateral shear that accommodates right-lateral motion between the Pacific and North America plates; the Mina deflection constitutes a 60-km-long right step in the Walker Lane (Faulds and Henry, 2008; Oldow, 1992, 2003; Stewart, 1988). The Bodie Hills volcanic field includes at least 31 volcanic rock units erupted from 21 significant volcanic eruptive centers. Four trachyandesite stratovolcanoes developed along the margins of the volcanic field and numerous silicic trachyandesite to rhyolite flow dome complexes erupted more centrally. Volcanism in the Bodie Hills volcanic field peaked at two periods, ~15.0 to 12.6 million years before present (Ma) and ~9.9 to 8.0 Ma, which were dominated by emplacement of large stratovolcanoes and large silicic trachyandesite-dacite lava domes, respectively. A final period of small-volume silicic dome emplacement began in the western part of the volcanic field at ~6 Ma and culminated at ~5.5 Ma (John and others, 2012).

California, Nevada↗
USGS2015

Geologic map of the Bodie Hills, California and Nevada

The Bodie Hills covers about 1,200 km 2 straddling the California-Nevada state boundary just north of Mono Lake in the western part of the Basin and Range Province, about 20 km east of the central Sierra Nevada. The area is mostly underlain by the partly overlapping, middle to late Miocene Bodie Hills volcanic field and Pliocene to late Pleistocene Aurora volcanic field (John and others, 2012). Upper Miocene to Pliocene sedimentary deposits, mostly basin-filling sediments, gravel deposits, and fanglomerates, lap onto the west, north, and east sides of the Bodie Hills, where they cover older Miocene volcanic rocks. Quaternary surficial deposits, including extensive colluvial, fluvial, glacial, and lacustrine deposits, locally cover all older rocks. Miocene and younger rocks are tilted &le;30&deg; in variable directions. These rocks are cut by several sets of high-angle faults that exhibit a temporal change from conjugate northeast-striking left-lateral and north-striking right-lateral oblique-slip faults in rocks older than about 9 Ma to north- and northwest-striking dip-slip faults in late Miocene rocks. The youngest faults are north-striking normal and northeast-striking left-lateral oblique-slip faults that cut Pliocene-Pleistocene rocks. Numerous hydrothermal systems were active during Miocene magmatism and formed extensive zones of hydrothermally altered rocks and several large mineral deposits, including gold- and silver-rich veins in the Bodie and Aurora mining districts (Vikre and others, in press).

California, Nevada↗
USGS2014

Succession of Laramide magmatic and magmatic-hydrothermal events in the Patagonia Mountains, Santa Cruz County, Arizona

This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40 Ar/ 39 Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40 Ar/ 39 Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73–68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bornite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argillic alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61–59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59–58 Ma) enargite-chalcocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61–59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from >10 m.y. to less than the geochronologic precision currently achievable.

Arizona↗
USGS2014

Modified expression for bulb-tracer depletion—Effect on argon dating standards

40 Ar/ 39 Ar geochronology depends critically on well-calibrated standards, often traceable to first-principles K-Ar age calibrations using bulb-tracer systems. Tracer systems also provide precise standards for noble-gas studies and interlaboratory calibration. The exponential expression long used for calculating isotope tracer concentrations in K-Ar age dating and calibration of 40 Ar/ 39 Ar age standards may provide a close approximation of those values, but is not correct. Appropriate equations are derived that accurately describe the depletion of tracer reservoirs and concentrations of sequential tracers. In the modified expression the depletion constant is not in the exponent, which only varies as integers by tracer-number. Evaluation of the expressions demonstrates that systematic error introduced through use of the original expression may be substantial where reservoir volumes are small and resulting depletion constants are large. Traditional use of large reservoir to tracer volumes and the resulting small depletion constants have kept errors well less than experimental uncertainties in most previous K-Ar and calibration studies. Use of the proper expression, however, permits use of volumes appropriate to the problems addressed.

Geochemistry, Geophysics, Geosystems↗
USGS2014

Preliminary geologic map of Black Canyon and surrounding region, Nevada and Arizona

Thermal springs in Black Canyon of the Colorado River, downstream of Hoover Dam, are important recreational, ecological, and scenic features of the Lake Mead National Recreation Area. This report presents the results from a U.S. Geological Survey study of the geologic framework of the springs. The study was conducted in cooperation with the National Park Service and funded by both the National Park Service and National Cooperative Geologic Mapping Program of the U.S. Geological Survey. The report has two parts: A, a 1:48,000-scale geologic map created from existing geologic maps and augmented by new geologic mapping and geochronology; and B, an interpretive report that presents results based on a collection of fault kinematic data near springs within Black Canyon and construction of 1:100,000-scale geologic cross sections that extend across the western Lake Mead region. Exposures in Black Canyon are mostly of Miocene volcanic rocks, underlain by crystalline basement composed of Miocene plutonic rocks or Proterozoic metamorphic rocks. The rocks are variably tilted and highly faulted. Faults strike northwest to northeast and include normal and strike-slip faults. Spring discharge occurs along faults intruded by dacite dikes and plugs; weeping walls and seeps extend away from the faults in highly fractured rock or relatively porous volcanic breccias, or both. Results of kinematic analysis of fault data collected along tributaries to the Colorado River indicate two episodes of deformation, consistent with earlier studies. The earlier episode formed during east-northeast-directed extension, and the later during east-southeast-directed extension. At the northern end of the study area, pre-existing fault blocks that formed during the first episode were rotated counterclockwise along the left-lateral Lake Mead Fault System. The resulting fault pattern forms a complex arrangement that provides both barriers and pathways for groundwater movement within and around Black Canyon. Regional cross sections in this report show that thick Paleozoic carbonate aquifer rocks of east-central Nevada do not extend into the Black Canyon area and generally are terminated to the south at a major tectonic boundary defined by the northeast-striking Lake Mead Fault System and the northwest-striking Las Vegas Valley shear zone. Faults to the west of Black Canyon strike dominantly north-south and form a complicated pattern that may inhibit easterly groundwater movement from Eldorado Valley. To the east of Black Canyon, crystalline Proterozoic rocks locally overlain by Tertiary volcanic rocks in the Black Mountains are bounded by steep north-south normal faults. These faults may also inhibit westerly groundwater movement from Detrital Valley toward Black Canyon. Finally, the cross sections show clearly that Proterozoic basement rocks and (or) Tertiary plutonic rocks are shallow in the Black Canyon area (at the surface to a few hundred meters depth) and are cut by several major faults that discharge most of the springs in the Black Canyon. Therefore, the faults most likely provide groundwater pathways to sufficient depths that the groundwater is heated to the observed temperatures of up to 55 °C.

Arizona, Nevada↗