The metamorphic history of exhumed high-grade rocks provides invaluable insight into the thermomechanical processes of subduction zones. While subduction in most orogens has been terminated by continent collision entailing variably strong overprint of related units, the Franciscan Complex of California allows studying a >150 Myr long subduction history that started at ~175 Ma and ended by transformation into a transform plate boundary (San Andreas fault) without significant metamorphic overprint. The highest grade metamorphic rocks of the Franciscan Complex of California are found as blocks in serpentinite and shale matrix mélanges. They include amphibolites, eclogites, blueschists, and blueschist facies metasediments. These Franciscan mélanges inspired the subduction channel return-flow model, but other processes e.g., buoyancy-driven serpentinite diapirism have been argued to be concordant with our current understanding of their metamorphic history, too. We investigate a suite of metabasite blocks from serpentinite and shale matrix mélanges of the Califonia Coast Ranges. Our new dataset consists of U-Pb dates of metamorphic zircon and 40Ar/39Ar dates of calcic amphibole and white mica. Combined with published geochronology, particularly prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. We find: (i) Exhumation from the eclogite-amphibolite facies occurred only in a short episode at 165­–160 Ma with an apparent southward younging trend. (ii) Exhumation of the blocks was uniform and fast in the eclogite-amphibolite facies with rates of 2–8 km/Myr. In the blueschist facies exhumation of the blocks was less uniform and slowed by an order of magnitude. (iii) The age of amphibole in a metasomatic reaction zone indicates that at least one amphibolite was enclosed in a serpentinite matrix by ~155 Ma. Considering the entire subduction zone system, the high-grade exhumation temporally correlates with a significant pulse of magmatism in the respective magmatic arc (Sierra Nevada) and termination of forearc spreading (Coast Range Ophiolite). Our findings do not support a steady-state process that is continuously exhuming high-grade rocks. Instead the subduction zone system changed with an eventlike character resulting in exhumation of high-grade rocks enclosed in serpentinite.

We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40Ar/39Ar dates of amphibole and phengite and U‐Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu‐Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole‐eclogite facies conditions occurred as a single episode at 165–157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to ~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one‐time event led to exhumation of amphibole‐eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist‐facies metamorphism of the Red Ant schist of the Sierra Nevada foothills.

Neogene, syn-collisional extensional exhumation of Asian lower-middle crust produced the Shakhdara-Alichur gneiss-dome complex in the South Pamir. The <1 km-thick, mylonitic-brittle, top-NNE, normal-sense Alichur shear zone (ASZ) bounds the 125 x 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km-thick, mylonitic-brittle, top-SSE South Pamir normal-sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22-17 Ma Alichur-dome-injection-complex leucogranites transition from foliation-parallel, centimeter- to meter-thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization-temperature constraints suggest Alichur-dome exhumation from 10-15 km depth. Thermochronologic dates bracket footwall cooling between 410-130 degrees C from 16-4 Ma; tectonic cooling/exhumation rates (42 degrees C/Myr, 1.1 km/Myr) contrast with erosion-dominated rates in the hanging wall (2 degrees C/Myr, <0.1 km/Myr). Dome-scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E-W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between 20-4 Ma to: (i) Mostly northward, foreland-directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non-underthrust Central Pamir.

The 35 Cl(n,p) and 35 Cl(n, ↵ ) cross sections at incident neutron energies between 2.42 - 2.74 MeV, were measured using the Berkeley High Flux Neutron Generator. The cross sections for 35 Cl(n,p) were more than a factor of three to five less than all of the values in the neutron absorption data libraries, while the 35 Cl(n, ↵ ) cross sections are in reasonable agreement with the data libraries. The measured energy-di ↵ erential cross section is consistent with a single resonance with a width of 293(46) keV. This result suggests that, despite the high incident neutron energy, any attempt to model (n,x) cross sections in the vicinity of the N = Z = 20 shell gap requires a resolved resonance approach rather than Hauser-Fesbach.

We designed and tested a compact deuteron-deuteron fusion neutron generator for application to 40Ar/39Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 39K(n,p)39Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 40Ar/39Ar dating.

The Sava-Klepa Massif represents an approximately 5 x 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times. In order to strengthen one or the other model, we concentrate on the Sava-Klepa Massif enclosing Paleozoic basement rocks, which mainly consist of a layered sequence of metamorphic marbles and mica schists. The metamorphic basement shows deformation structures clearly related to the Klepa event. Our results are based on paleostress and paleostrain analysis of twinned calcite from deformed marbles, and field observations, micro structural analysis, and 40Ar/39Ar dating of white mica. Paleostress and paleostrain analysis was enhanced by using an automated fabric analyser microscope, and developing the herein presented freeware standalone Windows (R) executable PACT software (Paleostress Analysis with Calcite Twins). Our results support the hypothesis that the Sava-Klepa Massif was formed during transtension as a pull-apart basin in the Late Cretaceous. This could have happened either after the collision of the European and Adriatic plates, or along a transtentional zone in the fore-arc region during subduction of the last Tethyan oceanic lithosphere. The Sava-Klepa Massif represents an approximately 5 x 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times. In order to strengthen one or the other model, we concentrate on the Sava-Klepa Massif enclosing Paleozoic basement rocks, which mainly consist of a layered sequence of metamorphic marbles and mica schists. The metamorphic basement shows deformation structures clearly related to the Klepa event. Our results are based on paleostress and paleostrain analysis of twinned calcite from deformed marbles, and field observations, micro structural analysis, and 40Ar/39Ar dating of white mica. Paleostress and paleostrain analysis was enhanced by using an automated fabric analyser microscope, and developing the herein presented freeware standalone Windows (R) executable PACT software (Paleostress Analysis with Calcite Twins). Our results support the hypothesis that the Sava-Klepa Massif was formed during transtension as a pull-apart basin in the Late Cretaceous. This could have happened either after the collision of the European and Adriatic plates, or along a transtentional zone in the fore-arc region during subduction of the last Tethyan oceanic lithosphere. The Sava-Klepa Massif represents an approximately 5 x 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times. In order to strengthen one or the other model, we concentrate on the Sava-Klepa Massif enclosing Paleozoic basement rocks, which mainly consist of a layered sequence of metamorphic marbles and mica schists. The metamorphic basement shows deformation structures clearly related to the Klepa event. Our results are based on paleostress and paleostrain analysis of twinned calcite from deformed marbles, and field observations, micro structural analysis, and 40Ar/39Ar dating of white mica. Paleostress and paleostrain analysis was enhanced by using an automated fabric analyser microscope, and developing the herein presented freeware standalone Windows (R) executable PACT software (Paleostress Analysis with Calcite Twins). Our results support the hypothesis that the Sava-Klepa Massif was formed during transtension as a pull-apart basin in the Late Cretaceous. This could have happened either after the collision of the European and Adriatic plates, or along a transtentional zone in the fore-arc region during subduction of the last Tethyan oceanic lithosphere.

Use of deuteron-deuteron fusion neutrons may substantially improve applicability and accuracy of the 40Ar/39Ar technique. We designed and tested a compact deuteron-deuteron fusion neutron generator for application to 40Ar/39Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 39K(n,p)39Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 40Ar/39Ar dating.

A next-generation, high-flux DD neutron generator has been designed, commissioned, and characterized, and is now operational in a new facility at the University of California Berkeley. The generator, originally designed for 40Ar/39Ar dating of geological materials, has since served numerous additional applications, including medical isotope production studies, with others planned for the near future. In this work, we present an overview of the High Flux Neutron Generator (HFNG) which includes a variety of simulations, analytical models, and experimental validation of results. Extensive analysis was performed in order to characterize the neutron yield, flux, and energy distribution at specific locations where samples may be loaded for irradiation. A notable design feature of the HFNG is the possibility for sample irradiation internal to the cathode, just 8 mm away from the neutron production site, thus maximizing the neutron flux (n/cm(2)/s). The generator's maximum neutron flux at this irradiation position is 2.58 x 10(7) n/cm(2)/s +/- 5% (approximately 3 x 10(8) n/s total yield) as measured via activation of small natural indium foils. However, future development is aimed at achieving an order of magnitude increase in flux. Additionally, the deuterium ion beam optics were optimized by simulations for various extraction configurations in order to achieve a uniform neutron flux distribution and an acceptable heat load. Finally, experiments were performed in order to benchmark the modeling and characterization of the HFNG.

The 35 Cl(n,p) and 35 Cl(n, ↵ ) cross sections at incident neutron energies between 2.42 - 2.74 MeV, were measured using the Berkeley High Flux Neutron Generator. The cross sections for 35 Cl(n,p) were more than a factor of three to five less than all of the values in the neutron absorption data libraries, while the 35 Cl(n, ↵ ) cross sections are in reasonable agreement with the data libraries. The measured energy-di ↵ erential cross section is consistent with a single resonance with a width of 293(46) keV. This result suggests that, despite the high incident neutron energy, any attempt to model (n,x) cross sections in the vicinity of the N = Z = 20 shell gap requires a resolved resonance approach rather than Hauser-Fesbach.

The Cadmium‐Lined Outer‐Core Irradiation Tube (CLOCIT) is a new irradiation facility for 40Ar/39Ar geochronology at the Oregon State University TRIGA® reactor. We report fluence (i.e., time‐integrated flux) parameters from the first four CLOCIT irradiations and compare them with the existing Cadmium‐Lined Inner‐Core Irradiation Tube (CLICIT). CLOCIT provides an average neutron flux equivalent of 1.45–1.53 × 10−4 J h−1; about 55% of CLICIT. Radial fluence gradients were on the order of 0.2–4.2% cm−1. A planar fit of J‐values results in residuals in the range of uncertainty in the J‐value, but systematic deviations resolve a non‐planar component of the neutron flux field, which has also been observed in CLICIT. Axial neutron fluence gradients were 0.6–1% cm−1, compared with 0.7–1.6% cm−1 for the CLICIT. Production rate ratios of interfering reactions were (40Ar/39Ar)K = (4 ± 6) × 10−4 and (38Ar/39Ar)K = (1.208 ± 0.002) × 10−2, (36Ar/37Ar)Ca = (2.649 ± 0.014) × 10−4, (38Ar/37Ar)Ca = (3.33 ± 0.12) × 10−5 and (39Ar/37Ar)Ca = (9.1 ± 0.28) × 10−4, similar to the CLICIT values.

Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over 30km of crustal section during the India-Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7-10km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90km. Two thrust sheets, comprising Cambro-Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite-facies metamorphic rocks within the domesequivalent to lower grade rocks outside the domesform fold nappes with dome-scale wavelengths. E-W stretching occurred contemporaneously with top-to-N imbrication and folding. At 22-12Ma, bivergent (top-to-N and top-to-S), normal-sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at 12Ma with opposite-sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant N-S shortening interacted with E-W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibetan Plateau and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India-Asia collision, and the Miocene-Recent dextral shear and E-W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.

Cross sections for the Ti-47(n,p)Sc-47 and Zn-64(n,p)Cu-64 reactions have been measured for quasimonoenergetic DD neutrons produced by the UC Berkeley High Flux Neutron Generator (HFNG). The HFNG is a compact neutron generator designed as a "flux-trap" that maximizes the probability that a neutron will interact with a sample loaded into a specific, central location. The study was motivated by interest in the production of Sc-47 and Cu-64 as emerging medical isotopes. The cross sections were measured in ratio to the In-113(n,n')In-113m and In-115(n,n')In-115m inelastic scattering reactions on co-irradiated indium samples. Post-irradiation counting using an HPGe and LEPS detectors allowed for cross section determination to within 5% uncertainty. The Zn-64(n,p)Cu-64 cross section for 2.76(-0.02)(+0.01) MeV neutrons is reported as 49.3 +/- 2.6 mb (relative to In-113) or 46.4 +/- 1.7 mb (relative to In-115), and the Ti-47(n,p)Sc-47 cross section is reported as 26.26 +/- 0.82 mb. The measured cross sections are found to be in good agreement with existing measured values but with lower uncertainty (<5%), and also in agreement with theoretical values. This work highlights the utility of compact, flux-trap DD-based neutron sources for nuclear data measurements and potentially the production of radionuclides for medical applications. (C) 2017 Elsevier B.V. All rights reserved.

Large domes of crystalline, middle to deep crustal rocks of Asian provenance make the Pamir a unique part of the India-Asia collision. Combined major-element and trace element thermobarometry, pseudosections, garnet-zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15-20km at 37-27Ma. The Central Pamir was then heated at similar to 10-20 degrees C/Myr and buried at 1-2km/Myr to 600-675 degrees C at depths of 25-35km by 22-19Ma. The Shakhdara Dome in the South Pamir was heated at similar to 20 degrees C/Myr and buried at 2-8km/Myr to reach 750-800 degrees C at depths of 50km by similar to 20Ma. All domes were exhumed at >3km/Myr to 5-10km depths and similar to 300 degrees C by 17-15Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large-scale, normal-sense shear zones, and with advection of near-solidus or suprasolidus temperatures into the upper crust, triggering exhumation-related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large-scale thickening and coincident similar to 20Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate-scale events. Plain Language Summary In the Pamir we can see deep into the crust of the India-Asia collision, something that cannot be done elsewhere. By measuring and interpreting mineral compositions, we determine that the Central Pamir was heated at similar to 10-20 degrees C/Myr and buried at 1-2km/Myr to 600-675 degrees C at depths of 25-35km by 22-19Ma. The South Pamir was heated at similar to 20 degrees C/Myr and buried at 2-8km/Myr to reach 750-800 degrees C at depths of similar to 55km by 20Ma. All domes were exhumed at >3km/Myr to 10-15km depths and 300 degrees C by 17-15Ma.

The accuracy of 40Ar/39Ar geochronology relies in large part on precise and accurate calibration of the ages and K-Ar isotopic compositions of standards. A widely used standard for Quaternary samples, the ∼1.2 Ma Alder Creek sanidine (ACs), has published ages spanning a range of ∼2%. New measurements of ACs co-irradiated with the Fish Canyon sanidine (FCs) standard and sanidines from astronomically dated Miocene tuffs in Crete and Morocco yield results that enable both (i) a direct calibration of ACs relative to FCs, and (ii) stepwise calibrations between these two standards employing the Miocene intermediaries. Results are summarized by the parameter R F C s A C s , defined as the ratio of (40Ar*/39ArK) of ACs to FCs, which embodies the fundamental age relationship between these standards that is independent of systematic variables such as decay constants or absolute ages of standards. Our new measurements, executed using three mass spectrometers and various irradiation and analytical protocols, yield a weighted mean R F C s A C s = 0.041702 ± 0.000014 (σ). This result can be combined with previously published determinations of R values for ACs relative to the Miocene tuffs and to FCs to yield a recommended interlaboratory value of R F C s A C s = 0.041707 ± 0.000011. The weighted-mean age of ACs using this interlaboratory value, based on astronomically-calibrated ages of FCs and the Miocene intermediary sanidines, is tACs = 1.1848 ± 0.0006 Ma (±0.05%). Applying this result to the most precise published 40Ar/39Ar data for the Matuyama-Brunhes geomagnetic polarity reversal yields tMBB = 780.1 ± 0.8 ka. In addition, these new data for ACs support previous conclusions that U-Pb zircon ages from the Alder Creek rhyolite incorporate ∼13 ka of pre-eruptive residence time.

Various interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed with the significant ones being routinely corrected for in 40Ar/39Ar geochronology. Interference of double charged 80Kr (80Kr++) with 40Ar has not yet been considered. Significant amounts of 80Kr are produced during neutron irradiation through the reaction 79Br(n,β−)80Kr. While previous workers reported a computed fission spectrum averaged cross section of ∼48mb—compared to ∼113mb for 39K(n,p)39Ar—we determined a ∼33-fold higher production rate of 80Kr from Br compared to 39Ar from K in the CLICIT facility of the OSU reactor. Low-K, high-Br phases, e.g., some amphiboles, and fluid or melt inclusion rich samples are prone to 80Kr++ interference biasing 40Ar/39Ar dates in the ‰ to % range. 80Kr++ resembles excess 40Ar in step-heating experiments. The interference can be corrected for by using the parallel reaction 81Br(n,β−)82Kr.

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before 35Ma and peaked at 23-20Ma, reflecting top-to-N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper 7-10km of the Asian crust. Multimethod thermochronology traces cooling through 700-100 degrees C between 22 and 12Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at 22-17Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at 3km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7-31 degrees C/Myr) starting between 23 and 12Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at 23-21Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at 12Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g.,slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.