The contribution of climate and tectonics to the Neogene exhumation history of the European Alps is studied in the Mont Blanc (MB) massif using low‐temperature thermochronology. Apatite fission track ...and (U‐Th‐Sm)/He data suggest that the MB massif was exhumed episodically: Rapid exhumation (2.5 ± 0.5 km/Ma) before 6 Ma is followed by an episode of slow exhumation and a period of accelerated exhumation (>1 km/Ma) after ∼3 Ma. Comparing the exhumation rates of the MB massif with reported exhumation rates of all other external crystalline massifs (ECM) shows that the MB massif is the only ECM that experienced rapid exhumation before 6 Ma, which is possibly related to NW and minor SE directed thrusting of the MB massif. The data demonstrate that the Messinian base level drop (∼5.5 Ma) and the increase in atmospheric moisture caused by an intensification of the Atlantic Golf Stream (4.6 Ma) did not affect the exhumation of the external Alps. All ECM, except the Gotthard massif, show an increase in exhumation rates at ∼3 Ma. We interpret this as the result of normal faulting along orogen‐parallel faults and beginning Alpine glaciation. The relative impact of these processes on the exhumation of the ECM can vary spatially and temporally, mainly depending on differences in geology and geomorphology. In the case of the MB massif we propose that the acceleration in exhumation rates at ∼3 Ma is caused by rapid valley incision related to glaciation, and that the recent relief of the MB massif is thus a young feature.
The Quaternary stratigraphy of the Alpine Foreland consists of distinct terrace levels, which have been assigned to four morphostratigraphic units: Höhere (Higher) Deckenschotter, Tiefere (Lower) ...Deckenschotter, Hochterrasse (High Terrace) and Niederterrasse (Lower Terrace). Here, we focus on the terrace gravels at Hohle Gasse, SSE of Pratteln near Basel, which are mapped as Tiefere Deckenschotter. Petrographic and morphometric data established from clasts allowed to infer the transport mechanisms and sources of the gravels. Sedimentological analyses indicate that the gravels were transported by a braided river and deposited in a distal glaciofluvial setting. In addition, it can be shown that the majority of the clasts display multiple reworking and only a minority maintained a distinct glaciofluvial shape. Cosmogenic multi-isotope dating using
10
Be and
36
Cl allowed direct dating of the sediments at the study site. A depth-profile age of
270
-
190
+
830
ka for
10
Be was achieved for the deposits at Hohle Gasse. Unfortunately, no age could be modelled from the
36
Cl concentrations as the blank correction was too high. Furthermore, this age proves that the studied terrace level should be assigned to the morphostratigraphic unit Hochterrasse.
The phonolitic Limberg t3 tephra (Kaiserstuhl Volcanic Complex, Germany) was previously dated by the conventional K/Ar method yielding inconsistent results. We have re‐dated this tephra layer with ...three independent methods. Fission Track (FT) external detector analyses on single apatite crystals (16.8 ± 1.3 Ma, 2s) and (U‐Th)/He measurements on titanite and apatite (16.5 ± 1.0 Ma, 2s and 16.8 ± 1.0 Ma, 2s, respectively) are in close agreement with laser Ar/Ar dates on incrementally heated single crystals of sanidine (16.3 ± 0.4 Ma, 2s). Due to very rapid cooling, the He, FT and Ar thermochronometers provide one single age representing the eruption event. The different minerals are characterised by favourable properties with respect to their chemical composition, grain size and shape. In particular for the t3 sanidine, homogeneity has been demonstrated by electron microprobe analysis and on a grain‐to‐grain and grain‐internal scale by single crystal incremental laser heating. Based on the agreement between independent methods and the mineral yield of this unit, the Limberg t3 tephra is proposed as multi‐method age reference material for single grain laser Ar/Ar, FT and (U‐Th)/He dating.
Le tephra phonolitique de Limberg t3 (complexe volcanique de Kaiserstuhl, Allemagne) a été daté dans le passé par la méthode conventionnelle K/Ar, avec des résultats incohérents. Nous avons daté de nouveau ce niveau de tephra par trois méthodes indépendantes. La datation par traces de fission (FT) avec détecteur externe sur des grains individuels d'apatite (16.8 ± 1.3 Ma, 2s) et les mesures U‐Th/He sur des titanites et des apatites (16.5 ± 1.0 Ma, 2s et 16.8 ± 1.0 Ma, 2s, respectivement) sont en très bon accord avec les datations Ar/Ar faites par laser et chauffage par palier de cristaux individuels de sanidine (16.3 ± 0.4 Ma, 2s). En raison du refroidissement rapide, les thermo‐chronomètres He, FT et Ar donnent un seul âge qui correspond donc à l'épisode d'éruption. Les différents minéraux présentent des propriétés très favorables, de composition chimique, de taille des grains et de forme. En particulier, l'homogénéité de la sanidine t3 a été démontrée par des analyses par microsonde électronique et par chauffage laser par palier, grain par grain ou à l'échelle d'un grain. En raison de cet accord entre des méthodes indépendantes et de la qualité des minéraux de cette unité, le niveau de tephra Limberg t3 peut être proposé comme matériau de référence de datation pour les méthodes d'analyse Ar/Ar par laser sur grain individuel, FT et datation (U‐Th)/He.
Sedimentology, petrography and the provenance of Holocene sediments from the Southern Chile Trench (36-47°S) were investigated in an integrated approach combining description of a collection of ...gravity cores, measurements of physical properties, quantitative X-ray petrography and modal analysis. The sediments studied were trench hemipelagic sediments, fan deposits, and more distal hemipelagic sediments from the Nazca Plate. The trench is mostly fed by multiple point sources via submarine canyons. Sandy turbidites show a southward increase in sediment maturity. Whereas volcanic lithic fragments and plagioclase represent the dominant fraction in the north, quartz content strongly increases in the southern part of the study area, in line with source lithologies. Further north, active volcanoes in the Main Cordillera represent almost the entire provenance signal as a result of a strong contribution of highly erodible volcanic rocks. Recurrence rates of sandy and silty turbidites in the trench fan sediments indicate a link to the palaeoseismic record on land. Our study documents the potential usefulness of proximal turbidites to reconstruct palaeoseismicity, even at a scale of single segments of the plate boundary.
A zircon fission track-annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in α-decay event damage-free zircon samples. Empirically derived ...parallel and fanning equations for this “zero-damage” model yield an excellent fit to the data, with the fanning model providing slightly better statistical parameters. A comparison between annealing models with fanning iso-annealing lines but different α-decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero-damage model.
Compilations of existing geologic constraints on the zircon partial-annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial-annealing zone and closure temperature. This finding is consistent with the fact that the annealing behavior of zircon from long-duration temperature evolutions is increasingly influenced by the accumulated α-decay event damage. Zircon samples of young age or low U content show a behavior closest to the predictions of the zero-damage model, and are in the predicted range of published models with low α-decay event damage density. For thermal events of more than 10 myr duration, however, constraints from field studies show marked differences from proposed partial-annealing zone boundaries of the zero- or low-damage models.
The applicability of the zero-damage model is threefold. (1) It predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no α-decay event damage is accumulated. (2) It predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age, as found in sedimentary samples, and in this case may be used as a thermometer. (3) It represents an important reference for the establishment of a more comprehensive model of zircon fission-track annealing that also includes the influence of α-decay event damage. For such a model, two different equations are discussed. However, additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of α-decay event-damage annealing.
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