Application of (U-Th)/He thermochronometry as a geothermal exploration tool in extensional tectonic settings: the Wassuk Range, Hawthorne, Nevada

Navy Geothermal Program Office at China Lake, C

Characteristics of pelleted wheat middlings that affect summer storage

Pelleted wheat middlings samples were collected from four Kansas flour mills in March, April, and May, 1997 to characterize their moisture content and bulk density as they would be purchased directly from the mills by a livestock producer. The average moisture content of pelleted wheat middlings was 14% as they left the mills but declined during the spring to 13.6%. Pellets purchased from Kansas mills during the summer months are likely to contain 13.0 to 13.5% moisture. The average bulk density was approximately 40 lb/ft3 , which is equivalent to about 50 lb/bu. Based on the equilibrium moisture contents determined from the collected samples, if air at typical Kansas summertime temperatures is above 65% relative humidity, pellets will absorb moisture during storage

Halogen (F, Cl, Br, and I) Devolatilization During Prograde Subduction: Insights From Western Alps Ophiolites

In order to examine the progressive chemical evolution of halogens (F, Cl, Br, I) in altered ocean crust (AOC) during prograde subduction, this study compares bulk and in situ halogen concentrations in mafic samples from three petrogenetically related exhumed terrains in the Western Alps (the Chenaillet ophiolite, the Queyras ophiolites of the Schistes Lustrés, and the Monviso ophiolite). Samples from the Chenaillet ophiolite represent oceanic crust unaffected by metamorphic halogen loss and define a protolith halogen content (122 μg/g F, 29 μg/g Cl, 82 ng/g Br, and 98 ng/g I). Samples from the Queyras ophiolites experienced blueschist facies conditions, undergoing recrystallization and halogen loss (74 μg/g F, 19 μg/g Cl, 70 ng/g Br, and 63 ng/g I). Eclogite facies samples from the Monviso meta-ophiolite exhibit markedly reduced Cl (8 μg/g Cl) and Br (42 ng/g Br) contents relative to samples from Chenaillet and Queyras. Using electron probe microanalysis (EPMA), F and Cl host minerals (e.g., amphibole, chlorite, epidote) are identified and characterized in selected samples, showing a broad distribution of F and Cl, lending support to the view that halogen devolatilization in the subducting slab occurs continuously and is not dependent on the breakdown of a particular phase. In situ Cl concentrations decrease significantly between sub-greenschist and blueschist assemblages. Fluorine is retained within subducting AOC and is decoupled from the heavy halogens (Cl, Br, I), which undergo continuous devolatilization during prograde metamorphism

Middle to Late Miocene Extremely Rapid Exhumation and Thermal Reequilibration in the Kung Co Rift, Southern Tibet

The Kung Co rift is an approximately NNW striking, WSW dipping normal fault exposed in southern Tibet and is part of an extensive network of active approximately NS striking normal faults exposed across the Tibetan Plateau. Detailed new and published (U-Th)/He zircon and apatite thermochronometric data from the footwall of the early Miocene Kung Co granite provide constraints on the middle Miocene to present-day exhumation history of the footwall to the Kung Co fault. Inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating (1) initiation of normal fault slip at ∼12–13 Ma and rapid exhumation of the footwall between ∼13 and 10 Ma, (2) acceleration of normal fault slip at rates of 21.9–6.9 mm/yr at ∼10 Ma, (3) rapid thermal reequilibration between 10 and 9 Ma, and (4) slow exhumation and/or quiescence from ∼9 Ma to the present day. Hanging glacial valleys in the footwall and fault scarps that cut late Quaternary till and moraine deposits indicate that fault slip continues today. Middle to late Miocene initiation of extension across the Kung Co rift is broadly the same as the documented initiation of EW extension across the south central Tibetan Plateau. Eastward flow of middle or lower crust from beneath Tibet accommodated by northward underthrusting of Indian crust beneath Tibet provides a plausible explanation for the onset of EW extension across the Tibetan Plateau

Velocity Dependence Of One- And Two-electron Processes In Intermediate-velocity Ar16++He Collisions

We report investigations of one- and two-electron processes in the collisions of 0.9-keV/u to 60-keV/u (vp=0.19-1.55 a.u.) Ar16+ ions with He targets. The cross sections for these processes were measured by observing the final charges of the Ar ions and the recoiling target ions in coincidence. The average Q values for the capture channels were determined by measuring the longitudinal momenta of the recoiling target ions. Single capture (SC) is the dominant process and is relatively independent of the projectile energy. The two-electron transfer-ionization (TI) process is the next largest and slowly increases with projectile energy. The Q values for both SC and TI decrease with increasing projectile energy. Our data thereby suggest that electrons are captured into less tightly bound states as the collision velocity is increased. Both double capture and single ionization are much smaller and fairly independent of the projectile energy. The energy independence of SI is somewhat surprising as our energy range spans the region of the target electron velocity where ionization would be expected to increase. Our analysis suggests that the ionization process is being suppressed by SC and TI processes. © 1993 The American Physical Society

Resonant Electron Transfer And Excitation In Two-, Three-, And Four- Electron Caq +20 And Vq +23 Ions Colliding With Helium

Significant new evidence is reported for resonant transfer and excitation in ion-atom collisions. This process, which is analogous to dielectronic recombination, occurs when a target electron is captured simultaneously with the excitation of the projectile followed by photon emission. Strong resonant behavior with structure, in agreement with theoretical calculations, is observed in the cross section for projectile K x rays coincident with single electron capture for 100-360-MeV Ca16+,17+,18+20 and 180-460-MeV V19+,20+,21+23 ions colliding with helium. © 1984 The American Physical Society

Constraining Deformation in the North Pamir and the Westernmost Tarim Basin

Abstract HKT-ISTP 2013 A

Regional Pliocene exhumation of the Lesser Himalaya in the Indus drainage

New bulk sediment Sr and Nd isotope data, coupled with U–Pb dating of detrital zircon grains from sediment cored by the International Ocean Discovery Program in the Arabian Sea, allow the reconstruction of erosion in the Indus catchment since ∼17&thinsp;Ma. Increasing εNd values from 17 to 9.5&thinsp;Ma imply relatively more erosion from the Karakoram and Kohistan, likely linked to slip on the Karakoram Fault and compression in the southern and eastern Karakoram. After a period of relative stability from 9.5 to 5.7&thinsp;Ma, there is a long-term decrease in εNd values that corresponds with increasing relative abundance of &gt;300&thinsp;Ma zircon grains that are most common in Himalayan bedrocks. The continuous presence of abundant Himalayan zircons precludes large-scale drainage capture as the cause of decreasing εNd values in the submarine fan. Although the initial increase in Lesser Himalaya-derived 1500–2300&thinsp;Ma zircons after 8.3&thinsp;Ma is consistent with earlier records from the foreland basin, the much greater rise after 1.9&thinsp;Ma has not previously been recognized and suggests that widespread unroofing of the Crystalline Lesser Himalaya and to a lesser extent Nanga Parbat did not occur until after 1.9&thinsp;Ma. Because regional erosion increased in the Pleistocene compared to the Pliocene, the relative increase in erosion from the Lesser Himalaya does not reflect slowing erosion in the Karakoram and Greater Himalaya. No simple links can be made between erosion and the development of the South Asian Monsoon, implying a largely tectonic control on Lesser Himalayan unroofing.</p

Late Cenozoic Foreland-to-Hinterland Low-Temperature Exhumation History of the Kashmir Himalaya

New apatite and zircon (U-Th)/He cooling ages quantify late Cenozoic exhumation patterns associated with fault activity across the Kashmir Himalaya. Apatite (U-Th)/He (AHe) cooling ages of detrital grains from the Sub-Himalayan foreland sediments indicate significant resetting. AHe data and thermal modeling reveal cooling and exhumation initiated by 4Ma at the deformation front and by 2-4Ma throughout other Sub-Himalayan structures. Exhumation rates for Sub-Himalayan structures are 1mm/year. In the hinterland, thrust sheet samples from the Main Boundary thrust and Main Central thrust yield AHe cooling ages between 5.1 and 21.1Ma. Published apatite fission track cooling ages (<3Ma) and high exhumation rates (3.6-3.2mm/year) across the Kishtwar window further to the north are consistent with AHe data from the Sub-Himalayan structures. The pattern of cooling ages and rates indicates that exhumation occurs in association with changes in the Himalayan basal decollement ramp geometry. Hinterland zircon (U-Th)/He (ZHe) data show a pronounced abundance and probability spike in cooling ages between 14 and 21Ma, a period when Main Central thrust motion is well documented throughout the Himalaya. ZHe single-grain ages from Sub-Himalayan samples contain a nearly identical cluster from 16 to 23Ma. Cooling patterns across the Kashmir Himalayas do not correlate spatially with modern monsoon precipitation, suggesting that climate-related precipitation and exhumation are decoupled. Coeval translation over the basal decollement and distributed imbricate thrust deformation of the foreland in the upper plate characterizes fault-related exhumation of the Sub-Himalayan orogenic belt after 4Ma. Our new data document the timing of cooling of rocks brought to the surface during mountain building of the Kashmir Himalaya. Mineral grains eroded from the Himalaya and deposited in the plain are now exposed in the Sub-Himalayan belt. The ages of these rocks that we have measured constrain the timing of burial and subsequent return to the Earth's surface (exhumation) during thrust fault-related deformation. Analysis of apatite grains reveals that cooling and exhumation initiated by 4Ma on the southernmost structure of the Kashmir Himalaya and by 2-4Ma on other distributed faults in the Sub-Himalayan belt. In the core of the mountain range rocks have young cooling ages (<3Ma) related to high uplift rates within the Kishtwar window, a zone of localized deformation in the High Himalaya. Thus, outward growth of the Sub-Himalayan belt occurred in concert with uplift in the hinterland over the past 4Myr. Precipitation rates vary systematically from south to north across the Himalaya, but these variations are not synchronous with the pattern of cooling and exhumation in Kashmir Himalaya. This result suggests that climate does not drive crustal deformation. Instead exhumation patterns primarily reflect the location, geometry, and partitioning of faulting within the Himalaya