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from different localities from the Schiener Berg in the context of the launching of map sheet Steckborn-Kreuzlingen of the Geological Atlas of Switzerland 1:25’000 (Zaugg & Geyer, in prep.) in order to understand the late stage of the tectonic evolution of the area. Prior to dating, fission track lengths were measured for all apatite samples to confirm their volcanic origin. Apatites were measured on an electron microprobe to check their composition. The limited time periods of volcanic activity at the Kaiserstuhl (16-19 Ma, Keller et al. 2002) and the Hegau area (15-7 Ma, Schreiner 1992) in combination of the significant differences in apatite composition (F-OH apatites from Kaiserstuhl, Cl-OH apatites from the Hegau) provide a basis for a clear distinction of the origin of the volcanic material. REFERENCES Bolliger, T. (1998): Age and geographic distribution of the youngest Upper Freshwater Molasse (OSM) of eastern Switzerland. Eclogae geologicae Helvetiae 91, 321-332. Gubler, T., Meier, M. & Oberli, F. (1992): Bentonites as time markers for sedimentation of the upper freshwater molasses: Geological observations corroborated by high-resolution single zircon U-Pb ages. SANW annual assembly, Basel, Abstract volume: 12-13. Hofmann, F. (1958): Vulkanische Tuffhorizonte in der Oberen Süsswassermolasse des Randen und Reiat. Kanton Schaffhausen. Eclogae geologicae Helvetiae 51, 371-377. Hofmann, F. (1959): Vulkanische Tuffhorizonte der Schienerbergeruptionen auf dem thurgauischen Seerücken. Eclogae geologicae Helvetiae 52, 462-475. Hofmann, F. (1961): Vulkanische Aschen in den Helicitenmergeln des baslerischen, aargauischen und badischen Tafeljuras. Eclogae geologicae Helvetiae 54, 133-136. Hofmann, F. (1975): Vulkanische Tuffe auf dem Wellenberg E von Frauenfeld und neue Funde auf dem thurgauischen Seerücken. Eclogae geologicae Helvetiae 68, 311-318. Hofmann, F., Büchi, U.P., Iberg, R. & Peters, T. (1975): Vorkommen, petrographische, tonmineralogische und technologische Eigenschaften von Bentoniten im schweizerischen Molassebecken. Beiträge zur Geologie der Schweiz, geotechnische Serie 54, 51pp. Kälin, D. (1993): Stratigraphie und Säugetierfaunen der Oberen Süsswassermolasse der Nordwestschweiz. Unpublished PhD thesis ETH Zürich, 238pp. Keller, J., Kraml, M. & Henjes-Kunst, F. (2002): 40Ar/39Ar single crystal laser dating of early volcanism in the Upper Rhine Graben and tectonic implications. Schweizerische Mineralogische und Petrographische Mitteilungen 82, 121-130. Pavoni, N. (1958): Neue Bentonitvorkommen in der Zürcher Molasse. Eclogae geologicae Helvetiae 51, 299-304. Pavoni, N. & Schindler, K. (1981): Bentonitvorkommen in der Oberen Süsswassermolasse und damit zusammenhängende Probleme. Eclogae geologicae Helvetiae 74, 53-64. Rahn, M. & Selbekk, R. (2007): Absolute dating of the youngest sediments of the Swiss Molasse basin by apatite fission track analysis. Swiss Journal of Geosciences 100, 371-381. Schreiner, A. (1992): Geologische Karte 1:50 000 von Baden Württemberg - Erläuterungen zu Blatt Hegau und westl. Bodensee. Geologisches Landesamt Baden-Württemberg, Freiburg, Stuttgart: 290p. Zaugg, A. & Geyer, M. (in prep.): Blatt 1033/1034 Steckborn-Kreuzlingen. Geologischer Atlas der Schweiz 1:25'000, Karte u. Erläuterungen. 1. Direct versus indirect thermochronology – what do we trully trace? Examples and implications for the Central Andes Ruiz, G.M.H. To quantify long-term denudation rates, research groups commonly applied low-temperature thermochronometric methods to rock now exposed at the surface. This approach on bedrocks from the hinterland is sometimes limited since erosion has often removed the record of earlier stages of orogenic growth. To overcome this shortcoming, researchers have increasingly studied since 20 years orogenic sedimentary records combining detrital thermochronological analyses with sedimentary petrography but also modelled detrital age populations from true bedrock catchments. We propose here to study the denudation history of regions located in the Central Andes of SE Peru. Our approach consists on analysing present-day erosional products along five different river catchments for the Apatite Fission-Track (AFT) thermochronometer. Up to four age populations were extracted from the analyses of 100 grains per sample. Age populations range between 80 and 0.5 Ma with a majority of age populations and grains younger than 10 Ma. These AFT analyses from Symposium 1: Structural Geology, Tectonics and Geodynamics
8 Symposium 1: Structural Geology, Tectonics and Geodynamics the ‘true’ present-day erosion product of the chain are compared with ones from an 'artificial' one we generated and this to investigate the recent evolution of the eastern Andes. The artificial detrital record was engendered by the combination of 197 individual grain ages we produced from a bedrock profile in the region (Ruiz in press). Interestingly, the 'artificial' sand express a clear homogeneous AFT signal with a single and pooled AFT age of 4.1 ± 0.1 Ma. This age is identical to the youngest age population (P1) we extracted from the 'true' sand within the same catchment (4.4 ± 0.4 Ma) and suggest that the ‘true’ dated grains of the P1 population were derived from, if not this one, a region with similar thermal record. Our results are of main importance because they indicate for the first time that a detrital age population, once statistically individualized and limitations of the method perfectly excluded, most likely reflects the erosion in a single part of a catchment. In the eastern Andes of Peru, the older age populations we extracted are probably derived from upper levels within the catchment that reflect by their presence, but not directly quantify, former denudation. Reversely, the youngest age populations for all present-day river sands are younger than 6.8 Ma. These data point towards lower levels of the eastern Andes that undergo rapid denudation and this since recent time (Ruiz in press) because of the preservation of older thermal record. The approach we developed is innovative and aims to reduce the amount of necessary analysis to constrain long-term denudation rates in different orogenic settings. It also hosts a methodological aspect by comparing results from direct (bedrock) and indirect (present-day river sands) thermochronological analyses within the same catchment. 1. 8 The Atlas Mountains: why there? Why now? Ruiz G.M.H., Negro F., Foeken J., Stuart Babault J., F., Ivy-Ochs S., Kober F., Saddiqi O., Stockli D., Champagnac J-D. & Frizon de Lamotte D. The target area is the WSW- ENE oriented intra-continental Atlas chain in Morocco located between the West Africa Craton and the Betic-Rif system. It is a key natural laboratory encompassing the Pre-Cambrian to recent evolution of the region even though the stratigraphic record is incomplete. The presence of high surface elevations in both the High-Atlas (>4000m) and Anti-Atlas (>2500m; Figs. 1 & 2) domains to the south is subject to discussions because there is little quantitative data available at present. Phases of uplift are thus ill constrained as places where the associated erosion products were accumulated. To better constrain the orogenic growth of the Atlas chain, we investigated the time-Temperature paths of bedrocks from two morpho-structural domains that are separated by the narrow Souss Basin, i.e. the High and Anti-Atlas (Fig. 1). Pre-Cambrian bedrocks from the Anti-Atlas domain yield old thermochronological Fission-Track ages on both zircon (390-300 Ma) and apatite (180-120 Ma) minerals that are associated with slightly younger Atlantic (U-Th)/He ages on apatite (120-110 Ma, Fig. 2). There are two possible (probably interfingering) interpretations for the preservation of such old thermal record: a) the Anti- Atlas was not affected by the 'Alpine' orogeny and (always) remain ‘stable’, or b) it is being affected since recent time but no level with such record is yet exposed. Bedrocks from the High-Atlas yielded (U-Th)/He ages on apatite comprised between 4 and 35 Ma (Fig. 1), which is a very different signal (Alpine?) than the one of the Anti-Atlas (Fig. 2). Age-elevation relationship suggests that denudation increased towards 1.0 km/my for the Late Miocene . Continental series of Cretaceous age from both Sub-Atlas domains indicate total resetting to temperatures greater than 80°C. Using a geothermal gradient of 25°C/km, this suggests that a post Cretaceous sedimentary pile of at least 3 kilometres in thickness is missing, which would be in agreement with sourcing from the High- Atlas region. The timing of the erosion of this pile is still unknown but it is being constrained by thermal modelling through Apatite Fission-Track analysis. Our thermochronological data provide for the first time constraints that evidence heterogeneous exhumation history between the different structural domains of the chain. Additional analyses using thermochronometers with higher temperature of closure (e.g. Fission-Track or (U-Th)/He on zircon) are being performed to investigate rates of medium to long-term processes. Our results are being compared with existing geological records from Morocco but also on the other side of the Atlantic ocean, to identify the relevant tectonic process that shaped the landscape of this mountain range in the frame of the Africa-Europe convergence but also, which is novel, opening of the Atlantic ocean.
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