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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.

Deep (> 500 m below ground) geothermal energy is generated by heat sources within the Earth, including unusually high lithospheric basal heat flow and/or intrusive bodies rich in radioactive isotopes, that heat the surrounding rocks and aquifers. This warm water can then be used for electricity production or to provide heat for buildings. These relatively high geothermal gradients can be found at depth in sedimentary basins where aquifers are surrounded by rocks with low thermal conductivity. Investigating the suitability of a basin for deep geothermal energy exploration requires, therefore, a thorough geological investigation of its spatially variable structure, stratigraphy and evolution. Low temperature thermochronology, namely apatite fission track and (U-Th-Sm)/He methods, are able to reconstruct the thermal structure of the shallow crust through time and, when data are available from boreholes, to quantify the evolution of the geothermal gradient, providing insights on the most promising areas where aquifers could be unusually warm. We have applied low temperature thermochronology to the study of the Midland Valley (MV) Basin, an extensive sedimentary basin onshore Scotland, hosting many potential energy consumers in the cities of Glasgow and Edinburgh. The Midland Valley mainly consists of alternating succession of sandstone and siltstone with mudstone, limestone and coal, predominantly of Carboniferous and Devonian age. The MV also experienced folding and faulting throughout its geological history; therefore, the succession is spatially highly variable, difficult to reconstruct by simply using the sparse borehole-derived stratigraphic constraints. Apatite fission track data from across the eastern sector of the basin and the UK Geoenergy Observatories borehole in Glasgow indicate a 1) rapid burial in the Carboniferous-Permian; 2) Permian-Mesozoic cooling and a 3) a relatively rapid early Cenozoic cooling, an event that is asynchronous across the basin. Using a combination of forward and inverse modelling techniques, we constrain the palaeo-geothermal gradients and highlight areas where the thermal structure of the shallow crust could still be relatively hot for aquifer geothermal energy.

Wetlands represent an excellent example of the interface between lithosphere, atmosphere, and hydrosphere. Despite being distributed throughout the planet, the pace of their destruction has increased, especially since the 1970s. In tropical regions, they may be associated with floodplains or topographical depressions permanently or temporarily flooded, with different size configurations and hydrogeomorphological dynamics. Among the major river systems in South America, the São Francisco river basin, located entirely in Brazilian territory, has strategic importance in providing energy and water resources to the semi-arid northeast region, with about 30 million people. In it, there are small wetlands, considered in extension, but with great frequency, located in areas of headwaters and floodplains, mainly. In this basin, the Pantanal of the Pandeiros watershed synthesizes the main environmental impacts are existing in the São Francisco River basin. For example, high frequency of accelerated linear water erosion processes, known locally as voçoroca (gullies), deforestation since the 1970s, which continues for the production of charcoal, the presence of a small hydroelectric plant, the reduction of water flow since the 1990s and the intense dynamics of sediment transport. In the São Francisco river basin, there is an abundance of rocks and regolith formed by siliciclastic material, where their contribution to the dynamics of wetlands is still not well understood. The Pantanal of the Pandeiros wetland located in the floodplain of São Francisco River has a singular occurrence. Where it is at risk of lose to provide environmental services, through silting, the Pantanal of the Pandeiros River has complex hydrogeomorphological dynamics from the delimitation of a depressed area concerning its surroundings, marked by slope rupture, with a contribution to its flooding, the increase, and maintenance of the São Francisco river level during flood pulses, determining longitudinal dynamics in the wetland. Were mapped four different hydrogeomorphologic zones from a multiproxy approach: Isotopic tracers 2H and 18O, together with piezometers, demonstrate connections between the wetland and the floods of the Rivers São Francisco and the Pandeiros, strongly influenced by past and current geomorphological forms and processes. Raster images of digital elevation models and satellite, during flood pulses, plus ground-penetrating radar and analyses of geocoverages records ranging from environments predominantly sandy to clayey from upstream to downstream, respectively. Levels sandy material up to 3m deep, followed by a layer of pebbles and paleochannels. Sediments collected up to 150 cm deep date back to 3,246 years B.P., with a predominance of humid environment, via analysis of 14C and 13C. The average denudation rate in the last 1.5 million years, using 10Be, a high contribution of sediments were identified, which circulate in the basin, coming from specific regions, where erosive processes are predominant, such as gullies.

Many mountain ranges survive in a phase of erosional decay for millions of years 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 subtropical, high-relief post-orogenic landscape in Brazil where active tectonics ended hundreds of millions of years ago. We show that catchment-averaged denudation rates are negatively correlated with mean values of 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 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 millions of years, 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.

The Midland Valley Basin of Scotland (MVS) is a major NE-SW trending, fault-bounded sedimentary basin in central Scotland, UK, comprised predominantly of Carboniferous and Devonian sedimentary rocks. Changing palaeo-environments of the MVS produced alternating successions of sandstone, siltstone, mudstone, limestone and coal. The MVS also experienced folding, fault inversion and development of a widespread unconformity during the latest Carboniferous culmination of the Variscan Orogeny and minor tectonic events thereafter. The MVS’ geological resources played a major role in driving Scotland’s economic, industrial, and cultural development in the 19th - 20th. The region was heavily exploited for coal and hydrocarbon energy resources and material for construction, manufacturing, and agriculture. The MVS basin remains as relevant in the 21st century having been identified as a viable source of low-carbon geo-energy resources (e.g., geothermal energy) and potential for subsurface energy storage (Heinemann et al., 2019). While the geology of the MVS has been well-studied, thermal and burial history reconstructions have typically relied on techniques focused on the maturation of organic matter (e.g., vitrinite reflectance, VR), which lack quantitative information on timing. Moreover, tracing sediment provenance can be challenging but crucial for understanding the tectonic evolution of the surrounding source region. Here, we present the results of a geochronological and thermochronological investigation of the MVS basin designed to better understand sediment pathways to the basin from surrounding upland regions and the post-depositional thermal history of the MVS. Our data includes zircon and apatite U-Pb data and apatite fission-track (AFT) data from across the basin and AFT data from a UK Geoenergy Observatories borehole in Glasgow. With our U-Pb data, we identify distant source areas in Greenland, more local source areas in the Scottish Highlands, and recycling of older sedimentary rocks and reworked material in the basin that change through the tectono-magmatic evolution of the basin. Our AFT data and associated thermal history modelling identify three main thermal events: i) Carboniferous-Permian heating; ii) Permian-Mesozoic cooling, and iii) relatively rapid Cenozoic cooling (McKenna, 2021; Hattie, 2021). These are attributed to post-Carboniferous burial followed by post-Permian exhumation. However, ambiguity in some of our models suggests some heating in the Mesozoic may have occurred and, due to the limitations on the temperature sensitivity of the AFT technique, the timing and rate of Cenozoic cooling is poorly resolved. Through our modelling we explore the influence changing palaeo-geothermal gradients has on our thermal history and whether the lower temperature thermochronometer apatite (U-Th)/He can better resolve the most-recent cooling event. Heinemann, N., Alcalde, J., Johnson, G., Roberts, J. J., McCay, A. T., & Booth, M. G. (2019). Low-carbon GeoEnergy resource options in the Midland Valley of Scotland, UK. Scottish Journal of Geology, 55(2), 93-106. McKenna, Eamon (2021) The Provenance and thermal histories of the Carboniferous Midland Valley of Scotland, PhD thesis, University of Glasgow. Hattie, Andrew (2021) Constraining the post-burial history of the central Midland Valley of Scotland using apatite fission track analysis: implications for geothermal energy. MSc(R) thesis, University of Glasgow.

Much of the interior of the Tibetan Plateau is characterized by internal drainage, low relief topography, and high altitude. How and when this landscape formed is controversial. In this study, we use new zircon U-Pb data and low-temperature thermochronological data (apatite and zircon [U-Th/He], apatite fission track [AFT]) from the Late Cretaceous to Cenozoic Nima Basin sedimentary rocks and Xiabie granite in the adjacent Muggar Thrust hanging wall (part of the regional Shiquanhe-Gaize-Amdo thrust system), to determine the paleodrainage and timing of exhumation in the region. Individual AHe and ZHe cooling ages range from 9 to 60 Ma and 58 to 118 Ma, and the AFT ages range from 30 to 90 Ma. The thermal history derived from the Northern Nima Basin sediments and Xiabie granite require a period of exhumation between 70 and 40 Ma in the thrust fault hanging wall, and 40 and 30 Ma in the Nima Basin. Across the region, this event was followed by low rates of exhumation and the deposition of locally sourced sediment, lacustrine, and evaporitic deposits that are indicative of an internal drainage system. We suggest that the exhumation event is associated with development of thrust-elevated relief that may have disrupted the drainage network favoring the development of an endorheic system. This system, sediment accumulation, and/or post-30 Ma tectonic quiescence led to the generation of low relief topography.

The presence of wetlands as a result of local fluvial and hydrological conditions constitutes a frequently observed feature of such rivers. Therefore, they are important elements of the basin, because besides functioning as buffer zones for CO2 and sediments they also house important ecosystems, playing an important role in the control of water circulation. Brazilian wetlands have different typologies and sizes, varying from huge swamplands such as the Pantanal do Mato Grosso, to flooded savannas called “veredas” or oxbow lakes. Their distribution in inland areas depends on the variety of flood pulses mainly linked to seasonality with the presence of distinct dry and wet seasons (Junk et al., 1989). This strong seasonality affects the São Francisco River (SFR), the 4th largest river in Brazil, which has frequent marginal lakes and swamps as it passes through five Brazilian states. This research aims to analyze the effect of the variation of the SFR level from 1925 to 2018, on the flow of the Pandeiros River which is one of many tributaries on the left side of SFR and on its wetland (“Pantanal Mineiro”). This wetland is hydrogeomorphologically linked to the SFR and receives water inputs during SFR flood periods. Measurements of the SFR water level performed once daily in the morning were obtained from gauging station no 44200000 belonging to the Companhia de Pesquisa de Recursos Minerais (CPRM) [altitude 445 m; 15°56'57.84"S; 44°52'4.68"W. The hydrological year starts at the end of the dry season on October 1st. Time series analyses (level duration curve, Seasonal Trend Decomposition (STL) of the daily level data, monthly level, mean, maximum, minimum level for each day of the year) were conducted to describe the hydrological regime and to assess temporal changes of the SFR levels and how these affect the magnitude, frequency and duration of flooding of the Pandeiros’s River wetland. Field observations (March 14, 2018) show that when SFR, which is Pandeiro’s base level, reaches a level of 5.0 m this leads to flooding conditions of the Pandeiros River wetland. Over the full period of record (1925-2018) the average level of the SFR was 3.86 m, with a minimum annual average of 2.43 m during the dry season (winter) and maximum of 5.98 m during the wet season (summer), with an average annual range of 3.55 m between both seasons. The SFR was above the 5.0 m threshold flooding level for 20% of the time 1925-2018, which corresponds to an average of 77.8 days of flooding per year in the wetland. The longest period of inundation was 178 days in 1926, when the SFR reached its maximum recorded level, and the shortest was 1 day in 2015, when it reached its minimum. The number of days per year of inundation have decreased over the full record, but that this is mainly due to a significant decrease since 1985. Prior to this, cyclic differences between wetter (1925 and 1985) and drier periods (1925 to 1945, 1945 to 1965) are observed.

Knowledge of the geological history of the Tibetan plateau is critical to understanding crustal deformation process, and the plateau’s influence on climate. However, the timing of Tibetan plateau development remains controversial. The Nima Basin along the Jurassic-Cretaceous Bangong Suture in central Tibet provides well-dated records of exhumation in this area. Here, we present detrital zircon U-Pb, apatite U-Th/He (AHe) and apatite fission track data (AFT) from upper Cretaceous and Oligocene red sandstones and conglomerates in the Nima Basin, as well as from the Xiabie granite in the hanging wall of the basin-bounding Muggar Thrust. 4 granite conglomerate clasts from the above yield zircon U-Pb ages ranging between 114-122 Ma, which likely come from the Xiabie granite. 7 granitoid/sandstone conglomerate clasts yield AHe ages ranging from 21-58 Ma, while AFT ages range from 34-83 Ma. Thermal history inversion modelling for five of the above samples show a consistent rapid cooling from 100 ℃ to 30 ℃ between 50-40 Ma, the cooling rate decreased significantly after 40 Ma. Implications of these data, integrated in the context of previously published data for the wider region (e.g. Rohrmann et al. 2012; Haider et al., 2013; Li et al., 2019) will be discussed. ReferenceRohrmann, A et al., 2012, Thermochronologic evidence for plateau formation in central Tibet by 45 Ma: Geology, v. 40, p. 187-190.Haider, V. L et al., 2013, Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co, Tibetan Plateau: Journal of Asian Earth Sciences, v. 70-71, p. 79-98.Li, H. A et al., 2019, The formation and expansion of the eastern Proto-Tibetan Plateau: Insights from low-temperature thermochronology: Journal of Asian Earth Sciences, v. 183, 103975.

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/ .

Upwelling mantle plumes beneath continental crust are predicted to produce difficult to quantify, modest uplift and denudation. The contribution of permanent and transient components to the uplift is also difficult to distinguish. A pulse of denudation in Britain in the Early Paleogene has been linked, although with some controversy, with the arrival of the proto-Iceland mantle plume. In this contribution we show that combining apatite and zircon (U-Th-Sm)/He and apatite fission track analyses from central west Britain with numerical modeling clearly identifies a pulse of early Cenozoic denudation. The data indicate that rock uplift and denudation were centered on the northern East Irish Sea Basin and 1.0–2.4 km of rocks were removed during the latest Cretaceous-early Paleogene. Uplift and erosion appears to have started a few million years before the earliest magmatism in the region. The regional denudation pattern mirrors the distribution of low-density magmatic rocks that has been imaged in the deep crust. However, the injection of the underplating melt is not enough to account for the total denudation. An additional regional uplift of at least 300 m is required, which is consistent with a transient thermal effect from the hot mantle plume. The rapid exhumation event ceased by ~40 Ma and the data do not require significant Neogene exhumation.

The timing and mechanisms involved in creating the elevated, low-relief topography of the South African plateau remains unresolved. Here we constrain the thermal history of the southwest African plateau since 300 Ma using apatite fission track (AFT) and (U-Th-Sm)/He (AHe) thermochronology. Archaean rocks from the centre of the Kaapvaal Craton yield AFT ages of 331.0 ± 11.0 and 379.0 ± 23.0 Ma and mean track lengths (MTL) of 11.9 ± 0.2 and 12.5 ± 0.3 µm. Towards the southwest margin of the craton and in the adjacent Palaeozoic mobile belt, AFT ages are significantly younger and range from 58.9 ± 5.9 to 128.7 ± 6.3 Ma, and have longer MTLs (>13 µm). The range of sampleAHe ages complements the AFT ages and single grain AHe ages for most samples are highly dispersed. Results from joint inverse modelling of these data reveal that the centre of the craton has resided at near surface temperatures (

Gondwana (Tauride/Kirsehir blocks) and Eurasia (Pontides) derived continental blocks delimit the Haymana basin, central Turkey, to the south and the north, respectively. The boundaries of these blocks define the Izmir-Ankara-Erzincan and Intra-Tauride Suture zones which are straddled by a number of Late Cretaceous to Oligocene marine to continental basins. The Haymana Basin is located at the junction of the IAESZ and ITSZ and comprises Upper Cretaceous to Middle Eocene basin infill deposited in response to the interaction of these blocks. The basin provides a unique opportunity to unravel spatio-temporal relationships related to the timing of late stage subduction history of Neo-Tethys Ocean and subsequent collision of the intervening continental blocks. We have conducted a multidisciplinary study in the region that includes mapping of major structures combined with fault kinematic analyses. E-W striking folds dominate the basin, cross-section balancing of these structures indicates around 25% roughly N-S shortening in the region. Paleostress studies indicate that the basin was initially subjected to N-S to NNE-SSW extension until the middle Paleocene (phase 1) and then N-S directed syn-depositional compression and coeval E-W directed extension until the middle Miocene (phase 2) implying strike-slip deformation and pure shear shortening in the basin. These different deformation phases are attributed to first fore-arc (subduction) basin development then foreland (collision) stages of the basin. Apatite (U-Th)/He dating of 5 samples indicate that exhumation of the SE segment of the basin started in early Oligocene, whereas the NW segment of the basin exhumed in the early Miocene. The differential uplift is possibly related to progressive north-westwards movement of Derekoy basin bounding fault at the north. We propose that the Haymana basin evolved from extensional forearc basin during the late Cretaceous to early Paleocene and foreland basin after the terminal subduction and subsequent collision of Tauride and Pontide blocks.

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 a.s.l.), 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 (DRC) 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 analysed. 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. Modelling 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-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.

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 ± 3.6Ma, 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 ± 31.4Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130Ma and 110–90Ma 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.

This study tests alternative models for the growth of the Tatry Mountains (Central Western Carpathians) by the application of low temperature thermochronology. Zircon (U + Th)/He ages from the north of the range are mostly between 48 to 37 Ma and indicate cooling prior to the onset of forearc sedimentation in the region (42–39 Ma). In contrast, zircon (U + Th)/He ages in the south of the range are around 22 Ma. Apatite fission track ages across the sampled sites range from 20 to 15 Ma. Apatite (U + Th)/He ages range from 18 to 14 Ma with little variation with elevation or horizontal location. Based on thermal modelling and tectonic reconstructions, these Miocene ages are interpreted as cooling in the hanging-wall of a northward dipping thrust ramp in the current location of the sub-Tatric fault with cooling rates of ~20 °C/Myr at ~22-14 Ma. Modeled cooling histories require an abrupt deceleration in cooling after ~14 Ma to

Over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite (U–Th)/He thermochronometry and it is now standard practice, and generally seen as best practice, to analyse single grain aliquots. These individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite. This is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains, and evidence of freshly broken ends when grains are viewed using a scanning electron microscope. This matters because if the 4 He distribution within the whole grain is not homogeneous, because of partial loss due to thermal diffusion for example, then the fragments will all yield ages different from each other and from the whole grain age. Here we use a numerical model with a finite cylinder geometry to approximate 4 He ingrowth and thermal diffusion within hexagonal prismatic apatite crystals. This is used to quantify the amount and patterns of inherent, natural age dispersion that arises from analysing broken crystals. A series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios. In addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets. The results indicate that natural dispersion of a set of single fragment ages (defined as the range divided by the mean) arising from fragmentation alone varies from c. 7% even for rapid (c. 10 °C/Ma), monotonic cooling to over 50% for protracted, complex histories that cause significant diffusional loss of 4 He. The magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius, and is of a similar magnitude to dispersion expected from differences in absolute grain size alone (spherical equivalent radii of 40–150 μm). This source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses (c. 6%) and standard deviations on multiple grain analyses from a single sample (c. 10–20%). Where there is a significant difference in the U and Th concentration of individual grains (eU), the effect of radiation damage accumulation on 4 He diffusivity (assessed using the RDAAM model of Flowers et al. (2009) ) is the primary cause of dispersion for samples that have experienced a protracted thermal history, and can cause dispersion in excess of 100% for realistic ranges of eU concentration (i.e. 5–100 ppm). Expected natural dispersion arising from the combined effects of reasonable variations in grain size (radii 40–125 μm), eU concentration (5–150 ppm) and fragmentation would typically exceed 100% for complex thermal histories. In addition to adding a significant component of natural dispersion to analyses, the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration (eU). Considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic. An important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 He distribution within the whole grains, thus providing an important source of information about the thermal history of the sample. We suggest that if the primary focus of a study is to extract the thermal history information from (U–Th)/He analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages, not minimise it, and should aim to analyse circa 20–30 grains from each sample. The key observations and conclusions drawn here are 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.

The question of the persistence of topography is not just a line in one of the most famous songs ever written, it is also one of the long-standing questions in geomorphology. Mountains created by tectonic processes hundreds of millions of years ago still stand proud in the landscape, posing intriguing questions of how topography can persist for such a long time, and if the mountains we see today are the result of more recent processes, and therefore not a direct relict of the original topography. Although models have been proposed to explain the antiquity of topography, the only way to test these models is by providing denudation rates over millions of year time scales. Such data can be uniquely provided by low temperature thermochronometers. In this paper we provide a brief review of these techniques and an example from western Scotland where the thermochronometers have been applied to determine the age of first order topography. The results indicate that although denudation rates have been temp...

Pseudotachylyte is the only fault rock that is known to form exclusively at seismic slip rates, so it is unique in preserving direct evidence of the dynamic processes in action during earthquakes. It is commonly assumed that pseudotachylyte is rare, and debate has centered on whether it is rarely generated or commonly generated but rarely preserved. We present field and electron microscope observations of eight new pseudotachylytes from faults in the Sierra Nevada that have previously been the focus of many detailed studies of fault growth and mechanics. These pseudotachylytes range from being abundant and easy to recognize in outcrop to being impossible to identify without microscope observations. Our data show that pseudotachylytes are much more common in the Sierra Nevada than has previously been reported. We suggest that pseudotachylytes may be present within many fault zones but remain unreported primarily due to difficulty in identifying very thin or reworked pseudotachylytes in the field; and therefore the use of these fault rocks to interpret dynamic earthquake processes must be revisited.

Apatite (U–Th)/He and fission track data from profiles in western Scotland constrain the timing and magnitude of denudation during the early Cenozoic when the north Atlantic region was the site of intense magmatic activity related to the proto-Icelandic plume. Apatite helium ages vary from 77 ± 8 to 265 ± 27 Ma (± 2σ) at Sgorr Dhonuill, Ballachulish, and from 104 ± 10 Ma to 166 ± 17 Ma at Clisham, Outer Hebrides. At both locations apatite fission track (AFT) ages are older than the corresponding He ages; at Clisham they vary from 189 ± 28 Ma to 242 ± 26 Ma, and from 186 ± 6 Ma to 257 ± 12 Ma at Sgorr Dhonuill. Apatite He ages increase linearly with elevation suggesting that the cooling rate remained constant in the late Mesozoic. However, the apatite He age profile requires a period of rapid cooling after ∼ 100 Ma. Apatite He ages predicted from the AFT-derived thermal histories are indistinguishable from measured He ages for a rapid cooling event of 1 to 10 Myr duration between 61 and 47 Ma at Sgorr Dhonuill and 65 to 49 Ma at Clisham. The combined apatite FT- and He-derived thermal histories constrain the early Cenozoic geothermal gradient at 39 ± 9 °C/km at Sgorr Dhonuill and 19 ± 6 °C/km at Clisham. Amounts of denudation related to the rapid cooling event vary from 1330 ± 230 m at Sgorr Dhonuill to 2250 ± 750 m at Clisham, in agreement with models that predict greater amounts of denudation where magmatic underplating is thicker. However, the direct correlation between underplating-driven surface uplift and denudation may only be apparent and a more complex link between spatial variation of surface uplift and denudation is suggested. The integration of results from multiple low-temperature thermochronometers, combined with inverse and forward modelling, provides a convincing and quantitative method to deduce onshore erosional histories, and provides critical information about the spatial distribution of erosion that cannot be derived from the offshore record.

The present study aims at testing the possibility of using the in-situ cosmogenic burial dating technique on deltaic deposits. The sequence analyzed is exposed along the Ligurian coast (north-west Italy) and is made of proximal marine and continental deposits previously considered Pliocene or Plio-Quaternary in age. In the study area two allostratigraphic units were recognized. The lower unit represents the evolution of a small coarse-grained delta developed in a fjord or embayment environment. The coarsening/shallowing upward trend observed within the sections, from bottom to top, suggests that the delta prograded rapidly in the landward portion of the canyon placed opposite to the paleo-river outlet. Within the deltaic sequence the transgressive and highstand system tracts were recognized. The unit 2 is composed by several alluvial fan systems deposited in small incised valleys developed within the previously, uplifted deltaic deposits and successively incised by a braided river system. In-situ produced cosmogenic nuclides were used in order to date the age of the deposition of the deltaic deposits. Results suggest that the studied deltaic sediments belonging to the unit 1 were deposited between 1,300,000 and 200,000 year ago thus during the Lower to Middle Pleistocene, whereas the unit 2 was deposited during the Middle Pleistocene as a consequence of a tectonically driven uplift phase. Furthermore samples collected within the prograding part of the delta show the higher denudation rates. The obtained results demonstrate that burial ages and related erosion rates inferred from cosmogenic nuclides concentrations can be considered as a very useful tool to reconstruct the sea level changes over the past 1 million year.

The post-Paleozoic tectonothermal evolution of the SW Iberian Variscides is poorly known mainly due to the scarce low-temperature geochronological data available. We have obtained new apatite fission-tracks and apatite (U–Th)/He ages to constrain the Mesozoic and Cenozoic tectonic evolution of this portion of the Iberian Massif located just north of the Betic-Rif Alpine orogen. We have obtained nine apatite fission-track ages on samples from Variscan and pre-Variscan granitoids. These ages range from 174.4 (± 10.8) to 54.1 (± 4.9) Ma, with mean track lengths between 10.3 and 13.9 μm. We have also performed 5 (U–Th)/He datings on some of the same samples, obtaining ages between 74.6 (± 1.6) and 18.5 (± 1.4) Ma. Time–temperature path modeling of these low-temperature geochronological data leads us to envisage four post-Paleozoic tectonically controlled exhumation episodes in the SW Iberian Variscides. Three of these episodes occurred in Mesozoic times (Middle Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous) at rates of ≈ 1.1 to 2.5 °C Ma− 1, separated by periods with almost no cooling. We relate these Mesozoic cooling events to the formation of important marginal reliefs during the rifting and opening of the central and northern Atlantic realm. The fourth exhumation episode occurred in Cenozoic times at rates of ≈ 3.2 to 3.6 °C Ma− 1, being only recorded in samples next to faults with topographic escarpments. These samples cooled below 80 °C at ≈ 20 Ma at rates of 3–13 °C Ma− 1 due to roughly N–S oriented compressional stresses affecting the whole Iberian plate, which, in the particular case of SW Iberia, reactivated some of the previous Late Paleozoic thrusts.

The apatite (U – Th)/He and fission track thermochronometers are combined to constrain the Mesozoic and Cenozoic denudational history of the Bathurst region in New South Wales. New apatite (U – Th)/He ages across the continental drainage divide range from 83 ± 14 to 114 ± 18 Ma (±2σ), supporting the evidence from previously published apatite fission track (AFT) data that the region underwent a period of rapid denudation during the mid-Cretaceous. The apatite He-derived thermal histories constrain the amount of erosion in the area and suggest that spatial variations in denudation of the Bathurst Batholith correspond to three broad regions with different histories. The area of the continental drainage divide separates the eastern flank where denudation was rapid at 120 – 90 Ma, and >3 km of crust was removed in about 30 million years, from the western flank, where denudation was less rapid and less prominent. The coincidence of some of the AFT and He ages with the timing of continental extension and Tasman ...

Apatite (U–Th)/He and fission-track thermochronometers are combined to constrain models of denudation and escarpment development of the high elevation Eritrean margin along the southern Red Sea. (U–Th)/He ages have been determined for apatites from two coast-perpendicular transects that were previously used for apatite fission-track analysis [E. Abbate, M.L. Balestrieri, G. Bigazzi, Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis, J. Geophys. Res., 107, B11, doi: 10.1029/2001JB001009, (2002)]. In central Eritrea near Asmara, He ages increase from c. 7 Ma at the coast to 152 Ma on the plateau along a transect. Further south, He ages from the margin border to the base of the escarpment span a narrower range (7 to 12 Ma). One sample from the top of a marginal high (2950 m) yields older ages (22–43 Ma). Forward modeling suggests that the He age distribution across the margin should allow scarp retreat and downwearing mechanisms to be distinguished in the case of a margin formed in the last twenty million years. The distribution of He ages suggests that the escarpment evolved by downwearing and that post-break up erosion of the escarpment was facilitated by in situ excavation rather than parallel retreat. This implies the existence of an inland drainage divide located seaward of the present day escarpment rim. We envisage that the inland divide formed during or immediately after continent break up, and that the eastern marginal belt represents a remnant of the apex structure. The short distance between the eastern marginal belt and the rim (c. 10 km) represents the total retreat of the escarpment. Comparison of the measured He ages from the coastal plain with those predicted from forward modelling indicates that the main phase of post-break up erosion started at about 15 Ma, closer to the initiation of seafloor spreading in the Red Sea than was thought previously.

The southeast Australian margin, like other high elevation passive margins, is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, escarpment and plateau to test the three prevailing models of SE Australian escarpment formation, namely retreat into a downwarped rift shoulder, escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid escarpment retreat or rapid in-place excavation of the escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.

Seismic data and core from the shallow cartographic Pilzno P-7 borehole were used to construct a new model of the Carpathian orogenic front between Tarnow and Pilzno, in the Pogorska Wola area (southern Poland). The most external, frontal thrust of the orogenic wedge (the Jaśniny structure) was identified as a syn-depositional fault-propagation fold detached above the Upper Badenian evaporites. Its formation was controlled by the presence of mechanically weak foredeep evaporites and by the morphology of the sub-Miocene Meso-Paleozoic foreland plate (Jaśniny and Pogorska Wola palaeovalleys). The frontal zone of the Carpathian orogenic wedge (the Skole thrust sheet and the deformed foredeep deposits of the Zglobice thrust sheet) is characterized by significant backthrusting of the foredeep succession towards the south, and by the presence of a triangle zone, with strongly deformed Upper Badenian evaporites of the Wieliczka Formation in its core. The triangle zone was formed during the latest thrusting movements of the Carpathians. An indication of the existence of the triangle zone in the vicinity of Debica has also been provided by reinterpretation of the archive regional geological cross-section. The youngest foredeep deposits, brought to the surface above the backthrust, have been dated as Sarmatian (NN7 nannoplankton zone), which indicates that the latest thrust movements within the frontal Carpathian orogenic in the vicinity of Tarnow–Debica took place approx. 11–10 million years ago. Thermochronological studies (AFT and AHe) indicated that the foredeep succession drilled by the Pilzno P-7 borehole has not been buried deeper than 1.5–2 km, which is compatible with reconstruction based on the seismic data.

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 400 μm and radius 75 μm, c. 6–50% dispersion), including the effects of grain size variation (for spherical equivalent grain radii between 74 and 122 μm, c. 10–70% dispersion) and the combined effects of fragmentation, grain size and radiation damage (for eU between 5 and 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.

Testing models of orogen exhumation requires precise constraint of the time–temperature paths of the exhumed rocks. The zircon (U–Th)/He (ZHe) thermochronometer has a closure temperature of ~ 140–210 °C, and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen. We performed ZHe analyses to test the existing contradictory models for the exhumation of the Ligurian Alps. The ZHe ages indicate a very rapid (1.3–6.8 mm/yr) and southward migrating exhumation of the orogen between ~ 32 and 25 Ma. These high exhumation rates are unique within the Alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the Ligurian Alps. We propose a model of tectonic denudation via detachment accomplished in the shallowest crust (

International audience; Thermochronologic data collected along age-elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates. However, thermochronologic age-elevation relationships are known to be perturbed by topographic effects and potential changes in relief, which are neglected in the inherently 1-D interpretation commonly applied. Such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging. In a companion paper, we develop a methodology for rigorously interpreting thermochronologic age-elevation profiles in terms of exhumation rates and relief development through time, and to independently quantify the resolution of these constraints. Here we test this approach using a thermochronological dataset consisting of apatite and zircon fission-track and (U-Th)/He data, collected at La Meije Peak in the Pelvoux-Ecrins massif (French Western Alps). Our data and models suggest a three-phase exhumation history in the Pelvoux-Ecrins massif, including a pulse of rapid exhumation at ˜ 6-5.5 Ma, preceded and followed by more moderate rates of denudation in the order of 0.3-0.4 km Myr‑ 1. This rapid exhumation event appears to occur coevally in other external crystalline massifs in the Alps but is not detected by qualitative inspection of the age-elevation relationships. Both our synthetic results and inversion of the La Meije data strongly suggest that apatite fission-track age-elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required. Combining apatite fission-track and (U-Th)/He ages and, particularly, including fission-track-length data greatly improves the resolution of the inferred exhumation histories. Although denudation rates through time and the timing of rate changes are generally well resolved, our data have insufficient resolution to satisfactorily constrain the relief history. Synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data.

[1] Several recent studies proposed an important increase in exhumation rate in the western European Alps since circa 5–4 Ma. In order to assess potential spatial differences in exhumation histories, we present new apatite fission track (AFT) and apatite (U-Th)/He (AHe) ages from the central Aar Massif (Guttannen area, Switzerland) and the western Lepontine Dome (Formazza area, Italy). Internal U/Th zoning in apatites explains alpha-ejection-corrected AHe ages that are older than the corresponding AFT ages in this study. A qualitative interpretation of AFT and AHe age-elevation relationships suggests a two-phase (9–7 and 5–3 Ma) exhumation scenario affecting the central Alps, with a stronger expression of the Pliocene signal in the Formazza area. However, a quantitative evaluation of exhumation scenarios using the 3-D heat equation solver Pecube highlights the existence of several other likely scenarios, casting doubt on the validity of a qualitative interpretation of the age-elevation relationships. In Formazza, scenarios suggested by quantitative modeling include continuous denudation at a rate of ∼750 m/Ma and a one-step exhumation rate change from 300 to 1000 m/Ma at 5 Ma. In Guttannen, they include continuous denudation at a rate of ∼400 m/Ma with valley deepening and two periods of higher exhumation rate (increasing from 300 to 700 m/Ma repeatedly at 9–7 and at 5–3 Ma). Contingent upon further flexural isostatic modeling, the magnitude of exhumation recorded in the axial region of the Alps since circa 5 Ma does not appear sufficient to solely explain the denudation recorded in the North Alpine Foreland Basin.

Despite the potential of zircon He thermochronometry for constraining rock thermal histories, it remains less commonly exploited than the apatite He chronometer. In part, this is due to the more challenging analytical techniques required to extract He, U and Th. Here we present a new method for the routine determination of zircon (U–Th)/He ages, and demonstrate how it can be used to constrain thermal histories and to quantify cooling in different tectonic settings. We present zircon (U–Th)/He ages that place a firm upper limit on the extent of denudation-induced cooling (c. 3 km) on the SE Australian passive margin; a region where synrift apatite fission-track and apatite (U–Th)/He ages have previously prevented quantitative constraint. We have also used the zircon (U–Th)/He thermochronometer to quantify the cooling of early Tertiary mafic plutons from Skye, Scotland, where the rate and timing of cooling cannot be determined using other thermochronometers.

International audience; The long-term evolution of the East Greenland fjord region is investigated using geomorphological and stratigraphical evidence to: (1) assess the nature of landscape modification caused by late Cenozoic ice sheet glaciation; and (2) relate patterns of glacial landscape modification to first-order (i.e. continent-margin scale) topography and geology. Geomorphological and stratigraphical evidence demonstrates evolution of the first-order topography and incision of at least part of the present first-order fjord system by 55 Ma. This hypothesis is tested using apatite (U­Th)/He ages for samples from two bedrock profiles near Kong Oscar Fjord. The thermochronology supports landscape evolution before 55 Ma, followed by relative tectonic stability, because it indicates rapid denudation around the time of rifting that occurred prior to continental breakup (i.e. between 75 and 55 Ma). The nature of landscape modification caused by late Cenozoic glacial erosion appears to have been controlled by first-order topography and geology, with selective ice sheet erosion in areas of high-elevation Caledonian basement and apparently little glacial erosion of low-elevation Mesozoic sedimentary strata. Nevertheless, fjord morphometry demonstrates systematic evolution of the first-order fjord system from confined and overdeepened fjords in Caledonian basement to wider and disproportionately larger fjords in Mesozoic strata. The latter indicates that changes in lithological strength enabled the development of more efficient fjord morphology under full glacial conditions that may have promoted fast ice flow.

[1] Apatite (U-Th)/He and fission track age determinations of samples from a 20-km-long, near-horizontal tunnel in the Hochalm-Ankogel Dome (eastern Tauern Window, Austria) are presented in order to determine the role of (paleo)-topography in perturbing isotherms in the shallow crust. Apatite fission track ages (26–8 Ma) show no systematic correlation with distance along the tunnel or elevation. Two age components in the fission track data indicate cooling through ∼120°C at approximately 20 Ma and ∼80°C at approximately 6 Ma. Surface and tunnel (U-Th)/He ages (17–9 Ma and 13–5 Ma, respectively) are consistently younger than the equivalent fission track ages. (U-Th)/He ages vary systematically along the tunnel with older ages at the northern and southern tunnel portals and younger ages in the central section. Geological factors (faulting, lithology) appear to have had little effect on this age distribution. The (U-Th)/He age pattern is inconsistent with rock cooling underneath the present-day Hochalm-Ankogel Dome topography. The age minimum is interpreted to coincide with a paleotopographic maximum in the Hochalm-Ankogel Dome and suggests that the 40–60°C isotherms were warped sufficiently to affect apatite (U-Th)/He ages. The tunnel apatite (U-Th)/He ages match synthetic He ages derived from a two-dimensional numerical model of landscape evolution. Integrating the thermal model results with the data allows the relief development to be constrained and we calculate that the present-day Hochalm-Ankogel Dome topography was formed at 7 to 10 Ma.

[1] The denudation history of the rapidly uplifting western part of the Spanish Sierra Nevada was assessed using apatite fission track (AFT) ages and 10Be analyses of bedrock and fluvial sediments. Major contrasts in the denudation history are recorded within the 27 km2 Rio Torrente catchment. Upland areas are characterized by low-relief, low slope angles, and locally the preservation of shallow marine sediments, which have experienced 2 km of denudation since circa 4 Ma. The minimum denudation rates of 0.4 mm yr−1 derived from AFT also contrast with the slow medium-term (104 years) erosion rates (0.044 ± 0.015 mm yr−1) estimated from 10Be measurements at high elevations. The local medium-long-term contrasts in denudation rates within the high Sierra Nevada indicate that much of the unroofing occurs by tectonic denudation on flat-lying detachments. In lower elevation parts of the catchment, rapid river incision coupled to rock uplift has produced ∼1.6 km of relief, implying that the rivers and adjacent hillslopes close to the edge of the orogen are sensitive to normal-fault-driven changes in base level. However, these changes are not transmitted into the low-relief slowly eroding upland areas. Thus the core of the mountain range continues to increase in elevation until the limits of crustal strength are reached and denudation is initiated along planes of structural weakness. We propose that this form of tectonic denudation provides an effective limit to relief in young orogens.

We have developed a diode laser (25 W, 808 nm) system for He extraction from minerals for (U-Th)/He chronometry. The laser beam is delivered via a 600 μm fiber cable and focused using a binocular microscope. Temperatures necessary for He release from apatite (500–600°C) and zircon (1100–1300°C) encapsulated in Pt-foil tubes are attained by heating to 0.5 W for 30 s and 1.25–2.5 W for 20 min, respectively, using a defocused beam. Heating at these powers does not result in measurable U and/or Th loss from apatite, as noted by the preservation of the distinct Th/U in multiple splits of two different Durango apatite crystals. Analyses of Durango and the California Institute of Technology internal standard apatite 97MR22 yield (U-Th)/He ages of 32.8 ± 1.8 Ma (1σ, n = 11) and 4.6 ± 0.5 (1σ, n = 5), respectively, well within accepted ages. The (U-Th)/He age and Th/U of five Fish Canyon Tuff zircon aliquots yield 29.3 ± 2.2 Ma (1σ) and 0.6 ± 0.03, respectively, and are indistinguishable from ages produced by resistance furnace He extraction. Heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals (e.g., hornblende, biotite, muscovite, garnet) and basalt groundmass, suggesting that diode lasers offer a cheap, small, low-maintenance alternative to Nd:YAG and Ar ion lasers for 40Ar/39Ar, cosmogenic noble gas, and stable isotope studies.

[1] First-order topographical features at high-elevation passive margins have the potential to store information about the early stages of continental rifting, which are usually lost in the sediment record. In this study we present a new combination of apatite fission track and (U-Th)/He analyses from two coast-perpendicular traverses to decipher the time of formation and evolution of the escarpment at the southeastern Australian margin. A combination of inverse and forward modeling of the apatite fission track (AFT) and He data indicates that the coast experienced a rapid denudational event starting at 110 ± 10 Ma, at least 15 Myr before seafloor spreading (85 Ma), in agreement with the hypothesis that rifting at magma-poor margins evolves slowly. Thus the enhanced denudation is not related to the synbreakup base level drop, but it may have been related to a thermally driven transient surface uplift. Considerations on isostatic flexure and rigidity of the lithosphere indicate that unless significant synrift brittle movements are assumed to have occurred along margin-parallel faults that are no more visible, the coast must have experienced a higher geothermal gradient than normal. The evolution of the rest of the coastal plain may have taken up to ∼90 Myr, in agreement with numerical models of passive margins evolution. The (U-Th)/He ages across the coastal plain are consistent with an in-place-excavation scenario of a high rift shoulder, and they rule out the possibility of a constant retreat of the escarpment. AFT + He data from the plateau indicate that the highlands remained stable throughout continental breakup, experiencing rates of erosion of 5–10 m/Myr since, at least, 200 Ma.

Thermal histories derived from apatite fission-track data are remarkably consistent irrespective of tectonic setting and overall rate of cooling. Rapid cooling through the partial annealing zone is typically followed by slower cooling, and such histories can only be explained by systematic errors in the experimentally derived annealing rates used to determine the thermal histories, or a consistent geotherm shape characterized by a relatively low geothermal gradient in the uppermost crust. Differences between the cooling paths characterizing individual tectonic settings indicate that geotherm shape influences the cooling histories. This suggests that crustal geotherms, especially those in orogenic belts, are characterized by a shallow zone of high permeability, allowing the rapid transfer of heat by fluid advection, perhaps together with a near-surface zone affected by enhanced heat loss due to topographic effects. The influence of such controls on cooling histories must be considered prior to using thermochronology data to constrain denudation histories.

CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico O Pantanal do rio Pandeiros (PRP), localizado na região Norte de Minas Gerais, Brasil, é uma importante wetland para a bacia do rio São Francisco contribuindo parcialmente com o aprisionamento de carbono e contenção de sedimentos transportados para o rio São Francisco, que, de outra maneira, contribuiriam para o seu assoreamento. Resultante da interação entre vários fatores como a geologia, geomorfologia e variações climáticas em região de transição de uma área tropical úmida para o semiárido, sua principal fonte de umidade são as monções que ocorrem na América do Sul, as quais possibilitam a ocorrência dos pulsos de inundação, alterando temporariamente seus níveis de base e permitindo a inundação das wetlands, demonstrando a importância de sua análise por meio de uma abordagem hidrogeomorfológica. O objetivo principal da pesquisa é o de compreender a dinâmica desta wetland como unidade hidrogeomorfológica e não apenas ecossistêmica, como normalmente ocorre na literatura. Foram realizadas análises dos pulsos de inundação com o uso de dados de estações fluviométricas e climatológicas instaladas no PRP, os quais apresentaram variações cíclicas no aporte de umidade. Os resultados indicam que nos últimos anos, foi registrada uma redução no nível do rio São Francisco e na vazão do rio Pandeiros implicando em uma redução das inundações do PRP em todos os aspectos investigados: duração, frequência, intensidade e magnitude, principalmente a partir da década de 1990. A alteração do nível de base, neste caso o nível do rio São Francisco na foz do rio Pandeiros, é fator importante pois por meio de análise de Modelos Digitais de Elevação com resolução de 12,5 m foi possível identificar que a wetland é topograficamente uma depressão cuja altitude é em grande parte, similar à do canal do rio São Francisco, o que implica em inundações muito mais longitudinais do que laterais no PRP. Foram aplicadas técnicas para caracterização da assinatura geoquímica da água superficial como: pH, alcalinidade, condutividade elétrica (CE), sólidos totais dissolvidos, turbidez, sílica e análise em ICP, que revelou abundância de cálcio e magnésio dissolvidos. A dinâmica de circulação da água com o uso de traçadores isotópicos 2H e 18O demonstrou as interações que ocorrem na foz do rio Pandeiros durante as diferentes fases da inundação. O monitoramento longitudinal do nível da água ao longo da área do PRP foi realizado com o monitoramento de 5 piezômetros cujos dados, analisados em conjunto às outras técnicas, indicaram a existência de quatro diferentes subunidades hidrogeomorfológicas no interior do PRP. Em cada uma destas, foram realizadas amostragens das geocoberturas superficiais, submetidas às análises laboratoriais de granulometria, pH, CE, carbono orgânico (CO), sódio, potássio, alumínio, ferro, cálcio e magnésio que também apresentam variação lateral e principalmente longitudinal ao longo do PRP. Subsuperficialmente, por meio do uso de ground penetrating radar, foi identificada a predominância de camadas horizontalizadas; contudo, a partir de 3-4 m de profundidade, há a ocorrência de diversos paleocanais e seixos angulosos, indicando ambiente de alta energia no período da deposição. Ainda foi analisada a dinâmica de desnudação da bacia do rio Pandeiros, através do uso do 10Be, com a estimativa da perda média de material alterado por milhão de anos (Ma) em quatro pontos da bacia, indicando um grande aporte de sedimentos oriundos da escarpa de dissecação das chapadas que contornam boa parte da bacia do rio Pandeiros foram, e continuam sendo direcionados para o PRP. Também foram datados sedimentos orgânicos, a 150 cm de profundidade, por meio do 14C (TPM), que apresentaram idades de até 3.246 anos A.P. e deposição em um ambiente paleoclimático muito úmido, relativamente constante, como indicou a composição isotópica de 13C. Os resultados indicam que esta wetland é dependente de condições multiescalares, em uma complexa dinâmica hidrogeomorfológica, em uma situação de equilíbrio ambiental sutil. The Pantanal of Pandeiros River (PRP), located in the northern region of Minas Gerais, Brazil, is an important wetland for the São Francisco River watershed, partially contributing to carbon trapping and containment of sediments transported to the São Francisco River, which, otherwise, they would contribute to silting. Resulting from the interaction between several factors such as geology, geomorphology, and climatic variations in a transition region from a humid tropical area to the semiarid region, its main source of moisture is the monsoons that occur in South America, which enable the occurrence of flood pulses, temporarily changing its base levels and allowing the flooding of wetlands, demonstrating the importance of its analysis through a hydrogeomorphological approach. The main objective of the research is to understand the dynamics of this wetland as a hydrogeomorphological unit and not just an ecosystem, as it usually occurs in the literature. Analysis of flood pulses was performed using data from fluviometric and climatological stations installed in the PRP, which showed cyclical variations in the moisture supply. The results indicate that in recent years, a reduction in the level of the São Francisco River and the flow of the Pandeiros river was registered, implying a reduction in the PRP flooding in all aspects investigated: duration, frequency, intensity, and magnitude, mainly from the 1990s. The change in the base level, in this case, the level of the São Francisco River at the mouth of the Pandeiros river, is an important factor because through the analysis of Digital Elevation Models with a resolution of 12.5 m it was possible to identify that the wetland is topographically a depression whose altitude is largely like that of the São Francisco River channel, which implies in flooding much more longitudinal than lateral in the PRP. Techniques were applied to characterize the geochemical signature of surface water, such as pH, alkalinity, electrical conductivity (EC), total dissolved solids, turbidity, silica, and ICP analysis, which revealed an abundance of dissolved calcium and magnesium. The dynamics of water circulation using 2H and 18O isotopic tracers demonstrated the interactions that occur at the mouth of the Pandeiros river during the different phases of flooding. The longitudinal monitoring of the water level along the PRP area was carried out with the monitoring of 5 piezometers whose data, analysed together with the other techniques, indicated the existence of four different hydrogeomorphological units within the PRP. In each of these, samples were taken from the regolith, submitted to laboratory analysis of particle size, pH, EC, organic carbon (CO), sodium, potassium, aluminium, iron, calcium, and magnesium, which also show lateral and mainly longitudinal variation along of the PRP. Subsurface, using ground penetrating radar, the predominance of horizontal layers was identified; however, from 3 - 4 m deep, there are several paleochannels and angular pebbles, indicating a high energy environment during the deposition period. The denudation dynamics of the Pandeiros river watershed was also analysed, using 10Be, with an estimate of the average loss of altered material per million years (Ma) in four points of the basin, indicating a large contribution of sediments from the escarpment of dissection of the chapadas that surround a good part of the Pandeiros river watershed and that were, and continue to be, directed to the PRP. Organic sediments at 150 cm depth were also dated using 14C (TPM), which showed ages of up to 3,246 years A.P. and deposition in a very humid, relatively constant paleoclimatic environment, as indicated by the 13C isotopic composition. The results indicate that this wetland is dependent on multiscale conditions, in a complex hydrogeomorphological dynamic, in a situation of subtle environmental equilibrium.