How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry: Reply

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A combination of apatite fission track and (U-Th)/He thermochronometers to constrain the escarpment evolution in south eastern Australia: a case study of high elevation passive margins

In this project apatite fission track and (U-T)/He thermochronometers are used to determine the cooling history of rocks from the coastal (south eastern New South Wales) and the interior (Bathurst area) regions of the eastern Australia high elevation passive margin. Two traverses across the coastal lowlands, escarpment and plateau top are used to determine the tempo and styles of response of the landscape to the continental breakup and sea-floor spreading of the Tasman Sea (85 Ma). The three prevailing models of escarpment evolution namely retreat into a downwarped rift shoulder, escarpment retreat and excavation in place on a high elevation rift shoulder with flexural rebound are described and tested using a previously untested combination of apatite fission track and (U/Th)/He data. The thermochronological data indicate that the coast was affected by a denudational pulse that peaked around 120-100 Ma and that was extinguished by the time of sea-floor spreading. The rapid denudational event caused the removal of 3-4.5 km (depending on the geothermal gradient) of crust at the coast and of approximately 2 km at the present base of the escarpment. The thermochronological data are inconsistent with the downwarped rift shoulder model and the apatite (U-Th)/He data indicate that, while the coast was denuded very rapidly, the coastal lowlands were excavated in place at a much lower pace, and the escarpment reached its present position no later than 60 Ma. This suggests that during continental extension and breakup, rates of denudation at the coast were approximately 80-30m/Myr (depending on the geothermal gradient), whereas at the base of the present escarpment they were about 10-5 m/Myr. The period after sea-floor spreading was characterised by stability and low rates of erosion. The pre-breakup topography, reconstructed using the backstacking technique, is characterised by a considerable relief in the area of the present escarpment. This result confirms the hypothesis that the escarpment evolved pinned to its present position

From source to sink in central Gondwana: Exhumation of the Precambrian basement rocks of Tanzania and sediment accumulation in the adjacent Congo basin

Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000 m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200 m below surface. Surface samples were also analyzed. AFT dates range between 317 ± 33 Ma and 188 ± 44 Ma. Alpha (Ft)-corrected AHe dates are between 433 ± 24 Ma and 154 ± 20 Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340–220 Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70 Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9 ± 2 km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes

How local crustal thermal properties influence the amount of denudation derived from low temperature thermochronometry

ranslating amounts and rates of rock cooling derived from low-temperature thermochronometry into denudation requires assumptions about the local geothermal gradient. The temperature gradient in the crust depends on many factors, including basal heat flow, crustal heat production, and thermal conductivity. Consequently, geothermal gradients may be variable on time scales over which rock cooling is tracked by thermochronometry. Using one-dimensional numerical modeling of heat transfer in rocks of varying thermal characteristics, we show that the geothermal gradient of the eroded layer is the most important factor for accurate estimation of denudation amounts. Using a three-dimensional numerical model (Pecube), we demonstrate the impact of crustal heat production and thermal conductivity on estimates of total denudation derived from apatite fission track data from central west Britain. We show that the regional variation in cooling ages measured in Caledonian granites can be explained by geothermal gradient variation due to the presence of a heat-producing granite batholith and removal of insulating sedimentary rocks, and does not require variable denudation. Neglecting the blanketing effect leads to twofold overestimation of the amount of denudation. The occurrence of heat-producing basement that was once covered by a sedimentary blanket is common, in particular in the core of mountain belts. Accurate determination of the amount and rate of denudation from thermochronometric studies in these situations must take into account the composition of the eroded rocks

The chronology and tectonic style of landscape evolution along the elevated Atlantic continental margin of South Africa resolved by joint apatite fission track and (U-Th-Sm)/He thermochronology

Atlantic-type continental margins have long been considered “passive” tectonic settings throughout the entire postrift phase. Recent studies question the long-term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U-Th-Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 µm, and mean (U-Th-Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130 Ma and 110–90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1–3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle-driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km

Rock Around the University - Transplanted Rock Exposures for On-campus Geoscience field Skills Training

Rock Around the University (RAU) is a teaching resource made up of 16 large (~2.5m) blocks of “local” Scottish rock which have been transplanted and orientated into carefully planned locations and elevations between the buildings of the University of Glasgow to look like natural exposures. RAU mimics a real-life fieldwork experience, on-campus, with the aim of enhancing the learning experience of undergraduate geoscience students. RAU allows progressive, reflective, and effective on-campus outdoor training of a wide-range of geological field skills and concepts, including: the description, analysis and measurements of rock features and structures; geological mapping; the use of structure contours to predict geological boundaries in terrains lacking abundant exposures; construction of cross-sections; and, the interpretation and reconstruction of 3D structure and geological history. Students visit the RAU exposures both during timetabled supervised ‘lab’ sessions and in their own time, providing an authentic fieldwork experience in a controlled location where key geological skills can be developed at the optimal rate for individual students. Being located on the campus means that there are no travel or expenses for students, fewer timetabling issues, and fewer general logistical complications and natural complexities than in remote fieldwork locations. In addition, students benefit from receiving ‘instant’ on-site feedback from staff on the challenges, problems and pedagogic issues that they encounter. RAU allows us to introduce rigorous field-based teaching at an early stage in geoscience courses and to stimulate and encourage reflective learning. Students locate, analyse and synthesise information in the field to provide effective solutions to problems and use RAU as a self-directed learning experience where they build confidence while working independently in a familiar environment. Hence the students reinforce their field skills before experiencing independent work in remote areas. In effect RAU uses the campus as a sustainable geoscience teaching resource. Experiences with all levels of undergraduate students over the eight years since RAU was established at the University of Glasgow have demonstrated that this on-campus resource is an ideal complement to the traditional programme of fieldwork classes. Students are much better prepared for their first major residential fieldwork having completed the RAU programme, and are much more confident in their field skills. RAU has allowed us to address more effectively the disconnect between laboratory and fieldwork skills, and remote fieldwork classes are now more focussed on the application, rather than the development, of field skills. RAU has also had the effect of enhancing the awareness of geoscience among the entire University community, due to the presence of students carrying out fieldwork on campus. Rock around the University is also used in recruitment and outreach, and is open to schools, amateur geoscientists, and anyone interested in Earth history. Printed leaflets are available and more information is available at https://www.gla.ac.uk/schools/ges/community/rockaround/

Validation of an elastic registration technique to estimate anatomical lung modification in Non-Small-Cell Lung Cancer Tomotherapy

<p>Abstract</p> <p>Background</p> <p>The study of lung parenchyma anatomical modification is useful to estimate dose discrepancies during the radiation treatment of Non-Small-Cell Lung Cancer (NSCLC) patients. We propose and validate a method, based on free-form deformation and mutual information, to elastically register planning kVCT with daily MVCT images, to estimate lung parenchyma modification during Tomotherapy.</p> <p>Methods</p> <p>We analyzed 15 registrations between the planning kVCT and 3 MVCT images for each of the 5 NSCLC patients. Image registration accuracy was evaluated by visual inspection and, quantitatively, by Correlation Coefficients (CC) and Target Registration Errors (TRE). Finally, a lung volume correspondence analysis was performed to specifically evaluate registration accuracy in lungs.</p> <p>Results</p> <p>Results showed that elastic registration was always satisfactory, both qualitatively and quantitatively: TRE after elastic registration (average value of 3.6 mm) remained comparable and often smaller than voxel resolution. Lung volume variations were well estimated by elastic registration (average volume and centroid errors of 1.78% and 0.87 mm, respectively).</p> <p>Conclusions</p> <p>Our results demonstrate that this method is able to estimate lung deformations in thorax MVCT, with an accuracy within 3.6 mm comparable or smaller than the voxel dimension of the kVCT and MVCT images. It could be used to estimate lung parenchyma dose variations in thoracic Tomotherapy.</p

Quantifying Holocene Relative Sea-level Changes and Paleoclimate Using the Scottish Speleothem Record

Speleothems, secondary cave carbonates, are valuable archives for reconstructing paleoclimate and relative sea-level changes where the caves are in coastal locations. Unlike the typical speleothems found in carbonate caves, speleothems were recently discovered in a meta-silicate sea cave in Iona, on the west coast of Scotland. Although speleothems have previously been reported from caves in volcanic rocks, speleothems in metamorphic caves have rarely been reported. The Iona speleothems are potentially crucial because paleoclimate reconstructions spanning the Holocene are scarce in Scotland due to a lack of material, particularly speleothems, which can be dated precisely using geochemical dating methods. In this research, the U-Th and 14C dating techniques will be used to constrain the precise age and growth history of the Iona speleothem. Results from pilot U-Th dating of the first speleothem sampled show it is about 1760 ~ 4780 years old (the data, however, have uncertainties up to 69.9%, due to the presence of non-authigenic Th). As for paleoclimate, oxygen isotopes indicate that the amount of precipitation was at a relatively low level between 3000 and 2000 years ago, then increased dramatically from ~2000 to 1760 years ago. These preliminary data indicate that the Iona speleothem has the potential to provide important insights into the Late Holocene relative sea-level changes and climate

Natural age dispersion arising from the analysis of broken crystals, part II. Practical application to apatite (U-Th)/He thermochronometry

We describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite (U-Th)/He ages. The approach explicitly exploits the information about the shape of the 4He diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals. Additional dispersion arising from differences in grain size and in U and Th concentration of grains, and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage, is explicitly included. In this approach we calculate the ingrowth and loss, due to both thermal diffusion and the effects of α-ejection, of helium over time using a finite cylinder geometry. Broken grains are treated explicitly as fragments of an initially larger crystal. The initial grain lengths, L0, can be treated as unknown parameters to be estimated, although this is computationally demanding. A practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long L0 value to each fragment. We found that a good value for L0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments. Currently whole crystals and fragments with one termination are taken into account. A set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories, and including realistic amounts of random noise (5-15%), are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion. These include experiments where dispersion arises only from fragmentation of a single grain (length 400μm and radius 75μm, c. 6-50% dispersion), including the effects of grain size variation (for spherical equivalent grain radii between 74-122 μm, c. 10-70% dispersion) and the combined effects of fragmentation, grain size and radiation damage (for eU between 5-150 ppm, c.10-107% dispersion). Additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases. The viability of the new technique is demonstrated for a real data set of 25 single grain (U-Th)/He apatite ages obtained for a gabbro sample from the BK-1 (Bierkraal) borehole drilled through the Bushveld Complex in South Africa. The inversion produces a well constrained thermal history consistent with both the (U-Th)/He data and available fission track analysis data. The advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes, such as the effects of temporally variable diffusivity, zonation of U and Th and arbitrary grain size variations, and it works equally effectively for whole or broken crystals, and for the most common situation where a mixture of both are analysed. For the routine application of the apatite (U-Th)/He thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15-20 single grain analyses are typically required to characterise the age dispersion pattern of a sample. The experiments also suggest that picking very short crystal fragments as well as long fragments, or even deliberately breaking long crystals to maximise the age dispersion in some cases, would ensure the best constraints on the thermal history models. The inversion strategy described in this paper is likely also directly applicable to other thermochronometers, such as the apatite, rutile and titanite U-Pb systems, where the diffusion domain is approximated by the physical grain size

Growing topography due to contrasting rock types in a tectonically dead landscape

Many mountain ranges survive in a phase of erosional decay for millions of years (Myr) following the cessation of tectonic activity. Landscape dynamics in these post-orogenic settings have long puzzled geologists due to the expectation that topographic relief should decline with time. Our understanding of how denudation rates, crustal dynamics, bedrock erodibility, climate, and mantle-driven processes interact to dictate the persistence of relief in the absence of ongoing tectonics is incomplete. Here we explore how lateral variations in rock type, ranging from resistant quartzites to less-resistant schists and phyllites and up to the least-resistant gneisses and granitic rocks, have affected rates and patterns of denudation and topographic forms in a humid semitropical, high-relief, post-orogenic landscape in Brazil where active tectonics ended hundreds of Myr ago. We show that denudation rates are negatively correlated to topographic relief, channel steepness and modern precipitation rates. Denudation instead correlates with inferred bedrock strength, with resistant rocks denuding more slowly relative to more erodible rock units, and suggest that the efficiency of fluvial erosion varies primarily due to these bedrock differences. Variations in erodibility continue to drive contrasts in rates of denudation in a tectonically inactive landscape evolving for hundreds of Myr, suggesting that equilibrium is not a natural attractor state and that relief continues to grow through time. Over the long timescales of post-orogenic development, exposure at the surface of rock types with differential erodibility can become a dominant control on landscape dynamics by producing spatial variations in geomorphic processes and rates, promoting the survival of relief, and determining spatial differences in erosional response timescales long after cessation of mountain building