Search

Downstream changes of the geochronological signature in the Po drainage allow to assess the relative sediment contributions shed from the orogenic source areas, in order to constrain the short-term (100 to 10000 years) erosion pattern in the Alps-Apennines orogenic couple. In this perspective, we collected modern sands in selected tributaries draining the major tectonic units exposed in the orogen, as well as along the Po River trunk and in different sites of the Po Delta. Zircon concentrates were separated according to specific quantitative procedures coupled with grain-size and grain-density analyses, which provide an evaluation of zircon content in source rocks, of hydraulic sorting effects, and related intersample/intrasample zircon variability. Our methodological approach allows to perform sediment budgets and evaluate mineral content in diverse sediment sources (mineral fertility). Budget calculations were performed by investigating geochronological signals upstream and downstream of major confluences. We employed, LA-ICPMS U-Pb geochronology on detrital zircons, because this technique allows a full control of zircon populations during separation procedures and data analysis; we found, instead, that fission track dating on detrital zircon is affected and biased by selective grain loss. Results of U-Pb analysis performed on single grains show that all samples from the Po Delta yield similar grain-age distributions, and include grain-age populations that are rather constant both in age and size. Major peaks are found at ca. 30 Ma, ca. 280 Ma, and 400-500 Ma. Peaks in these distributions can be traced upstream up to their sources. The bulk of the geochronological signature of the Po Delta sands is displayed already upstream of the confluence of the Northern Apennines rivers, suggesting that short-term erosion rates were much lower in the Northern Apennine than in the Western and Central Alps. Age peaks typical of the eastern Lepontine dome also characterize the Apenninic tributaries, reflecting Oligo-Miocene sedimentary transfer from the Central Alps to the Adriatic foredeep, and next tectonic accretion of turbidite wedges to the Apennine belt. U-Pb analyses were complemented by zircon typology of the whole dataset, integrated by SEM observations on selected samples. Major typological groups can be traced from entry points in the Po Plain down to the Po Delta. Euhedral zircons are largely shed from the External Massifs (Mont Blanc and Argentera) and the Periadriatic plutons. Zircon grains showing signs of chemical abrasion, possibly acquired under high-grade metamorphic conditions, were detected in tributaries draining the Lepontine area.; Dans le bassin versant du Pô les modifications vers l'aval de la signature géochronologique permettent d'établir les contributions relatives des zones orogéniques sources, afin d'apprécier l'érosion à court terme (100 à 10000 ans) dans les Alpes et l'Apennin. Des sables ont été échantillonnés dans divers affluents du Pô, choisis selon les unités tectoniques qu'ils drainent, et le long du cours principal du Pô jusqu'à son delta. Les concentrés de zircon ont été extraits suivant des procédures définies, couplées à des analyses granulométriques et densimétriques de façon à évaluer la teneur en zircon des sources, les effets du tri hydraulique, comme les variabilités intra-échantillon et inter-échantillons. Cette approche méthodologique permet de réaliser des budgets sédimentaires et d'évaluer le contenu minéralogique des diverses sources (fertilité minéralogique). Les calculs de budgets ont été réalisés par la détermination du signal géochronologique à l'amont et à l'aval des confluents majeurs. Nous avons utilisé la géochronologie U-Pb par sonde à ablation laser couplée à un spectromètre de masse (LA-ICPMS) sur zircons détritiques, parce que cette technique permet un contrôle complet des populations de zircon pendant les procédures de séparation et autorise l'analyse des données; nous avons pu constater que la datation par traces de fission du zircon détritique, trop affectée par les pertes sélectives de grains, n'est pas adaptée à ce contrôle. Les résultats d'analyses U-Pb sur grains individuels montrent des distributions d'âges similaires pour tous les sables du delta du Po. Les pics majeurs de ces distributions, observés vers 30 Ma, 280 Ma et 400-500 Ma, peuvent être suivis vers l'amont jusqu'à leurs sources. L'essentiel de cette signature géochronologique existe déjà à l'amont de la confluence avec les rivières drainant l'Apennin septentrional. Ceci montre que l'érosion à court terme a été bien moindre dans l'Apennin que dans les Alpes occidentales et centrales. Les pics d'âges typiques de l'Est du dôme lépontin se retrouvent en outre dans les affluents issus de l'Apennin, reflétant un transfert sédimentaire ancien, à l'Oligo-Miocène, des Alpes centrales vers la paléo-fosse adriatique, suivi de l'accrétion de ces prismes turbiditiques à la chaîne de l'Apennin. Outre les analyses U-Pb, la typologie morphologique des zircons a été estimée grâce à des observations par microscopie électronique à balayage. Des groupes morphologiques peuvent être suivis depuis leur point d'entrée dans la plaine du Pô jusqu'au delta. Les zircons automorphes proviennent principalement du Mont Blanc, de l'Argentera (massifs cristallins externes) et des plutons périadriatiques. Des zircons portant des traces de corrosion chimique, peut-être acquises lors du métamorphisme de haute température, ont été trouvés dans les affluents issus du domaine lépontin.

Provenance studies based on detrital geochronological techniques are useful to investigate the present-day pattern of erosion in mountain belts (e.g. Carter and Bristow, 2000). In particular, LA-ICPMS U-Pb geochronology allows rapid generation of large datasets, and laser offers high spatial resolution for grains with complex age structure. The aim of this work is to investigate the downstream changes of detrital zircon U-Pb ages in the Po drainage, in order to constrain the short-term (10e2-10e4 a) erosion pattern in the Alps-Apennines orogenic couple. We collected samples of modern sands in selected tributaries draining the major tectonic units exposed in the orogen, as well as along the Po River main trunk and in different sites of the Po River Delta. Zircon concentrates were separated according to a specific quantitative procedure coupled with grain-size and grain-density analyses, which allows an evaluation of zircon content in source rocks, of hydraulic sorting effects, and related intersample/ intrasample zircon variability. Such analyses are crucial to perform sediment budgets and erosion-rate calculations based on single-mineral budgets. Preliminary results show that all samples from the Po Delta yield similar grain-age distributions, and include grain-age populations that are rather constant both in age and size. Peaks in the distribution can be traced upstream up to their source. Major peaks are found at ca. 30 Ma, ca. 280 Ma, and 400-500 Ma. Ages defining minor peaks cluster at 100-200 Ma and 500-700 Ma. The geochronological signature of the Po delta sands is essentially displayed already upstream of the confluence of the Northern Appennine rivers, suggesting lower short-term erosion rates in the Northern Apennine than in the Western and Central Alps. Age peaks typical of the eastern Lepontine dome characterize Apenninic tributaries, reflecting Oligo-Miocene sedimentary transfer from the Central Alps to the Adriatic foredeep, and next tectonic accretion of turbidite wedges to the Apennine belt. U-Pb analyses were complemented by zircon typology for the whole dataset, integrated by SEM observation on selected samples. Major typological groups can be traced from entry points in the plain down to the delta. Euhedral zircons are largely shed from the External Massifs and Periadriatic plutons. Zircon grains showing chemical abrasion, possibly acquired under high-grade metamorphic conditions, were detected in tributaries draining the Lepontine area. Carter A., and Bristow C., 2000. Detrital zircon geochronology: enhancing the quality of sedimentary source information through improved methodology and combined U-Pb and fission-track techniques. Basin Research, v. 12, 47-57.

This study examines the role of erosion on tectonic exhumation and deformation history of accretionary orogens. Our focus is on Taiwan, where interaction between active tectonics, earthquakes and cyclonic storms, are responsible for the highest erosion and deformation rates in the world. Two experiments are used to investigate the effects of erosion in the active bivergent orogenic wedge of Taiwan. The first model of thrust wedge without erosion is used as reference model. The second has been submitted to erosion under flux steady state conditions (1), to simulate an erosion pattern close to what is expected in Taiwan. First model shows the structure, morphology and evolution of classical high friction thrust wedges. Analytical measurements of the second model with high basal friction submitted to erosion, outline that the presence of weak layers (made with glass microbeads) that mimic décollements in the entering sequence, favors strain partitioning. Basal accretion of thrust units develops an antiformal nappe stack, whose growth and location is enhanced by erosion. A rapid uplift of underplated material occurs in the rear part of the wedge whereas frontal accretion characterizes the front of the growing prism. A zone of high exhumation develops in the retrowedge and migrates toward the backstop with continued shortening. This model is compared to the recent morphostructural evolution of Taiwan. In Taiwan, underplating characterizes the core of the orogenic wedge, where continental units from the subducting eurasian margin develops a regional scale antiformal nappe stack (in the Central Range), (2). Here, metamorphic rocks have been rapidly exhumed (in less than 4 Ma.) from depth of about 15 km due to combined effects of basal accretion and erosion. Our results emphasize the role of crustal décollements the internal dynamics of thrust wedge and how the coupling between tectonics and surface processes controls this dynamics, particularly the exhumation processes. References 1 – S.D. Willett, Brandon, M.T., 2002. Geological Society of America, v.30, no. 2, p. 175-178 2 – C.Y. Lu and Malavieille J., 1994. Earth Planet. Sc. Lett., v.121, p.477–494