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5: Selected apatite grain under normal light for AHe analysis. White dotted line shows the grain boundaries and both terminations (Grain Ire-8 grain a).
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The main goal of this PhD thesis is to investigate the early Cenozoic exhumation history of Ireland and Britain. The causal mechanism for early Cenozoic exhumation on this sector of the NW European margin remains contentious, but broadly can be divided into two main competing hypotheses: i) exhumation caused by mantle-driven processes associated wi...
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The India-Asia continental collision zone archives a sedimentary record of the tectonic, geodynamic, and erosional processes that control the thermal history of the Himalayan orogenic interior since the onset of collision in early Paleogene time. In this paper, we present new (U-Th)/He thermochronometric cooling age data from 18 detrital zircons (Z...
Citations
... Long-lived protracted or accelerated Mesozoic rift-related exhumation of the landscape is consistent with interpretations from a number of studies from the region [103][104][105][106][107]. AFT dating and thermal history models from Devonian sandstones across the North Britain inferred a protracted cooling history from peak temperatures in the Late Paleozoic-Early Mesozoic [103]. ...
... Paired with apatite (U-Th)/He data, thermal history models from across NW Britain resolved multiple phases of cooling throughout the Paleozoic and Mesozoic attributed to the collapse of the Caledonian Orogeny and onset of spreading in the Atlantic [107]. Further work from Ireland and western Britain infers late Permian to mid-Jurassic cooling in thermal history models is rift-related exhumation [106], while similar work from western Ireland resolved a significant period of cooling, suggested to result from ~2.5 km rift-related exhumation, occurred in the late Jurassic-Early Cretaceous [105]. ...
The northeast (NE) Atlantic is one of the best-studied geological regions in the world, incorporating a wide array of geological phenomena including extensional tectonism, passive margin development, orogenesis, and breakup-related volcanism. Apatite fission-track (AFT) thermochronology has been an important tool in studying the onshore evolution of the NE Atlantic for several decades. Unfortunately, large regional-scale studies are rare, making it difficult to study geological processes across the whole region. In this work, a compilation of published AFT data is presented from across Fennoscandia, the British Isles, East Greenland, and Svalbard, with the goal of providing an accessible overview of the data and how this vast body of work has improved our understanding of the region’s evolution. Alongside a review of previous literature, interpolated maps of fission track age and mean track length (MTL) highlight regional trends in the data that may result from major first-order processes and areas of low sample density that should be targeted for future study. Additionally, in the absence of metadata required for thermal history modeling, apparent exhumation rate estimates are calculated from available elevation profiles and the timing of major exhumation events inferred from “boomerang plots” of fission track ages against MTL values. Across Fennoscandia, data suggests that the opening of the NE Atlantic and exhumation of the margin have clearly played a major role in the thermal history of the upper crust. The remaining areas of Britain, Ireland, East Greenland, and Svalbard all present more complex trends consistent with a combination of the NE Atlantic’s opening and the interplay between specific bedrock geology of sampling sites and localized geological processes. Areas of low sample density include southern Britain, NE Britain, southeast Greenland, southern Svalbard, and Eastern Fennoscandia, each of which provides the natural laboratory required to answer many unresolved questions.
... Long-lived protracted or accelerated Mesozoic rift-related exhumation of the landscape is consistent with interpretations from a number of studies from the region [103][104][105][106][107]. AFT dating and thermal history models from Devonian sandstones across the North Britain inferred a protracted cooling history from peak temperatures in the Late Paleozoic-Early Mesozoic [103]. ...
... Paired with apatite (U-Th)/He data, thermal history models from across NW Britain resolved multiple phases of cooling throughout the Paleozoic and Mesozoic attributed to the collapse of the Caledonian Orogeny and onset of spreading in the Atlantic [107]. Further work from Ireland and western Britain infers late Permian to mid-Jurassic cooling in thermal history models is rift-related exhumation [106], while similar work from western Ireland resolved a significant period of cooling, suggested to result from ~2.5 km rift-related exhumation, occurred in the late Jurassic-Early Cretaceous [105]. ...
The northeast (NE) Atlantic is one of the best-studied geological regions in the world, incorporating a wide array of geological phenomena including extensional tectonism, passive margin development, orogenesis, and breakup-related volcanism. Apatite fission-track (AFT) thermochronology has been an important tool in studying the onshore evolution of the NE Atlantic for several decades. Unfortunately, large regional-scale studies are rare, making it difficult to study geological processes across the whole region. In this work, a compilation of published AFT data is presented from across Fennoscandia, the British Isles, East Greenland, and Svalbard, with the goal of providing an accessible overview of the data and how this vast body of work has improved our understanding of the region’s evolution. Alongside a review of previous literature, interpolated maps of fission track age and mean track length (MTL) highlight regional trends in the data that may result from major first-order processes and areas of low sample density that should be targeted for future study. Additionally, in the absence of metadata required for thermal history modeling, apparent exhumation rate estimates are calculated from available elevation profiles and the timing of major exhumation events inferred from “boomerang plots” of fission track ages against MTL values. Across Fennoscandia, data suggests that the opening of the NE Atlantic and exhumation of the margin have clearly played a major role in the thermal history of the upper crust. The remaining areas of Britain, Ireland, East Greenland, and Svalbard all present more complex trends consistent with a combination of the NE Atlantic’s opening and the interplay between specific bedrock geology of sampling sites and localized geological processes. Areas of low sample density include southern Britain, NE Britain, southeast Greenland, southern Svalbard, and Eastern Fennoscandia, each of which provides the natural laboratory required to answer many unresolved questions.
... Finally, the apatite sample RC2168 (Sct-8) from Döpke (2017) was collected from a felsic intrusion in the eastern Grampian Terrane of Scotland. This apatite sample yielded central and pooled AFT ages of 316 ± 38 Ma and 309 ± 26, respectively, and a U-Pb age of 484 ± 21 Ma (2σ level, Döpke, 2017). There are no trace element data available for this sample in the literature. ...
... No AFT age was reported for the RM13 sample, but the central AFT age obtained in this study (9.9 ± 0.6 Ma; n = 78) agrees with central AFT EDM ages from Paros that range between 12.5 ± 2.8 and 10.5 ± 2.0 Ma (Brichau et al., 2006). Finally, the central and pooled AFT ages obtained from sample RC2168 (312 ± 23 and 306 ± 17, respectively) are in very good agreement with those (316 ± 38 and 309 ± 26; respectively) obtained by (Döpke, 2017; Table 2) with the spot ablation approach. Sample RC2168, unlike the samples which yield very young AFT ages, better illustrates the precision of our mapping approach. ...
Obtaining accurate and precise apatite fission-track (AFT) ages depends on the availability of high-quality apatite grains from a sample, ideally with high spontaneous fission-track densities (c. >1.10⁵ tracks.cm⁻²). However, many natural samples, such as bedrock samples from young orogenic belts or low-grade metamorphic samples with low U contents yield low spontaneous fission-track densities. Such apatites must be counted to avoid biasing the resultant FT age. AFT dating employing LA-Q-ICP-MS spot ablation works very well for grains with high spontaneous fission-track densities. This approach allows detection of potential U zoning, while also removing the need for an irradiation step and facilitating simultaneous acquisition of U-Pb and trace element data. The LA-Q-ICP-MS spot ablation thus offers several advantages compared to the External Detector Method (EDM). However, the spot ablation approach requires the counted area to mimic exactly the site and size of the laser spot, which for grains with low spontaneous fission-track densities (<10⁵ tracks.cm⁻²), implies fewer track counts and impairs the precision of the resultant AFT age. Here we present an alternative approach to LA-Q-ICP-MS spot analysis of low fission-tracks density grains by generating a U distribution (²³⁸U/⁴³Ca) map of the entire apatite surface by LA-Q-ICP-MS elemental mapping, which enables characterization of U zonation. The Monocle plugin for the Iolite LA-ICP-MS data reduction software is used to display elemental maps and extract mean ²³⁸U/⁴³Ca values of the same area counted for the fission-tracks. A typical grain-mapping session takes <5 h to map 80 grains. The method was employed on the Durango and Fish Canyon Tuff apatite reference materials, six bedrock apatite samples with low fission-track densities (≤1.10⁵ track.cm⁻²), and one bedrock apatite sample with high fission-track density (>1.10⁶ track.cm⁻²) to assess the precision and accuracy of our approach. Most apatite samples investigated here were previously dated by the EDM or the LA-Q-ICP-MS ablation spot method. The AFT grain-mapping ages agree with previously published EDM or LA-Q-ICP-MS spot ablation ages at the 2σ level. For each apatite sample, we simultaneously acquired U-Pb age and trace element data (Mn, Sr, La, Ce, Sm, Eu, Gd, Lu); here again the data agree with literature constraints (when available) within uncertainties. The mapping approach is therefore a practical solution to low-temperature thermochronology studies facing apatite grains with low spontaneous fission-track densities, while also facilitating investigation of the spatial relationships between thermo- and geochronometric ages and grain chemistry.
... Finally, the apatite sample RC2168 (Sct-8) from Döpke (2017) was collected from a felsic intrusion in the eastern Grampian Terrane of Scotland. This apatite sample yielded central and pooled AFT ages of 316 ± 38 Ma and 309 ± 26, respectively, and a U-Pb age of 484 ± 21 Ma (2σ level, Döpke, 2017). There are no trace element data available for this sample in the literature. ...
... No AFT age was reported for the RM13 sample, but the central AFT age obtained in this study (9.9 ± 0.6 Ma; n = 78) agrees with central AFT EDM ages from Paros that range between 12.5 ± 2.8 and 10.5 ± 2.0 Ma (Brichau et al., 2006). Finally, the central and pooled AFT ages obtained from sample RC2168 (312 ± 23 and 306 ± 17, respectively) are in very good agreement with those (316 ± 38 and 309 ± 26; respectively) obtained by (Döpke, 2017; Table 2) with the spot ablation approach. Sample RC2168, unlike the samples which yield very young AFT ages, better illustrates the precision of our mapping approach. ...
Obtaining accurate and precise apatite fission-track (AFT) ages is dependent on producing plentiful high-quality apatite grains from a sample, ideally with high spontaneous fission-track densities (c. >105 tracks.cm-2). Many natural samples, such as bedrock samples from young orogenic belts or low-grade metamorphic samples with low U contents yield low spontaneous fission-track densities. Such apatites must be counted to avoid biasing the resultant FT age. AFT dating employing LA-Q-ICP-MS spot ablation works very well for grains with high spontaneous fission-track densities which enable potential U-zoning to be detected, while also removing the need for an irradiation step and facilitating simultaneous acquisition of U-Pb and trace element data. The LA-Q-ICP-MS spot ablation thus offers several advantages compared to the External Detector Method (EDM). However, for grains with low spontaneous fission-track densities where U zoning cannot be observed, the LA-Q-ICP-MS spot ablation approach requires the counted area to mimic exactly the site of the laser spot, with the downside that this smaller counted area limits the precision of the resultant AFT age. Here we present an alternative approach to LA-Q-ICP-MS analysis of low fission-tracks density grains by generating a U distribution (238U/43Ca) map of the entire apatite surface by LA-Q-ICP-MS elemental mapping which enables characterization of U zonation. The Monocle plugin for the Iolite LA-ICP-MS data reduction software package is used to display elemental maps and extract mean 238U/43Ca values of the same area counted for the fission tracks. A typical grain-mapping session takes < 5 hours to map 80 grains. The method was employed on the Durango and Fish Canyon Tuff apatite reference materials, and on apatite from six bedrock samples with low fission-track densities (≤ 1.105 track.cm-2). Most apatite samples investigated here were previously dated by the EDM or the LA-Q-ICP-MS ablation spot method. The AFT grain-mapping ages agree with previously published EDM or LA-Q-ICP-MS spot ablation ages at the 2σ level. For each apatite sample, we simultaneously acquired U-Pb age and trace element data (Mn, Sr, La, Ce, Sm, Eu, Gd, Lu); here again the data agree with literature constraints (when available) within uncertainties. The mapping approach is therefore a practical solution to low-temperature thermochronology studies employing apatite grains with low spontaneous fission-track densities, while also facilitating investigation of the spatial relationships between thermo- and geochronometric ages and grain chemistry.
The continental crust of southeast Asia underwent from thickening, thinning to almost rifting during the Mesozoic era as the active continental margin transformed into a passive one. Such crustal thinning history is well-preserved in the Kinmen Island, as the lower crustal granitoids retrograded and rapidly exhumed to surface that were crosscutted by mafic dike swarm. Kinmen Island is situated on the SE coast of Asia, featured by the widespread Cretaceous magmatism as the Paleo-Pacific plate subducted and rollbacked underneath the South China block. Although these complex magmatism are well reported and studied, their associated structural evolution and plate kinematics have not been clearly deciphered. Detailed field mapping, structural measurement, and petrographic analysis of the Kinmen Island were conducted. Up to five deformation events accompanied with five relevant magmatic episodes as well as their corresponding kinematic setting are reconstructed. The ∼129 Ma Chenggong Tonalite (G1) preserved all deformation events identified in this study, which marks the lower bound timing of all reported events. D1 formed a gneiss dome with the Taiwushan Granite (∼139 Ma) at the core bounded by moderately dipping gneissic foliation (S1) as crust extended. D2 formed subhorizontal S-tectonite (S2) with further exhumation of D1 gneiss dome due to middle-to-lower crustal flow associated with further crustal thinning. D3 formed a sinistral ENE-WSW striking steeply S dipping shear belts with well-developed S/C/C’ fabrics. The moderately E-plunging lineation on C surface indicates its transtensional nature. Widespread garnet-bearing leucogranite (G2) associated with decompressional melting showed long lasting intrusion prior to D2 until post D3. D4 was the intrusion of biotite-bearing Tienpu Granite (∼100 Ma; G3) that truncated G1, G2, and all fabrics, which was followed by the intrusion of E-W striking, steeply dipping biotite-bearing pegmatite (G4) as the crust further extended. The youngest deformation event (D5) was NE-SW striking subvertical mafic dike swarm (G5; 90–76 Ma) due to mantle upwelling through significantly thinned crust. By integrating the structural evolution and the previously reported strain pattern, we delineate the slab rollback direction of the Paleo-pacific plate, which changed from northeastward (129∼114 Ma) to southeastward (107∼76 Ma). This plate kinematic movement switched during 114–107 Ma.
