Strong tectonic and weak climatic control on exhumation rates in the Venezuelan Andes

International audienceWe studied the relationships among present-day relief, precipitation, stream power, seismic energy, seismic strain rate, and long-term exhumation rates for the Venezuelan Andes. Average long-term exhumation rates were determined for seven large catchments in the Venezuelan Andes from fi ssion-track analysis of detrital apatite. A quantitative comparison between eight new detrital apatite fi ssion-track (AFT) age distributions presented here and previously published bedrock AFT age patterns shows that detrital AFT ages can be used for predicting exhumation patterns across the mountain belt. Catchment-averaged exhumation rates estimated from the raw data range from 0.48 ± 0.02 km m.y.-1 to 0.80 ± 0.26 km m.y.-1 Accounting for variable sediment yield and assuming that short-term sediment production rates scale with long-term exhumation rates, these rates vary from 0.33 ± 0.07 km m.y.-1 to 0.48 ± 0.08 km m.y.-1 No variation in rates is observed between the northwestern and southeastern fl anks of the mountain belt, despite a threefold increase in precipitation from the northwest to the southeast. Long-term exhumation rates are strongly correlated with relief in the different catchments, weak or negative correlations exist with precipitation data or present-day erosion indexes, while the correlation with seismic energy released by earthquakes is weak to moderate. This lack of correlation may be caused by the insuffi cient temporal range of the available precipitation and seismicity data, and the different time scales involved in the comparison. Long-term exhumation rates are, however, strongly correlated with seismic strain rates (which take the temporal earthquake magnitude-frequency scaling into account), suggesting that the moderate correlation with seismic energy is indeed related to the different time scales and that tectonic control on exhumation is signifi cant. In contrast, given that precipitation patterns in the Venezuelan Andes should have been installed during Miocene times, we suggest that decoupling of relief and exhumation from present-day climate explains the lack of correlation between exhumation and precipitation

Reconstitution de la morphogenèse Oligocène-Miocène des Alpes occidentales par une approche pluridisciplinaire

Le but de cette thèse est l'utilisation d'une approche multidisciplinaire qui combine des techniques d'analyse pétrologique, de la géochimie et de la thermochronologie afin de reconstituer l'évolution des Alpes occidentales pendant l'Oligocène et le Miocène et d'en déduire les implications géodynamiques. Ces techniques permettent à la fois d'identifier le bassin de drainage des sédiments et les taux d'exhumation dans ce bassin de drainage. L'enregistrement de cette évolution est préservé dans les bassins d'avant pays de chaque côté des Alpes occidentales en France et en Italie. Les techniques d'analyse pétrologique utilisées ici sont l'observation macroscopique, l'observation en lames minces, l'analyse par spectromètre Raman et l'étude de minéraux lourds. De nombreuses études ont été réalisées afin d'analyser les minéraux lourds des bassins alpins. Celles-ci permettent de déterminer la provenance des minéraux. Lors de ce projet, nous avons réalisé des analyses Raman sur des serpentinites permettant de distinguer les différents types de serpentinites. Or les Alpes internes montrent une gradation du métamorphisme croissant vers l'est, qui implique une variation des types de serpentinites vers l'est (association lizardites et antigorites dans les zones de basse température, antigorites exclusivement dans les zones de haute température). L'analyse de l'arrivée des différents types de serpentinites de part et d'autre de la chaîne permet de définir la position des réseaux de drainage dans les Alpes internes et de positionner la ligne de partage des eaux. La géochimie sur les basaltes détritiques permet d'analyser le type de basaltes et donc d'identifier leurs sources. Des basaltes non métamorphiques ont été identifiés en quantité importante dans les bassins d'avant-pays côté français démontrant la répartition importante de matériels océaniques obduits sur les Alpes internes à l'Oligocène. Les âges de thermochronologie détritique comparés à l'âge de dépôt permettent de déterminer le lag-time et donc le taux d'exhumation maximum de la zone érodée. En effet, la modélisation des isothermes permet de déterminer un taux d'exhumation à partir du lag-time. L'analyse des taux d'exhumation le long de la colonne stratigraphique à Barrême montre un pulse d'exhumation à partir d'une période très brève dans le temps : 30+-1 Ma à des taux d'exhumation compris entre 1,5 à 2 km/Ma, qui correspond à la mise en place des Alpes internes. Ces taux d'exhumation correspondent à des taux d'exhumation importants mais inférieurs à ce que l'on peut trouver dans l'Himalaya actuellement. Ils sont toutefois comparables à l'activité d'exhumation dans des montagnes jeunes. De récents travaux de modélisation montrent que le retrait de slab peut être consécutif à une rupture de slab profond. Notre équipe propose que dans les Alpes occidentales, la rupture et le retrait de slab a permis la mise en place du corps d'Ivrea comme un poinçon au dessus du slab.The aim of this dissertation was to use a multidisciplinary approach, combining petrologic, geochemical and geo-thermochronologic analyses, to reconstruct the topographic and exhumational evolution of the Western Alps during Oligocene and Early Miocene times, in relation to regional geodynamic events. Because the sedimentary record of this evolution is preserved in the foreland basins on the both sides of the Western Alps in France and Italy, this approach allows identifying sediment provenance and exhumation rates in the drainage areas. Petrological analyses used here were macroscopic observations in the field (pebbles counts), thin section analyses, and Raman spectrometry on detrital serpentinite pebbles and serpentine sand grains. The different serpentine species (antigorite, lizardite etc.), can be traced back to specific source lithologies because the metamorphic grade of the rocks exposed in the Western Alps increases eastward, with antigorite (HT serpentine) bearing rocks in the eastern piedmont complex and mixed lizardite-antigorite (LT serpentine) in the western piedmont complex. Analysis of serpentine species in the foreland basin deposits on both sides of the Alps allows determining changes in the paleo-Durance and paleo-Dora Riparia drainage areas and the position of the drainage divide, which have not changed since the Early Miocene. Major and trace element analyses of non-metamorphic basalt pebbles from the Barrême basin hint at the Chenaillet (or equivalent) obducted ophiolite in the internal Western Alps as the most likely source. Fission-track (FT) analysis of detrital apatite and zircon were used to determine maximum and average exhumation rates during the Oligocene. A pulse of fast erosional exhumation at about 30+-1 Ma had rates on the order of 1.5-2 km/Myr, while average rates were about 0.2-0.3 km/Myr. FT and U/Pb double dating of single zircons show that the signal of fast exhumation is not an artifact caused by volcanic contamination at around 30 Ma. The rapid creation of high relief and associated exhumation rates are related to isostatic surface uplift after slab break-off beneath the Western Alps at 35-30 Ma, followed by slab retreat which allowed emplacement of the Ivrea body vertical indenter that supports the high topography in the internal Western Alps.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Late Miocene – Recent exhumation of the central Himalaya and recycling in the foreland basin assessed by apatite fission-track thermochronology of Siwalik sediments, Nepal

International audienceThermochronological analysis of detrital sediments derived from the erosion of mountain belts and contained in the sedimentary basins surrounding them allows reconstructing the long-term exhumation history of the sediment source areas. The effective closure temperature of the thermochronological system analysed determines the spatial and temporal resolution of the analysis through the duration of the lag time between closure of the system during exhumation and its deposition in the sedimentary basin. Here we report apatite fission-track (AFT) data from 31 detrital samples collected from Miocene to Pliocene stratigraphic sections of the Siwalik Group in western and central Nepal, as well as three samples from modern river sediments from the same area, that complement detrital zircon fission-track (ZFT) and U-Pb data from the same samples presented in a companion paper. Samples from the upper part of the stratigraphic sections are unreset and retain a signal of source-area exhumation; they show spatial variations in source-area exhumation rates that are not picked up by the higher-temperature systems. More deeply buried samples have been partially reset within the Siwalik basin and provide constraints on the thermal and kinematic history of the fold-and-thrust belt itself. The results suggest that peak source-area exhumation rates have been constant at ~1.8 km Myr-1 over the last ~7 Ma in central Nepal, whereas they ranged between 1 and ~1.5 km Myr-1 in western Nepal over the same time interval; these spatial variations may be explained by either a tectonic or climatic control on exhumation rates, or possibly a combination of the two. Increasing lag times within the uppermost part of the sections suggest an increasing component of apatites that have been recycled within the Siwalik belt and are corroborated by AFT ages of modern river sediment downstream as well as the record of the distal Bengal Fan. The most deeply buried and most strongly annealed samples record onset of exhumation of the frontal Siwaliks along the Himalayan frontal thrust at ~2 Ma and continuous shortening at rates comparable to the present-day shortening rates from at least 0.3 Ma onward

Miocene to Recent exhumation of the central Himalaya determined from combined detrital zircon fission-track and U/Pb analysis of Siwalik sediments, western Nepal

International audienceFission-track (FT) analysis of detrital zircon from synorogenic sediment is a well established tool to examine the cooling and exhumation history of convergent mountain belts, but has so far not been used to determine the long-term evolution of the central Himalaya. This study presents FT analysis of detrital zircon from 22 sandstone and modern sediment samples that were collected along three stratigraphic sections within the Miocene to Pliocene Siwalik Group, and from modern rivers, in western and central Nepal. The results provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0 ± 1.4 Ma, and continuous exhumation at a rate of about 1.4 ± 0.2 km/Myr thereafter. The ~16 Ma cooling is likely related to a combination of tectonic and erosional activity, including movement on the Main Central thrust and Southern Tibetan Detachment system, as well as emplacement of the Ramgarh thrust on Lesser Himalayan sedimentary and meta-sedimentary units. The continuous exhumation signal following the ~16 Ma cooling event is seen in connection with ongoing tectonic uplift, river incision, and erosion of lower Lesser Himalayan rocks exposed below the MCT and Higher Himalayan rocks in the hanging wall of the MCT, controlled by orographic precipitation and crustal extrusion. Provenance analysis, to distinguish between Higher Himalayan and Lesser Himalayan zircon sources, is based on double dating of individual zircons with the FT and U/Pb methods. Zircons with pre-Himalayan FT cooling ages may be derived from either non-metamorphic parts of the Tethyan sedimentary succession or Higher Himalayan protolith that formerly covered the Dadeldhura and Ramgarh thrust sheets, but that have been removed by erosion. Both the Higher and Lesser Himalaya appear to be sources for the zircons that record either ~16 Ma cooling or the continuous exhumation afterwards

Differential exhumation driven by Tectonic processes in the Abancay deflection (Peruvian Andes)

The Abancay deflection (12-13.5°S), forming the northern edge of the Altiplano in the Central Andes of Peru, is a remarkable geomorphologic feature marking the along-strike segmentation of the Andes (Dalmayrac et al., 1980). Little is known about the timing and spatial distribution of exhumation in this peculiar part of the Eastern Cordillera for the last 40 Ma; however, it is characterized by 4 km-high relief and 2 km-deep gorges suggesting significant recent incision. To better constrain the exhumation and incision history of this northern edge of the Altiplano, we present apatite and zircon (U-Th)/He and fission-track data from 6 steep altitudinal profiles collected within the deflection (40 sampling sites). Thermochronology results highlight differential exhumation episodes between the Eastern Cordillera and the Altiplano with young thermochronological ages (15 Ma) respectively. We processed these ages into QTQt (Gallagher, 2012) and PECUBE (Braun, 2003) to discuss and unravel the exhumation timing, magnitude and settings of this area. Data inversion reveals the (re)activation of the crustal scale Apurimac fault system tilting the entire deflection like a pop-up structure (< 10 Ma) leading to differential exhumation between the Eastern Cordillera and the Altiplano. We speculate that he Abancay deflection, with its “bulls-eye” morphology and the abrupt increase in exhumation rate < 10 Ma, may represents an Andean proto-syntaxis, somewhat similar to the syntaxes described in the Himalaya or Alaska (Zeitler et al., 2001)

Control de la exhumación de la deflexión de Abancay (Andes del Perú) por procesos climáticos y/o tectónicos

La deflexión de Abancay se encuentra al norte del Altiplano peruano (12-13.5°S) y marca la transición entre los Andes estrechos del norte y los Andes anchos del sur. Es una anomalía excepcional de escala litosférica. En esta área peculiar de la Cordillera Oriental, numerosos macizos de rocas intrusivas afloran, conformando un relieve abrupto. Las quebradas muestran zonas de knickpoints ≥1000 m de desnivel, estas zonas en muchos casos coinciden con sistema de fallas regionales. Geológicamente, esta zona de transición arqueada, se extiende sobre ~200 km y presenta fallas regionales E-O, que cruzan el eje principal de elongación de la cadena. Esta área se caracteriza también por tener elevaciones >5500 m, siendo el pico más elevado el nevado del Salcantay (6271 m). A pesar de constituir una zona topográficamente importante en los Andes centrales, no existen suficientes trabajos que expliquen los procesos orogénicos de los últimos 40 Ma. En este sentido, es necesario utilizar técnicas modernas que nos permita entender el rol de la tectónica y del clima en la construcción de la topografía durante el Cenozoico. Varios autores desde los primeros tiempos de la geología indican que los paisajes activos a escala orogénica contienen un archivo importante que registra los patrones de la deformación diferencial y la erosión. Siguiendo esta indicación, primero nos enfocamos en cuantificar la exhumación, con el objetivo de determinar el levantamiento de la deflexión de Abancay. Para esto, realizamos trabajos de campo, llegando a obtener 40 muestras, que nos permitirán proporcionar nuevas edades de termocronología de baja temperatura ((U-Th)/He y trazas de fisión enapatitos). También realizamos análisis cuantitativos de geomorfología para estimar el estado de equilibrio del área estudiada a diferentes escalas de tiempo. Nuestros primeros resultados geomorfológicos (knickpoints, Ksn,χ) apuntan a un fuerte desequilibrio en el núcleo de la deflexión de Abancay, lo que sugiere un levantamiento de superficie muy reciente (15 Ma respectivamente

Representations and concepts of professional ethos among Swiss religious education teacher trainers

Over the past two decades, the organisation of religious education classes in Switzerland has undergone profound reforms. Amid the increasing secularisation and pluralisation of the religious landscape, many cantons have introduced a compulsory course that falls under the responsibility of the state and is aimed at teaching basic knowledge about a variety of religions. These reforms have enabled a harmonisation of the syllabi for religious education across the country and have prompted the adaptation of teacher training programmes. Because of the many diverse social expectations surrounding these new courses and the diverse academic tra- ditions in the field of religious education, however, a unified conception of these courses is still absent. In this article, we discuss the ongoing construction of religious education teachers’ professional ethos within this fluid context. In particular, we discuss the perspective of teacher trainers on pragmatic questions concerning religious plurality and the place of teachers’ and pupils’ personal (religious) experiences in the classroom, and pay attention to different representations of ‘religion’ and distinct ideas regarding the purpose of these courses as they have a major impact on the professional attitudes expected from teachers. Keywords: professional ethos; teacher trainers; Switzerland; concepts of religion; impartialit

Burial in the western Central Andes through Oligocene to Miocene ignimbrite flare-ups recorded by low-temperature thermochronology in the Cañete Canyon, Peru

Thermochronological data are essential to constrain thermal and exhumation histories in active mountain ranges. In the Central Andes, bedrock outcrops are rare, being blanketed by widespread late Palaeogene–Neogene and younger volcanic formations. For this reason, the exhumation history of the Western Cordillera (WC) in the Peruvian Andes has only been investigated locally along the mountain range. Dense thermochronological data are only available in canyons of the Arequipa (16° S) and Cordillera Negra regions (10° S). We present new apatite (U-Th)/He and fission-track data from the 1 km deep Cañete Canyon (13° S), where the Oligo-Miocene deposits are preserved lying conformably on an Eocene palaeo-topographic surface. Thermal modelling of thermochronological data indicate that the 30–20 Ma ignimbrite deposits overlying the bedrock were thick enough to cause burial reheating. We demonstrate that burial associated with thick volcanic formations should be taken into account when interpreting thermochronological data from the WC or in similar volcanic-arc settings

Early onset and late acceleration of rapid exhumation in the Namche Barwa syntaxis, eastern Himalaya

The Himalayan syntaxes, characterized by extreme rates of rock exhumation co-located with major trans-orogenic rivers, figure prominently in the debate on tectonic versus erosional forcing of exhumation. Both the mechanism and timing of rapid exhumation of the Namche Barwa massif in the eastern syntaxis remain controversial. It has been argued that coupling between crustal rock advection and surface erosion initiated in the late Miocene (8-10 Ma). Recent studies, in contrast, suggest a Quaternary onset of rapid exhumation linked to a purely tectonic mechanism. We report new multisystem detrital thermochronology data from the most proximal Neogene clastic sediments downstream of Namche Barwa and use a thermo-kinematic model constrained by new and published data to explore its exhumation history. Modeling results show that exhumation accelerated to ~4 km/m.y. at ~8 Ma and to ~9 km/m.y. after ~2 Ma. This three-stage history reconciles apparently contradictory evidence for early and late onset of rapid exhumation, and suggests efficient coupling between tectonics and erosion since the late Miocene. Quaternary acceleration of exhumation is consistent with river-profile evolution, and may be linked to a Quaternary river-capture event