Project

Discipline

geospeedometry; Lepontine dome; hi-precision geochronology; orogenic heat budget

Lead
Questo progetto vuole capire le reazioni del sistema terra alle sollecitazioni tettoniche delle zone di subduzione e di collisione come quella Alpina ed acquisire delle conoscenze teoriche che potranno aiutare a comprendere i processi di deformazione anche in regioni sismicamente attive.
Lay summary
Nella crosta inspessita di catene montuose la distribuzione e la durata di fronti termici influiscono sulla dinamica delle collisioni di continenti contribuendo, per esempio, alla localizzazione della deformazione lungo faglie, talvolta sismologicamente attive. Tali fronti originano da fonti alloctone e il calore è trasportato per diffusione o per avvezione di fluidi caldi o magmi o sono prodotte in-situ grazie alla trasformazione di energia meccanica in energia termica. Questo progetto ha per obbiettivo la distinzione delle fonti di calore sopracitate allo scopo di estendere la comprensione della loro influenza sulla genesi delle falde e in generale delle orogeni. Le differenti fonti di calore del metamorfismo regionale delle Alpi Lepontine saranno quindi studiate tramite un approccio multidisciplinare che combina il rilevamento di terreno, la petrologia, la geochimica e la geocronologia con specifici modelli numerici.

Direct link to Lay Summary

Last update: 13.05.2019

Natural persons

This proposal seeks funding for a PhD studentship to study the thermal budget in the Lepontine Dome (Central Alps), a structural and thermal anomaly that contains within its carapace shear zones that define the subpenninic nappes. The project aims to unravel a longstanding debate of nappe formation in the Central Alps and its relation to collisional heat budget that can be divided into two endmembers: (i) allochthonous heat source (conducted and/or advected from the mantle) and (ii) in-situ heat source (derived from mechanical work (shear heating) and/or radioactive decay). New field data along a shear zone suggests that shear heating may be more important that commonly assumed. However, its influence on the Lepontine regional metamorphism and its extent are unknown. Quantifying the relative importance of such endmembers becomes a paramount factor to understand rheology and deformation of crustal processes within orogens. We plan to constrain the parameters controlling the heat sources (e.g. diagnostic time-scale and spatial distribution of metamorphism) with a multidisciplinary method that combines: (1) Detailed geological and structural mapping combined with precise profiles of shear zones. The mapping builds on the new large-scale geological map of the Osogna Swiss National Map no. 1293 (1:10’000) that the PI and the co-PI produced at the University of Applied Sciences and Arts of Southern Switzerland (SUPSI) for Swisstopo. (2) Petrography and geochemistry combined with thermodynamic modelling to estimate the temperature and pressure evolution of the studied rocks. (3) Geospeedometry based on major-elements diffusion in garnet to monitor the timing of prograde to peak and retrogressed PT-path above ~500°C. (4) High-precision ion probe U-Pb geochronology on zircon metamorphic rims of syn-tectonic migmatites to pinpoint the time of peak temperature condition during deformation (closure temperature (Tc) > 650°C) and LA-ICPMS U-Pb dating on rutile to date the cooling of the rock (Tc = 600-500°C). Both Tc lay potentially in the stability field of garnet permitting to accurately constrain the timing and rate of diffusion of elements in garnet. (5) These results placed on the proper geological framework of geological maps create a continuum of structural, metamorphic and geochronological data that will be implemented in thermo-mechanical codes in order to gain physical insights of the geodynamic processes involved. In the Lepontine dome, the thermal conditions peaked (i) at ca. 2 GPa/650-750 °C around 40 Ma in the upper Adula/Cima Lunga nappe and (ii) at ca. 0.8 GPa/600-700 °C between 32 and 22 Ma in the migmatites of the Southern Steep Belt (SSB). To construct the paleo-heat flow(s) (temporal distribution of lateral vs vertical metamorphic gradients) used to discriminate the different heat sources and geodynamic scenarios, we plan strategic sampling (i) along almost vertical transects cutting the nappe pile from the Adula/Cima Lunga 5 and 10 km away from the SSB. (ii) along N-S transects from the migmatites of the SSB into the core of the Lepontine dome. This study deals with a geodynamic problem intrinsic to most orogens. It requires collaborations with the geodynamic group at University of Lausanne (UNIL) and the Jackson School of Geosciences (University of Texas) and it utilizes the detailed mapping and the regional studies made by SUPSI. This project arose from the interaction between SUPSI, UNIL and the Federal office of topography Swisstopo and will be an example of efficient and synergetic collaboration among the Swiss universities of applied sciences, Swiss universities and Federal institutions. The University of Lausanne takes the administrative role for the PhD defence.