Viola, ID, United States
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Bermudez M.A.,Joseph Fourier University | Bermudez M.A.,Central University of Venezuela | Kohn B.P.,University of Melbourne | Van Der Beek P.A.,Joseph Fourier University | And 3 more authors.
Tectonics | Year: 2010

The Venezuelan Andes formed by complex geodynamic interaction between the Caribbean Plate, the Panam Arc, the South American Plate and the continental Maracaibo block. We study the spatial and temporal patterns of exhumation across the Venezuelan Andes using 47 new apatite fission track (AFT) ages as well as topographic analyses. This approach permits the identification of at least seven tectonic blocks (Escalante, Cerro Azul, Trujillo, Caparo, Sierra Nevada, Sierra La Culata and El Carmen blocks) with contrasting exhumation and cooling histories. The Sierra Nevada, Sierra La Culata and El Carmen blocks, located in the central part of the Venezuelan Andes and separated by the Bocon fault system, cooled rapidly but diachronously during the late Miocene-Pliocene. Major surface uplift and exhumation occurred in the Sierra Nevada block since before 8 Ma. A second phase of uplift and exhumation affected the El Carmen and Sierra La Culata blocks to the north of the Bocon fault during the late Miocene-Pliocene. The highest topography and steepest relief of the belt coincides with these blocks. The Caparo and Trujillo blocks, located at the northeastern and southwestern ends of the orogen, cooled more slowly from the Oligocene to the late Miocene. These blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks. Unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional Caribbean geodynamics on the evolution of the Venezuelan Andes. Our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events (e.g., collision of the Panamá arc; rotation of the Maracaibo block). © 2010 by the American Geophysical Union.

Shen C.-B.,Wuhan University | Donelick R.A.,Apatite to Zircon Inc. | O'Sullivan P.B.,Apatite to Zircon Inc. | Jonckheere R.,TU Bergakademie Freiberg | And 4 more authors.
Sedimentary Geology | Year: 2012

Crystallization and cooling ages obtained from fission-track and U-Pb double dating of single zircons using LA-ICP-MS from five samples of Cretaceous Jianghan Basin strata, Yangtze block, central China, refine sediment sources and source-area exhumation. The Huangling basement massif of the northwestern Yangtze Craton and the Qinling-Dabie Orogen along the northern margin of the Yangtze block provided the U-Pb zircon-age populations. Two rapid cooling events at ca. 840-800 and ~. 206-151 to 43. Ma existed in the Huangling massif and the major exhumation period of the Qinling-Dabie Orogen at 221-195. Ma. The important tectonothermal event at 221-195. Ma of the Qinling-Dabie Orogen was characterized by the collision between the south China and the north China block and overprinted the whole Qinling-Dabie Orogen and the north Yangtze block. Together with the published geochronology, our data constrain the paleogeography of eastern China: (i) The Upper Triassic-Lower Jurassic strata have their sources in the Qinling-Dabie Orogen and the Cathaysia and Yangtze basement of the South China Block. (ii) In the Middle Jurassic, the Sichuan, Jianghan, Hengyang, and Mayang basins formed a large composite basin with identical provenance from the Qinling-Dabie Orogen and the Cathaysia. (iii) Since the Late Cretaceous, the Jiangnan Orogen and a mountain belt along the Pacific coast constituted important topographic boundaries in southern China. Within the Jianghan Basin, the maximum depositional ages of the Shimen, Wulong, Luojingtan, and Honghuatao Formations are approximately 145.53. ± 11.23, 113.09. ± 11.37, 96.00 ± 9.29, and 89.43. ± 9.29. Ma, respectively. This study demonstrates the feasibility of LA-ICP-MS zircon fission track and U-Pb double dating to refine the identification of sediment sources and determine the exhumation of source areas. © 2012 Elsevier B.V.

Moore T.E.,U.S. Geological Survey | O'Sullivan P.B.,Apatite to Zircon Inc. | O'Sullivan P.B.,United Road Services | Potter C.J.,U.S. Geological Survey | And 2 more authors.
Geosphere | Year: 2015

The Upper Jurassic and Lower Cretaceous part of the Brookian sequence of northern Alaska consists of syntectonic deposits shed from the north-directed, early Brookian orogenic belt. We employ sandstone petrography, detrital zircon U-Pb age analysis, and zircon fi ssion-track double-dating methods to investigate these deposits in a succession of thin regional thrust sheets in the western Brooks Range and in the adjacent Colville foreland basin to determine sediment provenance, sedimentary dispersal patterns, and to reconstruct the evolution of the Brookian orogen. The oldest and structurally highest deposits are allochthonous Upper Jurassic volcanic arc-derived sandstones that rest on accreted ophiolitic and/or subduction assemblage mafi c igneous rocks. These strata contain a nearly unimodal Late Jurassic zircon population and are interpreted to be a fragment of a forearc basin that was emplaced onto the Brooks Range during arc-continent collision. Synorogenic deposits found at structurally lower levels contain decreasing amounts of ophiolite and arc debris, Jurassic zircons, and increasing amounts of continentally derived sedimentary detritus accompanied by broadly distributed late Paleozoic and Triassic (359-200 Ma), early Paleozoic (542-359 Ma), and Paleoproterozoic (2000-1750 Ma) zircon populations. The zircon populations display fi ssion-track evidence of cooling during the Brookian event and evidence of an earlier episode of cooling in the late Paleozoic and Triassic. Surprisingly, there is little evidence for erosion of the continental basement of Arctic Alaska, its Paleozoic sedimentary cover, or its hinterland metamorphic rocks in early foreland basin strata at any structural and/or stratigraphic level in the western Brooks Range. Detritus from exhumation of these sources did not arrive in the foreland basin until the middle or late Albian in the central part of the Colville Basin. These observations indicate that two primary provenance areas provided detritus to the early Brookian foreland basin of the western Brooks Range: (1) local sources in the oceanic Angayucham terrane, which forms the upper plate of the orogen, and (2) a sedimentary source region outside of northern Alaska. Pre-Jurassic zircons and continental grain types suggest the latter detritus was derived from a thick succession of Triassic turbidites in the Russian Far East that were originally shed from source areas in the Uralian-Taimyr orogen and deposited in the South Anyui Ocean, interpreted here as an early Mesozoic remnant basin. Structural thickening and northward emplacement onto the continental margin of Chukotka during the Brookian structural event are proposed to have led to development of a highland source area located in eastern Chukotka, Wrangel Island, and Herald Arch region. The abundance of detritus from this source area in most of the samples argues that the Colville Basin and ancestral foreland basins were supplied by longitudinal sediment dispersal systems that extended eastward along the Brooks Range orogen and were tectonically recycled into the active foredeep as the thrust front propagated toward the foreland. Movement of clastic sedimentary material from eastern Chukotka, Wrangel Island, and Herald Arch into Brookian foreland basins in northern Alaska confi rms the interpretations of previous workers that the Brookian deformational belt extends into the Russian Far East and demonstrates that the Arctic Alaska- Chukotka microplate was a unifi ed geologic entity by the Early Cretaceous. © 2015 Geological Society of America.

Hickey K.A.,University of British Columbia | Barker S.L.L.,University of British Columbia | Dipple G.M.,University of British Columbia | Arehart G.B.,University of Nevada, Reno | Donelick R.A.,Apatite to Zircon Inc.
Economic Geology | Year: 2014

The duration of hydrothermal activity required to form ore deposits is poorly constrained. We demonstrate that thermochronology data, coupled with thermal modeling, can be used to constrain the duration of hydrothermal fluid flow. Apatite fission-track (AFT) thermochronology data define a conductive halo around an Eocene hydrothermal system that formed the Betze-Post gold deposit on the northern Carlin trend in Nevada. The premineralization Goldstrike stock acted as an essentially impermeable side to the auriferous Carlin hydrothermal system. The hydrothermal fluid conductively heated the intrusion over the time that it flowed past it. To derive first-order estimates for the maximum duration of this flow we numerically modeled one-dimensional conductive heat flow into the intrusion and used the results to forward model ensuing AFT annealing. Modeled levels of annealing were compared to AFT dates and track length data measured across the intrusion. Our results indicate that the episode of main ore-stage hydrothermal fluid flow (mean temperature of 200°C) that formed the ∼1,150 metric ton (t) Betze-Post gold deposit had a maximum duration of <15 to 45 ka. The average gold flux over this period was ∼80 to 30 kg yr-1, comparable to that measured in the deep reservoirs of several modern geothermal fields. Conservative estimates of gold concentration in the main ore-stage fluids imply that fluid upflow rates and total advective heat flow were also comparable to modern geothermal systems. This suggests that the most important factors for generating the large gold deposits of the northern Carlin trend were a large and/or continuous source of gold, and a very efficient means of removing it from the fluid, rather than the hydrologic system itself. The short duration of main ore-stage fluid flow is unlikely to represent a steady-state convective system. Instead, it most likely reflects a transient period of flow following slip and permeability generation along the steeply dipping Post-Genesis fault system that hosts many of the deposits along the northern Carlin trend. A sudden increase in the permeability of a fault may have led to a transitory period of peak fluid temperature as the fault initially tapped meteoric fluid that had resided at depth and had thermally equilibrated with the host rocks. With continued convection the flow drew cooler, less rock-buffered meteoric water down from higher in the system. © 2013 Society of Economic Geologists, Inc.

Benowitz J.A.,University of Alaska Fairbanks | Haeussler P.J.,U.S. Geological Survey | Layer P.W.,University of Alaska Fairbanks | O'Sullivan P.B.,Apatite to Zircon Inc. | And 2 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/ 39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFAT max negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains. Copyright 2012 by the American Geophysical Union.

Apatite to Zircon Inc. | Date: 2013-04-30

A method for determining the position and its statistical uncertainty of a confined fission track in a crystal based on detecting confined fission track tips in a series of transmitted light images. A computer software program for: detecting confined fission track tips in a series of transmitted light images and assessing the viability of each tip using a scoring equation; writing to and loading from a computer database of confined fission tracks; modifying the scoring equation for assessing confined fission track tip viability based on the contents of the computer database. A computer database consisting of transmitted light images of confined fission tracks. A method for determining the statistical probability that a confined fission track is a real confined fission track.

A method of determining the concentration of an element of interest in a solid of interest based on the ratio of the measured relative abundances of two isotopes in the solid of interest, one isotope of the element of interest and the second isotope from an element represented in the chemical formula of the solid of interest, and comparing this ratio to the ratio of the measured relative abundances of the same two isotopes for a reference solid for which the concentration of the element of interest is known. A method of calculating the concentration of the element of interest in the solid of interest. A method of executing a computer software program with instructions for calculating the concentration of the element of interest in the solid of interest.

Apatite to Zircon Inc. | Date: 2013-05-01

A method for determining the position and its statistical uncertainty of a fission semi-track in a crystal based on detecting the tip and etch figure of a fission semi-track in a series of transmitted light images. A computer software program for: detecting the tip and etch figure of a fission semi-track in a series of transmitted light images and assessing the viability of the tip using a scoring equation; writing to and loading from a computer database of fission semi-tracks; modifying the scoring equation for assessing fission semi-track tip viability based on the contents of the computer database. A computer database consisting of transmitted light images of fission semi-tracks. A method for determining the statistical probability that a fission semi-track is a real fission semi-track.

Apatite to Zircon Inc. | Date: 2013-04-12

Computer software for interpretation of chemical and isotope data (namely fission tracks, Helium, Argon, Lead and other daughter products which diffuse with temperature) from natural rock systems to extract temperature history information of the natural rock system and instructional user guides sold as a unit.

Chew D.M.,Trinity College Dublin | Donelick R.A.,Apatite to Zircon Inc. | Donelick M.B.,Apatite to Zircon Inc. | Kamber B.S.,Trinity College Dublin | Stock M.J.,University of Oxford
Geostandards and Geoanalytical Research | Year: 2014

Apatite incorporates variable and significant amounts of halogens (mainly F and Cl) in its crystal structure, which can be used to determine the initial F and Cl concentrations of magmas. The amount of chlorine in the apatite lattice also exerts an important compositional control on the degree of fission-track annealing. Chlorine measurements in apatite have conventionally required electron probe microanalysis (EPMA). Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is increasingly used in apatite fission-track dating to determine U concentrations and also in simultaneous U-Pb dating and trace element measurements of apatite. Apatite Cl measurements by ICP-MS would remove the need for EPMA but the high (12.97 eV) first ionisation potential makes analysis challenging. Apatite Cl data were acquired using two analytical set-ups: a Resonetics M-50 193 nm ArF Excimer laser coupled to an Agilent 7700× quadrupole ICP-MS (using a 26 μm spot with an 8 Hz repetition rate) and a Photon Machines Analyte Excite 193 nm ArF Excimer laser coupled to a Thermo Scientific iCAP Qc (using a 30 μm spot with a 4 Hz repetition rate). Chlorine concentrations were determined by LA-ICP-MS (1140 analyses in total) for nineteen apatite occurrences, and there is a comprehensive EPMA Cl and F data set for 13 of the apatite samples. The apatite sample suite includes different compositions representative of the range likely to be encountered in natural apatites, along with extreme variants including two end-member chlorapatites. Between twenty-six and thirty-nine isotopes were determined in each apatite sample corresponding to a typical analytical protocol for integrated apatite fission track (U and Cl contents) and U-Pb dating, along with REE and trace element measurements. 35Cl backgrounds (present mainly in the argon gas) were ~ 45-65 kcps in the first set-up and ~ 4 kcps in the second set-up. 35Cl background-corrected signals ranged from ~ 0 cps in end-member fluorapatite to up to ~ 90 kcps in end-member chlorapatite. Use of a collision cell in both analytical set-ups decreased the low mass sensitivity by approximately an order of magnitude without improving the 35Cl signal-to-background ratio. A minor Ca isotope was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. The 35Cl/43Ca values for each apatite (10-20 analyses each) when plotted against the EPMA Cl concentrations yield excellently constrained calibration relationships, demonstrating the suitability of the analytical protocol and that routine apatite Cl measurements by ICP-MS are achievable. © 2013 International Association of Geoanalysts.

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