peak pressure ; kinetics; stress variations; Monte Rosa nappe; heterogeneity; petrology; tectonics
Luisier Cindy, Baumgartner Lukas, Schmalholz Stefan M., Siron Guillaume, Vennemann Torsten (2019), Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm, in Nature Communications
, 10(1), 4734-4734.
Marger Katharina, Luisier Cindy, Baumgartner Lukas P., Putlitz Benita, Dutrow Barbara L., Bouvier Anne-Sophie, Dini Andrea (2019), Origin of Monte Rosa whiteschist from in-situ tourmaline and quartz oxygen isotope analysis by SIMS using new tourmaline reference materials, in American Mineralogist
, 104(10), 1503-1520.
Luisier Cindy, Baumgartner Lukas, Siron Guillaume, Vennemann Torsten, Robyr Martin (2019), H 2 O Content Measurement in Phengite by Secondary Ion Mass Spectrometry: A New Set of Reference Materials, in Geostandards and Geoanalytical Research
, 43(4), 635-646.
The tectonic and petrologic evolution of the Monte Rosa nappe, a major basement nappe in the Swiss/Italian Alps, is still incompletely understood. A main reason for this lack of understanding is that estimates of the Alpine peak pressure for the Monte Rosa nappe vary significantly, and published values range between 1.2 and 2.7 GPa. If these peak pressures are used to estimate the maximum burial depth of the Monte Rosa nappe, by assuming a lithostatic pressure, then the maximal burial depth ranges between approximately 40 and 90 km. Burial and exhumation from 40 km depth can be explained with orogenic wedge models, but for the exhumation from 90 km contrasting models are currently discussed. The peak pressures for the Monte Rosa nappe have been determined for different rock types, such as metamafics, pelites or whiteschists, and with different methods, such as geo-thermo-barometry or pseudosection modelling. There are three end-member possibilities to explain the peak pressure differences: (1) the Monte Rosa nappe experienced homogeneous metamorphic conditions, but different rock types recorded different thermodynamic pressures due to kinetic effects, for example, some rocks were so dry that certain prograde reactions did not take place. (2) The differences are due to different methods used to evaluate pressure and fluid conditions, or due to differences in thermodynamic data bases. (3) The Monte Rosa nappe was mechanically heterogeneous and experienced a heterogeneous stress so that the different peak pressure estimates are due to local stress deviations from the lithostatic pressure. The main aims of this project are (1) to map and quantify regional gradients of metamorphic conditions in the Monte Rosa nappe, (2) to investigate, with petrologic and petrographic tools, whether metamorphic gradients are due to a variation in chemical kinetic response, (3) to quantify potential stress variations in a mechanically heterogeneous Monte Rosa nappe with two-dimensional thermo-mechanical numerical simulations, (4) and finally, to compare petrologically-observed with mechanically-calculated pressure variations, and to provide a peak pressure estimate which has a regional significance providing a robust estimate for the maximum burial of Monte Rosa nappe. Chemical kinetic effects are studied by investigating equilibrium domains in mineral assemblages observed in the granite within individual thin sections. Water activity gradients will be assessed by H-concentration in biotite and phengite using secondary ion microprobe analytics. Strength contrast estimations for the Monte Rosa nappe will be based on a fold shape analysis of dikes which crosscut the Monte Rosa granite and the surrounding Schieferhülle. The dikes are folded only in the Schieferhülle but not in the granite indicating a significantly higher strength of the granite. The simulations will be performed with an existing finite element algorithm tailored to model lithospheric deformation for compressible and incompressible material, with viscoelastoplastic rheologies, gravity and thermo-mechanical coupling. Four mechanical model domains of the Monte Rosa nappe will be considered: strong granites / dikes and weak Schieferhülle / whiteschists lenses. Small lenses and dikes can be numerically resolved with the applied unstructured triangular finite element mesh. Three loading scenarios will be applied: loading with constant rate of shortening, loading with constant force and loading due to gravity only. We request three years funding for one PhD student to work on the thermo-mechanical modelling, and we request two and a half years funding for a PhD student who already started working on the petrological part of the project. The project will provide key constraints for the peak pressure in the Monte Rosa nappe which are essential for reconstructing the tectonic evolution of this nappe and the Alps in general.