Contributors:du Bray, Edward A; Ressel, Michael W; Barnes, Calvin G
Reference Number: 7950
GeoREF Number: 2007-051507
Publication Year: 2007
Abstract: "North-central and northeast Nevada contains numerous large plutons andsmaller stocks but also contains many small, shallowly emplacedintrusive bodies, including dikes, sills, and intrusive lava domecomplexes. Decades of geologic investigations in the study areademonstrate that many ore deposits, representing diverse ore deposittypes, are spatially, and probably temporally and genetically,associated with these igneous intrusions. However, despite the numberand importance of igneous intrusions in the study area, no synthesisof geochemical data available for these rocks has been completed. Thisreport presents a synthesis of geochemical data for these rocks. Theproduct represents the first phases of an effort to evaluate thetime-space-compositional evolution of Mesozoic and Cenozoic magmatismin the study area and identify genetic associations between magmatismand mineralizing processes in this region."
Descriptors: cartography; chemical composition; data bases; data processing;geochemistry; histograms; igneous rocks; intrusions; magmatism;Nevada; north-central Nevada; northeastern Nevada; statisticalanalysis; United States; USGS
Classification: "05A Igneous and metamorphic petrology; 02C Geochemistry of rocks,soils, and sediments"
Source: "U. S. Geological Survey Data Series, Report: DS-0244, 62 pp., 2007"
map_coordinates: "Latitude:N383000,N413000 Longitude:W1140000,W1183000"
Contributors:Blatter, D.L., Farmer, G. L., and Carmichael, I.S.E.
Reference Number: 2870
GeoREF Number: 2007-900087
Publication Year: 2007
Source: "Journal of Petrology, V. 48, No. 5, P. 901-950"
Contributors:Saleeby, J; Farley, K A; Kistler, R W; Fleck, R J
Reference Number: 8231
GeoREF Number: 2007-075911
Publication Year: 2007
Abstract: "The Tehachapi complex lies at the southern end of the Sierra Nevadabatholith adjacent to the Neogene-Quaternary Garlock fault. Thecomplex is composed principally of high-pressure (8-10 kbar)Cretaceous batholithic rocks, and it represents the deepest exposedlevels of a continuous oblique crustal section through the southernSierra Nevada batholith. Over the southern approximately 100 km ofthis section, structural/petrologic continuity and geochronologicaldata indicate that > or =35 km of felsic tointermediate-composition crust was generated by copious arc magmatismprimarily between 105 and 99 Ma. In the Tehachapi complex, thesebatholithic rocks intrude and are bounded to the west bysimilar-composition gneissic-textured high-pressure batholithic rocksemplaced at ca. 115-110 Ma. This lower crustal complex is boundedbelow by a regional thrust system, which in Late Cretaceous timetectonically eroded the underlying mantle lithosphere, and in seriesdisplaced and underplated the Rand Schist subduction assemblage bylow-angle slip from the outboard Franciscan trench. Geophysical andmantle xenolith studies indicate that the remnants of this shallowsubduction thrust descend northward through the crust and into themantle, leaving the mantle lithosphere intact beneath the greaterSierra Nevada batholith. This north-dipping regional structure recordsan inflection in the Farallon plate, which was segmented into ashallow subduction trajectory to the south and a normal steepertrajectory to the north. We combine new and published data from abroad spectrum of thermochronometers that together form a coherentdata array constraining the thermal evolution of the complex.Integration of these data with published thermobarometric andpetrogenetic data also constrains the tectonically drivendecompression and exhumation history of the complex. The timing of arcmagmatic construction of the complex, as denoted above, is resolved bya large body of U/Pb zircon ages. High-confidence thermochronometricdata track a single retrogressing path commencing from widelyestablished solidus conditions at ca. 100 Ma, and traversing throughtime-temperature space as follows: (1) Sm/Nd garnet approximately770-680 degrees C at ca. 102-95 Ma, (2) U/Pb titanite approximately750-600 degrees C at ca. 102-95 Ma, (3) Ar/Ar hornblende approximately570-490 degrees C at ca. 94-91 Ma, (4) Rb/Sr biotite approximately390-260 degrees C at ca. 90-86 Ma, (5) Ar/Ar biotite approximately320-240 degrees C at ca. 88-85 Ma, and (6) (U-Th)/He zirconapproximately 230-170 degrees C at ca. 88-83 Ma. Additionalstratigraphic constraints place the complex at surface conditions inPaleocene-early Eocene time (ca. 66-55 Ma). Integration of theseresults with thermobarometric and structural data, including publisheddata on the underlying Rand Schist, reveals a profound tectonic eventwhereby rapid cooling and exhumation at rates potentially as high as100s degrees C/m.y. and >5 mm/yr initiated at ca. 98 Ma and peakedbetween 96 and 94 Ma. Between 93 and 85 Ma, cooling rates remainedhigh, but decelerated with or without significant exhumation.Subsequent cooling and exhumation rates are poorly constrained butwere much slower and ultimately resulted in Paleocene-Eocene surfaceexposure. Initial rapid exhumation and cooling are hypothesized tohave been driven by abrupt flattening in the corresponding segment ofthe Farallon plate and the resulting tectonic erosion of theunderlying mantle lithosphere. Protolith as well as metamorphicpressure-temperature and age constraints on the Rand Schist indicateits rapid low-angle subduction between 93 and 88 Ma. Comparison of theRand Schist and Tehachapi complex pressure-temperature-time paths inconjunction with structural relations strongly suggest that the schistascended the equivalent of approximately 4 kbar relative to theTehachapi complex by low-angle normal displacement along the Randfault between 88 and 80 Ma to attain its current underplatedstructural position. Such exten
Descriptors: (U-Th)/He; absolute age; batholiths; California; cooling; Cretaceous;crust; dates; exhumation; geologic barometry; geologic thermometry;intrusions; Kern County California; magmatism; Mesozoic; metamorphicrocks; metamorphism; nesosilicates; orthosilicates; Paleozoic;protoliths; Rand Schist; rates; retrograde metamorphism; schists;Sierra Nevada; Sierra Nevada Batholith; silicates; tectonics;Tehachapi Mountains; thermal history; thermochronology; U/Pb; UnitedStates; zircon; zircon group
Classification: 16 Structural geology; 03 Geochronology
Source: "Special Paper - Geological Society of America, vol.419, pp.39-66,2007"
map_coordinates: "Latitude:N344500,N351500 Longitude:W1181500,W1185000"
Contributors:Frey, Holli M; Lange, Rebecca A; Hall, Chris M; Delgado-Granados,Hugo; Carmichael, Ian S E
Reference Number: 2044
GeoREF Number: 2007-014825
Publication Year: 2007
Abstract: "The Tepic-Zacoalco rift, a NW-trending corridor approximately 50Xapproximately 250 km, is one arm of a triple-rift system in westernMexico. Together with the Colima rift and the Middle America Trench,it bounds the Jalisco block, a portion of western Mexico that may bemoving independently of North America. The predominant basement rocktypes in the Tepic-Zacoalco rift are rhyolitic ash-flow tuffs andlavas, which were previously assumed to be Oligocene-Miocene in age,related to the Sierra Madre Occidental volcanic province, or older.New (super 40) Ar/ (super 39) Ar dates on 41 volcanic samples reveal apreviously unrecognized, voluminous flare-up of rhyolitic ignimbritesbetween 5 and 3 Ma throughout the entire corridor of theTepic-Zacoalco rift; they are often associated with Pliocene high-Tibasalts. The eruption rate during this Pliocene time period was anorder of magnitude higher (hundreds of m/m.y.) than that documented inthe Tepic-Zacoalco rift over the last 1 m.y. The P!"
Descriptors: absolute age; Ar/Ar; ash-flow tuff; basalts; Cenozoic; dates; digitalterrain models; East Pacific; eruptions; experimental studies;extension; geochemistry; igneous rocks; ignimbrite; Jalisco Block;lithosphere; major elements; Mexico; microplates; Miocene; Neogene;North America; North Pacific; Northeast Pacific; outcrops; PacificOcean; plate tectonics; Pliocene; pyroclastics; rates; rhyolite tuff;rhyolites; rift zones; rifting; Rivera Plate; Sierra Madre Occidental;Tepic-Zacoalco Rift; Tertiary; trace elements; triple junctions;volcanic rocks; volcanism
Classification: 18 Solid-earth geophysics; 05A Igneous and metamorphic petrology
Source: "Geological Society of America Bulletin, vol.119, no.1-2, pp.49-64, Jan2007"
map_coordinates: "Latitude:N180000,N220000 Longitude:W1030000,W1080000"
Contributors:Bachmann, O; Oberli, F; Dungan, M A; Meier, M; Mundil, R; Fischer, H