Deconvolution of fission-track length distributions and its application to dating and separating pre- and post-depositional components

To enable the separation of pre- and postdepositional components of the length distribution of (partially annealed) horizontal confined fission tracks, the length distribution is corrected by deconvolution. Probabilistic least-squares inversion corrects natural track length histograms for observational biases, considering the variance in data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can exist with or without measurements of angles to the c axis. In the latter case, 3D histograms are introduced as an alternative to histograms of c-axis-projected track lengths. Thermal history modelling of samples is not necessary for the calculation of track age distributions of corrected tracks. In an example, the age equations are applied to apatites with predepositional (inherited) tracks in order to extract the postdepositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post-depositional tracks. This complicates the calculation of the post-sedimentary thermal history, as the grains have experienced different thermal histories prior to deposition. Thereafter, the grains share a common thermal history. Thus, the extracted post-depositional histogram without inherited tracks may be used for thermal history calculation.

Role of thermal history on atomic structure and ductility of ion-irradiated metallic glasses

Maximizing the structural rejuvenation and ductility is one of the most heated debates in the field of metallic glasses (MGs). In this work, molecular dynamics (MD) simulation was implemented to model the ion irradiation effects on the Cu60Zr40 MG with different thermal histories and varied structural heterogeneities. The initial results indicated that the performance of an annealing-quench treatment on the MG induces the atomic configurations with different heterogeneities and potential energy values. The subsequent ion irradiation process also demonstrated that an optimized atomic structure was occurred for achieving maximum rejuvenation and ductility in the CuZr glassy alloy. It was unveiled that the intermediate initial heterogeneity provides an efficient pathway for maximizing the atomic rearrangements under the ion irradiation. It was also suggested that the medium population of Cu-centered clusters in the initial state facilitated the atomic rearrangements during the ion irradiation process. The structural characteristics and atomic reconfigurations for attaining the optimum ductility is discussed in details.

Thermo-tectonic studies of Mesozoic basement rocks, North Island, New Zealand

<p>The basement rocks of North Island, New Zealand, comprise metasedimentary terranes that were accreted onto the eastern Gondwana margin during Mesozoic subduction. Since the Oligocene, these terranes have been sitting at the leading edge of the Australian Plate, as the hanging wall of the Hikurangi subduction margin, overriding the subducting Pacific Plate. This thesis examines the thermo-tectonic histories of the basement rocks in North Island, using fission-track and (U-Th-Sm)/He thermochronology. In eastern North Island, thermochronological data from the basement rocks record the exhumation histories since the latest Jurassic, related to two subduction cycles. Zircon fission-track analysis yields detrital or slightly reset ages (264–102 Ma); apatite fission-track ages range from 122 to 7.9 Ma and (U-Th-Sm)/He from 33.3 to 6.0 Ma. In central North Island, modelled thermal histories suggest that the basement rocks were exhumed to shallow levels (<2 km) of the crust in the Early Cretaceous (~150–135 Ma). This was followed by a period of reheating until ~100 Ma, which is interpreted to be the result of burial by sedimentation above the accretionary wedge. From 100 Ma, models indicate thermo-tectonic quiescence until the Late Oligocene. During the late Cenozoic, exhumation of the basement rocks accelerated at ~27 Ma in the western margin of the axial ranges (Kaimanawa Mountains). This acceleration in exhumation rate is interpreted to reflect the initiation of the subduction of the Pacific Plate beneath central North Island. Since the Late Oligocene, basement exhumation in the axial ranges migrated towards the trough. Modelled thermal histories indicate significant eastwards reverse faulting on the margin-parallel Ngamatea Fault between ~27 and 20 Ma and on the Wellington-Mohaka Fault between ~20 and 10 Ma. In contrast to the activity in the axial ranges, in western North Island, the exhumational response of the basement rocks to the Cenozoic subduction was less significant and not revealed from the present thermochronological data. Since the Late Miocene, the exhumation rate in the axial ranges has varied significantly along-strike, lower in the centre and higher to the north and south. During the last 10 Myr, the total magnitude of exhumation has been ~4 km in the Wellington region in the south, >1 km in the Raukumara Range in the north and negligible (less than a few hundred metres) in the central axial ranges in the Hawke’s Bay region. Although the accumulation of underplated material at the basal upper plate may have contributed to the localised rock uplift and exhumation (e.g. in the Raukumara Range), margin-normal shortening of the upper plate in the forearc of the Hikurangi Margin has most likely dominated the unroofing process of the axial ranges. In northwestern North Island, the Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto in situ Mesozoic and early Cenozoic rocks. Detrital zircon and apatite fission-track ages reveal that the basal Northland Allochthon sequences and the underlying Miocene autochthonous sedimentary rocks were predominantly derived from the local Jurassic terrane (Waipapa Supergrop) and perhaps the Late Cretaceous volcanics. In addition, the Early Miocene autochthon contains significant sedimentary influx from the Late Oligocene volcanics related to the subduction initiation in northern New Zealand. Zircon and apatite fission-track data from the in situ Mesozoic basement were inverted using thermo-kinematic models coupled with an inversion algorithm. The results suggest that during the Late Oligocene, ~4–6 km thick nappes were emplaced onto the in situ rocks in the northernmost Northland region. Prior to basement unroofing in the Early Miocene, the nappes thinned towards the south. Following allochthon emplacement, eastern Northland was uplifted and unroofed rapidly over a period of ~1–6 Myr, leading to ~0.4–1.5 km erosion of the allochthon. Since the mid-Miocene, due to the decline in tectonic activity in this region, the Northland Allochthon and the underlying rocks have been eroded slowly. This thesis has documented variable exhumation and burial processes that occurred in the upper plates of both the Mesozoic Gondwana and late Cenozoic Hikurangi subduction margins. The results provide the foundation for future studies to investigate the kinematics and mechanism of the crustal exhumation and deformation of the North Island basement in further detail.</p>

Thermo-tectonic studies of Mesozoic basement rocks, North Island, New Zealand

<p>The basement rocks of North Island, New Zealand, comprise metasedimentary terranes that were accreted onto the eastern Gondwana margin during Mesozoic subduction. Since the Oligocene, these terranes have been sitting at the leading edge of the Australian Plate, as the hanging wall of the Hikurangi subduction margin, overriding the subducting Pacific Plate. This thesis examines the thermo-tectonic histories of the basement rocks in North Island, using fission-track and (U-Th-Sm)/He thermochronology. In eastern North Island, thermochronological data from the basement rocks record the exhumation histories since the latest Jurassic, related to two subduction cycles. Zircon fission-track analysis yields detrital or slightly reset ages (264–102 Ma); apatite fission-track ages range from 122 to 7.9 Ma and (U-Th-Sm)/He from 33.3 to 6.0 Ma. In central North Island, modelled thermal histories suggest that the basement rocks were exhumed to shallow levels (<2 km) of the crust in the Early Cretaceous (~150–135 Ma). This was followed by a period of reheating until ~100 Ma, which is interpreted to be the result of burial by sedimentation above the accretionary wedge. From 100 Ma, models indicate thermo-tectonic quiescence until the Late Oligocene. During the late Cenozoic, exhumation of the basement rocks accelerated at ~27 Ma in the western margin of the axial ranges (Kaimanawa Mountains). This acceleration in exhumation rate is interpreted to reflect the initiation of the subduction of the Pacific Plate beneath central North Island. Since the Late Oligocene, basement exhumation in the axial ranges migrated towards the trough. Modelled thermal histories indicate significant eastwards reverse faulting on the margin-parallel Ngamatea Fault between ~27 and 20 Ma and on the Wellington-Mohaka Fault between ~20 and 10 Ma. In contrast to the activity in the axial ranges, in western North Island, the exhumational response of the basement rocks to the Cenozoic subduction was less significant and not revealed from the present thermochronological data. Since the Late Miocene, the exhumation rate in the axial ranges has varied significantly along-strike, lower in the centre and higher to the north and south. During the last 10 Myr, the total magnitude of exhumation has been ~4 km in the Wellington region in the south, >1 km in the Raukumara Range in the north and negligible (less than a few hundred metres) in the central axial ranges in the Hawke’s Bay region. Although the accumulation of underplated material at the basal upper plate may have contributed to the localised rock uplift and exhumation (e.g. in the Raukumara Range), margin-normal shortening of the upper plate in the forearc of the Hikurangi Margin has most likely dominated the unroofing process of the axial ranges. In northwestern North Island, the Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto in situ Mesozoic and early Cenozoic rocks. Detrital zircon and apatite fission-track ages reveal that the basal Northland Allochthon sequences and the underlying Miocene autochthonous sedimentary rocks were predominantly derived from the local Jurassic terrane (Waipapa Supergrop) and perhaps the Late Cretaceous volcanics. In addition, the Early Miocene autochthon contains significant sedimentary influx from the Late Oligocene volcanics related to the subduction initiation in northern New Zealand. Zircon and apatite fission-track data from the in situ Mesozoic basement were inverted using thermo-kinematic models coupled with an inversion algorithm. The results suggest that during the Late Oligocene, ~4–6 km thick nappes were emplaced onto the in situ rocks in the northernmost Northland region. Prior to basement unroofing in the Early Miocene, the nappes thinned towards the south. Following allochthon emplacement, eastern Northland was uplifted and unroofed rapidly over a period of ~1–6 Myr, leading to ~0.4–1.5 km erosion of the allochthon. Since the mid-Miocene, due to the decline in tectonic activity in this region, the Northland Allochthon and the underlying rocks have been eroded slowly. This thesis has documented variable exhumation and burial processes that occurred in the upper plates of both the Mesozoic Gondwana and late Cenozoic Hikurangi subduction margins. The results provide the foundation for future studies to investigate the kinematics and mechanism of the crustal exhumation and deformation of the North Island basement in further detail.</p>

Tension testing of additively manufactured specimens of 17-4 PH processed by Bound Metal Deposition

In contrast to the traditional ways of subtractive manufacturing, additive manufacturing (AM), also known as 3D printing, adapts computer-aided design to iteratively build the component or part layer by layer. The technology has recently gained a high momentum, both within academia, but also within the industrial sector. However, it is common that parts produced by AM will have more defects than parts produced by traditional methods. The objective of this paper is to investigate a new method of additive manufacturing, namely the bound metal deposition method (BMD). This method seemed promising from the perspective that the metal is not iteratively being melted, similar to such as welding. In fact, the part is first printed, then washed, for then to be sintered. Consequently, avoiding the complex thermal histories/cycles. It was found that the material will exhibit anisotropic behaviour, and have a mesh of crack like defects, related to the printing orientation.

Thermochronologic constraints on the origin of the Great Unconformity

The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation of more than 3–4 km during Cryogenian (717–635 Ma) snowball Earth glaciations, or alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1000–540 Ma) due to plate tectonic reorganization from supercontinent Rodinia assembly and breakup. To test these hypotheses, the temporal pattern of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium-to-low temperature thermochronology and geologic information. We present inverse time-temperature simulations within a Bayesian modelling framework that record a consistent signal of relatively rapid, high magnitude cooling of ~120–200°C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3–5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents.

Optimization of Interfacial Energy for Langer-Schwartz Based Precipitation Simulations

Precipitation kinetics were investigated in select Fe, Ni, and Al alloys using a CALPHAD based precipitation model based on Langer-Schwartz theory. Thermodynamic and kinetic data are taken from commercially available CALPHAD software, but reliable interfacial energy data for precipitates needed for the calculations is often lacking. While models exist to approximate these interfacial energies, this study has focused on deriving more reliable estimates by comparison with experimental data. By performing simulations with thermal histories, nucleation sites, and precipitate morphologies that closely replicate experimental data found in literature, the interfacial energies were optimized until volume fraction and mean radius values closely matched the published data. Using this technique, interfacial energy values have been determined for carbides in Grade 22 low alloy steels, delta phase in Ni 625 and 718, SPhase in Al 2024, and Q’ and β’’ in Al 6111, and can be used for future predictive precipitation simulations.

Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA

The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth’s history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (~100 km) and short (tens of kilometers) spatial wavelengths. The spatially variable thermal histories implied by these data are best explained by Proterozoic syndepositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge. The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a “Great Unconformity.”

Simulating sedimentary burial cycles – Part 2: Elemental-based multikinetic apatite fission-track interpretation and modelling techniques illustrated using examples from northern Yukon

Compositionally dependent apatite fission track (AFT) annealing is a common but underappreciated cause for age dispersion in detrital AFT samples. We present an interpretation and modelling strategy that exploits multikinetic AFT annealing to obtain thermal histories that can provide more detail and better resolution compared to conventional methods. We illustrate our method using a Permian and a Devonian sample from the Yukon, Canada, both with complicated geological histories and long residence times in the AFT partial annealing zone. Effective Cl values (eCl; converted from rmr0 values), derived from detailed apatite elemental data, are used to define AFT statistical kinetic populations with significantly different total annealing temperatures (~110–245 °C) and ages that agree closely with the results of age mixture modelling. These AFT populations are well-resolved using eCl values but exhibit significant overlap with respect to the conventional parameters, Cl content or Dpar. Elemental analyses and measured Dpar for Phanerozoic samples from the Yukon and Northwest Territories confirm that Dpar has low precision and that Cl content alone cannot account for the compositional and associated kinetic variability observed in natural samples. An inverse multikinetic AFT model, AFTINV, is used to obtain thermal history information by simultaneously modelling multiple kinetic populations as distinct thermochronometers with different temperature sensitivities. A nondirected Monte Carlo scheme generates a set of statistically acceptable solutions at the 0.05 significance level and then these solutions are updated to the 0.5 level using a controlled random search (CRS) learning algorithm. The smoother, closer-fitting CRS solutions allow for a more consistent assessment of the eCl values and thermal history styles that are needed to satisfy the AFT data. The high-quality Devonian sample (39 single grain ages and 202 track lengths) has two kinetic populations that require three cycles of heating and cooling (each subsequent event of lower intensity) to obtain close-fitting solutions. The younger and more westerly Permian sample with three kinetic populations only records the latter two heating events. These results are compatible with known stratigraphic and thermal maturity constraints and the QTQt software produces similar results. Model results for these and other samples suggest that elemental-derived eCl values are accurate within the range, 0–0.25 apfu (rmr0 values of 0.73–0.84), which encompasses most of the data from annealing experiments. Outside of this range, eCl values for more exotic compositions may require adjustment relative to better constrained apatite compositions when trying to fit multiple kinetic populations. Our results for natural and synthetic samples suggest that an element-based multikinetic approach has great potential to increase the temperature range and resolution of thermal histories dramatically relative to conventional AFT thermochronology.