Constraints on the continental weathering feedback on carbon-cycle perturbations on northern Pangea during the end Triassic extinction (ETE) are sparse. Here, we use hyperspectral core imaging (HSI) applied to conglomeratic beds offshore central Norway which shows that enhanced degassing of basalt flows from the Central Atlantic magmatic province (CAMP) was concurrent with intense continental transformation during the ETE. We use well-constrained mercury pulses emitted in gaseous form during volcanism, and subsequently deposited in near-coastal sediments, to identify the ETE. Parallel to mercury pulses, HSI derived smectite was immediately replaced by kaolinite at the extinction level corroborating increased radiogenic run-off from the hinterland as inferred from osmium isotopes. Our new results suggest that, parallel with CAMP activity and with atmospheric carbon dioxide (pCO₂) up to four times the pre-extinction level, continental weathering instantaneously intensified, providing novel empirical knowledge that can be integrated in carbon-cycle models to underpin future warming assessments.

How to cite: Knies, J.: Instant weathering response to carbon-cycle perturbations during the end-Triassic extinction , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3024, https://doi.org/10.5194/egusphere-egu24-3024, 2024.

Time-temperature inversion is a critical step in the interpretation and application of thermochronologic data. However, the computational source code for such programs has typically not been freely available, limiting reproducibility. Here we present a new fully open-source (GPL-3.0) time-temperature inversion program, Thermochron.jl, developed in the Julia programming language. This program initially aims to invert zircon helium and apatite helium data via a Markov chain Monte Carlo approach, and allow for the propagation of uncertainty in diffusion parameters. Here we will present the testing and validation of this model, and consider the implications for some open problems in thermochronology, including the limits of resolution for deep-time thermochronology.

How to cite: Keller, B. and McDannell, K.: Development and implications of a new open-source time-temperature inversion program, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20432, https://doi.org/10.5194/egusphere-egu24-20432, 2024.

In sedimentary basins, the basement typically exhibits non-conformable contact with the overlying sedimentary strata. The basement surface undergoes a complex history of uplift, weathering, erosion, and sediment burial, all of which contribute to its reservoir structural composition. This study integrates microscopic observations, macroscopic well logging data, and seismic data to analyze the physical and weathering effects on the basement reservoirs with different lithologies and distinct structural features within the Erlian Basin. The basement rock types in the study area mainly include tuff, limestone, granite, cataclastic rock, and the basement has been affected by weathering, denudation, dissolution and structural transformation during the evolution of the basin. The basement has been subjected to long-term tectonic modification, forming network cracks on the macro scale and micro-fractures on the micro scale. Weathering and underground fluids along the fractures dissolve the matrix of tuff rock, feldspar in granite and limestone, thus forming dissolution fractures and dissolution pore in the basement rocks. These fractures and dissolution pores make the porosity and permeability of the basement rocks surface show obvious heterogeneity. According to the microstructure and physical property changes, the structure of different lithology basement surface is analyzed. Among them, the surface of the tuff basement rock has undergone multi-stage volcanic eruptions and weathering leaching, with a structure of multi-stage ancient weathering crust reservoir superposition, and the porosity and permeability of the gentle slope at the structural high part is large. The surface of granite and cataclastic rock basement is controlled by tectonic activity, weathering leaching, and formation fluid action. The double-layer structure comprises the top paleo-wind crust reservoir and the middle and lower fracture dissolution reservoirs, exhibiting high porosity and permeability in the elevated structural positions and the region proximate to the fault zone. Controlled by karstification, tectonic activity, and ancient landform, the limestone basement rock surface displays longitudinal variations in reservoir storage space types, featuring a structure with multiple sets of vertically arranged reservoirs, particularly characterized by high porosity and permeability in elevated structural positions and near fault zones. The concept and results of this work can be used for future studies on unconventional basement reservoirs in other regions.

How to cite: Shen, C. and Jiang, Y.: The reservoir development model of different lithology basement rocks in Erlian Basin，Northeast China., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4821, https://doi.org/10.5194/egusphere-egu24-4821, 2024.

In this study, we employed low-temperature thermochronology to investigate the depositional source and exhumation rates of the mountain belt in central Taiwan. We collected four sedimentary rock samples from the Chiunkongliao River and one sample from Wuxi. Utilizing zircon fission-track (ZFT) dating and uranium-lead (U-Pb) dating, we observed an increase in the percentage of partially reset zircon from the Chinshui Shale to the Toukoshan Formation. Additionally, we identified total reset zircon in the Toukoshan Formation. Furthermore, the probability density of U-Pb dating in the Toukoshan Formation leans more towards the Oligocene than the Miocene.

Through double-dating, we determined that ZFT ages less than 65 Ma are not the result of Cenozoic volcanic activity. These findings suggest a change in the origin of the depositional source from the Cholan Formation to the Toukoshan Formation. According to the lag time curve, the exhumation rate accelerated during the time period from 1.1 Ma to 0.5 Ma.

Comparing our results with previous studies in the Western Foothills of central Taiwan, we observed that the annealing zone was exposed earliest in the middle part than in the southern one. This may indicate that, in central Taiwan, the exhumation rate in the middle part was the fastest.

How to cite: Hsu, W.-C., Lee, Y.-H., Yang, K.-M., Lin, K.-W., and Chang, S.-P.: Study of Plio-Pleistocene Foreland Basin Provenance and Orogenic Exhumation History in Central Taiwan: Fission-Track and U-Pb Dating Analyses of Detrital Zircon from Western Foothills, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3322, https://doi.org/10.5194/egusphere-egu24-3322, 2024.

The age and formation of the Scandinavian mountains and the western coastal areas (the ‘strandflat’)  have long been the subject of debate. Some suggest that the present-day mountains are remains of the Caledonian orogen while others claim that the Caledonian nappes after denudation were covered by Mesozoic sediments and subsequently exhumed. We have tried to clarify these issues by studying remains of chemically weathered rocks (saprolites) along two profiles from the coast to the interior of central Norway. This multidisciplinary study includes the following data: digital topography, electrical resistivity tomography (ERT), XRD, XRF, palynological analyses and K–Ar dating of samples from outcrops, trenches and core drilling. The coastal areas are dominated by an outer ‘strandflat’ and an inner ‘joint-valley’ landscape, while the interior and mountainous areas are  characterised by smoother landscapes referred to as ‘palaeo-surfaces’. Remnants of pre–Tertiary weathering occur in the joint-valley landscape as well as on the palaeo-surfaces. The deep saprolites are found within fault- and fracture-zones and at depths exceeding 50 m in drillholes. It is suggested that the old saprolites were strongly eroded along the coast and in the fjords and valleys like Orkdalen and Sunndalen. K–Ar dating of clay from saprolites on the mainland commonly show Jurassic ages, seen along a profile that stretches from the coast to the Dovrefjell (approx. 1400 m a.s.l.).The age of the smectite- and kaolinite-containing saprolites seems to be almost contemporaneous along this profile, implying that the entire area was subject to weathering in a warm and humid climate, such as prevailed during the Late Triassic and Jurassic. Palynological remains in the clayey saprolites contain thermally altered pollen and spores from the Triassic and Jurassic, which supports the interpretation and dating of the saprolites.  It is therefore suggested that the Mesozoic landscape in central Norway was shaped by uplift and deep weathering in the Jurassic. However, saprolites occurring along a second profile south of the Trondheimsfjord show Carboniferous and Permian K-Ar ages, indicating that this area constitute a Permian or Triassic sediment basin that was eroded during the late Cenozoic. Thus, it is likely that the entire Trøndelag county was covered by Mesozoic sedimentary rocks, right up to the start of the Cenozoic erosion. Important processes that governed the shaping of the landscape were tectonic uplift and erosion throughout the Cenozoic, followed by extensive abrasion and erosion by glaciers and meltwater during Pleistocene. We therefore conclude that both the studied saprolites and the shape of the present-day landscape in central Norway are characterized by the landscape formed during the Jurassic. This includes the deep profiles of chemical weathering and a drainage pattern that changed in the Pleistocene.

How to cite: Olesen, O., Rueslåtten, H. G., Schönenberger, J., Smelror, M., van der Lelij, R., Larsen, B. E., Olsen, L., and Bjørlykke, A.: Jurassic heritance of the geomorphology in Mid Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3969, https://doi.org/10.5194/egusphere-egu24-3969, 2024.

The Borborema Province in northeastern Brazil is an ideal location for investigating the tectonic evolution of crustal-scale strike-slip shear zones. These structures exhibit an anastomosing network with numerous well-exposed mylonitic belts linked to the Neoproterozoic Brasiliano-Pan-African Orogeny. However, there is a gap in information on fluid-rock interaction related to both brittle-ductile and brittle deformation. This work aims to describe the control of brittle-ductile structures on the development of brittle fault zones with significant fluid interaction associated with the Cruzeiro do Nordeste shear zone, which limits the northern border of the Jatobá Basin. Our study is based on multiscale structural analyses, integrating aeromagnetic data, UAV images, outcrop-based measurements and microstructural characterization. We documented that the mylonitic foliation is represented by ENE-WSW magnetic positive anomalies (~14 nT/m) and is characterized by S-C fabrics indicating dextral kinematics. Brittle-ductile deformation is marked by dextral C'-type shear bands (WNW-ESE) and mesoscopic strike-slip faults (NW-SE and N-S). These structures exhibit bulges defined by fine-grained, peripheric quartz grains. C’ shear bands evolve into brittle fault zones composed of mosaic and chaotic breccias, veins filled by epidote, epidote+calcite, and calcite, which are associated with hydraulic brecciation. Cathodoluminescence analysis revealed variations in luminescence along the calcite-filled veins, suggesting at least two phases of fluid interaction. The older phase exhibits higher luminescence and is brecciated by the younger calcite fluid, which displays lower luminescence activation. This fluid-rock interaction can modify the permoporous system of analog reservoirs, which can be observed through the variation in cementation intensity in the Tacaratu Formation, sandstones of the Paleozoic sequence of the Jatobá Basin. Our results indicate that the Cruzeiro do Nordeste shear zone is an excellent example that preserves the record of ductile, brittle-ductile and brittle deformation due to exhumation. Furthermore, enhancing our knowledge of brittle deformation associated with the late stages of the Brasiliano-Pan-African cycle (Cambrian?) at shallow crustal levels may be the key to understanding the tectonic evolution of Paleozoic and Cretaceous sedimentary basins in northeastern Brazil. 

How to cite: Miranda, T., Barbosa, D., Correia Filho, O., Silva, A., Viegas, G., Pacheco, S., and Carvalho, B.: Evolution of brittle-ductile deformation to brittle fault zones and the role of fluid migration in Cruzeiro do Nordeste shear zone, Borborema Province, NE Brazil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10436, https://doi.org/10.5194/egusphere-egu24-10436, 2024.

Cenozoic collision between the Indian and Eurasian plates instigated significant intracontinental deformation in Central Asia, giving rise to the Tibetan Plateau and the Himalayan orogen. Simultaneously, it compelled the Pamir to undergo extensive northward movement, accompanied by tens to hundreds of kilometers of crustal shortening. Ultimately, this geological activity culminated in the collision of the Pamir with the South Tianshan. This collision may be a key factor influencing topography and climate change in Central Asia, yet comparatively little is known about the details of the tectonic evolution of the collision zone. In particular, precise determination of the timing of activation of different thrusts in the Main Pamir thrust (MPT), Pamir fold-and-thrust (PFT), and South Tianshan thrust (STT) system remains lacking. Here, we report new apatite (U-Th-Sm)/He (AHe) dates from fourteen samples collected from the hanging walls of these thrusts, situated at the westernmost tip of the Tarim Basin, NW China. Samples collected from the MPT record rapid cooling at ~ 11 ± 1 Ma, samples from the PFT show rapid cooling at ~ 7 ± 2 Ma and ~4-3 Ma, while samples from the STT reveal accelerated cooling at ~11 ± 1 Ma, ~7-6 Ma and ~3-2 Ma. We propose that the observed rapid cooling was caused by thrust-induced exhumation in this region, thus the rapid cooling represents the activity time of thrusts. Combined with previous studies on the onset deformation in the MPT and STT, we develop a model of the convergence between the North Pamir and South Tianshan in our study region since the late Oligocene. Late Oligocene to early Miocene (~20 ± 5 Ma) cooling ages from the MPT and STT hanging walls date the onset of thrusting, indicating the initiation of this convergence. Afterward, the MPT and STT experienced northward and southward propagation during the late Miocene (~11 ± 1 Ma), respectively. Subsequently, during the latest Miocene (~7 ± 2 Ma), the PFT started to form, while simultaneously, the STT propagated southward, resulting in the contact of these two thrusts at the Wuheshalu section. We suggest that the timing of contact of PFT with the STT represents the surface expression of the onset of collision between the Pamir and South Tianshan in the western Tarim basin. Following the initial collision, the PFT gradually propagated northward while the STT propagated southward during the Pliocene to Pleistocene (~3 ± 1 Ma), establishing the present-day superimposed and imbricated thrust system in the Pamir-South Tianshan convergence zone.

How to cite: Wang, F., R. Sobel, E., van der Beek, P., Zhu, W., Colleps, C., Gong, L., Rembe, J., Li, G., and Glodny, J.: New Constraints on Late Cenozoic Convergence between the Pamir and South Tianshan from Apatite (U-Th-Sm)/He Thermochronology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11185, https://doi.org/10.5194/egusphere-egu24-11185, 2024.

Recent progress of low-temperature thermochronology enables to analyze uplift-exhumation-cooling histories of the island-arc mountains with good confidence. This is particularly fruitful for studying the topographic evolution of the Japan Arc, because many of the Japanese mountains are started to uplift in recent time (e.g., late Pliocene to Quaternary) after an extended period of tectonic quiescence, and hence the resultant amount of total denudation is relatively small. The utility of the approach was first demonstrated by elucidating the uplift-exhumation-cooling process for some of the Japan Alps, in which average topographic changes of the tilted mountain block were quantitatively reconstructed by low-temperature thermochronology (Ref. 1-2). Such analyses also allow to estimate the background paleo-depth of neo-tectonic faulting episodes.

In this presentation, we highlight recent and ongoing important thermochronological research in and around the Japan Arc (Ref. 3-5). In addition, we will promote the Thermo2025 conference and introduce its preliminary plans. The International Conference on Thermochronology has been held biyearly around the world and the International Standing Committee on Thermochronology (ISCT) determined that the 19th conference (Thermo2025) will be held in Kanazawa, Japan, on September 14-20th, 2025 (https://isct.sedoo.fr/meetings-2/). Then, the local organizing committee, including the authors, have promoted the preparation of the conference in partnership with the domestic geoscience societies and international thermochronological communities. Pre-registration of Thermo2025 is now being accepted at the website (https://smartconf.jp/content/thermo2025). Those who are interested in the conference can soon receive the announcements by pre-registration.

References

1. Sueoka, S. et al. Island Arc 21, 32-52 (2012).

2. Sueoka, S. et al. Journal of Geophysical Research: Solid Earth 122, 6787-6810 (2017).

3. Sueoka, S. and Tagami, T., Island Arc 28, 1-8 (2019).

4. Fukuda, S. et al. Earth Planet Space, 72, 1-19 (2020).

5. King, G.E. et al. Geology, doi.org/10.1130/G50599.1 (2022).

How to cite: Tagami, T., Hasebe, N., and Sueoka, S.: Overview of thermochronological studies in and around the Japan Arc; towards Thermo2025 Conference in Kanazawa, Japan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13936, https://doi.org/10.5194/egusphere-egu24-13936, 2024.

The impact of a magmatic polyphasic intrusion on the empirical relationship between zircon metamict state as revealed by Raman spectroscopy (Zhang et al., 2000) and α-damage accumulation of the same grains is investigated in the eastern Adamello batholith (Central Italian Alps). Eighteen samples, spanning the contact between the Val di Genova pluton, the Sostino-Corno Alto pluton, and their host rocks – including Permian intrusives, were examined.

Zircons were analysed with a 100mW Nd:YAG solid state laser emitting at 532 nm Raman spectrometer to assess their metamict state based on the position and full width at half maximum of different Raman bands. For each zircon, on the same spot, LA-ICP-MS analyses were performed to determine the crystallization age and U, Th, and Pb concentrations. Crystallization ages and actinides content were used to calculate α-damage since zircon crystallization.

Magmatic zircon ages display a south to north younging trend from the Sostino pluton (44.79±0.35 Ma) to the Val di Genova pluton (34.18±0.20 Ma), consistent with literature data (Schaltegger et al, 2009 and 2019; Ji et al., 2019 and references therein). Moreover, magmatic zircons from the Adamello batholith exhibit a metamict state roughly consistent with calculated α-doses, indicating nearly full retention of decay-related damage. In contrast, zircons from the Paleozoic country rocks and Permian plutons show a nearly crystalline state conflicting with calculated high α-doses, thus suggesting a thermal overprint that annealed their crystalline structure, here identified with the thermal aureole of the Adamello magmatic intrusions. In fact, the measured metamict state of zircons in the host rocks is in overall agreement with α-damage accumulation calculated since intrusion rather than crystallization ages.

Raman analysis of radiation damage in zircon is being currently investigated as a new thermochronological method (Pidgeon, 2014; Nasdala et al., 2001; Nasdala et al., 2002; Hartel, 2021) with particular emphasis on unannealed zircons to test its feasibility. With this study, we present an application of Raman analysis of radiation damage in annealed zircons to trace their thermal history, thus providing independent validation of contact-metamorphic overprints.

How to cite: Favaro, S., Resentini, A., Tiepolo, M., Malusà, M. G., and Zanchetta, S.: Impact of magmatic intrusions on metamict zircon annealing as constrained by Raman spectroscopy in the eastern Adamello batholith (Central Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14975, https://doi.org/10.5194/egusphere-egu24-14975, 2024.

“Of what use at all is a lineament map?” constitutes a fair question posed with positively indecent irregularity, if ever at all. Here we suggest that because topographically derived lineament maps depict landscape elements that form under physically differing processes at different rates and times, they have historically conflated time-transgressive morphological evolution of the region at hand with a simplistic message, stated or implied, akin to “this map, then, represents the structural framework underlying such-and-such geological province.” This slippery slide down the razor presupposes two essential conditions: That incision occurs only where the substrate is most easily eroded and that all faults, fractures, and shear zones are less resistant than undeformed rock. It is a demonstrable fact that neither condition is necessarily true. Yet lineament mapping persists, from Earth to Mars and by now, presumably, on even more distant planets. What can we do with these things, and why do we bother?

Whilst quantifiable linearity does convey information that may be useful for landscape classification, abstracting topographic surfaces into lineaments does not, a priori, expose the structural template of the underlying bits of any given planet. Analyses of azimuth, density, length, or any other quantity provide little benefit unless one can constrain exactly what it was that one measured. Digital vectorization of modern DTM data does offer hope. Furthermore, linearity can be expressed in topographically positive features such as ridgelines. Individual vectors can (must!) be given local attributes (depth… width… sinuosity… slope…) that may be coupled to a postulated (theoretical, probably optimistic, and in most cases surely a relative) morphologically dependent ‘age’ of incision. Regional (‘environmental’) attributes pertaining to bulk properties of the area traversed by that very same vector (flat… inclined… U or V shape… carbonate… granite…) can be collected to provide external context. Equally important is a careful inventory of bias such that the mapping method generates reproducible vectors with representative and homogeneous coverage from the upper left-hand corner of the dataset to both the penultimate and ultimate pixels at the (assume southeastern) End Of File. Absent these, a lineament map is not dissimilar to a basket of tropical fruits plucked from one’s local Arctic haberdashery at or around Christmastime.

Because she exhibits a wide range of topographic styles, human-generated lineament maps have struggled to extract unbiased, homogeneous signals from Baltica’s tired old bones. Having experimented for some years with algorithms designed to map lineaments automatically from (x, y, z) data we feel inclined to present a few observations, interpretations, confessions of bias, and recommendations for how we might possibly do better. We will briefly describe one successful algorithm for computerized lineament mapping, present results that purportedly describe distinctly different Norwegian landscapes, illustrate some connections to certain known structures and disconnections with others, and attempt a sort of general, undoubtably conflicted synthesis of the potential use of lineament maps in assessing the landscape evolution of certain small parts of the Norwegian rifted margin.

“There are more things in heaven and Earth than are dreamt of in our Science….”

How to cite: Redfield, T. F., Torgersen, E., Svendby, A. K., Fabian, K., Dichiarante, A. M., and Øye, V.: Some thoughts on the usefulness of Lineament Maps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15953, https://doi.org/10.5194/egusphere-egu24-15953, 2024.

Lineament analysis from topographic maps is a well-known method to identify regional fracture systems and the potential for weakness zones, but the significance of lineaments can easily be misinterpreted. A topographic lineament (topolineament) is just an elongated depression, which may or may not contain important structural information. In which case lineaments represent faults or fractures, and do all brittle structures appear as a lineaments? This is the focus of our project.

In this study, we investigate the applicability of lineament analysis to characterize the fracture system in the Oslo region, Norway. Topolineaments, derived by automatic detection using an in house-developed algorithm (OttoDetect) on a 10x10m digital elevation model, are combined with field structural observations and measurements. Special emphasis was placed on comparing the orientation of brittle structures from field data with that of the detected topolineaments.

The fieldwork was carried out by measuring fractures at selected locations both along and away from larger topolineaments. In some areas, data collection also included measuring scanlines parallel and orthogonal to selected topolineaments. Structural measurements were split into different geographical areas of Oslo and plotted on stereonets. The topolineaments were analyzed and classified by parameters such as orientation, length, density (number of lineaments over a given distance) and width-depth ratio (within the lineament), etc. Rose diagrams of all lineaments within a given radius from a geographical center point were used to show the dominant lineament orientations in the studied area. We pay especially close attention to lineaments in regions dominated by bedrock, in order to represent only bedrock-incised topolineaments, so that the rose diagrams could be used to compare with fracture orientations.

Preliminary results show that both field data and topolineaments are dominated by two orthogonal sets: E-W and N-S. However, the relative dominance of either set in the two datasets seems less correlated. Further analysis is ongoing to also constrain the relationship between fracture and lineament densities.

How to cite: Svendby, A. K., Torgersen, E., Redfield, T., Dichiarante, A. M., and Arctander, K.: Lineament analysis for characterizing regional fracture system – A case study from the Oslo region, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16090, https://doi.org/10.5194/egusphere-egu24-16090, 2024.

Formation of the Norwegian rifted margin in the Mesozoic and Early Cenozoic and denudation of adjacent onshore areas resulted in preferential reactivation of regionally important Paleozoic faults and shear zones as indicated by a variety of published geochronological data. Lithospheric-scale necking impacted the later topographic evolution of the present onshore areas, including an escarpment topography and incision pattern that correlates with the seawards taper of the crystalline crust. The density of large, mapped landslides in turn reflects topographic and structural signals and tend to cluster inboard of sharply tapered areas. Margin formation also resulted in the impregnation of crystalline basement by swarms of smaller structures around multiply reactivated structures that made the bedrock prone for coastal erosion as well as onshore slope instability, with apparent maxima in glacially incised topography in areas inboard of sharply tapered margin segments in North- and Mid Norway as well as inboard of parts of the northern North Sea. Offshore, the sharply tapered Møre segment contains stacked submarine slide deposits in an anomaleously short progradational Quaternary wedge, illustrated spectacularly by the Holocene Storegga slide.  An important part of the geohazards distribution onshore and offshore Norway can thus be viewed as long-term responses to the rifting process through an interplay between crustal-scale inheritance, structural reactivation and saturation and mass redistribution by erosion, especially glacial transport. Other rifted margins that evolved by multiphase extension may display similar relationahips between ancient structural templates and the modern distributiion of geohazards.    

How to cite: Osmundsen, P. T. and Redfield, T. F.: From Margin to Menace: the role of structural inheritance in the geohazards distribution in Norway  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16413, https://doi.org/10.5194/egusphere-egu24-16413, 2024.

The late Oligocene Fish Canyon Tuff (FCT)—preserved within the San Juan volcanic field of southern Colorado—has long served as a reliable source of multiple accessory minerals for geochronology and thermochronology age standards. Whereas the ‘classic’ FCT sampling locality preserves near consistent ages of ~28–29 Ma across the sanidine ⁴⁰Ar/³⁹Ar, zircon U-Pb, zircon fission track, zircon (U-Th)/He, and apatite fission track systems, the average single-grain apatite (U-Th)/He (AHe) age at this site is notably younger at 20.8 ± 0.4 Ma. Considering that the classic sampling site is positioned at the bottom of a deeply incised valley with ~800 m of local relief, this AHe age has been proposed to record burial of the basal tuff to depths exceeding ~1000 m, with subsequent Early Miocene cooling reflecting valley incision. In contrast, an average single-grain AHe age of 28.5 ± 0.1 Ma was previously recorded from a newly proposed distal FCT locality where the upper-most tuffs are freshly preserved within a quarry. This AHe age is consistent with higher temperature geochronological ages from the same locality, which suggests that the distal FCT experienced no post-emplacement thermal disturbance. The observed, locality-specific difference in AHe ages provides a unique opportunity to calibrate and assess the potential of apatite ⁴He/³He thermochronology to (1) quantify the degree of post-emplacement burial and the rate of subsequent cooling at the classic FCT locality, and (2) record rapid late Oligocene cooling at the distal FCT locality. We respectively test the hypothesis that the classic FCT apatite will yield a comparatively diffusive ⁴He/³He degassing spectra, whereas the distal FCT apatite will preserve a near-uniform ⁴He/³He spectra that is solely affected by alpha-ejection. We consider and discuss newly derived ⁴He/³He results in light of (1) the geological history and landscape evolution of southern Colorado, (2) the potential use of distal FCT apatite as a coupled AHe and ⁴He/³He thermochronology reference material, and (3) the reproducibility of FCT apatite ⁴He/³He spectra using differing proton-irradiation procedures. 

How to cite: Colleps, C., van der Beek, P., and Amalberti, J.: Evaluating the classic and distal Fish Canyon Tuff localities with apatite 4He/3He thermochronology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10890, https://doi.org/10.5194/egusphere-egu24-10890, 2024.

Understanding the geodynamics of plateau evolution necessitates careful consideration of the spatial and temporal constraints associated with mountain building on the northeastern Tibetan Plateau. However, when and how the northeastern Tibetan Plateau grew remains highly debatable. Here we integrate apatite U-Pb, fission-track, and rare-earth element provenance indicators from the Oligocene-Miocene continental succession of the Xunhua Basin to establish a framework of drainage reorganization and topographic evolution of the Xunhua region. The results suggest three provenance changes at ca. 28 Ma, ca. 20 Ma, and ca. 12 Ma, that not only indicate topographic growth of the West Qinling, Laji Shan, and Jishi Shan, respectively, but emphasize the significance of apatite for provenance analysis. The compilation of our findings and deformations within the northeastern Tibetan Plateau reveals the Oligocene-Miocene stepwise expansion and the Middle Miocene stress reorganization within the northeastern Tibetan Plateau. Combined with regional evidences, we propose that the Early Cenozoic northward compression of the Indian continent shortened and thickened Tibetan lithosphere, and subsequently triggered the removal of thickened lithosphere beneath south-central Tibet in the Oligocene. This process not only induced Oligocene-Miocene progressive expansion across the northeastern Tibetan Plateau, but also facilitated the continuous northward injection of the Indian lithosphere. Simultaneously, accompanied by the southward insertion of the North China craton, the underthrusting of both the India and North China initiated sinistral strike-slip faults in the middle Miocene, driving a change in stress directions. The results of this study underline the contribution of both the lithospheric removal and continental underthrusting geodynamic processes in driving outward growth of plateau. 

How to cite: Guo, C., Zhang, Z., Lease, R., Malusà, M., Chew, D., Grasemann, B., and Xiao, W.: Apatite U-Pb, fission-track, and trace element provenance constraints on Oligocene-Miocene northeastern Tibetan Plateau growth and dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12215, https://doi.org/10.5194/egusphere-egu24-12215, 2024.

How did the continent-oceanic plate interact, and when did the initial west paleo-Pacific plate subduction beneath Eurasian continent occur, are still unknown. NE China deformation, volcanic eruptions and magmatic intrusions can give some constraints. Muling located in the Dunhua-Mishan fault zone in NE China, is a key area where the E-W-trending Eurasian domain changed to NE-trending west Pacific Plate domain in NE China. During the Mesozoic time, at least three stages of deformation occurred, including: (1) E-W-trending structures with extensive ductile shear deformation and south-verging folds which result from thrusting towards the south, followed the emplacement of granitic rocks. (2) NE- or NNE-trending thrust faults and strike-slip movement, accompanied by the formation of west-verging inclined and recumbent folds. This phase deformation changed the whole tectonic framework of eastern China from an E-W-trending Eurasian domain to a NE-trending west Pacific Plate domain. (3) NE-trending strike-slip faults and E-W-trending strike-slip motion. Field investigations of Mesozoic ductile shear zone and faults, granitic intrusions and dykes, combined with zircon U-Pb dating and muscovite ⁴⁰Ar/³⁹Ar plateau ages, reveal the age of the E-W-trending structures as ~254-209Ma, and NE–SW-trending tectonic belts as ~182–170 Ma. The tectonic transformation of the eastern China continent involved a change from E-W to NE-SW-trending structures was a response to the initial subduction of paleo-Pacific plate.

How to cite: Zhou, L. and Wang, Y.: Middle-late Mesozoic tectonic evolution of the NE China—corresponding to westward subduction of the paleo-Pacific plate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13898, https://doi.org/10.5194/egusphere-egu24-13898, 2024.